Influence of different practices on biogas sustainability

Biogas production and use are generally regarded as a sustainable practice that can guarantee high greenhouse gas (GHG) savings. However, the actual carbon footprint of biogas is strongly influenced by several factors. The aim of this study is to analyse the environmental performance of different biogas to electricity scenarios. Two criticalities are identified as important: the choice of feedstock and the operational practice concerning the digestate. Maize, manure and co-digestion of them are the different feedstocks chosen. Maize has higher yields, but its cultivation has to be accounted for, which consists of 28–42% of the GHG emissions of the whole process of producing electricity. Manure is considered a residue and as a result benefits from no production stage, but also from avoided emissions from the normal agricultural practice of storing it in the farm and spreading it as fertiliser, but has lower methane yields. Co-digestion combines the benefits and disadvantages of the two different feedstocks. Digestate storage in open or closed tanks and further use as fertiliser is analysed. The environmental impact analysis shows that a substantial reduction of GHG emissions can be achieved with closed digestate storage. The GHG emissions savings vary from about 3% in the maize pathways with open storage up to 330% in the manure pathway with closed storage. The biogas pathways, though, have worse environmental performances in all other environmental impacts considered but ozone depletion potential when compared to the European electricity average mix.