U N IV E R SIT Y O F N O R T H E R N BR ITISH C O LU M BIA BIO EN ER G Y PLANT:
PE R FO R M A N C E R E V IEW A N D O PPO R TU N ITIE S FO R IM PR O V E M E N T

by

N icholas G raham Finch
B.Sc., U niversity o f B ritish Colum bia, 1996
B.Sc., U niversity o f British Colum bia, 2001

THESIS SU BM ITTED IN PA RTIAL FU LFILLM EN T OF
TH E REQ U IREM EN TS FO R THE DEG REE OF
M A ST ER OF SCIEN CE
IN
N A TU R A L RESO U RCES A N D EN V IRO N M EN TA L STUDIES

U N IV ERSITY OF N O R TH ER N BRITISH CO LU M BIA
M ay 2014

© N ich o las G. Finch, 2014

UMI Number: 1526486

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A bstract
The U niversity o f N orthern British C olum bia com m issioned a biom ass gasifier to generate heat
to offset the use o f natural gas in 2011. At an average boiler output o f 6.9 G J/hr the average
therm al efficiency was determ ined to be 80% (LHV), the flue gas average tem perature was
134°C with an energy content o f 589 M J/hr.
O ptions investigated to im prove the efficiency o f the bioenergy system include: Installing a flue
gas condensing heat exchanger, reducing the flue gas O 2 percentage, pre-drying the fuel,
installing a chiller, and installing a therm al storage tank. The m ost viable opportunity that exists
is to add a flue gas condensing heat exchanger and connect the residences to the hot w ater loop.

Alternative technologies w ere com pared to the bioenergy plant in term s o f greenhouse gas
displacem ent, and the system w ith the greatest potential is a slow pyrolysis system producing
both heat and biochar for use in soils.

ii

Table of Contents
Table o f C ontents.............................................................................................................................................. iii
List o f T a b le s ...................................................................................................................................................... v
List o f F ig u re s.................................................................................................................................................... vi
G lo ssa ry ............................................................................................................................................................viii
Introduction.......................................................................................................................................................... 1
Section 1................................................................................................................................................................4
1.0 D ata A n aly sis............................................................................................................................................... 4
2.0 G asifier P erfo rm an ce ..................................................................................................................................6
3.0 O ptions to Increase Bioenergy Plant U tilizatio n ............................................................................... 10
3.1 Installing a Condensing H eat E x ch an g er.........................................................................................10
3.1.1 Flue G as Energy C o n te n t.................................................................................................................10
3.1.2 Condensing H eat E xchangers..........................................................................................................14
3.1.3 H eat U tilizatio n .................................................................................................................................. 17
3.1.4 Flue G as C ondensate........................................................................................................................ 22
3.2 Reducing the Excess O xygen Content in the Flue G a s ................................................................. 23
3.3 Fuel Pre-treatm ent................................................................................................................................ 25
3.4 Installation o f a C hiller........................................................................................................................ 27
3.5 Installation o f a Therm al Storage T a n k ............................................................................................30
4.0 Greenhouse Gas O ffse ts...................................................................................................................... 31
5.0 R ecom m endations to Im prove the Therm al Efficiency o f the Bioenergy P la n t.........................32
Section 2 ..............................................................................................................................................................34
6.0 Com parison o f U N B C ’s Bioenergy Plant to A lternative T echnologies........................................34
6.1 Biom ass B oilers..................................................................................................................................... 34
6.2 W ood Pellet B o ile rs ............................................................................................................................. 36
6.3 Pyrolysis T ech n o lo g ies...............................................................................

38

6.3.1 B io c h a r.................................................................................................................................................39
6.3.2 A ctivated C arbon............................................................................................................................... 40
6.3.3 Fuel F lex ib ility ...................................................................................................................................40
6.3.4 Carbon Offset P o ten tial....................................................................................................................41
6.3.5 T orrefaction........................................................................................................................................ 42
7.0 A vailable Biom ass Supplies in B C .......................................................................................................43
8.0 Greenhouse Gas Life-Cycle E m issio n s...............................................................................................45
8.1 D isplacem ent Factor C alculation....................................................................................................45

8.2 Carbon N eutrality o f B iom ass............................................................................................................47
8.3 Fossil Fuel Carbon In te n sity ..............................................................................................................48
8.4 Bioenergy E fficien cies........................................................................................................................ 48
8.5 Com parison o f D isplacem ent Factors Including U N B C ’s B ioenergy S ystem s..................... 50
8 .6 Bioenergy Potential in British C o lu m b ia........................................................................................ 54

9.0

C o n clu sio n ............................................................................................................................................... 55

A ppendix A ....................................................................................................................................................... 57
A ppendix B ........................................................................................................................................................81
A ppendix C ........................................................................................................................................................82
A ppendix D ........................................................................................................................................................83
R eferences........................................................................................................................................................110

List of Tables
Table 1.0 - Sum m ary o f A verage Bioenergy System D ata for M arch 2012 to N ovem ber 2 0 1 3 ....5
Table 3.0 - G asifier Perform ance during W inter and Sum m er M onths.............................................. 29
Table 4.0 - Potential Greenhouse Gas Offsets from D isplacing N atural Gas Consum ed in the
Student R esidences.....................................................................................................................32
Table 5.0 - Sum m ary o f Im pacts to Efficiency, Hog Fuel Consum ption and GHG Em issions
for Each Im provem ent O ption................................................................................................ 33
Table 7.0 - Sources o f Biom ass in British Colum bia and Bioenergy Potential as a Percentage
o f Total Fossil Fuel Dem and. (2006 data).............................................................................44
Table 8.0 - Electricity Generation in British C olum bia by Category from 2007 to 2 0 1 0 .............48
Table 8.1 - Electricity Generation Potential o f W ood C hips................................................................49
Table 8.2 - Calculated CO 2 D isplacem ent F actors.................................................................................. 51
Table 8.3 - Total Annual Greenhouse Gas D isplacem ent P otential................................................... 54
Table 8.4 - Greenhouse Gas Offset and Bioenergy Potential in British C olum bia by
A vailable Biom ass Supplies....................................................................................................54
Table A1 - D ata Table for Fig 3 .3 ............................................................................................................... 57
Table A2 - H og Fuel Properties....................................................................................................................57
Table A3 - Com bustion Calculations and Em pirical C orrelations.......................................................61
Table A4 - G asifier M ass & Energy B alances......................................................................................... 67
Table A5 - Com bustion Calculations, G asifier System Energy B alances.........................................70
Table A 6 - Constants for Enthalpy C alculations.....................................................................................71
Table A7 - Com bustion Calculations, Cooled Flue Gas (40°C ).......................................................... 73
Table A 8 - Com bustion Calculations, Cooled Flue Gas (55°C ).......................................................... 74
Table A9 - Antoine Constants for Saturated V apour Pressure and Condensation Tem perature
C alculations................................................................................................................................ 76
Table D1 - Size o f Therm al Storage tank as a Function o f Required Capacity and the
D ifference betw een Supply and Return W ater Tem peratures........................................ 106
Table E l - Potential Increase in Bioenergy System Y ield with W ood Pre-drying........................109

v

List of Figures
Figure 1.0 (A) - UN BC G asifier Process Flow ...........................................................................................2
Figure 1.0 (B) - Options for Increasing the U tilization Rate and Efficiency o f the
UNBC G asifier........................................................................................................................................... 2
Figure 2.0 - Cam pus Heating Supply and'D em and vs. H eating Degree D ays..................................... 6
Figure 2.1 - H eat D uration Curve - Cam pus H eat Supply and Dem and over One Y ear.................. 7
Figure 2.2 - Cam pus H eat D em and for N ovem ber 20th, 2012................................................................. 8
Figure 2.3 - Therm al Efficiency o f the G asifier as D eterm ined by Cum ulative H eat D elivered vs.
Cum ulative H og Fuel D elivered............................................................................................................. 9
Figure 3 . 0 - Seasonal B oiler Flue Gas Tem perature Fluctuation..........................................................11
Figure 3.1 - D istribution o f the Boiler Flue Gas Tem peratures............................................................. 12
Figure 3.2 - D istribution o f B oiler Flue Gas Energy C ontent (Sum o f Latent and Sensible Heat).
12

Figure 3.3 - Relationship betw een Boiler Output, Fuel M oisture and L atent Heat Loss in the Flue
G as................................................................................................................................................................14
Figure 3.4 - D irect and Indirect Flue Gas H eat Exchanger Schem atics.............................................. 15
Figure 3.5 - Target Flue Gas Tem peratures for M axim um Heat E xtraction....................................... 16
Figure 3.6 - Flue Gas H eat Capture as a Function o f Fuel M oisture.................................................... 17
Figure 3.7 —Annual H eating D em and for Each R esidence.................................................................... 18
Figure 3.8 - Flue Gas Latent and Sensible H eat Content vs. Residence H eat D em and................... 20
Figure 3.9 - B ox Diagram Illustrating Possible Upgraded Cam pus H ot W ater H eating L o o p .... 21
Figure 3.10 - R elationship betw een Exhaust Gas Tem perature and Losses w ith D ifferent Excess
A ir R atios................................................................................................................................................... 24
Figure 3.11 - Percent Oxygen in Flue Gas as a Function o f Boiler O utput........................................ 25
Figure 3.12 - Annual H eating and Cooling D em and on Cam pus and H eat Supply from the
G asifier and N atural Gas System ......................................................................................................... 28
Figure 8.0 - R anking o f D isplacem ent Factors for Bioenergy System s..............................................52
Figure A1 - Diagram Illustrating Data Points........................................................................................... 58
Figure A2 - Correlation betw een Heat Output, tonnes o f Biom ass Delivered and A uger
R otations.................................................................................................................................................... 60
Figure A3 - R elationship betw een Heat O utput and Bioenergy System Y ield................................. 6 6
Figure A4 - Correlation betw een Total Heat Captured, Flue Gas Tem perature and U sable Heat.
..................................................................................................................................................................... 79
Figure D1 - D aily Cam pus H eat Dem and over a 1 year Time P e rio d ................................................83
Figure D2 - H eat D uration Curve E x a m p le .............................................................................................. 84
Figure D3 - H eat D uration Curve for 6 M W system and 2.5 Turndow n............................................ 85

Figure D4 - H eat D uration Curve for a 3 M W system and 2.5 T urndow n.........................................85
Figure D5 - D aily Cam pus H eat D em and over a 1 year Tim e Period Show ing V ariability

86

Figure D 6 - H eat D uration Curve for 1 year Tim e Period..................................................................... 87
Figure D7 - H eat D uration Curve for 1 Y ear Tim e Period.....................................................................8 8
Figure D 8 - H ourly Cam pus H eat D em and over a 1 Y ear Time Period ............................................. 89
Figure D9 - H eat Duration Curve for H ourly H eat D em and D a ta .......................................................90
Figure DIO - H eat D uration Curve for 1 Y ear Tim e P eriod.................................................................. 91
Figure D l l - Cam pus H eat D em and for O ctober 25th, 2012.................................................................93
Figure D12 - Cam pus H eat Dem and for O ctober 27th, 2012.................................................................93
Figure D 13 - Cam pus H eat Dem and for O ctober 28th, 2012.................................................................94
Figure D14 - Cam pus H eat D em and for M arch 2nd, 2012..................................................................... 94
Figure D15 - Cam pus H eat Dem and for N ovem ber 10th, 2012............................................................ 95
Figure D16 - Cam pus H eat Dem and for N ovem ber 20th, 2012............................................................ 95
Figure D17 - Cam pus H eat Dem and for M arch 13th, 2013................................................................. 96
Figure D18 - Cam pus H eat Dem and for M arch 14th, 2013................................................................. 96
Figure D19 - Cam pus H eat Dem and for January 11th, 2013................................................................. 97
Figure D20 - Cam pus H eat Dem and for D ecem ber 22nd, 2012............................................................ 97
Figure D21 - Cam pus H eat Dem and for M arch 14th, 2012................................................................. 98
Figure D22 - Cam pus H eat Dem and for M arch 29th, 2012................................................................. 98
Figure D23 - Cam pus H eat D em and for July 7th, 2012.......................................................................... 99
Figure D24 - Cam pus H eat Dem and for February 12th, 2013............................................................... 99
Figure D25 - Cam pus H eat Dem and for D ecem ber 26th, 2012...........................................................100
Figure D26 - H eat D uration Curve with and w ithout Therm al Storage............................................103
Figure D27 - Heat D uration Curve with Thermal Storage and V aried O u tp u t............................... 104
Figure D28 - Im pact o f Therm al Storage Capacity on N atural Gas C onsum ption........................ 105

G lossary
Ash Fusion - In the context o f biom ass, w hen the inorganic constituents reach a high enough
tem perature to m elt and fuse together into a hard rock like m aterial (slag or clinker)
Bioenergy System Yield - The ratio o f useful energy out to the biom ass input
C oefficient o f Perform ance (CO P) - For a chiller, the COP is the ratio o f energy available for
cooling to the energy input ( Q cooiing / Q input)
C ondensate - The liquid produced from the condensable gases w ithin flue gas
D isplacem ent Factor - The C O 2 em issions avoided w ith the replacem ent o f fossil fuels with
bioenergy given in units o f kg CO 2 per tonne o f biom ass
Dry Basis - Referring to units that exclude m oisture. Exam ple: Flue gas flow rate dry basis
excludes the water vapour flow
Firing R ate - In the context o f a biom ass based energy system , it is the rate o f fuel consum ption
Flue Gas - Com bustion exhaust gases (Prim arily C O 2, N 2, and H 2O) released from the smoke
stack o f a com bustion system
G asification - Partial com bustion in an oxygen starved environm ent to generate syngas
com prised o f CO, H 2, and small concentrations o f CH 4, CO 2 and tars
H eating D egree Days (H D D ) - Relative to a reference tem perature, HD D is an indication o f the
heating dem and in a building. The colder the am bient conditions, the higher the HD D
H igher H eating Value (H H V) - The am ount o f heat released from com plete com bustion with
condensation o f the w ater vapour
H og Fuel - sawm ill residuals com prised o f bark, sawdust, shavings and chips
L atent H eat - The am ount o f heat released or absorbed by a substance undergoing a change in
state
L ow er H eating Value (LH V) - The am ount o f heat released from com plete com bustion without
condensation o f the w ater vapour (typical industrial com bustion conditions)
O xidizer - In the context o f gasification, oxygen is the oxidizer, which is the substance required
for a particular m aterial to com bust
Pyrolysis - Therm al decom position o f a m aterial in the absence o f or w ith low concentrations o f
oxygen. Products produced are: charcoal, gas and tars

Sensible H eat - The am ount o f heat required to raise the tem perature o f a substance, but does
not cause a change in state
Syngas - Gaseous m ixture o f CO, H2, and small concentrations o f CH4, CO2 and tars produced
from gasification o f biom ass, coal and from the reform ation o f natural gas
Torrefaction - Low tem perature pyrolysis (generally below 300°C) for the production o f an
energy pellet, w hich is m ore energy dense than traditional wood pellets and has w ater resistant
properties
Turndow n R atio - In reference to a boiler, it is the percent output that the system can be
reduced to w hile still operating. A turndow n ratio o f 2 m eans the boiler can operate at a
m inim um o f 50% o f the rated output. The turndow n ratio o f U N B C ’s gasifier is approxim ately
2.5
W et Basis - R eferring to units that include m oisture. Exam ple: Flue gas flow rate w et basis
includes the w ater vapour flow

ix

Introduction
The use o f bioenergy technologies for heating is becom ing an increasingly popular m eans for
achieving greenhouse gas reductions w hile m aintaining existing heating requirem ents. In B ritish
Colum bia w here public institutions are required to be carbon neutral, larger institutions such as
universities have been converting natural gas heating system s to biom ass based system s.
N um erous technologies exist and the suitability depends on the specific application and the
availability o f fuels. In northern British Colum bia, sawm ill residuals and logging w aste is readily
available and is therefore the fuel o f choice for bioenergy technologies in the region. W ood
waste boilers have been in use for decades at pulp m ills. Recent im provem ents in biom ass
gasification technologies are providing new opportunities due to the ability to produce a clean
fuel capable o f displacing natural gas (British C olum bia Bioenergy N etw ork 2010).

The U niversity o f N orthern British C olum bia in Prince George BC initiated a project in 2008 to
design a bioenergy system to displace 85% o f the cam pus natural gas usage in order to reduce
greenhouse gas em issions. The bioenergy plant w as com m issioned in 2011 and consists o f a
gasifier supplied by N exterra w hich converts sawm ill residuals (hog fuel) into syngas through a
process called gasification (See A ppendix B for a schem atic and A ppendix C for a cam pus m ap).
G asification is generally referred to as incom plete com bustion or partial oxidization w here
biom ass is converted to syngas in a controlled environm ent w ith lim ited oxygen. This produces a
gaseous m ixture com posed o f H 2, CO, CO 2 and CH 4 (W ang et al. 2008).

H eating to the m ain cam pus buildings is provided by a hot w ater loop with the N exterra gasifier
providing the m ajority o f the heat and four natural gas fired boilers providing the back-up.

1

1. Fuel Infeed
2. G asifier
3. Oxidizer
4. Boiler
5. Electrostatic Precipitator (ESP)
F igu re 1.0 (A ) U N B C G a sifier P rocess F low D iagram (S ou rce: N exterra W eb site)

A. Fuel pre-treatm ent (drying)
B. Oxygen addition control for m ore efficient com bustion
C. Flue gas heat exchanger to recover waste heat
D. Therm al storage on hot w ater loop to campus
E. Adsorption chiller (not shown)
F igure 1.0 (B ) O p tio n s fo r In creasin g th e U tilization R ate and E fficien cy o f th e U N B C G a sifier (S ou rce:
N exterra W eb site)

2

D uring peak heating dem ands in the winter, both the gasifier and the natural gas boilers operate
in order to provide enough heat output. H eating to the residences is provided by a separate
natural gas air handler and electric baseboards. The N orthern Sports Centre also has a stand­
alone heating system and is not connected to the cam pus hot w ater (See A ppendix C for cam pus
map).
In the U N BC bioenergy plant, the syngas is oxidized to generate heat for the cam pus w ater loop
via a heat exchanger. The syngas displaces natural gas w hich rem ains as the supplem ental and
backup fuel source for the campus. The rated capacity o f the N exterra gasifier is 4.4 M W o f
therm al energy and consum es betw een 500-1000 kg o f hog fuel per hour depending on the firing
rate. Typical gasifier efficiencies range from 70 to 95% (hot gas efficiency) depending on the
gasifier design (Q uaak et al. 1999), w ith the m ajority o f the losses in the flue gas. O ptions for
increasing the U N BC bioenergy plant efficiency and utilization rate are outlined in Figure
1.0(B).

There are a num ber o f technologies available for capturing residual heat in flue gas from
industrial heating system s. The m ost com m on technology is a condensing heat exchanger which
could be added to the flue gas stream in order to extract latent heat for use in an expanded hot
water loop (M arbe et al. 2004). The student residences are currently heated with a com bination
o f natural gas and electricity but there is the potential for connecting them to a new hot w ater
loop from the bioenergy plant. In order to m axim ize the therm al efficiency o f the condensing
heat exchanger, the return w ater tem perature needs to be below the condensation tem perature in
the flue gas heat exchanger. To reduce the w ater tem perature after the w ater exits the heating
loop in the residences, two options are discussed: One is to install a greenhouse which could then

3

be connected to the hot w ater loop; the second is to use the hot w ater to pre-heat the air for the
oxidizer.

An additional system w hich could be added to the bioenergy plant is a therm al storage tank
(V erda and Colella 2011). This tank w ould be installed in the m ain hot w ater loop betw een the
bioenergy plant and the cam pus buildings. Therm al storage tanks are a com m on m ethod to store
heat energy from a bioenergy plant so that the heat supply to the end users can be evened out and
the output o f the bioenergy system can rem ain at a steady rate.

D uring the sum m er m onths, when the heating dem ands are at their lowest, there is an
opportunity to install an adsorption chiller in the hot w ater loop (M araver et al. 2013). This
would enable the gasifier to rem ain at full output w here the efficiency is greatest, and supply air
conditioning to the cam pus buildings. This provides an option for m aintaining the efficiency at
peak levels w ithout the need to reduce the gasifier output due to seasonal demands.

The objectives o f this paper are to review data from the UNBC N exterra gasifier (bioenergy
plant), and the cam pus utilities in order to determ ine the therm al efficiency o f the system and
discuss opportunities for improvem ent. A detailed analysis o f the gasifier perform ance will be
carried out and a com parison will be m ade betw een the actual greenhouse gas savings and w hat
the potential savings w ould be using alternative biom ass conversion technologies. This will
gauge the success o f U N B C ’s overall objective o f reducing greenhouse gases.

