Energy Use Efficiency of Biogas Production Depended on Energy Crops, Nitrogen Fertilization Level, and Cutting System

Abstract

AbstractThe paper evaluates the relation between energy input (Ei) and output (Eo) of biogas production from six energy crops: maize, sorghum, sunflower, triticale, reed canary grass (RCG), and Virginia mallow (VM), cultivated in three different nitrogen fertilization levels. Furthermore, in the case of RCG, the impact of cutting system was examined. The results showed that raised N fertilization dose (in the range of 40–120 kg ha−1 and 80–160 kg ha−1, depending on the crops) increased biomass yield and methane productivity (MP) but simultaneously caused also the increase in Ei. Nonetheless, the application of higher N doses did not cause drastic decrease in energy use efficiency (EUE). The Ei was significantly lower for perennials than for annual crops. For this reason, EUE for RCG harvested in two cuts (5.0–5.2 GJ GJ−1) was close to EUE for maize (5.7–6.8 GJ GJ−1), despite the much lower MP (2027–2903 m3 ha−1 and 4409–5692 m3 ha−1, respectively) and Eo (73–105 GJ ha−1 and 159–205 GJ ha−1, respectively). Furthermore, the collection of RCG in more than two cuts turned out to be unjustified, due to increase in Ei and, simultaneously, decrease in MP.