Environmental Engineering Reference
Removal of CO 2 enriches the biogas in terms of methane content. Methane burns
faster and hence yields a higher specifi c output and thermal effi ciency compared to
raw biogas when used as engine fuel. Moreover absence of carbon dioxide in the gas
will enable an additional volume of air to be inducted into the engine cylinder
thereby improving the volumetric effi ciency of the engine. H 2 S usually amounts to
less than 1 % on volume basis in cattle dung based biogas, which is very harmful.
Its concentration more than this may cause corrosion in pipe lines and in engine.
Also, biogas usually contains water vapour which enhances corrosion and decreases
heating value of the fuel. Therefore, it is essential to enrich the biogas before com-
pression by removing CO 2 , H 2 S and water vapour to make it a suitable engine fuel.
Compressing biogas reduces storage space requirements, concentrates energy
content and increases pressure to the level needed to overcome resistance to gas fl ow.
Sometimes the production pressure of a biogas source does not match the pressure
requirements of the gas utilization equipment. Compression can eliminate the mis-
match of pressures and guarantee the effi cient operation of the equipment. Instead of
raw biogas if enriched biogas is compressed, it will reduce the cost of compression.
Also it will produce a gas with a high heating value, making it more suitable for uses
in internal combustion engines and in particular automobile engines.
Biogas Upgrading Processes
A variety of processes are available for upgrading of methane content in biogas by
removing insignifi cant contents for practical purposes i.e. carbon dioxide (CO 2 ),
and hydrogen sulphide (H 2 S). Most of these processes have been developed for use
in the natural gas, petroleum and petrochemical industries. As a consequence, some
of them may not be suited for biogas applications unless high fl ow rates are involved.
Commonly CO 2 removal processes also remove H 2 S.
Removal of CO 2 from Biogas
For an effective use of biogas as a vehicle fuel, it has to be enriched in methane. This
is primarily achieved by carbon dioxide removal, which also provides a consistent
gas quality with respect to energy value. A variety of processes are being used for
removing CO 2 from natural gas in petrochemical industries. Several basic mecha-
nisms are involved to achieve selective separation of gas constituents. The feasible
processes of CO 2 removal from biogas can be split into following four groups. At
present four different methods are used commercially for carbon dioxide removal,
but the most common technologies for biogas upgrading are the water scrubber
technology and the pressure-swing absorption technology.
Absorption into liquid (Physical/Chemical)
Adsorption on solid surface