# Influence of the Parameters of an Agricultural Biogas Plant on the Amount of Power Generated
## 2 Materials and Methods  
The research was carried out in an agricultural biogas plant derived from a substrate from dairy cows raised in a barnless system (a technological diagram of the biogas plant under study is shown in Figure 1). The agricultural biogas plant consisted of a digester with a diameter of 15.5 m and a wall height of 3.7 m. The total height of the digester, including the biogas balloon, was 8.7 m. It allowed for a nominal storage of 590 m\({}^{3}\) of substrate. Two agitators with a rated power of 9 kW each were installed in the chamber to maintain a homogeneous digestion mass. The agitators installed in the biogas plant were of the medium-speed type, and their speed did not exceed 700 rpm. Both agitators were switched on simultaneously every 30 min, and their operating time was 3 min. In excess foam formation, a mechanical foam beater was automatically activated. The water system installed in the chamber enabled a constant temperature to be maintained during the digestion process. The ratings showed that the biogas plant under study performed fermentation at a target temperature of 42 \({}^{\circ}\)C. Another component of the biogas plant was the container in which the combustion engine was connected to the generator, and other technical equipment for pumping and purifying the biogas and controlling the technological processes of the biogas plant were installed. The biogas was pre-treated in a gas filter and then flowed through a carbon filter filled with activated carbon. The biogas produced in the reactor was combusted in two internal combustion engines with a capacity of 20 kW each. The basic specifications of the generators are given in Table 1. The biogas-burning engine was cooled by circulating water in the engine and the cooling block. The final component of the biogas plant was a concrete, open-air digestate tank. The engine and generator were mounted on a rigid frame with belt drive, all fitted with vibration dampers. The exhaust system and manifold worked together with an exhaust gas heat exchanger. Slurry from the barn was pumped into the digester, where fermentation occurred under anaerobic conditions. The average length of time of the substrate in the fermenter \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{2}{c}{**Internal Combustion Engine**} \\ \hline Type & WG1605 \\ Cycle & Otto \\ Number of cylinders & 4 \\ Displacement capacity & 1537 L \\ Rated speed & 2700 rpm \\ Rated active power & 20 kW \\ Rated apparent power & 26 kVA \\ Primary energy consumption & 62.5 kW \\ Thermal power & 40.9 kW \\ Flue gas temperature & max 110 \({}^{\circ}\)C \\ Coolant temperature & max 95 \({}^{\circ}\)C \\ \hline \hline \multicolumn{2}{c}{Generator} \\ \hline Type & Asynchronous 4P/IE2 \\ Rated speed & 1500 rpm \\ Rated frequency & 50 Hz \\ Rated voltage & 3 \(\times\) 400 V \\ Winding connection & triangle \\ \hline \hline \multicolumn{2}{c}{Characteristics of the engine–generator group} \\ \hline Electrical efficiency & 32\% \\ Total efficiency & 97\% \\ Rated voltage & 400 V \\ Rated current & 29 A \\ Rated power factor (cos \(\phi\)) & 0.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Primary technical data on the generation equipment used in the agricultural biogas plant. Figure 1: Technological diagram of the analysed agricultural biogas plant [own elaboration]. 1—livestock facility, 2—slurry tank, 3—digester with biogas tank, 4—cogeneration unit, 5—digestate tank, 6—agricultural field. (hydraulic retention time) was between 2 and 3 weeks--the slurry spent no less than 12 days in the digester. The following formula describes the hydraulic retention time (HRT):  
\[HRT=\frac{V_{R}}{V} \tag{1}\]  
where _HRT_--hydraulic retention time [day], \(V_{R}\)--active digester volume [m\({}^{3}\)], and \(V\)--daily digester input volume [m\({}^{3}\)/day]. The bacteria's reproducing ability is essential when determining the hydraulic retention time. Methanogenic bacteria, for example, need up to several days to double their population.