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sustainability
Article
Life Cycle Environmental Impact of Biomass
Co-Firing with Coal at a Power Plant in the Greater
Houston Area
Raghava Rao Kommalapati 1, *ID , Iqbal Hossan 2, Venkata Sai Vamsi Botlaguduru 2, Hongbo Du 2
and Ziaul Huque 3
1Center for Energy & Environmental Sustainability, and Department of Civil & Environmental Engineering,
Prairie View A & M University, Prairie View, TX 77446, USA
2Center for Energy & Environmental Sustainability, Prairie View A & M University, Prairie View, TX 77446,
USA; ihossan@student.pvamu.edu (I.H.); vsbotlaguduru@pvamu.edu (V.S.V.B.); hodu@pvamu.edu (H.D.)
3Center for Energy & Environmental Sustainability, and Department of Mechanical Engineering,
Prairie View A & M University, Prairie View, TX 77446, USA; zihuque@pvamu.edu
*Correspondence: rrkommalapati@pvamu.edu; Tel.: +1-936-261-1660
Received: 26 April 2018; Accepted: 24 June 2018; Published: 27 June 2018
Abstract:
Electricity generation from coal is one of the leading contributors to greenhouse gas
emissions in the U.S. and has adverse effects on the environment. Biomass from forest residue can be
co-fired with coal to reduce the impact of fossil-fuel power plants on the environment. W. A. Parish
power plant (WAP, Richmond, TX, USA) located in the greater Houston area is the largest coal and
natural gas-based power generation facility in Texas and is the subject of the current study. A life
cycle assessment (LCA) study was performed with SimaPro
®
and IMPACT 2002+ method, for the
replacement of 5%, 10%, and 15% coal (energy-basis) with forest residue at the WAP power plant in
Texas. Results from the LCA study indicate that life cycle air emissions of CO
2
, CO, SO
2
, PM
2.5
, NO
X
,
and VOC could reduce by 13.5%, 6.4%, 9.5%, 9.2%, 11.6%, and 7.7% respectively when 15% of coal is
replaced with forest residue. Potential life cycle impact decreased across 9 mid-point impact categories
of, human/aquatic toxicity, respiratory organics/inorganics, global warming, non-renewable energy,
mineral extraction, aquatic acidification, and terrestrial acidification/nitrification. The potential
impact across damage/end-point categories of human health, ecosystem quality, climate change,
and resources reduced by 8.7%, 3.8%, 13.2%, and 14.8% respectively for 15% co-firing ratio.
Keywords:
W. A. Parish power plant; life cycle assessment; mid-point impacts; end-point impacts;
biomass co-firing
1. Introduction
Coal is a major source of electricity generation, and as of 2017, accounts for 30.1% of total
U.S. electricity production [
1
]. In Texas, about 30.6% of total electricity is generated from coal,
and less than 1% is generated from biomass [
2
]. Coal combustion creates significant environmental
impacts, and is responsible for 26.3% of total energy-related CO
2
emissions during 2016 in the U.S [
3
].
Co-firing of biomass with coal is a valuable process modification for reducing air pollutant emissions
and decreasing the overall environmental impact of coal-fired power plants [
4
–
9
]. Coal can be
replaced by 15% (mass-basis) biomass in an existing power plant with only minor modifications,
and co-firing with 10–25% (mass basis) biomass is possible without significant impact on heat release
characteristics of most boilers [
10
,
11
]. The use of biomass in existing electricity generating units
also reduces capital investment and the potential cost of the resulting renewable electricity [
12
].
Life Cycle Assessment (LCA), conducted according to ISO 14040, is an analytical tool that assists in the
Sustainability 2018,10, 2193; doi:10.3390/su10072193 www.mdpi.com/journal/sustainability
Sustainability 2018,10, 2193 2 of 18
comprehensive evaluation of the total life cycle environmental impact of a product/process [
13
].
Several studies were reported for assessing the economic and environmental impacts of direct-
and co-firing of biomass with coal from a life cycle perspective [
14
–
18
]. A study on LCA for
direct torrefied wood co-firing at 20% co-firing ratio, showed a reduction of 12% for global
warming, and 7% for acidification impact potentials [
5
]. Nine impact categories (acidification,
ecotoxicity, eutrophication, global warming, ozone depletion, photochemical oxidation, human
health-carcinogenic, non-carcinogenic, and respiratory effects) were studied in an LCA study
considering the co-firing of wood pellets with coal in the Southeastern United States and observed
significant reductions across all impact categories except ozone depletion [
9
]. The reduction in
environmental impacts due to co-firing of raw and torrefied wood pellets with coal at 20% co-firing
ratio in Chile were analyzed to be as high as 28–26% for acidification potential and 16–6% for global
warming potential [
19
]. Greenhouse gas emissions on a CO
2
-equivalent basis were observed to reduce
by 18.2% for 15% co-firing with wood residue at a 360 MW power plant, and other air pollutants such
as sulfur oxides (SO
2
) and nitrogen oxides (NO
X
) reduced by 12% and 8%, respectively [
20
]. At present,
low levels (5–15% co-firing) of co-firing are economically feasible if affordable biomass feedstocks are
available [21].
Local availability of sufficient quantity of biomass is a major controlling factor for determining
the cost-effectiveness of co-firing. Texas has a great resource of diverse biomass such as crop residues,
logging residues and mill residues [
22
–
25
]. Logging residue, the unused portions of harvested trees
left in the woods, are potentially available for co-firing including tops, limbs, and unutilized cull trees,
whereas stumps are not viable due to costs being prohibitively high [
22
]. Total logging residue in
Texas for the year 2008 is 2,906,361 t. In Northeast Texas, 50% of logging residue is from hardwood,
and 50% from softwood; in Southeast Texas, 78% is from softwood and 22% from hardwood [
22
–
25
].
Currently, logging residues are either burned or left in open fields by forestland owners, as markets for
logging residues are nonexistent [
24
]. This resource is also not utilized due to issues of harvest and
transportation. Integrating forest residue in coal-fired power plants is an attractive option due to lower
investment risk, low costs, and greater efficiency for reducing greenhouse gas (GHG) emissions [
24
,
26
].
The forest products industry produces considerable volumes of mill residue in the manufacturing
process, and this residue can be utilized as it is clean, uniform, on-site, and low in moisture content [
23
].
Currently, most of the East Texas mill residue has been utilized or marketed. In Northeast Texas, 74% of
mill residues came from softwood, and 26% came from hardwood; in Southeast Texas, 92% of mill
residue is from softwood [
23
]. The current study evaluates the environmental impact of co-firing forest
residue from Texas at the W. A. Parish (WAP) power plant, in Houston, TX. The eight counties of Texas
which comprise the Houston-Galveston-Brazoria (HGB) ozone non-attainment area, have 18 active
electricity generation facilities, of which the WAP power plant is the largest (3653 MW) [
27
]. WAP has
eight units, of which Units 1–4 operate on natural gas and generate 1191 MW; Units 5–8 generate
2462 MW electricity with coal consumption 36,000 t/day. WAP power plant is one of the significant
contributors to ozone precursor emissions in the Greater Houston area, and the leading point source
for emissions of four criteria air pollutants (CO, SO
2
, NO
X
, PM
2.5
) during peak summer ozone
episodes [
27
,
28
]. The objective of the current study is to evaluate the change in potential life cycle
environmental impacts due to co-firing of forest residue biomass from Texas, at the WAP power plant
in the Greater Houston area.
2. Materials and Methods
2.1. Life Cycle Assessment
A comprehensive Life Cycle Assessment (LCA) is conducted by SimaPro
®
8.3.0 software and
ecoinvent database is used in this analysis. IMPACT 2002+ method is used for life cycle impact
assessment at both mid-point and end-point impact categories. There are three co-firing techniques
commonly used: direct, indirect, and parallel co-firing [
29
]. Direct co-firing is taken into consideration
Sustainability 2018,10, 2193 3 of 18
in this study due to the lower investment requirements. Three co-firing scenarios (5%, 10%, and 15%,
energy basis) are analyzed along with base case (no co-firing). This study considers a functional unit as
one kWh of electricity produced in the power plant. The cradle-to-gate system boundaries are depicted
in Figure 1. Biomass supply chain included bundling, forwarding, transportation, and chipping.
