Is Anaerobic Digestion Energy Viable?

Anaerobic digestion (AD) is a biological process carried out by microorganisms that convert organic matter into microbial biomass and biogas, primarily composed of methane and carbon dioxide. AD is increasingly being implemented on farms for renewable energy production. For AD to be energetically viable, the energy generated from the biologically produced methane must exceed the energy spent for its production. The energy spent for the production of methane is related to the transport of biomass to the anaerobic digester, together with stirring and heating the bioreactor. The latter are influenced by the size and volume of the reactor. To facilitate the energetic assessment of various AD operations, publicly available data was utilized to develop an ExcelTM model. This model takes into account associated operations, such as biomass collection and transportation to the AD reactor. The model was used to determine the energetic break-even point for bio-methane generation. A feed mixture of 60% energy crops and 40% cow dung, with energy crops represented by sugarcane, was used for all simulations at different tonnages of feed per year. Across eighteen simulations, the model demonstrated an average annual energy productivity of 2.42 × 106 Wh/ton of volatile solids (VS). Notably, for all digester volumes exceeding 75 m³, the AD process was energetically efficient, producing more energy than required for its operation. The overall economic viability of an AD operation is highly dependent on local and national financial incentives and was not modelled. This tool enables the identification of optimal conditions concerning size and location for any AD operation, thereby enhancing its efficiency and profitability. Keywords: Energetic balance; Anaerobic Digestion; Biogas; Energetic viability; Circular economy; Renewable energy; Sustainability