Thermokinetic study of residual solid digestate from anaerobic digestion

Anaerobic digestion (AD) has become an effective waste management method in the agri-food sector to dispose of livestock and food wastes. As AD becomes more widely used new challenges emerge, such as the disposal of digestate by-products. Currently, the principal method for handling solid digestate (SD) is direct application to land as an organic fertiliser. However, as the sector grows, this option is less viable due to nutrition loading on the land and increased eutrophication. This paper explores the potential of using SD as a biofuel source. Additionally, the pyrolysis kinetic triplet of residual SD and thermal predictions was reported for the first time using Advanced Kinetics and Technology Solutions (AKTS) thermokinetics software to model and calculate the activation energy (E_a) and other kinetic parameters. ASTM-E698, Ozawa-Flynn-Wall and differential iso-conversional (model-free) methods were used and the Ea values calculated from each model were 169.8, 75 – 175.0, and 85 – 190.0 kJ mol⁻¹, respectively. The kinetic triplet can be used in the scale-up or designing of reactor systems considering SD as a feedstock. The kinetic prediction of isothermal pyrolysis of SD indicated that a temperature higher than 210 °C is required for onset of decomposition in the sample. Furthermore, SD has the potential to produce an additional ~27.9 MJ per day at a 500 kW standalone on-farm AD plant. This work has highlighted the potential of waste solid digestate as a potential solid biofuel that could add an additional revenue stream to AD plants and make them more sustainable.