Section 1

1.0 D ata Analysis
D ata from the gasifier control system for the tim e period M arch 2012 to N ovem ber 2013 w as
obtained and analyzed to review the perform ance and efficiency. D ata was available for 15
4

m inute intervals for hog fuel feed rate, air flow to the oxidizer, total air flow, tem peratures at
num erous points, and gasifier energy output to the campus. The m ain param eters calculated from
the control system data w hich were used in the analysis are provided in Table 1.0.
T ab le 1.0 - S u m m ary o f A v era g e B ioen ergy S ystem D ata for M arch 2012 to
N o v em b er 2013.

B oiler Flue Gas Tem p(°C )
B oiler Output (GJ/hr)
Hog Fuel Feed, Dry Basis (kg/hr)
Hog Fuel Feed, Wet Basis (kg/hr)
Hog Fuel Feed, Dry Basis (GJ/hr)
Hog Fuel Feed, Wet Basis (GJ/hr)
Hog Fuel Energy Density (MJ/kg)
Hog Fuel Moisture Content (%)
Flue Gas Flow, W et Basis (kg/hr)
Flue Gas Flow, Dry Basis (kg/hr)
Flue Gas Water Flow (kg/hr)
Flue Gas (kJ/hr)
Thermal Efficiency (HHV)

Average
134
6.9
495
693
7.9
4.7
16
40%
671
440
231
588,512
70%

D ata from the cam pus utilities was also obtained for the same tim e period which includes
dow nloads from m eters that record building heating and cooling dem ands. U sing a Pivot Table
in M icrosoft Excel 2010 (M icrosoft, R edm ond W ashington), the data was converted to daily
values and sorted into heating degree days (HDD) using a reference tem perature o f 15.5 °C.

H eating degree days are calculated using the follow ing form ula:

HDD= >
(15.5°C - T)
■£—'t=o

(1)

W here T = outside tem perature in °C and t = tim e in hours
If T > 15.5 then HDD = 0 for that interval

5

2.0 G asifier Perform ance
The heat delivered to the entire cam pus including the residences using heat supplied by both
natural gas and the gasifier is illustrated in Figure 2.0. The solid triangle data points represent the
gasifier output and the solid circles represent the com bined gasifier and natural gas supply. It can
be seen in Figure 2.0 that for lower heating degree days, the gasifier is able to supply m ost o f the
heat dem and for the cam pus. A bove a H D D o f approxim ately 20, the gasifier is not able to
supply all the heat to the cam pus so natural gas is used to m ake up the difference.

450

❖ Heat Utilized on Campus
400

* Total Heat Delivered to Campus
A Heat Delivered by Gasifier

350

1-9

o
■a

300

2
| 250
"3

o

O

-a

200

P 150
a
<u
H
100

0

5

10

15

20

25

30

35

Heating Degree Days (HDD)

F igu re 2.0 - C am p u s H eatin g Supply and D em an d vs. H ea tin g D egree D ays.

The overall heating supply and dem and is illustrated in Figure 2.1. An explanation on the style o f
heat duration curves and how to interpret the data is provided in A ppendix D. The closer the

sm oothed average gasifier output line is to the heat supply line, the better the gasifier is m atched
to the cam pus heating requirem ents. Since the gasifier was not sized for peak dem and during the
coldest w inter days, natural gas is used to supplem ent the gasifier output. Due to the low
turndow n ratio o f the gasifier, it w ould not have been cost effective to size the output to m atch
peak demand. Had it been sized to m atch the peak w inter heating dem and, the gasifier w ould not
have been able to reduce the output enough during sum m er m onths. In this case the gasifier
w ould have had to have been shut down entirely or operate with a larger percentage o f the output
exhausted.
6

5

£ 4
T3

S

«
E
<u
P

p — G asifier O utput

’Q 3
s

■z— R esidence #1 H eat D em and

— H eat Supply from G asifier & NG

— R esidence #2 H eat D em and

"EL
a

Sm oothed A verage G asifier O utput

1

_

0
0

50

100

/V'flf'YUflflA

150

200

JAjtAwA. — I

250

A

A, ■ A frA.

300

350

400

Tim e (D ays)
F igure 2.1 - H eat D uration C u rve - C am p u s H eat S u p p ly and D em and over O n e Y ear.

7

For these reasons, the gasifier was sized to displace 85% o f the cam pus’ natural gas usage. A s an
approxim ation, Figures 2.0 and 2.1 appear to illustrate this sizing so the data looks like it
m atches the original design criteria o f the bioenergy plant. The variability in the gasifier output
illustrated in Figure 2.1 is shown in m ore detail in Figure 2.2 where one 24 hour period was
focused on. The pattern o f spikes in the natural gas boiler and drops in the gasifier output
illustrated in Figure 2.2 is discussed in m ore detail in A ppendix D. Possible explanations for this
variation include: D ifferences in daytim e and nighttim e heating dem and, system dow ntim e,
bioenergy system operating below capacity, and control system issues. There appears to be a
pattern indicating a possible control or operations issue around balancing the gasifier output and
the natural gas boiler output. See A ppendix D for m ore details into the variability o f the
bioenergy plant output. G oing into detail about these possibilities was outside the scope o f this
study but is a recom m ended area for further research.

Time o f Day (hours)

F igu re 2.2 - C am p u s h eat d em an d fo r N ovem b er 2 0 th, 2012. A rea b elow the solid lin e is h ea t su p p lied by the
b ioen ergy system , area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas.

8

The rated capacity o f the gasifier boiler is 4.4 M W , but from the data presented in Figure 2.1, the
peak output is approxim ately 3.5 M W . The reason for this is intentional as the cam pus engineers
prefer to operate the boilers below 90% o f capacity to be able to better respond to normal
fluctuations in operation (Personal com m unication with Kevin Ericsson, UNBC C h ief Engineer,
M arch 2014).

A m ore detailed exam ination o f the gasifier efficiency was carried out by looking at the
relationship betw een the cum ulative hog fuel deliveries and the cum ulative heat delivered to the
cam pus. This relationship is illustrated in Figure 2.3 w here the upper line represents the data in
term s o f low er heating value (LFTV) and the lower line representing the data in term s o f higher
heating value (HHV). The efficiency is represented by the slope o f each line w here on a LHV
basis it is 80% and on a H H V basis it is 70%.

140

p 120

a.
c3 100
• vs HHV
• vs LHV
80

0

20

40

60

80

100

120

140

160

180

Cumulative Hog Fuel Delivered (TJ)
F igu re 2.3 - T h erm al E fficien cy o f the G asifier as D eterm in ed by C u m u la tiv e H ea t D elivered vs. C u m u lative
H og F u el D elivered .

9

In sum m ary, the gasifier appears to be sized correctly for the heat dem and profile. Specific issues
are: Low efficiency and variable output. The system currently covers approxim ately 85% o f the
heat load, but should be able to cover well over 90% given the rated capacity o f 4.4 M W (refer to
A ppendix D for details).

3.0 O ptions to Increase Bioenergy Plant Utilization
In this section, the follow ing system s or additions to the bioenergy plant are review ed in order to
determ ine the m ost viable opportunity to increase the efficiency o f the gasifier: Installing a
condensing heat exchanger, reducing excess oxygen in the flue gas, pre-treating the fuel,
installing a chiller, and installing a therm al storage tank.

3.1 Installing a C ondensing H eat Exchanger
The largest opportunity for im provem ent o f the U N BC bioenergy system efficiency is to extract
residual heat in the flue gas which accounts for the m ajority o f the losses in the system.

3.1.1 Flue Gas Energy Content
The flue gas has an average tem perature o f 134°C (Table 1.0) but varies seasonally w ith the
gasifier output as illustrated in Figure 3.0. The high variability illustrated can be explained with
the sam e reasoning as outlined in section 2.0 w here the gasifier output in Figure 2.1 was
described.

10

300

F all / W inter

Spring / S u m m er

Spring / Su m m er
g

150

3

100

w

50

^

^

^

^

~a<'

< ^'

^

^

O5

F igu re 3 .0 - S ea so n a l B oiler F lu e G as T em p eratu re F lu ctu ation .

A lthough there are a few gaps in the data, a general trend can be seen w here the tem perature
drops heading into the sum m er m onths w hen the gasifier is turned down. D uring w inter m onths
the gasifier is operating at a high rate and the flue gas tem perature increases.
A fter grouping the data into 20 degree increm ents (where 60 represents data from 0 to 60 and 80
represents data from 61 to 80 etc.), the m ajority o f the tem perature m easurem ents fall w ithin a
range o f approxim ately 120°C to 180 °C as illustrated in Figure 3.1.

11

50
45
A 35
A
A 30
o 25
a 20
o
•-C 15
u
2 io

I
60

80

100 120 140 160 180 200
Flue Gas Temperature (°C)

220

240

F igu re 3.1 - D istrib u tion o f th e B oiler F lu e G as T em p eratu res.

Using the latent heat o f condensation for w ater o f 2260 kJ/kg water, the average flue gas energy
content is 589 M J/hr (Table 1.0). The distribution o f the flue gas energy content is illustrated in
Figure 3.2. A pproxim ately 80% o f the data fell betw een 250 M J/hr and 750 M J/hr.

The flue gas energy content was determ ined by calculating the m ass flow o f w ater in the flue gas
from the calculated flue gas m ass flow and the m easured hum idity. M ultiplying the m ass flow o f
w ater by the latent heat o f condensation equals the energy content o f w ater vapour w ithin the
flue gas.

/—
"s 16
S
®
0s 14
C
3 12
« 10
A
«*-< 8
o
s

.©
<j
et
u

6
4
2
0
S'

cS'

cS'

iS ’ cS'

(S '

A*

Flue Gas (kJ/hr)
F ig u re 3.2 - D istrib u tion o f B oiler F lu e G as E nergy C o n ten t (Sum o f L aten t an d S en sib le H eat).

12

In order to improve the overall efficiency o f the gasifier, a greater percentage o f the energy
rem aining in the flue gas w ould have to be utilized. One opportunity to im prove the efficiency is
to install a flue gas condensing heat exchanger and connect new end users to a new heating loop.
This is a technology gaining attention throughout industry to extract both flue gas heat and w ater
from energy system s (Chen et al. 2012, Kilkovsky et al. 2014). The gasifier’s flue gas energy
content varies depending on the perform ance o f the gasifier and the hog fuel m oisture content.
As the m oisture content o f the fuel increases, the percentage o f latent heat increases relative to
the sensible heat (Sw ithenbank et al. 2011). In order to design a system to handle the variations
in biom ass fuel m oisture contents, the flue gas heat exchanger needs to cool the gas (sensible
heat transfer) and condense the water vapor (latent heat transfer) (Levy et al. 2011). A new hot
w ater loop w ould be required as the existing cam pus w ater loop is a high tem perature and
pressure system . The heat extracted from a flue gas heat exchanger w ould be a lower tem perature
system and not necessarily pressurized. This will be discussed further in subsequent sections.

The output o f the gasifier varies depending on a num ber o f factors including m aintenance and
operation issues. The variable with the largest im pact to the heat available in the flue gas is the
hog fuel m oisture content. In order to better understand the relationship, a com parison between
boiler output, fuel m oisture content and latent heat loss was carried out. This is illustrated in
Figure 3.3 w here it can be seen that other than low boiler output, the latent heat loss at any given
fuel m oisture rem ains relatively constant (see A ppendix A for data table and sam ple calculation).
But as the m oisture content o f the hog fuel increases, the latent heat loss percentage increases
significantly. The sizing o f the condensing heat exchanger w ould have to take this into account
so as to extract the m axim um heat available in the flue gas stream.

13

B o iler O utput (G J/hr)

F igu re 3.3 - R elation sh ip b etw een B oiler O u tp u t, F u el M oistu re an d L aten t H ea t L oss in the F lu e G a s. T he
typ ical b oiler o u tp u t ran ges from 4 to 11 G J/h r

3.1.2 C ondensing H eat Exchangers
In a condensing heat exchanger, a w ater loop passes through the heat exchanger, extracts heat
from the flue gas and can be used for heating (see Figure 3.4). The flue gas w ould then exit the
bioenergy plant at a m uch lower tem perature.

Flue gas heat exchangers can be o f tw o basic designs - direct or indirect. In a direct heat
exchanger, a w ater loop enters the hot flue gas stream w here heat is transferred to the w ater in
the pipe. In an indirect heat exchanger, cold w ater is sprayed into the hot flue gas and is collected
and pum ped back around through a pipe which is then used to heat another w ater loop (Chen et
al. 2 0 1 2 ).

14

Cool flue gas

Cooling
water
Cooling
water

Hot
water

Condensate & sludge

Direct Heat Exchanger

Hot
water

Condensate & sludge

Indirect H eat Exchanger

F igu re 3.4 - D irect an d In d irect F lu e G as H eat E x ch an ger S ch em a tics. H eat from th e flue g a s is tran sferred
to a w ater loop fo r h eatin g.

In order to m axim ize the heat extracted from the flue gas, there needs to be a properly sized heat
exchanger, an end user o f the heat, and a cool enough return tem perature back to the heat
exchanger. The relationship betw een the fuel m oisture content and the required final flue gas
tem perature in order to get the m axim um heat capture is illustrated in Figure 3.5 (see A ppendix
A, equations 54 to 58 for sam ple calculations). W ith the heat capture m axim ized, the overall
system efficiency is increased. A t approxim ately 3.5 M W boiler output, the flue gas would need
to be cooled to 40°C to extract the m axim um am ount o f the latent heat available assum ing a fuel
m oisture content o f 40% . The m ore w ater in the fuel, the more w ater there will be in the flue gas
and therefore a higher condensation tem perature will be able to extract the latent heat from the
flue gas.

15

♦30% moisture

140% moisture

A,50% moisture

50
45
■§ s i 40

20
0

1

2

3

4

B oiler output (M W )

F igu re 3 .5 - T arget F lu e G as T em p eratu res for M axim u m H eat E xtraction as a F u n ction o f B oiler
O u tp u t and F u el M oistu re.

The am ount o f work the heat exchanger w ould be required to do in order to m axim ize the heat
capture is illustrated in Figure 3.6 (see A ppendix A, equations 54 to 58 for sam ple calculations).
For exam ple, at a boiler output o f 3.5 M W and fuel m oisture o f 50% , the heat exchanger w ould
have to be sized for a 600 kW duty. The im pact boiler output has on flue gas heat capture is
illustrated in Figure 3.6. A t a 30% fuel m oisture content, increasing the boiler output from 2 to 3
M W increases the flue gas heat capture from 200 kW to 300 kW.

16

•30% moisture

«S»40% moisture

“ fip»50% moisture

700

s 500
« 400
*

300

C3

200

100
X
et

0
0

1

1

2

2

3

3

4

4

B oiler output (M W )

F igu re 3 .6 - F lu e G as H eat C ap tu re as a F u n ction o f Fuel M oistu re and B oiler O u tp u t.

3.1.3 H eat Utilization
There are a num ber o f opportunities for utilizing the additional heat available in a new hot w ater
loop such as connecting the student residences, the Enhanced Forestry Lab or the N orthern
Sports Centre. The Enhanced Forestry Lab and the student residences are relatively close to the
Bioenergy Building. The Enhanced Forestry Lab is already heated with biom ass (a pellet
system ). Therefore the buildings that m ake the m ost practical sense to connect to the heating
system are the residences. Currently the residences are heated by a com bination o f electric
baseboards and a natural gas air handler. The new heating loop w ould displace the existing
natural gas system which heats the hallways and the potable hot water.

The average heating dem and for each residence is 130 kW (obtained from the natural gas m eters)
w ith a distribution as illustrated in Figure 3.7. Each residence has a sim ilar heating dem and
profile peaking at approxim ately 250 kW .

17

01
s*W
V]
pj

100
50

0
1000

2000

3000

4000

5000

6000

7000

8000

6000

7000

8000

T im e (H ou rs)

1000

2000

3000

4000

5000

T im e (H ou rs)

F igu re 3.7 - A n n u al H ea tin g D em an d fo r E ach R esid en ce. D ata sorted based on total cam p u s h ea t d em an d .

Com paring this to the average flue gas energy content o f 589 M J/hr or 164 kW from Table 1.0,
there is enough energy rem aining in the flue gas to m eet the average heating dem and o f one o f
the residences. A com parison o f the daily heating dem and and the heat available in the flue gas is
illustrated in Figure 3.8. The distribution o f heat dem and for the residences ranges from 4 to 16
G J/day (50 to 200 kW ) with 35% o f the days having a heat dem and o f 12 GJ/day. The
distribution o f heat available from the flue gas ranges from 10 to 22 G J/day (120 to 250 kW )
with approxim ately 50% o f the days w ith 16 to 18 G J/day available. Com paring Figure 3.7 to
Figure D 9 in the A ppendix, the residence heat dem and m atches the gasifier output. How ever, it
is not practical to extract 100% o f the energy in the flue gas. To m axim ize the energy extraction,

the flue gas needs to be cooled as m uch as possible. The degree to w hich the flue gas can be
cooled depends on the return w ater tem perature in the heat exchanger. Therefore the lower the
w ater tem perature returning to the flue gas heat exchanger, a greater percentage o f energy can be
extracted from the flue gas (see A ppendix A, Figure A 4 for details).

D epending on the exit tem perature o f the w ater loop for the residence, it is possible that further
heat will need to be extracted in order to have a cool enough return tem perature to the bioenergy
plant. For this exam ple, an estim ated exit tem perature from the residence o f 55°C has been used.
To further lower the w ater tem perature, a few options have been illustrated in Figure 3.9. One
option is to install a greenhouse betw een the residences and the bioenergy plant w hich w ould use
the residual heat in the w ater loop. A second option could be to use the 55°C w ater for
preheating the air for the oxidizer. The com bustion calculations for the flue gas w hen cooled to
40°C using fuel w ith a m oisture content o f 40% are provided in Table A7. A t full gasifier output
the percentage o f flue gas heat captured is 74% but when flue gas is only cooled to 55°C, the
heat captured drops to 44% as illustrated in Table A8 (also using fuel at 40% m oisture).

19

■ Flue G as

2

4

6

8

H R esidence #1

10

12

14

* Residence #2

16

18

20

22

24

26

28

30

Heat Content of Flue Gas or Heat Demand of Residences
(GJ/Day)
F igu re 3 .8 - F lu e G as L aten t an d S en sib le H eat C on ten t vs. R esid en ce H ea t D em an d .

The box diagram (Figure 3.9) is for illustration purposes and shows a possible upgraded heating
loop w hich would increase the overall bioenergy plant efficiency. The com ponents o f the box
diagram will be discussed in subsequent sections (thick arrows represent w ater loops and the
thinner arrows represent flue gas flow). The follow ing discussion is based on dum ping heat from
the heating loop (to a greenhouse or cooling tow er) to achieve a final flue gas tem perature o f
40°C.

20

F igu re 3.9 - B ox D iagram Illu stra tin g P ossible U p grad ed C am p u s H ot W a ter H eatin g L oop. T he th ick a rrow s
rep resen t w a ter lines and the thin arrow s rep resen t flue gas.

The installation o f a greenhouse to utilize residual heat from a bioenergy fueled residence
heating loop has been studied as part o f the U niversity’s goal o f expanding biom ass heating to
parts o f the cam pus that are currently heated w ith natural gas. R enner (2011) notes that on
average 4,000 G J/yr o f heat is required per hectare o f greenhouse. Based on this rule o f thum b,
the potential size o f an additional greenhouse is 0.32 hectare based on the bioenergy p lan t’s hog
fuel m oisture content o f 30%, the final flue gas tem perature o f 40°C and a boiler output o f 3.5
M W . As this analysis is based on averages, during the coldest m onths w hen the heating demand
o f the cam pus buildings and residences are at the highest, there would be m ore residual heat
available for a greenhouse. This is illustrated in Figure 3.6 w here the m axim um flue gas heat
capture increases as the boiler output increases.

21

A condensing heat exchanger can improve the therm al efficiency o f a heating system by up to
20% (N euenschw ander 2011). The efficiency im provem ent w hen the flue gas heat is captured
based on two scenarios is shown in A ppendix A. The first scenario is 40% fuel m oisture content
with a final flue gas tem perature o f 40°C (Table A7), and the second scenario is 40% fuel
m oisture content with a final flue gas tem perature o f 55°C (Table A8). W ith the lower flue gas
tem perature, the efficiency im provem ent varies from 16.0% at a low boiler output to 18.5% at a
high boiler output. A t the higher flue gas tem perature, the efficiency im provem ent varies from
8.5% at a low boiler output to 11.6% at a high boiler output. This m eans that the gasifier
efficiency could increase from 80% (LHV) to betw een 87% and 95% . The condensing heat
exchanger w ould then feed a new w ater loop connected to the residences and potentially a
greenhouse operation as illustrated in Figure 3.9.

