Biomass Feasibility Project Final Report - Xcel Energy

Technology
Type
Table V-1: Emission Rates (lbs/MWh) for Various Energy Technologies
Natural
Gas (1)
Generic
Natural
Gas Fired
Turbine
Combined
Cycle
Plant
Coal (1) Oil (1) Direct
Combustion
Biomass (1)
Generic Generic Typical
Utility Utility Stoker
Boiler Boiler Grate
Firing Firing Wood-Fired
Bituminous Only Biomass
Coal Plant No. 6 Plant
Fuel
Gasification (2)
Combined
Cycle
Biomass
Gasification
Plant
Landfill
Gas/Biogas
(3)
Typical
Gas
Turbine (>3
MW)
Landfill
Gas/Biogas
(3)
Fuel Cell
(PAFC)
NOX 0.06 3.1 2.5 1.5 1.08 0.44-2.2 0.003
CO 0.03 0.21 0.35 3.556 0.001 0.6 0.015
SO2 0.02 15 10.4 0.4 0.58 0.65 0.006
PM 0.08 1.5 0.87 1.5 0.05 0.07 Negligible
CO2 750 2,296 1,708 3,407 1,962 1,958-3,132 1,078
(Xenergy and Energetic Management Associates, 2003)
1. Massachusetts Biomass Energy Working Group Technology Assessment, 2001
2. Mann, M & Spath, P. “Life Cycle Assessment of a Biomass Gasification Combined-Cycle System,” December 1997.
3. Arthur D. Little, Inc. “Profiles of Leading Renewable Energy Technologies for the Massachusetts Renewable Energy Trust
Fund,” October 10, 1998.
GASIFICATION
Biological Gasification
Anaerobic digestion falls within a general category called biological gasification, the process of
generating biogas from decomposing organic matter. It can take place in a vessel containing
oxygen, in which case it is called aerobic digestion, or in a vessel deprived of oxygen, in which
case it is called anaerobic digestion.
The process of anaerobic digestion is simple: a conveyor meters sludge, garbage and other
wastes into the end of a long revolving sealed cylinder. The inside of the cylinder is heated to
grow bacteria that decompose waste to produce methane. The cylinder slants down from the
infeed end. That combines with the tumbling action to move the waste slurry slowly from the
high end, where it has entered the cylinder, to the far end where the remaining waste is
removed.
As it usually is practiced, anaerobic digestion is an imperfect process. Most anaerobic digesters
are used to dispose of waste, not produce energy. So they just flare off methane and CO2 or
release them into the air (Xenergy and Energetic Management Associates, 2003). Not only does
that waste energy, it also puts two of the worst greenhouse gasses into the atmosphere. Two
other drawbacks of anaerobic digestion are its slow rate of decomposition and its incomplete
conversion of organic waste.
Nevertheless, anaerobic digesters do play a beneficial role in managing wastes. They take up
little space, use little energy, and dispose of nasty waste streams, like animal manure, sewage
sludge and industrial effluents, that we don’t want to see in our groundwater. Many municipal
wastewater treatment plants across the U.S. use anaerobic digesters to reduce solids in sewage
sludge (Dayton, 2001).
Concerns about the environmental impacts of livestock wastes have rekindled interest in
anaerobic digestion. Manures from dairy and swine farms make good feedstocks for anaerobic
Identifying Effective Biomass Strategies: Page 59
Quantifying Minnesota’s Resources and Evaluating Future Opportunities