a life cycle evaluation of wood pellet gasification for district heating

8/11/2019 A Life Cycle Evaluation of Wood Pellet Gasification for District Heating

1/11

A life cycle evaluation of wood pellet gasification for district heating

in British Columbia

Ann Pa a, Xiaotao T. Bi a,, Shahab Sokhansanj a,b

a Clean Energy Research Centre for University of British Columbia, 2360 East Mall Vancouver, BC, Canada, V6T 1Z3b Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

a r t i c l e i n f o

Article history:

Received 23 November 2010

Received in revised form 28 January 2011

Accepted 1 February 2011

Available online 5 February 2011

Keywords:

Life cycle analysis (LCA)

Wood pellets

British Columbia

District heating

Gasification

a b s t r a c t

The replacement of natural gas combustion for district heating by wood waste and wood pellets gasifi-cation systems with or without emission control has been investigated by a streamlined LCA. While stack

emissions from controlled gasification systems are lower than the applicable regulations, compared to

the current base case, 12%and 133% increases areexpected in the overall human health impacts for wood

pellets and wood waste, respectively. With controlled gasification, external costs and GHG emission can

be reduced by 35%and 82%on average, respectively. Between wood pellets andwoodwaste, wood pellets

appear to bethe better choice as it requires less primary energy and has a much lower impact on the local

air quality.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

As climate change due to greenhouse gas (GHG) emissions is

gaining recognitions, various methods of climate change adapta-

tion and GHG emission mitigation have been proposed, discussed

and explored. Replacing a fraction of the current fossil fuel by

alternative energy sources such as bioenergy is one of the many

approaches recommended by policy makers. For instance, ethanol

blending requirement in transport fuel in the United States

reaches 1.14 EJ in 2010 and will increase to 3.18 EJ by 2022 while

the European Union target for renewable energy in the transport

sector in 2020 is set to 10%, or 1.29 EJ of biofuel (IEA and OECD,

2009; European Commission, 2007). Other than in the transport

sector, there are numerous studies that emphasize the potentials

of renewable energy, or more specifically bioenergy, in district or

residential heating and in combined heat and power systems

(CHP) (Difs et al., 2010; Bjrklund et al., 2001). The importanceof policy developments to promote the use of bioenergy in these

sectors is also discussed (Kopetz, 2007; Rickerson et al., 2009).

However, the use of biomass for district heating has been quite

controversial due to concerns with possible increase in health im-

pact (Ries et al., 2009). This concern is especially true when the

fossil fuel to be replaced is natural gas and when the community

is densely populated. There are currently a few major district

heating systems in Vancouver. These include one located in the

stadium and entertainment district in the core of downtown

(Davis, 2004) and three in Vancouvers largest hospital sites (Roger

Bayley Inc., 2009; Ministry of Energy of British Columbia, 2010).

The most recent establishment is the Southeast False Creek Neigh-

bourhood Energy Utility (NEU) which provides hot water and heat

for all new buildings in the area, including the Olympic Village that

was built to accommodate Olympic athletes participating in the

2010 Winter Olympic (City of Vancouver, 2010). The downtown

system operates on natural gas while the NEU operates on a

base-load system utilizing sewer heat recovery pump along with

a natural gas peaking/back-up boiler. There was a debate at the

beginning on the energy source to be chosen for the base-load sys-

tem and the two contenders were biomass and sewer heat (Roger

Bayley Inc., 2009). In the end sewer heat recovery heat pump sys-

tem was selected because of the public concerns on local air qual-

ity and traffic inconvenience that may arise from biomassutilization.

Another district heating system in Vancouver is at the Univer-

sity of British Columbia (UBC), where more than 99% of the heat

is generated fromnatural gas and the rest fromfuel oil during peak

season. With UBCs ambitious plan of reducing GHG to 33%, 67%

and 100% below the 2007 level by 2015, 2020 and 2050, respec-

tively, the University has devised a detailed plan of action. Replac-

ing natural gas with renewable energy is an important part of the

actions to be taken (University of British Columbia, 2010a). In fact,

$26 million CAD has been allocated for the establishment of a bio-

mass gasification cogeneration system on campus for research and

0960-8524/$ - see front matter 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2011.02.009

Corresponding author. Tel.: +1 604 822 4408; fax: +1 604 822 6003.

E-mail address: [email protected](X.T. Bi).

Bioresource Technology 102 (2011) 61676177

Contents lists available at ScienceDirect

Bioresource Technology

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demonstration purposes (University of British Columbia, 2010b).

Given UBCs strong motive to become green and the large amount

of GHG emissions from the boiler house, it is interesting to inves-

tigate the complete replacement of fossil fuels in its boiler house

with bio-based fuels. Wood pellets, made of sawmill residue, burn

cleaner than biomass residue and are produced in large quantity in

BC are thus consideredas a potential candidate. In 2008, 9 out of 30

pellet plants in operations in Canada are located in BC and about 35

Canadian pellet mills are in the planning stage with 13 of them tobe located in BC (Melin, 2008, unpublished data). Overall, about

90% of the pellets produced in Canada were exported and 78% of

these pellets were shipped to Europe (Melin, 2008, unpublished

data; Spelter and Toth, 2009). Finding domestic applications for

these pellets would result in less transportation-related GHG emis-

sions. The technology to be evaluated is gasification as it is cleaner

than direct combustion.

The replacement of UBCs current natural gas boiler house

with a wood pellet gasification system is evaluated by a stream-

lined life cycle analysis (LCA). LCA is a powerful tool for scenario

comparisons as the incremental variation between each scenario

would provide valuable insights for decision making. Up to date,

LCA has been used to examine the benefits and impacts of various

new projects such as the wastewater treatment and reuse system

in China (Zhang et al., 2010). As there have been many concerns

on the true impacts and degree of sustainability of biomass en-

ergy systems, LCA has been used extensively in recent years to

evaluate a wide range of bioenergy systems and, sometimes for

comparison purposes, fossil fuel energy systems. Some examples

include the study on lignocellulosic ethanol production (Spatari

et al., 2010; Gonzlez-Garca et al., 2010), biofuel production from

microalgae (Campbell et al., 2011; Collet et al., 2011) and biomass

district heating systems (Eriksson et al., 2007). Eriksson et al.