3.1.4 Flue Gas C ondensate
Typically a boiler will produce 150 to 500 litres o f condensate and 0.01 to 0.3 kg (dry basis) o f
sludge per M W h o f therm al energy produced (Loo and K oppejan 2008). From Table 1.0, the
bioenergy plant’s average boiler output is 6.9 G J/h or 1.9 M W h/h o f therm al energy. Therefore
the estim ated condensate produced w ould be 300 to 1000 litres per hour and the sludge produced
w ould be 0.02 to 0.6 kg/hour. These estim ates agree with the calculated values outlined in Table
A4 in A ppendix A, w here the total H 2O out ranges from 335 to 1094 kg/hour (based on 40% fuel
m oisture).

Condensates from the flue gas o f biom ass based energy system s typically contain a m ixture o f
organics, inorganics and som e heavy m etals. M ost o f the heavy m etals are precipitated out in the
condensate sludge and can pose environm ental concern (Loo and Koppejan 2008). In
com parison, the condensate from the flue gas o f coal fired pow er plants contain a m uch higher
22

percentage o f harm ful chem icals such as sulfuric acid and m ercury resulting in the need for
expensive treatm ent system s (Levy et al. 2011). Tests could be com pleted to determ ine the
com position by taking a sam ple o f the flue gas and analyzing the constituents in order to verify
the chem icals that m ay be o f concern. Condensate sludge can be separated from the condensate
with a sedim entation tank, a wood filter, or a belt filter (Loo and Koppejan 2008).

The concentrations o f the com pounds in the condensate will vary depending on the m oisture
content o f the hog fuel. Previous research has dem onstrated that dry fuels have low er em issions
o f CO, hydrocarbons and polycyclic arom atic hydrocarbons (Atkins et al. 2010). Pre-drying the
hog fuel will be discussed in a subsequent section.

3.2 R educing the Excess O xygen C ontent in the Flue Gas
Reducing the excess oxygen in the flue gas is an effective m eans to increase the overall
efficiency in a biom ass com bustion plant (Loo and K oppejan 2008). The air required for
com plete fuel com bustion is referred to as the stoichiom etric air requirem ents. For exam ple, if
the form ula for biom ass is CHj 5 Oo.75 the stoichiom etry is as follows:

CH,.5 Oo75 + 0 2 -» C 0 2 + 0.75H2O

(2)

Based on this equation, 1 tonne o f biom ass w ould require 1.3 tonnes o f 0 2 for com plete
com bustion. To ensure com plete com bustion, excess air is norm ally added (Bain et al. 1998, Yin
et al. 2008). The higher the excess air the higher the flue gas flow and therefore m ore heat loss
w ith the flue gas (Figure 3.10).

23

20
100 °c

18

120 °C -tip-134 °C -# -1 8 0 °C

« 14
o 12

M

S 10
«

8

3
«

6

■a 4
W

2
0
1

1.5

2
Excess air ratio

2.5

3

F igu re 3 .1 0 - R elation sh ip b etw een E xh au st G as T em p era tu re and L osses w ith D ifferen t E xcess A ir R atios.

The ideal excess air ratio is 1.25 which results in a flue gas O 2 content o f 6% (Yin 2008). A t a
flue gas tem perature o f 180°C and an excess air ratio o f 1.25, the exhaust gas losses are
approxim ately 9% (Figure 3.10). A s the excess air ratio increases, so too does the oxygen content
in the flue gas (not shown). There is no oxygen sensor on the gasifier so a calculation w as m ade
using the hog fuel inputs and com bustion calculations (Table A4 in A ppendix A). The percent
oxygen varies betw een approxim ately 7% at low boiler output and 4.5 % at high boiler output
(Figure 3.11) (see A ppendix A, equation 32 for calculations).

24

8
7

6
.5 s® 5
V “
Dfi ^ 4
et
O 3

2
1
0
0

1

2

3

4

Boiler Output (MW)
F igu re 3.11 - P ercen t O xygen in F lue G as as a F u n ction o f B oiler O u tp u t.

The N exterra gasifier is not equipped w ith an oxygen sensor so it is not known w hat the oxygen
percentage in the flue gas currently is. Based on the com bustion calculations, it appears that the
percentage is already low and N exterra m ay require the oxygen percentage to rem ain fixed in
order for their technology to w ork as designed. A s a result, adjusting the excess oxygen level
may not be a practical option for im proving the system ’s efficiency.

3.3 Fuel Pre-treatm ent
A nother option to extract the residual energy in the flue gas could be to capture the waste heat to
pre-dry the hog fuel. If the hog fuel m oisture content was lowered and becam e m ore uniform as a
result, the overall gasifier efficiency w ould increase therefore increasing the heat output (Li et al.
2012). Low ering the fuel m oisture content from 50% to 30% has the potential to im prove the
therm al efficiency o f a biom ass com bustion plant by 8.7% (Loo and K oppejan 2008). The
increased efficiency is due to the reduced latent and sensible heat loss that occurs with high
m oisture content fuel (based on a system w ithout a flue gas condensing heat exchanger). The

25

sam ple calculations provided in A ppendix E show the increased yield which results from drier
fuel. The lower the fuel m oisture content, the greater the heat output o f the bioenergy system per
tonne o f biom ass. For exam ple, w ith a starting m oisture content o f 40% and a dried m oisture
content o f 25% , there w ould be a 4.2% yield increase (Table E l). This m eans that 4% less hog
fuel w ould be required for the sam e bioenergy plant output.

W ith uniform fuel m oisture content, the gasifier controls w ould not have to adjust for varying
inputs and w ould likely run m ore stable as a result. In term s o f ideal fuel m oisture contents, it
varies by gasifier design but fuel m oisture contents below 15% results in a m ore stable fuel bed
tem perature (Ruiz et al. 2013). In addition, dryer hog fuel w ould also improve the stack
em issions as the concentrations o f arom atic hydrocarbons will decrease as will the volum e o f
condensate (Atkins et al. 2010).

The challenge for this option would be that adding a hog fuel drying stage to the existing
bioenergy plant w ould be costly and not practical given that the existing hog fuel intake m etering
augers are close coupled to the gasifier. In addition, a drying stage in a bioenergy plant can
represent the highest capital cost com ponent (B ram m er 1999). A drying stage w ould need to be
installed betw een the hog fuel feed system and the gasifier, which in the existing design w ould
require costly m odifications. In addition to the high capital cost, dryer fuel w ould increase the
com bustion tem perature o f the fuel. As a result the ash fusion tem perature could be approached
resulting in slag form ation (A m os 1998). Slag (som etim es referred to as clinker) w ould cause
operational problem s if it built up inside the gasifier or the ash rem oval system and caused plug
ups. Due to the high variation in inorganic concentrations in biom ass, testing for the ash fusion
tem perature is the m ost com m on test for determ ining the m elting point o f fuels (V assilev 2014).

26

A series o f these tests w ould have to be com pleted as part o f any considerations o f installing a
biom ass dryer.

3.4 Installation o f a Chiller
A nother option for utilizing the unused capacity in the sum m er is to install a chiller. Chillers are
based on the principle o f a closed loop cooling cycle using either a liquid-vapor phase
(absorption) or a solid-liquid phase (adsorption) (Qian et al. 2013). The m ain differences
betw een adsorption and absorption chillers are the m aterials and substance state. In absorption
chillers a solution or solid absorbs a refrigerant becom ing a uniform solution or body, whereas in
adsorption chillers the refrigerant rem ains on the surface o f a solid adsorber (Pang et al. 2013). In
both cases, the refrigerant is regenerated by heating the absorbant or adsorber.

In term s o f capital cost, adsorption chillers have a higher capital cost but a low er m aintenance
and operating cost. The reason for the high costs o f adsorption chillers is due to their large
volum e and w eight and the fact that the technology is relatively new in com parison to absorption
chillers. The operating and m aintenance costs for absorption chillers are high because they rely
on corrosive salt solutions w hich require regular m aintenance to regenerate the desiccant (EcoM ax w hite paper). In addition, the purchase cost and environm ental costs to store the desiccant is
high m aking the adsorption chiller design a m ore attractive solution in m any cases as it sim ply
uses w ater as the refrigerant.

Currently, the cam pus air conditioning is provided by a cold w ater system powered by
electricity. The relationship betw een the cam pus cooling dem and and the gasifier output is
shown in Figure 3.12. During sum m er m onths w hen the heating dem and is low, the gasifier is
turned down and the air conditioning system is in use. If the gasifier w as to rem ain at full output

27

all year, it m ay be possible for a portion o f the cooling dem and to be m et w ith the addition o f a
chiller. The existing electric chiller system could rem ain as a back-up and once the adsorption
chiller is in place, the operating cost w ould be the incremental hog purchases.

6

5

"5.

a.
0

tzi

c*
s
"O
0
0
-a

4
-— Heat Supply from Gasifier & NG
,— Heat Consumed on Campus

3

0

,— Gasifier Output

1

OJ
a

— Campus Cooling Demand

an 1
0 z

o
o
U
"O
g 1
93

QJ

as

0
0

50

100

150

200

250

300

350

400

Time (Days)
F igu re 3.12 - A n n u al H ea tin g and C o o lin g D em and on C am p u s and H ea t S u p p ly from the G a sifier and
N atu ral G as System .

In order to look m ore closely at the potential for a chiller, the gasifier data was divided into
w inter and sum m er m onths. D uring the colder m onths o f the year, the gasifier is operating at or
near full output and is turned down during the w arm er m onths. This can be seen in Table 3.0
w here the average boiler output is 2,368 kW during the 6 m onths from October through M arch
and 1,527 kW April through September.

28

Table 3.0 - Gasifier Perform ance during W inter and Sum m er M onths.
Averages
Boiler Output (kW)
Hog Fuel (kg/hr dry basis)

Averages
Boiler Output (kW)
Hog Fuel (kg/hr dry basis)

Oct

Nov

Dec

Jan

Feb

Mar

W inter Average

1,935

2,621

2,584

2,348

2,325

2,397

2,368

503

679

669

608

603

621

614

Apr

May

Jun

Jul

Aug

S ep t

Summ er Average

2,182

1,692

1,418

1,250

1,308

1,313

1,527

565

438

367

324

339

347

397

E co-M ax A dsorption chillers distributed through Pow er Partners Inc. are designed for low
tem perature applications and are an exam ple o f a possible system to displace the existing air
conditioning system at UNBC. These particular chillers require a m inim um w ater tem perature o f
90°C and are available in capacities ranging from 35 kW to 1178 kW producing chilled water
betw een 3°C and 20°C (Eco-M ax specification sheet).

The cooling dem and illustrated in Figure 3.12 represents 177 days at an average dem and o f 406
kW and a peak o f 1300 kW . In order for the gasifier to m aintain the output required to m eet the
cooling dem and through a chiller, m ore hog fuel w ould have to be consum ed. Calculating the
required tonnage based on the average consum ption per kW o f output works out to 900 tonnes o f
incremental hog fuel annually (calculation below).
406 kW cooling dem and / 0.5 COP x (397 kg hog fuel / 1,527 kW output) x 24 hrs/day x 177
days/year / 1000 kg/tonne = 897 tonnes o f hog fuel
The coefficient o f perform ance (COP) for a chiller with a low tem perature heat source is
relatively low with a COP o f 0.5 considered relatively good (Garim ella S. 2012, M araver et al.
2013). A ssum ing the available heat input to the chiller to be the difference betw een w inter and
sum m er boiler outputs from Table 3.0 at 841 kW , there would be 420 kW o f available cooling
based on a COP o f 0.5. A chiller could therefore m eet the average cooling dem and for the
cam pus, but not m eet the peak dem and during the w arm est months.

29

3.5 Installation o f a Therm al Storage Tank
The use o f a therm al storage tank will provide a separation betw een the heat being generated in
the energy system and the heat being supplied to the cam pus. Excess heat generated from the
gasifier during periods o f low dem and could be stored until the dem and increases (Eynard et al.
2012). It w ould provide a buffer betw een b rief interruptions in the energy system operation and
the end user, with the benefit being that during b rie f periods o f dow ntim e on either the end user
side or on the fuel supply side, the heat generated w ould not be wasted (Stritih 2004).
Interruptions in the UNBC system occur on the supply side so if a therm al storage tank was in
place, the dem and could still be m et despite short periods o f downtim e at the gasifier. As the
efficiency o f the gasifier/boiler is highest w hen the system is operating at full output, the addition
o f a therm al storage tank will improve the overall therm al efficiency as the gasifier w ould not
have to be turned down during short interruptions in the system. K eeping the gasifier operating at
a steady output, when the dem and drops, heat w ould be put to storage and w hen dem and picks
up the heat can be delivered from the storage tank. A ppendix D goes into further detail on
therm al storage and how it could be used to smooth out the high variation in gasifier output
which is illustrated in Figure D27.

A s a rule o f thum b, a m inim um o f 10 m o f water storage are required for every 1 M W o f boiler
output. If the sizing was based on the peak output o f the gasifier (3.5 M W ) then the capacity o f
•>

the storage tank w ould need to be 35 m (Viessm ann presentation 2013). A further literature
review o f the recom m ended volum e o f w ater storage resulted in a w ide range o f
recom m endations. A ccording to BSI, the U nited K ingdom ’s N ational Standards Body, 25 to 50
m 3per M W o f peak load is recom m ended (BSI Technical Com m ittee BS E N 1536-4-7, 2008).

30

The calculations in A ppendix D are based on m odeling the existing bioenergy system with and
w ithout a therm al storage tank ranging in size from 1 GJ to 1,000 GJ, and determ ining the
natural gas savings. A 10 GJ tank w ould provide roughly 3 hours o f buffer betw een the supply
and dem and and decrease the natural gas consum ption by approxim ately 7%.

W hile the sizing o f the therm al storage system could be debated, they can im prove the overall
system output and act as a buffer betw een the supply and dem and o f heat. The location o f the
therm al storage tank w ould need to be betw een the bioenergy plant and the UNBC cam pus
(Figure 3.9).

Since biom ass energy system s do not have the same response tim e as fossil fuel system s that can
ram p up and down rapidly, a therm al storage tank should be considered when designing a
biom ass energy system . In addition, bioenergy system s do not have the sam e turndow n ratio as a
natural gas boiler and as such need to operate at or near peak capacity for m axim um efficiency.
But in the case o f the UNBC system, a therm al storage tank is not practical given the low
tem perature drop in the cam pus hot water loop. The resulting tank w ould have to be significantly
large (800 m 3) therefore m aking it uneconom ical (refer to A ppendix D for further details). The
tank volum e is m uch larger than the rules o f thum b indicated due to the low tem perature
differential in the cam pus w ater loop (3°C).

4.0 G reenhouse Gas O ffsets
Based on the data studied from M arch 2012 to N ovem ber 2013, the UN BC residences use 4,550
G J o f natural gas annually (see Table 4.0 below). A t an energy density o f 38.5 G J/m 3 (Fortis BC
H eat Values) and 1.916 tonnes CC^e/m 3 (Environm ent Canada, N ational Inventory Report), there
w ould be 225 tonnes o f C 02e offset each year if the residences were connected to a w ater loop

31

after the addition o f a flue gas condensing heat exchanger. This assum es 100% o f the natural gas
w ould be displaced, whereas in fact the natural gas system w ould rem ain in place and be used in
tim es when the bioenergy plant is not in operation.

T ab le 4.0 - P oten tial G reenh ou se G as O ffsets from D isp lacin g N a tu ra l G as C on su m ed in th e S tu d en t
R esiden ces.

N atural Gas
C onsum ption (GJ)

H eat C ontent
(G J/1000m 3)

Tonnes C 0 2/m 3

4,550

38.5

1.916

Tonnes C 0 2 / year
225

Since UNBC has a m andate to be greenhouse gas neutral (Greenhouse Gas Reductions Targets
Act), a reduction 225 tonnes o f C 0 2e has a value o f $12,375/year based on a carbon tax o f
$3 0/tonne and a carbon offset price o f $25.

A therm al storage tank could offset 50 tonnes C 0 2e/year the calculations for which are outlined
in A ppendix D. This w ould be achieved through the displacem ent o f som e o f the natural gas
spikes w ith heat from the storage tank. This w ould result in a 5% increase in hog fuel
consum ption. The system was m odeled with different therm al storage capacities and illustrated
in Figure D28. For a therm al storage tank w ith a 10 GJ capacity, there is an approxim ately 1,000
GJ drop in natural gas, w hich corresponds to 50 tonnes o f C 0 2e.

The installation o f a chiller to offset electric air conditioning would offset 6,000 GJ o f electricity.
Based on an em ission factor for BC Flydro o f 6.9 kg/G J (BC M inistry o f Environm ent, 2012),
there would be 41.4 tonnes o f C 0 2e offset valued at $2,277.

5.0 R ecom m endations to Im prove the Therm al Efficiency o f the Bioenergy Plant
I f the U niversity w ishes to pursue a project to increase the therm al efficiency o f the bioenergy
plant, the first step w ould be to com plete a detailed energy balance for the heat supply and

32

dem and for all cam pus buildings including the residences. This will confirm the available heat
and required dem and for heating and cooling on the cam pus during both sum m er and winter.
Then capital costs should be obtained for the m ore viable opportunities starting with the
installation o f a condensing heat exchanger in order to tie in the residences to the heating loop.
The condensing heat exchanger is likely the m ost viable opportunity for extracting residual heat
and im proving the overall therm al efficiency. Pre-drying the hog fuel using the w aste heat is
possible and w ould result in an efficiency im provem ent, but is likely not the m ost practical for an
existing installation. A dding a therm al storage tank is not recom m ended for the system as the
size requirem ents w ould m ake it cost prohibitive. The addition o f a chiller should be explored as
it provides a m eans to supply cooling to the cam pus w hile keeping the gasifier operating at full
capacity. The challenge for the chiller option will be the capital costs as they will be high and the
payback would be long given the relative low cost o f the existing air conditioning system and the
lim ited m onths o f use. A sum m ary o f the overall impacts o f the five options reviewed is outlined
in Table 5.0.

T ab le 5.0 - S u m m ary o f Im p acts to E fficien cy , H o g F u el C on su m p tion an d G H G E m ission s for E ach
Im p rovem en t O ption

Option
C ondensing H eat Exchanger
D ryer
Therm al Storage
Chiller
R educe E xcess A ir

Efficiency
10 - 20% increase
4% increase
No change
No change
No change

H og Fuel
No change
4% decrease
5% increase
15% increase
No change

G HG Em issions
225 tonnes C 0 2 / yr offset
No change
50 tonnes C 0 2 / yr offset
41 tonnes C 0 2 / yr offset
No change

Based on the com parison (Table 5.0), adding a flue gas condensing heat exchanger stands out as
show ing the greatest potential to im prove the existing system.

33

Section 2

6.0 Com parison o f U N B C ’s Bioenergy Plant to A lternative Technologies
The gasifier’s perform ance in term s o f efficiency and options for im provem ent w ere review ed in
Section 1. The following discussion will review alternative renew able technologies that could be
utilized to offset natural gas heating and achieve sim ilar results at U N BC, plus to com pare
alternative uses for hog fuel. Three technologies will be discussed: Traditional biom ass boilers,
wood pellet boilers and the em erging technology o f a pyrolysis system for the production o f heat
and charcoal. This review will then enable a com parison to be m ade betw een the gasifier’s
perform ance and alternatives in order to determ ine w here the bioenergy plant ranks w ith other
biom ass conversion technologies in term s o f greenhouse gas offsets.

6.1 Biom ass Boilers
Traditional biom ass fired boilers are considered a m ature technology that have w idespread use
throughout industry and in com m ercial operations. They are com m only used due to their ability
to handle a wide variety o f biom ass supplies w ith variable m oisture content (Yin et al. 2008).
The pulp and paper sector has used biom ass boilers for over a century to generate steam for use
in the pulp m ill and for pow er generation via a steam turbine. Technological changes over tim e
have m ade im provem ents to therm al efficiency, controls and em issions but the core technology
has not changed in recent years. Two com m on types are stoker grate and fluidized bed boilers
(Saidur et al. 2011). Stoker grate boilers can be subdivided into different designs depending on
the grate system w hich can be fixed, travelling or vibrating (Duo 2007, Bain et al. 1998). The
basic design is a fixed grate boiler where the biom ass is fed onto a grate where com bustion takes
place. Ash rem oval takes place via a stoker which rakes the grate after a period o f operation. In
this design the com bustion efficiency is the lowest o f the boiler designs and residual carbon in

the ash can be a problem depending on the control m echanism em ployed. Travelling or vibrating
grates are designed to spread the biom ass fuel evenly across the grate for im proved com bustion
characteristics. In general, these grate designs result in higher com bustion efficiency when
com pared to fixed grate boilers (Saidur et al. 2011).

