INNOVATIVE STRATEGIES FOR THE UTILIZATION OF BIOMASS ASH

by

Adrian James
BSc., University of West Indies, Mona, 2002
MSc., University of West Indies, Mona, 2006

DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTORATE OF PHILSOPHY
IN
NATURAL RESOURCES AND ENVIRONMENTAL STUDIES

UNIVERSITY OF NORTHERN BRITISH COLUMBIA
October 2013

© Adrian James, 2013

UMI Number: 3581390

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ABSTRACT

Bioenergy production using woody biomass is a fast developing application since this fuel
source is considered to be carbon neutral. The harnessing o f bioenergy from these sources
produces residue in the form of ash. As the demand for bioenergy production increases,
ash and residue volumes will increase. Major concerns arising from the management of
this byproduct include: storage availability, usage, product disposal and the implications
of the presence o f unbumed carbon. This research studies various ash types, identifying
specific fractions o f technological, environmental and economic viability. Fractions of
energy importance, inorganic distribution and catalytic properties of specific ash types are
investigated. Ash from three systems were investigated, an industrial boiler, a fixed bed
updraft gasifier and a wood pellet burner. Analyses of the boiler ash included particle
fractionation, proximate and ultimate analysis, Brunauer-Emmett-Teller (BET) surface
area, thermogravimetric analysis (TGA) and bulk density. Samples were separated into
various fractions based on particle sizes. The fixed carbon in the as-received boiler ash
samples was 30 and 50 % and the higher heating value (HHV) ranged from 5 - 2 5 MJ/kg
o f the different fractions. 68 % or more of the energy could be recovered in fractions >
425 pm. High carbon ash was successfully gasified in a fluidized bed reactor at low
temperatures and atmospheric pressure. The pH o f the as received samples for the gasifier,
boiler and pellet burner were 10.36, 12.49 and 13.46, respectively.

Ni with a

concentration o f 229 mg/kg in the pellet burner ash, exceeded the maximum limit for soil
amendments (in British Columbia, Canada) within the particle size fraction > 850 pm but
< 2000. All samples were significantly enriched in both Ca (50-61 %) and K (10-26 %).
Wood ash derived catalyst obtained from a gasifier and a wood pellet burner influenced

gasification reactivity. The pellet burner ash was a more effective catalyst than the gasifier
ash at similar catalyst to char loadings. High carbon ash could have potential end uses as
a fuel and soil additive, while ash may be used as a fertilizer, liming agent and as a catalyst,
in all cases promoting the common goal of sustainable development.

Contents
ABSTRACT

.....................................................................................

ii

LIST OF TABLES............................................. .............................. ........................................................... .

vi

LIST OF FIGURES

...............

NOMENCLATURE

........................................

vfii
x

ACKNOWLEDGEMENTS ........

xii

CO-AUTHORSHIP STATEMENT............................

xiii

THESIS OUTLINE.................................................................

xiv

Theoretical F ra m e w o rk

xvi

..............................................................

2
CHAPTER 1: Ash M an ag em en t Review — A pplications of Biom ass B ottom A s h ..............
1.1
Introduction...........................................................................................................................................2
1.2
Ash from Biomass Com bustion.......................................................................................................... 4
1.2.1 Estimate o f Potential Increase in Ash Production........................................................................5
1.3 Elements in Ash of Environm ental Significance.....................................................................................8
1.3.1 Presence o f Metals in Ash......................................................................................................................9
1.3.2 Applications o f Ash fo r Soil Am endm ent and Agriculture............................................................. 10
1.3
Technologies in Place for Processing Unburned Carbon in Ash as a Fuel............................... 15
1.4
Reviews and Suggestions of Proposed Ash Processing M ethods...............................................18
1.5
Technological Implications W hen Processing A sh ...................................................................... 22
1.6.1 Softening and Melting o f A sh ............................................................................................................ 23
1.7 Conclusion................................................................................................................................................... 25
CHAPTER 2: C haracterization of Biom ass B ottom Ash from an Industrial Scale Fixed-bed
Boiler by F ractio n atio n ..................................................................................................................................33
2.1 Introduction..............................................................................................................................................33
2.2. Experimental Section............................................................................................................................... 36
2.2.1 Particle size distribution.................................................................................................................... 36
2.2.2 Proximate and ultimate analysis o f wood ash................................................................................ 37
2.2.3 Thermal analysis................................................................................................................................. 38
2.2.4 Surface Area......................................................................................................................................... 39
2.2.5 Heating Value...................................................................................................................................... 39
2.2.6 Bulk Density.......................................................................................................................................... 39
2.3. Results and Discussion.............................................................................................................................40
2.3.1. Particle size distribution o f ash.........................................................................................................40
2.3.2. Results o f proximate analysis and ultimate analysis.....................................................................41
2.3.3. Thermogravimetric analysis.............................................................................................................47
2.3.4. Surface Area........................................................................................................................................ 51
2.3.5. Higher heating Value (HHV).............................................................................................................52
2.3.4. Bulk Density......................................................................................................................................... 53
2.4. Summary of Results.................................................................................................................................. 56
2.5. Conclusions................................................................................................................................................ 56

CHAPTER 3: C haracterization of Inorganic E lem ents in W oody Biom ass B ottom Ash from a
Fixed-bed C om bustion System , a D ow ndraft Gasifier an d a W ood Pellet B urner by
F ra c tio n a tio n ...................................................
63
3.1. Introduction............................................................................................................................................... 63
3.2. Experim ental..............................................................................................................................................65
3.2.1 Particle size distribution.................................................................................................................... 65
3.2.2. pH analysis...........................................................................................................................................66
3.2.3 Concentration and distribution o f trace elements and major ash-forming elem ents............... 66
3.2.4 Anion analysis...................................................................................................................................... 67
3.3. Results and Discussion.............................................................................................................................68
3.3.1. Particle size distribution o f ash.........................................................................................................68
3.3.2. pH analysis...........................................................................................................................................69
3.3.3 The concentration and distribution o f elements in ashes and anion distribution..................... 71
3.4 Conclusion...................................................................................................................................................79
CHAPTER 4: Investigation of Air and A ir-Steam G asification of High Carbon W ood Ash in a
85
Fluidized Bed R e a c to r ................................
4.1. Introduction............................................................................................................................................... 85
4.2. Experim ental..............................................................................................................................................87
4.2.1 Feed materials......................................................................................................................................87
4.2.2 Gasification setu p .........................................................................................................
88
4.2.3 Experimental procedure..................................................................................................................... 89
4.2.4 Gas analysis.......................................................................................................................................... 91
4.2.5 Analyses o f experimental results....................................................................................................... 91
4.3. Results and Discussion.............................................................................................................................92
4.3.1 Air Gasification.................................................................................................................................... 92
4.3.2 Air-Steam Gasification........................................................................................................................97
4.4 Conclusion.................................................................................................................................................104
CHAPTER 5: Catalytic Effect of Calcined W ood Ash during CO2 G asification of Biom ass Char
108
5.1 Introduction.............................................................................................................................................. 108
5.2 Experim ental............................................................................................................................................ 110
5.2.1. Char and Catalyst Preparation.................................................................................................... I l l
5.3 Results and Discussion.........................................................
114
5.4 Conclusion.................................................................................................................................................123
CHAPTER 6: Conclusion and R e c o m m e n d a tio n s..............................................

126

136
APPENDICES.....................
APPENDIX A M oisture Analysis of Boiler Ash Chapter 2 & 3 ............................................................. 137
APPENDIX B Higher Heating Value Correlation Chapter 2 .................................................................138
APPENDIX C Gasification S etu p ...............................................................................................................139
APPENDIX D
Experimental Procedure for Gasification (Chapter 4 ) ..........................................140
APPENDIX E Sample Spreadsheet of Gasification Param eters (Chapter 4 ) ..................................... 142
APPENDIX F Flow M eter Calibrations for Gasification (Chapter 4 ]..................................................144
APPENDIX G Experim ental Data for Gasification (Chapter 4 )............................................................147
APPENDIX H Elemental Analysis (Chapter 4 ) ....................................................................................... 149

v

LIST OF TABLES
Table 1.1
Table 1.2
Table 1.3

Properties of woody biomass samples
Bulk densities o f different types o f wood
Estimate o f potential ranges of net ash production
based on complete wood residue utilization in
global leaders o f wood residue generation, and
complete wood fuel utilization in global leaders of
wood fuel production.

3
6
7

Table 1.4

Showing particle size and bulk density in the
combustion o f sawdust and shredded wood

22

Table 2.1

Proximate analysis for fixed bed boiler bottom ash
samples (B1 & B2)

42

Table 2.2

Ultimate analysis of fixed-bed boiler bottom ash
samples (B1&B2)

43

Table 2.3

Combustion characteristic of fixed bed boiler
bottom ash (B l) samples

51

Table 2.4

Higher Heating value of boiler bottom ash samples
separated within particle fractions

52

Table 2.5

Properties of boiler bottom ash o f major ash
fractions, based on 100 kg sample

56

Table 3.1

Total carbon contents and pH of bottom ash
samples o f boiler, gasifier and pellet burner

69

Table 3.2

Concentration and distribution of trace elements
for samples of boiler, gasifier and pellet burner
bottom ash (dry basis)

70

Table 3.3

Percent total metal distribution in boiler, gasifier
and pellet burner ash

72

Table 3.4

The concentration o f major ash forming elements
within particle size fractions for boiler, gasifier and
pellet burner on a dry basis

73

Table 3.5

The concentration (dry ash basis) of water soluble
phosphates relative to particle size distribution for
boiler gasifier and pellet burner ash

75

Table 4.1
Table 4.2

Proximate and ultimate analyses o f hog fuel
The effect of temperature on various parameters
during air gasification. Biomass feed rate: 176 g/h;
ER: 0.12.

88

94

vi

Table 4.3

Effect of ER on higher heating value, carbon
conversion efficiency and gas yield during air
gasification Biomass feed rate: 176 g/h;
Temperature: 775°C

Table 4.4

Effect o f S/B ratio on higher heating value, carbon
conversion efficiency and gas yield during airsteam gasification for biomass feed rate: 176 g/h;
ER: 0.12; Temperature: 775°C

Table 5.1
Table 5.2

Proximate and ultimate analysis of char
Inorganic elemental distribution of GA catalyst
and PBA catalyst

Table 5.3

Semi-quantitative data of mineral distribution in
GA catalyst and PBA catalyst as determined by
XRD
Reactivity index of char and char-catalyzed CO 2
gasification reactions at 50 % char conversion

Table 5.4
Table A1

Moisture content o f boiler ash obtained

Table El

Sample spreadsheet o f gasification parameters for
gasifying boiler ash

Table G1

Gas analysis and operating conditions for air
gasification o f high carbon wood ash

Table G2

Gas analysis for air-steam gasification o f high
carbon wood ash

Table HI

Metal analysis for boiler ash

LIST OF FIGURES
Figure 2.1

Percent retained weight against particle size (pm)
for boiler ash samples on a dry basis.

40

Figure 2.2a

Distribution of fixed carbon (g) and ash (g) in 100
g (dry basis) of B1 sample based on particle size
fraction distribution.

44

Figure 2.2b

Distribution of fixed carbon (g) and ash (g) in 100
g (dry basis) of B2 sample based on particle size
fraction distribution.

45

Figure 2.3

TGA graph of weight change as a function of
temperature for B1 boiler ash sample from a fixed
bed boiler.

47

Figure 2.4

DTGA graph displaying rate o f mass loss as a
function of temperature for B1 boiler ash sample
from a fixed bed boiler.

48

Figure 2.5

Bulk density as a function o f particle size (pm) for
boiler ash.

54

Figure 2.6

Bulk density versus percent fixed carbon for boiler
ash.

55

Figure 3.1

Fixed Carbon as a function of particle size
distribution for boiler, gasifier and pellet burner
ash samples

68

Figure 3.2a
and 3.2b

Correlation o f calcium (Ca) and phosphorus (P)
concentrations, respectively, as a function o f total
carbon found in boiler ash

78

Figure 4.1

Schematic diagram o f biomass air and air-steam
gasification in a bubbling fluidized bed

90

Figure 4.2

Effect o f temperature on gas composition. Biomass
feed rate: 176 g/h; ER: 0.12.

93

Figure 4.3

Effect of ER on gas composition. Biomass feed
rate: 176 g/h; Temperature: 775 °C

95

Figure 4.4

Effect of S/B ratio on produced gas concentrations.
Biomass feed rate: 176 g/h; Temperature: 715 °C,
ER: 0.12.

98

Figure 4.5

H2 /CO molar ratio as a function of temperature for
air gasification. Biomass feed rate: 176 g/h; ER:
0 . 12.

99

viii

Figure 4.6

H2 /CO molar ratio as a function o f ER for air
gasification. Biomass feed rate: 176 g/h;
Temperature: 775°C

100

Figure 4.7

H2/CO molar ratio as a function of S/B for airsteam gasification. Biomass feed rate: 176 g/h;
Temperature: 715 °C; ER:0.12

101

Figure 5.1a

Figure 5.1a. X-ray diffraction patterns of pellet
burner ash catalyst

116

Figure 5.1b
Figure 5.2

X-ray diffraction patterns o f gasifier ash catalyst
Observed char conversion during CO 2 gasification
with and without the addition o f PBA-catalyst and
GA-catalyst between 0-36 wt% catalyst loadings.

117
118

Figure 5.3

Gasification reactivity as a function o f char
conversion during C 02 gasification at 800 °C
using varying percent catalyst loadings for GAcatalyst and PBA-catalyst.

119

Figure B1

Higher heating value as a function of particle size
(pm) for boiler ash

138

Figure C l

Schematic diagram o f biomass air and air-steam
gasification in a bubbling fluidized bed

139

Figure FI
Figure F2

Calibration of screw feeder for high carbon ash
Calibration o f water pump at 50 % stroke

144

Figure F3

Calibration of the flow controller for air introduced
at the bottom of the reactor.

146

145

ix

NOMENCLATURE

(FAO)

Food and Agriculture Organization

AI

Alkali index (AI)

AR

As-received

BA

Boiler ash

BET

Brunauer-Emmett-T eller

d.w.

Dry weight

DTG

Differential thermogravimetric

EA

Elemental Analyzer

ER

Equivalence ratio

FC

Fixed carbon

GA

Gasifier ash

H2/CO

Hydrogen to carbon molar ration (mol/mol)

HHV

Higher heating value (MJ/kg or MJ/m3)

I.D.

Internal diameter (m)

IC

Ion Chromatography

ICP-MS

Inductively coupled plasma mass spectrometry

LHV

Lower heating value (MJ/kg or MJ/m3)

PA (PBA)

Pellet burner ash

PAH

Polycyclic aromatic compounds

r

Char reactivity (s‘1)

Rb/a

Base-to-acid ratio

Rs

Reactivity index (m in 1)

S/B

Steam/biomass

t

Time (s)

T

Temperature (°C)

to.s

Gasification time taken to reach a carbon conversion o f 50 %.

TG

Thermogravimetric

TGA

Thermogravimetric Analyzer)

VM

Volatile matter

Wash

Mass of ash in char sample after gasification

W0

Initial mass of char at the beginning of gasification (mg)

Wt

Mass o f sample at time t (mg)

wt.%

Weight percent

X

Carbon conversion

XRD

X-ray Diffraction

Pb

Bulk density (kg/m3)

xi

ACKNOWLEDGEMENTS

I am most grateful to my supervisors, Dr. Ronald W. Thring and Dr. Steve Helle for the
financial, motivational and supervisory support given to me throughout this research. Thanks
should also be extended to my committee members who also ensured I was guided towards the
right direction.

I would like to thank the University o f Northern British Columbia Central

Equipment Laboratory and the University o f British Columbia for their assistance in allowing me
to conduct sections o f this research in their laboratories. I would also like to acknowledge funding
support from Natural Sciences and Engineering Research Council of Canada (NSERC) and
Canfor Pulp Limited Partnership (CPLP). Thanks should also be extended to Drs. N. Ellis, J.
Grace and A. P Watkinson o f UBC for allowing me to use their fluidizing gasifier. I am thankful
to James Butler and M Masnadi for their assistance with the experiments and M. Sakaguchi for
the design, construction and testing o f the original equipment. None of this would have been
possible without the love and support o f my mother and father as well as those who motivated
me throughout the toughest stages of the research and writing process.
Gurkaran Sarohia, “yuh a dih boss”.

CO-AUTHORSHIP STATEMENT

I conducted all experimental work, data analysis, and prepared all the drafts o f manuscripts in
this thesis with the exception of the italicized comments. All drafts of manuscripts and
publications have been reviewed and strengthened through input given by my supervisors Dr.
Steve Helle and Dr. Ronald Thring. Other persons that have been instrumental in the successful
completion and editing of specific chapters are listed below:
Chapter 1 - H. Ghuman (contributedparts o f pages 4-8)
Chapter 2 - Dr. P. Rutherford
Chapter 3 - Dr. P. Rutherford, G. Sarohia
Chapter 4 - Dr. P. Rutherford, Dr. J. Grace, M. Masnadi
Chapter 5 - Dr. P. Rutherford, Dr. J. Grace, M. Masnadi (M. M asnadi assisted in carrying out
aspects o f the TGA experiments)

THESIS OUTLINE

The thesis is presented in a manuscript-based format, presenting various applications for different
types of ash, identifying specific fractions o f technological, environmental and economic
viability. Each chapter addresses specific concerns and identifies suitable process and
applications. Chapter 1 entitled “Ash Management Review - Applications of Biomass Bottom
Ash”, serves as an introduction as well as a comprehensive review paper, addressing issues related
to the management o f ash, specifically bottom ash. The second chapter, “Characterization of
Biomass Bottom Ash Obtained from an Industrial Scale Fixed-bed Boiler by Fractionation”,
discusses how the presence o f unbumed carbon in some bottom ash suggests potential beneficial
uses, for example as an energy source. A comparative study characterizes two bottom ash samples
obtained from an industrial scale fixed-bed boiler. The physical and chemical properties of each
bottom ash, as well as their respective particle fractions obtained by sieving, are analyzed and
discussed. “Characterization of the Inorganic Elements in Woody Biomass Bottom Ash from a
Fixed-bed Combustion System, a Downdraft Gasifier and a Wood Pellet Burner”, Chapter 3,
directs the reader’s attention to the environmental significance o f ash, particularly as a soil
additive. Chapter 3 is focused on identifying suitable applications of bottom ash based
predominantly on its chemical properties. Having identified and characterized ash with high
unbumed carbon content in Chapter 2, Chapter 4 investigates the feasibility o f gasifying high
carbon wood ash particles smaller than 3 mm. This chapter is entitled “Investigation of Air and
Air-steam Gasification of High Carbon Wood Ash in a Fluidized Bed Reactor”. Chapter 5
investigates the use of two types o f combustion ash, gasifier ash and a pellet burner ash, for their
catalytic effects on woody biomass CO2 gasification by varying the percent catalyst loading. This
chapter “Catalytic Effect of Calcined Wood Ash During CO 2 Gasification o f Biomass Char” will

report the pertinent findings. The thesis concludes with Chapter 6, providing a comprehensive
overview of the technological, environmental and economic importance of biomass ash. Chapter
6 contextualizes the findings of this research within the geographical framework of British
Columbia, Canada, but can be applied in other geographical regions. The theoretical framework
serves to conceptualize the research by outlining the objectives o f each chapter.

xv

T h e o re tic a l F ra m ew o rk
Investigation o f Air and Air-Steam
Gasification o f High Carbon Wood Ash
in a Fluidized Bed Reactor

Current State..
Increasing demandfo r
bioenergy.
■ Increasing ash
production and ash
volumes.
’

To d eterm in e :
• The b e h a v io r o f high carb on ash d u rin g g asification
• I he potential o f p ro d u c in g a low to m e d iu m calorific
value p ro d u c e r gas
• O p t im a l o p e ratin g con d itio n s d u rin g g asifica tio n

Catalytic Effect o f Calcined Gasifier and
Pellet Burner Ash during C 0 2 Gasification o f
Biomass Char
To d e te rm in e :
• T h e e f fe c tiv e n e s s o f u s in g w o o d as h as a
ca ta ly st
• The ra te o f c a rb o n c o n v e r s i o n s
• G a sific a tio n re a c tiv itie s

Ash Management
Challenge

Characterization o f Biomass Bottom Ash
from an Industrial Scale Fixed-bed
Boiler by Fractionation.
To determ ine:
• T h e p hysical and chem ical properties o f w o o d
ash and fractions o f specific particle sizes
• T h e therm al b e h a v io r using th e rm o g ra v im e tric
an aly sis t 1 G A )
• T h e fractions o f energy im p ortance

1How to manage and
utilize ash sustainably
and economically?

Primary Concerns
' Storage
>Disposal
' Usage
' Unburned carbon

Characterization o f Inorganic Elements in
Biomass Bottom Ash from a Fixed-Bed
Combustion System, Downdraft Gasifier and
a Wood Pellet Burner by Fractionation.
To d e te rm in e :
• T h e in o rg an ic e le m e n ta l d is trib u tio n o f
sp ecific p a r tic le siz e fra c tio n s
• T h e pH a n d a n io n d is trib u tio n
• C o rre la tio n s b e tw e e n c a rb o n c o n te n t
an d in o rg a n ic d is trib u tio n

xvi

Preface

Chapter 1 of this thesis is a version of the published article:
James, A. K.; Thring, R.W.; Helle, S.; Ghuman, H. S. Ash Management Review—Applications
o f Biomass Bottom Ash. Energies 2012, 5, 3856-3873.

1

CHAPTER 1: Ash M a n a g e m e n t R eview — A pplications o f B iom ass
B o tto m Ash

1.1 Introduction

In industrialized countries, it is expected that the future generation of electricity will be from
the direct combustion of residues and wastes obtained from biomass (1). Biomass boilers are one
medium for efficiently combusting the biomass and obtaining its energy. According to Demirbas
et al. (1), increased efficiencies can be attributed to large scale combustion processes, thus
improving heat recovery. Many combustion technologies are available for biomass combustion
such as fixed bed, fluidized bed and pulverized bed combustion (2). According to Saidur et al. (2),
fluidized bed combustion is the best technology to bum a fuel with low quality, high ash content
and low calorific value. In addition, the authors noted that the other firing systems present
limitations and are techno-economically unviable to meet the challenges o f biomass fuel
properties. Sandberg et al. (3) also noted that fluidized bed systems are the most suitable for
converting biomass into energy, because o f their ability to handle different fuels, flexibility, low
operating temperature and low emissions.
Because of the ash content that is present in biomass (see Table 1.1), boiler combustion
processes are known to produce large amounts o f ash. Also, as the demand for bioenergy
production increases the ash and residue volumes will increase. Major challenges will arise relating
to the efficient management o f these products. The primary concerns are ash storage, ash disposal,
ash usage and the presence of unbumed carbon. The continual increase in ash volume will result
in decreased ash storage facilities (in cases of limited room for landfill expansion), as well as
increased handling, transporting and spreading costs.

2

Table 1.1. Properties o f woody biomass samples (4)(5).
Proximate Analysis
(wt %)
Moisture
Volatile matter
Ash
Fixed carbon

Rice husk Rice husk pellet Larch dust Willow Miscanthus Pine
3.6
60
16.3
20.1

9.2
65.1
9.3
16.4

2.6
76.7
0.8
19.9

7.2
78.1
1.0
13.7

6.1
67.9
12.9
13.1

5.5
81.2
1.2
12.1

Due to the variety of biomass fuel sources with differing ash properties, finding one
application that will be suitable for all o f the ash is unlikely. Identifying the characteristics o f the
ash will provide valuable information as to the likely methods for processing. Gomez-Barea et al.
(6) proposed three main utilization categories for fly ash derived from biomass: (1) Use in
agriculture; (2) Use as fuel and (3) Use in construction.
The potential utilization of ash is influenced by contaminants such as heavy metals and the
extent to which the ash is sintered (7). Clean biomass contains minerals and important trace
elements and therefore can be recycled to forest grounds, however, these trace metals must be
clearly quantified and their impacts studied if they are to be applied to soils. Effective
environmental monitoring and protection must be carried out to ensure that ash disposal does not
become an environmental hazard.
In addition, high levels of unbumed carbon can be found in the ash produced from boilers.
According to Demirbas (8), the fly ash from biomass-fired grate boilers contain high levels of
unbumt carbon and is not suitable for recycling to the forest. Grate boilers often produce a fly ash
with 50% or more of unbumt carbon. The presence of this carbon indicates inefficient fuel use and
can reduce ash stabilization (chemical hardening) and significantly increases ash volume. If the
carbon contents are to be reduced it would become necessary to rebum the ash.