(2007) conducted a LCA study of district heating and CHP system

in Sweden using three different fuels: waste incineration, biomass

combustion and natural gas combustion. Another study at-

tempted to use LCA to investigate which of natural gas combus-

tion, wood pellet combustion, sewer heat recovery and

geothermal recovery would be the best choice for a district heat-ing system in Vancouver, BC, Canada (Ghafghazi et al., in press).

The study reveals that none of the energy sources has absolute

advantages over the others in all the impact categories considered

although by using renewable energy at least 200 kg of CO2-eqv

can be avoided per MWh of heat produced. Furthermore, the per-

formance of each type of energy source depends on many factors

such as electricity mix and types of energy utilized for producing

pellets.

For this study, an in-house life cycle inventory (LCI) database of

BC pellets (Pa et al., 2009) is utilized to evaluate a total of five sce-

narios for district heating at UBC. The base scenario is the current

installation and the others are wood waste gasification, wood pel-

let gasification and each of the two gasification operations with

emission controls. The wood waste gasification scenario utilizes

emission factors from the industry for wood waste gasificationwhile the pellet scenario uses estimated wood pellet gasification

emission factors based on literature values and wood waste gasifi-

cation emission factors from industry. For the scenarios with emis-

sion controls, an electrostatic precipitator (ESP) for dust control

and a selective catalytic reduction (SCR) unit for NOXcontrol are

included. The overall impacts on human health, ecosystem quality

and primary energy consumption in addition to GHG reduction

resulting from using wood waste and wood pellets are compared

to demonstrate the pros and cons of wood waste and wood pellet

utilization when replacing natural gas. The externality analysis

based on variations in emission profiles in different scenarios is

also performed to quantify the economical benefits for each

option.

2. Method and calculation

2.1. In-house BC pellet LCI Database

The functional unit for the in-house BC pellet LCI database is

one tonne (t) of wood pellets. Allocations are mass-based. The

streamlined life cycle consists of harvesting, transportation of har-

vested material to sawmill, sawmill processing, transportation of

sawmill by-products such as planer shavings and sawdust to pelletmill, pellet mill operations, pellet transportation in bulk via heavy

duty trucks (HDV, class 8, which has a gross vehicle weight rating

of above 15 t) and train to port in North Vancouver. For this study,

the transportation of pellets from port in North Vancouver to UBC

campus and the pellet usage in the UBC gasification/combustion

boiler are also included. The 20.2 km transportation from port in

North Vancouver to UBC is by HDV. Emissions from infrastructures

and land use changes are not included in the database in view that

pellets in BC are made from sawmill residue and forest residue.

The pollutants investigated are CO2, CH4, N2O, CO, non-methane

volatile organic compound (NMVOC), NOX, SOX and particulate

matters (PM). Other pollutants in trace amounts, although avail-

able in some databases, are not included in this study for consis-

tency reasons. CO2

is categorized as either fossil or biogenic.

However, CH4and CO are separated into biogenic or generic where

generic may contain a small amount of biogenic emissions as not

all emission data used segregated fossil and biogenic emissions

of CH4 and CO. Note that all indirect emissions are also included

in the analysis. For instances, emissions produced during produc-

tion and transportation of fuels are all accounted for in the

database.

Energy consumption data during harvesting and sawmill oper-

ations are obtained from Sambos (2002) and Nyboers (2008)

work, respectively. Information gathered from a few member com-

panies of Wood Pellet Association of Canada provided energy con-

sumption data for pellet mill and port operations. The different

types of energy considered are electricity, natural gas, heavy fuel

oil (HFO), middle distillates (diesel), propane, steam, wood waste

and gasoline. The primary energy consumptions are included inthis database and the electricity mix used is specific to BC. Details

regarding various transportation segments are also obtained from

the surveys.

The pellet LCI database was presented in the 8th World Con-

gress of Chemical Engineering in Montral (Pa et al., 2009) and will

be released in a follow-up publication with more details. The

methodology used to establish this database is used to construct

all the scenarios in this study.

2.2. UBC district heating system

For this study, a total of five scenarios will be investigated. The

base case is the current operation and the four woody biomass gas-

ification systems are wood waste, wood pellets and each of these

two systems equipped with ESPs with 99% PM removal efficiencyand SCR with 80% NOXremoval efficiency.

The values presented in this work are either per MJ of fuel input

or per year of operation. The annual operation is based on the

amount of heat that is currently generated in the base scenario

on a yearly basis, which is 974 TJ. This is chosen as the functional

unit because the amount of heat to be produced in a year is iden-

tical for all scenarios thus allowing for scenario comparison, which

is also equivalent to the functional unit of per unit of energy

produced.

2.2.1. Base scenario

The current facility configuration consists of boilers where the

fuels (natural gas and fuel oil) are fed into. The fuels are combusted

6168 A. Pa et al./ Bioresource Technology 102 (2011) 61676177


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to heat up the entering water stream to produce steam at 165 psig

(1138 kPa) that is then distributed around the campus. The flue gas

from the boilers is directed to an economizer to preheat the water

entering the boiler before the flue gas exits the facility. For the base

case, the stages in the LCA of both natural gas and oil include pro-

duction of fuels, their transportation or transmission to UBC, and

emissions during the end usage at the current facility. The emis-

sion factors for natural gas and fuel oil production and transmis-

sion are obtained from GHGenius v3.17 (Delucchi and Levelton,2010) and are referred to as upstream emission factors. In GHGe-

nius, CH4 and CO emissions from both fossil and biomass origins

are not segregated and are both reported simply as CH4 and CO.

