Life cycle assessment of greenhouse gas mitigation of production and use of bio-methane

Biogas and bio-methane that are based on energy crops are renewable energy carriers and therefore potentially contribute to climate protection. However, significant greenhouse gas emissions resulting from agricultural production processes must be considered. Among those, the production and use of fertilizer, and the resulting leaching of nitrous oxide (N2O) are crucial factors. This paper provides an integrated life cycle assessment (LCA) of biogas (i.e. bio-methane that has been upgraded and injected into the natural gas grid), taking into account the processes of fermentation, upgrading and injection to the grid for two different types of biogas plants. The analysis is based on different feedstocks from crop rotation systems for different locations in Germany. A special focus is on the sensitivity of assumptions on nitrous oxide emissions on overall greenhouse gas emissions. Much research exists on the measurement or modeling of the actual N2O emissions that result from farming processes. Since there is as yet no precise regional data, most analyses use tier-1 data from the IPCC national greenhouse gas inventories as a default. The present paper coincides with recent research in indicating that this data varies at the regional level. However, it is not the scope of the article to evaluate the quality of existing data for N2O emissions, but to show the effects of different assumptions to the life cycle assessment of greenhouse gases from biomethane. Thus, a link between the provision of emission data and the practical implementation of biogas technology is provided. The main result is that the supply chain of substrates from agricultural processes appears to contribute the most to the greenhouse gas emissions of bio-methane. The “worst case” scenario where 5% of the nitrogen fertilizer used is emitted in form of N2O shows that the greenhouse gas mitigation potential of bio-methane versus natural gas is very small, so there is not much margin for error in the plant technology.