3

The utilization o f ash has also seen its application in the construction industry. According to
Gomez-Barea et al. (6), fly ash can be used as a cement replacement in concrete, for soil
stabilization, as a road base, structural filler in asphalt and asphalt base products, light weight
bricks and synthetic aggregate.
While much research has been conducted on fly ash utilization, a lot still remains to know
about the effective management and utilization of bottom ash.

1.2 Ash from Biomass Combustion

The ash content of wood chips normally depends on the bark content of the mixture since
the minerals are usually more concentrated in that region (9). Ash is the inorganic uncombustible
part of fuel left after complete combustion, and contains the bulk of the mineral fraction o f the
original biomass (7). Ash is an integral part o f the plant structure and consists o f a wide range of
elements (10). In wood, ash represents less than 2 percent, while in agricultural crop materials it
can be 5 - 10 % and up to 30 - 40 % in rice husks and milfoil.
Biomass-based products produce solid residue ash, a result o f thermochemical degradation.
These thermochemical processes include combustion, pyrolysis, and incineration of woody
biomass. Bottom ash and fly ash are usually the two types o f ash produced and may vary in
properties due to the different types of biomass available, operating conditions and the type of
system used. High ash contents significantly reduce the energy output derived from a specific
biomass source.

4

1.2.1

Estimate of Potential Increase in Ash Production
An estimate o f the potential ash production may be derived by carefully studying the

amounts of woody biomass that are used or may be used for processes that produce ash. The
analysis of Table 1.1 presents an example of the percentage composition o f ash based on different
types o f woody biomass. Subsequently this composition along with the quantity o f biomass
produced can be used to estimate the total amount of ash produced.
Wood residue forms a significant input for energy related uses such as in gasification,
pyrolysis, combustion and other systems based on harnessing the energy potential o f woody
biomass. Wood residues are defined by the FAO (11) as wood by-products which have not been
reduced to small pieces. They consist principally o f industrial residues, e.g., sawmill rejects, slabs,
edgings and trimmings, veneer log cores, veneer rejects, sawdust, bark (excluding briquettes),
residues from carpentry and joinery production, etc. Residues produced at industrial processing
sites, like bark and sawdust in sawmills, are the largest commercially used biomass source (12).
According to the Food and Agriculture Organization (FAO) (11), approximately 98.2 x 107
m3 o f wood residue was generated globally, as a yearly average from 1992 to 2010. For the sample
period the top five wood residue generating countries produced 15.3 * 107 m3 from China, 14 x
107 m3 from Brazil, 13 x 107 m3 from USA, 7.9 x 107 m3 from the Russian Federation and 7.7 x
107 m3 from France. These residues have the potential for supplementing current wood fuel
consumption. Comparing these statistics to coniferous wood-fuel used, USA had 9.5 x 107 m3 of
wood-fuel, 91 x 107 m3 for China, 13 x 107 m3 for Brazil, 19 x 107 m3 for the Russian Federation
and 2.7 x 107 m3 for France in 2010. This data can help us draw some conclusions about ash
production from the current wood-fuel use, and the potential increase in ash production from the
combustion of wood residues. Ash produced from wood residue or wood chips has distinct

5

chemical and physical properties that vary in part due to factors such as origin of biomass, type of
energy harnessing process, chemical reactions occurring during high heat conditions in the furnace
and storage and treatment of fuel (13). Literature reported values vary between 1% (wt %) ash
content for clean wood without bark to 5 - 15 % ash content for contaminated bark (14). An
estimate of the ash generated from potentially using wood residue may be obtained. Table 1.2
reports bulk densities for different kinds o f residual woody biomass. We may use these values and
arrive at an approximate value of bulk density for wood residue, equivalent to 0.16 and 0.21 ton/m3
for wood chips and wood fuel.

Table 1.2. Bulk densities o f different types o f wood (15).
Wood

Bulk density (ton/m3) Dry ash free tonnes

Hardwood chips
Softwood chips
Sawdust

0.23
0.18-0.19
0.12

Planer Shavings

0.10

Assuming that the entire wood residue produced in the world were to be incinerated, gasified
or combusted to harness energy we can use the bulk density of residue and weight percentage of
ash to establish an approximation of the ash produced. Table 1.3 shows the ranges of ash produced
from fuel wood and the potential addition by increasing the use of residual woody biomass in
combustion, incineration or pyrolysis processes.

6

Table 1.3. Estimate of potential ranges of net ash production based on complete wood residue
utilization in global leaders o f wood residue generation, and complete wood fuel utilization in
global leaders o f wood fuel production. The lower and upper limits use 5 and 10 % (wt %) ash
respectively.
Country
China
Brazil
USA
Russia
France

Ash from wood residue combustion Ash from wood fuel combustion
(105 tons)
(105 tons)
1.2-2.4
9.5-19.1
1.1-2.2
1.4-2.7
1.04-2.1
0.99-2.0
0.63-1.3
1.99-4.0
0.61-1.2
0.28-0.57

The ash is fixed at 5 - 10 % (wt %) for wood used in a commercial and large scale energy
systems. The lower limit o f 5 % and the upper limit of 10 % will give a good range for the quantities
of ash produced. This range is relatively higher for clean wood without bark (<1 % ash), but seems
to fit the values for the ash content of dominant types wood present in the wood available for use.
These include: bark (3 - 4 % ash), contaminated bark ( 5 - 1 5 %), contaminated reject wood (0.5
- 19 %) and clean reject wood (0.5 - 3 %) (14). We must also consider studies that suggest the
actual amount o f ash generated is higher due to inefficiencies in the boilers and furnaces. This
range will be used to calculate the upper and lower approximates o f ash produced from fuel-wood
and wood residue. These values for the upper and lower limits o f ash produced are reported in the
Table 1.3.
According to Obenberger et al. (16), while the 2005 production o f ash in European Union
amounted to 5.6 x 107 tons, the future trend in biomass for energy is expected to double by 2020,
and might lead to production of 15.5 * 107 tons of ash in the EU-27. Our estimates of current ash
production fall into proportion, as Europe produced 140 x 107 m3 of fuel wood for 2005. This
translates to about a range of 1.6 x 107 to 3 x 107 tonnes of ash produced in 2005 from domestically
produced fuel wood. Fuel wood imports and industrial utilization o f wood residues are not

7

considered in this estimate. They also contribute positively to the net ash production of a region.
A detailed calculation o f the total ash produced is difficult to determine. This is due to the lack of
understanding and information about all biomass sources and their net contribution to ash
production processes.
Literature reported values for current biomass use as a fuel compared to its potential use,
vary from 16 % in North America, 12 % in Latin America, 22 % in Europe and 108 % in Asia
(12)(17)(18). This averages to 38 % for the world (12)(17)(18). This implies that countries such
as USA, Canada, and other European nations have immense potential for developing bio-energy
based technologies. These figures point out clearly to a future increase in exploitation of this
potential, and a subsequent increase in ash generation. Limited understanding o f ash behaviour
and its environmental impacts acts as a hindrance for the complete utilization o f combusted wood
residue. The probable alternative fate for most wood-processing residues currently used for power
production is landfill disposal (19).

1.3 Elem ents in Ash of Environmental Significance

The major inherent ash forming elements in biomass include Ca, Si, Al, Ti, Fe, Mg, Na, K,
S and P (7)(9). The composition of ash affects its behaviour under high temperatures of
combustion and gasification reactors (10). These problems may include clogged ash-removal
caused by slagging ash, sintering, deposition, erosion, corrosion and pollutant emissions that are
mainly created by the presence o f alkali metals, alkaline earth metals, silicon, chlorine and sulphur
in the ashes (10)(20)(21).

8

1.3.1 Presence of Metals in Ash
The presence o f volatile heavy metals contained in ash residue may also have negative
environmental impacts if irrationally managed and disposed, due to the possible leaching into
underground and surface waters (20). According to Khan et al. (7), the potential utilization o f ash
is influenced by contaminants such as heavy metals which are often present depending on the
biomass source. According Demirbas (8), the composition o f ash is dependent on the plant species,
growth conditions and ash fraction. For example, Vamvuka (20) suggested that the high
concentration o f the Ni and Cr present in the olive kernel ash under study was most likely due to
the soil parent material. According to the author the soil type is laden with Ni/Cr which is
transferred to the plant through rootlets. Wood ash generally has a higher concentration of As, Cd,
Pb and Hg than agricultural residue, such as the ash from wheat, straw and fruit shells.
While the Khan et al. (7) review pointed out that the heavy metals are typically concentrated
in fly ash, these metals, though lower in concentration, are also present in bottom ash. With the
large quantities of bottom ash being generated annually, their metal concentrations and the
cumulative metal concentrations from deposition and landfilling must be investigated. Vamvuka
(20) investigated the thermal behaviour of olive kernel ash that was produced in a fixed and
fluidized bed combustor. The environmental impacts o f the ash upon disposal to local soils were
also analyzed. A 150 cm long with 7 cm inner diameter cylindrical stainless steel lab scale reactor
tube was used. For the fluidized bed, olive kernel with 1 % moisture was fed at a rate o f 480 kg/h
to a bed temperature of 900 °C. A batch o f 0.5 kg fuel was loaded for the fixed bed and air was
supplied at 6 m3/h with an excess o f 20 % to ensure complete combustion. Bottom and fly ash
samples were collected and analyzed for each reactor. According to Vamvuka (20), the elements
Cr, Cu, Ni and Mn were enriched in olive kernel ash derived from fixed bed experiments, while

9

the toxic elements Se and Pb were below 9 ppm. Cr had the highest concentrations o f 2000 ppm
which the researcher attributed to the soil parent type of the fuel. The results also showed that trace
elements were very low in cyclone ash and may have escaped in the flue gas due to the short time
for re-condensation during fluidised bed combustions. The leachates analysis produced negligible
quantities (ppb) o f all constituents except Cr, Se and Pb levels, <3 ppb were found to be the lowest
and Mn the highest at 5872 ppb.

1.3.2 Applications of Ash for Soil Amendment and Agriculture
However, some mineral nutrients o f the ashes may have a vitalizing effect on its application
to agricultural or forests soils. Olanders et al. (9) in their work reported that the ash from biomass
fuel contains only trace amounts o f heavy metals, which makes them fairly easy to dispose of and
they can be good fertilizers. A two part research was carried out by Gomez-Barea et al. (6) in
which they looked at the optimization of the operating conditions to achieve better ash quality and
then assessing the ash quality in order to explore its potential utilisation. Two types o f biomass,
orujillo and meat and bone meal (MBM) were gasified in a bubbling fluidized bed gasifier with
bed materials ofite and limestone. The operating temperatures ranged from 700 to 850 °C with a
fuel feed rate of 6-35 kg/h. The potential utilisation of ash as a soil conditioner, soil rehabilitation
and plant growing medium, soil nutrient and fertiliser and as a neutralizing agent and liming agent
was investigated.
High concentrations o f P and Ca were found in MBM ashes while high K levels were found
in orujillo ashes but low solubility levels were obtained for these elements for both fuel types
(Gomez). P in MBM ashes measured a solubility level o f less than 1 % in DIN leaching test. The
research concluded that due to this low solubility o f P and Ca, the use of these ashes in common

10

soils were doubtful for use as a fertilizer. In relation to the heavy metals, both orujillo and MBM
ash had a high Cr concentration but was thought to be as a result of the decomposition of the steel
in the reactor from abrasion. Of concern in this research according to Gomez-Barea et al. (6), was the
Cl content of the orujillo ash, which they highlighted as probably the main handicap of this ash. A Cl
content of 0.5 -1 .5 wt % was obtained for the fuel. Both MBM and orujillo had a high PAH, around
100 mg/kg. While sustainable methods are constantly being sought for the utilisation of fly ashes
Gomez-Barea et al. (6) concluded that fly ash from these two waste were not suitable for some
applications because of the high carbon content, chlorine content, alkali content and in some cases
heavy metal content. Physical or thermal pre-treatment of the ashes were proposed so as to make
the ash more usable. A few suggestions included: washing so as to remove alkali or chlorine
content, applying low temperature combustion for carbon removal and using high temperature
treatments for more persistent contaminants. The fly ash from waste material gasification did not
meet the requirements for fertilisers. These requirements were in accordance with the utilization
standards for, Metal limit values for ash utilization in cultivation in Finland and recommended
minimum and maximum values for components in ash produced in Sweden.
In similar research conducted by Nurmesniemi et al. (22), the physical and chemical
properties o f bottom ash and fly ash obtained from a 115 MW bubbling fluidized bed combustion
plant were investigated. One problem cited as influencing the research was the rapid increase in
large amounts of fly and bottom ash generation due to the increase use of wood-based biomass for
energy production. These energy sources are considered to be carbon neutral. Other problems
included the increased costs of landfill disposal in the form of waste tax or deposit fee as well as
the difficulties in acquiring new landfill sites and stricter EU landfill directives. Hence, the need
to find recycling options for ash. Ash from the bubbling fluidized bed boiler operated at 800 °C

11

was withdrawn and stored at 4 °C in a refrigerator. A strong alkaline pH value of 11.9 was recorded
for the bottom ash. This was attributed to some of the dissolved metals occurring as basic metal
salts, oxides and carbonates.
In relation to the liming effect o f the ash, Nurmesniemi et al. (22) investigated the acid
neutralizing value (NV) by looking at the cations of Ca, Mg and K. A NV 8.7 % (Ca equivalents,
d.w.) was obtained for bottom ash which they suggested that a ca. of 4.4 tonnes of this residue
would correspondingly be required to replace 1 tonne of commercially ground limestone. The
research concluded that fly ash would act as a better soil liming agent to neutralize soil acidity
than bottom ash. While the research agrees that fly ash is a better forest fertilizer, plant nutrient
agent and soil improvement agent than bottom ash, the large quantities o f bottom ash generated
makes it of environmental importance. For the bottom ash, elements such as Mg, Ca and K
concentrations (d.w.) were 0.6,6.0 and 2.6 % respectively while fly ash concentrations were 2.6%,
20.5% and 3.9% respectively. The researchers noted that the Cl levels were also below the Finnish
maximum limit value (2.0 %; d.w.) recording <0.1 % for bottom ash and 0.5 % for fly ash.
A total P of 0.3 % (d.w.) with a negligible water soluble P content was recorded for bottom
ash. The research noted that water soluble P is the amount of P that is readily available to plants.
The poor water solubility of P was highlighted as a draw back in the application o f wood ash to
soils since only a small portion P is extractable and available for plants when used in forest
fertilizers. On the other hand, the paper referenced Moilanen et al. (23), who suggested that waterinsoluble forms of P in forest fertilizers minimise the risk of P leaching into water bodies.
The research highlighted two heavy metals, Cr and Pb having concentrations that were 1.8
and 110 times higher in fly ash than bottom ash, respectively. The bottom ash concentrations for
Cr and Pb were 39 mg/kg and <3.0 mg/kg, respectively. Nurmesniemi et al. (22) concluded that the

12

concentrations o f all elements in the fly and bottom ash were lower than the Finnish limit values
and therefore does not restrict the use o f wood ash as a forest fertilizer.
Dahl et al. (24) conducted a series of similar studies on the heavy metal concentrations in
bottom ash and fly ash fractions from a large-sized (246 MW) fluidized bed boiler with respect to
their Finnish forest fertilizer limit in values. The study assessed whether the physical and chemical
properties, nutrients and heavy metals concentrations in the various ash fractions supported their
use as a forest fertilizer. Fifty percent o f the fuel was from forest residue while the other 50 % was
from commercial peat fuel. The results were in agreement with many o f the other studies, obtaining
high nutrient concentrations for the ashes. This would suggest the possibilities o f utilizing bottom
ash and fly ashes as a forest fertilizer. Dahl et al. (24) indicated that these ashes should be put to
better use than to be deposited at a landfill. In addition, the authors suggested that ash utilization
should be seen as an example of sustainable utilization of industrial residue since minerals would
be returned to the forest environment and would reduce the need for fertilizers. In all cases, the fly
ash concentrations were higher for all the heavy metals and alkali metals under study than bottom
ash. However, a point o f interest in the results was that the Hg content in the bottom ash was too
low for detection. With Hg being one of the elements of extreme environmental scrutiny a low
concentration is undoubtedly preferred in biomass ash.
Another interesting finding of this research related to the element As. A slightly elevated As
concentration of 40 mg/kg (d.w.) was seen in one of the fly ashes under study (24). According to
the paper, the Finnish limit for this metal was 30 mg/kg (d.w.), which makes the fly ash containing
40 mg/kg of As unsuitable for use as a forest fertilizer. On the other hand, if both fly ashes were
combined the residue could be suitable for use as a fertilizer since all other fly ash at different
particle sizes were all below 10 mg/kg (d.w.).

13

Most o f these researchers mentioned above seem to consider ash as an effective fertilizer
and soil amendment. While fly ash in some cases seems to contain high amounts of heavy metals,
alkali metals and PAH which may exceed the allowable environmental limits, bottom ash appears
to fall within the allowable limit. However, larger quantities of this bottom ash residue would be
required to provide the adequate soil nutrients.
Considering the facts that ash fractions have varied concentration o f metals, the application
of a mixture of bottom and fly ash to soils could also be explored. The utilization of bottom ash
has the advantage o f lower heavy metal concentrations but the disadvantage o f higher nutrient
losses. Mixtures o f fly ash and bottom ash may be useful to achieve optimum nutrient delivery
within limits for heavy metal concentrations (16). Also, if there is significant ash recycling to soil,
bottom ash and some fly ash could be combined while the additional fly ash would be directed to
landfill or other uses in order to prevent build-up of heavy metals.
It is also important to consider the origin of the biomass source and the characteristics of the
ash that will be produced. For example, Rejinders et al. (25) referenced the Minnesota Office o f
Environmental Services (26) who reported that the wet disposal of coal ash has been related to
abnormalities in animals. The elemental concentrations may vary with ashes from biomass or coal
or other combustion materials. In light of this, the ash from each fuel source must be carefully
analyzed before land applications are considered. Perhaps, constant testing and monitoring o f
landfills could also provide valuable information as to the cumulative long-term impacts that ash
storage could have. This could provide valuable information as to likely changes over time if
applied to soils.
Questions relating to the authenticity of the leachate test have also arisen. Reijnders et al.
argue in a review (25) that it is possible that an accurate analysis o f the leaching test conducted in

14

the laboratory may not accurately reflect the leaching behaviour in the field. The review noted that
reactions such as weathering, the dissolution o f amorphous phases, the formation of minerals, the
effect o f flow conditions, ionic strength o f pore solutions and kinetically determined processes can
never be truly determined from the laboratory phase.

1.3 Technologies in Place for Processing Unburned Carbon in Ash a s a Fuel

As previously mentioned, high contents of unbumed carbon in bottom ash or fly ash
indicates inefficient fuel use (8). This unbumed carbon concentration often varies for combustion
systems and has led a number of researchers to investigate the varying reasons for this change as
well as to determine the amounts of carbon that may be present in combustion ashes. According to
Bahadori et al. (27), when coal is combusted a potential significant loss is that of unbumed carbon.
According to Gomez-Barea et al. (6), the carbon present in fly ash is generally present in large
amounts, typically 10 - 60 % of the ash mass while Duan et al. (28) suggested a range o f 10 30%. Their work referenced Turner et al. (29) supporting the idea that all coal-fired steam
generators and coal-fired vessels inherently suffer an efficiency debit attributable to unbumed
carbon. Duan et al. (28) highlighted two possible reasons for the high carbon content in fly ash of
Circulated Fluidized Bed (CFB) boilers as the short residence time, resulting in incomplete burnout
o f char and the high ash content which covers the char and prevents the free movement of gases
to the core o f the char. There is a need to find efficient technologies that could be used to reduce
unbumed carbon content in both fly ash and bottom ash. Additionally, optimizing the use o f
existing technologies could also improve combustion efficiency.
Demirbas (8) proposed that the fly ash could be rebumed to remove additional carbon and
that CFB boilers are suitable for doing so since they are fuel-flexible and produce well burnout

15

ashes. The review noted that unbumt carbon has replaced 1-2 % of the fuel input to a CFB boiler,
reducing fuel costs and NOx emissions by about 20 - 30 % depending on the amount of ash.
However, Gomez-Barea et al. (6) looked at the idea of recycling fly ash in gasifiers but concluded
that this processing method would be technically impossible. While the researchers considered fly
ash from biomass to be a rational option as a fuel source in boilers and power plants, the heavy
metals, as well as Cl and K would severely limit the mixing o f these ashes with specific biomass
before feeding the boiler. The paper cited corrosion-derived problems caused by K 2 O and HC1 as
major reasons for rejecting the technique. However, useful suggestions were offered as to how to
effectively reduce the carbon content in the cyclone fly ash by their investigation o f the impact of
bed temperature on carbon content. When orujillo was gasified at different temperatures, the
carbon content in the fly ash decreased when temperature was increased (6). At 700 °C the carbon
content was 20.18 % while at 820 °C the carbon content reduced to 9.09 %.
Duan et al. (28) investigated a fly ash recirculation technique called fly ash recirculation by
bottom-feeding (FARBF). The fly ash was recirculated from the bottom o f the dense bed to the air
plate. A FARBF system was installed on a 75 t/h boiler burning mixture o f 60 % coal sludge and
40 % Chinese medium coal and was operated between the 980-1050 °C for the dense bed and 850
°C for the secondary zone. The experiments were accomplished by varying the recirculation rate,
making it 0, 25, 50, 75 and 100 % of the total fuel ash fed into the furnace. Each test was carried
out for 4 h until a stable condition was achieved. The boiler dense bed temperature decreased from
960 to 880 °C with a 70 % recirculation rate. This change was considered to be as a result o f the
fly ash absorbing heat when recirculated from the bottom of the boiler. The unbumt carbon in fly
ash saw a decrease from 14.1 to 7.5% as the recirculation rate increased from 0 to 8 t/h and from
14.1 to 8.8 with a 70 % recirculation. Duan et al. (28) explained that these results were obtained

16

due to the intensive material turbulence inside the dense bed, crashing into the shell of the ash and
exposing the unbumt carbon to the high temperature environment. Longer residence times were
also said to influence burning out o f the coal particles. The researchers concluded that FARBF can
help to reduce the unbumt carbon in fly ash thus improving combustion efficiency.
Batra et al. (30) looked at characterizing the unbumed carbon in bagasse fly ash obtained
from two different sugar mill boilers in India and found more than 25 % present. They noted that
the unbumed carbon posed disposal problems, presented obstacles when used in cement
compositions and would therefore be better if used for other applications. The research showed
that, industrially, carbon separation can be carried out using industrial scale sieve shakers for
removal of the coarse carbon rich fraction. Three fractions were investigated, “as-received”
fraction, sieve fraction greater than 425 pm and fractions between 150 and 425 pm. Maximum
carbon removal was obtained when a sieve fraction greater than 425 pm was used or when a
separation by fraction floatation in water was used. This conclusion was carried out by thermal
gravimetric analysis for the different fractions. The study concluded that over 25 wt % of unbumed
carbon is present in bagasse fly ash. They proposed that the high carbon content present in boiler
fly ash could be used as household fuel or gasifier feed after briquetting or pelletizing.
Carbon content in wood-ash has a direct impact on process efficiency and ash recycling in
that, as more carbon is converted the higher the efficiency and the lower the volume o f ash
generated. While there is obvious potential for bottom ash to be recycled as fuel not much research
has gone into the area. The researches mention mostly the idea o f recycling ashes into a CFB.
However, the need for more critical analysis of the unbumed carbon present in bottom ash and
effective ways of capturing this energy as a fuel need to be investigated. Most papers seem to
suggest that there is only energy potential in ash if pelletized or briquetted. As mentioned

17

previously, the carbon content in ash varies depending on the processing conditions, types of
combustion equipm ent and fuel source. Subsequently, process efficiencies will vary,
affecting the degree of unburnt carbon th at may be present in ash. Therefore, a backup
system or technology should be in place to deal with variations of carbon that may be present in
the waste or combustion residue. This will not only improve efficiency but will reduce the volumes
o f ash produced as well as the additional costs incurred for disposal.