By default, GHGenius does not display biogenic emissions of CO 2but this can be altered so biogenic CO2emission values can be ex-

tracted. The combustion emission factors for the current installa-

tion are from the Combustion Test Report provided by UBC boiler

house (Northwest Instrument Systems Inc., 2009), EMEP CORINAIR

Emission Inventory Guidebook (European Environment Agency,

2007) and US EPA AP-42 documents (US Environmental Protection

Agency, 1995). For the CombustionTest Report, the boiler was fired

with different fuels and at different capacities. The emissions were

higher if the equipment was operated at a lower capacity. For the

purpose of this study, the emission factors at 50% capacity are se-

lected. The emissions were reported as concentrations (in ppmv) of

the flue gas so material balance is carried out to determine the flue

gas flow rate. For natural gas firing, SOXemission is assumed to be

0 as sulfur content in the BC natural gas is negligible. Table 1lists

the total emission factors of the UBC boiler running on natural gas

and fuel oil with the sources of emission factors specified. The cur-

rent amount of steam production is further explained in detail in

the following section for easy comparison with the woody biomass

scenarios.

2.2.2. Woody biomass gasification scenarios

The system boundary and stages for the wood pellet scenario

have already been described earlier in the In-house BC pellet LCI

Database section. The life cycle stages for the wood waste gasifi-

cation scenarios include the production of two types of wood res-idues and their transportation, and the final usage at UBC. The two

types of wood residues are forest harvesting residues and sawmill

residues.

The forest residue production started from harvesting operation

with data taken fromSambo (2002). The forest residue is chopped

in the forest using mobile chopper andthe emissions related to this

process are from US-EI (Swiss Centre for Life Cycle Inventories

et al., 2008). The chopped residue is then transported to railhead

via HDV over a distance of 150 km. From the railhead to the North

Vancouver shipping port, the train would travel 350 km. From the

shipping port, the forest residue would be delivered to UBC district

heating facility via HDV over a distance of 20.2 km.

For the sawmill and planer mill residue, the harvesting of wood

for the forest and sawmill operations are all included and emission

data are based on literature used for the pellet LCI but convertedaccordingly so that the functional units are per tonne of wood res-

idue with 51% moisture content, dry basis. The sawmill residue

would be transported to the railhead via HDV over a distance of

25 km. The residue then travel by train for 350 km before arriving

the North Vancouver port. The residue is then delivered to UBC via

HDV over a distance of 20.2 km. The distances used in the calcula-

tions are estimated based on harvest field and sawmill locations in

BC, Canada (Natural Resources of Canada, 2003a,b) and opinions

from the local industry (Melin, 2010, personal communication).

The final wood waste to be gasified is assumed to have a mois-

ture content of 60%, dry basis, as that is maximum moisture con-

tent allowed for smooth operation of the gasifier. Due to this

limitation, some natural drying/aging is assumed to happen at

UBC and the moisture content difference between the fuel deliv-

ered and the fuel fed into the system is taken into account.

The proposed biomass utilization system for woody biomass

gasification is a retrofitted air gasification system because gasifica-

tion generally produces lower PM, CO, VOC (volatile organics) and

NOXemissions compared to direct combustion (European Environ-

ment Agency, 2007; Sparica, 2009, personal communication). The

syngas produced is combusted in the existing natural gas combus-

tor to heat up water in the boiler to generate steam. The flue gas

can be treated with an ESP to remove PM and/or a SCR unit to re-

move NOXif required.

The thermal efficiency of this system depends on the moisture

content of the biomass fuel. Typical thermal efficiency for biomass

fuel with approximately 60% moisture content (dry basis) is 62%

(Sparica, 2009, personal communication) and this is the thermal

efficiency assumed for the wood waste scenarios. For biomass with10% moisture content, the thermal efficiency is 78% (Sparica, 2009,

personal communication). This number is used for wood pellet sce-

narios despite the moisture content of BC wood pellets is actually

around 6%. Combining thermal efficiency and the amount of steam

produced in 2008, it is deduced that 126,015 t of wood waste, with

a gross calorific energy content at 12.50 MJ/kg (Forest Product

Table 1

Estimated total emission factors for UBC boiler house and their sources.

Fuel oil-firing boiler Natural gas-firing boiler

Total emission

factor (g/GJ of

fuel used)

Source of emission factor Total emission

factor (g/GJ of

fuel used)

Source of emission factor

Upstream Combustion Upstream Combustion

CO2, fossi l 88,593 Delucchi and

Levelton (2010)

European Environment

Agency (2007)

53,393 Delucchi and

Levelton (2010)

Northwest Instrument Systems

Inc. (2009)

CO2, biogenic 475 0 79.9 0

CH4 120 European Environment

Agency (2007)

72.3 European Environment

Agency (2007)N2O 7.97 1.67

CO 26.1 US Environmental Protection

Agency (1995)

9.14 Northwest Instrument Systems

Inc. (2009)

NMVOCa 22.8 European Environment

Agency (2007)

5.05 European Environment

Agency (2007)

NOX 89.8 Northwest Instrument Systems

Inc. (2009)

36.1 Northwest Instrument Systems

Inc. (2009)

SOX 245 Mass balance based on input S

content fromPodolski et al. (2008)

6.09 Mass balance based on input S

content

PM 5.73 European Environment Agency (2007) 0.49 European Environment

Agency (2007)

a Non-methane volatile organic compounds.

