Environmental impacts concerning flexible power generation in a biogas production

Abstract Combined heat-and-power units (CHPUs) of biogas plants can be operated at partial load or in intervals to respond to market conditions or interventions by the grid operators. In this study, we present calculations for the specific greenhouse gas (GHG) emissions of electricity from biogas driven CHPUs in dependence of engine load. Following the methodology of life cycle assessment and using measured data from five real-world biogas plants in Bavaria, we investigated the following scenarios for the operation of the CHPU: (1) full load, (2) 80% load response and (3) 60% load response. Our system boundary included the whole biogas production chain, starting from the production/supply of the input materials and ending at the supply of electricity (functional unit: 1 kWh of electrical energy fed into the grid). As electrical efficiency and engine load are positively correlated, partial-load operation results in higher specific GHG emissions of electricity from biogas. The impact of decreasing efficiency of the CHPU under partial load on overall specific GHG emissions turned out to be higher than the influence of methane slip from the engine. Furthermore, the increase of further efficiencies such as higher yields, efficient use of synthetic and organic fertilizers, as well as minimization of fuel consumption and the use of regenerative fuels also lead to further decrease of GHG emissions.