Margaret K. MannNational Renewable Energy Laboratory

Golden, Colorado USA

A Comparison of the Environmental Consequences of Power from Biomass,

Coal, and Natural Gas

Purpose of LCAs conducted

System descriptions• Biomass IGCC• Average coal• Coal/biomass cofiring• Natural gas combined cycle

Comparative Results• Energy• Greenhouse gases• Other air emissions• Resource consumption

Outline of Presentation

Life Cycle Assessment: Definition

LCA, in the context of novel systems, 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 so that research

can be focused on mitigating them

(no show-stopping surprises)

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.

Systems Examined

Indirectly-heated gasification

Dedicated hybrid poplar feedstock

Zero carbon sequestration in base case

Pulverized coal / steam cycle

Illinois #6 coal - moderate sulfur, bituminousSurface mining

Biomass IGCC

Average coal

15% cofiring by heat input

Biomass residue (urban, mostly) into PC boiler0.9 percentage point efficiency derating

Credit taken for avoided operations including

decomposition (i.e., no biomass growth)

Biomass / coal

cofiring

Biomass residue

Avoided emissions credit as with cofiring

Direct-fired biomass

Combined cycleUpstream natural gas losses = 1.4% of gross

Natural gas

Life Cycle Energy Balance

0

5

10

15

20

25

30

net energy ratio

external energy ratio

D edicatedbiom ass

IG C C

AveragePC coal

C oal/b iom asscofiring

D irect-firedbiom assresidue

N G C C

Energy Balance Oddities

Key question: why are the energy results so poor for the fossil systems?

Answer: Upstream Energy Consumption is High

Non-feedstockenergy

(kJ/kWh)

% of non-feedstock energy related to:

Flue-gascleanup

Transportation Natural gasproduction orcoal mining

Biomass IGCC 231 0% 16% N/A

Direct biomass 125 0% 49% N/A

Coal 702 35% 32% 25%

Natural gas 0.5% N/A 98.3%1,718

Carbon Cycle (GHG Emissions)

Example flows:

• Biomass energy crop - photosynthesis, carbon sequestration in soil

• Biomass residue - avoided decomposition emissions

• Coal - coal mine methane, coal mine waste

• Natural gas- fugitive emissions, leaks

• General - incomplete combustion, upstream fossil fuel consumption

Key question: On a life cycle basis, what are the net greenhouse gas emissions of these systems?

Life Cycle Greenhouse Gas Emissions

-600

-400

-200

0

200

400

600

800

1000

1200

D edicatedbiom ass

IG C C

AveragePC coal

C oal/b iom assC ofiring

N G C C

GW

P (

g C

O-e

qui

vale

nt /

kW

h)

2

D irect-firedbiom assresidue

Other Air Emissions

Biomass IGCC also emits isoprene at 21 g/kWh

-15

-5

5

15

Particulates SOx NOx CO NMHCs

-41 g/kWh

Average PC coal

15% Coal / biomass cofiring

Direct biomass residue

Dedicated biomass IGCC

NGCC

CH4

Resource Consumptiong

/kW

h

-

50

100

150

200

250

300

350

400

450

500

Coal Limestone Oil Natural Gas

Average PC coal

15% cofiring

Direct-fired residue biomass

Dedicated biomass IGCC

NGCC

Summary

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

Energy:• Coal and natural gas: negative system energy balance• Even neglecting the energy content of coal and natural gas, biomass systems are more energy efficient• NGCC: natural gas extraction and losses account for 21% of total energy

Air emissions:• Biomass: few particulates, SO2, NOx, and methane• Coal: upstream CO and NMHC emissions lower• NGCC: system methane emissions high

Resource consumption: Biomass systems << fossil systems

Cofiring:• 15% cofiring reduces GWP of coal system by 18%• Reduction in emissions, resource consumption, and energy use

Credits

Co-author: Pamela L. Spath, NREL

Funding from the U.S. Department of Energy Biomass Power Program