A. Pa et al. / Bioresource Technology 102 (2011) 61676177 6169


4/11

Laboratory, 2004), is required annually to produce the same

amount of steam as the base case. For the wood pellet scenarios,

64,257 t of wood pellets, with a gross calorific energy content at

19.4 GJ/t (Accredited Laboratory, 2007), is required. Just for com-

parison, in 2008 the boiler house consumed 1034 TJ of natural

gas and 7.84 TJ of fuel oil to generate 350 kt of steam at 165 psig

(1138 kPa), translating to 974 TJ of heat produced (UBC Utilities,

2009). These numbers correspond to a 93% overall thermal

efficiency.

2.2.1.2. Woody biomass gasification emission factors. For the wood

waste scenarios, the gasification emission factors utilized were

based on wood waste gasification in a commercial fixed bed gas-

ifier. The biogenic CO2 emission is calculated based on the carbon

content of wood. It was assumed that the carbon content in dry

wood is 50% and the moisture content of wood waste, a mixture

of forest residue and sawmill and planner mill residue, is 60%,

dry basis, which is the maximum moisture allowed for the gasifier.

At 60% moisture content, the mixed waste would give off 0.092 kg

of biogenic CO2 emission per MJ of pellets gasified. The CH4, N2O

and SOXemission factors are not available so they are estimated

by the emissions of wood waste combustion in boiler from US

AP42 document (US Environmental Protection Agency, 1995). The

annual emissions are obtained by multiplying the emission factors

by the annual fuel consumption.

It is speculated that wood pellet gasification emission factors

may vary quite substantially given that the combustion emission

for wood waste and wood pellets do vary considerably as shown

in the literature or published database (Swiss Centre for Life Cycle

Inventories, 2008; Johansson et al., 2004; Wierzbicka et al., 2005;

Lillieblad et al., 2004). In attempt to better represent wood pellet

gasification emissions in the wood pellet scenarios, which are not

available in the literature, the emission factors are estimated using

two types of ratios. The first ratio is the ratio between wood and

pellet combustion emission factors from literature and database.

This first ratio together with the wood gasification emission factors

from the industry can yield a set of estimated emission factors for

the wood pellet gasification system. The second ratio is the ratiobetween published wood combustion emissions and the wood gas-

ification emission from the industry. This ratio can then be applied

to pellet combustion emission factors from literature and database,

resulting in another set of estimated emission factors for pellet

gasification, provided that the values of pellet and wood combus-

tion emissions are different from those used to calculate the first

ratio, as that would yield two identical sets of wood pellet gasifica-

tion emission factors.

In order to carry out this approximation process based on ratios,

it is crucial to compare data with similar set-up in terms of emis-

sion controls, system type and type of biomass used. Different

emission data are matched based on considerations mentioned

and whenever possible, data from the same article or database

are compared. For the calculation of the first type of ratio, no unit

conversion is required as the units used are usually consistentwithin a single source. However, when calculating the second type

of ratios, unit conversions need to be performed as industrial emis-

sion data for wood gasification are provided in mass of pollutant

per energy unit of wood utilized while most literature report their

data in mass of pollutant per volume of flue gas with the O2% or

CO2% of flue gases provided along with the specification on dry

or wet gas basis. Conversions of units are performed as described

in The Handbook of Biomass Combustion and Co-firing (van Loo

and Koppejan, 2007). Since gasification emission factors from the

industry do not include CH4and N2O, their ratios are not calculated

and pellet combustion emission factors from the US-EI database

are used in the estimated pellet gasification emission data.Table 2

lists all the emission factors used in the calculation of the ratios. Table

2

Listofpelletandwoodcombustionemissionfac

torsfrom

literatureinkgofpollutantemittedperMJoffuelutilized.

Source

Wierzbickaetal.

(2005

)

Pagelsetal.

(2003)

Johanssonetal.

(2004)

USEnviron

mental

ProtectionAgency

(1995)

SwissCentrefor

LifeCycleInventories

etal.

(2008)

Lilliebladetal.

(2004)

Fuel

Forestresidue

Pellet

Forestresidue

Mixedwood

Pellet

Wetwood

Mixedwood

chip

fromforest

Pellet

Shaving

chips,

andsawdust

Pellet

Load

Medium

High

Medium

80%

60%

45%

Low

Low

Emissioncontrol

Multicyclone

Multicyclone

None

Multi

cyclone

M

ulticyclone

Typeofequipment

1.5

MW,

movinggrate

1MW,

movinggrate

Emission

median

forpelletboiler

Emission

median

forpelletboiler

Furnace,

50kW

1.5

MW,

m

ovinggrate

BiogenicCO2

9.1

7E02

9.1

7E02

1.0

3E01

9.6

5E028.9

3E02

7.8

5E02

BiogenicCH4

4.3

5E05

1.7

7E06

9.0

3E06

9.0

3E06

7.0

0E07

3.0

0E07

N2O

5.5

9E06

5.5

9E06

3.0

0E06

2.5

0E06

BiogenicCO

4.1

0E03

3.2

0E04

2.5

8E04

2.5

8E04

1.1

8E04

6.5

0E054.0

0E04

3.4

4E05

NMVOC

2.8

5E05

2.5

0E06

7.3

1E06

7.3

1E06

9.0

0E07

1.5

0E06

NO

X

7.2

0E05

6.7

0E05

9.4

6E05

9.4

6E05

1.1

0E04

7.4

0E053.3

1E05

4.6

7E05

SOX

1.0

7E05

1.0

7E05

2.5

0E06

2.5

0E06

PM2.5

3.4

0E05

2.0

0E05

PM

4.4

6E05

6.8

8E05

1.8

7E05

8.1

0E05

5.5

9E05

4.5

4E05

8.8

0E05

1.9

0E05

1.4

2E04

9.4

6E05

4.3

0E05

2.3

7E052.1

6E05

2.1

6E05



5/11

Table 3 summarizes the two set of emission factors obtained

from the two types of ratios and their average values. The average

estimated pellet gasification emission factors are used for the cal-

culation in uncontrolled pellet scenario in this study. Note that SOXis manually set to zero since SOXemission depends mostly on the

sulfur content of the fuel and wood pellet contains negligible sulfur

at less than 0.01%, dry basis (Johansson et al., 2004).