The m ost m odem boiler design is a fluidized bed boiler w hich was introduced due to its higher
com bustion efficiency and lower SO 2 and N O x em issions (US Environm ental Protection A gency
2007). The EPA report m akes a com parison betw een stoker grate and fluidized bed boilers and
found that at the same excess air ratio and exhaust tem perature, the stoker grate boiler efficiency
is 77% while the fluidized bed boiler efficiency is 80% (HHV). These num bers vary depending
on operating conditions and fuel m oisture content. A n FPInnovations presentation by Duo (2007)
reports a 68% (HHV) therm al efficiency in a stoker grate boiler using a fuel m oisture o f 50%
(Duo 2007). The higher efficiency reported for a fluidized bed boiler is significant but comes
w ith a higher capital cost. Fluidized bed boilers are also m ore suited for fuels with a low energy
density and high ash content (Saidur et al. 2011).

K raft pulp m ills in Canada are all relatively old having been constructed in the 1960’s so the
m ajority o f the boilers in operation are either fixed grate or m oving grate designs. If a new boiler
was installed today, it is likely that a fluidized bed (som etim es referred to as bubbling fluidized
bed) boiler w ould be considered given the efficiency, im proved em issions and the environm ental
benefits o f low er ash volum es to be disposed of.

A lthough boilers are available in a wide range o f sizes, the econom ics tend to be m ore favorable
on a larger scale when looking at the capital cost per tonne o f steam or kW o f therm al energy
produced (K um ar et al. 2003, Svanberg et al. 2013). A s a result biom ass boilers are the

35

technology o f choice for the m ajority o f the pulp and paper m ills in operation today. In addition,
they are well suited to handle a w ide range o f fuel m oisture contents which is typical when
utilizing hog fuel. In com parison, dealing with variable fuel m oisture contents is not som ething
current gasification system s are well suited to handle. The reason is the m oisture can lead to an
increased need for gas cleanup especially if the syngas is being produced for pow er generation
(A sadullah 2014).

Biom ass gasification has not gained m arket acceptance in large capacities and with the exception
o f a few larger installations, gasification system s are generally sm aller in scale than the biom ass
system s required for a pulp mill. The tw o largest biom ass gasifiers are in Finland; the one in
Lahti is 160 M W th and the one in V aasa is 140 M W th (IEA Task 40). H og fuel fired biom ass
boilers at pulp and paper m ills are typically m uch larger by com parison and can be up to 500
M W in size (Preto 2011).

Sm aller boilers used in com m ercial heating operations are not as sophisticated as the boilers used
in the pulp and paper sector (Alakangas et al. 2006). Both hot w ater and steam can be generated
for use in therm al applications. Sm all-scale equipm ent does not usually have advanced control
and gas cleaning and is often located in more populated areas. For these reasons, high quality
fuel with low m oisture (< 30% ) and low ash (e.g. high quality pellets < 0.7% ) is preferred
(Alakangas et al. 2006).

6.2 W ood Pellet Boilers
A new er technology that is utilized in sm aller applications for renew able heating is a w ood pellet
boiler. A lthough this technology has been in use for decades in Europe, it is only starting to
m ake inroads in N orth Am erica. Pellet boilers are typically used in sm aller com m ercial

36

operations or district heating applications due to their availability in a variety o f small to m edium
capacities. The advantage o f a w ood pellet boiler over a traditional biom ass boiler is that they are
designed around a fuel w hich is very uniform in both size and m oisture content. This greatly
im pacts the ability to optim ize the controls and m axim ize the efficiency (Alakangas et al. 2006).
A nother benefit is that wood pellets have a transportation advantage over unprocessed
biom ass/hog fuel due to the fact that they are dried and as such have a higher energy density.
This enables jurisdictions that are not located near a source o f unprocessed biom ass to use w ood
pellet boilers as a viable source o f renew able heat (M agelli et al. 2009). Y ellow knife is an
exam ple o f such a jurisdiction w here they have an initiative to install w ood pellet boilers and are
im porting the pellets as they do not have the biom ass sources locally (City o f Y ellow knife N W T
new s article, Arctic Energy A lliance 2009).

T ransporting biom ass great distances impacts the lifecycle greenhouse gas em issions (M agelli et
al. 2009, Dw ivedi et al. 2014). Sawmill residuals or logging debris w ith a m oisture content o f
50% has an energy density o f approxim ately 8 - 1 0 GJ/tonne, w et basis (inform ation from
C anfor Pulp). W ood pellets are typically 5 - 10% m oisture and have an energy density o f 18.5 20 G J/tonne (M urray 2011). The m ore energy dense the fuel is the m ore efficient the
transportation costs becom e on a per tonne basis due to there being less w ater. This is the m ain
reason w hy it is not econom ical to transport sawm ill residuals or logging debris long distances.
In term s o f the U N BC gasifier, the system is designed around hog fuel w ith a m oisture content o f
up to 60% , which is typical o f the hog fuel w ithin the Prince George region. A lthough wood
pellets are dry and o f a uniform size, the costs are significantly higher than the available sawm ill
residuals so it w ould not m ake econom ic sense to design a sim ilar gasifier to use wood pellets as
the fuel. H og fuel is delivered to UN BC for $60/tonne (dry basis) whereas wood pellets w ould

likely cost m ore than $100/tonne. Current delivered prices into D enm ark are $180/tonne w hich
includes shipping and rail (Argus Biom ass M arkets, 2014). B ulk pellet costs in W estern Canada
range from 105 to $ 160/tonne (Arctic Energy A lliance 2009).

6.3 Pyrolysis Technologies
A grow ing technology is the use o f pyrolysis for the com bined generation o f heat and charcoal
production. Pyrolysis is the therm al destruction o f biom ass in an inert atm osphere producing
charcoal (biochar), oil and gas products (Ryu et al. 2007). D epending on the tem perature and
residence tim e, the w eight fraction o f charcoal, oil and gas products varies.

Pyrolysis technologies are distinguished as being either a fast or slow pyrolysis system . Fast
pyrolysis system s target bio-oil as the prim ary product which is either used as a liquid fuel or
further processed in to chem ical by-products. For these system s, approxim ately 75% o f the
original m ass is converted to bio-oil, 13% to gas and 12% to biochar. For a slow pyrolysis
system , approxim ately 30% o f the original m ass is converted to bio-oil, 35% to gas and 35% to
biochar (Sohi et al. 2009). These percentages vary depending on the reactor tem perature and
residence tim e. Increasing the reactor tem perature from 350°C to 700°C decreases the charcoal
yield from 35% to 20% (Ryu et al. 2007). As the yield o f charcoal drops at higher tem peratures,
the w eight percentage o f carbon in the biochar increases as m ore volatiles are driven o ff (Ronsse
et al. 2013).

D epending on the end use o f the charcoal and w hether it is to be used as an energy pellet or in
agricultural applications, the pyrolysis system s are designed and operated differently. For a
system designed for producing both charcoal and heat, a slow pyrolysis system is used w here the
bio-oil and gas generated are used as fuels for therm al applications.

38

6.3.1 Biochar
B iochar has been used in agricultural applications w here it is used as a soil enhancer to improve
crop yield (Lehm ann 2007). M uch research has gone into biochar w here it has been shown to
improve w ater retention, plant nutrient uptake, and low er soil bulk density all o f which have
resulted in increased crop yield (Laird 2008). B iochar has also been used to reclaim soils
follow ing m ining activities, cleanup tailings ponds and other soils heavily im pacted by industrial
activity (Fellet et al. 2011). For exam ple, w hen open pit m ines are constructed, the biom ass
m aterial that is rem oved could be converted to charcoal for use in soil reclam ation or w ater
filtration on site. B iochar is also claim ed to be a form o f long-term carbon sequestration, as the
carbon in biochar is resistant to degradation (G urw ick et al. 2013, Roberts et al. 2010, Crom bie
et al. 2013). There are also discussions in publications about the role biochar could play in
carbon-negative bioenergy production (Kauffm an et al. 2014). This is due to the highly stable
carbon that rem ains in the soil when biochar is used in agriculture.

The availability o f biochar is currently lim ited to small quantities from various pilot plants
w orking to com m ercialize their technology. The m arket is new and a full acceptance and
understanding o f the pros and cons are still in the early stages. A local study show ed that biochar
addition could enhance soil properties and the early grow th o f pine and alder in som e sub-boreal
forest soils (Robertson et al. 2012). Flowever, m ore research is required to be able to fully
quantify the benefits and opportunities as not only are soil conditions highly variable, so too are
the properties o f biochar. B iochar research is sum m arized in Lehm ann (2007). A recent review
highlights the uncertainty about how biochar production and application affect w hole-system
greenhouse gas budgets (G urw ick et al. 2013).

39

6.3.2 Activated Carbon
There is also grow ing interest in the use o f charcoal to produce activated carbon for w ater and air
filtration. A ctivated carbon is a form o f charcoal that has undergone further processing, or
activation, resulting in a m aterial that has a very high surface area (M ohan and Pittm an 2006).
Currently, activated carbon is produced from a variety o f sources including wood, coconut shells,
w heat straw (Schroder et al. 2007) and petroleum coke (Kawano et al. 2008). A ctivated carbon
can be produced econom ically from biom ass w hich is favorable over coal based products due to
the grow ing interest in organic farm ing utilizing renew able m aterials.

U sing activated carbon for flue gas cleanup and m ercury rem oval in a coal fired pow er plant is a
com m on practice (Yang et al. 2007). W ith the grow ing aw areness o f oil sand developm ent
impacts, activated carbon could also be used for contam inated w ater cleanup. A ctivated carbon is
a very large industry and one w ould expect it to grow given the sensitivities around air em issions
from industry, w aste w ater cleanup and the production o f clean drinking water.

6.3.3 Fuel Flexibility
Pyrolysis system s are not lim ited to using w ood waste as a fuel; any source o f biom ass can be
broken dow n with pyrolysis. Agriculture waste products, bio-solids and other organics can all be
utilized therefore taking a w aste product that w ould have otherwise been com posted or landfilled and using it to generate heat and biochar. There is great potential for such a conversion
technology due to the fact that there is a significant supply o f fuels that are currently being
disposed o f at a cost. This includes all organic m aterial that is currently ending up in landfills,
w hich are costly to operate and have significant environm ental impacts.

40

6.3.4 Carbon O ffset Potential
Generating renew able heat from a pyrolysis based system could offset natural gas in the same
m anner as other technologies, but producing biochar for agricultural use can reduce greenhouse
gas em issions (G urw ick et al. 2013, Roberts et al. 2010, Crom bie et al. 2013). The reason is that
w hile the flue gases and tars produced from the pyrolysis system can be used for heat and/or
pow er generation, biochar, which has a high carbon content, can be considered a form o f carbon
sequestration w hen applied to soils. Efforts are underw ay to develop a carbon offset
m ethodology through an organization com prised of: The Clim ate Trust, The Prasino Group, the
International Biochar Initiative and Carbon Consulting (w w w .biochar-intem ationai.org). This is
being subm itted to the Am erican Carbon R egistry (ACR), w hich is a voluntary carbon m arket
offset program . The protocol centers on developing an accepted scientific test to quantify biochar
stability in soils so as to verify the lifespan o f the carbon w hen m ixed in soils. Once the carbon
offset protocol is in place, it w ould be expected that the m arket for biochar will improve as there
w ould then be a process to verify the benefits and quantify the carbon credits.

The m ethane that is produced w hen organics breakdow n in a landfill has a global w arm ing
potential 21 tim es that o f carbon dioxide (BC M inistry o f Environm ent, 2012). B iochar is high in
carbon w hich does not leach out in the soil; rather it rem ains in the soil as dem onstrated in the
A m azon w here biochar has rem ained in the soil for thousands o f years (Lehm ann 2007). This
persistence in the soil can therefore be view ed as avoided m ethane and carbon dioxide
generation.

A pyrolysis system w hich produced a clean source o f heat could offset natural gas plus the
biochar could sequester carbon when used in agriculture applications. A pproxim ately 50% o f the
carbon contained in the biom ass rem ains in the biochar as com pared to biom ass com bustion
41

which releases all but around 3% in the form o f carbon dioxide (M cH enry 2009).
Conservatively, 80% o f the carbon in biochar (40% o f the carbon in the original biom ass) is
predicted to be stable over very long (100’s o f years) tim e periods (Roberts et al. 2010, Crom bie
et al. 2013).

6.3.5 Torrefaction
A low tem perature pyrolysis system that is gaining interest in the wood pellet industry is
torrefaction, which produces an energy pellet with im proved properties. A t a tem perature o f
betw een 250°C and 300°C, hem icellulose decom poses leaving the rem aining cellulose and lignin
w hich can then form a pellet w ith water resistance and a higher energy density (Prins 2006). One
o f the m ajor challenges that the w ood pellet industry faces is the lack o f w ater resistance. W ood
pellets are transported in bulk and their lack o f w ater resistance results in the need for higher
transportation and storage cost. Torrefied pellets have w ater resistance due to the fact that when
heated, hydroxyl groups are destroyed in the biom ass by dehydration reactions resulting in the
inability to form hydrogen bonds w ith w ater (U sla et al. 2008). In addition, the low tem perature
pyrolysis process results in approxim ately 30% w eight loss so m ore biom ass is required per
tonne o f final product. The higher energy density com es from the fact that 70% o f the initial
m ass rem ains with 90% o f the energy (Dutta and Leon 2011, W annapeera et al. 2011, Yan et al.
2009).

The required heat for the torrefaction process is provided by either com busting waste w ood or by
com busting a com bination o f natural gas and the volatiles released during the process (UBC
Biom ass Pelletization W orkshop, 2011). This is an area o f product developm ent which requires
further research and will greatly im pact the overall m ass and energy balance o f the system . In a
high tem perature pyrolysis system , there is enough energy contained within the volatiles driven

o ff to com bust and provide all the necessary heat for the system to operate w ithout the need for
natural gas (other than a pilot) (Lee et al. 2010). Torrefaction on the other hand, only drives o ff a
portion o f the volatiles so additional heat sources are required to keep the system operating.
Fuels with m oisture contents greater that 15-20% w ould require a separate dryer heated w ith
another fuel due to the lim ited volatiles available as a heat source (Kiel et al. 2008).

The target m arket for torrefied wood pellets is the export m arket; predom inantly Europe. There
is a transportation advantage due to the higher energy density, plus w ater resistance (Uslu et al.
2008). A lthough torrefied pellets are not available yet on a large com m ercial scale, pricing is
expected to be higher than wood pellets due to the added processing costs (U slu et al. 2008).

There have been a num ber o f projects announced for the construction o f a com m ercial scale
torrefied w ood pellet plant, but none appear to be in full operation. One such com pany w hich has
m ade headlines in the Canadian Biom ass M agazine in early 2014 is Zilkha Biom ass Energy
w hich has announced a 275,000 tonne per year torrefied pellet plant for Selm a, A labam a
scheduled to be com pleted in 2015 (Canadian Biom ass M agazine online article).

7.0 A vailable Biom ass Supplies in BC
Biom ass supplies cannot be considered an unlim ited source o f fuel. This section is a b rie f review
o f available biom ass supplies in BC.

British C olum bia’s forest industry generates significant volum e o f biomass that is utilized in the
pulp and paper industry and num erous industrial and com m ercial energy system s. B ut forestry
residuals are not the only source o f biom ass in the province. Table 7.0 breaks dow n the available
biom ass supplies and the percent o f fossil fuel energy that it could displace. H arvesting
sustainably as well as m ountain pine beetle tim ber that no longer has value as saw logs or pulp

43

wood could displace h a lf o f the fossil fuels consum ed in the province. The m ountain pine beetle
tim ber w as forecasted to be available for 20 years from 2006 when the data in Table 7.0 was
published. This does not assum e that all the pine beetle tim ber w ould be econom ical to harvest
so the total econom ically available pine beetle tim ber is likely less. The sustainable forestry
estim ates are net o f tim ber processed into lum ber and other solid w ood products. A detailed
analysis on the availability o f m ountain pine beetle tim ber is outside the scope o f this report.

T ab le 7 . 0 - S ou rces o f B iom ass in B ritish C olu m b ia an d B ioen ergy P oten tial as a P ercen ta g e o f T otal F ossil
F uel D em an d . (2 0 0 6 data).

Municipal Solid
W aste
Sustainable
Agriculture
Sustainable Forestry
Mountain Pine Beetle
Total
Total Fossil Fuel
Demand

Tonnes/year

PJ/yr

% o f Potential

% o f Fossil Fuel
Energy

948,450

15.2

2.9

1.6

3,266,505
17,114,615
11,014,618
32,344,188

52.1
273.8
176.2
517.4

10.1
52.9
34.1
100

5.7
29.8
19.2
56.2

920

(R eproduced from th e B ritish C olu m b ia B ioen ergy N etw ork: A n In form ation G u id e on P u rsu in g B iom ass
E n ergy O p p ortu n ities and T ech n ologies in B ritish C olu m b ia. A u gu st 2010)

M ountain pine beetle biom ass is not be sustainable over the long term , and m ay not be
econom ical over the short term. This leaves over 10 m illion tonnes o f forestry biom ass per year
available for bioenergy. The sustainable forestry category in Table 7.0 includes 11.9 m illion
tonnes o f annual harvest residues, based on the assum ption that 70% o f forestry residues (17.1
m illion tonnes/year) can be harvested sustainably. Dym ond et al. (2010) estim ate sustainable
residues at 8 m illion tonnes per year (50% o f the 15.5 m illion tonnes total clearcut residues).
Kum arappan et al. (2009) estim ate 8.7 m illion tonnes o f forest residues and 8.4 m illion tonnes o f
m ill residues available at a cost o f less than $ 1 0 0 /tonne.

44

The forest industry including pulp and paper are large consum ers o f biom ass and are significant
producers o f bioenergy using residuals from their operations. But there are large untapped
sources available such as m unicipal solid waste and agricultural residuals. W hile they m ay not
m ake up a large portion o f the province’s inventory o f biom ass, they are readily available and are
suitable for sm aller renew able energy systems. Such biom ass supplies are often suitable for
universities and com m ercial operations which have m uch sm aller energy needs than that o f a
pulp mill. In addition, sm aller bioenergy system s generally cannot com pete for forestry residuals
as they are tied up in contracts w ith the pulp and paper m ills.

8.0 G reenhouse Gas Life-C ycle Em issions
The greenhouse gas intensity o f biom ass energy is highly dependent on a num ber o f variables
such as transportation distances, harvesting m ethods, and processing requirem ents. In term s o f
which bioenergy option has the lowest greenhouse gas life-cycle cost, it w ould likely involve
locally sourced biom ass w ith the least am ount o f processing (when com paring to the same fossil
fuel displaced). In other words hog fuel w ould have a lower life-cycle cost than wood pellets
sourced from the same area due to the energy required to produce w ood pellets. D ue to the lower
m oisture content wood pellets have the advantage w hen long transportation distances are
required (U slu et al. 2008). Locally sourced biom ass is generally preferable when sim ply looking
at fuel costs. This m ay change in the future depending on biom ass availability, fossil fuel prices,
financial incentives and a greater adoption o f renew able technologies.

8.1 D isplacem ent Factor Calculation
A com m on m ethod to com pare bioenergy system s is through the use o f displacem ent factors.
The displacem ent factor is the am ount o f fossil fuel em issions that is directly offset by replacing

45

the fossil fuel w ith biom ass (Schlam adinger and M arland 1996). D isplacem ent factor units are
typically kg C O 2 per tonne o f biom ass (Laser at al. 2009).

A num ber o f form ulas can be used to calculate the displacem ent factor, depending on the data
available. The form ula used in this study is based on the one presented in Laser et al. (2009)
(equation 3). This form ula was chosen as it uses variables for which data w as either available or
could be calculated.

