Biogas upgrading – Review of commercial technologies - SGC
Biogas upgrading – Review of commercial technologies - SGC
Biogas upgrading – Review of commercial technologies - SGC
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<strong>SGC</strong> Rapport 2013:270<br />
4 Cryogenic separation and liquefaction<br />
The technical field <strong>of</strong> cryogenic gas treatment comprises a lot <strong>of</strong> different processes<br />
and have been used for the following three tasks for biogas:<br />
Removal <strong>of</strong> trace contaminants, mainly in the landfill gas context<br />
Removal <strong>of</strong> main components such as CO2, N2 etc (gas <strong>upgrading</strong>)<br />
Condensation <strong>of</strong> upgraded biomethane to bio-LNG (LBG)<br />
As a consequence, cryogenic processes cannot be treated as a homogeneous<br />
family <strong>of</strong> gas <strong>upgrading</strong> techniques. Furthermore, most processes currently available<br />
are under development, so little sound data is available.<br />
4.1 Technical background<br />
Cryogenic processes make use <strong>of</strong> low temperatures in order to achieve the wanted<br />
results. There is no fixed temperature level below which processes are considered<br />
“cryogenic”. However, the processes described here operate at temperatures<br />
well below -50 °C (approx 220 K), i.e. in an area where common gases become a<br />
liquid. Figure 34 illustrates the condensation point <strong>of</strong> some common gases in the<br />
area <strong>of</strong> biogas.<br />
Figure 34 Boiling (condensation) point at atmospheric pressure for some pure<br />
gases. Pure CO2 sublimates under these conditions at -78 °C while it condensates<br />
at elevated pressure. For diluted gases, other temperatures may be valid.<br />
4.1.1 Cooling options<br />
Common to cryogenic processes is the need to generate low temperatures. Cooling<br />
<strong>of</strong> a stream can be achieved either indirectly (by heat exchangers with colder<br />
media) or directly. For the indirect option, one possibility is the usage <strong>of</strong> liquid nitrogen<br />
in order to liquefy another gas (biomethane), thereby consuming the liquid<br />
nitrogen. This is evidently the most basic setup for a liquefaction process and has<br />
been used in some pilot plants for a pro<strong>of</strong> <strong>of</strong> concept since very little technique is<br />
needed for the supply <strong>of</strong> cooling energy. However, this is not viable in a larger<br />
scale because the running costs get too high.<br />
For the direct cooling alternative, a lot <strong>of</strong> different processes have been developed,<br />
all consisting <strong>of</strong> a combination <strong>of</strong> compressors, heat exchangers and ex-<br />
56 Svenskt Gastekniskt Center AB, Malmö <strong>–</strong> www.sgc.se
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