2.2.1.3. Emission controlled woody biomass gasification scenarios.

Table 3also includes the current air emission limits, in kg per MJof fuel consumed, for biomass boilers and heaters in Metro Van-

couver (Metro Vancouver, 2008) and the wood waste gasification

emission factors used in this study. It is apparent that emission

control units need to be in place in order to stay below the local

air emission limit. The numbers show that NOX and PM need to

be reduced by 37% and 66%, respectively for wood pellet and 44%

and 87% for wood waste gasification. Both can be easily achieved

by technologies such as SCR for NOXreduction and ESP for PM re-

moval as the typical removal efficiencies for these units are

approximately 80% and 99%, respectively (Forzatti, 2001; De

Nevers, 2000). These efficiencies are applied for the controlled

woody biomass gasification emission scenarios.

2.3. Life cycle impact assessments of the scenarios

Biogenic CO, CO2 and CH4 have the same impacts as non-bio-

genic emissions in terms of human and ecosystem quality as the

chemical structure does not depend on the origin of emissions.

However, in terms of climate change impact, biogenic CO, CO2and CH4are considered to have less impact than their fossil-origin

counterparts as biogenic carbon emissions are considered to be

carbonneutral and do not result in net-increase in the carbon con-

tent of the atmosphere. By definition PM is for all particulate mat-

ters while PM2.5 refers to those less than 2.5lm and PM10 are

those less than 10 lm. PM2.5 are the ones with significant health

impacts (Humbert et al., 2005). The impact factors for PM are esti-

mated based on an average PM2.5to PMratio in ambient air of 0.33

(Humbert et al., 2005; Dockery and Pope, 1994). Throughout this

study, care was taken to avoid double accounting. When only PMemission factors are available, such as from GHGenius database

(Delucchi and Levelton, 2010), they are used and the impacts are

calculated using PM impact factors. When both PM and PM2.5emission factors are available, only the PM2.5 emission factors

and thus impacts are utilized. In the result section, sometimes both

PMand PM2.5are listed, but it is important to acknowledge that for

each process, only one of the PM or PM2.5 emission factors, and

thus their respective impact factors, is used, not both.

The emission factors and energy consumption data are im-

ported into a commercial LCA software, SimaPro, to allow for the

use of various life cycle impact assessment (LCIA) methods. IM-

PACT 2002+ (Jolliet et al., 2003) is selected for impact assessment

in this case study. IMPACT 2002+ includes both midpoint and

end point impacts by linking all life cycle inventory data via 15

midpoint categories to four damage, or end point, categories, as

illustrated inFig. 1. The dashed lines indicate that the conversion

into damage categories has not yet been properly established.

The units used for each impact category, such as DALY and

PDFm2 yr, are defined inFig. 1.

The most current version of IMPACT 2002+ at time of analysis

(v2.06) is adapted for analysis with two extra categories added to

keep track of the primary energy consumption and external costs

throughout the entire life cycle. These two new end point catego-ries are also presented inFig. 1. The impacts of biogenic CH4 and

CO are added under the categories of respiratory organic and inor-

ganic, respectively, using the impact values of their fossil fuel ori-

gin counterparts. Six endpoint categories, human health,

ecosystem quality, climate change, primary energy consumption,

external cost and resources can be obtained. Only the first five will

be evaluated in this study. It is important to keep in mind that IM-

PACT 2002+ was developed in Europe so the values of parameters

used for the compilation of human toxicity are at a continental le-

vel for Western Europe. Due to this reason, the final values to be

presented here only serve as indicators for scenario comparisons

as the absolute values may not be so meaningful due to geograph-

ical and geological differences in Western Canada and Western

Europe.

External cost, also known as externality, is the unaccounted anduncompensated impact on a group arising from the social or eco-

nomic activities of other groups (European Commission, 2003).

Table 3

Estimated wood pellet gasification emission factors and air emission limits for biomass boilers in Vancouver, Canada.

Pol lutant Wood waste

gasification emission

factors (kg/MJ)

Estimated pellet gasification

emission factors based on

ratio 1 (kg/MJ)

Estimated pellet gasification

emission factors based on

ratio 2 (kg/MJ)

Average estimated pellet

gasification emission

factors (kg/MJ)

Vancouver air emission

limit for biomass boilers

(kg/MJ)

Biogenic CO2 9.17E02 8.50E02 8.22E02 8.36E02

Biogenic CH4 9.03E06 3.00E07 3.00E07 3.00E07

N2O 5.59E06 2.50E06 2.50E06 2.50E06

Biogenic CO 1.46E05 1.26E06 1.14E06 1.20E06 1.59E04

NMVOC 4.30E06 3.77E07 3.02E07 3.39E07

NOX 7.31E05 6.80E05 6.33E05 6.56E05 4.10E05

PM 4.00E05 1.92E05 1.14E05 1.53E05 5.13E06

Fig. 1. Overall scheme of the IMPACT 2002+ framework, linking LCI results via the

midpoint categories to damage categories with modifications implemented for this

study (based onJolliet et al., 2003).



6/11

Therefore, externality reflects the impact on environment and hu-

man health. Table 4 lists three sets of reported external costs for air

pollutants investigated in this study. The costs for biogenic CO2,

CH4 and CO are estimated using the impact factors for different

pollutants in IMPACT 2002+. The global warming factors listed in

IMPACT 2002+ are mostly based on IPCC 2001s 500-year time

horizon values. Since CO2 only has an effect on climate change

and biogenic CO2 has no impact, biogenic CO2 has been assigned

a zero external cost. It is noted that the average external cost for

CH4 in the literature is close to $0.23, which is equal to the cost

of CO2 multiplied by the impact factor of CH4, a value of seven.