1.4 Reviews and Suggestions of Proposed Ash Processing Methods
Most o f the work on ash and the presence o f unbumed carbon have been on fly ash especially
from coal. However, more research needs to be undertaken relating to bottom ash as the volumes
significantly increase industrially. Effective recycling and processing measures must be identified
to ensure maximum energy use so as to eliminate some of the ash related storage and disposal
problems.
The presence o f unbumed carbon in bottom ash suggests its potential for uses other than as
a waste product. In this dissertation, we present the separation of unbumed carbon in wood biomass
bottom ash from a fixed-bed combustion system by sieve fractionation, followed by the application
of the gasification technology to particle sizes of energy importance.
Demirbas et al. (1) in a review pointed out that biomass gasification is the latest generation
of biomass energy conversion processes and is being used to improve the efficiency and to reduce
investments costs o f bioelectricity generation. According to Quaak (10), gasification produces
gaseous fuel that is easily handled, produces very little excess air when combusted and contains
low levels of contaminants. For this reason, a gasifier may be used to harness any additional energy
that the conventional boiler could not obtain by using the boilers waste as a fuel for the gasifier.

18

Biomass gasification is one o f the technologies o f energetic use o f biomass as heat and
electricity may be produced from using such process (31)(32). It is a thermochemical process of
gaseous fuel production by partial oxidation of a solid fuel (32). In this process, the chemical
energy o f the solid fuel is converted into the chemical and thermal energy o f the product gas. The
result of gasification is the producer gas, containing carbon monoxide, hydrogen, methane and
some other inert gases (31).
An updraft fixed gasifier may be used to combust the unbumed carbon. Fixed bed gasifiers
are relatively simple, high charcoal burnout and an internal heat exchange that leads to low gasexit temperatures and high energy efficiencies (10)(32). The system also allows for variation in
fuel particle sizes as it is able to process relatively small fuel particles (10). The presence of tars
in the product gas is usually a concern in this system. However, it is anticipated that the boiler ash
will have very little negative impacts on the gasifier as it relates to the presence of tars. This is
because most of the tar would have been consumed from the boiler combustion stage. O f particular
concern is the potential slagging that may result due to the high ash content of the feedstock.
According to Quaak et al. (10) the updraft gasifier is able to accommodate fuels with a maximum
moisture content o f 60 %, particle size 5-100 mm, maximum ash content of 25 % and should be
able to produce 5 - 6 kJ/Nm3.
The energy recovery may be measured based on the composition of the gases produced.
Hence, the ash-carbon content will be reduced using gasification with energy recovery in the form
o f heat and syngas production. The application o f gasification technology in the reduction o f ashcarbon content and the ability to recover any useful energy in the combustion residue presents
promising prospects.

19

In order to separate and characterize ash and unbumed carbon based on its particle size, the
“as-received” ash must be separated in its fractions by sieving. Sieve sizes ranging from 0 to 2000
pm may be used but the upper limit is based on the size of combustion residue obtained. Each
separated fraction must be characterized in terms of its physical and chemical properties in order
to identify fractions in bottom ash with high organic/carbon content and those with high mineral
content as well their impact on the overall volume of ash stored or disposed of. Based on their
characteristics, suitable applications for each fraction can be determined.
Limited information and results were found relating to the sieving of ash for unbumed carbon
recovery. Alternate suggestions for effective utilization o f inorganic content in ash included the
pre-treatment o f ash whether by washing or sieving (6). It is hoped that some metals could be
washed and Cl in particular could be removed from these ash fractions using these techniques.
According to Dahl et al. (24) sieving methods have been applied to process ash containing
heavy metals, separating them into various fractions. This separation produces ash fractions with
low heavy metal concentrations, making them applicable in road construction and cement blends.
Sieves were used to separate ash into particle sizes 2.0 - 0.5, 0.5 - 0.125 and <0.125 mm. The
elemental concentrations o f the heavy metals were noted based on the particle fraction. The bottom
ash remained within the limits of elemental concentrations for all the particle sizes, however, one
fly ash of particle size lower than 0.125 mm exceeded the limit value o f As, while Pb exceeded its
environmental limit within particle size 0.5-2.0 mm. However, it should be note that any pre­
treatment will incur additional processing costs.
It is anticipated that fractions containing high levels o f unbumt carbon could be a useful fuel
source in the gasification process. As previously mentioned, some researchers have proposed
pelletizing or briquetting bottom ash for fuel use. Research results for pelletization of bottom ash

20

for use as a fuel could not be found, even after an extensive literature search. However, limited
papers and studies exist as it relates the pelletization of bottom ash from biomass for other uses.
In must be noted that this process comes with inherent challenges. According to Lovgren et al.
(13), a high content o f unbumt organic matter interferes with the agglomeration o f ash particles.
To deal with this situation, pelletizing (roll pelletizing) has proven to be an efficient method. This
method helps to prepare ash for recycling o f basic cations, and acid neutralizing lime components
back to the forest (33)(34).
Several studies have described hardening and carbonation (treatment with carbon dioxide)
o f ash as an effective method to handle ash. Accelerated carbonation positively affects the
hardening o f an ash product, which may then be easily transported and used as a fertilizer. Also,
well hardened products show slow leaching patterns, which are considered good for applications
in fertilizing land (35)(36)(37). However, it is difficult to harden ash with high carbon content.
In a TNO Report (14), it pointed out that if ash is to be re-used it is important to know its
particle size and particle density. These will better aid in understanding the potential of ash for
environmental applications and its contribution to reducing ash volumes. Obtaining the bulk
densities for various particle sizes could give valuable information on dumping and storage cost.
According to the TNO-report, the dumping costs in The Netherlands can be relatively high. A
point of interest is that ashes produced from different wood sources may vary in bulk densities.
The report suggested that the difference in bulk densities were relative to the fuel source and the
combustion process and technology employed (See Table 1.4).

21

Table 1.4. Showing particle size and bulk density in the combustion of sawdust and shredded
wood (14).
Type of fuel ash fraction
Grate fire ash
Sawdust
^
„
Cyclone fly ash
ou
j
, Grate fire ash
Shredded Wood
Cyclone fly ash

Particle size (pm)
10-30.000
2-100
15-15.000
2-160

Bulk density (kg/m3)
662
283
960
430

1.5 Technological Implications When Processing Ash
While there are obvious benefits to some soil properties, other ash related problems remain
as some o f the main obstacles to the economical and viable applications of biomass gasification
(21). According to Vamvuka (20), the successful design of a combustion system using agro­
residue as feedstock will partly depend on the ability to control the technical and environmental
problems associated with the inorganic constituents.
During combustion some of the inorganic species formed are a result of the interactions with
the organic portion of the biomass. According to Olanders et al. (9), the organic structures are
decomposed and the ash formers are released. The alkaline earth metals leave the combustion zone
as solid particles while the alkaline metals are transported in vapour form as chlorides, hydroxides
and oxides. These species can react with SO2 to form sulphate particles which results in the
formation of hard deposits on surfaces leading to corrosion problems (9)(38). Olanders’ et al. (9)
research showed that the first steps in ash-forming process involves calcium, potassium, silicon,
sulphur and chlorine forming carbonates, sulphates, chlorides and small amounts of silicates. A
fixed-bed furnace with temperature 1100 - 1200 °C for wood fuels and 1000 - 1100 °C for straw
was used to make such conclusions.

22

Obemberger et al. (39) in an analysis of the various inorganic elements in biomass fuels,
noted that Si in combination with K can lead to the formation of low melting silicates in fly-ash
particles, K is relatively volatile, forming chlorides, hydroxides and sulphates which plays an
important role in corrosion. Ca also forms chlorides and sulphates but is less volatile than K and
generally increases the melting point of ashes which are also true for Mg. It was also observed that
at higher temperatures and longer heating times, oxides and silicates become dominant. A notable
trend also was that silicon and iron are more effectively bound in bottom ash under oxidizing
conditions than reducing conditions while Ca was less (9).

1.6.1 Softening and Melting of Ash
If ash is to be rebumed, the likely conditions under which it softens and melts must be
known. Olanders et al. (9) used TGA/DTA determination on ash collected from the same fixedbed furnace mentioned above, to understand the various temperatures at which softening and
melting would occur relative to the biomass type. The research showed that straw ash softened at
a much lower temperature than wood/bark. Initial softening of straw ash was at 700 °C and melting
occurred at 975 - 1025 °C. On the other hand, wood bark softened at 1000 - 1200 °C and had a
melting point of 1480 °C. Ohman et al. (40) showed initial melting at 850 - 1025 °C. According
to the research, the lowest melting temperature when stored bark was used as fuel, occurred at 866
± 7 °C. All other samples had temperatures between 980 °C and 1025 °C. The research concluded
that the total ash and critical inorganic elements in some raw materials could result in ash related
problems such as slagging and the forming of deposits on burners. They showed that the elemental
distribution in the slag samples varied significantly between samples for different fuels. According
to Vamvuka (20), fixed bed ash softens at 1221 °C and fluidises at 1258 °C for olive kernel ash.

23

On the other hand, fluid bed fly ash softens at 1293 °C and fluidises at 1360 °C for the same ash.
When combustion takes place in fixed beds, these should be operated at temperatures below 1100
°C, to avoid ash melting and accompanying problems.
A number of researchers have tried to determine the likelihood o f slagging and fouling
occurring. While a definite number cannot be ascertained, equations have helped in such
predictions. According to Ohman et al. (40) slagging is considered to be the melting of ash. Two
important parameters are the alkali index (Al) and the base-to-acid ratio (Rb/a) of wood-ash (24).
Vamvuka (20) noted that the alkali index (Al) expresses the quantity o f alkali oxides in the
fuel per unit of fuel energy:
AI = kg(K20 + Na20)/GJ
According to Vamvuka (20), when Al values are in the range 0.17 - 0.34 kg/GJ fouling and
slagging is probable, while when these values are greater than 0.34 fouling or slagging is virtually
certain to occur.
The base-to-acid ratio (Rb/a) can also be used to help determine the likely hood o f fouling of
the ash. This can be written as follows (24):
% (Fe20 3 + CaO + MgO + KzO + Na20)
_
% (Si02 + Ti02 + A120 3)
The label for each compound makes reference to its weight concentration in the ash. As Rb/a
increases, the fouling of a fuel ash increases.
A number o f researchers have also investigated the addition o f other compounds to minimize
or prevent softening or melting of the ash. Ninomiya et al. (41) showed that efficient gasification
of a particular coal depends, sensitively on the melting behaviours of the ash produced from that
coal and that the high melting temperature ash can be controlled by adding basic oxides which

24

cause a decrease in ash melting temperatures and slag viscosity. The work showed that CaCC>3
additive is an efficient fluxing element for the control o f ash melting, particularly, AhCh-rich ash
melting (20). Wilen et al. (42) also suggested that the addition o f a powdery additive, kaolin, talc
and ceramic feldspar would increase the fusion temperature. While this would increase the cost of
pellets by 5% and also increase the ash content thereby lowering the heating value, it would also
result in cheaper maintenance and equipment cost.

1.7 Conclusion

Biomass based energy systems for heat and electricity will have an important place in the
overall energy setup to meet increasing consumer demands. Woody biomass in the form of wood
chips, wood residue, planer shavings, sawdust etc. forms the basis for most combustion processes
that uses the energy value o f this waste material. All o f these combustion systems produce a
significant amount of ash, which varies from 5 to 15 % (by weight) o f biomass processed. A careful
evaluation of data for wood residue and wood chips, the two major sources o f wood biomass
available for energy generation, shows us that current use levels are well below potential. These
facts along with the shift towards cleaner and carbon neutral fuels are expected to contribute to an
exponential increase in ash production across the world. This has significant implications for waste
management and handling. The limited understanding of ash behaviour, properties and its long­
term environmental impacts pose a risk in the scenario of excessive ash generation. The importance
of better technologies for producing energy from biomass is also significant. Improved
technologies that produce less ash volumes and increase carbon reduction should be considered to
assist in ash management related issues.

25

Ash utilization is limited by the presence o f heavy metals and other inorganic compounds,
which are formed as a result of the thermochemical reactions that the biomass undergoes when
combusted. The variability in heavy metal concentration in ash arises from the differences in
properties of feedstock, and hence no singular inorganic composition profile for ash is true.
Subsequently, no one application will be suitable for all kinds of ash. Inefficiencies in boilers and
furnaces also result in high percentages of unbumed organic matter in ash. This carbon content
may be recycled to the boiler or furnace to improve energy output and increase the process
efficiency. Suggested uses for ash includes the application of ash as agricultural fertilizers, as a
fuel due to the presence o f high unbumed carbon content and/or as an additive in construction
materials.
The presence o f alkali metals, alkaline earth metals, chlorine, sulphur and silicon influences
the reactivity and leaching to the inorganic phases. Ash may be utilized as a neutralizing and liming
agent. Research also indicates that bottom ash has significantly lower concentrations o f heavy
metals than fly ash, as such, a mixture o f fly ash and bottom ash may be suitable for application as
a soil amendment to forest soils. This should assist in maintaining the nutrient cycling instead of
landfilling these important nutrients.
Unbumed carbon present in ash allows for the exploration o f using ash as a fuel. This
unbumed organic matter has been investigated as a fuel source in some studies that suggest
recirculation of ash, increasing residence times and increasing material turbulence inside the
boiler. The presence of carbon in ash limits its applications as it decreases its binding properties
in construction material. The high carbon content also presents challenges for pelletization and
briquetting as it decreases the binding properties.

26

The need for other ash processing methods should be investigated. We propose sieve
fractionation as a suitable method for the separation of unbumt carbon present in bottom ash
obtained from a fixed-bed combustion system, followed by the application of the gasification
technology to particle sizes o f energy importance. The use of the gasification technology in the
reduction o f ash-carbon content and the ability to recover any useful energy in the combustion
residue presents promising prospects. Sieve fractionation may also assist in identify fractions in
bottom ash with high organic/carbon content and those with high mineral content as well their
impact on the overall volume o f ash stored or disposed of.
If ash is to be rebumt, the likely conditions under which it softens and melts must be known
since this may cause severe negative impacts on the combustion equipment leading to corrosion.
It is expected that the production o f bottom ash will increase greatly; therefore, continued research
is needed to find suitable applications and processing technologies for ash.

27

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Preface

Chapter 2 o f this thesis is a version of the article accepted for publication:
James, A. K.; Thring, R.W.; Rutherford, P. M.; Helle, S. Characterization of Biomass Bottom
Ash from an Industrial Scale Fixed-bed Boiler by Fractionation. Energy and Environment
Research 2013.

32

CHAPTER 2: C h a ra cte rizatio n o f B iom ass B o tto m A sh fro m a n
In dustrial Scale Fixed-bed Boiler by F ractio n atio n

2.1 Introduction
Increased use o f bioenergy has significantly increased ash generation in many countries.
Large-scale utilisation of wood and wood residues for district heating plants, process heating plants
and combined heat and power plants has resulted in large volumes o f ash production. It is projected
that by 2020, the thermal application of biomass in European countries will result in an annual ash
production o f approximately 15.5 million tons per year (1).
Ash formed during biomass combustion can be divided into bottom ash and fly ash (2).
Bottom ash is the fraction produced on the grate in the primary combustion chamber. While many
studies have investigated fly ash, bottom ash usually accounts for 60 to 90 % of the total ash
generated (1).
A number of barriers to the utilization of wood ash have been identified, including heavy
metal concentrations and the presence of organic pollutants (1). High amounts o f organic carbon
in the ash indicate incomplete combustion o f biomass, which suggests inefficient fuel use (2)(3).
The presence of carbon in ash limits its application in forest soils since carbon dilutes the liming
and fertilization effects and also restricts the hardening o f the ash (3)(4). Ash hardening is used to
improve handling, reduce dust and to decrease the ash dissolution rate. Unbumed matter, as
indicated by high carbon content, increases ash volumes and results in higher handling,
transportation, disposal and spreading costs (3)(5). For many environmental applications, it has
been shown that the carbon content should be below 5 wt % (dry basis) (6). In addition, the high

33

combustibility o f char, restricts the application of high carbon ash to some lands due to increased
forest fire risk (7).
According to Emilsson, a high content o f uncombusted material in ash leads to difficulty in
the ash hardening for ash use in construction materials and also suggests that high charcoal content
in the ash can be recombusted (8). High content of uncombusted material also poses problems
when used in cement compositions.
The relationship between bulk density and the presence o f unbumt carbon in ash is important
when considering ash transport and storage. High bulk densities may reduce ash volumes, thus
decreasing the need for additional storage space and reducing transportation costs. Bulk density
decreases with the fineness o f the ash fraction (6). The authors reported that ash from straw and
cereal combustion showed low density due to its specific chemical matrix, containing more salts
and less minerals when compared to wood ash. The bulk density o f bottom ash from bark
combustion and woodchip combustion, both from a moving grate underfeed stoker, had densities
of 950 kg/m3 (9). On the other hand, the bulk density of bottom ash from sawdust combustion
(from an underfeed stroker) was 650 kg/m3. Thus, fuel type may influence the bulk density of the
produced ash.
While improving combustion system efficiency is integral to reducing the carbon content o f
bottom ash, identifying effective recycling methods are also essential in dealing with high carbon
ash. The unbumed carbon content in bottom ash varies amongst combustion systems. Short
residence time (10) and high ash content are considered as major factors for the incomplete burnout
of char. High ash content may result in an ash layer covering the char and preventing the free
movement of gases to the core of the char during combustion. State-of-the-art fluidized bed
combustion systems have combustion efficiencies of over 95 % irrespective of ash content (11).

34

In these systems, the only ash fraction that may contain a high carbon content is the coarse fly ash
fraction (as precipitated in cyclones) which is reintroduced into the combustion chamber.
A possible use of ash, depending on the carbon content, is as a fuel (5). Preliminary research
on this idea has been carried out predominantly on fly ash (10). Gasification o f high-carbon fly
ash has been investigated and the resulting ash shows positive results in the production o f C-fix
blocks, a concrete-like material that uses a heavy petroleum residue as a binder (9). The high
carbon content in some forms of ash could also make it useful as a fuel after briquetting or if
pelletized (12)( 13). Combustion of the high carbon ash presents a number o f operational problems.
Related problems include corrosion (14) and scouring. Based on the design of the boilers, the ash
may be carried with the flue gas through the boiler tubes (11). The variability in particle sizes as
the feedstock becomes more burnt-out also causes inherent problems in fixed bed systems. Fixed
bed systems usually require a feedstock that is as uniform as possible so as to avoid channeling
(15) (16).
This study investigates the high carbon bottom ash from fixed bed boilers generated by a
large pulp and paper producer in Canada. Large volumes of high carbon ash are produced and are
typically sent to landfill due to the low potential economic and environmental benefit for other
applications. The costs associated with changing the system to a more recent boiler design are
considerably high. If the high carbon fraction can be separated from the low carbon fraction o f the
ash other applications may become viable. Separation methods for isolating carbon from ash
include floatation in a continuous mode and fractionation by sieving (5). Industrially, carbon
separation can be carried out using industrial scale sieve shakers for removal o f the coarse carbon
rich fraction or by scaling up floatation methods.

35

The objective of this research is to characterize bottom ash from an industrial scale fixed
bed boiler and to identify the ash fractions with potential energy importance. This work examines
the physical and chemical properties o f the ash and ash fractions. Fractions with high carbon
contents are identified and an analysis of the combustion behavior is carried out. The data obtained
enhances knowledge about the utilization and storage of woody biomass bottom ash.

2.2. Experimental Section
Wood ash from a Canfor Pulp Limited Partnership pulp mill fixed-bed boiler in Northern
British Columbia was used in the study. Boiler feed is hog fuel, a mixture o f bark and sawdust
primarily derived from pine wood with variable particle sizes. Based on an ultimate analysis, the
hog fuel is 49 % C and 6 % H on a dry basis. Hog fuel moisture content varies depending on
sawmill feedstock, ranging from 25 to 50 %; ash content is approximately 2.5 %. Typically fixed
bed boilers operate within 850 to 1400 °C (17). This fixed bed boiler produces approximately 27
MW of power and has a lower grate temperature o f 255 °C with an outlet flue gas temperature of
170 °C. Ash samples, denoted B1 and B2 were collected from the system on November 17, 2010
- B 1 and on April 27,2012 - B2. The boiler has a dry ash removal system, and the ash was sampled
from the ash bins shortly after removal from the boiler. The samples were separated into different
particle fractions as described in section 2.1.

2.2.1 Particle size distribution
Approximately 100 g of “as-received” (original sample obtained from boiler) ash was
separated into different size fractions using a stack o f five sieves (2000 pm, 850 pm, 425 pm, 250
pm, 150 pm) arranged in decreasing diameter openings (18). The ash was poured on the top sieve

36

(largest opening), which was then covered; the sieve stack was placed on an automatic shaker for
15 minutes after which the stack was removed. Each sieve with the retained material was gently
tapped on the sides before being removed from the stack and weighed. Retained ash was removed
and stored, while the sieve trays were thoroughly cleaned and reweighed to obtain the mass o f ash
retained on each tray. The analyses were repeated twice and an average value calculated and
reported.
The following retained fractions were used for subsequent analyses:
AR = as-received wood-ash,
> 2000 pm,
> 850 pm and < 2000 pm,
> 425 pm and <850 pm,
> 250 pm and < 425 pm,
> 150 pm and less than 250 pm
<150 pm.
For convenience, the above fractions will simply be denoted as 2000 pm, 850 pm, 425 pm,
250 pm, 150 pm and <150 pm.
Each o f the retained fractions were weighed and then stored for further analysis.

2.2.2 Proximate and ultimate analysis of wood ash
The proximate analysis of each sample fraction was carried out according to American
Society for Testing and Materials (ASTM) method D 1762-84. The fixed carbon (FC), volatile
matter (VM) and ash content were determined on a dry basis. Samples were dried for 3 h at 105

37

°C to remove all moisture. VM analyses was carried out at 950 °C for 7 min in capped crucibles
and then ash content was determined at 550 °C for 6 h in open crucibles. The FC was determined
by difference as shown in Equation (2.1). The ash content is defined as the remaining inorganic
contents after the complete removal of fixed carbon, volatile matter and moisture. All analyses
were performed on three replicates and the averages presented.
FC = 100 - VM - A s h

(2.1)

where FC, VM and Ash content are wt.% expressed on a dry basis.
Ultimate analysis o f carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) was conducted
using a Costech™ Elemental Analyzer, ECS 4010 Elemental Combustion System. Reactor
conditions were: 1000 °C, helium carrier gas at 105 mL/min, Gas Chromatography (GC) Column
#051080, SS 5 mm x 2 m, at 100 °C, 450 L x 18 mm reaction tube, and HayeSep Q 60/80GC
column packing. A Thermal Conductivity Detector was used. Results are presented in wt.% and
on a dry basis.