The coal supply chain consists of two stages: coal mining and coal transportation. Biomass chipping is
considered to be conducted at the power plant to account for cost and energy effectiveness reported
for plants larger than 300 MW [
4
]. This study does not consider biomass production, as available forest
residue is directly taken into account for co-firing. Harvesting of biomass is excluded from the system
boundary, and biogenic carbon neutrality is assumed as in Zhang et al., (2010) [
30
]. In the power plant,
consumption of materials and energy that are needed in excess for co-firing are excluded from system
boundary, due to negligible contribution to emissions [
20
,
31
]. Thus, water consumption is assumed to
be constant throughout the base- and co-firing cases.
Sustainability 2018, 10, x FOR PEER REVIEW 3 of 18
energy basis) are analyzed along with base case (no co-firing). This study considers a functional unit
as one kWh of electricity produced in the power plant. The cradle-to-gate system boundaries are
depicted in Figure 1. Biomass supply chain included bundling, forwarding, transportation, and
chipping. The coal supply chain consists of two stages: coal mining and coal transportation. Biomass
chipping is considered to be conducted at the power plant to account for cost and energy effectiveness
reported for plants larger than 300 MW [4]. This study does not consider biomass production, as
available forest residue is directly taken into account for co-firing. Harvesting of biomass is excluded
from the system boundary, and biogenic carbon neutrality is assumed as in Zhang et al., (2010) [30].
In the power plant, consumption of materials and energy that are needed in excess for co-firing are
excluded from system boundary, due to negligible contribution to emissions [20,31]. Thus, water
consumption is assumed to be constant throughout the base- and co-firing cases.
Figure 1. System boundary for Life Cycle Assessment (LCA) study.
2.2. Inventories
2.2.1. Biomass Collection
Logging residues are partially piled along the roadside or left dispersed in the harvesting area,
and Forwarder is typically used for collection and piling of residues. The removal volume attributed
to logging residues is directly related to harvesting areas. In East Texas, 43.5 t per acre were utilized
while 10.1 t per acre were left as logging residue, excluding the residual stumps in 2008, that were
only for trees taller than 5 inches [32]. Assuming a 20% recovery rate for trees shorter than 5 inches
as per Mathison et al., (2009), an additional 0.9 t per acre was added that made total logging residue
11.0 t per acre in 2008 [32]. In this study, John Deere 1010E was considered as forwarder with an
engine power of 115.5 KW. Forwarders of 80–120 kW output power (class II) with a load capacity of
10–12 t were considered, and fuel consumption was estimated as per Equation (1) [33,34]. Y is the
fuel consumption (L/h), and X is the engine output power (kW). Productivity of forwarder largely
depends on hauling distance and can be calculated as in Equation (2), P is productivity (m3/PMH),
and L is the average hauling distance (m) [35]. It is assumed that average distance per trip is 300 m,
as per Akay et al., (2004) and SimaPro input data is prepared by considering 8 h/day machine
operation and lubricants consumption of 0.349 L/green t. Table 1 describes the inventory data [36].
Bulk density of biomass is relatively low and has an effect on transportation stage emissions.
Bundling allows for achieving maximum bulk density, which is important for transportation and is
referred to as composite residues logs (CRL) or bundles. John Deere 1490D is a common bundler for
CRL operations, and the maximum productivity of John Deere 1490D bundler is 30 bundles/PMH,
which consumes 3 gal/h fuel [37,38]. This bundler compacts and wraps slash into 10 ft long bundles
with an average diameter of 27 inches. The volume of one bundle is approximately 0.7 m3 and average
bundle weight is 0.55 t [39]. Moisture content, harvested tree species, forest residue density, forest
residue arrangement, and operator skill are critical parameters for productivity of bundler [40].
Inventories for SimaPro® are prepared considering 8 h per day of operation, as described in Table 1.
Figure 1. System boundary for Life Cycle Assessment (LCA) study.
2.2. Inventories
2.2.1. Biomass Collection
Logging residues are partially piled along the roadside or left dispersed in the harvesting area,
and Forwarder is typically used for collection and piling of residues. The removal volume attributed
to logging residues is directly related to harvesting areas. In East Texas, 43.5 t per acre were utilized
while 10.1 t per acre were left as logging residue, excluding the residual stumps in 2008, that were only
for trees taller than 5 inches [
32
]. Assuming a 20% recovery rate for trees shorter than 5 inches as per
Mathison et al., (2009), an additional 0.9 t per acre was added that made total logging residue 11.0 t per
acre in 2008 [
32
]. In this study, John Deere 1010E was considered as forwarder with an engine power
of 115.5 KW. Forwarders of 80–120 kW output power (class II) with a load capacity of 10–12 t were
considered, and fuel consumption was estimated as per Equation (1) [
33
,
34
]. Yis the fuel consumption
(L/h), and Xis the engine output power (kW). Productivity of forwarder largely depends on hauling
distance and can be calculated as in Equation (2), Pis productivity (m
3
/PMH), and Lis the average
hauling distance (m) [
35
]. It is assumed that average distance per trip is 300 m, as per Akay et al.,
(2004) and SimaPro input data is prepared by considering 8 h/day machine operation and lubricants
consumption of 0.349 L/green t. Table 1describes the inventory data [
36
]. Bulk density of biomass
is relatively low and has an effect on transportation stage emissions. Bundling allows for achieving
maximum bulk density, which is important for transportation and is referred to as composite residues
logs (CRL) or bundles. John Deere 1490D is a common bundler for CRL operations, and the maximum
productivity of John Deere 1490D bundler is 30 bundles/PMH, which consumes 3 gal/h fuel [
37
,
38
].
This bundler compacts and wraps slash into 10 ft long bundles with an average diameter of 27 inches.
The volume of one bundle is approximately 0.7 m
3
and average bundle weight is 0.55 t [
39
]. Moisture
content, harvested tree species, forest residue density, forest residue arrangement, and operator skill are
Sustainability 2018,10, 2193 4 of 18
critical parameters for productivity of bundler [
40
]. Inventories for SimaPro
®
are prepared considering
8 h per day of operation, as described in Table 1.
Y=46.4 ∗10−3X+7.222 (1)
P=17.0068 ∗L13.2533
L(2)
Table 1. Inventory data for biomass collection.
Item Value Unit References and Assumptions
Forwarding of residue (1 h)
Forwarder 6.85 ×10−5P * Assuming service life is 14,600 PMH * [4]
Lubricating oil 0.515 kg [41]
Diesel, low sulfur
10.6 kg Density of diesel 0.84 kg/L (Ecoinvent database).
Bundling of residue (1 h)
Bundler 6.85 ×10−5P * Assuming service life is 14,600 PMH * [4]
Diesel, low sulfur
9.54 kg Density of diesel 0.84 kg/L
Lubricating oil 0.608 kg Consumption rate of lubricating oil were taken from and density is
0.98 g/cm3[35]
Packaging film,
low-intensity
polyethylene
2.4 kg Used to fix bundles, 0.08 kg of PA per bundle (Ecoinvent database)
Vegetable oil 0.354 kg Used for lubricating chainsaw [41]
* P is unit of machinery as per SimaPro®library; PMH is productive machine hour.
2.2.2. Biomass Transportation and Chipping
Long haul truck is commonly used for transporting biomass from forest site to power plant, and
the current study considers a long long-haul truck that can carry 41 t of biomass with fuel consumption
of 0.0319 L/tonne-km (t-km), as specified in Table 2[
7
,
42
]. There are different types of chippers that
grind the residues, but a large-scale chipper that can reduce the cost, along with terminal on-site
chipping is considered in the current study [
43
]. An open drum chipper, Biber 92 that has 358 kW
power and sieve size of 50, with fuel consumption of 2.8 L/t and productivity of 25.8 t/h is used,
as specified in Table 2[
43
]. The higher heating values (HHV) of logging residues are 12,401 kJ/kg
and 13,951 kJ/kg for hardwood and softwood, respectively [
44
]. The percentage of hardwood logging
residues and softwood are 33.80% and 66.20%, respectively [
23
]. Residues collected from different
counties in Texas have variable distance from the W. A. Parish (WAP) power plant, and a weighted
average distance of 183.9 miles is calculated for transportation stage. Details on the counties and
distances to WAP plant are provided in Tables 1and 2of the Supplementary Information.