The same observation is made for N2O where the calculated valuebased on its impact factor of 156 is $5.15. Based on these observa-

tions, the external cost of biogenic CH4is estimated by multiplying

its impact factor of 4.25 by the cost of CO2 to yield $0.14. This is

plausible as CH4 only has effects on global warming according to

IMPACT 2002+. For biogenic CO, it also has impact on human

health, which is the total cost of CO minus the cost of climate

change. With the cost of climate change for CO being estimated

as the cost of CO2 multiplied by the climate change impact factor

for CO in IMPACT 2002+, which is 1.57, the health cost for CO is

found to be $0.67, which applies to CO from all sources. Since

the climate change impact factor of biogenic CO is 0, the total cost

of biogenic CO is equal to the health cost of CO. The emission

reduction achieved from replacing natural gas and fuel oil with

wood pellets can then be combined with external cost for each pol-

lutant inTable 4to derive the reduction in external costs.Since health impact depends heavily on the emission location

and its proximity to population, the health impact associated with

end usage alone for all five scenarios are compared as the point of

usage is at UBC campus, where the risk of exposure to pollutant is

much higher compared to pellet mills in suburban areas. The end

stage health impacts for all scenarios are normalized by the value

of the base scenario as it is the relative, not the absolute, values of

the health impact that are relevant for this comparison.

3. Results and discussion

Using values presented inTable 1, the current annual emissions

from UBC boiler house are calculated and presented in Table 5,

together with the emissions from all the biomass gasification sce-

narios. The emission factor for PM instead of PM2.5is provided for

all processes in the life cycle except for steam generation, where

the PM2.5 emission factor is provided instead of PM. Due to this

reason, there are both PM and PM2.5emissions reported inTable 5

but there is no overlapping between them as the emission factor

for PM during steam generation was not used in the calculation.

From Table 5 it is apparent that the estimated biogenic CO,

NMVOC, NOXand PM emissions for pellet gasification are lower

than wood waste gasification, as observed in pellet and wood

waste combustion. It appears that the most obvious advantage of

switching to woody biomass gasification is the drastic reductionof CO2emissions of fossil fuel origin. However, this is coupled with

a substantial increase in biogenic CO2emission, particularly due to

lower thermal efficiency for the biomass gasification system and

the high carbon intensity of biomass energy. Another emission

reduction lies in generic CH4. Even though there is a slight increase

of biogenic CH4 emission, there is a net CH4 reduction of 64% and

77% when accounting CH4from all origins for wood waste and pel-

let gasification, respectively. The high CH4emission from the cur-

rent scenario arises from the upstream processing of natural gas

as well as the leakage and loss during pipeline transmission. This

observation is more noticeable inFig. 2where the stage-wise dis-

tribution of each pollutant for the current scenario is illustrated. It

is evident from the same figure that natural gas burns very cleanly

with most of the emissions from upstream, with the exception of

fossil CO2, N2O and NMVOC. Despite that natural gas combustionand upstream operations seem to be contributing the most to the

emissions and environmental impacts, it is important to note that

more than 99% of the energy input was from natural gas thus Fig. 2

does not suggest that fuel oil burning is cleaner than natural gas.

However, it is noted that there is a significant SOXemission from

oil combustion despite that only less than 1% of the energy input

was from fuel oil.

Other than generic CH4emission and CO2emission of fossil ori-

gins, all other emissions would increase when the boiler is

switchedfrom natural gas to woody biomass gasification. The most

significant increase, other than in biogenic CO2, is in PM emissions,

reaching approximately 130- and 77-folds for wood waste and

wood pellets, respectively. Even with an ESP unit, the increase

Table 4

Summary of external costs from literature and the external costs used in this study.

Bi and Wang (2006) Average values from various

states in the US a Golay (2005)

Dones et al. (2005) Values used in this analysis Calculation and remarks

CAD $/kg CAD $/kg CAD $/kg CAD $/kg

CO2, fossil 0.04 0.03 0.03 0.03 Average of all values

CO2, biogenic 0 Estimated using impact factors in

IMPACT 2002+ method

CH4 1.05b 0.25 NA 0.25 State average value is used as it is

not just estimation based on GWP

CH4, biogenic 0.14 Estimated using impact factors in

IMPACT 2002 + method

N2O 12.52b 4.73 NA 4.73 State average value is used as it is

not just estimation based on GWP

CO 0.41 1.02 NA 0.72 Average of all values

CO, biogenic 0.67 Estimated using impact factors in

IMPACT 2002 + method

NMVOC NA NA 1.78 1.78

VOC NA 3.76 NA 3.76

NOX 5.23 6.41 4.59 5.41 Average of all values

SOX 5.46 2.30 4.64 4.14 Average of all values

PM 14.70 3.14 18.52c 12.12 Average of all values

PM2.5 NA NA 30.87 30.87

a Based on values from New York State Public Service Commission, Department of Public Utilities of Massachusetts, Public Service Commission of Nevada and

California Public Utilities Commission and presented in Golays lecture slides (Golay, 2005).b Estimated by source based on CO2cost multiplied by specific pollutants 100 years time horizon GWP (global warming potential) value from the 2007 IPCC report.c Estimated by source based on typical PM2.5/PM ratio.



7/11

would still be approximately 17- and 43-folds for wood waste and

pellet, respectively. ESP appears to be less effective for the pellet

scenario because a large portion of PM emission is released from

the upstream fuel preparation process, which is not controlled by

the ESP installed for the gasification plant. For wood pellet sce-

nario, approximately 31% of the total PM emission is from pellet

mill where wood residue is burned for biomass drying. Thus,

removing PM from gasification process alone would achieve a less

significant PM reduction over the entire life cycle. However, it isimportant to point out that the zero emission of PM2.5under cur-

rent natural gas operation results from the fact that all emission

factors related to base scenario are only for PM but not for PM2.5specifically. With emission controls in place, the increase in other

pollutant emissions ranges from 215% (for NMOVC) to 448% (for

N2O) for wood waste and 42% (for SOX) to 393% (for all CO), for

wood pellets.