2.2.3 Thermal analysis
Thermal characteristics were determined using a TA Instrument TGA-50/50H Shimadzu
thermogravimetric analyzer. The analyses were conducted using air at a flow rate of 50 mL/min,
under atmospheric conditions. For each test, approximately 10 mg of the sample was heated
from room temperature to 800 °C at a heating rate of 10 °C/min. A thermocouple was located on
the inside o f the analyser above the sample. The TGA analysis was done only on B1 sample for
the AR portion, > 425 pm fraction and < 425 pm fraction. The thermogravimetric (TG) plots
describe the weight loss of the sample with increasing temperatures while the differential
thermogravimetric (DTG) plot illustrates the derivative of sample mass with time at a specific

38

temperature. The analysis were repeated three times to ensure that the thermal distributions,
trendlines, weight losses and peaks were similar for the repeats. Also, when the flow rate of air
was varied (data not shown) no difference was seen in the thermal distribution of the char.
2.2.4 Surface Area
Brunauer-Emmett-Teller (BET) surface area measurements were carried out with N 2 at 196 °C using a single-point Micromeritics FlowSorb 11 2300 surface area analyzer. The surface
area was determined only on char particles that were > 2000 pm for both B1 and B2 samples.
Particles in this fraction were more homogeneous due to the absence o f finer ash particles and
more char-like particles.

2.2.5 Heating Value
The gross calorific value was determined for each fraction using a Parr Oxygen Bomb with
approximately 1 g of boiler ash. Results were determined in accordance with ASTM D-5865/E711
and were calculated relative to sample weight on an oven dried basis.

2.2.6 Bulk Density
The bulk density for each fraction was determined as follows. Approximately 50 ml of
sample was placed into a 100 ml graduated measuring cylinder (1 ml divisions). The cylinder was
then tapped 6 times on a rubber surface from a height o f approximately 1.5 in. The mass and
volume o f the sample were then obtained and used to calculate bulk density. This procedure was
repeated three times and an average value obtained for each sample. All bulk density
determinations were expressed on a dry basis.

39

2.3. R esults and D iscussion

2.3.1. Particle size distribution of ash

35.00

30.00

% Retained weight

25.00

20.00

-+~r B1 % Retained

15.00

• - p 2 % Retained
10.00

I

5.00

0.00
<150

150

250

425

850

2000

Particle size (Mm)

Figure 2.1 Percent retained weight against particle size (pm) for boiler ash samples on a dry basis.

40

Distribution of the particles in the boiler samples are shown in Figure 2.1. Fractions > 250
pm showed a similar distribution for both ash samples. The B2 sample was approximately 10 %
higher in mass than B1 for the fractions 250 pm, 425 pm and 850 pm. As the particle sizes became
smaller, that is for fractions 150 and < 150 pm, the mass of B 1 was higher than B2. For B 1, fraction
< 150 pm accounted for the highest weight with 25 % o f the total mass passing through the 150
pm sieve. The same fraction for B2 was approximately 20 % lower than B1. The difference in the
particle size distributions o f the samples is likely associated with the variation in the char contents
of the AR samples. As char becomes more combusted, the sample particle sizes will decrease due
to an increase in ash formation. Therefore, B2 is expected to have greater carbon content in the
AR sample.

2.3.2. Results of proximate analysis and ultimate analysis
The results of the proximate analysis and ultimate analysis of the wood ash samples are
shown in Table 2.1. Percent volatile matter ranged from 0 to 20 % for all samples. The volatile
matter for the samples appeared to slightly increase as particle size decreased. This may be due to
an increase in inorganic content such as carbonates and oxides that are usually present in wood
ash (19). Ash content increased with decreasing particle size. Fractions 425 pm and higher for B1
and B2 ranged from ~ 17 to 34 % ash content, while fractions 250 pm and below ranged from ~
60 to 70 %. The ash content in the 425 pm fractions o f B1 and B2, respectively, were 32 % and
23 % lower when compared to their 250 pm fractions. In general, the FC content increased with
increasing particle size. B2 showed a gradual increase from 10 to 70 % in FC as particle size
increased. For both B1 and B2, fractions 425 pm and larger contained 50 % or more FC while

41

fractions 250 jam and smaller had ~ 26 % or less FC. The high FC seen in the larger fractions are
a result o f incomplete combustion o f the sample and the presence o f char-like particles.
Table 2.1. Proximate analysis for fixed bed boiler bottom ash samples (B1 & B2).
B2

B1
FC

Vm

Ash

(wt.%)

(wt.%)

<150

13.8

150

(wt.%)

FC
(wt.%)

Vm
(wt.%)

Ash
(wt.%)

19.1

67.1

10.0

19.6

70.4

9.5

17.5

73.0

10.6

17.1

72.3

250

16.0

17.4

66.6

25.3

17.4

57.4

425

51.1

14.9

34.0

51.9

13.8

34.3

850

63.6

16.7

19.7

67.8

13.5

18.7

2000

59.0

14.8

26.2

67.4

14.9

17.7

AR

29.3

15.9

54.8

54.0

15.8

30.2

Particle size
(pm)

All results are on a dry weight basis
AR = as received (i.e. not separated into particle size fractions)

Table 2.2 displays the results of the ultimate analysis, and shows the total C, H, and N for
each particle fraction. In general, the total % C increased with increasing particle size fraction. The
weight percent distribution of C for most samples was greater than those from the proximate
analysis (i.e. C greater than FC). This is anticipated since the total carbon includes the organic
and inorganic C such as carbonates. All samples had H concentrations o f 1 % or less, and N
concentrations of less than 0.3 %. S was below detection limits. These low concentrations o f H are
expected since the H present in the wood is converted to H2 O during the combustion process. Also,
sulphur is usually not present or is only present in small amounts in woody biomass samples unless
the soil is very high in sulphur content.

42

Table 2.2 Ultimate analysis of fixed-bed boiler bottom ash samples (B1&B2).
B2

B1

Particle

C

H

N

c

H

N

pm

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

<150

22.70

0.41

0.06

17.17

0.65

0.04

150

15.99

0.27

0.04

13.85

0.56

0.03

250

13.13

0.21

0.01

21.56

0.59

0.01

425

55.32

0.81

0.70

52.76

0.79

0.06

850

63.92

0.98

0.06

66.41

0.71

0.18

2000

68.15

0.76

0.12

70.73

0.53

0.25

AR

38.17

0.42

0.11

51.79

0.68

0.08

size

All results are on a dry weight basis

43

Mass (g)

25.00

m

■ Mass of C/fraction g

m

ill

250

* Mass of Ash/fraction g

425

Particle Size (|jm)
Figure 2.2.a Distribution o f fixed carbon (g) and ash (g) in 100 g (dry basis) of B1 sample
based on particle size fraction distribution.

44

16.00

■ Mass of C/fraction g
i Mass of Ash/fradion g

<150

150

250

425

850

2000

Particle Size (|im)
Figure 2.2.b Distribution of fixed carbon (g) and ash (g) in 100 g (dry basis) o f B2 sample based
on particle size fraction distribution.

45

Figure 2.2a and 2.2b shows the distribution of fixed carbon (FC) in 100 grams o f boiler ash
samples B1 and B2. The 850 pm fraction for both B1 and B2 were the highest in FC content, 12 g
and 20 g respectively. Fraction 425 pm had the second highest FC contents of 7 and 10 g
respectively. The two boiler ash samples showed a variation in FC distribution. This could be due
to a number of factors such as variations in retention time in the reactor, incomplete combustion,
temperature variations and fluctuations, moisture content of fuel etc. Also, the moisture content
and composition o f the biomass feedstock may vary depending on when and where it was sourced.
Higher moisture content will reduce the rate of combustion o f the fuel. Also, because o f the
variability in energy demand o f the plant, ash may be removed from the grate before complete
combustion. This is in addition to the other ash related challenges that occur in the boiler as
previously mentioned in the introduction. The 250 pm fractions for Bland B2 had the highest ash
contents of approximately 14 and 26 g. As previously seen in the proximate analysis, fractions 425
pm and higher in both boiler samples contain more than 50 % FC. Combined, fractions 425, 850
and 2000 pm contained ~ 72 % of the total FC in sample B1. These same fractions accounted for
only ~ 36 % o f the total sample mass. For B2, the 425 pm and higher fractions contained ~ 82 %
of the total FC and accounted for 59 % of the total mass. The high FC present in boiler ash provides
an opportunity for re-burning or reusing its energy.

46

2.3.3. Thermogravimetric analysis

100

**
00

JZ

1

0

200

400
600
T em perature (oC)

800
AR ------ t * 5 * *

1000

Figure 2.3. TGA graph of weight change as a function of temperature for B1 boiler ash sample
from a fixed bed boiler. Lines displayed represents AR sample, fractions > 425 pm and < 425
pm.

47

Tem perature (°C)

200

500

800

-0.3
-0.5
AR
• - < 425 |im

-

1.1

-1.3
-1.5

Figure 2.4. DTGA graph displaying rate o f mass loss as a function o f temperature for B1 boiler
ash sample from a fixed bed boiler. Lines displayed represents AR sample, fractions > 425 pm
and < 425 pm.

48

Figure 2.3 shows the thermal behaviour of the as-received, combined fractions > 425 jam
and combined fractions < 425 pm for boiler ash B1 when heated under air at atmospheric
conditions. The results are only shown for B l, however, the results for B2 as received show similar
thermal distributions (not shown). The TGA analyses showed that the mass of the boiler sample
decreased as temperature increased. The weight loss occurred in phases which is linked to the
removal of specific components of the sample such as water, carbonates, aluminates, silicates,
oxides, organic carbon and other components (20). All three samples displayed a similar pattern
in thermal distribution. The first weight loss was seen between ~ 25 - 110 °C and accounted for
the removal of moisture from the sample. A further mass loss was seen up to ~ 350 °C. This change
is generally attributed to the removal o f moisture, carbon monoxide, and volatiles such as tars and
other organic carbons that may have condensed on the sample (14). The most noticeable change
in mass of the AR, > 425 pm and < 425 pm fractions was seen between the temperatures 350 to
440 °C and accounted for ~ 32,43 and 14 % mass loss, respectively. This is assumed to be organic
carbon (char). Further mass losses occurred between 440 to 610 °C and between 610 to 670 °C.
The decomposition of mineral components such as carbonates (CaCC>3 ) and some of the other
metal carbonates and oxides present in the sample may occur during this phase. For the range of
temperatures studied, the AR, > 425 and < 425 pm fractions lost ~ 44, 56 and 26 % o f initial mass
by 800 °C, respectively. Therefore, fractions > 425 pm had the highest combustible content. The
sample burnout time corresponding to the burnout temperatures, that is, the temperature that shows
no further mass loss on the TGA (21)(22) are shown in Table 2.3. The fraction > 425 pm had a
slighter shorter burnout time but was not significantly different from the other two samples under
study.

49

The DTGA analysis in Figure 2.4 shows two distinct peaks. These peaks are associated
with the thermal oxidation of different components in the particles such as carbonized material,
heavy organic molecules, condensed tars and inorganic compounds (23). As seen in Table 2.3, the
AR sample, fractions > 425 and < 425 pm had the highest rate of mass losses at ~ 1.2082 mg/min
at 383 °C, 1.3843 mg/min at 362 °C, and 0.6642 mg/min at 405 °C, respectively. Lignocellulosic
samples heated in air typically have well resolved peaks at ~ 400 °C (24)(25). The fractions
containing higher carbon contents were observed to have lower peak temperatures. Peak
temperature determines how easily a fuel is ignited and is defined as the maximum rate of weight
loss due to volatilization accompanied by the formation o f carbonaceous residue, on the DTGA
curve (21). Low peak temperatures mean that the fuel is easier to ignite. Since the > 425 pm
fraction was found to have the highest rate of mass loss at a lower temperature, faster reaction
times and lower ignition temperatures could be observed if re-burned, when compared to the other
two samples. The samples showed clearly defined thermal phases when heated and oxidized and
contained substantial amounts of combustible material, particularly char for the AR sample and
particle fractions > 425 pm.

50

Table 2.3. Combustion characteristic of fixed bed boiler bottom ash (B l) samples

Sample

Temperature
range (°C)

Weight
Loss

Peak
Temperature
Tp (°C)

DTGmax
(mg/min)

Sample
burning
time (min)

(wt. %)
AR

> 425 pm

s
=1

V

110-350

4.28

350-440

31.98

440-610

3.54

610-670

3.79

670-800

0.36

110-350

6.49

350-440

43.59

440-610

3.71

610-670

2.25

670-800

0.4

110-350

2.94

350-440

14.44

440-610

4.81

610-670

3.83

670-800

0.15

35.33
383

1.2082

649

0.2023

34.23
362

1.3843

648

0.1357

34.47
405

0.6642

646

0.1815

Results are on a dry basis.

2.3.4. Surface Area
The N 2 -BET surface area absorption B l and B2 (> 2000 pm fraction) were 850 and 770
m2/g, respectively. The high surface areas obtained are expected for wood ash chars (26)(27)(3).
This may influence the reactivity rate during combustion or gasification processes. A larger
available surface area provides more sites for oxygen to bond with carbon (28) increasing

51

oxidation. It has also been reported that wood chars that are produced at higher temperatures have
a higher surface area (29). Lower pressures also increase the surface area o f charge (27).
2.3.5. Higher heating Value (HHV)
Table 2.4 displays the higher heating values for the various size fractions of the B l and B2
ash samples. Heating values within each size class were relatively similar between the two ash
samples. A range o f approximately 5 - 25 MJ/kg was obtained for both samples. As the particle
size fraction increased, the heating value o f the fraction also increased. A slight decrease in the
heating value was observed when moving from 850 to 2000 pm for both Bl and B2. The obtained
HHVs suggest that the waste being disposed of has good energy potential and is comparable to
some forms of biomass used in energy production processes (30).

Table 2.4. Higher Heating value o f boiler bottom ash samples separated within particle fractions
Particle size (|im)

B l MJ/kg

B2 MJ/kg

<150

7.02

8.89

150

5.58

6.86

250

7.40

10.40

425

15.77

19.33

850

22.96

24.56

2000

20.83

23.54

AR

10.77

14.42

All results are on a dry weight basis.

52

2.3.4. Bulk Density
The bulk density was calculated for each fraction and is represented as a function of particle
size in Figure 2.5. B2 has lower bulk densities in all fractions. The as-received samples o f Bl and
B2 were 244 and 172 kg/m3 respectively. For both Bl and B2, fractions 425 pm and higher had
significantly lower bulk densities than fractions 250 pm and below. B l fractions 425 pm and
higher had a bulk density range of ~ 100 - 150 kg/m3. Bulk density was ~ 50 % less for fractions
250 pm and lower. For B2, bulk density was also 50% less for fractions 250 pm and lower
compared to fractions 425 pm and higher.
The bulk densities were lower with samples o f higher FC contents. Figure 2.6 shows the
correlation o f bulk density with percent FC within the various particle fractions. A linear
relationship is established and shown in Equation 2.2. While the value o f R2 is 0.8692, the graph
shows a significant reduction in bulk densities of fractions with higher FC contents. If the carbon
is reduced or eliminated in ash, the bulk density increases. This may be important in the
transportation and storage o f ash. With increasing bulk densities, greater amounts of ash could be
transported from combustion plants to landfills.

pB= -4.9579 FC + 406.73 R2 0.8692

(2.2)

pB- Bulk density

53

500
450
400

% 350
^

300

?

250

g 200
a*

-a 150
3
CQ

100
50
0

<150

150

250

425

850

2000

Particle size (urn)
Figure 2.5. Buik density as a function o f particle size (pm) for boiler ash.

54

500
450
400

♦ ♦

£ 350
y = -4.9579x+406.73
R2 = 0.8692

5 300
6

•g 250
«
Q 200
£

I 150
100

50
0

20

40

60

80

Fixed Carbon
Figure 2.6. Bulk density versus percent fixed carbon for boiler ash.

55

2.4. Sum m ary of R esults

Table 2.5. Properties o f boiler bottom ash o f major ash fractions, based on 100 kg sample.
Parameter

B l AR

B l >425 pm

B l <425 pm

B2 AR

B2 >425 pm

B2<425 pm

Mass (kg)

100.0

37.2

62.8

100.0

60.0

40.0

Energy density (MJ/kg)

10.8

20.0

6.7

14.4

22.4

9.7

Energy content (GJ)

1.08

0.74

0.42

1.44

1.34

0.39

FC (%)

29.3

58.3

13.2

54.0

61.6

21.5

Ash (kg)

54.8

9.7

42.8

30.2

14.8

24.4

Bulk density (kg/m3)

244.0

106.6

448.3

172.0

81.5

385.3

Volume (m3)

0.41

0.35

0.14

0.58

0.74

0.10

Bulk density calculated from Equation 2.2
Table 2.5 summarizes some of the main results by reporting data obtained for the as-received
fractions, combined fractions > 425 pm and combined fractions < 425 pm for Bl and B2. When
samples are collected from fractions > 425 pm, the energy density can be increased by
approximately 50 % and 64 % for Bl and B2 respectively when compared to the AR for both
samples. In addition, the majority o f the energy content is in the larger fraction (> 425), 64 % for
Bl and 78% for B2. On a mass basis, less than half o f the ash is in the larger fractions (> 425).
The bulk density analysis shows that the low carbon fractions o f ash have four times the bulk
density, compared to the high carbon fractions. This suggests a good opportunity for volume
reduction. By rebuming the larger size fraction, ash volumes can be decreased by over a half, while
recovering more than two-thirds of the energy present in the as-received sample.

2.5. C onclusions
The boiler ash studied shows variability in carbon contents that may be a result of variations
in retention time in the reactor, incomplete combustion, temperature variations and fluctuations
and variations in moisture content. However, unique to both ash samples is a high carbon content

56

in the larger size fractions, specifically for the fractions > 425 pm. Separating and re-using particle
sizes 425 pm and higher would recover over 50 % of the unbumt carbon that is present in this
bottom ash. Sieve fractionation may allow for easy accessing o f the carbon and may present itself
as a cost effective pre-treatment method for rebuming high carbon ash, in order to obtain the
highest energy components. The reintroduction of ash in the combustion system may present
challenges due to problems related to corrosion, scouring and fouling as experienced with the fixed
bed boiler that produces this high carbon ash. Thermogravimetric analysis shows that this high
carbon ash may be combusted and displays thermal phases similar to the combustion of
lignocellulosic biomass. The highest rate o f mass loss was 1.3843 mg/min for fractions > 425 pm.
This occurs at a lower peak temperature when compared to the other samples, resulting in lower
ignition temperatures. There was no significant difference in the burn-out time for all 3 samples.
The high surface area o f the char particles could increase combustion or gasification reactivity
rates, since a large surface area is available for oxidation. A linear correlation was identified
between the FC and bulk density, where samples with higher concentrations o f fixed carbon have
lower bulk densities.
In an effort to ensure environmental sustainability and sustainable use of our biomass
resources, efficient use must be realized. The use o f woody biomass in bioenergy process must be
such that all possible energy is extracted. It is preferred that bioenergy processes are able to so.
However, should that be not possible due to system limitations or fuel type etc, other options must
be employed in conjunction with existing technologies to ensure maximum use of the resource.
Finally, the need for an in-depth analysis o f the inorganic distribution within fractions o f bottom
ash should also be investigated for possible environmental and technical utilization. For example,
there are likely to be similarities in the properties of ash and biochar. Biochar may have beneficial

57

soil applications, for example biochar has been suggested to enhance seedling growth (31)(32).
Perhaps high carbon ash could also have similar results for soil applications.

58

References

1. Obemberger, I.; Supancic, K. Possibilities o f ash utilisation from biomass combustion plants.
Proceedings o f the 17th European Biomass Conference & Exhibition. Hamburg : ETA
Renewable Energies, June/July 2009.
2. Picco, D. Technical assistance for the development and improvement o f technologies,
methodologies and tools for enhanced use o f agricultural biomass residues. Gorizia : s.n.,
February 2010.
3. Demirbas, A. Potential applications o f renewable energy sources, biomass combustion
problems in boiler power systems and combustion related environmental issues. Progress in
Energy Combustion Science. 2005, Vols. 31171-192.
4. Sarenbo, S. Wood ash dilemma-reduced quality due to poor combustrion performance.
Biomass and Bioenergy. 2009, Vol. 33, 1212-1220.
5. Gomez-Barea, A.; Vilches, L.; Campoy, M.; Femandez-Pereira, C. Plant optimization and ash
recycling in fluidised waste gasification. Chemical Engineering Journal. 2009, Vol. 146, 227236.
6. Van Loo, S.; Kopejan, J. The handbook o f biomass combustion and co-firing, s .l.: Earthscan
Publications, 2008. ISBN:9781844072491.
7. Dimitrakopoulous, A. Thermogravimetric analysis of Mediterranean plant species. Journal o f
Analytical and Applied Science. 2001, Vol. 60, 123-130.
8. Emilsson, S. International handbook. From extraction of forest fuels to ash recycling.
Jonkoping : Swedish Foresty Agency, 2006. ISBN 91-975555-1-7.
9. Pels, J.; de Nie, D.; Kiel, J. Improvement of the economics of biomass/waste gasification by
higher carbon conversion and advanced ash management. [Online] n.d. [Cited: October 20,
2012.] http://www.ecn.nl/docs/library/report/2006/c06038.pdf.
10. Duan, L.; Liu, D.; Chen, X.; Zhao, C. Fly ash recirculation by bottom feeding on a circulating
fluidized bed boiler co-burning coal sludge and coal. Applied Energy. 2012, Vol. 95, 295-299.
11. Saidur, R.; Abdelaziz, E.; Demirbas, A.; Hossain, M.; Mekhilef, S. A review on biomass as a fuel
for boilers. Renewable and Sustainable Energy Reviews. 2011, Vol. 15, 2262-2289.
12. Batra, V.; Urbonaite, S.; Svensson, G. Characterization o f unbumed carbon in bagasse fly
ash. Fuel. 2008, Vol. 87, 2972-2976.

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13. Cetin, E.; Moghtaderi, B.; Gupta, R.; Well, T. Influence of pyrolysis conditions on the structure
and gasification reactivity of biomass chars. Fuel. 2004, Vol. 2004,2139-2150.
14. Khan, A.; Jong, W.; Jansens, P.;& Spliethoff, H. Biomass combustion in fluidized bed boilers:
potential problems and remidies. Fuel Processing Technology. 2009, Vol. 90, 21-50.
15. Ryu, C.; Yang, Y.; Khor, A.; Yates, N.; Sharifi, V.; Swithenbank, J. Effect of fuel properties on
biomass combbustion: Part 1. Experiments - fuel type, equivalence ration and particle size.
Fuel. 2006, Vol. 85,1039-1046.
16. Wamecke, R. Gasification of biomass: comparison o f fixed bed and fluidized bed gasifier.
Biomass and Bioenergy. 2000, Vol. 18, 489-497.
17. Quaak, P.; Knoef, H.; Stassen, H. Energy from biomass. A review o f combustion and
gasification technologies. Washington, D .C .: World Bank Technical Paper; 422. Energy
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18. Herrman, T.; Baker, S. Evaluating particle size. Kansas : Kansas State University
Agricultural Experiment Station and Cooperative Extension Service, 2002.
19. Liao, C.; Wu, C.; Yan, Y. The characteristics o f inorganic elements in ashes from a 1 MW
CFB bimass gasification power generation plant. Fuel Processing Technology. 2007, Vol. 88,
149-156.
20. Zhang, H.; Zhao, Y.; Jingyu, Q. Termal characterization o f fly ash from municipal solid
waste incinerator (MSWI) in Shanghai. Process Safety and Environmental Protection. 2010,
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21. Idris, S.; Rahman, N.; Ismail, K. Combustion characteristics o f Malaysian oil palm biomass,
sub-bituminous coal and their respective blends via thermogravimetric analysis (TGA).
Bioresource Technology. 2012, Vol. 123, 581-591.
22. Miranda, T.; Esteban, A.; Rojas, S.; Montero, I.; Ruiz, A. Combustion analysis of different olive
residues. International Journal o f Molecular Science. 2008, Vol. 9, 512-525.
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applications of solid particles produced at a biomass gasification plant. Biomass and
Bioenergy. 2012, Vol. 47, 134-144.
24. Elder, T.; Kush, J.; Hermann, S. Thermogravimetric analysis of forest understory grasses.
Thermochimica Acta. 2011, Vol. 512, 170-1771.
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26. Link, S.; Arvelakis, S.; Hupa, M.; Yrjas, P.; Kulaots, I.; Paist, A. Reactivities of the biomass
chars originating from reed, douglas fir and pine. Energy and Fuels. 2010, Vol. 24, 65336539.
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61

Preface

Chapter 3 of this thesis is a version o f the article accepted for publication:
James, A. K.; Helle, S.; Thring, R.W.; Sarohia, G.; Rutherford, P. M. Characterization o f
Inorganic Elements in Woody Biomass Bottom Ash from a Fixed-bed Combustion System, a
Downdraft Gasifier and a Wood Pellet Burner by Fractionation: Proceedings o f the Third
International Conference on Environmental Pollution and Remediation, July 15-17, 2013,
Toronto.

