## 1 Introduction  
The electricity and transport sectors are responsible for most of the world's CO\({}_{2}\) emissions from fossil fuels [1]. Despite greenhouse gas (GHG) emission policies, the transport sector is the only major European economic segment in which GHG emissions have grown since 1990, with all other sectors achieving emission reductions over the same period [2]. European emission mitigation goals include a 60% reduction in greenhouse gas emissions by 2050 compared to 1990 and a 20% reduction in emissions by 2030 compared to 2008 [3]. To achieve these goals, it is necessary to replace fossil fuels with more sustainable and low-emission fuels. Alternative fuels which could reduce greenhouse gas emissions could be compressed natural gas (CNG) [4], biogas (BG) [5], biodiesel [6], alcohols [7], their blends [8] or hydrogen [9], which are considered to be cleaner and more environmentally friendly compared to conventional fuels. Gaseous fuels mix easily with air and forms homogeneous, highly flammable mixtures. Therefore, harmful exhaust emissions are reduced [10]. Biogas, consisting mainly of methane, has a high ratio of hydrogen/carbon (H/C) atoms. Less carbon atoms in biogas ensures lower emissions of CO2 after the same energy input [11]. Biogas is one of the most promising alternative fuels for reducing GHG emission [12]. Properly cleaned biogas could be used as a renewable fuel for vehicles [13]. Biogas could be used in high compression ratio engines. An increased compression ratio leads to a higher thermal efficiency of the engine, but at the same time, emissions of NOx and HC increase [14]. The British Petroleum Company's Energy Outlook predicts that by 2035, usage of renewable energy sources will grow the most in OECD (Organization for Economic Cooperation and Development) countries [15]. Of all renewable fuels, biofuels (including biogas) have been recognized as an excellent source of energy, which is equally suitable for developed countries as a means of reducing CO2 emissions and for developing countries by providing electricity to rural areas, where local people have easy access to biomass [16]. Additionally, biogas production provides greater benefits for the manufacturer, because it is usually spread through a variety of industries, such as energy, transport and waste management, as biogas could be produced from decomposing organic waste from landfills, agricultural or industrial waste, food waste or other biodegradable materials, reducing the negative impact on the environment and methane emissions [17]. The number of biogas plants has increased over the last 10 years, leading to increased usage and production of biogas. In Europe, 6227 biogas plants were registered in 2009 and the number of biogas plants doubled to 13,812 in 2012. Within five years, the number of BG plants increased by 30% as 17,783 biogas plants were registered [18]. The distribution of biogas plants in top 15 countries of Europe is shown in Figure 1, which based on data from [18; 19]. The rising number of biogas plants indicates an increased demand of biogas. In the EU-28, production doubled from 93 to 187 TWh between 2008 and 2016 [19]; further doubling could be expected before 2030, and even greater changes could be recorded in individual countries [20]. In view of the growing production of biogas and its potential to reduce GHG emissions, biogas can be an alternative to fossil fuels in the energy, heat and transport sectors [21]. Figure 1: Number of biogas plants in top 15 countries of Europe, arranged in descending order. At the same time, biogas production would increase the disposal of waste, as biogas could be produced from different feedstock, such as the domestic sewage, citrus pulp, manure, whey, silage and algae [22; 23; 24; 25], and increase the usage of biofuels. Depending on feedstock, biogas could be produced as a first-, second-, third- or fourth-generation alternative fuel [26]. Mainly biogas composition consists of methane (CH4) and carbon dioxide [27].