Two-Fraction Kinetic Modelling of Methane Recovery and Energy Potential During Batch Anaerobic Digestion of Agricultural and Agro-Industrial Substrates

Anaerobic digestion (AD) of agricultural and agro-industrial substrates depends on feedstock properties and degradation rate. Biochemical methane potential (BMP) gives the final methane potential, but it does not show how much methane is recovered within a given digestion time. A two-fraction kinetic model was used to estimate methane recovery and energy potential during batch digestion. The input data included total solids, volatile solids, experimental BMP, the shares of rapidly and slowly biodegradable fractions, their rate constants, and lag time. Ten substrates were assessed using archived experimental datasets from laboratory studies conducted between 2018 and 2025. The specific contribution of this study is the re-analysis of these datasets using the same two-fraction fitting and calculation procedure, enabling direct comparison of methane recovery at defined digestion times. Methane recovery after 20, 30, and 40 days was expressed per tonne of fresh matter and converted to chemical energy. An illustrative mixture of eight substrates was also analysed at 20, 25, 30, and 40 days by summing the methane contributions of the individual components, without considering interactions between substrates. After 30 days, calculated methane recovery ranged from 11.4 to 350.0 Nm3 CH4 t−1 fresh matter. The highest values were obtained for crude glycerol, food waste, maize silage, dissolved air flotation sludge, and sugar beet pulp. Animal slurries gave lower values because of their low volatile solids content. For the mixture, methane recovery increased from 60.9 to 65.7 Nm3 CH4 t−1 fresh matter between days 20 and 30 and reached 68.6 Nm3 CH4 t−1 after 40 days. Chemical energy increased from 607 to 655 kWh t−1 fresh matter and reached 684 kWh t−1 after 40 days. The smaller increase after day 30 resulted from the declining contribution of the rapidly biodegradable fraction. The model separates final methane potential from methane recovered at a specified batch digestion time. It allows feedstocks with different solids contents, BMP values, and degradation rates to be compared on the same basis. The approach can therefore support preliminary feedstock screening and the selection of substrate and mixture variants for further laboratory testing and detailed process assessment.