DF = (X 1) x (X2) x (X3) - X 4

(3)

X I = fossil fuel carbon intensity (kg CO 2/G J displaced)
X2 = bioenergy efficiency / fossil fuel efficiency - processing losses (GJ displaced/G J biom ass)
X3 = biom ass energy density (GJ biom ass/tonne dry biom ass)
X4 = transportation em issions (kgCCVtonne dry biom ass)
D isplacem ent factors are used in carbon balance m odels. For exam ple, they have been used to
com pare carbon sequestration in forests vs using short rotation forestry to grow biom ass for
bioenergy and coal displacem ent (Barala and Guhab 2004, Schlam adinger and M arland 1996,
Yem shanov and M cK enney 2008). These studies used displacem ent factors o f 1,330, 1,100, and
1,750 kg fossil fuel CO 2 displaced per tonne o f biom ass for coal displacem ent. The large
variability is due to different assum ptions on bioenergy and fossil fuel efficiencies. The larger
num ber is based on the assum ption that bioenergy is as efficient as coal. The higher the
displacem ent factor, the m ore favourable bioenergy use com pared to carbon sequestration in
forests (M arland and Schlam adinger 1997). For liquid fuel production, displacem ent factors
range from 970 to 1,260 kg fossil fuel C 0 2 displaced per tonne o f biom ass (Laser et al. 2009).
These displacem ent factors are not appropriate for use in BC due to the low carbon electricity
generation and lim ited coal use.
46

8.2 Carbon N eutrality o f Biom ass
Com paring the full life-cycle assessm ents o f biom ass based renew able energy to fossil fuels has
undergone m uch debate recently. The US Environm ental Protection A gency (EPA ) is currently
review ing regulations surrounding the carbon neutrality o f biom ass and biogenic em issions
accounting (US Environm ental Protection Agency 2014). Different life-cycle assessm ent
m ethodologies do not alw ays have the same results, and as such there is debate as to the
appropriate regulations to use for biom ass energy (Sedjo 2013). The debate in the US centers
around the point that there are C O 2 em issions at a biom ass energy plant in the sam e m anner as
there w ould be in an energy system based on fossil fuels. In addition, land use changes m ay
result in large carbon em issions over the short and m edium term (Johnson 2009). The key is that
over the longer term , the CO 2 em issions are considered neutral as long as the biom ass consum ed
is regrow n so as to com plete the carbon cycle. W hile there is controversy over the carbon
neutrality o f m any biom ass sources, wood waste that w ould have decom posed or som ehow
returned its carbon to the atm osphere anyw ay is usually considered carbon neutral (Johnson
2009). D iffering opinions on the full life-cycle em issions for biom ass have created confusion
and, as a result, one m ust use existing life-cycle assessm ent tools w ith caution.

A t the United N ations Clim ate Change Conference in Durban South A frica in 2011 (BC M inistry
o f Environm ent, June 2012), new rules w ere agreed upon for carbon accounting in forestry. Key
updates include the point that wood used for bioenergy w ould be accounted for as an imm ediate
release o f greenhouse gas, and forests that have been negatively im pacted by outbreaks such as
the pine beetle or w ild fires will not negatively im pact carbon accounting (BC M inistry o f
Environm ent 2012). It is expected that these updates from the United N ations will be

47

incorporated into revised greenhouse gas reporting in all reporting countries, and w ill help to
clarify the questions around carbon accounting in bioenergy.

8.3 Fossil Fuel Carbon Intensity
The net greenhouse gas life-cycle cost savings when using biom ass are dependent on the fossil
fuel displaced. D isplacing coal w ith biom ass for exam ple will displace m ore greenhouse gas than
displacing natural gas due to the higher carbon content o f coal. The carbon intensity for coal is
69 kg CO 2 per G J com pared to 50 for natural gas (Table 8.2). Electricity supply in British
Colum bia is prim arily hydroelectricity which has a low carbon intensity o f 6.9 kg C 0 2 per GJ
(Table 8.2). The carbon intensity o f electricity generation does vary year to year depending on
the m ix o f generation plants in use. The range is 2.5 to 10 kg C 0 2 per GJ (Environm ent Canada
2014). The breakdow n o f electricity generation by category in British C olum bia for the periods
2007 through to 2010 is show n in Table 8.0.

T ab le 8.0 - E lectricity G en eration in B ritish C olu m b ia by C ateg o ry from 2007 to 2010.

Electricity Generation (GWh/yr)
2007

2009

2008

2010

70

70

100

80

Natural Gas

2,660

3,080

2,610

2,430

Hydro

54,700

48,600

46,300

44,400

670

560

400

630

58,100

52,310

49,410

47,540

R efined Petroleum Products

Biom ass
Total

(R ep rod u ced from th e B ritish C olu m b ia G reenh ou se G as In ven tory R ep ort 2010)

8.4 Bioenergy Efficiencies
A report by Envirochem Services Inc. determ ined the electricity generation potential per bonne
dry tonne (BDT) o f wood chips at 50% m oisture content. A selection o f the conversion
technologies is provided below in Table 8.1 (Tam pier et al. 2004).

48

T ab le 8.1 - E lectricity G en eration P oten tial o f W ood C h ip s (R ep rod u ced from T ab le 2 .4 .6 T a m p ier et al.
2004).

Small Steam
Large Steam

Bio-Oil

Gasification

Condensing

Condensing Cycle

Conversion

Conversion

Cycle

kW h/BDT wood

1,659

363

440

563

Efficiency (wood to electricity)

30%

6.5%

7.9%

10%

The large steam condensing cycle has the greatest specific electricity generation per bone dry
tonne (BDT) o f wood chips due to econom ies o f scale and the high efficiency o f steam turbines.
This refers to larger biom ass boilers com bined w ith a condensing steam turbine and com m only
found in a pulp mill. They are only econom ical on a large scale so are not suitable for sm aller
com m ercial operations. The bio-oil conversion has the potential to produce the least am ount o f
electricity due to the fact that only a percentage o f the biom ass ends up as oil (Singh et al. 2010).
This is reflected in the efficiency o f 6.5% (Table 8.1). The balance o f energy in a bio-oil
conversion process is m ade up o f volatiles and a small percentage o f charcoal. The value for both
the gasification conversion and the small steam condensing cycle assum es all the heat generated
is utilized for pow er generation (Tam pier et al. 2004).

The em issions for w ood pellets include 16.2 kg CC^e for drying and 7.9 kg CC>2e for processing
per tonne o f pellets produced (M urray 2013, Bradley 2006) Burning wood pellets is m ore
efficient than burning w et biom ass, especially if there is no flue gas condensing heat exchanger.
Production o f wood pellets typically consum es 10 - 15% o f the total biom ass supply to dry the
incom ing biom ass (M agelli et al. 2009). This results in a 15% processing loss in the production
o f w ood pellets and torrefied wood pellets (Table 8.2).

O ther bioenergy options such as the production o f liquid fuels (bio-oil, gasoline, and diesel) and
biochar do not require external energy sources for production (except for transportation). A

49

portion o f the energy contained in the wood is used to pow er the process. For liquid fuels
(gasoline and diesel), 35 - 50% o f the original energy from the biom ass rem ains in the final
product (Singh et al. 2010). For bio-oil, 65 - 70% o f the original energy rem ains in the final
product (Singh et al. 2010). D uring the production o f biochar, 30 - 55% o f the original biom ass
energy rem ains in the final product (M atovic 2010, W eifu et al. 2010, Roberts et al. 2010). For
all o f these options, there can be some excess energy that could be used for heating or electricity
generation.

8.5 Com parison o f D isplacem ent Factors Including U N B C ’s B ioenergy System s
A n analysis was carried out to com pare the life-cycle greenhouse gas displacem ent potential for
a selection o f bioenergy options as com pared to the existing gasifier. D isplacem ent Factors were
calculated using fossil fuel em ission factors (8.3) and bioenergy system efficiencies (8.4). Details
are outlined in Table 8.2. U N B C ’s bioenergy plant can be com pared to alternative technologies
in term s o f CO 2 displacem ent. The existing bioenergy plant, referred to as the base case in Table
8.2, displaces 782 kg CO 2 per tonne o f hog fuel consum ed. This can be increased to 882 kg C O 2
w ith the addition o f a flue gas condensing heat exchanger. The addition o f a chiller to the gasifier
w ater loop only increases the CO 2 displaced by 3.5 kg due to the fact that it displacing
hydroelectricity w hich already has a low greenhouse gas intensity.

50

Table 8.2 - Calculated CO 2 Displacement Factors

kg C02/GJ
Displaced

Ratio o f
Efficiency of efficiency (GJ
Conversion
exBting
displaced/GJ
sytem
Efficiency
biomass)

Processing
Losses (%)

Energy Density Processing
ofbiomass input & Transport Displacement
Net GJ
(GJ
for Export
Factor (kg
dsplaced/GJ biomass/tonne
C02/tonne
(kg
biomass
biomass)
C02/tonne)
biomass)

Gasifier
Base Case (natural gas displacement) “

49.9

80%

80%

100%

100%

16

16.0

782

Flue Gas Cond. Heat Exchanger (nat. gas displacement)a

49.9

90%

80%

113%

113%

16

16.0

882

Chiller (hydro electricity displacement)a

6.9

50%

Wood Pellets (natural gas displacement) a'b

49.9

90%

70%

129%

109%

18

51.7

930

Liquid bioliiel (gasoline & diesel displacement) °’c d
Slow Pyrolysis A

66.8

50%

18

16.0

585
1108

Heat (natural gas displacement) D’bc

49.9

45%

18

17.1

404

Biochar for soils (carbon capture)b,c,r
Slow Pyrolysis B

97.8

40%

18

16.5

704
959

Heat (natural gas displacement) abc

49.9

15%

18

5.7

135

69

70%

18

45.2

824
885

Bio-oil (natural gas dBplacement)a-b,c

49.9

75%

18

17.1

674

Biochar for soils (carbon capture)b c f
Biomass Cogeneration

97.8

12%

18

16.5

211
422

Heat (natural gas displacement) °'b

49.9

50%

16

16.0

399

Electricity (hydro electricity displacement) a’b

6.9

35%

16

16.0

23

Biochar pellets for export (coal displacement) b,c'B
Fast Pyrolysis Plant

3.5
15%

Pellets for Export (coal displacement)b,s

69

85%

15%

72%

18

81.3

816

Torrefied Pellets for Export (coal displacement) b,g

69

100%

15%

85%

18

81.3

974

a - British Columbia Ministry of Environment, 2012 BC Best Practices Methodology for Quantifying Greenhouse Gas Emissions. September 2012
b - Bradley D. GHG Impacts of Pellet Production from Woody Biomass Sources in BC, Canada. Climate Change Solutions. May 24,2006
c - Van Vliet O.P.R., et.aL Fischer-Tropsch diesel production in a well-to-wheel perspective: A carbon, energy flow and cost analysis. Energy Conversion and Management 50 (2009) 855-876
d - Piekarczyk W.,Czamowska L., Ptasinski K., and Stanek W. Thermodynamic evaluation of biomass-to-biofuels production systems. Energy 62(2013) 95-194
e - Discussions with Phil Marsh, Chief Technology Officer, BC Biocarbon
f - Assume charcol is 80% fixed carbon, 44 kg C 0 2/12 kg C, & 30 GJ/tonne
g - Environment Canada, National Inventory Report Greenhouse Gases Sources and Sinks Part 2. The Canadian Government Submission to the UN Framework Convention on Climate Change
All units in lower heating value (LHV)

IN atural G as

B H ydro E lectric

a C oal

Biomass for E lectricity only

23

Biomass Cogen H eat & Electricity

6038

e C arbon C apture

H G as & D iesel

Liquid Biofuel
782

G asifier Base C ase
G asifier with Chiller

785

Pellets fo r Export

J 816

Fast Pyrolysis B io-oil & B iochar for Soils

i 826

G asifier w ith Flue G as Cond. Heat Exchanger

882
■ 93 3

W ood Pellets
Slow Pyrolysis H ea t& Biochar Energy Pellets

959

Torrefied Pellets fo r Export

974

Slow Pyrolysis Heat & B iochar for Soils
200

400

600

800

1000

1200

kg C O z d isp la ced / tonne biom ass

F igu re 8.0 —R an k in g o f D isp la cem en t F actors fo r B ioen ergy System s.

The displacem ent factors from Table 8.2 w ere ranked in order in Figure 8.0 and it can be
observed that the system w ith the potential to displace the greatest am ount o f greenhouse gases is
the slow pyrolysis system producing heat and biochar for soils. This system could offset 404 kg
CO 2 for heating and 704 kg CO 2 w hen the biochar is utilized as a soil am endm ent for a total o f
1,108 kg CCVtonne biom ass. The biochar com ponent is based on 80% o f the biochar’s m ass
being stable carbon which rem ains in the soil, w ith an energy density o f 30 G J/tonne and
converting the carbon to CO 2 (U sla et al. 2008). I f a slow pyrolysis system had been designed in
place o f the existing U N BC gasification plant, m ore biom ass w ould have been consum ed to
output the sam e heating load because only bio-oil (30% ) and gas (35%) w ould have been
available for heating. Therefore, 35% m ore biom ass w ould have to be purchased in order to m eet
the same heating dem ands o f the campus. The biochar produced could have becom e a by-product

52

for sale and/or used for research activities such as in greenhouses or field trials on or o ff cam pus,
potentially offsetting the additional cost o f hog fuel.

The next two highest ranked system s are the slow pyrolysis heat and biochar energy pellet
com bined system , and torrefied pellet for export for coal displacem ent. These rank high due to
the com ponent o f coal that is offset, despite the transportation penalty when shipping pellets to
Europe. The im pact o f the transportation is 81.3 kg CO 2 per tonne o f biom ass w hich is
significant, but overall there is still a net reduction when com paring to coal (Bradley 2006). The
greatest displacem ent factor w ould be if w ood pellets w ere used dom estically to displace coal
(e.g. in A lberta) as the im pact o f the transportation w ould be greatly reduced.

W ood pellet boilers like the one installed in U N B C ’s Enhanced Forestry lab (EFL), are higher in
efficiency w hen com pared to a biom ass gasifier and as such they have a higher displacem ent
factor. The displacem ent was calculated to be 930 kg CC^/tonne biom ass as com pared to 782 kg
CCVtonne biom ass for the gasifier. The wood pellet boiler conversion efficiency is higher than
that o f the gasifier plus it is displacing a less efficient natural gas system . The old natural gas
boiler in the EFL only has an efficiency o f 70% com pared to 80% for the m ain cam pus natural
gas boilers (Table 8.2).

The system w ith the lowest displacem ent factor is a biom ass based pow er plant displacing
hydroelectricity, w hich m akes sense given the low displacem ent factor for hydroelectricity. A
liquid biofuel displacem ent factor was calculated to be 585 kg CCVtonne biom ass w hich is lower
than literature values (Laser et al. 2009). A significant by-product o f liquid biofuel production is
electricity. The British C olum bia electricity em issions intensity is only 6.9 kg CO 2/G J o f
electricity versus the literature value o f 180 kg CO 2/GJ o f electricity (Laser et al. 2009).

53

UN BC uses approxim ately 4,300 tonnes o f hog fuel per year, based on the average hourly
consum ption from Table 1.0 o f 495 kg/hr and assum ing 365 days o f operation per year. Using
this annual consum ption estim ate, the total greenhouse gas offset potential for selected options
are outlined in Table 8.3.

T ab le 8.3 - T otal A n n u al G reen h ou se G as D isp lacem en t P oten tial.

T onnes C 0 2
G a sifie r B a se C ase

3,364

G a sifie rw ith Flue G as C o n d e n se r

3,794

Pellets fo r E xport

3,509

T orrefied Pellets fo r E xport

4,188

Slow Pyrolysis H e a t & B io ch ar fo r Soils

4,765

A t 3,364 tonnes o f CO 2 displaced annually, U N B C ’s gasifier dem onstrates a viable option for
reducing greenhouse gas em issions. As new er technologies m ature, other options m ay becom e an
attractive alternative for projects with a sim ilar scope.

8.6 Bioenergy Potential in British Colum bia
U sing a conservative estim ate o f 10 m illion tonnes o f biom ass available each year (7.0), the
GHG offset potential was calculated for four bioenergy options (Table 8.4). This analysis does
not include pine beetle tim ber.

T ab le 8.4 - G reen h o u se G as O ffset and B ioen ergy P oten tial in B ritish C olu m b ia by A v a ila b le B iom ass
Sup plies.

Displacement
Factor (kg

Conversion E n ergy/Y ear
Efficiency
(PJ)
C 0 2/tonne biomass)

GHG Offeet
Potential (tonnes
C 0 2/year)

Electricity

30%

54

23

230,000

Liquid Fuels

50%

90

585

5,850,000

H eat

80%

144

880

8,800,000

Biochar

45%

81

1108

11,080,000

54

Based on the total electrical dem and in 2010 o f 170 PJ/year (Table 8.0 where 1 GW h = 3.6 x 10' 3
PJ) biom ass based electricity generation could supply 32% o f the provinces electricity. B ased on
the total liquid fuels dem and o f 460 PJ/year (N RCan 2014) biom ass based liquid fuels could
supply 20% o f the provinces liquid fuel demand. Based on the total natural gas dem and o f 260
PJ/year (N RC an 2014), biom ass could supply 56% o f the provinces heating demand. This
assum es natural gas is prim arily used for heating. The largest GH G offset potential could com e
from the com bined slow pyrolysis for heat and biochar which could offset 11 m illion tonnes o f
CO 2 through com bined carbon storage in biochar and natural gas displacem ent. This represents
18% o f B C ’s 62 m illion tonnes o f GHG em issions (British Colum bia G reenhouse Gas Inventory
R eport 2010).

9.0 Conclusion
The analysis o f U N B C ’s bioenergy plant has revealed that at an average output o f 6.9 G J/hr and
a therm al efficiency o f 80% (LHV), the system displaces 3,364 tonnes o f CO 2 through displacing
the use o f natural gas.

Options to im prove the efficiency were explored and it was determ ined that the m ost viable
opportunity w ould be to install a flue gas condensing heat exchanger and extract m ore residual
heat. B y connecting the student residences to a new hot w ater loop, a greater percentage o f the
gasifier’s output w ould be utilized. It is estim ated that an efficiency im provem ent o f 8.5% to
18.5% could be possible w ith the use o f a flue gas condensing heat exchanger. This w ould
increase the CO 2 displaced to 3,794 tonnes. A n additional system w orth exploring is the addition
o f a chiller w hich w ould enable the gasifier to m eet the cooling dem and o f the cam pus. This
w ould allow the gasifier to operate at or near capacity all year m eeting the heating dem and

55

during the w inter and cooling dem and during the sum m er. The chiller w ould have a negligible
im pact to CO 2 displacem ent.

The installation o f a dryer or therm al storage system could increase the gasifier utilization by
approxim ately 5%. The m ore significant issue is the variable output o f the gasifier. This should
be addressed first before the dryer and therm al storage option can be fully evaluated. If the
gasifier could operate at a steady output, there is the potential for the bioenergy system to m eet
94% o f the cam pus heating dem and, as opposed to the current 85%.

A review o f alternative renew able technologies and their potential for greenhouse gas
displacem ent showed that slow pyrolysis system s with com bined heat and biochar output have
the greatest offset potential. W ood pellets exported to Europe also have a high displacem ent
factor due to the fact that they are used to offset coal. In BC , for bioenergy to play the largest
role in reducing GH G em issions, natural gas displacem ent is the best option w ith current
renew able technologies. The UN BC system is a good exam ple o f current technologies displacing
natural gas. A s pyrolysis and biochar technologies m ature, greater GHG offset potential will
emerge. U sing available forestry residuals, up to 18% o f the provinces GHG em issions could be
offset.

Future w ork into the perform ance o f the bioenergy plant should focus on the following: Establish
the reason for the high variation in the gasifier output and the spikes in the natural gas boiler
output (Figures 2.1, 2.2 and A ppendix D), then develop a m ore effective control strategy to
balance the output from the bioenergy plant to the cam pus demand.

56

A ppendix A
Note: all calculations re p re s e n te d in appendix A a ssu m e a final flue gas te m p e r a tu r e of 40 °C an d a hog
fuel m oisture c o n te n t o f 40%
T ab le A l - D ata T ab le for F igure 3 .3 . L aten t H eat L oss % as a F u n ction o f F uel M C % an d B o iler O u tp u t

_______________________________________________________________________________

(MW).