Table 2. Inventory data for biomass transportation and processing.
Item Value Unit References and Assumptions
Transport, long haul truck (1 t-km)
Diesel 2.68 ×10−2kg Density of diesel 0.84 kg/L (Ecoinvent database).
Chipping of biomass (1 h)
Diesel, low sulfur 60.7 kg Density of diesel 0.84 kg/L (Ecoinvent database).
Lubricating oil 0.925 kg Ecoinvent database
2.2.3. Coal Mining and Transportation
WAP uses powder basin river sub-bituminous (PRB) coal from Wyoming, with a moisture content
of 27.66%, gross calorific value of 19,119.72 kJ/kg [
45
]. PRB coal has 36% of fixed carbon, 30.10% volatile
Sustainability 2018,10, 2193 5 of 18
matter, and 0.25% of organic sulfur. This study considers standard mining inventories from ecoinvent
database in SimaPro. Fuel requirement for coal transportation was taken as the default US standard
from U.S. life cycle inventory (LCI) database in SimaPro, and details of the calculation are provided in
Appendix A. Diesel-powered train is selected as the mode of transportation, with diesel consumption
of 0.006482 L/t-km. The inventories in Table 3were calculated based on the distance from Wyoming
to WAP power plant (1378 miles). Coal losses in the supply chain are considered to be 4% [46].
Table 3. Inventory of coal at power plant.
Item Amount Unit Comment
PRB sub-bituminous coal at Power plant 1 Kg
Moisture content 0.277 Kg
Moisture content of PRB coal is 27.66% [
45
]
Ash content 6.44 ×10−2Kg Ash content is 6.44% [45]
Energy, gross calorific value 19,119 kJ
Coal, at mine 1.00 Kg Considering coal loses in supply chain [45]
Transport, train, diesel powered/US 2.23 t-km tonne-kilometer is expressed as t-km
2.2.4. Co-Firing at Power Plant
The average emissions of all air pollutants for the WAP power plant was obtained from Airs
Facility Subsystem (AFS) file for inventory of base case (no co-firing) operational stage emissions.
AFS file was developed by Texas Commission on Environment Quality (TCEQ) and contains hourly
emissions from power plant [
27
]. For the base case, the average emissions of CO, NH
3
, SO
2
, NO
X
,
PM
2.5
, and VOC are 0.235107 kg/MWh, 0.0001467 kg/MWh, 2.142404 kg/MWh, 0.213116 kg/MWh,
0.089681 kg/MWh, and 0.006434 kg/MWh, respectively. These emissions are used as emissions from
combustion stage for the base case. The amount of water and fuel were taken from SimaPro US
LCI database. The CO
2
emissions from coal-fired electricity production are 939 kg/MWh [
30
,
42
].
The coal amount (0.554 kg/kWh) was calculated by assuming a 34% overall efficiency and a calorific
value of 19,120 kJ/kg. In this study, biomass co-firing was considered on an energy basis. If HV
b
is
average heating value of biomass, HV
C
is the average heating value of coal, and H
b
is the heating
ratio, then biomass requirements based on the heat basis (M
b
) in percentage are represented in
Equation (3) [
47
]. Here, HV
C
= 19,120 kJ/kg and HV
b
= 13,426 kJ/kg, which gives 6.973%, 13.662%,
and 20.084% on mass basis for energy basis ratios of 5%, 10%, and 15%, respectively.
Mb=Hb×
1
HVb
Hb
100×HVb+1−Hb
100
HVc
(3)
More than 100 successful field demonstrations in 16 different countries have taken place
for co-firing that use major type of biomass (wood, animal waste, herbaceous waste) combined
with various types of coal in a pulverized fuel boiler (tangential, wall, and cyclone fired) [
48
,
49
].
The estimates for SO
2
emissions reduction were 3.84% for 5% co-firing ratio (energy basis) and 6.89%
for 10% co-firing (energy basis) at Albright Generating Station and Michigan City Generation Station,
respectively, by using PRB coal and woody biomass [
11
,
50
]. A study conducted with wood waste
co-firing with PRB coal in Michigan provided the relation (RNOx = 0.75B) of NO
x
emissions reduction
in combustion stage, where RNO
x
is the NO
x
emissions reduction, and B is the percentage of biomass
in the fuel blend (mass basis) [
11
]. Carbon monoxide (CO) reductions of 1% and 5% for 5% and
15% energy-based co-firing were obtained from Mann and Spath’s 2002 study that considered wood
residues as biomass [
20
]. Another study reported 10.05% reduction of CO for 20% energy basis co-firing
coal with woody biomass [
19
]. Decrease in particulate matters (PM) emissions was reported with
co-firing of forest residue with coal, and a linear equation was developed (y= 0.9x
−
1.5633, where xis
the co-firing ratio, and yis the percentage reduction of emission) by considering emissions reduction as
3%, 7.31%, and 12% for 5%, 10%, and 15% co-firing, respectively. Volatile organic compounds emission
Sustainability 2018,10, 2193 6 of 18
reduced 11.20% for 20% co-firing of wood pellets with coal [
19
]. The estimates for CO
2
reductions due
to co-firing (9.82%, 15%, and 27.23% reduction for 10%, 20%, and 25% co-firing) were obtained from
previously published studies [
19
,
20
,
51
–
53
]. Ammonia emissions do not have any significant change
due to co-firing [51]. The inventories for base case are summarized in Table 4.
Table 4. Co-firing stage inventories for base case (0% co-firing, energy basis) for 1 kWh electricity.
Item Amount Unit Comment
Output
Electricity, sub bituminous coal, at power plant
1 kWh
Inputs
(materials/fuels)
Light fuel oil [18] market for/Conseq,U 1.58 ×10−4kg
Default data from electricity [WECC]/US,
SimaPro database. Used for start-up the
power plant.
Water, decarbonized, at user {GLO}/market
for/Conseq,U 1.39 kg Default data from electricity[WECC]/US,
SimaPro database
NOXretained, by selective catalytic
reduction{GLO}/market for/Consec,U 1.05 ×10−3kg Default data from electricity[WECC]/US,
SimaPro database
Water, completely softened, from decarbonized
water, at user {GLO}/market for/Conseq,U 5.57 ×10−2kg Default data from electricity[WECC]/US,
SimaPro database
PRB sub-bituminous coal at Power plant 0.554 kg Calculated by considering plant
efficiency 34%.
Forest residue at power plant 0 kg Base case (0% co-firing)
3. Results
3.1. Emissions from Coal and Biomass Supply Chains
The summary of emissions from biomass and coal supply chains is presented in Table 5.
Forwarding, bundling, and chipping machines use low-sulfur diesel, and the transportation process
in SimaPro database uses varying grades of diesel fuel. Transporting 1 t of biomass to power plant
requires 7.85 kg of diesel fuel, whereas forwarding, bundling, and chipping requires 1.0162 kg,
0.6069 kg,
and 2.4679 kg
, respectively. Transportation of biomass is the stage responsible for highest
CO
2
emissions followed by other stages of biomass handling such as chipping. The lowest CO
2
emissions are from the forwarding stage. Forwarder and bundler operated over distances of 25 km
and 60 km per day, respectively, that contributed to air emissions presented in Table 5(SimaPro
database). Even though diesel consumption in operational stage of bundling is lower than the
forwarding stage, emissions for bundling are higher due to higher material input and more processes.
Transportation emits highest CO
2
followed by chipping in accordance with previous studies [
4
].
Carbon monoxide emissions follow a similar trend as CO
2
emissions, with transportation stage being
the largest contributor and forwarding stage being the lowest, as CO emissions are from incomplete
combustion where the oxidation process is not stoichiometrically balanced [54].
Table 5. Summary of emissions from 1 t of biomass and coal supply chains.
Pollutant
Coal (1 t) Biomass (1 t)
Mining Transportation Forwarding Bundling Transportation Chipping
CO2(kg) 63.1 47.2 4.28 5.00 26.5 10.1
CO (g) 268 290 15.0 16.5 160 30.9
NOx(g) 168 1150 34.8 21.0 193 33.0
SO2(g) 219 22.4 6.94 7.76 16.6 14.8
PMc(g) 4.48 28.5 0.77 1.58 0.90 0.84
Sustainability 2018,10, 2193 7 of 18
NO
x
emissions are highest for transportation and lowest for bundling. A study in California
noted that NO
x
emissions from 1 t forest biomass collection and processing are 123.64 g excluding
transportation; all the off-road equipment consumed 12.5496 L of diesel for 1 t of biomass [
55
].
