# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Biogas and Biomethane as Energy Resources
Biogas is a gaseous biofuel obtained from the fermentation of biomass, in the absence of oxygen and at a controlled temperature, by microorganisms. It is a mixture of several gases, made up especially of two main components, that is, methane (CH4) and carbon dioxide (CO2), and to a lesser extent of water vapor (H2O), hydrogen sulfide (H2S), nitrogen (N2) and oxygen (O2). In addition to these, trace elements may also be present. The typical composition of biogas is reported in Table 1. Biogas contains variable concentrations of components as a function of the biogas source. The relative content of CH4 and CO2 in biogas mainly depends on the substrate being digested. CO2 is now considered the main greenhouse gas responsible for global warming, but CH4 has a Global Warming Potential (GWP) 85 times that of CO2 over a 20-year timescale [33;34]. Thus, methane (CH4) is the second largest contributor to global warming [35], and after CO2, it is the gas with the greatest impact on emissions, contributing 1/4 of today's man-made warming [36]. Moreover, methane is a potent air-polluting agent and contributor to ozone formation, which itself causes serious health \begin{table}
\begin{tabular}{l c} \hline \hline
**Mixture Components** & **Percentage (\%)** \\ \hline \(\bullet\) & Methane (CH4) & 40–80 \\ \(\bullet\) & Carbon dioxide (CO2) & 30–50 \\ \hline \(\bullet\) & Hydrogen sulfide (H2S) & 1–4 \\ \(\bullet\) & Nitrogen (N2) & 0–1 \\ \(\bullet\) & Oxygen (O2) & 0–1 \\ \(\bullet\) & Water vapor (H2O) & 0–1 \\ \hline \(\bullet\) & Iron (Fe) & \\ \(\bullet\) & Nickel (Ni) & \\ \(\bullet\) & Cobalt (Co) & Trace \\ \(\bullet\) & Selenium (Se) & \\ \(\bullet\) & Molybdenum (Mo) & \\ \(\bullet\) & Tungsten (W) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Biogas constituents [31; 32]. problems. Methane can be released at various times during production and distribution, but it is still not fully understood where, when and how much methane is released. CH\({}_{4}\) emissions are increasingly recognized as a key factor for the climate; therefore, a careful assessment of any fugitive and diffuse emissions is needed [37]. Mitigation of these gases is critical to supporting the global goal of limiting warming by reaching a global temperature limit of 1.5 or 2 \({}^{\circ}\)C [38]. The global average temperature in 2020 was 1.2 \({}^{\circ}\)C higher than pre-industrial temperatures, and the effects of this warming are being felt globally [38]. Based on current climate data, there is an urgent need to accelerate our efforts to reduce atmospheric concentrations of greenhouse gases to reverse global climate change [10]. Biogas originates from a biochemical process, the anaerobic digestion (AD) of biodegradable biological materials through microbial activity. Both the composition of biogas and its properties depend on the type of substrate to be digested, the type of plant and the process conditions. Its chemical composition also influences the production operations to maximize its utilization characteristics [39]. Other elements that affect its composition are pH, operating temperature, organic loading rate, hydraulic retention time and reactor structure [40]. In any case, the choice of the starting substratum is extremely important, as the activities of microorganisms are directly related to its nutritional composition. Substrates used for biogas production vary considerably from region to region and often vary with the availability of specific feedstocks in a given area. The composition of substrates influences biogas yield, the amount of methane, biodegradability and the biomass degradation rate [41]. The most important nutrients in substrates are carbohydrates, proteins and lipids [42]. Usually, materials with large amounts of fat and protein have a higher methane yield than those with high carbohydrate content. High fat content, however, can cause failure of anaerobic digestion due to the formation of long-chain fatty acids [43].