Fuel M C %
vs. Boiler
Output
(M W )

0%

10%

20%

30%

40%

50%

0.83
1.11

11.5%

16.7%

20.3%

25.2%

32.1%

10.1%

13.8%
12.1%

14.6%

17.8%

22.1%

28.1%

1.39

9.3%

11.2%

13.5%

16.5%

20.4%

26.0%

1.67
1.94

8.9%
8.6%

10.6%
10.3%

12.8%
12.5%

15.7%
15.2%

19.5%
18.9%

24.8%
24.1%

2.22

8.5%

10.2%

12.3%

23.7%

8.4%

10.1%

12.2%

15.0%
14.9%

18.6%

2.50

18.5%

23.6%

2.78

8.5%

10.1%

12.2%

14.9%

18.5%

23.6%

3.06

8.5%

10.2%

12.3%

15.0%

18.6%

23.7%

3.33
3.61

8.6%
8.7%

10.3%
10.4%

12.4%
12.6%

15.2%
15.3%

18.8%
19.0%

23.9%
24.2%

0 1 I f- i ! ) f l l l f

Latent heat loss (%) =

^ h ljn fp y if

100

'
1 ,0 0 0

Qdelivered

T able A 2 - H o g F u el P rop erties.
Fuel properties Dry wt%
49.3%
Xc

HHVtd

20.27 GJ/tonne

laten t heat of steam

2.26

GJ/t

Molecular w eigh ts (g/m ol)

xH
Xs

6.2%

Mc
Mh

0.0%

MH2

Xo
XN

40.2%

T ref

0.0%
4.3%

R

Xash

Pfg

K
298
8314.32 J/kmol K
1
atm

12.0112
1.0080
2.0159

Mc02

32.0640
44.0100

Mh20
Ms02

18.0153
64.0628

Mo2
MN2
Mh2o/M h2

31.9988
28.0134
8.9364

Ms

57

Em pirical biom ass, air and flue gas tem perature correlations:
Based on operating data, em pirical relationships w ere developed for biom ass input, air input, and
flue gas tem perature for a range o f gasifier operating conditions.

G asifier Data Points:

D ata used for calculations are indicated in the following figure:

TnueFlue gas
tem perature (°C)
t \

Energy out
(GJ/hour)

Biomass in
{% auger speed)

Gasifiei

Oxidizer

T

mcas Gasifier air flow
(lbs/hour)
Teas Gasification air T (°C

moxid Oxidizer air
flow (kg/hour)

*»

TRedrc Recirculation
air flow T (°C)

TFresh Fresh air T (°C)

The fresh air flow rate (mFresh) and the recirculation air flow rate (niRedrc) are unkown.

F igu re A l - D iagram Illu stra tin g D ata P oints.

U nit conversions from the supplied data to m etric units used M icrosoft Excel C onvert function.

58

A ir flow calculations
The incom ing fresh air flow rate is unknown, but can be found using an energy balance on the
gasifier air, recirculation air and fresh air:

m Gas CP ( T g as

T’r EF^ ~ m F re sh ('p (T F re sh

'^REf ') T m Recirc c p (T R e a r c - T r e f )

(5 )

I f T ref is taken as 0 , and Cp is assum ed to be constant, then:
m Gas cpT'c as ~ ^-Fresh^-PTfresh T m Recirc CP^Retire

(6)

Also, assum ing constant density:
m Gas = m Fresh + m Recirc

(7)

Com bining, and solving for Q f :
m G asTR ecirc~r>lGas'FGas

" '■Fresh

T

_T

' Recirc ' Fresh

(8)

Total air flow into gasifier/oxidizer system:
m air,in = m Fresh T f^-oxid

(9)

The uncertainty in the fresh air calculation is large, how ever the fresh air flow rate is small
com pared to the oxidizer air flow rate (fresh air flow is 2 to 5% o f oxidizer flow).

Biom ass calibration
Biom ass feed is m easured w ith the feed auger, given as % o f m axim um auger rotation speed.
The m axim um auger rotation speed is 210 rotations/hour. The volum e o f biom ass m oved per
rotation depends on m oisture content, but is approxim ately 1630 kg/hour at m axim um rotation
speed according to data supplied by Nexterra.

59

Cum ulative hog fuel deliveries w ere com pared w ith cum ulative GJ delivered and cum ulative
auger rotations. From this data a value o f 8.94 kg hog fuel/rotation was determ ined. A lso, shown
by the data is the close correlation betw een heat out and auger rotations and hog fuel deliveries.

/

r

, / kg \

na

hog
fu e l (\nour)
t 2- ) = 8.94 x
u J

auger rotations
■
—----------hour

(10)
'

8400

60,000

7900
50,000 -

7400
6900

"O 40,000 -

6400
•£ 30,000 -

5900
a c 2 0 >000 '

5 4 0 0 TJ

tiJ 10,000 -

4900 <2
bn
4400 3
3900

Cumulative auger rotations
F igu re A2 - C orrelation b etw een H ea t O u tp u t, ton n es o f B iom ass D elivered and A u g er R o ta tio n s.

E m pirical R elationships
U sing data collected at 15 m inute intervals from the gasifier, JM P 8 was used to develop
em pirical relationships betw een hog fuel consum ed and heat output, betw een total air input and
hog fuel consum ed, and betw een flue gas tem perature and heat output. The em pirical
relationships are:

m biomass,in [^our] = ( l 2 .4 2 + 1.35 Qdelivered [/Mwr] )

0^)
60

m air,in

1/2our. ~ 5 6 0 ' 9 + 7 , 1 6 m biom ass,in [ ^ J

(1 2 )

Tpiue [°C] = 92.9 + 6.18 Qdelivered [ ^ ]

(13)

The follow ing tables represent the output from the JM P 8 Analysis:

T ab le A 3 - C om b u stion C alcu la tio n s and E m p irical C orrelation s.

Air-toBiomass (kg
Flue gas T Thermal Thermal Yield air/kg dry
Yield (GJ/t) HHV (GJ/GJ)
tiiel)
(K)

Heat output Heat output
(GJ/hour)
(MW)

Biomass input
(kg dry
fiiel/hour)

Air input
(kg/hour)

Flue gas T
(°C)

Qdelivered

Qdelivered

^biomass,in

tt^air.in

Tflue

Tflue

3
4
5
6
7
8
9
10
11

0.83
1.11
1.39
1.67
1.94
2.22
2.50
2.78
3.06

273.57
319.52
369.03
422.11
478.75
538.96
602.74
670.08
740.98

2517.09
2845.51
3199.40
3578.78
3983.64
4413.98
4869.81
5351.11

111.43
117.61
123.78
129.96
136.14
142.32
148.50
154.68

10.97
12.52
13.55
14.21
14.62
14.84
14.93
14.92

5857.90

160.86

384.58
390.76
396.93
403.11
409.29
415.47
421.65
427.83
434.01

12
13

3.33
3.61

815.45
893.49

6390.16
6947.91

167.04
173.22

440.19
446.37

Y

1biomass

Yhhv

^air/biomass

14.85

0.54
0.62
0.67
0.70
0.72
0.73
0.74
0.74
0.73

9.20
8.91
8.67
8.48
8.32
8.19
8.08
7.99
7.91

14.72
14.55

0.73
0.72

7.84
7.78

61

Q livarlate Fit o f Mean(Hogfuel (kg/h)) By Total heat
1 0 0 0 -i

/■ .‘I .

£ ot 700
« 600-

* • *

•

< ■ :■,

.

iEv;
A ' •„" • * * * , * • • . • .
f- •• ;: a ... .* .•. • •
200

r-'T—T'"7"'1l" Tr-,"7- , | 1 f , | ,—| I \ I ;■

'I | »

1

•

2

3

4

5

6

7

8

9 10 11 12 13

14 IS 16

Total heat
B - —-Transformed Fit Sqrt ]
T l l T r a n s f d r r ^ f f T O ''! ^ r t ' ...... .............. ....... __ ’

.......

Sqrt(Mean(Hog fuel (kg/h») ^ 1^421661 + l348944*TotaI
heat
.......

RSquare
RSquare Adj
Root Mean Square Error
Mean of Response
Observations (or Sum Wgts)
► ila c jr a r R t

0.702773
0.7027S
2.144S37
22.12781
12792

1

▼fAnaTysisofVariance
Source
Model
Error
C. Total

OF
1

12790
12791

Sum Of
Squares Mean Square
139080
139080.08
4,599039
S8821.70
197901.78

* Param eter Estimates
Term
Intercept
Total heat

Estimate
12.421661
1.348944

F Ratio
30241.12
Prob > F
0 .0 0 0 0 *

□

Std Error t Ratio Prob>|t|
0.0S8947 210.72 0.0000*
0.0077S7 173.90 0.0000*

62

▼HBivariate Fit o f Mean(Total Air (kg/hr)) By Mean(Hog fuel (kg/h))
lOOOO'i

§

E

c
1$

9000*!
800070006000-i
5000*:
40003000*:
2000 1 0 0 0 -i
0 *i

*

•

*.

.

.

* *•

;

.

.

•

* •«

' • . . <•,# * ! i . s'*
* ‘

•

"' V , •V '•

;K
■(•••*

I

r .,

100

,

200

,

|

t >

300

,

|

f

,

400

f

|

|

500

!

v |

* i*

r*.,*
•I. . •" » *•

£if v-

r

,

t < .* V«*»

r

r

600

r

p

r

700

|

800

i

f

i

|

Mean (Hog fuel (kg/h))
s E3 — U n e ar Fit j

▼Linear Fit
Mean(Tota! Air (kg/hr)) * 560.85391 +
7.1633518*Mean(Hog fuel (kg/h))

▼
I Summary of Fit
RSquare
RSquare Ad)
Root Mean Square Error
Mean of Response
Observations (or Sum Wgts)

0.727421
0.7274
878.69SS
4052.061
132S8

►Lack Of Fit 1
▼Analysis of Variance
S um o f

Source
Model
Error
C. Total

OF
Squares Mean Square
1 2.7314e+10 2.731e+10
132S6 1.023Se+10 77210S.84
132S7 3.7549e+10

F Ratio
3S37S.74
Prob > F
0.0000*

▼Parameter Estimates
Term
Intercept
Mean(Hog fuel (kg/h))

Estimate Std Error
S60.8S391 20.06942
7.1633518 0.038086

i

t

i

|

900 1000 1100

t Ratio Prob>|t|
27.9S <.0001*
188.08 0.0000*

T 0 Bivariate Fit o f Mean(Boi!er Flue Gas Temp (°C)) By Total heat

••

180

>■

> « £140“

! *3
j
120
“

!

100“

i

80-

Total heat

0 — *Linear Fit

T'tineaf'Fit................................................
Mean(Boiler Flue Gas Temp C O )« 92.890192 +
6.178879*Total heat

T Summaryof lit
RSquare
RSquare Adj
Root Mean Square Error
Mean of Response
Observations (or Sum Wgts)

0.694381
0.694357
10.02084
137.3495
12792

►Lack Of Fit ]
Analysis o f Variance
Source
Model
Error
C. Total

OF
1

12790
12791

Sum of
Squares Mean Square
2918075.2
2918075
1284336.7
100
4202411.9

F Ratio
290S9.S0
Prob > F
0 .0 0 0 0 *

T Param eter Estimates
Term
Intercept
Total heat

Estimate Std Error
92.890192 0.27S44S
6.178879 0.036246

1
t Ratio Prob>|t|
337.24 0.0000*
170.47 0.0000*

H eat output unit conversion:

QtoUnre* I MW] =

1

(14)

The gasifier output range is approxim ately 3 to 13 G J/hour (0.8 to 3.6 M W ). All o f the follow ing
calculations were over this range o f gasifier output.

Biom ass input [kg/hour] as a function o f gasifier heat output [GJ/hour]:
in b io m a s s ,in

= (1 2 .5 3 4 5 5 5 + 1 .3 3 5 1 2 9 5 Qdeu vered) 2

(15)

A ir input [kg/hour] as a function o f biom ass input [kg/hour]:

n ^ b io m a s s ,in

5 6 1 .7 6 6 4 9 + 7 .1 4 7 4 4 9 6 m b io m a s s ,i n

(16)

Flue gas tem perature [°C] as a function o f gasifier heat output [GJ/hour]:
Tfme = 9 2 .8 9 0 1 9 2 + 6 .1 7 8 8 7 9 Qdelivered

(17)

Flue gas tem perature conversion:
Tfiue[K] ~ Tfiue[°C] + 2 7 3 .1 5

(18)

B ioenergy system yield [GJ heat delivered per tonne biomass]

= 1 “ “'" “ 1 .0 0 0
nLb io m a ss,in

(1 9 )

B ioenergy system yield [GJ heat delivered per G J biom ass [HHV]]

t'™ > '= £!^

2£

<2 0 >

The bioenergy system yield decreases at low gasifier output as illustrated below:

65

16
14
TJ

12
5?
£> *«>

10

=3 I

73 o

8

b 2
a

6
4

2

2

2

3

3

Boiler Output (MW)
F igu re A 3 - R elation sh ip b etw een H ea t O u tp u t and B ioen ergy System Y ield .

A ir to biom ass ratio:

■ ^air/biom a ss

^biomass, in

(21)

This is the ratio o f total air input to total biom ass input.

Table A4 on the follow ing page has the results o f the gasifier m ass and energy balances

66

Table A4 - Gasiller Mass & Energy Balances.

C in
(kg/hour)

Heat output
(GJ/hour)

Hin
(kg/hour)

O in
(kgltour)

N in

ash in

C 0 2 out

total H 2 0
out

ash out

(kg/hour)

(kg/hour)

(kgfliour)

0 2 out
(kg/hour)

N 2 out

(kg/hour)

(kg/hour)

(kgftiour)

mo.i„

mN.in

tt\ish.in

1rl02,out

ttV2.out

ttllsh.out

O 2 in flue H20 in flue stoichiometric
air (kg/hour)
gas (%)
gas (%)

Y02

Y h20

excess air
ratio

X

IT1C02,out

HtaO.out

3

134.9

17.0

693.2

1919.9

11.8

494.5

335.0

197.9

1919.9

11.8

7.19

0.18

1663.05

1.51

4

157.5

19.8

787.8

2170.4

13.7

577.6

391.3

209.3

2170.4

13.7

6.73

0.18

1942.37

1.46

Qdelivercd

mc.in

akir.stoich

5

181.9

22.9

889.7

2440.4

15.9

667.1

451.9

221.5

2440.4

15.9

6.34

0.19

2243.37

1.43

6

208.1

26.2

999.0

2729.7

18.2

516.9

234.7

2729.7

18.2

2566.04

236.0

29.7

1115.6

3038.6

20.6

586.3

248.7

3038.6

20.6

6.00
5.72

0.19

7

763.0
865.4

0.19

2910.38

1.39
1.37

8

265.7

33.4

1239.5

3366.8

23.2

974.3

660.1

263.6

3366.8

23.2

5.47

0.20

3276.40

1.35

9

297.1

37.4

3714.5

25.9

1089.5

738.2

279.4

3714.5

25.9

5.25

0.20

330.3

41.5

4081.6

28.8

1211.3

820.6

296.1

4081.6

28.8

5.07

0.20

3664.08
4073.44

1.33

10

1370.8
1509.4

11

365.3

45.9

1655.3

4468.2

31.9

1339.4

907.5

313.6

4468.2

31.9

4.90

0.20

4504.48

1.30

12
13

402.0
440.5

50.6
55.4

1808.6
1969.2

4874.2

35.1
38.4

1474.1

998.7

332.1

4874.2

35.1

4.76

0.20

4957.18

1.29

1615.1

1094.2

351.4

5299.6

38.4

4.63

0.20

5431.56

1.28

5299.6

1.31

Mass flow of carbon into gasifier [kg/hour]:
W c.in = ^-biom ass, in^C

(2 2 )

M ass flow o f hydrogen into gasifier [kg/hour]:

(23)

m H,in = m biomass,in^H

M ass flow o f oxygen into gasifier [kg/hour]:

m 0 in — ^ -b io m a s s ,in X 0 + 0.21 Wlair,in M

(24)

m air

Oxygen input includes the oxygen content o f the biom ass and oxygen content o f the air. Oxygen
content o f w ater is not included in the calculation since the w ater does not react to oxygen.

M ass flow o f nitrogen into gasifier [kg/hour]:

™N>in = 0.79 m airiin ^

Mair

(25)

N itrogen input is from the air (biom ass N content is ignored).
M ass flow o f ash into gasifier [kg/hour]:
m -a sh ,in ~

m b io m a s s ,in -^ a s /l

(26)

M ass flow o f CO 2 out o f the oxidizer [kg/hour]:
MC02

™-C02,out = ™Cli n - ^ r

(27)

All o f the carbon input w ith the biom ass is converted into CO 2 . Carbon content o f ash is ignored,
as is CO 2 content o f input air. I f biom ass is 5% ash, an this ash has 20% carbon (both high
estim ates), then carbon lost with the ash is 1%.

68

Mass flow of H 2 O out of the oxidizer [kg/hour]:
MCwb 1

________

H20 ,o u t ~ ^ -b io m a s s ,in i- M Q wb

MH20

^o\

™ H ,in

K ^°)

W ater out in the flue gas is equal to the w ater com ing in w ith the biom ass (m oisture content) plus
the w ater form ed from hydrogen oxidation during com bustion.

M ass flow o f O2 out o f the oxidizer [kg/hour]:

m 0 2,o u t = m O,in ~ ( j n C02lo u t ~ m C ,in ) ~ ( j* 1H ,in

m H ,in J

(2 9 )

Oxygen out in the flue gas is equal to the oxygen in (oxygen in biom ass + oxygen in air), m inus
the oxygen that is converted into CO 2, m inus the oxygen that is converted into H 2O.

M ass flow o f N2 out o f the oxidizer [kg/hour]:

™-N2,o u t ~

N2,in

(3 0 )

M ass flow o f ash out o f the gasifier & oxidizer [kg/hour]:
(31)

m a s h ,o u t ~ m a s h ,in

M ole fraction O 2 in flue gas (dry basis):
m 02> O U t

v

2

~ ^ T

(32)

mC02,out m0 2,out mN2,out
MC02
m02
m N2

M ole fraction H 2O in flue gas:
m H 2 Q ,o u t

y

— _____________mh2o_____________
2^

m

C 0 2 ,O U t

m C02

m 0 2 ,O U t

m N 2 lO U t

m H 2 Q ,O U t

m o2

mn2

m H20

'

^

69

Stoichiometric air requirement [kg/hour]:
•m

-

„

(2Xc

,J j£ _ _ X o \

™-air,stoic/t~ rnbiomass,in{Mc ^ 2Mh

Mair

Mq) 2x 0.21

l

J

2 m oles 0 required for every m ole o f C, 1/2 m ole O for every m ole o f H, m inus 1 m ole o f 0 for
every m ole o f 0 already in the biom ass. D ivided by 2 to convert m ole O to m ole 0 2.

Excess air ratio:
X=

g a is ia mair,scoic/i

(35)

T ab le A S - C om b u stion C a lcu la tio n s, G a sifier System E n erg y B alan ces.

Heat output
(GJ/hour)
Q d c liv c r e d

3
4

C02
(kJ/hour)

H 20
(kJ/hour)

02
(kJ/hour)

Q c02

Q h20

Q o2

Q n2

-37904

-54488
-68225

-15881
-18007

-173105
-209712

-47581

N2
(kJ/hour)

flue gas
flue gas latent
flue gas
flue gas latent Other tosses
sensible heat sensible heat heat toss
(GJ/hour)
heat loss (%) (%)
loss (GJ/hour) loss (%)
Q s c n s ib ie

^ s e n s ib le

0.28
0.34

5.07%

Q la te n t

5.30%

^ la te n t

0.76
0.88

Q o th e r

13.66%
13.66%

27.17%
19.28%
13.95%
10.42%

5
6

-58798

-84102

-20348

-251566

0.41

5.55%

1.02

13.66%

-71677

-102274

-22919

0.50

7

-86339

-25738

0.59

5.80%
6.05%

1.17
1.33

13.66%
13.66%

8
9
10

-102905
-121498
-142243

-122899
-146137

-299049
-352541

-172144

-28820
-32180

-412423
-479075

-35835

-552881

6.32%
6.59%
6.86%

1.49
1.67

-201082

0.69
0.80
0.93

11

-165264

-233108

-39801

-634223

1.07

7.14%

12

-190689

-268385

-44094

-723483

1.23

7.42%

13

-218643

-307072

-48730

-821046

1.40

7.71%

2.26
2.47

flue gas total
heat toss
(GJ/hour)
Q flu c

1.04
1.23
1.44

8.15%

1.66
1.91

13.66%
13.66%

6.79%
6.09%

2.18
2.47

1.85

13.66%

5.85%

2.79

2.05

13.66%

5.96%

3.12

13.66%
13.66%

6.32%
6.85%

3.48
3.87

Flue gas energy content for each gas [kJ/hour]:

Q„as = ^
f

( “ .(T V .,) + f ( T r e f f + f [ T „ , f + * (Tre/f + f { T „ , f + a 6 - n ( f „ J -

CJ/h^) - ^ ( T f l u e )

~

a,J) TYo

This form ula calculates the therm al energy content o f each gas com pared to a reference
tem perature (energy content at the reference tem perature is defined as 0).
•

a l through a6 are constants specific for each gas.

