# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Biogas and Biomethane as Energy Resources
However, carbohydrate-rich substrates can counteract the process, negatively impacting the C/N ratio (the optimal ratio of carbon to nitrogen for the AD process is between 20 and 30) [40]. The C/N ratio of fermentation substrates is important for an optimal biogas process. In order for microorganisms to form new cells, nitrogen must be present. For this reason, the ratio should not be higher than 30. If it is, the nitrogen deficit leads to a decrease in biomass production. In contrast, if the ratio is less than 20, there could be problems with process inhibition due to the presence of ammonia. Common production sources for biogas production are included in Table 2.  
Besides these, because of their abundance, lignocellulosic biomass materials such as straw and shells are taken into consideration for methane production [46]. Cattle and pig waste in the form of manure is often used as the main source for biogas production, because it contains the microorganisms needed to degrade the substrate and produce biogas [47]. The use of biogas on farms has a positive effect on the daily operations of farms. Biogas plants can reduce the carbon footprint of farms, improve and aid in manure management, and provide a source of income for owners. The biogas produced has a wide range of applications, which are mainly associated with energy generation. Recently, there has been a growing interest in biogas purification for the production of biomethane, which is characterized by higher calorific value (i.e., 20-30 MJ/m\({}^{3}\) for \begin{table}
\begin{tabular}{l l} \hline \multicolumn{2}{l}{**Biogas Production Matrices**} \\ \hline - & Agricultural waste \\ - & Livestock manure \\ - & Sludge \\ - & Sewage sludge \\ - & Industrial by-products \\ - & Food waste \\ - & Wastewater and more \\ \hline \end{tabular}
\end{table} 
Table 2: Use of various matrices for biogas production [44; 45]. biogas, \(>\)35 MJ/m\({}^{3}\) for biomethane) [48]. Biomethane is defined as all methane of organic/renewable origin. It can be obtained by means of separation from biogas or through methanation of hydrogen and carbon dioxide. Biomethane has characteristics comparable to those of natural gas, making it attractive for feeding into the natural gas grid for a variety of applications. To be defined as such, biomethane must be at least 98% pure. In the literature, however, gas with a concentration of CH\({}_{4}\) >95% is defined as biomethane. Biomethane can be used as fuel for automotive transportation. In fact, compared with other biofuels, such as ethanol and biodiesel, which require blending with conventional gasoline and diesel, respectively, biomethane can be directly used in existing engines. The use of biogas and biomethane as energy sources presents environmental benefits, ranging from decreasing greenhouse gas emissions to replacing fossil fuels and increasing efficiency in energy production. The uses of biogas and biomethane are shown in Table 3.  
Currently, biogas and biomethane are considered the centerpiece of the circular economy [51, 52]. In fact, although wind and photovoltaic are the most used technologies so far (with 432.56 GW and 227 GW of global installed capacity, respectively) and are already able to supply electricity at low cost, they are intermittent and difficult to predict due to their nature [53]. On the other hand, biogas provides a continuous and stable source of energy, as it is produced using the consolidated technology of anaerobic digestion of organic material [48]. Moreover, biogas can be stored and used on request. This is an important advantage of biogas over solar and wind systems. Biogas production is also climate-independent (solar radiation and wind) and is thus a very reliable renewable energy source. Biogas as an intermediate fuel and as a final fuel is highly versatile in all energy sectors [54]. In any case, coordination between stakeholders, and energy, environmental (including waste management), transport, agriculture and food policies is essential to the success of biogas and biomethane.