InFig. 3, the stage-wise contributions to the total emissions are

illustrated for wood waste gasification (Fig. 3a), wood pellet gasifi-

cation (Fig. 3b), wood waste gasification with emission controls

(Fig. 3c) and wood pellet gasification with emission controls

(Fig. 3d), respectively.Fig. 3 reveals that the top contributor to bio-

genic CO2and N2O emissions is the gasification stage for all woody

biomass gasification scenarios. For the uncontrolled wood waste

scenario, more than 80% of the generic CO and approximately

50% of the NMVOC emissions are emitted during the harvesting

stage. The gasification stage is the main contributor to the remain-

ing pollutants except for fossil-origin CO2and generic CH4as more

than 40% of each of these pollutants are emitted during harvesting.

For wood waste gasification with emission control units, the har-

vesting stage also becomes the main contributing stage for NOXand PM emissions throughout the life cycle and remains to be

the main contributor for fossil-origin CO2 and generic CH4.

For both uncontrolled and controlled wood pellet scenarios, the

harvesting stage is the main contributor to fossil-origin CO2, gener-

ic CO, NMVOC, and SOXwhile pellet mill is where the majority of

biogenic CH4and CO is emitted due to the burning of wood residue

within the mill. In the uncontrolled pellet scenario, 42% of the NOX

emission in the life cycle is emitted in the harvesting stage and 40%from gasification. Moreover, gasification is responsible for 44% of

the life cycle PM emission. However, with emission control, gasifi-

cation stages contribution to PM and NOXare reduced to 0.8% and

12%, respectively, with pellet mill becoming the new hot-spot for

PMemission. Note that the PM categories in both Figs. 2 and 3 refer

to All PM inTable 5.

The external costs from each scenario are also presented inTa-

ble 5. By switching to wood waste gasification, there is actually an

increase of $450,000 CAD in external costs while wood pellet gas-

ification would result in an $87,000 CAD saving. It was stated ear-

lier that in order to satisfy the air emission limits in Vancouver for

biomass boiler theNOX and PMemissions need to be reduced. With

the installation of SCR and ESP units, these two pollutants can be

reduced by 80% and 99% thus achieving emissions much lower

than what is required. With the emission control units, the exter-nal costs can be reduced by 38% and 31% from the base case for

wood waste and wood pellets, respectively. Note that no spatial

variation of the external cost has been considered in the current

analysis in which the emissions released in densely populated ur-

ban area and less populated remote area are given the same exter-

nal cost for each gas pollutant.

Since emissions are increased for all major pollutants when the

boiler house is switched from natural gas to wood pellet gasifica-

tion, it is hard to comprehend the relative overall impacts from

each scenario based on emission inventories only.Fig. 4compares

each of the five scenarios impacts on human health, ecosystem

quality and climate change, as well as a breakdown of these

Table

5

Annualairemissionsfrom

currentandwoodpe

lletscenarios.

Uncontrolledwoodybiomassgasification

Controlledwoodybiomassgasificationa

Emissionsforbase

Scenario(t/yr)

Emissionsfor

woodwaste(t/yr)

Reductioninexternal

cost($1000CAD)

Emission

sfor

woodpe

llet(t/yr)

Reductioninexternal

cost($1000CAD)

Emissionsfor

woodwaste(t/yr)

Reductioninexternal

cost($1000CAD)

Emissionsfor

woodpellets(t/

yr)

Reductioninexternal

cost($1000CAD)

AllCO2

55,9

97

154

,205

1,5

06

121,5

52

1,5

52

154,2

05

1,5

06

121,5

52

1,5

52

CO2,

fossil

55,9

11

8

,629

1,5

06

8,8

77

1,5

52

8,6

29

1,5

06

8,8

77

1,5

52

CO2,

biogenic

86.3

0

145

,575

0.0

0

112,6

76

0.0

0

145,5

75

0.0

0

112,6

76

0.0

0

AllCH4

75.7

0

27.1

8

13.8

3

17.5

2

14.7

9

27.1

8

13.8

3

17.5

2

14.7

9

CH4

75.7

0

12.8

5

15.8

4

16.3

6

14.9

6

12.8

5

15.8

4

16.3

6

14.9

6

CH4

b,b

iogenic

0.0

0

14.3

3

2.0

1

1.1

7

0.1

6

14.3

3

2.0

1

1.1

7

0.1

6

N2O

1.7

9

9.8

1

37.8

9

4.6

3

13.4

0

9.8

1

37.8

9

4.6

3

13.4

0

AllCO

9.6

5

47.7

3

25.9

7

47.6

0

25.9

8

47.7

3

25.9

7

47.6

0

25.9

8

COb

9.6

5

21.6

2

8.5

9

23.4

3

9.8

9

21.6

2

8.5

9

23.4

3

9.8

9

CO,b

iogenic

0.0

0

26.1

0

17.3

8

24.1

7

16.1

0

26.1

0

17.3

8

24.1

7

16.1

0

NMVOC

5.4

0

17.0

1

20.6

2

11.7

9

11.3

4

17.0

1

20.6

2

11.7

9

11.3

4

NOX

38.0

4

219

980

203

893

127.0

0

481

137.5

6

539

SOX

8.2

2

127.0

0

77.8

1

11.6

4

14.1

1

27.0

4

77.8

1

11.6

4

14.1

1

AllPM

0.5

6

72.5

5

882

43.5

5

522

10.1

8

126

24.6

3

292

PM

0.5

6

72.0

6

867

43.5

2

521

9.6

9

111

24.5

9

291

PM2.5

c

0.0

0

0.4

9

15.2

0

0.0

3

1.0

1

0.4

9

15.2

0

0.0

3

1.0

1

Totalchangesin

externalcost

450

87

804

671

aselectivecatalyticreduction(SCR)hasaremovalefficiencyof80%whileelectrostaticprecipit

atorhasaPMremovalefficiencyof99%.

bmayincludesomebiogenicemissionsasw

ell.

cfromsteamgenerationonlyasnoPMem

issionfactorwasavailableforthisprocess.



