# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Purification and Improvement of biogas  
Biogas upgrading is a widely studied and debated topic. Its use as a substitute for natural gas has gained, in fact, considerable interest in recent years. Biomethane is produced through anaerobic digestion, followed by an upgrading of biogas that aims to increase the concentration of CH4 by separating it from other biogas compounds. Biomethane is typically 97-99% methane and 1-3% CO2. It is a renewable gas that can be transformed into \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substrate** & **C/N Ratio** & **Biogas Yields** & **References** \\ \hline Manure, straw, energy crops, & & Increase in biogas yields by & \\ food waste, slaughterhouse & Balanced C/N ratio & 1.27–3.46 times compared with & [121, 122, 123, 124] \\ waste and residual fat & & mono-digestion of the same substrates & \\ \hline Quail manure and & & Enhancement in methane production & \\ alkali-pretreated corn stover & Balanced C/N ratio & (296.5 mL CH4 gVSadd \({}^{-1}\)) and high & [122, 125] \\ (NaOH 2.5\%) & & energy recovery & \\ \hline Poultry manure with & & Enhancement in biogas production with & \\ alkali-pretreated corn stover & Balanced C/N ratio & the maximum biogas yield of & [126, 127, 128, 129] \\ (6\% NaOH) & & 745 mg/g TS & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of different types of co-digestion and their impact on biogas yields. a resource by means of its conservation in depots. Storage of biomethane allows it to be used at any time and place; thus, it is useful as transportation fuel or for injection into the national natural gas grid [130]. To increase the exploitation of biogas, it is necessary to remove various impurities using upgrading technologies. We usually refer to impurities such as water (H\({}_{2}\)O), carbon dioxide (CO\({}_{2}\)), hydrogen sulfide (H\({}_{2}\)S), dioxygen (O\({}_{2}\)), dinitrogen (N\({}_{2}\)), carbon monoxide (CO), ammonia (NH\({}_{3}\)), volatile organic compounds (VOCs), siloxanes, hydrocarbons, and aromatic and halogenated compounds. The biogas upgrading process generates new possibilities for its use, as it can replace natural gas, which is widely used in many countries. Most of the biomethane produced worldwide is obtained with the AD process, followed by the processes of carbon dioxide removal and desulfurization of raw biogas. However, upgrading processes add to the costs of biogas production. Therefore, it is important that the upgrading process be optimized in terms of low energy consumption, high efficiency and high methane yield [131]. Biogas upgrading technologies are mainly based on absorption, adsorption and membrane separation processes. These purification techniques allow for high methane concentration and high efficiency of removal of various impurities. The primary objective is to enhance the low calorific value of biogas and convert it into valuable fuel. The end result of biogas upgrading is pure methane [132]. Currently, different upgrading technologies, such as solvent scrubbing, adsorption processes and so on, are commercially available. The biogas upgrading technologies are summarized in Table 7 and are then discussed below. The upgrading processes of biogas can be conducted using several techniques, such as (1) solvent scrubbing, (2) adsorption processes, (3) cryogenic separation, (4) hydrogenation processes and (5) membrane separation. Advantages and disadvantages of CO\({}_{2}\) removal techniques are indicated in Table 8.  
Solvent scrubbing is the most commonly used process. It can occur either by physical absorption, with water or specific solvents that absorb the contaminants of biogas, or by chemical absorption, with the use of chemical means. The cleaning procedure using water is based on the higher solubility of H\({}_{2}\)S and CO\({}_{2}\) in water than CH\({}_{4}\)[136]. For instance, CO\({}_{2}\) is 26 times more soluble in water than methane at 25 \({}^{\circ}\)C. Physical washing with water is carried out at a pressure between 6 and 10 bar [137].