Distribution of specific greenhouse gas emissions from combined heat-and-power production in agricultural biogas plants

Abstract

From model and case studies based on small samples it is clear that specific greenhouse gas (GHG) emissions of energy supply from biogas are strongly dependent of system characteristics and scope. We derive prescriptive statistics for the GHG balance of electricity production from agricultural biogas systems on the basis of a large audit data set. System boundaries include upstream processes, the production of energy crops (EC), the anaerobic digestion process, the storage of digestate, and the utilization of biogas in a combined heat-and-power-unit (CHPU). For our sample of 593 biogas systems the calculated specific CO2-equivalent-emissions of electricity fed into the public grid range from −1,730 to 821 g kWh−1 (mean value ± standard deviation: 307 ± 125 g kWh−1; interquartile range: 249–384 g kWh−1). For the sample as a whole, the mix of input materials on a mass basis consists of 58% EC and 42% animal manure (AM). With this mix, the substrate supply chain contributes 56.3% to the total GHG-emissions of the biogas systems. To fully compensate GHG-emissions from EC production by avoided emissions from AM storage, the ratio AM/EC would need to be increased about fivefold. This result shows that in order to be sustainable, a biogas system in agriculture needs to be understood more as a servicing function to farming rather than the purpose of farming. Other dominant sources of GHG-emissions are the methane slip from the CHPU, biogas losses and parasitic electricity demand.