8/11

impacts into different stages to signal out hot-spots throughout

their life cycles.

By switching to woody biomass, both impacts on human health

and ecosystem quality increase significantly. For human health, the

current impact is 4 DALY and it would increase by 6.2- and 8.6-

folds for wood pellets and wood waste, respectively. Even with

emission control units, the increase would still be 3.3-folds on

average for both woody biomass fuels. Since the parameters used

in the impact assessment method are based on Western Europe,care should be exercised in the interpretation of human health

impact.

The current impact on ecosystem quality is 2.26E5 PDF m2 yr

and it would increase by around 4.7 and 4.2 times when switched

from natural gas to wood waste and wood pellet gasification sys-

tems, respectively. With SCR and ESP, the increase can be lowered

to an average of 2.4-folds for both fuel types.

FromFig. 4it becomes apparent that the harvesting of woody

material and the gasification stage contribute greatly to humanFig. 2. Stage-wise emission distribution for current natural gas boiler scenario.

Fig. 3. Stage-wise emission distribution for (a) wood waste gasification, (b) wood pellet gasification, (c) wood waste gasification with SCR and ESP units with 80% NOXand

99% PM removal efficiency, respectively, and (d) wood pellet gasification with identical emission control units.



9/11

health impact. It is evident that by adding emission control units,

the pellet scenarios human health impact for the entire life cycle

can be further reduced by 35% while the health impact associated

with gasification alone can be further reduced by 87%. However,

since wood waste requires little upstream processing, the addition

of emission control can effectively reduce the overall health impact

by 59% and the reduction in gasification stage alone would be 88%.

For the effect on ecosystem quality, the main contributions are

from the harvesting and gasification stages as well. With emissioncontrol, gasification only contributes 20% and 12% to the entire life

cycles impact on ecosystem quality for wood waste and pellet,

respectively. Moreover, with emission controls the ecosystem

quality impact for the entire pellet gasification life cycle can be fur-

ther lowered by 32% while impacts associated with gasification

alone can be reduced by 80% when compared to uncontrolled pel-

let scenario.

Lastly,Fig. 4 confirms that the key advantage associated with

switching to woody biomass is the reduction in GHG emissions.

Fig. 4c clearly illustrates that impact on climate change can be re-

duced by 82% and 83% from the current 56.7 kt of CO2-equvalent

per year when wood waste and wood pellets are used, respectively.

Another scenario performance indicator is primary energy con-

sumption. To generate 974 TJ of usable heat annually, the current

natural gas scenario consumes 1284 TJ of primary energy and this

number is slightly higher for the pellet scenarios at 1516 and 1725

TJ for wood waste scenarios. Primary energy takes into account the

energy resource required to produce fuels, power or products.

These include the heating value of raw materials such as harvested

wood and crude oil, energy required to produce fuels such as die-

sel, and energy required to convert different fuels, such as natural

gas or diesel, to electricity.

It is important to acknowledge that human health impact is

more of local concern as compared to theglobal climate change im-

pact. As the UBC district heating systemis located in a densely pop-

ulated area, the stack emissions from the boiler house will have the

most significant impact on human health. The end usage contribu-

tions to human health impact for all five scenarios are normalizedby the base case value andare comparedin Fig. 5. It is apparent that

the human health impact directly linked to the end usage increases

substantially when switched from natural gas to woody biomass as

it would be augmentedby 18- and7.7- folds for uncontrolled wood

waste and pellet gasification, respectively. Thisvalueis lowered to a

133% increase for controlled wood waste scenario and a mere 12%

increase for controlled wood pellet scenario. As a result, it is

strongly recommended that both the PM and NOXemission control

units be installed in biomass combustion/gasification district heat-

ing systems to prevent the deterioration of local air quality and

drastic increase in local health impact, in addition to meeting the

local emission standards. It should be noted that adopting wood

pellet gasification may have other impacts on the community that

are not accounted for in this study, such as the noise and inconve-

nience associated with pellet delivery traffics.

In this study, the gasification plant produces only heat so the

LCA study of a CHP plant in place of the existing boiler house might

yield different results. As UBC also aims to become a net energy

exporter by 2050, the CHP option is readily pursued as UBC has

Fig. 4. Stage-wise impact analysis in terms of (a) human health, (b) ecosystem quality, and (c) climate change for curren t and woody biomass gasification with and without

emission control units.



10/11

already decided to establish a CHP demonstration unit on campus

using a system developed by Nexterra and GE to provide green

heat and electricity while serving as a research facility.

4. Conclusions

Replacing fossil fuels with biomass may not always be desirable

and the decision would depend heavily on the priorities of the spe-

cific project. Natural gas combustion outperforms emission-con-

trolled woody biomass gasification scenarios in primary energy

consumption and all impact categories considered other than cli-

mate change while all woody biomass gasification scenarios yield

significant reduction on GHG and external costs. Pellet gasification

is superior to wood waste as it has lower primaryenergy consump-

tion and the health impact associated with stack emission for the

controlled waste wood gasification is 133% higher than the base

scenario, compared to 12% for pellets.

Acknowledgements

The authors would like to thank UBC Sustainability office, UBC

Utility, and Dr. Anthony Lau for providing the air emission data

for the existing natural gas district heating facility. The authors

are also grateful to the financial support from Agriculture Canadas

ABIN program and the UBC Graduate Fellowship (UGF) program.

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