Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
pansion devices forming a cooling cycle. Direct cooling is also the method used to
lower the temperature of the refrigerants in chillers. Therefore, a combination of
direct cooling (of the refrigerant) and indirect cooling (heat transport from the biogas
to the refrigerant) is often used in the case of biogas cooling. Details on cooling
cycles can be found in Appendix II.
4.2 Purification of landfill gas
Landfill gas is very similar to digester gas, being composed mainly of methane and
carbon dioxide. The main challenge in the use of landfill gas as a fuel is the varying
and elevated content of trace contaminants such as siloxanes and halogen
compounds, some of them being known and others not even being identified. Furthermore,
landfill gas can contain elevated amounts of nitrogen which cannot be
removed by most conventional gas upgrading techniques. Techniques capable of
removing nitrogen from biogas/landfill gas include PSA and cryogenic upgrading to
liquefied methane (see further on in this chapter).
One cryogenic technique specifically developed for the removal of trace contaminants
is the CO2 Wash® process developed by US based Acrion Technologies. In
this process, the raw gas stream is cleaned by liquid carbon dioxide, even called
“CO2 Wash”. The idea is that many impurities such as halogenated hydrocarbons
or siloxanes have a much higher solubility in carbon dioxide than methane. The
process is mostly interesting for the treatment of landfill gas with its high and varying
contents of impurities.
Usually, the CO2 Wash® process is combined with other gas treatment steps in
order to produce CBG (compressed biogas) or LBG. A possible setup for LBG
production is shown in Figure 35. After compression, H2S removal and drying, the
gas stream enters a cryogenic column at the bottom and is then led upwards,
meeting a stream of liquid CO2. Impurities are dissolved in the liquid CO2 and bled
off the system at the sump. The gas stream leaving the head of the column contains
methane, a part of the carbon dioxide of the raw biogas as well as the oxygen
and nitrogen from the raw gas. The rest of the carbon dioxide leaves the system
in the upper half of the column as a liquid, food grade CO2 stream.
Raw biogas
Compressor
H2S removal
Permeate (CO2)
Membrane separation
Refrigeration
Food grade CO2
Water CO2 and VOCs
Refrigeration
Figure 35 Schematic overview of LBG production with the CO2 Wash process.
LBG storage
The purified gas can be directly used in boilers and CHP engines; in this case, the
CO2 Wash® system leads to less wearing of the installation and a longer lifetime.
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