The separation of carbon dioxide from the raw biogas is necessary to increase the calorific value of the gas. Various processes are used, with the following processes being particularly popular on the European market:
Adsorption is the attachment of molecules from a fluid to a solid surface. This effect is used in PSA to remove CO2 and trace amounts of gases from the raw biogas. Before adsorption, sulphur compounds and water vapour must be removed from the raw biogas, as these substances can damage the material used in adsorption (e.g. molecular sieve).
Pressure water washing makes use of the different solubility of CH4 and CO2 in water. The raw biogas is passed through a washing column in countercurrent to the washing solution, whereby the CO2 contained in the biogas dissolves in the water. In order to improve the solubility of CO2 in water, the process is carried out under a pressure of several bar above atmospheric pressure. Hydrogen sulphide (H2S) and ammonia (NH3) can also be partially removed from the biogas in this way. If the concentration of hydrogen sulphide in the raw biogas is very high, an upstream coarse desulfurization is required. The loaded washing liquid can be regenerated and reloaded by reducing the pressure.
Physical washing with polyglycols is also based on the principle of gas scrubbing. In contrast to DWW, however, a scrubbing solution made of various polyglycols is used here. This scrubbing liquid has a higher absorption capacity and selectivity towards CO2 compared to water. The CO2-laden scrubbing liquid is regenerated after scrubbing at an elevated temperature (50 - 60°C).
Various amines and their mixtures are used in chemical washing. CO2 reacts with the amines in a chemical reaction. This allows a much higher load of the washing liquid than with other washing processes. The detergent is regenerated by heating to temperatures of approx. 110 - 160 °C. The temperature level depends on the pressure level of the absorption process. The necessary heat can be provided, for example, by the waste heat from a combined heat and power plant. In Germany, the pressureless process with high temperatures has been predominantly implemented for detergent regeneration.
Processing using membrane separation techniques is a physical process. The raw biogas is compressed to a pressure of several bar and passed through a membrane. The CO2 passes through the membrane and can be separated from CH4. This increases the methane concentration in the biomethane. As a rule, up to three membrane stages are necessary to achieve the methane concentration required for feeding.
CO2 can also be separated from biogas in liquid form under the influence of low temperatures (cryogenic separation). The different boiling points of CO2 and CH4 ensure a very clean separation with very little methane slip. The process also offers another advantage. The CO2 obtained as a by-product is very pure and can be used in the food industry, for example. Processes based on the principle of pure cryogenic separation have rarely been implemented on a large scale due to the high energy expenditure. In contrast, the combination of low-temperature cooling and membrane processes represents an innovative hybrid process. In a first step, the CH4 in the biogas is enriched on a membrane. The CH4 is then cooled to low temperatures with the remaining CO2 and separated. This hybrid process is already being used in practice on an industrial scale.
