# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Purification and Improvement of biogas
During the upgrading process, the biogas is pressurized to 8-12 bar up to 40 \({}^{\circ}\)C and fed through the bottom of the scrubber, while water is fed into the scrubber from above. The absorption column is usually filled with random packing material to increase the gas-liquid mass transfer. There is a countercurrent interaction between gas and water, which increases the contact time and mass transfer between gas and water. CO\({}_{2}\) is absorbed in the downward-flowing water, and biomethane is obtained as over-90%-pure CH\({}_{4}\). The exhaust water containing the absorbed CO\({}_{2}\) and H\({}_{2}\)S is sent to a flash column to recover the CO\({}_{2}\)-rich exhaust gas. There are two types of water scrubbing, namely, single-pass water scrubbing, where water comes from sewage treatment plants, and regenerative absorption, where water can be \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Solvent Scrubbing** & **Adsorption Processes** & **Cryogenic Separation** & **Hydrogenation Processes** & **Membrane Separation** \\ \hline It uses a gas–liquid & & & & The selective \\ reverse flow to improve & It is based on three & It is known that CO\({}_{2}\) & & permeability \\ selectivity in the & separate approaches for & and H\({}_{2}\)S are liquefied & CO\({}_{2}\) is used to produce & characteristics of the \\ contaminants using & selective adsorption of & at different pressures & synthetic methane in & membranes, which \\ water or other organic & bigas contaminants: & and temperatures. The & combination with H\({}_{2}\) & enable separation of the \\ liquids, or with the & The most widely used & diversity of boiling & produced from clean & various biogas \\ chemical reaction & technique for biogas & points of the gases in & renewable sources. & components, are the \\ between biogas contaminants and a & upgrading is PSA. & the biogas is exploited. & & foundation of this \\ chemical solution. & & & technique. \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of technologies in use in biogas upgrading [133; 134; 135]. regenerated in a desorption column by decompressing it at atmospheric pressure, resulting in the removal of CO\({}_{2}\) and H\({}_{2}\)S. In single-pass water scrubbing, the treated water is used to absorb CO\({}_{2}\) and H\({}_{2}\)S from the raw biogas and is then returned to the treatment systems. In regenerative absorption, fresh water is used for the absorption of impurities from raw biogas. Water regeneration is crucial to the economic sustainability of this technology, due to the considerable water quantities required for biogas upgrading [138]. Subsequent to washing with water, the drying of biomethane is necessary due to the increase in water content [139]. Finally, after a drying phase, CH\({}_{4}\) can reach a purity of up to 99%. It has recently been revealed that it is possible to lower the cost of high-pressure water washing (HPWS) by including residual heat recovery [139]. In addition, CO\({}_{2}\) desorption in the biogas upgrading process has recently been improved by using vacuum and ultrasound, positively influencing the effectiveness and efficiency of the HPWS process [140]. Washing can also be performed with organic solvents. This procedure is a physical absorption process similar to water scrubbing; however, it uses organic solvents instead of water not only to absorb CO\({}_{2}\) from the biogas but also for the absorption of H\({}_{2}\)S and H\({}_{2}\)O. Polyethylene glycol ether (PEG), methanol and N-methyl pyrrolone are the commonly used organic solvent mixtures to absorb CO\({}_{2}\) and H\({}_{2}\)S from raw biogas [141]. These organic solvents have higher solubility of CO\({}_{2}\) in the solvent than in water. During this organic solvent-scrubbing process, raw biogas is initially compressed to 7-8 bar and cooled to 20 \({}^{\circ}\)C, before being injected from the bottom of the absorption column [138]. The organic solvent is also cooled before being injected.