Our study shows that biomass collection and processing emits 88.8 g excluding transportation where
fuel consumption by off-road equipment is 4.87 L. The difference could be due to the varying nature of
forestry equipment between the two studies and geographic differences. In our study, PM
2.5
is the
major pollutant that needs to be analyzed. No PM
10
emissions were reported for transportation stage,
but emission of PM
C
(>2.5, and <10) was 0.904 g. Chipping emits all three sizes of particulate matter,
and combined emissions are the highest source for PM. The bundling process has higher emissions for
both PM
10
and PM
C
. The dominant source of PM is the use of diesel in the biomass supply chain [
56
].
According to SimaPro database, the diesel selected for transportation does not result in quantifiable
PM
10
emissions during combustion. Sulfur dioxide emissions are also highest during transportation
stage and lowest in the forwarding stage. Non-methane volatile organic compounds (NMVOC) and
methane also follow the same trend as SO
2
. Methane emissions are about nine times higher during
transportation stage compared to forwarding stage. NMVOC and methane both are lowest during
forwarding of residue and highest during transportation.
Coal cleaning is part of the coal mining process and one t coal washing requires 6.52 kWh of
electricity, which can be converted into 2.18 kg of coal [
8
]. One t of coal mining emits 63.1 kg of
CO
2
, and transport to WAP from Wyoming emits 47.2 kg. Emissions during mining are greater than
transportation for coal life cycle. Diesel combustion and electricity used in mining results in emission
of 24.4 kg and 22.4 kg of CO
2
, respectively. The remaining process of mining emits 15.7 kg. In the case
of transportation, 42.2 kg of CO
2
emits from diesel consumption. The average carbon dioxide emission
from 1 t-km of diesel based train is 22 g [
57
]. Considering this rate, 1 t of coal transportation from
Wyoming to WAP should result in 44.4 kg of CO
2
. Gasoline combustion in the mining stage results in
emission of 116 g of CO, which is the single process responsible for maximum contribution, followed
by emissions from diesel consumption at the refinery. Nitrogen oxides also follow a similar trend
with respect to contribution of individual stages, due to electricity obtained from bituminous coal
needed for mining. SO
2
emissions from transportation are lower than mining. According to this study,
only PM
C
is emitted during coal mining and transportation, with transportation stage for 1 t coal
resulting in emissions of 28.5 g of PM
C
. The higher emissions from transportation stage is principally
due to diesel combustion by train (27.6 g). Mining of coal emits more VOC than transportation stage,
but NMVOC emissions are higher in transportation stage.
3.2. Life Cycle Emissions for 1 kWh Electricity Generation
WAP power plant uses NO
x
control by selective catalytic reduction (SCR) system, baghouse for
PM control, and flue gas desulfurization (FGD) for SO
2
control. Considering these processes from
SimaPro database, along with water use that is needed for cooling the system and fuel oil for start-up
of the power plant, life cycle emissions are calculated and presented in Table 6. It is assumed that all
these systems will remain functional for co-firing scenarios. More than 90% of emissions during the
life cycle are from combustion stage. Life cycle emissions of CO
2
were reported to be 1050 g/kWh,
considering PC boiler, FGD, and SCR [
58
]. However, the varying nature of coal (sub-bituminous) that
affects the carbon content is responsible for the deviation in our results. Life cycle emission of CO
2
reduces by 13.45%, 8.31%, and 3.26% for 15%, 10%, and 5% co-firing, respectively, as shown in Figure 2.
This result is comparable to the reduction in CO
2
emissions of 8% for 10% co-firing with forest biomass
as per a study in Colorado [
53
]. The life cycle emissions of CO reduce by 6.40%, 3.90%, and 1.41% for
15%, 10%, and 5% co-firing, which is relatively lower reduction compared to CO
2
emissions. A study
conducted in Vietnam with wood residue co-firing with bituminous coal reported that, for 5% and 15%
co-firing ratios, CO emissions reduce by 1% and 5%, respectively [59].
Sustainability 2018,10, 2193 8 of 18
Table 6. Life cycle emissions (air) from biomass co-firing in WAP power plant.
Pollutant Base Case 5% Co-Firing 10% Co-Firing 15% Co-Firing
CO2(g/kWh) 1010 977 927 875
CO (g/kWh) 0.558 0.550 0.536 0.523
SO2(g/kWh) 2.410 2.320 2.250 2.180
NOx(g/kWh) 0.949 0.913 0.876 0.839
PM2.5 (g/kWh) 9.140 ×10−28.960 ×10−28.630 ×10−28.300 ×10−2
PM (>2.5, <10) (g/kWh) 2.420 ×10−22.350 ×10−22.270 ×10−22.200 ×10−2
VOC (g/kWh) 2.480 ×10−22.420 ×10−22.350 ×10−22.290 ×10−2
NMVOC (g/kWh) 7.210 ×10−27.090 ×10−26.970 ×10−26.840 ×10−2
Methane, fossil (g/kWh) 1.840 ×10−21.880 ×10−21.910 ×10−21.950 ×10−2
NH3(g/kWh) 3.720 ×10−33.730 ×10−33.750 ×10−33.770 ×10−3
Sustainability 2018, 10, x FOR PEER REVIEW 8 of 18
Figure 2. Percentage reduction of life cycle air emissions due to co-firing.
NOX is a major pollutant that causes ground level ozone in the Greater Houston area. Co-firing
of biomass with coal decreases life cycle emissions of NOx by 3.80%, 7.69%, and 11.59% for 5%, 10%,
and 15% co-firing, respectively [28]. Biomass contains higher oxygen content and is more volatile fuel
than coal. The volatile content establishes a fuel-rich zone early in the flame and leads to reduction
in NOx emissions. The combustion stage is responsible only for 22.45% of NOx emissions, while coal
processing and transportation accounts for the major portion. This result is in contrast to the co-firing
study conducted in Vietnam, that reported that the highest NOX emissions are from combustion stage
[59]. However, the discrepancy could be due to the consideration of the SCR system for NOx control
in the current study that can reduce more than 80% of NOX emissions from WAP power plant [60].
Life cycle emission of SO2 reduces by 3.73%, 6.64%, and 9.54% for 5%, 10%, and 15% co-firing ratio,
respectively, and the primary reason is low-sulfur content in the biomass.
Life cycle PM2.5 emission reduces by 9.19% for 15% co-firing, due to the combination of
baghouses at WAP power plant and low sulfur content of biomass. Volatile organic compounds are
mostly emitted from upstream processes, and life cycle VOC emissions reduced by 2.42%, 5.24%, and
7.66% for 5%, 10%, and 15% co-firing ratios, respectively. There is no significant change in ammonia
emission due to co-firing, in accordance with previous studies [51]. The emissions reduction from the
combustion stage, which has a direct impact on the air quality of the Greater Houston area, is
depicted in Figure 3.
0
1
2
3
4
5
6
7
8
9
10
11
12
51015
% reduction of emissions
Co-firing ratio
SO2 PM2.5 VOC CO NOx
Figure 2. Percentage reduction of life cycle air emissions due to co-firing.
NO
X
is a major pollutant that causes ground level ozone in the Greater Houston area. Co-firing
of biomass with coal decreases life cycle emissions of NO
x
by 3.80%, 7.69%, and 11.59% for 5%, 10%,
and 15% co-firing, respectively [
28
]. Biomass contains higher oxygen content and is more volatile fuel
than coal. The volatile content establishes a fuel-rich zone early in the flame and leads to reduction in
NO
x
emissions. The combustion stage is responsible only for 22.45% of NO
x
emissions, while coal
processing and transportation accounts for the major portion. This result is in contrast to the co-firing
study conducted in Vietnam, that reported that the highest NO
X
emissions are from combustion
stage [
59
]. However, the discrepancy could be due to the consideration of the SCR system for NO
x
control in the current study that can reduce more than 80% of NO
X
emissions from WAP power
plant [
60
]. Life cycle emission of SO
2
reduces by 3.73%, 6.64%, and 9.54% for 5%, 10%, and 15%
co-firing ratio, respectively, and the primary reason is low-sulfur content in the biomass.