62

CHAPTER 3: C h aracterizatio n o f Inorganic E lem en ts in W o o d y
B iom ass B o tto m Ash fro m a Fixed-bed C o m b u stio n S ystem , a
D o w n d raft G asifier a n d a W o o d P ellet B u rn er by F ra ctio n a tio n

3.1. Introduction
The employment o f wood combustion technologies have resulted in a rapid increase in the
use of woody-biomass residues for energy production. Boilers, gasifiers and pellet burners are but
some of the systems available for wood energy production. A disadvantage o f using biomass for
energy production is that large amounts o f residual ash are generated (1).

Ash from these

combustion processes vary in quality depending on the fuel type, operating conditions of the
system and the type of combustion system (2). Ideally, biomass ash should be recycled whenever
possible, but large amounts of wood ash are typically landfilled (3). Countries such as Sweden and
the United States of America (USA) use landfills to dispose the majority of their ash (4)(5)(6).
The quality of the ash in part determines recycling options; some options such as land spreading
require low trace element concentrations to prevent environmental pollution (7). The application
of wood ash to soils may counteract soil acidification and correct for nutrient deficiencies; this is
widely used in regions that carry out extensive forest harvesting, such as Northern Europe and
parts of North America (6)(8)(9). In Canada, biomass ash is mainly landfilled but land applications
are more common in some provinces than others. In British Columbia the use o f ash for soil
applications is relatively limited due to current provincial regulations. Until adequate research is
carried out the majority o f ash generated in B.C. may be limited to landfilling or other uses outside
of soil applications.
Limiting factors for the use of wood ash in soils include trace element concentrations and
potential organic pollutants (10)(11). Elevated concentrations of trace elements in ashes may limit

63

utilization o f ash in soils and therefore also the recycling of nutrient elements (e.g. Ca, Mg, K, P),
too. The handling and application of ash is improved by hardening or aggregation; but, ash with
a high organic C content does not harden properly (12).
The physical and chemical characteristics o f wood ash may vary with particle size fraction
(1)(13)(14). Particle size fractions high in undesirable trace element concentrations could be
separated from some ashes, thereby improving the overall quality o f the residual ash material (and
therefore improving recycling options). Previous research shows that arsenic concentrations in fly
ash exceeded the Finnish environmental limits for the particle size fraction less than 0.125 mm
(1). Particle separation may be carried out by sieve fractionation and may influence variables such
as trace element concentrations, pH, organic carbon concentration, bulk density and other
chemical-physical properties.
Should the properties vary significantly in particle size fractions, greater applications of ash
as a soil amendment or as a raw material for products requiring specific properties could be
employed. Ash-producing industries may find this particularly useful when considering disposal
or utilization methods for ash.
The objective of this research was to characterize ash from the three systems. A comparative
study o f the chemical properties o f each bottom ash, as well as their respective fractions, obtained
by sieving, was carried out. This paper will identify the inorganic elemental distribution of specific
particle size fractions, pH and anion distribution o f boiler, gasifier and wood pellet burner bottom
ash. It is hoped that these results will assist in finding specific applications for bottom ash.

64

3.2. Experimental
Wood ash from from three types o f combustion systems, a fixed-bed boiler (Canfor Pulp
Mill), a downdraft gasifier (University of Northern British Columbia - UNBC) and a wood pellet
burner (UNBC) were used in the study. The downdraft fixed bed gasifier is capable of producing
a thermal output of 5 MW and operates at ~ 1250 °C. The wood pellet burner 0.4 MW and an
industrial scale fixed bed boiler, 27 MW. The fuel for the gasifier and boiler is hog fuel comprised
predominantly of softwood sawmill waste and is primarily derived from pine wood. The pellet
burner utilizes wood pellets made locally from soft-wood saw dust. Two ash samples were
collected from each system. The boiler bottom ash sample was collected on April 27,2012 - BA.
The gasifier bottom ash sample was collected July 18, 2012 - GA. Pellet burner sample was
collected March 5, 2010 - PA. Henceforth, the boiler sample is denoted as BA, gasifier sample
GA and pellet burner sample, PA. The ash samples were separated into different particle fractions
as follows.

3.2.1 Particle size distribution
Approximately 100 g of air-dried “as-received” ash original sample obtained from boiler,
gasifier, or pellet burner was separated into different size fractions using a stack o f 5 sieves (2000
pm, 850 pm, 425 pm, 250 pm, 150pm, arranged in decreasing diameter openings (15). Ash was
poured on the top sieve (largest opening), which was then covered; the sieve stack was then placed
on an automatic shaker for 15 minutes after which the stack was removed. Each sieve with the
retained material was gently tapped on the sides before being removed from the stack and weighed.
Retained ash was removed and stored, while the sieve trays were thoroughly cleaned and
reweighed to obtain the mass of ash retained on tray. The analyses were repeated twice and an

65

average o f the results taken. The following retained fractions were used for subsequent analyses:
as-received wood-ash, > 2000 pm, > 850 pm but < 2000 pm, > 425 pm but <850 pm, > 250 pm
but < 425 pm, > 150 pm but less than 250 pm and <150 pm.
For reporting data purposes, the fractions will be denoted as 2000pm, 850pm, 425pm,
250pm, 150 pm and < 150pm. Each o f the retained fractions were weighed and then stored for
further analysis.

3.2.2. pH analysis
The pH o f ash fractions were determined following methods described in Kalra and Maynard
(16). The pH was measured potentiometrically using the pH of saturated paste method. A 400 ml
beaker was half-filled with ash, then sufficient deionized water was added to saturate sample. The
sample was left to sit for 1 hour after which pH readings were taken. Analysis of all fractions were
repeated twice and an average of the results taken.

3.2.3 Concentration and distribution of trace elements and major ash-forming elements
Selected elements within each fraction were determined by ICP-MS at UNBC. Samples were
prepared by microwave digestion, using a Milestone MLS 1200 Mega digestion system, with
concentrated HNO 3 . Metal characterization was done by an inductively coupled plasma (ICP-MS)
on an Agilent 7500 ICPMS machine. This was used to determine the alkali, alkali earth and trace
elements.

66

3.2.4 Anion analysis
Anion concentrations were determined using the Dionnex IC-5000 system. Ten (10) ml of
deionized water was added to ~ 400 mg o f samples and shaken for 12 hrs. The samples were then
removed from shaker and centrifuged for 15 min, after which ~ 2 ml o f sample was obtained for
analysis using Ion chromatography. All results shown are considered to be only water soluble
amounts. Duplicate analyses were conducted and an average concentration reported.

67

3.3. R esults and D iscussion
3.3.1. Particle size distribution of ash
_ _

60
50
3? 40
+■*

S30

20

10
0

<150

150

250

425

850

Particle size (|im)
Figure 3.1. Retained Weight as a function o f particle size distribution for boiler, gasifier and
pellet burner ash samples (Data referenced) (14).

68

Table 3.1. Mean total carbon contents and pH o f bottom ash samples o f boiler, gasifier and
________________ pellet burner.________________________
*Carbon
Particle
size/
pm
2000
850
425
250
150
<150
AR

pH

BA

GA

PA

wt.%

wt.%

wt.%

BA

GA

PA

10.41
70.73
3.07
9.69
66.41
3.71
5.94
10.49
10.05
11.16
5.76
6.62
10.74
52.76
10.28
12.13
19.16
21.56
3.24
12.12
10.36
13.2
10.41
13.36
6.11
36.08
12.56
13.85
17.17
3.67
5.75
10.43
13.53
12.22
5.64
13.46
51.79
5.13
12.49
10.36
*Data referenced from (15) and on a dry basis

3.3.2. pH analysis
Table 3.1 shows the distribution of the pH for the PA, GA and BA samples. Ash from all
three systems was high in pH but recorded different pH readings. Ash from the gasifier was the
least alkali, followed by the boiler then the pellet burner. The as-received (AR) samples for the
gasifier, boiler and pellet burner were 10.36, 12.49 and 13.46 respectively. It was observed that
the pH increased for all ash samples as the particle size fraction decreased, with the exception of
the particle size fractions < 150 pm for the for BA. The increase in pH may have been due to the
increase in concentrations of the alkali earth metals (described later) as the particle size fractions
decreased potentially leading to higher concentrations base-forming metal salts.

69

Table 3.2. Concentration and distribution o f trace elements for samples o f boiler, gasifier
and pellet burner bottom ash (dry basis).
Particle Size
As
Cd
V
Co
Mo
Cr
Cu
Ni
Pb
(p)m
mg/kg
mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
PA
0.49
0.50
7.22 14.70
6.74
3.13
49.58 61.23 229.59
850
47.32 108.91 175.40
3.55 15.03
6.90
3.55
0.80
1.91
425
0.85
3.69
4.17 14.30
7.67
4.01
79.91 135.53 109.48
250
7.54
0.84
4.45
52.09 153.60 69.70
2.07 12.85
4.52
150
7.93
0.98
9.46
52.38 170.72 40.65
2.74 10.18
5.24
<150
7.71
0.44 10.55
7.20
0.92
54.13 132.47 54.50
4.82
AR
Gasifier ash
2000
850
425
250
150
<150
AR

23.40
16.66
12.81
37.75
18.54 31.30
60.97
35.33
44.54 66.37
47.57 207.57
33.51
43.38

20.00
19.09
27.23
68.13
67.74
78.81
60.74

0.63
7.36
1.72
5.12
4.10
4.20
1.64

29.37
21.95
20.71
29.65
28.57
33.41
28.53

5.29
4.17
4.63
7.35
7.04
7.86
6.78

0.48
1.15
0.74
4.30
3.33
5.33
4.13

1.39
1.76
1.40
6.31
4.85
7.49
5.42

0.03
0.04
0.07
0.30
0.31
0.44
0.27

Boiler ash
2000
850
425
250
150
<150
AR

5.84 34.01
4.64 81.52
10.36 50.42
19.87 77.59
30.00 107.49
28.97 419.58
11.82 32.85

6.89
6.31
11.06
19.05
25.07
25.36
13.39

3.34
9.59
7.09
9.99
9.00
13.86
4.89

3.50
4.14
12.16
18.54
24.49
24.66
12.44

1.56
1.86
3.23
5.17
6.70
6.99
3.79

1.51
1.40
1.61
2.29
2.71
2.79
1.88

0.52
0.69
1.14
1.82
2.07
2.37
1.45

2.21
2.85
4.11
6.18
6.18
7.52
4.76

*Environmental
150
20
75
2200
180
500
1060
Limits
*Environmental limits for ash intended for land application in B.C., Canada (17)
(Expressed on an AR basis)

20

70

3.3.3 The concentration and distribution of elements in ashes and anion distribution

Trace elements
The concentrations o f elements for the as-received ash and within specific particle size
fractions of Cr, Cu, Ni, Pb, V, Co, Mo, As and Cd are shown in Table 3.2. O f the as-received ash
samples analysed, Cu from the Pellet burner system was highest in trace element concentration;
132 mg/kg for Cu (Table 3.2). Cu was enriched in pellet burner ash by ~ 3 and ~ 4 times that o f
the gasifier and the boiler samples, respectively. This could have been from the contamination of
metals used in manufacturing the pellet burner. The elements Ni and Cr followed next in
concentrations, ranging from 33 - 55 mg/kg for the pellet burner and gasifier as-received ash. The
boiler samples were ~ 5 times lower than the pellet burner ash in both Ni and Cr contents. In most
results, the boiler samples were slightly lower in trace metal concentrations. All elements in the
as-received fractions of the 3 ash samples were well within the British Columbia soil amendment
limits (17).
Table 3.2 also shows the distribution o f trace elements within particle size fractions for the
respective ash. For most elements, a slight increase in concentration was observed as the particle
size fraction decreased in all ash types. All fractions for each element were within the
environmental limits for soil amendments in British Columbia, except the 850 pm fraction o f Ni,
obtained from the pellet burner ash; concentration o f 229 mg/kg. While not exceeding the limit,
the 425 pm fraction also had a high concentration o f 176 mg/kg Ni. It is evident that some
elements are more concentrated in specific fractions o f ash.

The results suggest that fraction

separation can be a useful method to isolate fractions containing higher amounts o f some metals.

71

This method may be a useful technique for isolating elements exceeding environmental exposure
limits and at the same time rendering the residual ash useful.
Table 3.3. Percent total metal distribution in boiler, gasifier and pellet burner ash.
Elements
BA
PA
GA
wt.%
wt.%
wt.%
Ca
60
61
55
K
10
15
21
6
Mg
6
11
Al
4
7
2
4
Mn
3
6
Fe
6
3
1
P
3
3
2
Na
2
2
1
Remaining ?
I1
9
elements
(Expressed on an AR basis)

Table 3.3 displays the percent o f total ash forming elements in each ash sample. These
percentages were obtained by taking the sum o f all the total metal content found in 1 kg (d.b) o f
ash. All samples were significantly enriched in both Ca (50-61 wt.%) and K (10-26 wt.%). The
high weight percent metal content found in these ash are expected for woody biomass ash. The
elements Mg, Al, Mn, Fe, P and Na each contributed 10 % or less to the total elemental portion of
the ash. The other inorganic elements present in ash, though not mentioned in the table
cumulatively contributed 2 % or less to the total metal content o f the ash under study and are not
discussed in this paper. All analysis were based on the top eight most concentrated elements Ca,
K, Mg, Al, Mn, Fe, P and Na.
The elements Al, P, Mg, Mn, Na, Fe, K, and Ca are shown in Table 3.4. The table shows
that the concentration of P, Mn, Mg, K and Ca were found to be higher in pellet burner ash when
compared to the other two ash types. The elements Al, Fe, and Na were present in higher quantities
in the gasifier ash when compared to the other two ash types. Additionally, the pellet burner ash
contained ~ 4 times higher concentrations o f K, Mg and Mn, than the other ash types. However,
72

Ca was only ~2 times higher in pellet ash. Some degree of variation in concentrations is expected
due to the likely variation o f fuel types, temperatures and other factors. Research has shown that
calcium concentrations increase at temperatures below 900 °C which is primarily due to the
decomposition of calcium carbonates (18). Above 900 °C, the Ca concentration also increases due
to the dissociation and volatilization of potassium oxide formed after dissociation. The variation
in oxide and/or carbonate concentrations bound to the ash will affect the concentration o f the
elements in samples when concentration is calculated on a weight basis.

Table 3.4. The concentration o f major ash forming elements within particle size fractions
______________ for boiler, gasifier and pellet burner on a dry basis.___________________
Particle Size
P
Mn
Al
Mg
Na
K
Fe
Ca
(p)m
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
PA
850
425
250
150
<150
AR

136000
196000
240000
253000
263000
246000

9270
10500
9760
9190
7560
7690

25100
35200
46100
51400
56500
50900

1610
2760
3460
3410
2850
2850

61800
99300
113000
106000
95700
95200

3880
6610
8110
8260
8550
8080

11200
16900
22600
25200
28100
25800

9970
9060
8880
7990
6170
6660

GA
2000
850
425
250
150
<150
AR

20900
19800
27400
141000
113000
167000
125000

11900
9900
11900
16100
16300
15900
14900

4970
4210
5360
14000
12200
15700
12300

4970
4210
5360
14000
12200
15700
12300

5000
9120
12900
20800
20700
18400
20100

932
1080
1380
6960
5570
8180
6030

854
1020
1510
7270
6070
8460
6520

12700
10700
11900
12700
12900
12800
12300

BA
2000
850
425
250
150
<150
AR

46400
51600
62700
122000
153000
167000
92600

2420
2730
6210
10000
12100
12100
6760

4520
4950
6450
12200
14900
16300
8970

2460
2550
2470
2620
2790
2790
2510

26000
25300
22500
22400
20800
21500
23400

2090
2420
3060
6480
8260
9140
4850

2690
2970
3620
7230
9080
10100
5390

1560
1800
4730
7610
10100
9760
5190

73

Major elements general trends (AR)
Major elements general trends (within fraction)
Within fractions, it was observed that P, Mg, Mn, Na, and Ca either increased with
decreasing particle size or remained relatively constant. Ca was highest in concentration within
each fraction and for all ash types (Table 3.4). The highest values reported for Ca were seen in
Pellet burner ash, from -136,000 to -262,000 mg/kg moving from fractions 2000 pm to <150 pm.
Additionally, K was found to be highest in the pellet burner ash, containing concentrations ranging
from -62,000 to -112000 mg/kg when moving from 2000 pm to 250 pm fractions. While most of
the elements were present in higher quantities in pellet burner ash as compared to other ash types,
Al, Na and Fe were present in higher concentrations in gasifier ash.
The concentration of each element not only varies across different systems but within
fractions as well. This variation in concentrations poses a difficulty in the general application of
bottom ash to soils due to the lack o f standardization in ash quality. According to (19), ash
recycling to agricultural lands can help reduce the use of artificial fertilizers and close the natural
mineral cycle. This is largely due to the presence o f N, P and K in biomass ash. However, (7)
suggests that the use of biomass ash as a soil fertilizer is limited due to a number o f deficiencies
in ash. The author noted that biomass ash can only be a source o f K because ash from thermal
sources is low in N and the P present has a very low solubility at soil conditions. In previous
research on the same ash, the ultimate analysis showed N content all less than 1 wt.% on a dry
basis for all ash samples (14). Wood naturally has low N contents. Furthermore, due to the
conversion of most of the wood N to NH3, NOx and/or N2 during the combustion of the wood, it
is expected that the nitrogen content be low (18)(20)(11).

74

Table 3.5. The concentration (dry ash basis) o f water soluble phosphates relative to
particle size distribution for boiler gasifier and pellet burner ash.
sieve
BA
GA
PA
size/
mg/kg
mg/kg
mg/kg
pm
0.74
2000
2.29
2.57
0.15
0.29
850
0.35
0.32
3.64
425
0.22
1.67
250
0.1
1.87
150
0.23
0.66
<150
0.08
AR
0.83

While the percent mass o f phosphorus in the metal contents o f the samples ranged from 13%, the amount that is available for plant uptake or that is water soluble must be considered, this
was measured in the form of phosphates (PO'34). Phosphorous, may exist in both organic and
inorganic forms (17).

The total and plant available phosphorous may vary. Water soluble

phosphorus is the amount of phosphorus that is readily available to plants (21) (22). Research
conducted on wood based biomass in an incinerator reported that the water soluble phosphorus
content in bottom ash sample is negligible (22). The research concluded that the poor water
solubility o f phosphorus was a drawback in the application of wood ash to soils since only a small
portion P is extractable and available for plants when used in forest fertilizers. Our research
showed some amounts o f water soluble phosphorus present in the form of phosphates, though low
in concentrations (Table 3.5). The as-received ash for the pellet burner and gasifier were 0.83
mg/kg and 0.08 mg/kg, respectively, while the boiler ash was below the detection limit.
Concentrations varied within particle size fractions. The pellet ash ranged from 0.66 - 3.6 mg/kg;
lowest observed in particle size fraction < 150 pm and the highest in 425 pm fraction. The gasifier
ash ranged from 0.1 - 0.74 mg/kg for the different fractions. The water soluble phosphate were

75

only detected within the three largest particle size fractions for the boiler ash and ranged between
0.35 - 2.57 mg/kg. While the three samples fractions of the boiler samples had higher
concentrations of phosphorus, water soluble phosphorus were not detected for these fractions. It
does suggest that the phosphorus present in these fractions may be insoluble in water.
The presence o f relatively high Ca and Mg, typically in the form o f oxides, hydroxides and
carbonates, suggests use o f these ashes as potential liming agents. This could help in controlling
soil pH. As the particle size fraction decreased, the concentrations of Ca and Mg increased and so
did the pH. Therefore, specific fractions could be used to achieve appropriate liming requirements.
Inorganic relationship to total carbon content
The results were also analysed to identify the possibilities o f any relationship that may exist
between the elemental distribution of major inorganic components and the total carbon content
(14). Figures 3.3a and 3.3b shows a graphical display of the elements Ca, and P versus total
carbon. These two elements showed a linear correlation for boiler sample; most of the carbon (over
60 %) in this sample was organic carbon. A strong correlation was obtained with R2 value o f
0.9272 and 0.9218 for Ca and P, respectively. The equations are displayed in equations 3.1 and
3.2. As a general trend, as the total carbon content increases the elemental concentrations for Ca
and P decrease. These equations may help in determining the approximate amount o f certain
nutrients that may be present in ash. It would be worth removing as much carbon as possible to
increase the concentration o f nutrients.

y = —1969C + 182159

y = —113.92C + 9973.5

R 2 = 0.9272

R2 = 0.9218

(3.1)

(3.2)

76

No major trend or correlation was observed when samples from the gasifier and pellet burner
were analysed. This may have been due to the very low carbon distribution within the particle size
fractions for each sample.
The presence of organic carbon in ash, if recycled to forest soils presents a challenge.
Unbumed carbon creates a dilution effect which reduces the concentration of inorganic nutrients
available in the ash (10). This is evident from the two correlations above where the metal content
increases with the decreasing carbon contents. If a high concentration o f carbon is present in ash,
greater amounts o f ash will be required to fulfill the nutrients demand by the soil. It should be
noted that high carbon ash has also been associated with elevated concentrations of
environmentally harmful products of incomplete combustion, such polycyclic aromatic
compounds (PAH) (10)(23); other work with these ashes has shown very low concentrations o f
PAHs, dioxins and furans (data not shown). The presence of carbon also reduces the ability o f the
ash to harden, which is integral to improving the handling of ashes and reducing its solubility (10).
This property limits its application to soil. Perhaps, if suitable methods for recirculating and
rebuming high carbon ash are determined, higher nutrient concentrations could be obtained while
at the same time tackling some of existing ash related issues.

77

1.8E+5
1.6E+5

y = -1969.1x+182159
R2 = 0.9272

J? 1.4E+5
1.2E+5
i,

c

.2
♦3
2

+4

c

©
u

1.0E+5
8.0E+4
6.0E+4

c 4.0E+4
o

u

2.0E+4
0.0E+0
0

20

40

60

80

Wt.% Total Carbon Content (d.b)
(a)

1.0E+4

y = -113.92x + 9973.5
R2 = 0 .9 2 1 8

9.0E+3
00 8.0E+3
CUD

E

c

7.0E+3
6.0E+3

.2 5.0E+3
2 4.0E+3
4-#
c

«u
u 3.0E+3

c
o

u

2.0E+3
l.OE+3
O.OE+O
20

40

60

80

W t.% Total C arbon C o n te n t (d.b)
(b)

Figure 3.2a and 3.2b Correlation of calcium (Ca) and phosphorus (P) concentrations,
respectively, as a function of total carbon found in boiler ash.

78

On the other hand, should high carbon ash possess similar properties as biochar then it could
be used to obtain positive results in soil applications. Research has shown that biochar addition to
soil may enhance properties, increasing plant growth nutrient availability, cation exchange
capacity, water retention and with a reduction in the release o f some greenhouse gases
(24)(25)(26). Further studies are needed to determine if high carbon ash could also have these
positive effects.

