Purification Technologies for Biogas Generated by Anaerobic Digestion
Q. Zhao, E. Leonhardt, C. MacConnell, C. Frear and S. Chen
Biogas is produced in many different environments, including in landfills, sewage
sludge and during anaerobic degradation of organic material. Biogas is comprised of
methane (CH4, about 45-75% by volume), carbon dioxide (CO2, 25-55%), and other
compounds including hydrogen sulfide (H2S, present in concentrations from several
hundred to a couple of thousand parts per million), water, and other trace gas
compounds. Methane is a powerful greenhouse gas if emitted into the atmosphere,
but can also represent a valuable renewable energy source, with the potential to
reduce GHG emissions when it is collected and substituted for fossil fuels.
Background
Biogas can be used directly to generate power, but the large volume of CO2 reduces
the heating value of the gas, increasing compression and transportation costs and
limiting economic feasibility to uses that occur at the point of production.
Purification allows for a wider variety of uses, either for heat and electricity, or for
vehicle fuels. For use as a fuel, purification to remove carbon dioxide (CO2) and
hydrogen sulfide (H2S) is required, because H2S corrodes vital mechanical
components within engine generator sets and vehicle engines if it is not removed.
Purified biogas provides reductions in GHG emissions as well as several other
environmental benefits when used as a vehicle fuel. Biogas emits less nitrogen
oxide, hydrocarbon and carbon monoxide than gasoline or diesel, and engines
fueled by purified biogas are quieter than diesel engines. Refueling with biogas
presents fewer environmental risks than refueling with gasoline or diesel, because it
can be done at small units located at an owner’s home or business, minimizing the
potential impacts if leaks or spills occur. Potential negatives include the high cost
($3-6/GJ) to upgrade the biogas, reduced driving range for vehicles dependent on
specialty fuel, and less cargo space due to biogas storage.
Feasible biogas purification technologies exist for large-scale sewage and biowaste
digesters, and the technologies for upgrading biogas, compressing, storing and
dispensing biomethane are well developed. If cost-effective methods for upgrading
biogas could be developed for the farm-scale, biogas purification could provide
dairy farmers with revenue to complement (or replace) electrical power sales. This
is especially critical in the Pacific Northwest, where low power rates have prevented
cost competitive power from farm-scale anaerobic-digesters, limiting total dairy-
derived power.
Engine conversion to accommodate biogas also represents a potential barrier, but
because biogas has the same properties as natural gas, it can be easily used by
vehicles which are configured for natural gas. Worldwide, there are about 10,000
biogas driven cars and buses, plus an additional 3.8 million natural gas fuelled
vehicles (representing 0.5% of the world vehicle stock), mainly in Argentina, Brazil,
Pakistan, Italy, India and the U.S. (ENGVA, 2004).
To help develop appropriate biogas purification technologies for farm-scale
anaerobic digesters, Washington State University evaluated various methods for
removing acidic impurities, and developed and tested absorption tower
technologies for application to a farm-scale anaerobic digester. In addition, Western
Washington University has begun the process of building a full-scale pilot system.
This pilot system will purify biogas from Vander Haak Dairy (Lynden, Washington),
and sell it to Airporter Shuttle/Belair Charters for use by buses running along the
Interstate 5 “Green” corridor from the Seatac airport south of Seattle to Ferndale,
north of Bellingham. This is a new project for the diary industry, fuel users, and the
community, as there is currently only one operational
transportation facility in North America, at the Hilarides Dairy in Lindsay, California,
which began operation in the summer of 2009.
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Q. Zhao, E. Leonhardt, C. MacConnell, C. Frear and S. Chen
Biogas is produced in many different environments, including in landfills, sewage
sludge and during anaerobic degradation of organic material. Biogas is comprised of
methane (CH4, about 45-75% by volume), carbon dioxide (CO2, 25-55%), and other
compounds including hydrogen sulfide (H2S, present in concentrations from several
hundred to a couple of thousand parts per million), water, and other trace gas
compounds. Methane is a powerful greenhouse gas if emitted into the atmosphere,
but can also represent a valuable renewable energy source, with the potential to
reduce GHG emissions when it is collected and substituted for fossil fuels.
Background
Biogas can be used directly to generate power, but the large volume of CO2 reduces
the heating value of the gas, increasing compression and transportation costs and
limiting economic feasibility to uses that occur at the point of production.
Purification allows for a wider variety of uses, either for heat and electricity, or for
vehicle fuels. For use as a fuel, purification to remove carbon dioxide (CO2) and
hydrogen sulfide (H2S) is required, because H2S corrodes vital mechanical
components within engine generator sets and vehicle engines if it is not removed.
Purified biogas provides reductions in GHG emissions as well as several other
environmental benefits when used as a vehicle fuel. Biogas emits less nitrogen
oxide, hydrocarbon and carbon monoxide than gasoline or diesel, and engines
fueled by purified biogas are quieter than diesel engines. Refueling with biogas
presents fewer environmental risks than refueling with gasoline or diesel, because it
can be done at small units located at an owner’s home or business, minimizing the
potential impacts if leaks or spills occur. Potential negatives include the high cost
($3-6/GJ) to upgrade the biogas, reduced driving range for vehicles dependent on
specialty fuel, and less cargo space due to biogas storage.
Feasible biogas purification technologies exist for large-scale sewage and biowaste
digesters, and the technologies for upgrading biogas, compressing, storing and
dispensing biomethane are well developed. If cost-effective methods for upgrading
biogas could be developed for the farm-scale, biogas purification could provide
dairy farmers with revenue to complement (or replace) electrical power sales. This
is especially critical in the Pacific Northwest, where low power rates have prevented
cost competitive power from farm-scale anaerobic-digesters, limiting total dairy-
derived power.
Engine conversion to accommodate biogas also represents a potential barrier, but
because biogas has the same properties as natural gas, it can be easily used by
vehicles which are configured for natural gas. Worldwide, there are about 10,000
biogas driven cars and buses, plus an additional 3.8 million natural gas fuelled
vehicles (representing 0.5% of the world vehicle stock), mainly in Argentina, Brazil,
Pakistan, Italy, India and the U.S. (ENGVA, 2004).
To help develop appropriate biogas purification technologies for farm-scale
anaerobic digesters, Washington State University evaluated various methods for
removing acidic impurities, and developed and tested absorption tower
technologies for application to a farm-scale anaerobic digester. In addition, Western
Washington University has begun the process of building a full-scale pilot system.
This pilot system will purify biogas from Vander Haak Dairy (Lynden, Washington),
and sell it to Airporter Shuttle/Belair Charters for use by buses running along the
Interstate 5 “Green” corridor from the Seatac airport south of Seattle to Ferndale,
north of Bellingham. This is a new project for the diary industry, fuel users, and the
community, as there is currently only one operational
transportation facility in North America, at the Hilarides Dairy in Lindsay, California,
which began operation in the summer of 2009.
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