# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Purification and Improvement of biogas
After washing, the organic solvent is regenerated by heating and depressurizing it in a secondary desorption column [142]. Another technology is chemical washing. Chemical washing is based on a chemical reaction in a countercurrent flow between the absorbed gases and a chemical solvent (amine reagent). The following chemical solvents are used: mono-, di- and tri-ethanolamines, mainly mono-ethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), and 2-amino -2-methyl-1-propanol (AMP) or piperazine (PZ). These react with gases to separate them from the gaseous mixture. MEA is among the most commonly used, thanks to its greater selectivity for CO\({}_{2}\) and greater absorption capacity, ensuring efficiency of CO\({}_{2}\) removal of 80-95% [142; 143; 144]. Chemical washing requires a considerable amount of heat compared with the physical absorption process. Adsorption processes are based on the selective adsorption of biogas contaminants. Pressure Swing Adsorption (PSA), Temperature Swing Adsorption (TSA) and the Oscillating Vacuum Adsorption (VSA) are used. PSA is the most frequently used method for biogas upgrading [133]. It is based on the ability of a porous material to adsorb target molecules from a gas mixture under the application of high pressure; these are then released at lower pressures [134]. Basically, the process consists of an adsorption phase during which the biogas, after preliminary removal of H\({}_{2}\)S and water vapor/moisture, is injected from the bottom of an adsorption column. During passage through the column, CO\({}_{2}\) and/or O\({}_{2}\) and/or N\({}_{2}\) can be adsorbed, and the gas is then expelled as a flow of more or less pure CH\({}_{4}\). The cryogenic separation process is different, as it exploits the diversity of the gas boiling points of the gases in biogas. This process is, in fact, based on the principle that gases such as CO\({}_{2}\) and H\({}_{2}\)S are liquefied at different pressures and temperatures. The boiling point of CH\({}_{4}\) is the lowest of all biogas constituents. As a result, a gradual decrease in temperature allows for the removal of impurities in order to obtain extremely pure biomethane. Methane has a boiling point of \(-161.5\)\({}^{\circ}\)C, while CO\({}_{2}\) has a boiling point of \(-78.2\)\({}^{\circ}\)C, and this ensures the separation of CO\({}_{2}\) as a liquid by cooling biogas at high pressures [141]. In this separation, the temperature is reduced (to \(-170\)\({}^{\circ}\) C), and the pressure (80 bar) is increased incrementally to achieve pure CO\({}_{2}\) and CH\({}_{4}\) flows. A combination of direct and indirect cooling is used to ensure optimal operating conditions [136]. All variations of these technologies have recently been examined [145; 146]. Cryogenic technology is, among all, the most energy- and cost-consuming technology. In hydrogenation processes, CO\({}_{2}\) is used to produce synthetic methane in combination with H\({}_{2}\) produced from clear renewable sources. At the present time, the technology for obtaining methane on a large scale is to produce natural gas from synthetic gas (H\({}_{2}\), CO\({}_{2}\) and CO) obtained from the conversion of coal using metal-based catalysts at a temperature between 150 and 500 \({}^{\circ}\)C and pressure between 1 and 100 bar. The most commonly used catalysts are nickel, copper, iron, cobalt, palladium, ruthenium, rhodium, platinum, molybdenum and tungsten, supported by a support material consisting of silicon oxide and carbide, or oxides of aluminum, cerium, titanium and zirconium. Temperature, pressure and physical state are fundamental factors for these elements, which would otherwise gradually hinder the reaction and generate problems of selectivity, cause the production of harmful by-products and the formation of other hydrocarbons other than methane and finally show poor efficiency. Due to the remarkable selectivity, it is indeed possible to achieve complete transformation of CO\({}_{2}\) and H\({}_{2}\)[142].