Characterisation and variability of greenhouse gas emissions from biomethane production via anaerobic digestion of maize

Biomethane is a renewable gas that can be used in existing infrastructure to reduce dependency on natural gas and lower greenhouse gas (GHG) emissions. Policy incentives have promoted a rapid implementation of biomethane production facilities using anaerobic digestion (AD). A range of feedstocks are used in AD including crops which have a higher GHG burden than most wastes and residues. The purpose of this research is to characterise and assess GHG emissions from typical operational biomethane facilities. It is imperative that GHG savings are obtained therefore quantifying emissions using a robust methodology is paramount. This study uses maize as a case study utilising data from several farms and AD facilities. Results show that calculated emissions for biomethane production from maize are 33.8 gCO2e/MJ of biomethane using the Renewable Energy Directive (RED) methodology. Key emission sources include N-fertiliser production, soil N2O emissions, imported electricity use, and fugitive methane. Sensitivity analysis performed assessed key data inputs and demonstrates how input inventory parameters affect the GHG balance and highlights variability in results. For the desired GHG savings to be achieved it is important that operators minimise fertiliser use, use nitrogen inhibitors, minimise imported electricity, and undertake close management of methane loss. This paper shows that although biomethane is considered a renewable, low carbon fuel, the inputs need to be carefully managed in order to achieve this