Environmental Engineering Reference
In-Depth Information
Membrane separation
Cryogenic separation
Selection of the appropriate process for a particular application depends on the
scale of intended operation, composition of the gas to be treated, degree of purity
required and the need for CO 2 recovery.
Existing biogas upgrading systems are suitable and optimised for large-scale
operations and hence demand high capital investment costs. Improvements in new
and traditional techniques can lower investment and operational costs. For biogas
upgrading, technologies such as water scrubbing, pressure swing absorption (PSA),
chemical and physical absorption and cryogenic processing are commercially
available and many others are at the pilot phase study level. The most widely used
technologies for biogas upgrading are water scrubbing, PSA, organic physical and
chemical scrubbing. Out of these technologies, water scrubbing and PSA are consid-
ered to be most appropriate at a small scale due to low cost and easy maintenance.
Hydrogen Sulphide Removal
Hydrogen sulphide is always present in biogas, although concentrations vary with
the feedstock. It has to be removed in order to avoid corrosion in compressors, gas
storage tanks and engines. Several methods have been developed. Based on the
Swedish experience, air-oxygen dosing in the biogas and iron chloride dosing to the
digester slurry are the most suitable for small-scale operations. For larger scale
operations when upgrading to natural gas quality is the objective, chemical absorp-
tion of H 2 S might become more feasible.
Moisture Removal
Several methods are available based on separation of condensed water or using gas
drying. For upgrading to natural gas quality, gas drying techniques like refrigera-
tion, PSA silica gel adsorption etc. is preferred.
Versatility of Biogas Use
Biogas can be divided into two grades: raw biogas (CH 4 55-65 % and CO 2 35-45 %)
and upgraded (CH 4 > 90 % and <10 % other gases). Both forms of biogas have dif-
ferent approaches for utilisation as shown in Fig. 2 . Raw biogas is a low-grade fuel
as it has a lower percentage of methane and hence it is the cheapest fuel for rural
people. It can be utilised on the site of production itself or nearby for cooking with
biogas cook stoves and for electricity production by using it in dual fuel 100 %
biogas engines. If raw biogas needs to be utilised at a distance from the production
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