70

Table A6 - Constants for Enthalpy Calculations.
E nthalpy C on stan ts

a l( T )

a2 (TA2)

a3 (TA3)

a4 (TA4)

a5 (TA5)

-7.12356E-06

2.45919E-09

-1.437E-13

-48371.9697

6.5204E-06 -5.48797E-09

1.77198E-12

-30293.7267

a6

C02

2.35677352

0.0089846

H 20

4.19864056

-0.0020364

S02

2.911438

0.00810302

-6.90671E-06

3.32902E-09

-8.77712E-13

-36878.81

02

3.78245636

-0.0029967

9.8473E-06

-9.6813E-09

3.24373E-12

-1063.94356

N2

3.298677

0.00140824

-3.96322E-06

5.64152E-09

-2.44485E-12

-1020.8999

Flue gas sensible heat loss [GJ/hour]:

n

v sensible

_ <?co2+Qff2o+<?02+<?N2
1,000,000

Sensible heat loss calculates the heat loss due to the flue gas being w arm er than the fuel input
and the surroundings. The am ount o f sensible heat loss depends on the reference tem perature,
w hich was taken as 25 °C (this is a conservative estim ate, heat loss will be higher using lower
reference tem peratures).

% Flue gas sensible heat loss [GJ/hour]:

y »nsiite =

nLb io m a ss ,m n n v

1,000 X 100

(38)

Flue gas latent heat loss [GJ/hour]:

Qlatent = rll^

i ^ la te n t

(39)

This form ula calculates the energy loss for w ater leaving the system in vapour state (as opposed
to liquid). For w ater, AHiatent= 2.26 GJ/t.

% Flue gas latent heat loss [GJ/hour]:

W

= i.o o o x io o
mbiomass.in™Hv

(40)

71

Unaccounted for heat loss [GJ/hour]:
Qother = 1

Qdellvered 1,000 - Sensible _ Qlatent
™-biomass,inHHV
Qdelivered Qdelivered

(4 ])

Q o th er ~ 1

Yh h v

(4 2 )

Vsen sib le

^ la te n t

Q other includes radiant heat loss and uncertainties in heat loss calculations.

Flue gas total heat loss [GJ/hour]:

Q f l u e = Q s e n s lb le 4" Q l a t e n t

(4 2 )

The m ajority o f the heat loss is due to latent heat loss, especially at higher hog fuel m oisture
content.

72

Table A7 - Combustion Calculations, Cooled Flue Gas (40°C).
Flue gas

Water
partial
pressure

Water
saturation
Condensation pressure

Portion o f
water that

Flue gas
latent heat
capture

T(°C )

(mmHg)

condenses

(GJ/hour)

capture
(GJ/hour)

% o f flue gas
heat captured

% Efficiency Improvement

P re o

" O o tid

i^ H 2 0 .sa !

^ H 2 0 ,c o n d

Q l a l e n t .c o o l

Q f l u e .c o o l

Q f l u c .c o o l / Q f l u c

Q f l u e . c o o i / ( Q f l u e , c o o i " ^ Q d e liv e r e d )

135.23
138.93

57.86

55.33

0.591

0.45

0.68

65.47%

58.43

55,33

0.602

0.53

0.82

66.79%

18.5%
17.0%

0.610

0.62

0.98

67.92%

16.3%

0,617
0.623

0.72
0.83

1.15
1.33

68.90%
69.77%

16.0%
16.0%

0.628
0.632
0.636

0.94
1.05
1.18

1.54
1.76
2 .0 0

70.55%
71.26%
71.91%

16.1%
16.4%
16.7%

55.33

0.639

1.31

2.26

72.51%

17.1%

55.33
55.33

0.641
0.643

1.45
1.59

2.55
2.85

73.06%
73.58%

18.0%

Heat
output

Cooled
fluegasT

(GJ/hour)

(K)

C 02
(kJ/hour)

H 20
(kJ/hour)

sensible heat
capture
N2
02
(kJ/hour) (kJ/hour)
(GJ/hour)

Q d e liv e r e d

T flu e .c o o l

Q c 0 2 .c o o l

Q h 2 0 , coo!

Q o 2,c o o l

Q n 2 . coo 1

Q s c n s ib le .c o o l

3
4

313

-45015

0.23

-57161

-13124
-15092

-142902

313

-31527
-40132

-175568

0.29

5

313

-50195

-71322

-17262

55.33

-61837
-75178

-87657
-106321

-19651
-22274

0.35
0.43
0.51

58.91

313
313

-213176
-256107
-304741

142.03

6
7

144.66
146.89

59.30
59.63

55.33
55.33

8
9

313
313

-90340
-107447

-127473
-151272

-25148
-28288

-359458
-420641

148.80
150.44

313

-126622

-177878

-31711

-488672

151.87

59.91
60.14
60.35

55.33

10

0.60
0.71
0.82

55.33
55.33

11

313

-147990

-207449

-35433

-563933

0.95

153.12

60.53

12

313
313

-171679
-197814

-240146
-276131

-39469
-43836

-646806
-737677

1.10
1.26

154.21
155.17

60.68
60.81

13

(mmHg)

All calculations based on fuel with a moisture content of 40% and cooling the flue gas to 40°C

UJ

Flue gas
total heat

17.5%

Table A8 - Combustion Calculations, Cooled Flue Gas (55°C).

Heat
output
(GJ/hour)

Cooled
flue gas T

Flue gas

Water

Water

sensible heat

partial

saturation

pressure
(mmHg)

(1C)

C 02
(kJ/hour)

H 20
(kJ/hour)

02
N2
(kJ/hour) (kJ/hour)

capture
(GJ/hour)

Q d e liv e r e d

T flu c .c o o l

Q c 0 2 .e o o l

Q h 2 0 . co o 1

Q o 2 .c o o i

Q n 2 . coo I

Q s e n s ib le .c o o l

3
4

313

-45015

0.23

-57161

-13124
-15092

-142902

313

-31527
-40132

-175568

0.29

5

-50195
-61837
-75178

-71322

-17262

7

313
313
313

-87657
-106321

-19651
-22274

-213176
-256107
-304741

0.35
0.43
0.51

6

Condensation

■t*

Flue gas

Flue gas

latent heat

total heat

T (°C )

pressure
(mmHg)

condenses

capture
(GJ/hour)

capture
(GJ/hour)

% offlue gas
heat captured

% Efficiency Improvement

PH20

ic o n d

P H 2 0 .s a t

^ H 2 0 .c o n d

Q l a t e n t .c o o l

QfluC.COOl

Q f l u e .c o o l / Q f l u e

Q f l u e .c o o l /( Q f l u e .c o o l " * ~ Q d e liv e r e d )

135.23
138.93

57.86

0.591

0.45

0.68

65.47%

18.5%

58.43

55.33
55.33

0.602

0.53

0.82

66.79%

17.0%

142.03

58.91

55.33

0.610

0.62

144.66
146.89

59.30
59.63

55.33
55.33

0.617
0.623

0.72
0.83

0.98
1.15
1.33

67.92%
68,90%
69.77%

16.0%
16.0%

70.55%
71.26%

16.3%

8

313

-90340

-127473

-25148

-359458

0.60

148.80

59.91

55.33

0.628

0.94

1.54

9
10

313
313

-107447
-126622

-151272
-177878

-28288
-31711

-420641
-488672

0.71
0.82

150.44
151.87

60.14
60.35

55.33
55.33

0.632
0.636

1.05
1.18

1.76
2.00

71.91%

16.1%
16.4%
16.7%

11

313

-147990

-207449

-35433

-563933

0.95

153.12

60.53

55.33

0.639

1.31

2.26

72.51%

17.1%

12

313

-171679

-240146

-39469

-646806

1.10

60.68

55.33

0.641

313

-197814

-276131

-43836

-737677

1.26

60.81

55.33

0.643

1.45
1.59

2.55

13

154.21
155.17

73.06%
73.58%

17.5%
18.0%

All calculations based on fuel with a moisture content of 40% and cooling the flue gas to 55“C

■vj

Portion o f
water that

2.85

H eat exchanger energy capture is from 1) cooling the flue gas (sensible heat capture); and 2)
condensing the w ater vapour (latent heat capture). For latent heat capture, the am ount o f water
that will condense in the heat exchanger needs to be calculated.

Potential sensible energy recovery for each gas [kJ/hour]:

Q g a s, cool = Mgas ( a l ( T f l u e . c o o l ) "F ~
f l u e ,c o o l )

( j'flu e , c o o l )

+ a- 6 ~ a l ( T f i u e ) — - ^ ( j f i u e )

flu e,co o l)

~ ~ (f//u e )

"b “

f t flu e,co o l)

~ ~ ^ (jflu e )

+

~ ~ ^ (jflu e )

a 6) —

~

(44)

1,000

v

'

This form ula calculates the difference in therm al energy content o f each gas betw een the original
(current) flue gas tem perature and the flue gas tem perature after a heat exchanger. Tem peratures
m ust in in Kelvin.

Potential total sensible heat recovery [GJ/hour]:
n

—Q c 0 2, c o o I + Q h 20 , c o o I + Q o 2, c o o I + Q n 2.c o o I

^ s e n s ib le ,c o o l ~

j-.o

j 000 000

W ater vapour pressure in flue gas [mm Hg[:
Ph2o = ^H2oPfiue x 760

(46)

Pjiue is the pressure o f the flue gas, and is assum ed to be 1 atm (760 mm Hg).
Tem perature at which w ater vapour w ill condense [°C]:

'c o n d ~ a _ l OG(PH2o )

(47)

P m o m ust be in m m Hg, see below for constants A, B and C.
Saturated w ater vapour pressure at cooled flue gas exit tem perature [mm Hg]:

75

(48 )
A, B, and C are constants, and depend on the final tem perature o f the flue gas (Tj]ue cooi). Tjiue cooi
m ust be in °C. If the flue gas is cooled below Tcond, then some or all o f the w ater vapour will
condense.
T ab le A 9 - A n toin e C o n stan ts fo r S atu rated V a p o u r P ressu re and C on d en sation T em p era tu re C alcu la tion s.

0 to 60 °C

60 to 150 °C

A

8.10765

7.96681

B

1750.286

1668.21

C

235

228

Antoine Constants

Fraction o f w ater vapour that w ill condense:

I F T flu e,cool > Tcond

cond

-» * h 2o = 0 ( n o c o n d e n s a t i o n )

x H20

(49)

(50)

As the flue gas is cooled below the w ater vapour condensation tem perature, w ater vapour will
start to condense. The further the flue gas is cooled, the greater the am ount o f w ater that will
condense.

Potential total latent heat recovery [GJ/hour]:
n
V la te n t, coo I ~ x Hz O

rnH20out
i 000

L^n la t e n t

(51)

Potential flue gas total heat recovery [GJ/hour]:

Q flu e ,c o o l ~ Q sen sible,cool T Q la te n t,co o l

(52)

Potential flue gas total heat recovery [%]:
Qflue,cool 2 0 0
Qflue

( 53 )

76

The heat captured using a heat exchanger com pared to the total heat loss in the flue gas w ithout a
heat exchanger.

Flue gas heat capture as a function o f available cooling w ater tem perature:
D epending on the tem perature o f the heating load, not all o f the heat captured from the flue gas
can be used. In order to capture the m axim um am ount o f heat from the flue gas, the cooling
water needs to be as cold as possible. If the flue gas needs to be cooled below the tem perature o f
the external heat load, then a significant fraction o f the captured heat will need to be dum ped in a
cooling tow er (or greenhouse).

Fraction o f captured flue gas used for heating:

From flu e g a s h e a t e x c h a n g er to h e a t load

T hot

H e a t load

From h e a t lo a d

T cool
C o o lin g

T o flu e g a s h e a t ex ch a n g er

I#

T cold

Total energy captured from flue gas heat exchanger:
Qflue,cool ~ ^-cooling water^C^HOT '~ T cold )

(54)

T h o t is the tem perature o f the hot w ater leaving the counter current flue gas heat exchanger.

For

m ost calculations, T h o t is assum ed to be 85 °C T h o t needs to be below the incom ing tem perature
o f the flue gas (120 °C).

77

T c o ld

is the tem perature o f the w ater returning from the cooling tow er (or greenhouse) to the flue

gas heat exchanger. T c o l d is varied from 55 to 15 °C. The flue gas exit tem perature ( T j] ue cooi ) is
assum ed to be 5 °C w arm er than T c o l d (a 5 °C tem perature difference is required to drive the
heat transfer from the flue gas to the w ater loop).

Energy delivered to heat load:
Q u s e d — ttl co olin g w a t e r c ( T h o t ~ T c o o l )

Tcool

(55)

is the tem perature o f the w ater leaving the heat load (e.g. the residences). For m ost

calculations, T Co o l is assum ed to be 55 °C. A typical space heating load (heating a room to 20
°C) will return the cooling w ater at 25 to 30 °C. A hot w ater heating load (60+ °C) will return
cooling w ater at 65 to 70 °C.

Fraction o f captured flue gas heat used for heating:
Q used

_

Q flu e ,cool

O ’H O T - T c O O l )
(J h o t - T c OLd )

U sable flue gas therm al energy [GJ/hour]:
f)

—n

Reused “ V J
flue,cool

(THOT-TCOOL)

(cn\

7, cold)x
U HOT-T

w ')

U sable flue gas therm al energy [kW]:

n

—n

(ThOT-TcOOl)1.000

Vused - Vfluexool ( T h o t - t c o l d )

3.6

/rm
^

The captured heat from the flue gas (Qjiue.cooi) increases as T c o l d decreases - the m ore the flue
gas is cooled the greater the energy capture (solid line in Figure A4). H ow ever, the fraction o f

78

useful heat (dashed line in Figure A 4) increases as T c o l d approaches T c o o l (decreasing the need
to waste heat in a cooling tower). The system can be optim ized to find the m axim um Qused by
perform ing the calculations at a range o f flue gas exit tem peratures. In Figure A5, w hich has
calculations for m axim um heat output at 40% hog fuel m oisture, the optim um tem perature to
m axim ize flue gas heat utilization is 40 °C. This was obtained by m ultiplying the tw o curves
from Figure A 4 (the total heat captured tim es the fraction o f the heat that is usable).

% U sed fo r H e a tin g

'T o ta l H e a t C a p tu r e (G J /h o u r )
Hi
63
O
<u
J3
5

100

4

90

3.S

st

-t-t

st

a>
w

3

70
60

5 b’

50
40

1.5

IX

+*
st
Si

3

30

1
20
0.5

£

T3

a>

80

o
2.5
tin
t*—
i
"O t.
a
u
2
a
•t-t
o
a
U

«2

10

I f
5* «

u pq
Cm
O

S

o
u
63
U
fa

0
15

25

35

45

55

65

F lu e g a s tem p e r a tu re (°C )

Figure A4 - Correlation between Total Heat Captured, Flue Gas Temperature and Usable Heat (for 40%
moisture fuel).

79

500 i
450 400 350 300 -

«u 0w 200

-

150 100 -

F lu e g a s tem p era tu re (°C )

F igure A 5 - U sab le H ea t C ap tu red as a F u n ction o f F in al F lu e G as T em p eratu re. F o r 40% m oistu re co n ten t
fu el (T h e m axim u m usab le h ea t is o b tain ed if the flue gas is cooled to 40°C ).

80

Highlights

• First university owned district heating system using
biomass heat
• Capacity: 15 MMBtu/hr
• Fuel: bocal wood residue
• Integrated research laboratory
• LEEDGold bulldhg

Performance

Research

• Avoided: 3600 tomes of COs
• Paniculate: less than 10 mg/hi3(contract)
• Operational: January 2011

• Btergy balance
• Syngas composition

Biomass Gasification
Main Cam pus District Heating
Heat to campus
district energy
loop

Sawmill
Residue

Fly Ash

Goals
To displace 85% of natural gas used for cons campus
heating.
To demonstrate syngas production and biomass
cempus heathg

Bottom ash

Statistics
6.000 geen tonnss/yr
Fuel:
Fuel moisture content: bp to 60%
Heat:
80.000 GJ/Vr
Coital cost:

Exhaust
Stack

$15.7 M

Components

Partners

4.3 m diameter gasifier
4.4 MWflue gas boiler
601hog fuel storage
Electrostatic precipitator
1400 m! bioenergy building

UNBC
Negderra System s Corp.
Knowledge Infrastructure Rogram

Public Sector Energy Conservation Agreement
Innovative Clean Energy Fund

A p p e n d ix C
Cam pus M ap

A ppendix D
H eat D uration Curves, G asifier O utput V ariability and Therm al Storage
H eat duration curves are a useful tool to analyze data that has a significant degree o f process
variation, and are often used to size biom ass heating system s. Heat duration curves are
dem onstrated using cam pus heat dem and data in Figure D1 and Figure D2. In Figure D l, the
daily heat dem and is graphed chronologically. The cam pus heat dem and increases in the winter,
and is highly variable (Figure D l). To construct a heat duration curve, the heat dem and data is
sorted from highest to low est and graphed versus tim e (Figure D2). The heat duration curve is
used to determ ine the num ber o f days that the gasifier is operating at high output versus low
output. By sim ply looking at the raw data points in Figure D l, it is difficult to extract the same
information.

6
5
4

3

2
1
0

Day of year
F igu re D l —D aily C am p u s H eat D em and over a 1 year T im e P eriod .

83

6

5

4

a 3
C3

£
o
©

2

C3
tS
1

0
120

150

180

210

240

270

300

330

360

T im e (d a y s)

F igu re D2 - H eat d u ration cu rve ex a m p le, u sin g th e sa m e d aily total cam p u s h ea t d em an d d ata p resen ted in
F igu re D l.

One use for heat duration curves is for sizing biom ass heating system s. D ue to lim ited turndow n
ratios, biom ass heating system s tend to have a small range over w hich they can be efficiently
operated. W hen looked at on an hourly basis, cam pus heat dem and can be as high as 6 M W . A 6
M W biom ass heating system w ith a 2.5 turndow n ratio is capable o f producing from 6 M W to
2.4 M W . This system w ould not run efficiently during the sum m er m onths and w ould cover only
64% (shaded area o f chart) o f the total cam pus heat dem and (Figure D3). A sm aller heating
system w ould be needed during the sum m er months. In com parison, a 3 M W biom ass heating
system w ith a 2.5 turndow n ratio w ould cover m uch m ore o f the cam pus heating needs (Figure
D4). This system would need supplem ental heating on cold days (heat dem and > 3 M W ), but
over the year w ould provide 94% (shaded area o f chart) o f the campus heat dem and. A 3.5 M W
system w ith a 2.5 turndow n ratio would supply 97% o f the cam pus heating needs (not shown).

6 i

5 -

0

30

60

90

120

150

180

210

240

270

300

330

360

T im e (d ays)

F igu re D 3 — H eat D uration C u rve for 6 M W system an d 2.5 T u rn d ow n . T he d ark er sh ad ed area rep resents
the heat d em an d covered by a 6 M W biom ass h eatin g system w ith a 2.5 tu rn d ow n ra tio .

5 -

T im e (d ays)

F igu re D 4 - H eat D uration C u rve fo r a 3 M W system an d 2.5 T u rn d ow n . T he d a rk er sh ad ed area rep resen ts
the h eat d em an d covered by a 3 M W biom ass h eatin g system w ith a 2.5 tu rn d ow n ratio.

85

The campus heat dem and graphed in Figure D l is supplied by the N exterra gasifier system and
by the older natural gas boilers (Figure D5). There were days in the shoulder season (October)
when the bioenergy system was not operating w ell, and there were days in the fall through spring
when natural gas was used to supply heat above w hat the bioenergy system was supplying
(Figure D5). I f the data is sorted using the total heat dem and, from highest to lowest, a heat
duration curve is obtained (Figure D6). The heat duration curve in Figure D6 is sim ilar to the
theoretical 3 M W bioenergy system graphed in Figure D4, but w ith m ore variability. The large
deviations o f the biom ass heating system curve from the total heat dem and curve are due to the
days o f low biom ass system output in October.

6 i
£

W

m

m

m

D a y o fY ea r

Figure D 5 - D aily C am p u s H eat D em an d over a 1 year T im e P eriod S h o w in g V aria b ility . T h e sh ad ed area
below th e solid lin e rep resen ts th e h eat supplied by th e U N B C b ioen ergy system (gasifier). T h e sh ad ed area
ab ove the solid line rep resen ts th e h eat d em an d su p p lied by n atu ral gas.

86

If the data is sorted using the gasifier heat output, from highest to lowest, a heat duration curve is
obtained (Figure D7). The m axim um gasifier output on a daily basis is approxim ately 3.3 M W ,
w ith a noticeable range betw een 2.1 to 3.0 M W covering the high heating dem and season. The
gasifier output in Figure D7 represents 85% o f the total cam pus heat dem and (area under the
solid line com pared to the total shaded area). This com pares to 94% theoretical for a 3.0 M W
bioenergy system . The heat dem and during the low gasifier output on the far right side o f the xaxis in Figure D7 is supplied by natural gas. This natural gas represents 4% o f the heating
dem and that could otherwise have been covered by the gasifier, had it been operating effectively.

