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The typical composition of raw biogas does not meet
the minimum CNG fuel specifications. In particular, the CO2 and
sulfur content in raw biogas is too high for it to be used as vehicle fuel
without additional processing. Biogas that has been upgraded to biomethane by
removing the H2S, moisture, and CO2 can be used as a vehicular
fuel. Biomethane is less corrosive than biogas, apart
from being more valuable as a fuel. Since production of such fuel typically exceeds immediate
on-site demand, the biomethane must be stored for future use, usually either as
compressed biomethane (CBM) or liquefied biomethane (LBM).
Biomethane
can be liquefied, creating a product known as liquefied biomethane (LBM). Two
of the main advantages of LBM are that it can be transported relatively easily
and it can be dispensed to either LNG vehicles or CNG vehicles. Liquid biomethane is transported in
the same manner as LNG, that is, via insulated tanker trucks designed for
transportation of cryogenic liquids.
Biomethane
can be stored as CBM to save space. The gas is stored in steel cylinders such
as those typically used for storage of other commercial gases. Storage
facilities must be adequately fitted with safety devices such as rupture disks
and pressure relief valves. The cost of compressing gas to high pressures
between 2,000 and 5,000 psi is much greater than the cost of compressing gas
for medium-pressure storage. Because of these high costs, the biogas is
typically upgraded to biomethane prior to compression.
