Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
brane systems used for gas separation is available in SGC report 270 [47]. A
membrane is a dense filter that can separate the components in a gas or a liquid
down to the molecular level. The membranes used for biogas upgrading are mainly
manufactured from polymeric materials. The membranes used for biogas upgrading
retain methane while the carbon dioxide is able to permeate through the
membrane. During the separation of carbon dioxide from the raw gas other compounds
such as water vapor, hydrogen and to some degree oxygen are removed
from the biomethane. The permeation rate through a glassy polymer, commonly
used in the biogas applications, is mainly depending on the size of the molecules,
which gives the relative permeation rates shown in Figure 15.
Figure 15. Relative permeation rate of different molecules through a membrane
used for upgrading of biomethane.
The gas stream going into a membrane is called the feed stream. The feed is separated
into permeate and retentate inside the membrane. Retentate is the gas
stream that do not pass through the membrane while permeate is the gas stream
that pass through the membrane. An example of a membrane process for upgrading
of bio-SNG is depicted in Figure 16. The membrane unit can consist of several
membrane stages. The producer gas is cleaned from pollutants, e.g. water and
hydrogen sulphide, before it is compressed and fed to the membrane unit. In cases
where ammonia and volatile organic carbons are expected in significant concentrations
in the producer gas, these components are also removed before the
biogas upgrading. Additional to this cleaning, it is also common to have a particle
filter to protect the compressor and the membranes.
After gas cleaning, the biogas is compressed to 5-20 barg. Since oil lubricated
compressors are commonly used, it is important to have efficient oil separation
after compression. This oil separation is important not only for the oil residues from
the compressor but also for removing oil naturally occurring in the biogas. The oil
will otherwise foul the membrane and decrease its lifetime. In the membrane unit
itself the carbon dioxide is separated from the main gas stream, but since a minor
share of the methane and other compounds will permeate, the off-gas stream from
one stage is recirculated to the compressor to reach an acceptable yield.
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