Life cycle PM
2.5
emission reduces by 9.19% for 15% co-firing, due to the combination of baghouses
at WAP power plant and low sulfur content of biomass. Volatile organic compounds are mostly emitted
from upstream processes, and life cycle VOC emissions reduced by 2.42%, 5.24%, and 7.66% for 5%,
10%, and 15% co-firing ratios, respectively. There is no significant change in ammonia emission due
to co-firing, in accordance with previous studies [
51
]. The emissions reduction from the combustion
stage, which has a direct impact on the air quality of the Greater Houston area, is depicted in Figure 3.
Sustainability 2018,10, 2193 9 of 18
Sustainability 2018, 10, x FOR PEER REVIEW 9 of 18
Figure 3. Percentage reduction of air emissions from combustion stage.
3.3. Mid-Point Impact Assessment
Life Cycle Impact Assessment (LCIA) is a method that assesses the environmental aspects and
potential impacts associated with a goods or a service [13]. LCIA results can be linked to damage
categories via midpoint categories. There are 14 mid-point impact categories in the IMPACT 2002+
method, where 9 categories (human toxicity, respiratory inorganics, respiratory organics, aquatic
ecotoxicity, terrestrial acidification/nitrification, aquatic acidification, non-renewable energy, mineral
extraction, and global warming) show a reduction in potential impact due to co-firing with forest
residue at the WAP power plant. The five impact categories of ionizing radiation, ozone layer
depletion, terrestrial ecotoxicity, land utilization, and aquatic eutrophication show an increase in
potential impact. Human toxicity includes both carcinogenic and non-carcinogenic effects; processing
of coal is mostly responsible for non-carcinogens and heavy metals such as mercury [60]. In co-firing,
use of coal decreases, causing overall reductions in heavy metals and thereby leading to lowering of
human toxicity. This result aligns with the finding that co-firing of coal with woody biomass reduces
impact on human toxicity [19]. Acidification is driven by the release of acidic gases like SO2, and NH3.
Biomass has lower sulfur content, which results in lowering oxides of sulfur due to combustion, and
thereby reduces the potential for aquatic and terrestrial acidification. In addition, life cycle impacts
from the current study show that PM reduction can contribute to lowering of human toxicity. Coal
mining and transportation are responsible for higher emissions of VOCs, and co-firing with lower
mass of coal would have thus contributed to lowering of respiratory organics. Mining of coal is
mostly responsible for aquatic ecotoxicity; lower coal use reduces aquatic ecotoxicity impact. Coal
processing is responsible for more than 99% of non-renewable energy impacts. For instance, coal
processing has contributed 49.5 MJ primary/kWh in the 15% co-firing case. Using lower amounts of
coal has low non-renewable energy impacts. Mineral extraction category also follows the same trend
as non-renewable energy impact. The biggest decrease at midpoint impact is the non-renewable
energy category, followed by mineral extraction and global warming potential (GWP). Global
warming potential decreased by 13.24% for 15% co-firing. The atmospheric accumulation of
greenhouse gases, such as CO2, N2O, and CH4 will be lowered due to co-firing due to the lack of net
CO2 accumulation from existing forest residue. GWP reduces by 15.63% for 20% raw pellets co-firing
with coal. Table 7 summarizes the midpoint impacts.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
51015
% reduction of emissions in combustion
Co-firing ratio
CO SO2 NOx PM2.5 VOC
Figure 3. Percentage reduction of air emissions from combustion stage.
3.3. Mid-Point Impact Assessment
Life Cycle Impact Assessment (LCIA) is a method that assesses the environmental aspects and
potential impacts associated with a goods or a service [
13
]. LCIA results can be linked to damage
categories via midpoint categories. There are 14 mid-point impact categories in the IMPACT 2002+
method, where 9 categories (human toxicity, respiratory inorganics, respiratory organics, aquatic
ecotoxicity, terrestrial acidification/nitrification, aquatic acidification, non-renewable energy, mineral
extraction, and global warming) show a reduction in potential impact due to co-firing with forest
residue at the WAP power plant. The five impact categories of ionizing radiation, ozone layer depletion,
terrestrial ecotoxicity, land utilization, and aquatic eutrophication show an increase in potential impact.
Human toxicity includes both carcinogenic and non-carcinogenic effects; processing of coal is mostly
responsible for non-carcinogens and heavy metals such as mercury [
60
]. In co-firing, use of coal
decreases, causing overall reductions in heavy metals and thereby leading to lowering of human
toxicity. This result aligns with the finding that co-firing of coal with woody biomass reduces impact
on human toxicity [
19
]. Acidification is driven by the release of acidic gases like SO
2
, and NH
3
.
Biomass has lower sulfur content, which results in lowering oxides of sulfur due to combustion,
and thereby reduces the potential for aquatic and terrestrial acidification. In addition, life cycle
impacts from the current study show that PM reduction can contribute to lowering of human toxicity.
Coal mining and transportation are responsible for higher emissions of VOCs, and co-firing with
lower mass of coal would have thus contributed to lowering of respiratory organics. Mining of
coal is mostly responsible for aquatic ecotoxicity; lower coal use reduces aquatic ecotoxicity impact.
Coal processing
is responsible for more than 99% of non-renewable energy impacts. For instance,
coal processing has contributed 49.5 MJ primary/kWh in the 15% co-firing case. Using lower amounts
of coal has low non-renewable energy impacts. Mineral extraction category also follows the same
trend as non-renewable energy impact. The biggest decrease at midpoint impact is the non-renewable
energy category, followed by mineral extraction and global warming potential (GWP). Global warming
potential decreased by 13.24% for 15% co-firing. The atmospheric accumulation of greenhouse gases,
such as CO
2
, N
2
O, and CH
4
will be lowered due to co-firing due to the lack of net CO
2
accumulation
from existing forest residue. GWP reduces by 15.63% for 20% raw pellets co-firing with coal. Table 7
summarizes the midpoint impacts.
Sustainability 2018,10, 2193 10 of 18
Table 7. Mid-point impacts of biomass co-firing at WAP power plant.
Impact Category Unit Base Case 5% Co-Firing 10% Co-Firing 15% Co-Firing
Human toxicity kg C2H3Cl eq/kWh 7.18 ×10−37.10 ×10−37.02 ×10−36.93 ×10−3
Respiratory inorganics kg PM2.5 eq/kWh 4.04 ×10−43.92 ×10−43.80 ×10−43.67 ×10−4
Ionizing radiation Bq C-14 eq/kWh 5.63 ×10−25.99 ×10−26.35 ×10−26.69 ×10−2
Ozone layer depletion kg CFC-11 eq/kWh 3.02 ×10−93.16 ×10−93.28 ×10−93.40 ×10−9
Respiratory organics kg C2H4eq/kWh 7.48 ×10−57.28 ×10−57.08 ×10−56.87 ×10−5
Aquatic ecotoxicity kg TEG water/kWh 22.9 22.4 21.9 21.3
Terrestrial ecotoxicity kg TEG soil/kWh 0.300 0.314 0.327 0.340
Terrestrial acidification
and nutrification kg SO2eq/kWh 7.72 ×10−37.48 ×10−37.25 ×10−37.03 ×10−3
Land occupation m2org.arable/kWh 5.67 ×10−57.05 ×10−58.37 ×10−59.64 ×10−5
Aquatic acidification kg SO2eq/kWh 3.45 ×10−33.34 ×10−33.25 ×10−33.16 ×10−3
Aquatic eutrophication kg PO4P-lim/kWh 1.20 ×10−61.30 ×10−61.39 ×10−61.48 ×10−6
Non-renewable energy MJ primary/kWh 58.3 55.4 52.5 49.6
Mineral extraction MJ surplus/kWh 9.35 ×10−38.90 ×10−38.45 ×10−38.00 ×10−3
Global warming kg CO2eq/kWh 1.030 0.998 0.946 0.893
Five midpoint impact categories have an increase in potential impact due to co-firing of which
aquatic eutrophication and ionizing radiation are the most concerning. The higher level of nutrients
and ionizing radiation in the aquatic system might be due to the sub-processes in the machinery used
for forest residue collection and the use of electricity generated from nuclear power in manufacturing of
the machinery [
61
]. In co-firing, forest residue uses more land, which is the main reason for increasing
impact. In the case of biomass, eutrophication impacts mainly come from use of fertilizer in forestry
process, while the mining stage is responsible for eutrophication impacts from coal [
6
]. It occurs due to
phosphate emissions from a different stage of LCA. Eutrophication potential of electricity production
increased by 16% from 20% biomass (rice, wheat) co-firing with coal in a study conducted in Turkey [
6
].