6 -i
£

0

30

60

90

120

150

180

210

240

270

300

330

360

Time (days)
F igure D 6 - H eat D u ration C u rve for 1 y ea r tim e p eriod . U sin g th e sam e d aily total cam p u s h ea t d em an d an d
b ioen ergy system d ata p resen ted in F igu re DS. A rea below th e solid lin e rep resen ts h ea t su p p lied by the
bioen ergy system . S h aded area ab ove the solid line rep resen ts h ea t su p p lied by n atu ral gas.

87

If hourly data is used in place o f daily data, m uch greater variability is seen (Figures D 8 and D9).
It appears to be com m on for the bioenergy system output to decrease dram atically for an hour or
several hours at a tim e (Figure D8). The heat duration curve for the UNBC bioenergy system
(Figure D9) is different from the theoretical 3 M W bioenergy system represented in Figure D4,
w ith less o f the total heat dem and covered by the actual system due to the large variability in
output. Sim ilar to Figure D7, the hourly data was sorted based on gasifier output (Figure DIO).
The results are sim ilar to those described in Figure D 7, with the exception that the hourly gasifier
output is slightly higher reaching 3.8 M W .

6 -i
S'

T im e (d ay s)

F igure D7 - H eat D u ration C u rv e fo r 1 y ea r T im e P eriod . U sing th e sam e d aily total cam p u s h ea t d em an d
an d b ioen ergy system d ata p resented in F igu re D 5. A rea b elow th e solid line rep resen ts h ea t su p p lied by the
b ioen ergy system . Sh ad ed area ab ove the solid line rep resen ts heat su p p lied by n atu ral gas. S o rt based on the
b ioen ergy system o u tp u t.

88

6 i

Hours of Year
F igu re D 8 - H o u rly C am p u s H ea t D em and over a 1 Y ea r T im e P eriod. T he sh ad ed area b elow the solid line
rep resents th e h ea t su p p lied by th e b ioen ergy system . T h e sh ad ed area ab ove th e solid line rep resen t th e h eat
d em and sup p lied by n atu ral gas.

89

6 i

/•"s
£

mm m
0

750

1500 2250 3000 3750 4500 5250 6000 6750 7500 8250
T im e (h o u r s)

F igu re D 9 - H eat D uration C u rve for H ou rly H eat D em and D ata from F igu re D 8. A rea b elow th e solid line
rep resen ts h ea t su p p lied by the b io en erg y system . S h ad ed area a b ove th e solid line rep resen ts h ea t su p p lied
by natu ral gas. W h ite line is a m ovin g a v era g e o f the b ioen ergy system ou tp u t.

90

750

1500 2250 3000 3750 4500 5250 6000 6750 7500 8250
T im e (hours)

F igu re DIO - H eat D u ration C u rve fo r 1 Y ear T im e P eriod . U sin g th e sam e d aily total cam p u s h eat d em and
and bioenergy system d ata presen ted in F igu re D 8. A rea below th e solid lin e rep resen ts h eat su p p lied by the
b ioen ergy system . S h aded area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas. S o rt based on the
bioen ergy system ou tp u t.

V ariability in G asifier O utput
The hourly heating data presented in Figures D8 and D 9 is difficult to interpret due to the large
variability in heat dem and and in bioenergy system output, and the 1 year tim e scale o f the
graphs. Select days are represented in Figures D l l through D25 for a closer investigation o f the
variability. W hile no definitive conclusions can be m ade, there are several patterns in the
variability that help explain the data presented in Figures D5 to D10.

One com m on occurrence is a m orning spike in heat dem and (Figures D l l to D 20, D24 and
D25). The m ajority o f the increase in m orning heat dem and is usually supplied by natural gas,
w hich appears to have a faster response tim e than the bioenergy system . Som etim es a spike in

91

natural gas usage is followed by a decrease in gasifer output as illustrated in Figures D l l to D14
This m ay be due to the control systems. W hen natural gas adds heat to the m ain w ater loop, the
tem perature o f the w ater in the loop increases and the bioenergy system s responds by lowering
output. These behaviours contribute to the variability in the heat duration curves since high heat
dem and is often m atched w ith low bioenergy heat output.

On som e days both the heat dem and and the bioenergy system output are highly variable
(Figures D15 to D20). The reason for this is unknow n, but appears to be due to operational
problem s w ith the bioenergy system . These operational problem s often seem to follow increases
in bioenergy system output and m ay be due to increases in tem perature in the gasifier leading to
over-tem perature alarm s and subsequent slow downs.

On other days the bioenergy system obviously has operational and/or m aintenance issues and is
turned down for hours at a tim e (Figures D21 to D24). There are also days that the bioenergy
system is not operational at all due to yearly m aintenance (not shown). The final exam ple o f
variability is shown in Figure D 25. Here the bioenergy system has steady output (significantly
below the rated capacity o f 4.4 M W ) and rem ains at this output level all day even as the heat
dem and increases.

Explanations for variability in the heat duration curves include: a non-optim ized control system
betw een the natural gas and bioenergy system s; short-term (< 1 hour) operational issues with the
bioenergy system ; long-term operational issues with the bioenergy system ; and the bioenergy
system often running well below capacity.

92

Tim e o f D ay (hours)

F igu re D l l - C am p u s H eat D em an d for O ctob er 2 5 >h, 2012. A rea b elow th e solid line is h ea t su p p lied by the
b ioen ergy sy stem , area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas.

Tim e o f D ay (hours)

Figure D12 - Campus Heat Demand for October 27th, 2012. Area below the solid line is heat supplied by the
bioenergy system, area above the solid line represents heat supplied by natural gas.

93

99999999999999999

m m z
Time o f Day (hours)

F igu re D 13 - C am p u s H eat D em an d fo r O ctob er 2 8 th, 2012. A rea b elow th e solid lin e is h eat su p p lied by the
b ioen ergy system , area ab ove th e solid line rep resen ts h eat su p p lied by n atu ral gas.

12

16

20

Time o f Day (hours)
Figure D14 - Campus Heat Demand for March 2nd, 2012. Area below the solid line is heat supplied by the
bioenergy system, area above the solid line represents heat supplied by natural gas.

94

w
399999999999999999999999999

fin

mm

•Ti'TT'fH'Tr f f n f r r r r

Time o f Day (hours)
F igu re D1S - C am p u s H ea t D em an d for N ovem b er 10* , 2012. A rea below th e solid line is h ea t su p p lied by
th e b ioenergy system , area ab ove th e solid line rep resen ts h eat su p p lied by n atu ral gas.

m

m

m

m

m

*

m lm
k
mm.
rWmmk-

JHMMH
Time o f Day (hours)

Figure D16 —Campus Heat Demand for November 20th, 2012. Area below the solid line is heat supplied by
the bioenergy system, area above the solid line represents heat supplied by natural gas.

95

s

1

9MM '-mm m
WmKm mmm

W W ®m

«0 m

9

m

9

W

im

m

m
Tim e o f D ay (hours)

F igu re D 17 - C am p u s H eat D em an d for M arch 13th, 2013. A rea below th e solid line is h eat su p p lied by the
b ioen ergy system , area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas.

899999999999999999999

Time o f Day (hours)

Figure D18 - Campus H eat Demand for March 14* , 2013. Area below the solid line is heat supplied by the
bioenergy system , area above the solid line represents heat supplied by natural gas.

96

Time o f Day (hours)

Figure D 19 - C am p u s H ea t D em an d for Jan u a ry 11th, 2013. A rea below th e solid lin e is h ea t su p p lied by the
b ioen ergy sy stem , area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas.

Time o f Day (hours)

Figure D20 - Campus H eat Dem and for December 22nd, 2012. Area below the solid line is heat supplied by
the bioenergy system , area above the solid line represents heat supplied by natural gas.

97

mm
Time o f Day (hours)

F igu re D21 - C am p u s H eat D em an d fo r M arch 14th, 2012. A rea b elow th e solid line is h ea t su p p lied by the
bioen ergy sy stem , area a b ove th e solid line rep resen ts h ea t su p p lied by n a tu ra l gas.

8

12

Time o f Day (hours)
Figure D22 - Campus Heat Demand for March 29,h, 2012. Area below the solid line is heat supplied by the
bioenergy system, area above the solid line represents heat supplied by natural gas.

98

Time o f Day (hours)

F igu re D 23 - C am p u s H eat D em an d for Ju ly 7 th, 2012. A rea b elow th e solid line is h eat su p p lied by the
bioen ergy system , area ab ove th e solid line rep resen ts h ea t su p p lied by n a tu ra l gas.

9999999999991

m m

m

m m .
m m

Time o f Day (hours)

Figure D24 - Campus Heat Demand for February 121 ,2 0 1 3 . Area below the solid line is heat supplied by the
bioenergy system, area above the solid line represents heat supplied by natural gas.

99

Time o f Day (hours)

F igu re D 25 - C am p u s H ea t D em and for D ecem b er 2 6 th, 2 0 1 2 . A rea below th e solid line is h ea t su p p lied by the
bioen ergy system , area ab ove th e solid line rep resen ts h ea t su p p lied by n atu ral gas.

Therm al Storage

One option to decrease variability in the bioenergy system output w ould be to add a therm al
storage system . A therm al storage system is basically ju st a large tank o f water. W hen building
heat dem and is low, hot w ater is added to the top o f the storage tank. W hen building heat
dem and is high, hot w ater from the storage tank is used to help m eet the heat dem and. For the
current UN BC bioenergy system , this m ay address tw o issues. First, short term outages (e.g.
Figures D17 and D18) could be covered by heat in the therm al storage tank instead o f natural
gas. Second, the bioenergy system w ould be able to run at a steady rate (it w ould not be turned
down at night but would be used to fill the therm al storage tank). The increase in m orning heat
dem and could be supplied from the therm al storage tank and the bioenergy system could

continue running at a steady rate. This m ay avoid operational problem s illustrated in Figures D 1 1
to D14. Therm al storage should help sm ooth the bioenergy system output on days sim ilar to
those shown in Figures D15 to D20, how ever this will depend in part on w hat the operational
issues are. In all o f these cases, by im proving operation o f the bioenergy system and by
providing a heat buffer for the m orning dem and, therm al storage should decrease natural gas
consum ption. Unless the tank was very large, therm al storage w ould not help w ith prolonged
bioenergy system shutdow ns shown in Figures D22 to D24. Therm al storage w ould not help with
situations w hen the bioenergy system is running below capacity (Figure D25).

To determ ine the im pact o f therm al storage on natural gas consum ption, a m odel w as used. The
m odel assum es that the bioenergy system can operate at rated capacity indefinitely. W hen heat
dem and is less than the rated capacity, heat is added to storage. W hen heat dem and is greater
than the rated capacity, heat is taken from storage.

The am ount o f heat in the therm al storage system for each 15 m inute interval is calculated as
follows:

The bioenergy system output is assum ed to run at either the heat dem and or at the rated capacity
( if the heat dem and is equal to or greater than the rated capacity)

B IO qut = M IN {HD, 0.9 X B10MW)

(59)

For a 15 m inute interval:
1 M W = 1 M J/s x 60 s/m in x 15 m in x 1 GJ/1000 M J = 0.9 GJ
A dding heat to therm al storage (up to STOm ax):

IF {HD < 0.9 x MWbio a n d IF STO < STOMAX) t h e n STO = STO + (0.9 x MW mo - HD)

(60)

101

Rem oving heat from therm al storage (as long as there is heat available STO > 0):

IF (HD > 0.9 x MWmo and IF STO > 0) t h e n STO = STO + (0.9 x MWBI0 - HD)

(61)

All other cases: STO = STO + 0

W here HD = heating dem and (GJ); M W Bio = rated capacity o f bioenergy system (M W );

STO = heat stored in therm al storage tank (GJ); STO m ax = m axim um heat storage capacity in
therm al storage tank (GJ);

N atural gas is used to m eet high dem and beyond the capacity o f the bioenergy and therm al
storage systems:

IF IF (HD < 0.9 x MWBi 0 and IF STO < STOMAX)

H D - B I 0 OUT- ( 0 . 9 X M W B I O - H D )

N u = --------------------------------------------

(62)

Bng

else (if there is no dem and from storage or if storage is empty):

H D —B I O q u t

NG = ----------- —

(63)

Bn g

W here N G = natural gas consum ption (GJ) and t|Ng = efficiency o f the natural gas boilers
(assum ed to be 0.8).

R esults from therm al storage calculations are presented in Figures D 26 and D27. The calculated
heat duration curves are m uch sm oother than the actual bioenergy system heat duration curve.
This is predom inantly due to the assum ption that the bioenergy system can operate at the rated
capacity. This assum ption has a m uch greater im pact on the results than the size o f therm al
102

storage. The heat duration curves for a m odeled 3.5 M W biom ass system with and w ithout 10 GJ
o f therm al storage alm ost overlap in Figure D26. Both curves are significantly above the actual
bioenergy system heat duration curve (dotted line).

6

M odeled 3.5 MW system w ith 10 GJ therm al storage

5

s
o.
2
4
o
5V

M odeled 3.5 MW system w ith o u tth erm al storage

s
Cl 3
a
-a
e
«

2

4>
Q
■m
a

1

S

sS
0
0

30

60

90

120

150

180

210

240

270

300

330

360

Time (Days)
F igu re D 26 - H ea t D uration C u rve w ith and w ith o u t T h erm al S torage. J a g g ed d otted line rep resen ts d aily
b ioen ergy system ou tp u t.

103

0 5-

■o

120

150

180

210

240

270

300

330

360

Time (days)

F igu re D 27 - H eat D u ration C u rv e w ith T h erm al S torage an d V aried O u tp u t. J a g g ed d otted lin e rep resen ts
d aily b ioen ergy system o u tp u t. H orizon tal lines rep resen t m od eled b ioen ergy system s w ith 10 G J o f th erm al
storage and a m axim u m ou tp u t o f 4 , 3.S, 3 ,2 .5 and 2 M W from top to bottom .

A s can be seen in Figure D27, the UNBC bioenergy system appears to operate betw een 2.5 and 3
M W for m uch o f the tim e even though the heat dem and is higher.

A ssum ing an ideal bioenergy system (as m odeled), increasing therm al storage size decreases
natural gas consum ption as heat is stored during low dem and periods and used in high demand
periods in place o f natural gas. A 10 GJ therm al storage tank size should decrease natural gas
consum ption by ~ 7% (Figure D28). This should also provide a ~ 3 hour heat supply buffer for
system operational difficulties. Further increases in therm al storage tank size have dim inishing
returns in decreasing natural gas consum ption. The higher the bioenergy system capacity, the
lower the natural gas consum ption.
104

& 10

100

200

300

400

500

600

700

800

900

1000

Storage Size (GJ)
F ig u re D 28 - Im p act o f T h erm al S torage C ap acity on N atu ral G as C on su m p tion . 2 .5 M W (solid lin e), 3 M W
(d ash ed line) and 3 .5 M W (d otted line) b ioen ergy system s.

The size o f the therm al storage tank depends on both the am ount o f heat to be stored and the
tem perature difference betw een the hot w ater and cold water:

jj

,

ST O x 106

Vo l = -----------4.2 X A T

(65)
v

'

•>

W here Vol = volum e o f therm al storage tank (in m , assum ing density o f water is 1,000 kg/m );
STO = capacity o f therm al storage tank (GJ); 4.2 = the heat capacity o f w ater (kJ/kg/°C); and AT
= the tem perature change in the w ater loop (°C).

For the U N BC bioenergy system , the tem perature difference on the bioenergy system hot w ater
loop is 3 °C. For the m ain cam pus hot w ater loop the tem perature difference is 5 to 10 °C. These

105

are small values and m ake therm al storage system im practically large (Table D l). Even with a 10
°C tem perature change on the hot w ater loop, a 10 GJ therm al storage tank w ould require a
volum e o f 238 m 3. This corresponds to a 6.2 m x 6.2 m x 6.2 m tank (although therm al storage
tanks are cylindrical). This is a very large tank, indicating therm al storage is probably not
econom ically viable unless there is a larger tem perature drop in the heating system.

T ab le D l —Size o f T h erm al S to ra g e tan k as a F u n ction o f R eq u ired C ap acity and th e D ifferen ce b etw een
Su p p ly and R etu rn W a ter T em p eratu res.

Vol (m3)

Vol (m3)

Vol (m3)

(if AT = 3 °C)

(if AT = 5 °C)

(if AT = 10 °C)

1

79.4

47.6

23.8

3

238

143

71.4

5

397

238

119

10

794

476

238

100

7,940

4,760

2,380

500

39,700

23,800

11,900

Thermal storage
capacity (GJ)

106

A p p en d ix E
Hog Fuel Drying Calculations

Drying calculations are similar to the calculations presented in Appendix A.

For bioenergy calculations, wood moisture content is usually given on a wet basis (mass o f water per total
mass). To convert from wet basis to dry basis (mass o f water per mass o f dry wood):

(66)

MCdb ~~

Where M Cdb is the % moisture on a dry basis (tonne water/tonne dry wood) and M Cwb is the % moisture
content on a wet basis (tonne water/total tonne).

In addition to free water (water that can be removed in an oven), wood also contains hydrogen which
forms water during combustion:

C H u O ojs + 0 2

C 0 2 + 0.75H2O

From this reaction, for every tonne o f dry wood burnt, 1.3 tonnes o f 0 2 is consumed, 1.8 tonnes o f C 0 2 is
produced, and 0.55 tonnes o f H20 is produced. The water released (in tonnes) per tonne o f dry wood
during combustion is:

H20 = 0.55 -1

MCwb
100-M Cwft

(67)

The increase in efficiency o f a bioenergy system due to drying the wood prior to combustion can be
estimated by calculating the decrease in energy loss due to less water in the flue gas. W ater in the flue gas
contains both latent and sensible heat, and if there is no flue gas heat exchanger this heat is lost to the
atmosphere.

107

Difference in flue gas water content (in tonnes water per tonne dry biomass) between burning wet and
dried wood:

MCwb.w

MCWb,d

(68)

Where MCwb,w is the % moisture content o f the wet wood on a wet basis (tonne water/total tonne) and
MCwb,d is the % moisture content o f the dried wood on a wet basis (tonne water/total tonne).

Latent heat loss with water (in GJ per tonne dry biomass):

latent ~ AmH20

2.26

(69)

Where 2.26 is the latent heat o f water in GJ/tonne water.

Sensible heat loss with water (in GJ per tonne dry biomass):

= AmH2o x 0.00188 x AT

(70)

Where 0.00188 is the heat capacity o f water vapour in GJ/tonne water/°C and AT is the temperature o f the
flue gas above ambient temperature. AT is assumed to be 125 °C

Increase in efficiency due to less water in the flue gas:

„ _________

Hgain ~

latent*sensible 1 n n

2o

/' 7 1 ’v

v '1/

Where 20 is the energy content o f 1 tonne o f dry wood (in GJ/tonne dry wood)

Results from several calculations are listed in Table E l. The higher the moisture content o f the incoming
wood, and the lower the moisture content o f the dried wood, the less water in the flue gas and the higher

108

the yield o f the bioenergy system. For a typical case o f 40% wet wood dried to 25%, the bioenergy
system yield should increase by 4% compared to a system without drying. This will result in 4% less
wood required for the same heat output. There is still energy loss with the flue gas due to water vapour.

T ab le E l - P oten tial In crease in B ioen ergy System Y ield w ith W ood P re-d ryin g.

Wet wood
moisture content
(wet basis)

Dried wood
moisture content
(wet basis)

Difference in
flue gas water
content (tonne
water/tonne dry
wood)

Heat loss due to
water (GJ/tonne
dry wood)

% increase in
bioenergy system
yield

50

25

0.67

1.66

8.3

40

25

0.33

0.83

4.2

30

25

0.095

0.24

1.2

50

20

0.75

1.87

9.4

40

20

0.42

1.04

1.0

30

20

0.18

0.45

0.4

W ater vapour in the flue gas for burning 40% moisture content wood is 1.2 tonnes water/tonne dry wood,
with a corresponding efficiency loss o f 15% due to latent and sensible heat o f the water vapour. Water
vapour in the flue gas for burning 25% moisture content wood is 0.87 tonnes water/tonne dry wood, with
a corresponding efficiency loss o f 11% due to latent and sensible heat o f the water vapour. The addition
o f a dryer decreases the heat losses associated with water vapour in flue gas from 15 to 11%, for a 4%
increase in yield.

109

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