3.4 Conclusion
The inorganic element contents, pH and phosphate distribution o f wood ash samples
obtained from a fixed-bed boiler, fixed-bed gasifier and pellet burner were characterised. The
samples showed variations in pH values both within particle size fractions and across the three
systems. The as-received (AR) samples for the gasifier, boiler and pellet burner ash had a pH of
10.36, 12.49 and 13.46, respectively. The pH increased with decreasing particle size fractions.
This increase in pH may have been due to the increase in concentrations o f the alkali earth metals
as the particle size fractions decrease, forming higher concentrations o f base-forming metal salts.
The trace element contents for the AR samples o f the three ash types were all within the
environmental limits for soil amendments in British Columbia, Canada. However, when analyzed
within particle fractions, Ni with a concentration o f229 mg/kg from the pellet burner ash, exceeded
the limit within the particle size fraction > 850 pm but < 2000. The AR samples were significantly
enriched in both Ca (50-61 wt.%) and K (10-26 wt.%) on a total metal composition. The highest
values reported for Ca were seen in Pellet burner ash, from -136,000 to -262,000 mg/kg. Very
low concentrations of water soluble phosphates were obtained for all ash types in some case below

79

the detection limit. A strong inverse correlation was developed between the total carbon present
and the metal contents for Ca and P.
These results suggest that fraction separation can be a useful method to isolate fractions
containing higher amounts o f some metals. This method may be a useful technique for elements
exceeding environmental exposure limits.
However, the variation in concentrations poses a difficulty in the general application of
bottom ash to soils due to the lack o f standardization in ash quality.

As shown from the research,

the inorganic content o f the ash samples varied across systems and within fractions. In addition
to known factors such as fuel type and temperature, the research showed that other factors such
the carbon content and particle size could play a role in determining the concentrations o f the
available inorganics. It is therefore difficult to determine standard compositions o f ash.

80

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ash and fly ash fractions from a large sized (246MW) fluidized bed boiler with repect to
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Agricultural Experiment Station and Cooperative Extension Service, 2002.
16. Kalra, Y.; Maynard, D. Methods manual for forest soil and plant analysis. Alberta : Forestry
Canada, 1991. Report No. NOR-X-319.
17. BC Ministry of Environment. Land application guidelines for the organic matter recycling
regulation and the soil amendment code of practice. New Westminister, BC : SYLVIS
Environmental, 2008. Report No. 758-08.
18. Misra, M.; Ragland, K.; Baker, A. Wood ash composition as a funtion of furnace
temoperature. Biomass and Bioenergy. 1993, Vols. 4, No2, pp. 103-116.
19. Obemberger, I.; Supancic, K. Possibilities o f ash utilisation from biomass combustion
plants. Proceedings o f the 17th European Biomass Conference & Exhibition Hamburg:
ETA Renewable Energies, June/July 2009.
20. Steenari, B.; Karlsson, L.; Lindqvist, O. Evaluation of the leaching characteristics of wood
ash and the influence of ash agglomeration. Biomass and Bioenergy. 1999, Vol. 16, 119136.
21. Fuhrman, J. K.; Zhang, H.; Schroder, J. L.; Davis, R. L. Water-soluble phosphorus as
affected by soil to extract ratios, extraction times and electrolyte. Communication in Soil
Science and Plant Analysis. 2005, Vol. 36, 925-935.
22. Nurmesniemi, H.; Manskinen, K.; Poykio, R.; Dahl, O. Forest fertilizer properties o f the
bottom ash and fly ash from llarge-sized (115 MW) industrial powerplant incinerating
wood-based biomass residues. Journal o f the Univeristy o f Chemical Technology and
Metallurgy. 47, 2012, Vol. 1, 43-52.

82

23. Bundt, M.; Krauss, M.; Blaser, B.; Wilcke, W. Forest fertilzation with wood ash: effect on
the distribution and storage o f polycyclic aromatic hydrocarbons (PAHs) and poly
choloronated biphenyls (PCBs). Journal o f Environmental Quality. 2001, Vol. 30,12961304.
24. Robertson, S.; Rutherford, M.; Lopez-Gutierrez, J.; Massicotte, H. Biochar enhances seedling
growth and alterss root symbioses and properties o f sub-boreal forest soils. Canadian
Journal o f Soil Science. 2012, Vol. 92, 329-340.
25. Laird, D. The charcoal vision: a win-win-win scenario for simultaneously producing
bioenergy, permanent sequestering carbon, while improving soil and water quality. Agron.
Journal. 2007, Vol. 100, 178-180.
26. Lehmann, J. A handful o f carbon. Nature. 2007, Vol. 447, 143-144.

83

Preface

Chapter 4 o f this thesis has been prepared as a manuscript draft and will be submitted for
possible publication shortly.

84

CHAPTER 4: In v estig atio n o f Air an d A ir-Steam G asification o f High
C arbon W o o d Ash in a Fluidized Bed R eacto r

4.1. Introduction
The pulp and paper industry produces large volumes of high carbon ash obtained from
boilers. While the carbon content is relatively high in this residue, the energy content of all of the
ash produced is approximately 1 % o f the energy content of the wood. Combustion of the high
carbon ash presents a number of operational problems such as corrosion and scouring, and due to
boiler design the ash may be carried by the flue gas through the boiler tubes. The variability of
particles also causes inherent problems in a fixed bed system. Fixed bed systems usually require
a feedstock that is as uniform as possible to avoid channeling (1) (2). The costs associated with
changing the system to a more recent design is high.
High carbon ash is considered to have very little potential economic and environmental
benefit at this stage and is typically sent to landfills.
additive is restricted in British Columbia.

The application of bottom ash as a soil

Other options for utilizing high carbon ash must be

explored, including use as a low cost feedstock for gasification to recover as much energy as
possible, while reducing ash volume.
Biomass gasification is one technology which applies biomass to produce heat and electricity
(3)(4). Biomass gasification is a thermo-chemical process of gaseous fuel production by partial
oxidation o f a solid fuel (3). Gasification results in producer gas containing CO, H 2 , CnHx and
some other gases (4). The main objective is to generate a combustible gas rich in CO, H2 and CH 4
with medium to high lower heating value (LHV) (5)(6). Operating conditions such as temperature,
equivalence ratio (ER) and steam/biomass (S/B) ratio play important roles in biomass gasification.

85

Bed temperature is one of the most important operating parameters in gasification. Affecting
both the heating value and the producer gas composition (5). The heat needed for air gasification
is provided by the heat o f combustion of the biomass. Therefore high temperatures improve
biomass combustion and consequently increase CO 2 production, lowering the heating value o f the
produced gas. A high bed temperature improves carbon conversion and steam cracking and
reforming o f tars, resulting in less char, reduced tar formation and higher gas yields (5)(7)(8).
The equivalence ration (ER) strongly influences the type o f gasification. More combustion
occurs at high ER, increasing CO 2 production (5)(9). A higher air flow rate results in higher gas
velocities, improving the combustion of solid char due to improved oxygen diffusion (9)(10). An
air ratio o f 0.2 - 0.3 is most favourable for producing CO-rich gas (11). When steam is the
gasifying agent, H 2 and CO2 increase, while CO decreases due to the water gas shift reaction (12).
Fluidized bed reactors have been widely applied for gasification, pyrolysis and combustion
of a wide range o f particulate materials including biomass (13). Distinct advantages include high
heat transfer, uniform and controllable temperatures, favourable gas-solid contact and the ability
to handle a wide range o f particulate properties such as particle diameter. Fluidized bed reactors
also accommodate wide variations in fuel quality.
Air-blown biomass gasification produces low calorific value gases, with higher heating values of
4-7 MJ/Nm3, whereas oxygen and steam-blown processes result in a HHV of 10-18 MJ/Nm3 (11).
Circulating fluidized bed gasifier (CFB) test using various feedstocks such as, spruce-pine-fir
sawdust mixture, 1:1 ratio of pine bark and spruce whitewood mix, cypress, hemlock and cedarhemlock mixtures have produced gases with HHV from 2.43 - 6.13 MJ/kg, with either air or airsteam as the gasifying agent. For example, in a fluidized bed experiment at atmospheric

86

temperature, the gasification of pine saw dust produced a LHV of 6.74 - 9.14 MJ/Nm3 in an airsteam medium at ER = 0.22 (14).
The research carried out in this study was intended to determine the feasibility of gasifying
high carbon wood ash particles smaller than 3 mm to identify whether they behave similarly to
unbumed wood when gasified. Test were carried out to,
1. Determine the range o f equivalence ratios for stable operation.
2. Determine the calorific value o f the producer gas with air and air-steam agents to ascertain the
potential o f producing a low to medium calorific value syngas.
3. Measure the carbon conversion efficiency.
4. Calculate the product gas yield.

4.2. Experimental

4.2.1 Feed materials
Wood ash particles from an industrial scale fixed-bed boiler (Canfor Pulp Mill, Prince
George, BC) were the feedstock in the study.

Hog fuel is used in this boiler comprised

predominantly o f softwood sawmill waste derived from pine wood. Silica sand was the inert bed
material.

The proximate and ultimate analyses of the ash are provided in Table 4.1.

87

Table 4.1. Proximate and ultimate analyses o f boiler ash
Higher heating value (MJ/kg)

11.60

Proximate analysis (wt.% dry basis)
Volatile Matter
Fixed
Ash

21.5
28.8
49.7

Ultimate analysis (wt. % dry basis)
C
48.5
H
0.9
O
33.4
N
0.2
S
0
Other ash forming elements________________ 17.0
O - Calculated by difference

4.2.2 Gasification setup
Air gasification and air-steam gasification were carried out in a lab-scale bubbling fluidized
bed reactor. Constructed from 310 stainless steel 3-inch diameter (nominal) pipe (I.D. = 77.9 mm)
of height o f approximately 800 mm. Two electric heaters supplied heat to the reactor. The reactor
was charged with 1.4 kg o f sand as bed material. A pressure tap located in the biomass feeder was
used to control and facilitate the discharge of feedstock. The bed was fluidized by air and nitrogen
introduced below the distributor. Water was pumped to the reactor then vapourized, with its flow
rate measured by a steam flow meter. The biomass feedstock was fed from the side o f the reactor
through an atomizer nozzle, covered by a cooling jacket to keep the feedstock temperature below
80°C, to avoid plugging by thermal decomposition. The produced gas flow exited the reactor at
the top and passed through a cyclone to prevent tar condensation. Excess steam in the product gas
was separated by a condenser, while fine ash and char particles were captured by an internal

88

cyclone, supplemented by a filter after the condenser and a waste bin. The product gas flow rate
was measured by a rotameter combined with a thermocouple and a pressure transducer.

4.2.3 Experimental procedure
The feedstock was added to the hopper prior to the experimental run. The gasifier and
furnace heaters for air preheating were turned on, and controllers were set at the selected operating
temperatures. With sand as the bed material, the reactor was charged with ~ 7 L/min o f nitrogen
to assist with fluidization and aid heat transfer. The feedstock was then fed at 176 g/h, with an air
supply o f 0.282 Nm3/h from the bottom o f the reactor to provide an ER o f 0.12. When the system
reached steady state, gas samples were taken at 4 min intervals. Experiments were conducted at
various bed temperatures within the range o f 650 - 770°C. The reactor was then operated at a
fixed temperature of 775°C while varying the ER.
For air-steam gasification, water was introduced to the reactor at varying steam/biomass
(S/B) ratios and a fixed temperature o f 715°C and a fixed ER o f 0.12, with gas sampling as for air
only.

89

Particle Filter

Heating Tape

Bubbling
Fluidized
Bed

vent

Condenser

B to n w ss
H opper

Flow Meter
Water Cooled
Screw Feeder
Condensate
Collector

0—
^1 |1—©

Micro-GC

Rotameters
R9

RS

R!

AAA
Air

N>

Pump

Figure 4.1. Schematic diagram of biomass air and air-steam gasification in a bubbling fluidized
bed. T - Thermocouple P- Pressure sensor R- Rotameter

90

4.2.4 Gas analysis
The concentrations of H 2 , N 2 , CO, CO 2 and CnHx were measured by a micro-gas
chromatograph CP-4900 (Varian Inc.) equipped with a COx column and a thermal conductivity
detector.

4.2.5 Analyses of experimental results
To assess the gasification process, variables such as Equivalence Ratio (ER), carbon conversion
efficiency and higher heating values (HHV) were determined as follows.

_ w eig h t o f a ir /w e ig h t dry biom ass

( 4 1)

stoich iom etric air/b io m a ss ratio
_ steam feed rate+ m oisture introduced w ith fuel (g /h )

„

'

,.

..

Total fuel feed rate (g /h )

gas v elo city xlOOO[CO%+C02%+3(C3H8 %)]

Carbon conversion efficiency = ----------- -------- — —---------- —------ z±23J

biomass feed flow rate x C%

(4.3)

where produced gases are in volume %, gas flow rate is (Nm3/h), feed flow rate(g/h) and C %, is
the biomass percent carbon based on the ultimate analysis. Thehigher heating value isestimated
from equation 4.4.

HHV = (12.75 H2 + 12.63 CO + 39.82 CH4 + 63.43C2H4+ 99C3H8...) /100

(4.4)

where the species contents are in mol % and their heats o f combustion in MJ/Nm3 (11) (15).

91

4.3. R esults and D iscussion

4.3.1 Air Gasification

Effect o f reactor temperature.
Analysis o f the gas produced was carried out for reactor temperatures (electronically
displayed) ranging from ~ 650 to ~ 770°C in increments of approximately 30°C. From Figure 4.2,
it can be seen that the CO concentration increased with temperature. All other gas concentrations
remained nearly constant, except for CO 2 whose concentration decreased with increasing
temperature. The gases produced were predominantly influenced by the reactions:

C + j 0 2 ->C0

partial oxidation

C + 0 2 -* C 02

complete combustion (4.6)

C + H20 -* CO + H2

water-gas shift

(4.5)

(4.7)

Hence the C present in the fuel as char reacted directly with the O supplied by the air to
produce CO, an exothermic reaction. CO production favoured higher temperatures, resulting in
loss CO2 generation with increasing temperatures. The reactions were being carried out at ER =
0.12, below the ideal ER range of 0.2 - 0.3 (Li X, 2004). The limited O2 fed resulted in a high
CO : CO2 ratio. This would result in greater concentrations of CO instead o f CO2. The H

concentration remained low and relatively constant, in part because there was very little H in
the fuel. CH4 could not be detected but propane was found at very low concentrations. Table
4.2 summarizes the results when temperature was varied during air gasification. The carbon

92

90

% Molar C o n c e n tra tio n

80
70
C 02

60

H2

50
40
30

C3H8

20
10
0

650

700

750

800

Tem perature °C
Figure 4.2. Effect of temperature on gas composition. Biomass feed rate: 176 g/h; ER: 0.12.

93

conversion efficiency increased from 31.2 to 52.9 %, with increasing temperature, limited by the
low H content and the lack o f O to the reactor. The higher heating values increased with increasing
temperature from 0.77 to 1.64 MJ/Nm3. The percentage increase in HHV, increased by ~ 40%
between 657 and 675°C, thereafter approximately 10% for each temperature rise studied. The
monotonic increase resulted from the improved carbon conversion, at higher temperatures. The
gas yield ranged from 2.26 to 2.53 Nm3/kg and increased for the first two temperature rises then
slightly decreased at 771°C.

Table 4.2. The effect of temperature on various parameters during air gasification. Biomass feed
rate: 176 g/h; ER: 0.12.__________________
771
698
740
675
Lowed Bed temperature (°C) 657
1.64
1.01
1.39
1.54
HHV (MJ/Nm3)
0.77
Carbon conversion efficiency
~
49.9
52.9
45.6
48.3
(%)
2.53
2.48
2.49
2.53
Gas yield (Nm3/kg)__________ 2.26

94

CO2
-H2

C3H8

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

0.27

Figure 4.3 Effect of ER on gas composition. Biomass feed rate: 176 g/h; Temperature: 775 °C

95

Effect o f ER.
The effects o f ER on a number of factors were studied, with ER ranging from 0.12 to 0.25
and the reactor temperature 775°C. No significant variation was seen between ER of 0.12 to 0.25.
While CO concentrations remained relatively constant, the concentration o f H decreased, while
that of CO 2 increased, with increasing ER (Figure 4.3). The CO concentration was higher than
that of the other gases for all ER studied. The most significant variation in the gases under study
was in CO 2 . The other gases showed very little change in concentration. The carbon conversion
efficiency increased from 52.9 to 89.9 % when ER increased from 0.12 to 0.25 (Table 4.3). This
increase in carbon conversion efficiency resulted from increased CO 2 (14). The calculated HHV,
including N ranged from 1.64 to 2.38 MJ/Nm3 as shown in Table 4.3. The HHVs recorded when
ER was varied were higher than when the effect of temperature was investigated, where HHV
increased with increasing temperature. The gas production yield ranged from 2.48 to 2.73 m3/kg.
The gas production was highest for ER = 0.25. Based on carbon conversion efficiency, the
optimum point for gasification was at an ER of 0.25.

Table 4.3. Effect of ER on higher heating value, carbon conversion efficiency and gas yield during
air gasification Biomass feed rate: 176 g/h; Temperature: 775°C.____________________
0.23
0.25
0.16
0.19
0.12
ER
2.22
2.38
1.90
1.96
1.64
HHV (MJ/Nm3)
Carbon conversion efficien
79.7
63.8
68.2
89.9
52.9
(%)

Gas
Yield
(Nm3/kg)_________

2.48

2.51

2.44

2.64

2.73

96

4.3.2 Air-Steam Gasification
Effect o f steam-biomass ratio.
Analysis of the gas produced was carried out at S/B ratios from 0.4 to 2.2 and temperature
of 715 ± 5°C with ER of 0.12. It was difficult to maintain a fixed reactor temperature as some heat
from the reactor converted the water into steam. Likewise steam gasification is an endothermic
process. Due to the water gas shift reaction, the concentrations o f CO 2 and H2 increased with
increasing S/B ratio, while the CO concentrations decreased, as seen in Figure 4.4.
As the S/B ratio increased the water gas shift reaction became more integral in the process,
resulting in higher concentrations of H2 and CO2 .
The carbon conversion efficiencies increased when the S/B ratio increased from 0.4 to 1.3 and
decreased thereafter (Table 4.4). The highest carbon conversion efficiency o f 69.7 % was at a S/B
ratio o f 1.3, a steam flow rate of 0.216 kg/h, while the lowest value, 51.3 % was at S/B = 0.4.
HHV ranged from 1.95 to 2.50 MJ/Nm3. The heating value (including N) reached a maximum at
S/B = 1.3 due to increased production of CO 2 and H, reducing the calorific value o f the producer
gas (Gabra M, 2001). The volume of gas produced ranged from 2.45 to 3.19 m3/kg.

Table 4.4. Effect o f S/B ratio on higher heating value, carbon conversion efficiency and gas yield
during air-steam gasification for biomass feed rate: 176 g/h; ER: 0.12; Temperature: 775°C.
SB
0.4
0.8
1.3
1.7
2.2
HHV (MJ/Nm3)
1.95
2.29
2.26
2.50
2.11
Carbon conversion efficiency (%)
51.3
63.2
69.7
66.7
54.2
2.93
3.06
3.19
2.90
Gas Yield (Nm3/kg)______________ 2.45

97

70
C02

% Molar Concentration

60
50

• C3H8

40
30

20

10
_tn

0
0.3

0.8

1.3

1.8

2.3

S/B
Figure 4.4. Effect of S/B ratio on produced gas concentrations. Biomass feed rate: 176 g/h;
Temperature: 715 °C, ER: 0.12.

98

0.28

0.26

Hj/CO

0.24
0.22
0.2

0.18
0.16
0.14
640

660

680

700

720

740

760

780

T em perature (°C)
Figure 4.5. H 2/CO molar ratio as a function o f temperature for air gasification. Biomass feed
rate: 176 g/h; ER: 0.12

99

0.17

0 .16

HJCO

0.15

0.14

0.13

0.12
0.11
0.1
0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

0.27

ER

Figure 4.6 H 2/CO molar ratio as a function o f ER for air gasification. Biomass feed rate: 176 g/h;
Temperature: 775°C

100

1.8
1.6

1.4

Hz/CO

1.2
1

0.8
0.6

0.4
0.2
0.2

0.7

1.2

1.7

2.2

S/B

101

Effect o f temperature, ER and steam-biomass ratio on H 2/C0
The H 2/CO molar ratio was analysed based on the impact o f temperature, ER and S/B ratio.
As shown in Figure 4.5, molar ratio o f H2 /CO was less than 0.3 for the temperature range
investigated and decreased with increasing temperature during air blown gasification. At 775 °C,
all H2/CO molar ratios were below 0.17 and decreased with increasing ER. For the air-steam
blown process, as S/B ratio increased the H 2/CO molar ratio also increased. The ratios were higher
when compared to the air-blown processes and ranged from 0.4 to 1.75 (Figure 4.7). The injection
o f steam as a gasifying agent increased the H 2/CO molar ratio because moisture promotes both
steam gasification and the water gas shift reaction (11) (16).
While all three runs were at different operating conditions, the HHV can be compared at
similar conditions. At ER = 0.12 and 698°C for the air blown process and at 714°C for the steamair gasification (S/B ratio of 0.4), the HHVs (including N) were 1.39 MJ/m3 and 1.95 MJ/m3
respectively. The heating value for the steam fed process was approximately 29 % higher than
without steam.
As previously discussed, the HHV increased with increasing ER and the optimum carbon
conversion efficiency was found at ER = 0.25. The increase in HHV from ER of 0.12 to 0.25 was
approximately 31 % for the air blown process. Since the air-steam gasification was carried out at
an ER 0.12, it is likely that the HHV would increase if the ER was increased to 0.25, producing
gas with a higher calorific value. An increase in the highest calorific value of 2.50 MJ/Nm3 might
be seen. Research on sugar cane residue (bagasse) showed that a gas generated at 3.5 - 4.5 MJ/Nm3
did not present any problems when burned (16). However, in order to get good burning o f the gas
to fuel a turbine, combustion should be close to the stoichiometric conditions. The potential of an
even higher heating value suggests that the producer gas could be useful.

102

A high superficial gas velocity may cause entrainment of fine ash and carbon particles in the
producer gas while too low a value may encourage defluidization in the reactor. While oversized
particle sizes may not be fluidized and cause agglomerate (17), the feedstock under study had a
low density relative to sand. No noticeable fusing of ash or ash-sand particles were observed on
visually inspecting the particles after opening the reactor after each run. Hence, the possibility of
reducing the fluidizing gas velocity could be considered. Gasification with N as a fluidizing gas
dilutes the produced gas (18). Therefore, the calorific values were calculated on a N-free basis to
give an idea of what range of heating values could be achieved. The minimum fluidizing flow
used for the experiments was approximately 15.0 L/min. This was achieved by adding N to give
the desired operating fluidization conditions. Further investigation should be carried to reduce the
fluidizing flow in an effort to produce a higher calorific value producer gas.

103

4.4 Conclusion

1) High carbon ash gasification in a bubbling fluidized bed reactor was successfully gasified at
low temperatures and atmospheric pressure. Ash and woody biomass showed some similar
trends for gasification and product gas formation.
2) The higher heating value o f the producer gas at equivalence ratios from 0.12 to 0.25 were in the
range o f 0.77 - 2.50 MJ/Nm3 with gas yields from 2.26 - 3.27 Nm3/kg.
3) The carbon conversion efficiency increased with increasing temperature and reached a
maximum at an ER of 0.25.
4) The HHV increased with increasing temperature, 650 - 770°C.
5) The heating value for the steam fed process was approximately 30 % higher than without steam
at otherwise similar operating conditions.
6) There may be some potential in producing a gas o f higher calorific value from the air-steam
process, since the air-steam run was carried out at a low ER. This would encourage additional
uses o f high-carbon ash. Also, if possible to lower the fluidizing velocity a gas o f better HHV
could be achieved.
7) The H2/CO molar ratio increased with the addition o f air-steam over the range o f S/B ratios
studied, compared to air blown processes where the ratio decreased with increasing ER and also
with increasing temperature.
8) No noticeable fusing of ash or ash-sand particles was observed on visual inspection after run
completion.