Ozone layer depletion (OLD) also increases with co-firing. This finding is consistent with a study
of co-firing with using raw wood, which suggested OLD could increase by 22.67% for 20% co-firing
ratio [
19
]. Terrestrial ecotoxicity and land occupation also has greater impact in co-firing than base
case. The relative increase and decrease in mid-point impact potentials with reference to base case
(treated as 1.00) is described in Figure 4.
Sustainability 2018, 10, x FOR PEER REVIEW 10 of 18
Table 7. Mid-point impacts of biomass co-firing at WAP power plant.
Impact Category Unit Base Case 5% Co-Firing 10% Co-Firing 15% Co-Firing
Human toxicity kg C
2
H
3
Cl eq/kWh 7.18 × 10
−3
7.10 × 10
−3
7.02 × 10
−3
6.93 × 10
−3
Respiratory inorganics kg PM
2.5
eq/kWh 4.04 × 10
−4
3.92 × 10
−4
3.80 × 10
−4
3.67 × 10
−4
Ionizing radiation Bq C-14 eq/kWh 5.63 × 10
−2
5.99 × 10
−2
6.35 × 10
−2
6.69 × 10
−2
Ozone layer depletion kg CFC-11 eq/kWh 3.02 × 10
−9
3.16 × 10
−9
3.28 × 10
−9
3.40 × 10
−9
Respiratory organics kg C
2
H
4
eq/kWh 7.48 × 10
−5
7.28 × 10
−5
7.08 × 10
−5
6.87 × 10
−5
Aquatic ecotoxicity kg TEG water/kWh 22.9 22.4 21.9 21.3
Terrestrial ecotoxicity kg TEG soil/kWh 0.300 0.314 0.327 0.340
Terrestrial acidification
and nutrification kg SO
2
eq/kWh 7.72 × 10
−3
7.48 × 10
−3
7.25 × 10
−3
7.03 × 10
−3
Land occupation m
2
org.arable/kWh 5.67 × 10
−5
7.05 × 10
−5
8.37 × 10
−5
9.64 × 10
−5
Aquatic acidification kg SO
2
eq/kWh 3.45 × 10
−3
3.34 × 10
−3
3.25 × 10
−3
3.16 × 10
−3
Aquatic eutrophication kg PO
4
P-lim/kWh 1.20 × 10
−6
1.30 × 10
−6
1.39 × 10
−6
1.48 × 10
−6
Non-renewable energy MJ primary/kWh 58.3 55.4 52.5 49.6
Mineral extraction MJ surplus/kWh 9.35 × 10
−3
8.90 × 10
−3
8.45 × 10
−3
8.00 × 10
−3
Global warming kg CO
2
eq/kWh 1.030 0.998 0.946 0.893
Five midpoint impact categories have an increase in potential impact due to co-firing of which
aquatic eutrophication and ionizing radiation are the most concerning. The higher level of nutrients
and ionizing radiation in the aquatic system might be due to the sub-processes in the machinery used
for forest residue collection and the use of electricity generated from nuclear power in manufacturing
of the machinery [61]. In co-firing, forest residue uses more land, which is the main reason for
increasing impact. In the case of biomass, eutrophication impacts mainly come from use of fertilizer
in forestry process, while the mining stage is responsible for eutrophication impacts from coal [6]. It
occurs due to phosphate emissions from a different stage of LCA. Eutrophication potential of
electricity production increased by 16% from 20% biomass (rice, wheat) co-firing with coal in a study
conducted in Turkey [6]. Ozone layer depletion (OLD) also increases with co-firing. This finding is
consistent with a study of co-firing with using raw wood, which suggested OLD could increase by
22.67% for 20% co-firing ratio [19]. Terrestrial ecotoxicity and land occupation also has greater impact
in co-firing than base case. The relative increase and decrease in mid-point impact potentials with
reference to base case (treated as 1.00) is described in Figure 4.
Figure 4. Cont.
Sustainability 2018,10, 2193 11 of 18
Sustainability 2018, 10, x FOR PEER REVIEW 11 of 18
Figure 4. Relative change in impact across mid-point impact categories (base-case impact treated as
1.00 for each category), due to co-firing: (a) categories that showed reduction in impact with co-firing,
and (b) categories that showed increase in impact due to co-firing.
3.4. End-Point Impact Assessment
All the midpoint categories can be classified into four damage categories as presented in Table 8
and Figure 5. Some midpoint categories have an increase in impact due to co-firing, but all the end-
point/damage categories show a reduced impact. This is primarily due to the low-weighting
associated with categories such as land occupation that are not of primary concern in a state like
Texas. The maximum reduction for 15% co-firing is in the resources category (14.85%), followed by
climate change (13.24%), as described in Figure 5. Previous end-point assessments made for biochar
co-firing with coal suggested that the potential impacts of all damage categories except human health
would improve [61]. Potential impact from the human health category increased for biochar, due to
processing of biochar being associated with increase of carcinogens, non-carcinogens, and respiratory
organics categories [61]. Our result indicates that forest residue would lead an improvement across
all damage categories. The current key constraint to the commercial scale use of biomass for electricity
production is profitability [62]. The market price of biomass-based energy often exceeds the fossil
fuel-based energy. The main reasons for exceeding market price are collection of biomass,
transportation, conversion and other costs. A study of co-firing (up to 15%) forest residue with coal
for electricity generation in East Texas estimated that logging residue costs of $21.01–$26.95 per t are
competitive with coal cost of $27.30/t considering average hauling distance of 200 miles [63]. The
study also reported that, for distances greater than 200 miles, the forest residue cost is not competitive
for energy production and increases with increase of co-firing ratio. Additionally, hauling distance
increases with the increase in required quantity of biomass, due to availability constraints. In our
study, the average hauling distance is 183.9 miles. This could be a limiting factor that determines the
cost-effectiveness of large-scale implementation of forest residue co-firing.
Table 8. Damage category (end-point) impact of co-firing.
Damage Category Unit Base Case 5% Co-Firing 10% Co-Firing 15% Co-Firing
Human health DALY/kWh 3.03 × 10
−7
2.95 × 10
−7
2.86 × 10
−7
2.77 × 10
−7
Ecosystem quality PDF * m
2
* yr/kWh 1.16 × 10
−2
1.15 × 10
−2
1.13 × 10
−2
1.12 × 10
−2
Climate change Kg CO
2
eq/kWh 1.03 1.00 0.95 0.89
Resources MJ primary 58.3 55.4 52.5 49.6
* DALY: Disability-Adjusted Life Years; PDF: Potentially disappeared fraction of species.
Figure 4.
Relative change in impact across mid-point impact categories (base-case impact treated as
1.00 for each category), due to co-firing: (
a
) categories that showed reduction in impact with co-firing,
and (b) categories that showed increase in impact due to co-firing.
3.4. End-Point Impact Assessment
All the midpoint categories can be classified into four damage categories as presented in Table 8
and Figure 5. Some midpoint categories have an increase in impact due to co-firing, but all the
end-point/damage categories show a reduced impact. This is primarily due to the low-weighting
associated with categories such as land occupation that are not of primary concern in a state like Texas.
The maximum reduction for 15% co-firing is in the resources category (14.85%), followed by climate
change (13.24%), as described in Figure 5. Previous end-point assessments made for biochar co-firing
with coal suggested that the potential impacts of all damage categories except human health would
improve [
61
]. Potential impact from the human health category increased for biochar, due to processing
of biochar being associated with increase of carcinogens, non-carcinogens, and respiratory organics
categories [
61
]. Our result indicates that forest residue would lead an improvement across all damage
categories. The current key constraint to the commercial scale use of biomass for electricity production
is profitability [
62
]. The market price of biomass-based energy often exceeds the fossil fuel-based energy.
