# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Biogas and Biomethane as Energy Resources  
Carbon dioxide or in general greenhouse gas emissions represent the main cause of global climate change. The scientific community is, in fact, significantly focused on the search for alternative and permanent solutions to achieve in view of the looming energy crisis. Current global policies and guidelines for country development show an increasing focus on global change. The latter is mainly caused by consumerism, liable for accelerating the natural resource extraction rate and generating enormous quantities of waste, which, at the current rate of production and consumption, are estimated to double by 2050 [15]. Environmental and energy security issues are the main reasons for stimulating the search for alternative fuels. In the current scenario, renewable technologies produce electrical, thermal or mechanical energy using biomass (i.e., energy crops, agricultural or forest residues, municipal waste, etc. ), wind, solar (i.e., thermal and photovoltaic), hydroelectric (i.e., river flow, tidal, wave) and geothermal energy and are considered the best alternative sources [16; 17]. Therefore, it is reasonable to assume that gases produced from other sources (such as biomass) will continue to grow in importance as the need to replace fossil fuels as an energy source continues to grow. Biomass is the most abundant renewable resource on Earth [18]. It is considered the leading emerging alternative to fossil fuel resources, and it can provide energy and multiple products [19]. Indeed, it will play an increasingly important role not only in the future global energy infrastructure for the generation of energy and heat but also in the production of gaseous and liquid products for the production of fuels [16; 20]. In recent years, the production of energy from biomass has shown significant growth and increasing weight among renewable energies [21]. Efficiently converting biomass into energy requires investment in research and the development of innovative technologies. The knowledge of the biological and technical mechanisms that are at the basis of biomass production and transformation processes is essential to favoring the introduction of increasingly advanced and efficient techniques and methods in terms of energy, economic and environmental sustainability. There are various ways to use agricultural biomass, and scholars worldwide are busy studying methods for using agricultural biomass resources to obtain high yield and maximum benefit from the products. Various processes can be used to convert biomass into energy. Biomass can be burned and transformed into fuel gas through partial combustion; into biogas through fermentation; into bioalcohol through biochemical processes; into biodiesel; and into bio-oil or syngas, from which chemicals and fuels can be synthesized. Among the various energy production chains, biogas has the greatest potential for development due to its strong compatibility with the Italian agricultural system [22]. Furthermore, novel approaches matured under the European Green Deal, such as EU Methane Strategy [23], Energy System Integration Strategy [24] and From Farm to Fork Strategy [25], put biogas in a leading view [26]. In this regard, biogas has indeed emerged as a promising green energy source and appears to have high future potential for replacing fossil fuel-based energy sources [27]. The increasing use of by-products and residues for energy production is the basis for a sustainable bioeconomy to reduce dependence on fossil fuels [28]. The correct management and transformation of by-products and organic waste represents a winning strategy, since disposal costs are reduced and the resources considered waste are valorized through the production of renewable energy and biofuels [29]. Therefore, biofuel and biomass are renewable sources that can directly replace fossil fuels for present and future energy restriction, due to their environmentally friendly and renewable energy nature [30].