104

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10. Natarajan, E.; Nordin, A.; Rao, A.; Overview of combustion and gasification of rice husk in
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106

Preface

Chapter 5 o f this thesis has been prepared as a manuscript draft and will be submitted for
possible publication shortly.

107

CHAPTER 5: C atalytic Effect o f C alcined W o o d Ash d u rin g C 0 2
G asification o f B iom ass Char

5.1 Introduction
With increasing use of bioenergy technologies comes increasing effort to improve process
efficiencies. With a range of outputs, interest in biomass gasification is growing. A primary aspect
of the gasification process is the variation in the reaction rate during char conversion (1). This
variation may be due to a change in the reaction area as well as the change and distribution of the
catalyst during gasification.
Research on catalyst for use in gasification process is often carried out specifically in relation
to gasifier design and biomass feed type (2). The use of inexpensive, strong and easily regenerated
catalysts can help produce a tar-free product gas, suitable syngas composition and higher
gasification reactivity. Wood ash has been investigated for its catalytic effect where the presence
o f alkali and alkali earth metals have been reported to promote gasification (2)(3). However,
research carried out on wood ash as a catalyst is not as extensive as research on catalysts made
from salts or metals such as potassium. Potassium is considered to be a good catalyst, but needs to
be recycled for cost efficiency (4). The porosity of ash combined with high metal and mineral
content makes ash a suitable catalyst (5). Some of the minerals present in wood ash include CaO,
MgO and K 2 O.
A mechanism suggested for catalytic gasification consists of an oxidation-reduction cycle in
which oxygen is transferred to the carbon matrix through the catalytically active alkali species
followed by the rate determining decomposition of the oxidised carbon site producing CO in the

108

process (6)(7)(8). An oxygen containing alkali species active for oxygen transfer to a free carbon
site results in a surface oxygen intermediate.
Research has shown that the use o f wood ash as a catalyst during gasification o f bituminous
coal increased the reactivity by a factor o f 9 and of wood by 32 at 700 °C, when compared to the
uncatalyzed reaction (2)(9). Recent work has also shown that impregnating 9.5 wt% KOH in pine
wood resulted in complete char conversion within 12 min under steam gasification conditions
carried out at 700 °C (10).

The use of the impregnated catalyst resulted in an increase in

gasification rate by a factor o f 30 compared to char that was not impregnated. The research also
ranked the catalytic activities o f a number of salts which are stated in decreasing order, KNO 3 >
KHCO 3 ~ K2 CO 3 52 KOH > NaOH > CaO > K2 HPO4 > KBr > KC1. In another work, the presence
o f alkali carbonates on biomass steam gasification at 650 °C and higher, resulted in a decrease in
carbon conversion to gas, during volatilization but the rate and total amount o f gas produced during
gasification stage increased (2) (11). CO 2 co-gasification of switch grass with coal and fluid coke
at 750 °C and 950 °C also showed that the biomass addition enhanced the rate o f gasification (3).
It was also believed that the formation of potassium aluminosilicate (KAlSiaOs) may deactivate
the catalyst. The catalytic effect o f hardwood ash containing 12 wt% K and 47 wt% Ca was
investigated during steam-char (coal) TGA experiments at 700 °C and higher (12). The findings
suggested that dry mixed wood ash increased the reactivity of the process but was slightly less
than an equivalent amount of K 2 CO 3 . The application of a dry aqueous extract of the ash
containing 39 wt% K provided an equivalent increase in reactivity to K 2 CO 3 .
The percentage distribution by weight o f these ash forming compounds will vary depending
on the type of feedstock and the combustion process that may have been employed. Agricultural
and woody biomass often vary in metal concentrations and some metals are volatilized as the

109

temperature increases during combustion. It is for this reason that a minimum standard for the
type o f ash and their concentrations should be determined if wood ash is to be considered a suitable
catalyst.
The need to continuously find cheap, readily available and reusable catalysts is an important factor
when considering catalyst application in energy production processes. This work investigates the
use of two types o f combustion ash, gasifier ash and a pellet burner ash, for their catalytic effects
on woody biomass CO 2 gasification by varying the percent catalyst loading. The research seeks
to identify the effectiveness o f using ash as a catalyst by determining rate o f carbon conversions,
gasification reactivities and the reactivity indices as well as identifying which o f the ash could be
a more effective catalyst.

5.2 Experimental

Wood ash samples
Wood ash from a downdraft gasifier and a wood pellet burner (both located at the University
of Northern British Columbia - UNBC) and char-ash from an industrial scale fixed bed boiler were
used in this study. The fuel for both the gasifier and the industrial boiler is hog fuel, comprised
predominantly of softwood sawmill waste derived from pine wood. The pellet burner utilizes wood
pellets made locally from soft-wood saw dust.

110

5.2.1. Char and Catalyst Preparation
Approximately 100 g of char-ash (containing significant amounts o f unbumed carbon)
obtained from the industrial fixed -bed boiler was sieved using a 2000 pm sieve. The particles >
2000 pm (char) were retained and the rest discarded. The retained fraction (henceforth referred to
as char) was crushed to particle sizes ranging from 6 3 -1 5 0 pm.
Approximately 100 g o f gasifier ash was separated using a 150 pm sieve and the particle
fraction <150 pm was retained for additional preparation while the rest was discarded. The sample
was calcined by placing it in a muffle furnace at 105 °C for 3 h, after which the furnace was ramped
up to 500 °C and set at a heating rate of 10 °C/min up to 800 °C, and then held at 800 °C for 4 h.
After calcination was completed, the calcined ash was placed in 20 ml vials and stored in a
desiccator for use as a catalyst. The procedure was repeated for the pellet burner ash. The prepared
catalysts were denoted as GA catalyst for gasifier ash catalyst and PBA catalyst for pellet burner
ash catalyst and these abbreviations will be used hereafter in the paper.

5.2.2. Characterization Methods
The char sample was analyzed by proximate and ultimate analysis. The calcined wood ash
(GA and PBA) catalysts were characterized by ultimate analysis, X-ray Diffraction (XRD), surface
area and metal analyses. The proximate analysis of char was carried out according to ASTM
method D 1762-84. Ultimate analysis of C, H, N, S was conducted using a Costech™ Elemental
Analyzer, ECS 4010 Elemental Combustion System. Reactor conditions were: 1000 °C, helium
carrier gas, 105 mL/min, GC Column #051080, SS 5 mm x 2 m, at 100°C, reaction tube: 450 L x
18 mm, GC column packing: HayeSep Q 60/80. A Thermal Conductivity Detector was used.
Properties, such as volatile matter (VM), ash content, fixed carbon (FC) and C, H, N, S, are

111

expressed on a dry weight basis. To determine alkali metals, alkali earth metals and trace elements
in the catalysts, samples were prepared by microwave digestion using a Milestone MLS 1200
Mega digestion system with concentrated HNO 3 . Metal characterization was done by an
inductively coupled plasma (ICP-MS) on an Agilent 7500ICPMS m achine..
X-ray diffraction (XRD) powder patterns o f the catalyst were obtained on a Bruker D8
Advance Series II using Cu-Ka1radiation at a wavelength of 1.5406 A with 2 0 being varied from
10° to 90°.
BET surface area measurements were carried out with N 2 at 77 K using a single -point
Micromeritics FlowSorb 11 2300 surface area analyzer.

5.2.3. Gasification
A thermo-gravimetric analyzer (TGA, Cahn Thermax 500) was used to measure weight loss
of the char during CO 2 gasification. The gasification experiments were performed isothermally at
800 °C and at atmospheric pressure. Approximately 20 mg of char was weighed into a crucible
(half sphere with 12 mm diameter) and then heated at 15 °C/min to 800 °C under the flow o f N 2
(400 mL/min). The gas was switched from N 2 to CO 2 (400mL/min) after reaching 800 °C and a
stabilized mass.

Gasification was carried out until complete conversion o f the char.

The

experiment was repeated by adding varying percentages of the catalyst (36, 18 and 10 wt.%, dry
basis) while keeping the mass of char as constant as possible.

112

Analyses o f experimental results

Evaluation o f reactivity:
Carbon conversion in CCh-char gasification, X, is defined as,
X =

Wo-Wt
■Wo-Wash

(5.1)

where W0 is the initial mass of char, Wash is the mass of ash in char sample after gasification and
W, is the mass of sample at time t.

The gasification rate per unit mass o f residual fixed carbon varies with carbon conversion and can
be represented as,
r(X ) = ^ S

(5.2)

where r is the char reactivity.

The reactivity index, Rs (14) (15) is defined as,

(5.3)
Where to.s is the gasification time taken to reach a carbon conversion o f 50 %.

113

5.3 Results and Discussion

Table 5.1. Proximate and ultimate analysis o f char
Proximate analysis (wt.% dry basis)
Volatile Matter
Fixed Carbon
Ash

14.8
59.1
26.1

Ultimate analysis (wt. % dry basis)
C
H
O (calculated by difference)
N
Ash

68.15
0.76
22.57
0.12
8.40

Table 5.2. Inorganic elemental distribution of GA catalyst and PBA catalyst
PBAGACatalyst
Catalyst
Element

Ca
K
Mg
Mn
P
A1
Fe
Other

mg/kg
167000
18400
15700
8460
8180
15900
12800
9120

mg/kg
263000
95700
56500
28100
8550
7560
6170
8720

Characterization of catalyst
Woody biomass is known for its high concentrations of alkaline earth metals, particularly
Ca. Table 5.2 shows the metal concentrations for the major elements in the PBA-catalyst and the
GA-catalyst. The PBA-catalyst is significantly more concentrated in Ca, K, Mg and Mn than the
GA-catalyst. These elements are known to contribute to the catalytic effects of some processes.

114

Ca, K, Mg and Mn are respectively, approximately 1.6, 5.2, 3.6 and 3.3 times more concentrated
in the PBA-catalyst than in the GA-catalyst.
Analysis of the XRD results shown in Figure 5.1(a) highlights a number of dominant
peaks. The XRD pattern o f PBA-catalyst shows sharp peaks at 20 = 32.4, 37.5, 54.0 and 64.5.
These peaks were linked to CaO (Lime). The peaks at 43.3 and 62.5 were linked to MgO
(Periclase). Other minerals identified included Ca(Si04) (Calcium Silicate), Fe 3MmSi3 0 i2 (Iron
Manganese Silicate) and Fe2 0 3 (Hematite). Analysis o f the patterns indicates that CaO has the
highest mineral content in the ash. The XRD pattern of the GA-catalyst seen in Figure 5.1(b)
shows CaO (Lime) and MgO (Periclase) at similar angles to those detected in the PBA-catalyst.
Gehlenite Magnesian Ca2(Mgo.25Alo.75)(Sii.25Alo.7507) was identified at 32.5 and also at some
smaller peaks. Potassium aluminium silicate (KAlSi206) was also found to be present in the
GA-catalyst. The BET surface area of the PBA-catalyst and GA-catalyst were 3.1 m2/g and 6.7
m2/g, respectively.

Table 5.3. Semi-quantitative data o f mineral distribution in GA catalyst and PBA catalyst as
determined by XRD_____________________________________________________________
GA - Catalyst
PBA -Catalyst
Formula
%
Formula
%
CaSi03
CaO
MgO

79
70.9
1.9

Ca2(S i04)
CaO
MgO

15.2
44.4
24.6

KAlSi20 6

50

Fe3Mn2Si30i2

8.9

C a2(M go.25A lo.75)(Sii.25Alo.7507

3 .4

F e203

6 .8

S i0 2

34

Ca5(P 04)3F

7-4

115

rawo«w -

raw •

c woo

o
u
c saw'
raw•
raw -

raw ~
sow*$L
»

1«*#4
»
2-Th«ta - Scale

jpM

L«W M »6*0 S M

*

Figure 5.1a. X-ray diffraction patterns o f pellet burner ash catalyst

116

»

m

<•«

»

«&

*e

*e

2-Th#ia - Seal*
Figure 5.1b. X-ray diffraction patterns o f gasifier ash catalyst

117

(X)
Char Conversion

Char
Char-PBA36%

0.4

Char-GA36%
Char-PBA 18%
— ■ Char - PBA 10 %

0

500

1000

1500
Time (sec)

2000

2500

3000

Figure 5.2. Observed char conversion during CO 2 gasification with and without the addition o f
PBA-catalyst and GA-catalyst between 0-36 wt% catalyst loadings.

118

0.002

Gasification reactivity (sec1)

0.0018
0.0016
0.0014

0.0012
0.001
0.0008
0.0006
0.0004

0.0002

0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1

0.9

C arbon Conversion (X)
Char

Char - PBA 36 % —

Char - 6A 36 % —*— Char - PBA 18 % —

Char - PBA 10 %

Figure 5.3. G asification reactivity as a function o f char conversion during C O 2 gasification at
800 °C using varying percent catalyst loadings for G A -catalyst and PB A -catalyst.

119

The effect of catalyst on char conversion and reactivity.
Catalyst loading was varied by changing the mass o f catalyst addition from 0 to 36 wt. %.
Figure 5.2 shows the gasification of char with catalyst was faster than without catalyst. Carbon
conversion was different between the two catalysts, with the PBA-catalyst having a much greater
effect. With the addition o f PBA-catalyst or GA-catalyst, 50% char conversion took ~ 5 min and
~ 19 min respectively. For the same degree of conversion the non-catalysed char reaction took
~21 min. 90 % char conversion with 36 % PBA-catalyst loading, 36 % GA-catalyst loading, or
with no catalyst loading took ~ 10 min, ~ 32 min and ~ 34 min respectively. While most o f the
carbon (> 90 %) was converted rapidly in the PBA-catalyst reaction, the reaction became
significantly slower thereafter. This resulted in the PBA-catalyst taking the same time as the other
two reactions for full carbon conversion.
Due to the low impact o f the GA-catalyst on char conversion, no further investigation o f this
catalyst was carried out. Figure 5.2 also shows the char conversion with PBA-catalyst o f loadings
18 % and 10 %. The char conversion decreased when the percent catalyst loads decreased. For 50
% carbon conversion, the conversion times with 18 % and 10 % loading were ~ 8 min and ~ 12
min, respectively. For 90 % conversion, the conversion times were ~ 14 min and ~ 23 min for the
18 % and 10 % catalyst loadings, respectively.
Figure 5.3 illustrates the gasification reaction as a function of char conversion. In all cases,
the addition o f a catalyst increased the gasification reactivity o f the char. The highest char
gasification reactivity was 4.81 x 10'4 /sec. The samples showed significant differences in
reactivity with the PBA-catalyst loading of 36 % being the highest at 1.82 x 10"3/sec. The reactivity
remained relatively constant until most of the char was converted. The high gasification reactivity
seen for the PBA-catalyst gasification explains the rapid carbon conversion for that sample. The

120

gasification reactivity decreased with decreasing catalyst loading for the PBA-catalysts of loadings
18 % and 10 %. At 18 %loading, the highest reactivity was 1.27 x 10'3/sec and decreased gradually
throughout the reaction while at 10 % loading the highest reactivity was significantly lower, 7.23
x 1O'4/sec. The reactivity of the GA-catalyst at 36 % loading was the lowest for all the catalysts
studied, though slightly higher than char reactivity without catalysts. The highest gasification
reactivity of this catalyst was 4.82 x
lO^/sec.

Table 5.4. Reactivity index of char and char-catalyzed CO2 gasification reactions at 50 % char
conversion.
Rs (min'1)
Sample
Char
Char-GA 36 %
Char-PBA 36 %
Char-PBA 18 %
Char-PBA 10 %

0.023
0.026
0.100
0.062
0.042

The reactivity index at 50 % char conversion is shown in Table 5.4. The char with PBAcatalyst loading of 36 %had the highest reactivity index of 0.100 min'1. The GA-catalyst was only
slighter higher in reactivity compared to the non-catalysed char. Although the reactivity indices
are small, the activity ranking can be identified in terms of percent ash loading, PBA 36 %> PBA
18 %> PBA 10 %. When comparing ash types: PBA 36 %» > GA 36 %. This suggests that the
higher concentrations of specific elements in the PBA-catalyst, as seen from the ICP analysis,
results in increased gasification reactivity. Elements of importance may be Ca, K, Mg and Mn
Both Ca and K are significantly higher in concentration in the PBA-catalyst. The XRD results seen
in Table 5.3 identified no minerals of K but instead identified CaO as the major mineral (44.4 %)
followed by MgO (24.6 %) and Ca(Si04) 15.2 %. With these observations, it is likely that CaO

121

and MgO play a major part in the catalytic effect on gasification reactivity. Further work on the
catalytic effects of CaO, MgO and Ca could be done to investigate the validity of these
assumptions. Wood ash is usually rich in metal-oxide content and may provide the necessary
oxygen radicals to bond with C.
When comparing the PBA-catalyst and the GA-catalyst at the same catalyst loading, the
concentration of Ca in the PBA-catalyst was 1.6 times higher (obtained from Table 5.2). This
could have an impact on the gasification reactivity since analysis o f the work showed that higher
concentrations of the PBA-catalyst increased reactivity. The other elements o f importance such
as K and Mg in the GA catalyst were approximately 5.2 and 3.6 times lower in concentrations than
in the PBA-catalyst. The lower concentrations of these alkali and alkali earth metals may also
account for the low gasification reactivity of the GA-catalyst. The XRD results seen in Table 5.3
identified CaO as the major mineral (70.9 %) in the GA-catalyst. KAlSi206 was also identified in
the mineral content (5 %). It is possible that the presence o f this mineral could inhibit the activity
of the catalyst (deactivating the catalyst) as presented in similar research (3). The elements A1 and
Fe in the GA-catalyst were two times higher in concentration than in the PBA-catalyst. This could
possibly explain the likelihood of KAlSi2 0 6 being present as a mineral.
The K/Al concentration ratio was significantly higher in the PBA-catalyst compared to the
GA-catalyst, ~12.7 and ~1.2 respectively. The lower ratio of the GA-catalyst suggests that A1 is
likely to have a stronger influence on the effectiveness of the GA-catalyst. The catalytic effect of
K may have been limited due to A1 bonding with K as seen in the formation o f KAlSi206. On the
other hand, this mineral was not identified in the PBA-catalyst.

122

5.4 Conclusion

CO2 gasification of biomass char at 800 °C, using wood ash derived catalyst obtained from
an updraft fixed gasifier and a wood pellet burner influences gasification reactivity. Both catalysts
were enriched in Ca, K , Mg and Mn but were significantly higher in the PBA-catalyst. The XRD
results identified CaO as the major mineral in both catalyst and also MgO in the PBA-catalyst.
The order of reactivity for the catalysts studied, reported as percent catalyst added to char were,
PBA 36 % > PBA 18 %> PBA 10 % > GA 36 % and PBA 36 % » > GA 36 % . Therefore, the
highest gasification reactivity was achieved when 36 % pellet burner ash catalyst was added to
char.

It was concluded that larger catalyst loadings for the PBA-catalyst, hence higher

concentrations of the alkali and alkali earth metals, increases the gasification reactivity. The GAcatalyst showed significantly lower reactivities which may have been due to the lower
concentrations of elements of catalytic importance as well as the presence of potassium aluminum
silicates, which are considered to inhibit catalytic activity. The K/Al ratio in both catalyst showed
that the PBA-catalyst was significantly higher -12.7 than the GA-catalyst, -1.2. The catalytic
effect of K may have been limited due to A1 bonding with K as seen in the formation of KAlSi206.

123

References

1. Hamilton, R.; Sams, D.; Shadman, F. Variation o f rate during potassium-catalysed CO 2
gasificaion of coal char. F u e l. 1984, Vol. 63,1008 -1012.
2. Sutton, D.;Kelleher, B.; Ross.; J. Review o f literature on catalyst for biomass gasification. Fuel
Processing Technology. 2001, Vol. 73,155-173.
3. Habibi, R.; Kopyscincki, J.; Masnadi, M.; Lam, J.; Grace, J.; Mims, C.; Hill, J. Co-gasification
of biomass and non-biomass feedstocks : synergistic and inhibition effects of switchgrass mixed
with sub-bituminous coal and fluid coke during CO 2 gasification. Energy and Fuels. 2013, Vol.
27,494-500.
4. Hauserman, W. Relalting catalytic coal or biomass gasification mechanisms to plant capital cost
components. Internation Journal o f Hydrogen Energy. 1997, Vol. 22, 4,409-414.
5. Klinghoffer, N.; Castaldi, M.; Nzihou, A. Beneficial use o f ash and char from biomass
gasification. Proceedings of the 19th Annual North America Waste-to-Energy Conference.
Lancaster, Pennsylvania : ASME, 2011. 1-5.
6. Meijer, R.; Weeda, M.; Kapteijn, F.; Moulijn, J. Catalyst loss and retention during alkalicatalysed carbon gasification in CO 2 . Carbon. 1991, Vol. 29, 7, 929-941.
7. Kapteijn, F.; Moulijn, J. Kinetics o f potassium carbonate-catalysed CO 2 gasification o f activated
carbon. Fuel. 1983, Vol. 62, 2,221-225.
8. Kapteijn, F.; Peer, O.; Moulijn, J. Kinetics of alkali carbonate catalysed gasification of
carbon:l.C02 gasification . F u el. 1986, Vol. 65, 10, 1371-1376.
9. Hauserman, W. High-yield hydrogen production by cataltic gasififcation o f coal or biomass.
International Journal o f Hydrogen Energy. 1994, Vol. 19, 5, (413-419).
10. Nanou, P.; Murillo, G.; van Swaaij, W.; Rossum, G.; Kersten, S. Intinsic reactivity o f biomassderived char under steam gasification conditions-potential o f wood ash as catalyst. Chemical
Engineering Journal. 2013, Vol. 217, 289-299.
11. Hallen, R.; Sealock, L.; Cuello, R.; Bridgewater, A. Research in thermochemical biomass
conversion. London : Elsevier Applied Science, 1988. 157.
12. Hauserman, W.; Kulas, R.; Timpe, R. Catalytic effect on the gasification of a bituminous
argonne premium coal sample using wood ash or taconite as additive. [Online] [Cited: April
11,2013.]

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http://web.anl.gov/PCS/acsfiiel/preprint%20archive/Files/36 3 NEW%20YQRK OS91 0892.pdf.
13. Quaak, P.; Knoef, H.; Stassen, H. Energy from biomass. A review o f combustion and
gasification technologies. Washington, D .C .: World Bank Technical Paper; 422. Energy series,
1999. 0-8213-4335-1.
14. Ye, D.; Agnew, J.; Zhang, D. Gasification of Souh Australian low-rank coal with carbon
dioxide and steam: kinetics and reactivity studies. F u el. 77,1998, Vol. 11,1209-1219.
15. Wang, J.; Yao, Y.; Cao, J.; Jiang, M. Enhanced catalysis of K 2CO 3 for steam gasificationn of
coal char by using Ca(OH )2 in char preparation. F u el. 2010, Vol. 89, 310-317

125

CHAPTER 6: C onclusion an d R e c o m m e n d a tio n s

Forest biomass in British Columbia (BC) is considered to be the most abundant biomass
resource and could theoretically replace almost half o f the Provinces yearly fossil energy
consumption (1). A number of biomass feedstocks exist in the Province.