The main reasons for exceeding market price are collection of biomass, transportation, conversion
and other costs. A study of co-firing (up to 15%) forest residue with coal for electricity generation
in East Texas estimated that logging residue costs of $21.01–$26.95 per t are competitive with coal
cost of $27.30/t considering average hauling distance of 200 miles [
63
]. The study also reported that,
for distances greater than 200 miles, the forest residue cost is not competitive for energy production and
increases with increase of co-firing ratio. Additionally, hauling distance increases with the increase in
required quantity of biomass, due to availability constraints. In our study, the average hauling distance
is 183.9 miles. This could be a limiting factor that determines the cost-effectiveness of large-scale
implementation of forest residue co-firing.
Sustainability 2018,10, 2193 12 of 18
Table 8. Damage category (end-point) impact of co-firing.
Damage Category Unit Base Case 5% Co-Firing 10% Co-Firing 15% Co-Firing
Human health DALY/kWh 3.03 ×10−72.95 ×10−72.86 ×10−72.77 ×10−7
Ecosystem quality PDF * m2* yr/kWh 1.16 ×10−21.15 ×10−21.13 ×10−21.12 ×10−2
Climate change Kg CO2eq/kWh 1.03 1.00 0.95 0.89
Resources MJ primary 58.3 55.4 52.5 49.6
* DALY: Disability-Adjusted Life Years; PDF: Potentially disappeared fraction of species.
Sustainability 2018, 10, x FOR PEER REVIEW 12 of 18
Figure 5. Reduction in Impact for Damage categories due to co-firing.
3.5. Uncertainty Analysis
Results presented in Table 9 describe the effect of introducing uncertainty in the transportation
of forest residue supply chains on the potential mid-point impacts. An analysis for 15% uncertainty
in the transportation of biomass at 10% and 15% co-firing ratios suggests that aquatic ecotoxicity,
human toxicity, respiratory organics/inorganics, and terrestrial acidification/nitrification would be
impact categories most affected. The emissions from diesel used in trucks that transport biomass to
the WAP Power plant is the major factor that results in higher impact for respiratory
organics/inorganics and human toxicity when distances increase. Negligible change was observed
for categories such as ozone layer depletion, ionizing radiation, land occupation, and terrestrial
ecotoxicity because truck transportation is not a major source of chlorofluoro carbons and
solid/hazardous landfill waste, in comparison to coal combustion that generates fly ash at the power
plant. The emissions of air pollutants such as NOx and NH3 increase with distances which biomass
is transported, as listed in Table 10, thereby causing higher levels of aquatic acidification. Changes in
VOC and PM2.5 described in Table 10 also contribute to the mid-point impact categories of human
toxicity and respiratory organics/inorganics. The relative changes observed for increase or decrease
in transportation distances are not symmetrical, as noticed for the respiratory inorganics mid-point
category. For an increase of +15% transportation distance, there is an increase of 0.58% impact
potential, whereas lowering the transportation distance by 15%, only lowers the impact by 0.27%.
The variation in fuel efficiency of trucks, and non-proportional changes in emissions of VOC,
NMVOC, and CO could be the reasons for this asymmetrical behavior. The changes in emissions of
NOx, PM2.5, and NH3 are symmetrical with uncertainty in transportation.
Table 9. Relative change in mid-point impacts due to uncertainty in transportation of biomass
Impact Category Unit 10% Co-Firing Ratio 15% Co-Firing Ratio
+15% −15% +15% −15%
Human toxicity Kg C2H3Cl eq/kWh 0.28% −0.28% 0.58% −0.43%
Respiratory inorganics kg PM2.5 eq/kWh 0.26% −0.26% 0.54% −0.27%
Ionizing radiation Bq C-14 eq/kWh 0.00% 0.00% 0.00% 0.00%
Ozone layer depletion kg CFC-11 eq/kWh 0.00% 0.00% 0.00% 0.00%
Respiratory organics kg C2H4 eq/kWh 0.14% 0.00% 0.29% −0.44%
Aquatic ecotoxicity kg TEG water/kWh 0.46% −0.91% 1.41% −0.94%
Terrestrial ecotoxicity kg TEG soil/kWh 0.00% 0.00% 0.00% 0.00%
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
51015
% reduction of damage categories
Co-firing ratio
Human health Ecosystem quality Climate change Resources
Figure 5. Reduction in Impact for Damage categories due to co-firing.
3.5. Uncertainty Analysis
Results presented in Table 9describe the effect of introducing uncertainty in the transportation
of forest residue supply chains on the potential mid-point impacts. An analysis for 15% uncertainty
in the transportation of biomass at 10% and 15% co-firing ratios suggests that aquatic ecotoxicity,
human toxicity, respiratory organics/inorganics, and terrestrial acidification/nitrification would be
impact categories most affected. The emissions from diesel used in trucks that transport biomass to the
WAP Power plant is the major factor that results in higher impact for respiratory organics/inorganics
and human toxicity when distances increase. Negligible change was observed for categories such
as ozone layer depletion, ionizing radiation, land occupation, and terrestrial ecotoxicity because
truck transportation is not a major source of chlorofluoro carbons and solid/hazardous landfill waste,
in comparison to coal combustion that generates fly ash at the power plant. The emissions of air
pollutants such as NOx and NH
3
increase with distances which biomass is transported, as listed in
Table 10, thereby causing higher levels of aquatic acidification. Changes in VOC and PM
2.5
described
in Table 10 also contribute to the mid-point impact categories of human toxicity and respiratory
organics/inorganics. The relative changes observed for increase or decrease in transportation distances
are not symmetrical, as noticed for the respiratory inorganics mid-point category. For an increase of
+15% transportation distance, there is an increase of 0.58% impact potential, whereas lowering the
transportation distance by 15%, only lowers the impact by 0.27%. The variation in fuel efficiency of
trucks, and non-proportional changes in emissions of VOC, NMVOC, and CO could be the reasons for
this asymmetrical behavior. The changes in emissions of NOx, PM
2.5
, and NH
3
are symmetrical with
uncertainty in transportation.
Sustainability 2018,10, 2193 13 of 18
Table 9. Relative change in mid-point impacts due to uncertainty in transportation of biomass.
Impact Category Unit 10% Co-Firing Ratio 15% Co-Firing Ratio
+15% −15% +15% −15%
Human toxicity Kg C2H3Cl eq/kWh 0.28% −0.28% 0.58% −0.43%
Respiratory inorganics kg PM2.5 eq/kWh 0.26% −0.26% 0.54% −0.27%
Ionizing radiation Bq C-14 eq/kWh 0.00% 0.00% 0.00% 0.00%
Ozone layer depletion kg CFC-11 eq/kWh 0.00% 0.00% 0.00% 0.00%
Respiratory organics kg C2H4eq/kWh 0.14% 0.00% 0.29% −0.44%
Aquatic ecotoxicity kg TEG water/kWh 0.46% −0.91% 1.41% −0.94%
Terrestrial ecotoxicity kg TEG soil/kWh 0.00% 0.00% 0.00% 0.00%
Terrestrial acidificationand
nutrification kg SO2eq/kWh 0.41% −0.28% 0.43% −0.57%
Land occupation m2org.arable/kWh 0.00% 0.00% 0.00% 0.00%
Aquatic acidification kg SO2eq/kWh 0.00% −0.31% 0.00% −0.32%
Aquatic eutrophication kg PO4P-lim/kWh 0.00% 0.00% 0.00% 0.00%
Non-renewable energy MJ primary/kWh 0.00% 0.00% 0.00% 0.00%
Mineral extraction MJ surplus/kWh 0.00% 0.00% 0.00% 0.00%
Global warming kg CO2eq/kWh 0.00% 0.00% 0.11% 0.00%
Table 10. Relative change in air pollutant emissions due to uncertainty in transportation of biomass.
Pollutant
10% Co-Firing Ratio 15% Co-Firing Ratio
+15% −15% +15% −15%
CO2(g/kWh) 0.01% −0.10% 0.01% −0.05%
CO (g/kWh) 0.37% −0.30% 0.19% −0.76%
SO2(g/kWh) 0.00% 0.00% 0.00% 0.00%
NOx(g/kWh) 0.23% −0.23% 0.36% −0.36%
PM2.5 (g/kWh) 0.12% −0.12% 0.12% −0.12%
PM (>2.5, <10) (g/kWh) 0.00% 0.00% 0.00% −0.45%
VOC (g/kWh) 0.85% −0.43% 0.87% −0.87%
NMVOC (g/kWh) 0.29% −0.29% 0.00% −0.44%
Methane, fossil (g/kWh) 0.00% 0.00% 0.00% −0.51%
NH3(g/kWh) 0.27% −0.27% 0.27% −0.27%
4. Discussion
Profitability is a key current constraint to the commercial use of biomass for electricity production.
