*6.1.1 Physical absorption method*

Physical absorption method uses water scrubbing system. Water scrubbing is the most commonly used technology for biogas cleaning and upgrading [170].

This process depends on the extraction of H2S and CO2 from the biogas because of their raised solubility in water compared to CH4 (i.e., according to Henry's law, the solubility of CO2 in water at 25°C is roughly 26 times higher compared to methane); whereas, physical absorption method is using organic solvents. This method relies on the same principle as water scrubbing; however, the absorption of CO2 and H2S is accomplished by the use of organic solvent instead of water.

### *6.1.2 Chemical absorption method*

Various methods are used to bind the CO2 molecules contained in the biogas, such as chemical scrubbers, utilize aqueous amine solutions (i.e., mono-, di-, or triethanolamine); chemical absorption method; and using amine solutions. One of the benefits of this technology is that H2S can be totally absorbed in the amine scrubber. Amine scrubbing systems mostly contain a stripper and an absorber unit.

### *6.1.3 Pressure swing adsorption (PSA)*

Pressure swing adsorption (PSA), which extracts the various gasses from biogas, relies on their molecular properties and the compatibility of the adsorbent matters. The adsorbents can be zeolites (Zeolite 13X, Zeolite 5A), carbon molecular sieve, activated carbon, and other substances with high surface area [171]. The major principle of PSA system depends on the properties of pressurized gasses to be appealed to solid surfaces. Thus, under high pressure, huge quantities of gas will be adsorbed, whereas, a decline of pressure will result in gas discharge. The PSA technology follows four different or equal duration stages, namely pressurization, adsorption, blow-down, and purge [171].

### *6.1.4 Membrane technology*

Membrane technology is considered as an alternative to the traditional absorption-based biogas upgrading technology. The major principle of the membrane technology depends on the selective permeability characteristics of membranes allowing the biogas components to separate [172].

### *6.1.5 Cryogenic technique*

The bases of this technology are the different liquefaction temperatures for biogas compounds [173]. It is conducted through a gradual decrease of biogas temperature allows the selective separation of CH4 from both CO2 and rest components. Thus, a high-purity biomethane is obtained in agreement with the quality standards for Liquefied Natural Gas (LNG). The easiest path to remove the impurities contained in biogas by means of cryogenic methods employs a constant pressure of 10 bar [9, 174–176]. The liquefaction is carried out by declining the temperature successively in order to get rid of each pollutant or mitigate them in different steps. The first step is often set up at −25°C, where mostly siloxanes, H2O, and H2S are obtained. A second set step is assigned at −55°C to partially liquefied CO2, accompanied by a new decline until −85°C to totally get rid of

### *Recent Advances of Biogas Production and Future Perspective DOI: http://dx.doi.org/10.5772/intechopen.93231*

the remaining CO2 by a solidification step [177]. The liquefied CO2 gained in the second temperature stage can be sold as high-purity by-products to improve the whole economic process performance. Another more normally used option contains a preparatory dry of the gas accompanied by a multistep compression up to 80 bar. This permits preserving a higher operational temperature of between −45 and −55°C, containing as major drawback a needful intermediate cooling in the multistep compression [178]. Cryogenic techniques represent a good option to be optimized because these techniques yield high-purity products, ranging between 95 and 99% [13, 179].
