Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
solubility of methane according to Eq. 4. The water that is transported to the desorption
column will contain the main part of the carbon dioxide but less than 1%
of the methane in the raw biogas.
The pressure in the flash column has to be decreased to maintain the same methane
slip if the methane concentration in the raw biogas increases. The reason is
that more methane and less carbon dioxide is transported with the water into the
flash column, resulting in a changed composition – more CH4 and less CO2 – in
the flash column gas volume. If the pressure is kept constant, the partial pressure
of methane will increase significantly resulting in higher solubility in the water according
to Eq. 4. For a system working at 8 bar(a), the flash pressure has to be
decreased from about 3 bar(a) to about 2 bar(a) when the methane concentration
is increased from 50% to 80% in the incoming raw biogas.
The flash column has no packing and is designed with a diameter wide enough
to decrease the vertical speed of the water to such an extent that even small gas
bubbles are able to rise instead of being dragged into the desorption column. The
top of the flash column should be designed so that water is not sucked into the gas
going back to the compressor. The volume of this gas stream going back to the
compressor is usually 20-30% of the incoming raw gas flow.
After removing most of the methane from the water in the flash column, the carbon
dioxide is released from the water in the desorption column. The water enters
the top of the desorption column, while air is entering at the bottom. This column is
also filled with random packing to increase the contact surface between the air and
the water. The low percentage of carbon dioxide in the air in combination with decreased
pressure results in a partial pressure of carbon dioxide close to zero and
thus a very low solubility of carbon dioxide in the water. The water leaving the desorption
column is virtually free from carbon dioxide and is pumped back into the
top of the absorption column. The time it takes to circulate a specific volume of
water one time in a water scrubbing system is around 1-5 minutes depending on
design and current load.
Table 9 shows how much water is needed to upgrade 1000 Nm 3 of biogas per
hour to less than 2% CO2 in the upgraded biogas. Values are given for various
temperatures and pressures. The methane concentration in the raw biogas has no
influence on the water flow, as described previously. The pressure is directly proportional
to the needed water flow, as shown in Eq. 4 and a water flow corresponding
to another pressure can therefore easily be calculated. The required water
flow depends also on the temperature of the water. A few degrees lower temperature
will decrease the required water volume several percentage units. This
relation is shown in Eq. 7 and Figure 19.
Table 9 Typical water flow needed to upgrade 1000 NM 3 /h raw biogas.
Pressure Water temperature Water flow
[bar(a)] [°C]
[m 3 /h]
8 20 210-230
8 14 180-200
6.5 14 210-230
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