The market price of biomass-based energy often exceeds that of fossil-fuel-based energy [
62
]. The main
reasons for exceeding market price are costs involved with collection of biomass, transportation,
and conversion. A study of co-firing (up to 15%) forest residue with coal for electricity generation by
Ismayilova, (2007), estimated that logging residue costs $21.01–$26.95 per ton are competitive with coal
cost of $27.30/t considering average hauling distance of 200 miles [
63
]. The study also reported that
for distances greater than 200 miles, the forest residue cost is not competitive for energy production,
and cost is increaseds with increase of co-firing ratio, because hauling distance increases with the
increase in required amount of biomass. The same study also reported that using forest residue
can increase the new jobs that help the local economy. In our study, the average hauling distance is
183.90 miles. It suggests that co-firing at WAP will be economical up to 15%.
The majority of impact categories have lower life cycle environmental impact in co-firing,
except ionizing radiation, ozone layer depletion, terrestrial ecotoxicity, land utilization, and aquatic
eutrophication. In co-firing, use of coal decreases, causing an overall reduction in human toxicity.
Co-firing of coal with woody biomass reduces impact on human toxicity [
19
]. Acidification is the
response of acid gases like SO
2
, NH
3
, and NO
x
. As biomass contains less amount of sulfur, reduction in
acidification potential is intuitive. Also, the life cycle analysis of this study concludes that PM reduced
Sustainability 2018,10, 2193 14 of 18
in co-firing cases. These factors are responsible for the reduction of impact categories: respiratory
inorganics, respiratory organics, terrestrial acidification and nitrification, and aquatic acidification.
Coal mining and transportation emits higher respiratory organics than biomass case. Mining of
coal is mostly responsible for aquatic ecotoxicity; lower coal use reduces aquatic ecotoxicity impact.
Coal processing is responsible for more than 99% of non-renewable energy impacts. Coal processing
has contributed 49.5 MJ primary/kWh in the 15% co-firing case. The biggest decrease of midpoint
impacts is in non-renewable energy impact followed by mineral extraction and global warming
potential (GWP). The atmospheric accumulation of GHG, such as CO
2
, N
2
O, and CH
4
due to biomass
does not lead to a net increase in GHG accumulation. There is no significant ionizing radiation impact
from coal mining and transportation. But forest residue collection and processing has a contribution
to this category. One of the possible reasons is the use of electricity generated from nuclear power
in the sub-process of machinery [
61
]. Ozone layer depletion (ODP) also increases with co-firing
which is 12.47% for 15% co-firing. A study of co-firing by using raw wood and coal concluded that
ODP increases 22.67% for 20% co-firing ratio [
19
]. Terrestrial ecotoxicity and land occupation also
has greater impact in co-firing than base case. In co-firing, forest residue uses more land which is
the main reason for increasing impact. Coal mining and transportation does not have significant
contribution in this impact category. In case of biomass, eutrophication impacts mainly come from
use of fertilizer in forestry process, while mining stage is responsible for eutrophication impacts
from coal [
19
]. It occurs due to phosphate emissions from a different stage of LCA. Eutrophication
potential of electricity production increased by 16% from 20% biomass (rice, wheat) co-firing with
coal as per Huang et al., (2013). Results from our study also show agreement with eutrophication
potential increasing with co-firing [
61
]. The variation involved with transportation in the biomass
supply chain represents the largest source of uncertainty in estimating life cycle impacts in the current
study. The uncertainty analysis conducted for a
±
15% change in the transportation distances indicates
that global warming potential would not vary significantly, although emissions of air pollutants such
as CO, NOx, and VOC contribute to increases in human toxicity and respiratory organics/inorganics.
This finding is particularly significant for the Greater Houston Area, often confronted with high levels
of ground-level ozone. The interpretation of results from the current study should also consider
some of the limitations involved, such as geographical constraint, use of built-in Simapro machinery,
and exclusion of waste, ash and recycling outputs of the power plant from the system boundary.
5. Conclusions
Co-firing of forest residue with coal at the WAP power plant results in significant reduction
in emissions of all criteria air pollutants. The maximum reduction in life cycle emissions was
observed for CO
2
(13.45%), followed by NO
x
(11.70%) for the 15% co-firing scenario. For the
combustion stage, emissions reduction was highest for NO
x
(15.06%), followed by CO
2
(13.79%),
indicating the potential alleviation of ozone precursors in the Greater Houston area. Nine midpoint
impact categories (human toxicity, aquatic ecotoxicity and acidification, global warming, respiratory
organics and inorganics, terrestrial acidification/nutrification, non-renewable energy, and mineral
extraction) showed reduction in potential impact due to co-firing with forest residue, with GWP
decreasing by 13.24% for 15% co-firing. The life cycle impact increased across five midpoint
impact categories (ionizing radiation, land occupation, aquatic eutrophication, ozone layer depletion,
terrestrial ecotoxicity); the maximum increase (69.93%) was for the land occupation category, due to
biomass accumulation from forest residue. All four damage categories showed reduction of potential
impact due to co-firing. The end-point impact category of climate change resulted in 13.24% lowering
of impact, suggesting the positive contribution that forest residue can make toward a more sustainable
energy production in Texas. The primary limitations of co-firing with forest residue include commercial
wood demand and regulatory concerns, and any associated increases in the market price of electricity.
In addition, the uncertainty induced due to the varying transportation distances in the biomass
supply chain could increase the impact of co-firing in categories such as human toxicity and aquatic
Sustainability 2018,10, 2193 15 of 18
ecotoxicity, thereby controlling the environmental costs-benefit ratio of co-firing forest residue at the
WAP power plant.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2071-1050/10/7/2193/s1.
Author Contributions:
R.R.K. conceived the idea, R.R.K. and H.D. designed the study outline; I.H. performed
the literature review and conducted the life cycle assessment with SimaPro
®
; I.H. and V.S.V.B. analyzed the data
and drafted the paper.
Acknowledgments:
This work is supported by the National Science Foundation (NSF) through the Center
for Energy and Environmental Sustainability (CEES) at Prairie View A&M University, an NSF CREST Center,
Award No. 1036593.
Conflicts of Interest:
The authors declare no conflict of interest. The sponsors had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to
publish the results.
Appendix A
Table A1. Fuel requirement in kg/MWh, for different co-firing scenarios.
Fuel
Co-Firing Ratio
Base Case (0% Co-Firing) 5% Co-Firing 10% Co-Firing 15% Co-Firing
Coal 553.8 526.1 498.4 470.7
Biomass 0 38.62 75.66 111.2
Table A2. Inventory of coal at power plant.
Amount Unit Comment [45]
Output
PRB sub-bituminous coal at Power plant 1 kg
Input: (From nature)
Moisture content 0.277 kg Moisture content of PRB coal is 27.66%
Ash content 0.064 kg Ash content of PRB coal is 6.44%
Energy, gross calorific value 19,120 kJ
Inputs: (From technosphere, materials/fuels)
Coal, at mine 1.004 kg Coal losses in supply chain is considered to be 4%
Transport, train, diesel powered/US 2.227 t-km tonne-kilometre is expressed as
Table A3. Inventory of forest residue at power plant.
Amount Unit Comment [32,44,64]
Output
Forest residues at power plant 1 t Wood category
Input (from nature)
Biomass 1.05 t Assuming 5% loss
Energy, gross calorific value 12,180 MJ
Transformation, from forest land, extensive 367.9 m2
11 t per acre residues were utilized at 2008 in Texas
Ash content 0.03 t Ash content 3%
Moisture content 0.33 t Moisture content 33.3%
Input (material and fuels)
Forwarding of forest residues 0.096 h Using Tables 4and 5
Bundling of forest residues 0.064 h Using Tables 6and 7
Transport, long-haul truck 281.9 t-km Weighted average distance is 183.9 mile
Chipping of biomass by Biber 92 0.041 h
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