CO2 Separation

On the European market, pressure swing adsorption and pressure water washing are the most widely used processes for CO2 separation.

Additional processes are currently in the pilot stages; some of these must still be intensively researched. The following provides a short, non-exhaustive description of some selected CO2 separation processes:

Pressure Swing Adsorption - PSA

"Adsorption" refers to the exit of molecules from fluids and their subsequent attachment to solid surfaces.  This process is used in PSA to remove the CO2 and any remaining traces of other gases from raw biogas. Before adsorption, sulfur and water vapor must be removed from the raw biogas, since these substances can damage the active carbon required in the process. As a rule, water separation succeeds when raw biogas is cooled nearly to the point of freezing. For the separation of sulfur from biogas, various other processes may be used.

Pressurised Water Scrubbing (PWS)

Pressure water washing is a matter of physical adsorption processes. Raw biogas is routed through a water-filled pressure tank, in which the gases present in the biogas are absorbed by the water through the application of physical force. Besides CO2, this process can also remove some of the hydrogen sulfide (H2S) and ammonia (NH3) present in raw biogas. When biogas contains more hydrogen sulfide than can be removed through pressure water washing, this is an indication that an additional sulfur removal process should be implemented “upstream” of the pressure water wash. Following the completion of the pressure water wash, the gas must be dried and dehumidified before being fed into the natural gas grid. 

There are other promising processes for CO2 removal in addition to Pressure Swing Adsorption and pressure water washing. The following passage briefly describes each of these processes:


Like pressure water washing, this biogas purification process hinges around a physical adsorption, though washing fluid that is highly absorptive of CO2 and H2 is also involved. The washing fluid Genosorb can regenerate itself after high-temperature washes. The washing fluid has a shelf life of about 10 years.  Because of this, an essentially larger load of washing fluid is possible.

The amine washes are processes of chemical adsorption. In contrast to pressure water washing and Genosorb absorption, amine washes use chemical reactions to remove extraneous gases present in biogas. Because of this, a large range of washing fluids can be used.

Monoethanol Amine Washing – MEA

Cleaning biogas using mono ethanol amine is suitable when only CO2 must be removed. The process can be implemented with only minimal pressure, though it requires a standard minimum temperature of approximately 40°C. The process is therefore especially appealing for sites that have ready access to heat. 

Diethanol Amine Washing - DEA

The process of diethanol amine washing is very similar to that of monoethanol amine washing. As a washing fluid, though, diethanol amine differs from monoethanol amine. Its CO2 adsorption capacity is greater than that of monoethanol amine, though its environmental impacts  (specifically its hazards for waterways)  are more serious.

The following table provides an overview of the individual purification processes and their most important parameters.

Criteria PSA PWS Genosorb MEA DEA
Adsorption process   physical physical chemical chemical
Pre-cleaning necessary? Yes No No Yes Yes
Required pressure (bar) 4-7 4-7 4-7 depressurized depressurized
Methane loss 3-10% 1-2% 2-4% <0,1% <0,1%
Methane content of the gas product >96% >97% >96% >99% >99%
Required power consumption [kWh/Nm³] 0,25 <0,25 0,25-0,33 <0,15 <0,15
Required temperature range [°C] No No 55-80 160 160
Range of controllability as percentages of total load +/- 10-15% 50-100% 50-100% 50-100% 50-100%
Processes already underway >20 >20 3 3 1