Grass and cover crops for biogas production and climate change mitigation : A life cycle perspective

Abstract

One strategy to mitigate climate change and increase energy security is to replace fossil fuels with bio-based alternatives. Upgraded biogas (biomethane), is a renewable energy carrier that can be readily integrated into existing infrastructure, e.g. for heat and electricity generation, or as vehicle fuel. Life Cycle Assessment (LCA) is a frequently used methodology for studying the environmental impact of bioenergy systems, but spatial and temporal variations in emissions, emissions from soil organic carbon dynamics and nitrous oxide emissions are often inadequately described in LCA. In this thesis, LCA methodology was used to explore the climate mitigation potential of grass and cover crop cultivation and their integration into biogas systems in Sweden. Various approaches were employed in data inventory, including agro-ecosystem modelling, simple carbon modelling, empirical approaches for estimating nitrous oxide emissions, and data from short- and long-term field experiments. Alongside the conventional GWP metric, a dynamic impact assessment method was applied to consider the timing of emissions. The results revealed considerable mitigation potential for grass- and cover crop based-biogas systems. Introducing a grass-based biogas system using fallow land in Uppsala Municipality doubled the biogas production, leading to mitigation potential of 9950 tonnes CO2-eq per year. However, the biogas mitigation potential exhibited large variation (79 to 102% compared with diesel fuel), depending on where in the region grass was cultivated. Cover crop cultivation had higher climate mitigation potential when the cover crop was harvested, primarily through fossil fuel substitution and a reduced risk of elevated nitrous oxide emissions during winter. These findings offer valuable insights that can hopefully be used in creating sustainable crop-based bioenergy systems in Sweden and other regions with similar conditions.