47    MEMBRANE SEPARATION OF C02 AND H2S FROM
BIOGAS FOR INDUSTRIAL COGENERATION
PURPOSES
Lawrence A. Calabro, Water Resources Engineer
Department of Environmental Management
Providence, Rhode Island 02908-5767
A.A. Friedman, Professor
Department of Civil and Environmental Engineering
S. Alexander Stern
Donald Gage Stevens Research Professor
Department of Chemical Engineering and Materials Science
B. Krishnakumar, Senior Research Associate
Department of Chemical Engineering and Materials Science
Syracuse University, Syracuse, New York 13244
INTRODUCTION
Membrane separation processes are being used on an increasingly larger scale for a wide variety of gas mixtures. The objective of this study was to assess the feasibility and economics of a
new membrane separation process for upgrading the CH4 content of biogas produced by anaerobic processes in industrial wastewater treatment plants. The upgraded biogas can be utilized for
the cogeneration of electricity and heat, which could result in substantial savings in plant operation costs.
Biogas is produced by the anaerobic fermentation of high-strength organic wastes, and typically is composed of 60-65 mole-% CH4, 30-35 mole-% C02, 0.1-0.5 mole-% H2S, and traces
of NH3, amines, mercaptans, and other gases. The energy value of biogas ranges from about 500
to 650 Btu/ft3, as compared with 960 Btu/ft3 for pure CH4 or 1,050 Btu/ft3 for pipeline quality
natural gas.1"3 Typically, about 14-15 ft3 (STP) of biogas is formed for every pound of BOD5 removed from wastewater.
Depending on site-specific industrial requirements, a fraction of the biogas produced is used
for process heating purposes with the balance being used in boilers or wasted by flaring. When
biogas is flared, a substantial amount of energy is wasted. By contrast, if the CH4 content in raw
biogas is enhanced to over 90 mole-% by removing most of the C02 and H2S, the upgraded biogas can be used as fuel for an internal combustion engine. Such an engine in conjunction with a
generator can produce electrical power as well as heat. Raw biogas can also be used in internal
combustion engines, but its high content of inert C02 markedly lowers engine efficiency. Moreover, both the C02 and H2S present in raw biogas yield corrosive products when burned in the
presence of moisture.
Conventional methods of separating acid gases (C02 and H2S) from mixtures with CH4 include: absorption of the acid gases in liquid solvents, adsorption on solids, and chemical conversion of other compounds. These separation processes are energy-intensive, require significant
capital investment and support services, and the process equipment typically has a large "foot
print", i.e., it occupies large spaces not always available at smaller facilities. Membrane separation processes, by contrast, offer energy efficiency and in some applications reduced capital investment costs. Moreover, the membrane process equipment is simple, compact, and modular,
and can be easily scaled up or operated at reduced capacity. Last, but not least, membrane separation processes adapt easily to variations in feed composition, flow rate, and pressure.4"7 Such
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press. Inc., Chelsea. Michigan 48118.
Printed in U.S.A.
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