Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
thane loss. It is also beneficial if methane in the off-gas can be used in an efficient
way by e.g. cogeneration in a boiler or CHP. The second design (ii) is used in
most biogas upgrading units built with membranes from Air Liquide Medal TM . This
design increases the methane recovery compared to design (i). In this case the
permeate (the gas passing through the membrane) from the first membrane stage
is removed from the system while the permeate from the second membrane stage
is recirculated back to the compressor to minimize the methane slip. This will however
increase the energy consumption. The third design (iii) is used with membranes
from Evonik Sepuran®. The retentate (the gas not passing through the
membrane) from the first stage is polished in the second membrane stage, in a
similar way as in design (ii) to obtain a product gas of with a purity of more than
97% methane. Additional to design (ii), also the permeate of the first stage is polished
in a third membrane stage, to minimize the CH4 concentration in the off-gas
and the volume of gas circulated back to the compressor. The permeate stream of
the second stage and the retentate of the third stage are combined and recycled to
the compressor.
In a membrane unit, the main part of the remaining water after compression is
separated from the biomethane together with the carbon dioxide. Therefore, a gas
dryer is commonly not needed to further decrease the dew point. Figure 16 shows
the biogas upgrading plant in Poundbury in UK based on membrane technology.
Figure 16 The membrane biogas upgrading plant in Poundbury with a capacity of
650 Nm 3 /h raw biogas. Images from DMT.
2.3.2 Theoretical background
The gas stream going into a membrane is called the feed stream. The feed is separated
into permeate and retentate inside the membrane module. Retentate is the
gas stream that does not pass through the membrane while permeate is the gas
stream that passes through the membrane.
The transport of a gas molecule through a dense polymeric membrane can be
expressed by Eq. 3 (Baker 2004).
݆ ௜ = ஽ ೔௄ ೔∆௣ ೔
௟
Eq. 3
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