LCA of biomass digestion for biogas and biomethane

In order to improve the yield of biogas plants, operators often purchase or cultivate substrates with high energy content. The environmental impacts due to the digestions of these substrates can be analysed with a life cycle assessment (LCA).

ESU-services was commissioned by the Swiss Federal Office of Energy (SFOE) to investigate life cycle inventory datasets of biogas production from the following substrates: maize silage, sugar beets, fodder beets, beet residues, molasses, and glycerine. Furthermore, biogas from a grass refinery was analysed. The life cycle inventories were based on literature data and a survey of 16 biogas plant operators, which was conducted within another project of the SFOE “Biomass Research Programme”.

The setup of life cycle inventory datasets allows for a detailed assessment and comparison of the environmental impacts of using biogas from different substrates and products of biogas operated cogeneration units. If considering the total environmental impacts assessed with the ecological scarcity method (2006), driving with biogas from energy crops has higher environmental impacts compared to driving with conventional natural gas.
Electricity produced from biogas generated from energy crops has considerably higher environmental compared to the average electricity from the Swiss grid, whereas electricity produced from biogas generated from waste substrates has lower environmental impacts and can be considered as green electricity.

At present, biogas in Switzerland is mainly produced from sewage sludge, slurry, and biowaste. If the co-digestion with higher shares of substrates made from energy crops increases significantly in future, the produced biogas cannot comply with the thresholds for a fuel tax reduction anymore.