Biogas Crops – Part II: Balance of Greenhouse Gas Emissions and Energy from Using Field Crops for Anaerobic Digestion

Abstract

Several field crops, cultivated under north-eastern German conditions, are analysed for their ecological benefit if used for anaerobic digestion. The analyses is based on the assessment of cumulated energy demand necessary for the cultivation at different fertilisation levels, transport and storage of these crops as well as on the assessment of greenhouse gas emissions deriving from these processes. Although the values obtained are quite similar to each other this picture completely changes if considering the methane formation potential and hence the electricity available from these renewable energy sources. Cereals like rye, triticale, barley and maize as well as alfalfa show relatively low values of GHG emissions, and cumulated energy demand whereas hemp and Jerusalem artichoke have a considerable worse balance. In the case of high fertiliser input during cultivation the value of GHG emissions for Jerusalem artichoke even approaches the value for electricity produced within the German power-mix. Open-top tanks for digested energy crops may be a serious source of additional methane emissions.