Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
2.3.1 Process description
A typical and simplified design of a biogas upgrading unit based with membranes
is shown in Figure 14.
Figure 14 Typical design of a biogas upgrading unit with membranes
The raw biogas is normally cleaned before compression to remove water and hydrogen
sulfide. In cases where ammonia, siloxanes and volatile organic carbons
are expected in significant concentrations, these components are also commonly
removed before the biogas upgrading. The water is removed to prevent condensation
during compression and hydrogen sulfide is removed since it will not be sufficiently
separated by the membranes. The water is commonly removed by cooling
and condensation while hydrogen sulfide commonly is removed with activated
carbon. 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 6-20 bar(a). The pressure that is
used depends on requirements on the specific site as well as the design and manufacturer
of the upgrading unit. Since oil lubricated compressors are commonly
used, it is important to have an 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.
The membrane separation stage is designed differently depending on the manufacturer
of the system and the membranes they are using. Three of the most
common designs on the market today are shown in Figure 15.
Figure 15 Three different designs of the membrane stage, that are available on the
market today.
The first design (i) includes no internal circulation of the biogas and therefore lower
energy consumption for the compression. However, the methane loss will be higher
and it is important to use membranes with high selectivity, i.e. large difference
between the permeation rate of methane and carbon dioxide, to minimize the me-
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