**3. Energy content and potential of biogas**

The energy content of biogas varies but a typical normal cubic meter of methane has a calorific value of around 10 kWh, but CO2 being noncombustible has zero, making methane the ultimate determinant of the energy value of biogas. At 60% methane composition, then, the energy content of biogas would be less than 10 kWh per normal cubic meter, as CO2 and other noncombustible impurities absorb instead of generating heat [1, 2, 94]. The primary factor influencing the thermal energy content of biogas is the methane composition within the gas mixture. The volumetric composition of methane in biogas widely varies based on factors like the feedstock used and process control [49, 95, 96]. This disparity suggests that biogas's LHV ranges between 16 and 28 MJ/m<sup>3</sup> , rather than remaining stable at a single value. Combustion of biogas releases thermal energy that can be used for cooking or other uses like internal combustion engines [3]. Raw biogas has undesirable components like hydrogen sulfide, ammonia, and various volatile organic compounds whose presence and composition are determined by the feedstock used and process control applied [33, 97].

#### **3.1 Biogas for engine applications**

Biogas can be used as a fuel internal combustion engine although it requires some modifications on the engine design and ignition to handle gaseous biogas fuel instead of the traditional liquid fuels. Biogas has higher octane rating (MON = 130) enabling it to sustain higher compression ratios without engine knocking, hence better engine performance [51, 98–100]. Historically, the use of biogas as an engine fuel started during the Second World War (2WW), as Germany and its Nazi territories had challenges sourcing fuel vehicles and farm machinery [44, 62]. This forced them to resort to sewage gas and biogas from manure and digesters, which were bottled and used as fuel during the crises.

Biogas is an efficient, sustainable, and environmentally friendly fuel that can perform optimally in spark ignition engines modified to suit its specific properties. The performance and utility of biogas as engine fuel is increased by methane enrichment or the addition of hydrogen to improve its flame quality [1]. The main challenge with biogas as engine fuel is the gaseous nature, which means low volumetric density. Biogas as an engine fuel is improved by biogas upgrading, methane enrichment, prechamber combustion, alteration of ignition parameters, higher compression ratio, and addition of hydrogen for better performance and emissions management [98, 101].

The following are some guidelines to consider when selecting biogas engine fuel.

