## 1 Introduction  
The transport sector is facing the challenge of reducing emissions, while demand for transport is increasing. As several European Commission reports state, the road transport is the second largest source of carbon dioxide (CO\({}_{2}\)) in the European Union (EU), accounting for around 25% of total CO\({}_{2}\) emissions [1, 2]. Without ambitious steps towards decarbonization, the EU-2016 Basic Standard states that by 2050, road transport may account for the largest share of CO\({}_{2}\) emissions [3]. Therefore, European emission reduction targets obligates a 60% reduction in greenhouse gas emissions by 2050 compared with 1990 and a 20% reduction in emissions by 2030 compared to 2008 [4]. To achieve these targets, further efforts have been made in the research and development of clean and alternative fuels, such as compressed natural gas (CNG) [5], biogas [6], biodiesel [7], rapeseed oil [8], alcohols [9] and hydrogen (H\({}_{2}\)) [10; 11], which are considered to be cleaner and more environmentally friendly when compared to conventional fuels. Biogas is one of the most promising types of biofuel with the potential to reduce greenhouse gas emissions and use of fossil fuels. Biogas is produced by anaerobic digestion of organic compounds, such as agricultural wastes, municipal waste, animal waste, green waste, or food waste. The three main components of biogas (BG) are methane (CH\({}_{4}\)), carbon dioxide, and nitrogen (N\({}_{2}\)). However, it may contain traces of hydrogen sulfide (H\({}_{2}\)S), hydrogen, ammonia (NH\({}_{3}\)), oxygen (O\({}_{2}\)), and carbon monoxide (CO) [12]. A more detailed composition of biogas is given in Table 1.  
As shown in Table 1, if the biogas is produced in a sewage digester, the concentration of the harmful hydrogen sulfide is close to zero. In other cases, for example, those described in [12; 13], the concentration of H\({}_{2}\)S could reach up to 400 ppm and cause corrosion (reaction with water leads to formation of sulfuric acid), the formation of Sulphur oxides (SO\({}_{\chi}\)) as a result of combustion, and the formation of combustible mixtures (4.5-45.0 vol%. H\({}_{2}\)S in air). In Europe, according to Eurostat 2016, more than 9.67 million tons of raw material for biogas production in the sewage digester were produced, and more than 4.82 million tons were produced in Italy alone. The data include only sludge and liquid waste from waste treatment. Europe produced in total more than 680 million tons of possible feedstock for biogas production [14]. Depending on the used raw materials, biogas production efficiency can variate. As shown in Table 2, one kilogram of wheat straws can produce 0.483 cubic meters of biogas, but different feedstocks provide various results. Converting all feedstocks to biogas with a ratio of 0.1 kg of feedstock per cubic meter of biogas would produce 68 Gm\({}^{3}\) of biogas or more than 58 Mtoe (total oil equivalent) [25]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Biogas** & **CH\({}_{4}\) (vol\%)** & **CO\({}_{2}\) (vol\%)** & **N\({}_{2}\) (vol\%)** & **O\({}_{2}\) (vol\%)** & **H\({}_{2}\)S (ppm)** \\ \hline Landfills & 45–62 & 24–40 & 1–17 & 1–2.6 & 15–427 \\ Sewage digester & 58–65 & 33–40 & 1–8 & \textless{}1 & 0–24 \\ Organic waste digesesters & 60–70 & 30–40 & 1 & 1–5 & 10–180 \\ \hline \end{tabular}
\end{table} 
Table 1: Biogas composition depending on raw material [12].