The applicability of the renewable energy directive calculation to assess the sustainability of biogas production

Biogas production processes are often multifunctional systems, which also produce fertilizers from digested sludge. The environmental impacts of such systems are usually determined using life cycle assessment (LCA). There are alternative approaches to conduct the LCA, e.g., allocation of emissions based on a product's and co-product's energy content according to the Renewable Energy Directive (RED), or substitution without allocation according to the ISO 14040 standard. We calculated the climate change impacts of biogas production using these two alternative methods, whilst also considering process modifications of the base case biogas production process. The aim was to find out whether the production system achieves the saving targets for greenhouse gas (GHG) emissions set by the RED. Since the RED enables different interpretations of its calculation rules, we created four case studies representing alternative ways to allocate the emissions to the reject water and solid fractions separated from the sludge. Consequently, our emission estimates for the base case vary between 16.9 and 47.7 g CO2/MJ, while the emission savings range from 42% to 80%. Most of the case studies achieved the most stringent saving target (60%).

► Substitution approach and RED allocation approach produce different emission estimates. ► Different RED allocation alternatives show different emission savings. ► Whether the secondary material flows are wastes or co-products is a major issue. ► It is questionable whether the RED calculation rules are suitable for multifunctional production systems.