Margaret K. MannCo-author: Pamela L. Spath
National Renewable Energy LaboratoryGolden, Colorado USA
A Comparison of the Environmental Consequences of Power from Biomass,
Coal, and Natural Gas
Short discussion of life cycle assessment (LCA)
Purpose of LCAs conducted
System descriptions• Biomass IGCC• Average coal• Coal/biomass cofiring• Natural gas combined cycle
Comparative Results• Air emissions• Greenhouse gases• Energy
Outline of Presentation
Life Cycle Assessment: Definition
LCA is:• a systematic analytical method• used to quantify the environmental benefits and drawbacks
of a process• performed on all processes, cradle-to-grave, resource
extraction to final disposal• ideal for comparing new technologies to the status quo• helps to pinpoint areas that deserve special attention• reveals unexpected environmental impacts
(no show-stopping surprises)
Systems Examined
Indirectly-heated gasificationDedicated hybrid poplar feedstockZero carbon sequestration in base case
Pulverized coal / steam cycleIllinois #6 coal - moderate sulfur, bituminousSurface mining
Biomass IGCC
Average coal
15% cofiring by heat inputBiomass residue (urban, mostly) into PC boiler0.9 percentage point efficiency deratingCredit taken for avoided operations including decomposition (i.e., no biomass growth)
Biomass / coal cofiring
Biomass residueAvoided emissions credit as with cofiring
Direct-fired biomass
Combined cycleUpstream natural gas losses = 1.4% of gross
Natural gas
Purpose of Studies
Biomass LCA was conducted to answer common questions:• What are the net CO2 emissions?• What is the net energy production?• Which substances are emitted at the highest rate?• What parts of the system are responsible for the greatest impacts?• What should biomass R&D focus on?
Coal and natural gas LCAs the foundation for quantifying the benefits of biomass power.
Direct-fired biomass system describes current biomass power industry.
Cofiring LCA examined near-term option for biomass utilization.
Each assessment conducted separately - common systems not excluded.
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Dedicated BiomassIGCC
Average PC Coal Coal/biomass cofiring(15%)
Direct-f ired biomassresidue
Natural gas combinedcycle
g/kW
h
Operating plantTotal system
Life Cycle & Plant CO Emissions
Life Cycle & Plant Particulate Emissions
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Dedicated BiomassIGCC
Average PC Coal Coal/biomass cofiring(15%)
Direct-fired biomassresidue
Natural gas combinedcycle
g/kW
h
Operating plantTotal system
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Dedicated BiomassIGCC
Average PC Coal Coal/biomass cofiring(15%)
Direct-fired biomassresidue
Natural gas combinedcycle
g/kW
h Operating plantTotal system
Life Cycle & Plant SO2 Emissions
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Dedicated BiomassIGCC
Average PC Coal Coal/biomass cofiring(15%)
Direct-fired biomassresidue
Natural gas combinedcycle
g/kW
h
Operating plantTotal system
Life Cycle & Plant NOx Emissions
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Dedicated BiomassIGCC
Average PC Coal Coal/biomass cofiring(15%)
Direct-fired biomassresidue
Natural gas combinedcycle
g/kW
h Operating plantTotal system
Life Cycle & Plant NMHC Emissions
Life Cycle Air Emissions
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CO Particulates SO2 NOx NMHCs
g/kW
h
Average PC CoalCoal/biomass cofiring (5%)Coal/biomass cofiring (15%)Natural gas combined cycleDirect-fired biomass residueDedicated Biomass IGCC
100 kg biomass (bone dry)(50.6 kg carbon)
46%landfilled
54%mulched
90.0 kg CO2(24.6 kg carbon)
90%aerobic
54 kg biomass(27.3 kg carbon)
14.8 kg CO2(4.05 kg carbon)
5.4 kg CH4(4.05 kg carbon)
46 kg biomass(23.3 kg carbon)
anaerobicdecomposition
50% to CO2
50% to CH4
40.5% capturedand combusted 59.5% released
as CH4
5.4 kg CO2(1.5 kg carbon)
2.9 kg CH4(2.2 kg carbon)
of the non-ligninlignin and 50%
resistant to degradation
15.2 kg carbon
degradation of50% of cellulose& hemicellulose
8.1 kg carbon
10% oxidizedby soil
microbes 1.5 kg CO2(0.4 kg carbon)
90% notoxidized by
soil microbes
4.9 kg CH4(3.65 kg carbon)
10%anaerobic
decomposition
3.6 kg CH4(2.7 kg carbon)
Avoided Biomass Decomposition
Avoided per 100 kg of biomass:111.7 kg CO2 + 6.5 kg CH4
= 248.2 kg CO2-equiv
If 100 kg biomass were to completely decompose aerobically: 185.4 kg CO2
Life Cycle Greenhouse Gas Emissions
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Dedicatedbiomass
IGCC
AveragePC coal
Coal/biomassCofiring
NGCCGW
P( g
CO
- equ
ival
ent/
kWh)
2
Direct-firedbiomassresidue
Summary
Air emissions:• Biomass: few particulates, SO2, NOx, and methane• Coal: upstream CO and NMHC emissions lower• NGCC: system methane, NMHC, CO emissions high
Greenhouse Gases:• Biomass IGCC nearly zero net GHGs• Average coal system: ~1,000 g CO2-equiv/kWh• NGCC system: ~500 g CO2-equiv/kWh• Today’s biomass systems remove GHGs from
atmosphere• Cofiring: greater reduction than rate of biomass input