Harvest residue

accounts for 37 % of the total biomass energy potential and is the single largest biomass resource.
Saw mill residues are readily available. Pine beetle killed wood, construction and demolition
waste are also other sources of wood residue. Other sources o f biomass in BC include agricultural
wastes (e.g.; crop residues, liquid and solid animal wastes that can be used for energy purposes),
municipal bio-waste and landfill gas, food waste, energy crops, oils and animal fat.
Securing a long-term supply of biomass feedstock is a major consideration when selecting
biomass as an energy system. At this point, the pine beetle infestation has left large amounts of
wood available for use. The B.C. Ministry o f Forests, Lands and Natural Resource Operations
estimates that the mountain pine beetle has affected a cumulative total o f 18.1 million hectares o f
cubic meters of timber since the current infestation began (2). To put this in context, the Ministry
noted that 18.1 million hectares is more than five times the size o f Vancouver Island. Much of
this, however is too remote to harvest economically. A major challenge as we increase bioenergy
production within the Province will be the long-term availability o f these woods due to: rapid
decay and utilization rates, access rights to residues from existing tenures, harvesting costs
including road construction, replanting and transportation (1).
The sustainable use of biomass as a resource during harvesting and utilization must also be
considered, for example when biomass is used in energy production processes. The use of woody
biomass in bioenergy processes must be such that all possible energy is completely extracted.
However, this is likely not possible due to specific problems such as: fuel types, variations in

126

retention time in the reactor, incomplete combustion, temperature variations and fluctuations,
moisture content o f fuel and other factors. For these reasons, other viable energy processing
options must be identified. Ideally, these options will be explored in conjunction with existing
energy technologies so as to ensure maximum use of the resource.
A major residue that is produced during bioenergy production is ash. The management of
this resource often poses problems as it relates to its utilization and storage. Bottom ash usually
accounts for the largest portion of ash; that is, approximately 60 to 90 % o f the total ash being
generated (3). In some industries the presence o f large amounts o f carbon has rapidly increased
ash volumes, creating concerns relating to short term storage, transportation and landfill
management. It is for these reasons that alternative methods for the utilization of ash o f varying
physical and chemical characteristics must be identified.

Identifying specific fractions of

technological, environmental and economic importance could assist in dealing with this “waste”.
However, the quality of ash obtained from combustion systems varies in unbumed carbon content
and in inorganic distribution.
A potential method to obtaining and characterizing the unbumed carbon in ash is sieve
fractionation; a technique employed in this research. Sieve fractionation allowed for easy
accessing of the carbon and may present itself as a cost effective pre-treatment method for
rebuming high carbon ash in order to obtain the highest energy components. If large amounts of
unbumed carbon are present in ash, it could be used as a fuel source which would have positive
impacts on reducing ash volumes, increasing bioenergy processing efficiencies and also ensuring
the sustainable use of biomass.

This research showed that in some ash types the unused energy

potential was 10.77 and 14.42 MJ/kg.

When separated into particle fractions by sieving, the

energy content within the largest particle size fraction was as high as 25 MJ/kg resulting from the

127

carbon-carbon bonds in char. The obtained calorific values suggest that the waste being disposed
of has good energy potential and is comparable to some forms o f biomass used in energy
production processes (4). According to Liao et al. (4), the calorific value of biomass is in the
range of 16-20 MJ/kg, while bituminous coal achieves 34 MJ/kg. Data obtained from Natural
Resource Canada showed that, in 2010, the average energy intensity for residential space heating
in BC was 0.29 GJ/m2 (5). Assuming an average house is approximately 150 m2, the energy
required to satisfy this demand would be 43.5 GJ/yr. Since the high carbon ash has -1 4 MJ/kg
or 14 GJ/tonne, it would require -3 .1 tonnes/yr of high carbon ash to meet the energy demands
of heating an average residential household. Using high carbon ash for space heating should be
considered if there is an adequate supply due to energy content, cost saving effects and potential
for environmental sustainability.
This research also showed that using a lab-scale fluidized bed gasifier was effective in
reducing the high carbon content in ash. Air-steam gasification o f this fuel showed that there is
tremendous potential in producing a gas o f a low to medium calorific value. A gas heating value
o f 2.5 MJ/Nm3 was obtained with the potential to be even higher by increasing the equivalence
ratio and reducing the volume of fluidizing gas (nitrogen). For comparison, natural gas energy
density is 36 MJ/Nm3. Research carried out on sugar cane residue (bagasse) showed that a gas
generated at 3.5 - 4.5 MJ/Nm3 did not present any problems when it was fired and burned (6).
However, in order to acquire efficient combustion of the gas so as to fuel a turbine, combustion
should be done close to the stoichiometric conditions. The possibilities to obtain a producer gas
higher than 2.5 MJ/m3 using gasification technology suggests the fuel potential of high carbon
ash. This research suggest that there are many opportunities for continued research, such as

128

identifying other suitable methods and technologies for processing this fuel as well as the need
for continued optimization o f a product gas.
Returning high carbon ash to energy conversion systems could also be beneficial in
increasing process efficiencies. The need for a detailed life cycle analysis is essential in
determining the cost and benefits of reusing this fuel. Wood ash could be an effective catalyst in
woody biomass gasification processes by increasing the gasification reactivities. The reactivities
measured in this research, varied based on the ash type, elemental concentrations and mineral
content of the ash.

Ash obtained from a pellet burner, when used as a catalyst increased

gasification reactivities with increasing catalyst loadings. This was attributed to the high
concentrations o f the alkali and alkali earth metals. The high reactivity o f biomass char allowed
fast and efficient investigation into the effectiveness of wood ash as a catalyst during biomass
gasification. Due to the lower gasification reactivity o f some forms o f coal when compared to
wood char, wood char catalyzed reaction may assist in understanding ash catalyst reactions which
may could be extended to some coal gasification research. The findings from this research could
be beneficial in coal gasification processes.
It must be noted that, should high carbon ash be gasified, issues related to ash carry-over in
the producer gas could be a challenge due to the fine nature and low density o f some ash particles.
This could result in the constant need to remove and replace gas filters, thus increasing overhead
costs. The possibilities of returning high-carbon ash to existing gasifiers through co-combustion
with unbumed woody biomass could also be an effective way of processing the unbumed carbon.
Other options could be using a different type o f gasifier or making pellet or briquettes.
The limited understanding o f the behaviour o f ash, its properties and long-term
environmental impacts also pose a problem. Ash utilization is limited by the presence of heavy

129

metals and other inorganic compounds (7) (8), which are formed as a result o f the thermochemical
reactions that the biomass undergoes when combusted. The variation in inorganic concentrations
pose a difficulty in the general application of bottom ash to soils due to the lack of a standardized
ash quality. Sieve fractionation could be helpful in obtaining and/or eliminating fractions of
environmental importance. When ash from a downdraft fixed-bed gasifier, wood pellet burner
and an industrial-scale fixed-bed boiler were investigated, the trace element contents for the
samples were all within the environmental limits for soil amendments in British Columbia,
Canada (9). On the other hand, when analyzed within particle fractions, Ni with a concentration
o f 229 mg/kg from the wood pellet burner ash, exceeded the limit within the particle size fraction
> 850 but < 2000 pm.
The increase in ash volumes from increase bioenergy use, could result in its use as a
fertilizer and or as a soil conditioner, should adequate concentrations of specific elements be
identified. According to Obemberger et al. (10), ash recycling to agricultural lands can help
reduce the use of artificial fertilizers and close the natural mineral cycle owing to the presence of
N, P and K in biomass ash. This could have positive impacts on forests and farm economies
assuming adequate nutrients are provided to improve tree growth. While nitrogen was absent or
extremely low in all samples, high concentrations o f K and P were present. Yet, the research
agreed with other authors who have suggested that very little o f the P may be water-soluble (11),
limiting adequate P plant uptake. The need for further greenhouse trials to investigate the impact
o f varying concentrations of wood ash and high carbon wood ash on biomass growth with
different soil types in British Columbia would be an asset in understanding the benefits o f ash.
High pH’s were found in the ash types studied, which may have been due to high
concentrations of alkali and alkali earth metals being present. The pH increased as the particle

130

size fractions decreased which may have been due to the higher concentrations of base-forming
metal salts in smaller particle fractions. A major benefit o f ash could be as a liming agent in soils.
A strong linear relationship of Ca and P to C content was identified. This agrees with previous
work which suggested that unbumed carbon creates a dilution effect and reduces the
concentration of inorganic nutrients available in the ash (7). Should ongoing studies continue to
demonstrate the positive effects of ash and/or high carbon ash on soils and soil properties, farmers
and foresters could possibly consider the use of ash as a cheaper fertilizing and soil amendment
alternative in BC.
In addition, establishing smaller biomass plants where energy demand is lower could be
considered for rebuming high carbon ash. For example, wood pellet burners bum efficiently and
produce very little to no unbumed carbon in the ash with very little overhead costs. Therefore, if
a combustion system that could rebum high-carbon ash was to be developed, this could possibly
harness the left over energy in the ‘waste’ while dealing with the storage, transportation and
handling cost related to high-carbon ash. An economic feasibility study would need to be carried
out on this idea. As with most development, small biomass plants will present a relatively higher
transition costs when compared to large scale biomass plants because o f the increased time, effort
and money to secure projects (12). Nevertheless, continued biomass supply and security must be
considered and may require a diverse biomass energy portfolio.
Some communities may benefit significantly from small biomass energy technologies as
these communities could become involved in operating and managing their bioenergy facilities,
once they are trained. Perhaps, small scale bioenergy plants could assist in offsetting the high
demand for diesel by providing some amount o f heat energy. These bioenergy facilities could be
a source o f employment for both skilled and unskilled workers, creating much needed

131

employment for rural areas. The plants could be used for heat generation.

Based on the

widespread availability o f biomass, the technology would lend itself across BC. Bioenergy
developments would need to consider issues such as technical experience, education, community
profile, climate change mitigation, employment, health and emissions and landscape and
biodiversity (12).

High carbon ash generated by some industries located close to these

communities could be supplied with this feedstock to be used as heating fuel. For example, the
City o f Prince George, British Coumbia could benefit from the large volumes o f high carbon ash
produced by Canfor Pulp Limited Partnership.
High carbon ash and ash with no organic carbon presents positive environmental, social
and economic implications. The research has uniquely characterized ash of varying physical and
chemical compositions and has shown viable applications for many of these ash types. High
carbon ash can be a viable energy source, while acting as a diluent for specific metals present in
ash. The fuel possesses the ability to be gasified providing additional energy in the form of
producer gases. Ash is also an effective catalyst in biomass gasification processes. Fractionation
is a scientifically sound method and could also be cost effective in obtaining specific chemical
and physical properties of ash, possessing useful environmental benefits.
While this research has provided valuable options for forest biomass “waste”, there is still
room for additional research on specific findings emerging from the study. Very little research
has been done on the possibilities o f returning high carbon ash to energy production systems,
therefore, additional work should be done on optimizing the processes in cases where a high
carbon ash is being produced.

Through optimization and improved gasification process

efficiencies a gas of higher calorific value could be realized. The benefits of applying varying
ash types to different soils is still an area requiring further knowledge.

132

If British Columbia is to produce firm and sustainable energy, as well as ensuring
environment sustainability, technologies should be efficient in extracting as much energy from
our bioenergy processes as possible. Viable alternatives for obtaining the additional left over
energy should also be developed for those processes that are inefficient. In addition, finding all
possible ways to reuse waste in an environmentally safe manner must be the focus o f
development. While it may be unrealistic to expect forest biomass to replace half of the Province’s
current annual consumption o f fossil fuels, there is potential for products o f this resource to be
utilized sustainably. High carbon ash could have potential end uses as a fuel and soil diluent,
while ash may be used as a fertilizer, liming agent and as a catalyst, in all cases promoting the
common goal of sustainable development.

133

References

1. Envint Consulting. An information guide on pursuing biomass energy opportunities and
technology in British Columbia [Online] March 2011. [Cited: June 3,2013.]
http://www.bcbioenergy.eom/wp-content/uploads/2011/05/Bioenergv-Guide-2010-finalupdated-Mav-2011.pdf.
2. British Columbia Ministry of Forests, Lands and Natural Resource Operations. Facts about
B.C.'s mountain pine beetle. [Online] May 2012. [Cited: June 3,2013.]
http://www.for.gov.bc.ca/hfp/mountain pine beetle/Updated-Beetle-Facts Mav2012.pdf.
3. Obemberger, I.; Supancic, K. Possibilities o f ash utilisation from biomass combustion
plants. Proceedings of the 17th European Biomass conference & Exhibition. Hamburg : ETA
Renewable Energies, June/July 2009.
4. Liao, C.; Chuangzhi, W.; Yanyongjie, Haitao, H. Chemical elemental characteristics of
biomass fuels in China. Biomass and Bioenergy. 2004, Vol. 27, 119-130.
5. Natural Resoucre Canada. Comprehensive energy use database table. [Online] [Cited: June 3,
2013.]
http^/oee.mcan.gc.ca/corporate/statistics/neud/dpa/showTable.cfm^type^CP&sector^res&iu
ris=bc&m=5«fepage=4&CFlD=30408844&CFTQKEN-efe64231a5887a09-0BA67FDB9A2 2-A467-EC 86BFFE AE4E40D7.
6. Gabra, M.; Pettersson, E.; Backman, R.; Kjellstrom, B. Evaluation o f cyclone gasifier
performance for gasification of sugar canre resifue - Part 1: gasification of bagasse. Biomass
and Bioenergy. 2001, Vol. 21, 351-369.
7. Sarenbo, S. Wood ash dilemma - reduced quality due to poor combustion performance.
Biomass and Bioenergy. 2009, Vol. 33, 1212-1220.
8. Knapp, B.; Insam, H. Recycling o f biomass ashes: current technologies and future research
needs, [book auth.] Insam H. Knapp B. Recycling o f Biomss Ashes. New York : Springer,
2011 .

9. BC Ministry of Environment. Land application guidelines for the organic matter recycling
regulation and the soil amendment code o f practice. New Westminister, BC : SYLVIS
Environmental, 2008. Report No. 758-08.
10. Obemberger, I.; Supancic, K. Possibilities of ash utilisation from biomass combustion
plants. Proceedings of the 17th European Biomass conference & Exhibition. Hamburg : ETA
Renewable Energies, June/July 2009.

134

11 Nurmesniemi, H.; Manskinen, K.; Poykio, R.; Dahl, O. Forest fertilizer properties o f the
bottom ash and fly ash from llarge-sized (115 MW) industrial powerplant incinerating woodbased biomass residues. Journal o f the Univeristy o f Chemical Technology and Metallurgy.
47,2012, Vol. 1,43-52.
12. OECD/IEA. Bioenergy project development and biomass supply. [Online] [Cited: August
11,2011.] http.7/www.iea.org/textbase/nppdf/free/2007/biomass.pdf.

135

APPENDICES

136

APPENDIX A

Moisture Analysis of Boiler Ash Chapter 2 & 3

Table A1. Moisture content of boiler ash obtained
November 17, 2010 -B 1 and April 2 7 ,2 0 1 2 - B 2.
B2
B1
Sieve
Average
Average
Moisture
Tray/
Moisture
Content
Content
jim
%
%
33.3
8.8
2000
10.0
32.3
850
8.4
27.9
425
35.2
4.7
250
3.8
26.0
150
23.9
4.9
<150
8.8
6.9
AR

137

APPENDIX B

Higher Heating Value Correlation Chapter 2

y = 0 .2 9 7 6 x + 3.2687
R2 = 0.9694

HHV (M J/k g )

25
20
15
10
5
0

0

20

40

60

% Fixed C a rb o n - B1 a n d B2
Figure B 1. Higher heating value as a function o f particle size (pm) for boiler ash.

Gasification Setup

APPENDIX C

Fatjcle Filter

Heatms Tape

Bubbling
Fluidized
Bed

B ionw ss
I ta p p e r

r

vent

Flow Meter
Water Cooled
Screw Feeder

0

Condensate
Collector

—

Micro-GC

I
R9

Rotameters
R1

R8

A

AA

Air

N;

CO-

Pump

Figure C l. Schematic diagram of biomass air and air-steam gasification in a bubbling fluidized
bed. T - Thermocouple P- Pressure sensor R- Rotameter

139

APPENDIX D

Experimental Procedure for Gasification (Chapter 4)

Before heating up
•

Set up micro-GC and connect the gas line.

•

Turn on the micro-GC computer and open the controlling software (Galaxy)

•

Set up the micro-GC acquisition sequence

•

Open N 2 valves o f pressure regulator on gas cylinder

•

Open air valve on the wall and set pressure to 100 psi

•

Open cooling water valve to reading o f 30

•

Open 3 way valve after condenser to flow meter

•

Turn on computer for recording
o

•

Open recording valve software (PDAVIEW) and start recording

Open N 2 valves

Water Pump
•

Set water pump to desired stroke (steam gasification)

After Burner
•

Turn on after burner and set final temperature to 650°C

•

Open air valve for after burner

Heating up reactor
•

Set final temperatures o f top and bottom furnaces to gasification temperatures.

Feeding Fuel
•

Start micro-GC acquisition sequence

140

•

Turn on water pump (steam gasification)

•

Open N 2 and air valve for the reactor and hopper to the required flow.

•

Turn on screw feeder

•

Record feed rates of N 2 , air and fuel

•

Adjust flow meter pressure around 3 psi-g by adjusting the valve before the afterburner
to make steady flow to the micro-GC

After Gasification
•

Stop screw feeder and water pump

•

Open air valve to bum left over carbon

Shutdown
•

Turn off micro-GC

•

Keep cooling water on

•

Stop all gas flow and cooling water when all monitoring temperatures reach ambient
temperature

•

Turn off computer

141

APPENDIX E

Sample Spreadsheet of Gasification Parameters (Chapter 4)

Table E l. Sample spreadsheet of gasification parameters for gasifying boiler ash.

g/min
kg/s
g/hr
Screw
Feeder

2.93
4.88E-05
176

Tamb
Tfur

°c
°c

Furnace Pressure

Pfur

K
kPa
kPa

20
700
973
20
121.33

Biomass N2 Purge Flow

Qpurge

mL/min
mA3/s
kg/s

1000
3.24E-05
2.32E-05

Fuel Flow Rate

Ambient Temperature
Furnace Temperature

Gfuel

Gpurge
■AS
Equivalence Ratio
Steam to Biomass Ratio
N2 Fraction

ER
RH 2 O
RN 2

%
kg/kgfuel
kg/kgfuel

150

SF = (Gfuel+0.418)/0.0223

12%
2.20
2.40

Stoichiometric Dry Air
Actual Dry Air To
Gasifier
Stoichiometric Wet Air
Actual Wet Air To
Gasifier

Mda

kg/kgfuel

5.204

11.53*C+34.34(H0/8)+4.34S

Tda
Mwa

kg/kgfuel
kg/kgfuel

0.625
0.633

Mda*EAC
Mda*(l+Ma)

Twa

kg/kgfuel

0.08

Mwa*EAC

Steam

Gsteam

kg/s
g/min
mA3/s
mL/min
strokes/min

1.04E-04
6.24
3.85E-04
6.3
12.8

Qsteam
Qwater

Gfuel *RH20 - Gfuel
*[H20]

Gsteam /pwater
SR = (F - 0.7676)/0.4291

142

kg/s
mA3/s
L/min
Rotometer
kg/s
mA3/s
L/min
Rotometer

3.09E-05
7.83E-05
1.291
22.7
1.17E-04
2.79E-04
5.043
7.9

GFG
QFG

kg/s
mA3/s

2.75E-04
7.75E-04

Qsyn gas
Qtotal gas

mA3/s
mA3/s

1.39E-05
7.89E-04

UFG
Ug

m/s
m/s
m/s

0.173
0.188
-0.015

dp
pbio

pm
m
kg/mA3

400
0.0004
1100

Carbon
Hydrogen
Oxygen
Nitrogen
Sulphur
Ash
Moisture

[C]
[H]
[O]
[N]
[S]
[ASH]
[H20]

%
%
%
%
%
%
%

48.50%
0.90%
33.40%
0.20%
0.00%
17.00%
5.00%
100.0%

Higher Heating Value

HHV

kJ/kg

11600

Fluidizing Air

Fluidizing N2

Total Fluidizing Gas

Syn Gas Flow
Total Gas Flow
Calculated Gas Velocity
Design Gas Velocity
Excess Velocity

Gair
Qair

GN2
QN2

2.82E-01
R = (F+0.0757)/0.0602
Gfuel *RN2

R = (F-1.6736)/0.4251

BIOMASS

Density
Particle Diameter

143

Flow Meter Calibrations for Gasification (Chapter 4)

APPENDIX F

y = 0 .0 2 2 3 x - 0.418
R2 = 0.9872

T3
u.

100

200

300

400

500

600

Screw Feeder
Figure F I. Calibration o f screw feeder for high carbon ash.

144

60

50

40

30

20

10

0
0

10

20

30

40

50

60 70
Stroke/inin

80

90

100 110 120 130

y = 0.4291 x + 0.7676
Figure F2. Calibration o f water pump at 50 % stroke (IWAKI metering pump, adjustable stroke)

145

18
17
16

15
14
13
12
’’e ’ 11

1 10
i-

* 7

£ 6
5
4
3
1
-----

0

0

10

20

30

40

50

60

70 80 90
Reading [mm]

100 110 120 130 140 150

Figure F3. Calibration o f the flow controller for air introduced at the bottom o f the reactor.

146

APPENDIX G

Experimental Data for Gasification (Chapter 4)

Table G1. Gas analysis and operating conditions for air gasification o f high carbon wood ash
Input
Temperature ER
Gas
Avg
Avg
Avg
Avg
flow
gas
Avg
flow
o2
n2
out
CO
c 3h 8
C 0 2 Avg H2
°C
L/min
%
%
%
%
%
L/min
%
A
657 0.12 8.418
1.97
0.89
0.13
89.57
3.44
0.22
15.69
675

0.12

8.418

2.14

1.06

0.13

88.35

6.05

0.11

15.69

698

0.12

8.418

1.09

1.33

0.12

89.07

6.76

0.37

15.69

740

0.12

8.418

0.34

1.41

0.12

89.01

7.78

0.38

15.69

771

0.12

8.418

0.11

1.39

0.12

88.53

8.67

0.38

15.69

775

0.12

8.418

0.11

1.39

0.12

88.53

8.67

0.38

15.69

775

0.16

8.366

0.16

1.41

0.12

86.60

10.59

0.39

15.73

775

0.19

8.643

0.39

1.35

0.12

85.88

11.13

0.39

15.81

775

0.23

8.172

0.59

1.51

0.12

83.85

12.96

0.39

15.91

775 0.25 7.936
Biomass feed rate: 176 g/h;

1.24

1.55

0.11

81.87

14.13

0.40

15.95

B

147

Table G2. Gas analysis for air-steam gasification o f high carbon wood ash
Input gas
Gas flow
flow
Avg H2
Avg CO Avg C 3 H 8 out
S/B
Avg CO 2
AvgN2
L/min
L/min
%
%
%
%
%
0.4
7.152
0.34
0.67
3.93
85.45
8.80
14.337
0.8
7.153
1.99
7.22
80.58
9.19
0.21
15.755
7.154
78.69
0.44
1.3
8.49
2.08
9.30
16.133
1.7
7.155
8.22
79.99
8.09
0.43
16.506
2.49
81.14
0.42
2.2
7.157
4.08
8.50
4.84
15.657
Biomass Feed Rate -176 g/h; ER - 0.13; Temperature - 775°C, Steam/Biomass Ratio (S/B)

148

APPENDIX H

Li
mg/kg

Elemental Analysis (Chapter 4)

Table H I . M etal analysis for boiler ash collected January 10,2013
Al
K
Ca
Mn
Na
Si
Be
Mg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg

Fe
mg/kg

273.9

0.1

3339.5

62648.5

8455.8

301.4

102347.0

269367.2

31016.7

7102.6

Ni
mg/kg

Cu
mg/kg

Zn
mg/kg

As
mg/kg

Se
mg/kg

Mo
mg/kg

Cd
mg/kg

Ba
mg/kg

Hg
mg/kg

Pb
mg/kg

44.2

158.7

451.6

1.0

0.5

6.1

8.7

1558.2

0.1

4.0

149