Abstract
The 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.
Highlights
One of the main requirements for energy crop cultivation is the lack of the competition with food and feed production in arable land areas [1]
We should strive to ensure that the energy input (Ei) and energy output (Eo) ratio, i.e., the so-called energy use efficiency (EUE), is as high as possible
Jankowski et al [3] even assessed energy efficiency of biogas production from crops grown in Poland, but Eo was calculated based on calorific value of biomass, not on its biogas yield
Summary
One of the main requirements for energy crop cultivation is the lack of the competition with food and feed production in arable land areas [1]. In addition to the right selection of species, appropriate agrotechnical measures are applied, such as increased fertilization doses or more frequent mowing of perennials and grasses All these measures, result in an increase in energy input on cultivation, which in consequence may result in lower energy efficiency. Jankowski et al [3] even assessed energy efficiency of biogas production from crops grown in Poland, but Eo was calculated based on calorific value of biomass, not on its biogas yield. They did not evaluate neither the impact of various nitrogen fertilization levels, nor cutting frequency
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