For biogas scrubbing physical/chemical absorption method is generally applied as they
are effective even at low flow rates that the biogas plants are normally operating at. Also
the method is less complicated, requires fewer infrastructures and is cost effective.
One of the easiest and cheapest method involves the use of pressurized water as an
absorbent. The raw biogas is compressed and fed into a packed bed column from bottom;
pressurized water is sprayed from the top. The absorption process is, thus a counter-current
one. This dissolves CO2 as well as H2S in water, which are collected at the bottom of the
tower. The water could be recycled to the first scrubbing tower [3]. This perhaps is the
simplest method for scrubbing biogas.Bhattacharya et al. [4] developed one such water scrubbing system. The process
provides 100% pure methane but is dependent on factors like dimensions of scrubbing
tower, gas pressure, composition of raw biogas, water flow rates and purity of water used.
Vijay [5] developed a packed bed type scrubbing system using the locally available
packing materials removing 30–40% more CO2 by volume compared with the scrubbing
systems without a packed bed.
Khapre [6] designed a continuous counter-current type scrubber with gas flow rate of
1.8 m3/h at 0.48 bar pressure and water inflow rate of 0.465 m3/h. It continuously reduced
CO2 from 30% at inlet to 2% at outlet by volume.
Dubey [7] tried three water scrubbers having diameters 150 mm (height: 1.5 m),
100 mm (height: 10 m) and 75 mm (height: 10 m) to absorb CO2 present (37–41%) in the
biogas. He found that the CO2 absorption is influenced by the flow rates of gas and water
than different diameters of scrubbers.
The G.B. Pant University of Agriculture and Technology, Pantnagar, India [8]
developed a 6 m high scrubbing tower, packed up to 2.5 m height with spherical plastic
balls of 25 mm diameter. The raw biogas compressed at 5.88 bar pressure was passed at a
flow rate of 2 m3/h while water was circulating through the tower. A maximum of 87.6%
of the CO2 present could be removed from the raw biogas.
Water scrubbing method is popular for CO2 removal in sewage sludge based biogas
plants in Sweden, France and USA. The results show that 5–10% CO2 remains in biogas
after scrubbing [9].