Biogas upgrading to biomethane - The Biogas: Science, Production and Applications - page 359

components can also be separated in a cryogenic process. Using the example
of the practice of one manufacturer, the following steps take place [13].

. First a dew point decrease to 6 C takes place. At this temperature, H 2 S

and siloxanes can be (partly) removed by catalytic adsorption.

8

. After this pretreatment, the raw gas is compressed up to 18-25 bar.

. The temperature is then decreased to C. At this temperature, the

gas is dried and possible remaining siloxanes may also be condensed.

25 8

. The final step of desulphurization (polishing) takes place.

. Temperature decrease to C causes liquefaction of the CO 2,

which is then removed from the system.

50 to 59 8

A methane loss of 0.1-1% can be expected [19]. In 2008, one manufacturer
expected a methane loss of

0.5% [13]. According to new information
(2012), methane losses are limited to 2% [24]. The demand for electricity has
been reported to be 0.18-0.25 kWh el /m n 3 related to raw biogas [13]. Because
the off-gas contains significant amounts of CH 4 , emissions reductions in the
off-gas flow may be necessary.

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15.3.3 Off-gas treatment

Depending on the off-gas composition, one or several off-gas treatment
steps may be needed. The most relevant contaminants are CH 4 and H 2 S
respectively sulphur oxides SO x . Depending on particular loads and/or
concentrations of these contaminants, respective national legal requirements
can demand degradation or separation of these substances. As an example
of possible legal requirements, Table 15.3 shows limitations of emissions
according to German 'technical instructions on air quality control' (TA Luft
2002); these technical instructions are also partly applied in other European
countries [47].
Hydrogen sulphide (H 2 S) will occur in concentrations greater than
5mg/m n 3 in the unoxidized off-gas if no preliminary removal has been
carried out in the raw gas flow; this is normally what can be expected using
water scrubbers or Genosorb ® scrubbers for biogas upgrading. If these off-
gas flows, containing relevant concentrations on H 2 S, are oxidized, SO x are

￿ ￿ ￿ ￿ ￿ ￿

Table 15.3 Emission limits according to German TA Luft 2002 for relevant

substances in the off-gas of biogas upgrading plants [47]

Mass
flow (g/h)

Mass concentration
(mg/m 3 )

Comments

CH 4 500 50 Indicated as total carbon

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H 2 S 15 3

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SO x 1800 350 Indicated as sulphur dioxide

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