## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline
**Raw Material** & **Biogas Production (m\({}^{3}\)/kg)** & **Ref.** \\ \hline Corn residues & 0.563 & [15] \\ Wheat straws & 0.408–0.483 & [15; 16] \\ Rice straws & 0.468–0.503 & [15; 17] \\ Sugarcane wastes & 0.463 & [15] \\ Lignocellulose biomass & 0.178–0.402 & [17; 18] \\ Switch grass & 0.208 & [19] \\ Pine wood & 0.033 & [19] \\ Food wastes & 0.683–0.725 & [20; 21] \\ Nonedible oil seed cakes & 0.242 & [22] \\ Bagasse & 0.330 & [22] \\ Dry leaves & 0.118 & [22] \\ Manure & 0.129 & [23] \\ Brewer’s yeast & 0.430 & [24] \\ Dry maize & 0.397 & [25] \\ Cattle slurry & 0.370 & [26] \\ Maize silage, whole-crop rye silage, & 0.610 & [26] \\ and fodder beet silage & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Amount of biogas produced from one kilogram of feedstock. Data from EurObserv'ER 2017 suggest that in 2016, twenty-eight European Union (EU-28) countries used only a part of their feedstock to produced just under 16.1 Mtoe of biogas [27]. However, every year, the number of biogas plants increases. The European Banking Authority (EBA) Annual Report 2019 states that by the end of 2018, 18,202 biogas plants were operating in Europe after a decade of sustained growth, which was largely stable during periods of political turmoil in different countries [28]. Statistic data demonstrating the increasing number of biogas plants in the European Union and distribution by countries are shown in Figure 1.  
Due to the increasing number of biogas plants and many possible raw materials, biogas can be used as a green energy source for the use of electricity and heat and as a transport fuel. Biogas power plants are often located near the sources of raw materials needed for biogas production (farms, slaughterhouse, and water or waste treatment plants) that are not included in the natural gas pipeline network. In this case, the produced biogas is typically used in heavy-duty cogeneration plants located near biogas plants to generate electricity and heat [30; 31]. However, there are many opportunities in the market to transport biogas and connect local manufacturers with potential consumers. Production of electricity becomes less demanding of power, and smaller stationary engines can be used. Therefore, opportunities arise to use biogas for generating electricity in heavy-duty and smaller engines. Replacing fossil fuels with biogas would reduce greenhouse gas emissions, because CO\({}_{2}\) emissions of biogas (made from biomass) combustion are considered as biogenic and calculated to be neutral with regard to their impact on the climate [32; 33]. Researchers also describes a reduction in sulfur, nitrogen, and carbon compounds in exhaust gasses when changing fossil fuel to biogas [34]. Standard internal combustion after the necessary changes could be adapted to use biogas, which can be used as a renewable energy source in a spark ignition [35] or compression ignition [36] (CI) internal combustion engine. As described by [PERSON] et al. [37], due to the higher autoignition temperature, biogas cannot be used as a standalone fuel for CI engines. Therefore, biogas is mostly used in a dual-fuel approach with diesel or biodiesel [34; 38]. Because of higher autoignition temperature and higher antiknock index, the biogas-fueled spark ignition (SI) engine can have a high compression ratio (CR), which enhances the thermal efficiency of the engine [39]. Moreover, biogas has the potential for complete replacement of fuel in SI engines. However, it is highly recommended that  
Figure 1: (a) Change in the number of biogas plants in Europe. (b) Distribution of biogas plants per 1 million people [29]. the exact CR, air-fuel ratio, and ignition timing be maintained in order to achieve low emissions and the best fuel conversion efficiency [40]. The presence of carbon dioxide in biogas reduces engine performance and brake thermal efficiency, but the compression ratio is directly proportional to these parameters [41].