Dynamic analysis and investigation of the thermal transient effects in a CSTR reactor producing biogas

In the field of primary energy saving, biomethane is becoming more and more attractive. The need to manage urban waste, combined with the increasing use of biofuels, makes this resource crucial for sustainability objectives. Therefore, a detailed estimation of biogas production from an anaerobic digester is an important aspect for the related research activity. This work presents a thermal transient model developed in MATLAB® that can accurately predict heat transfer and biological phenomena occurring within the digester. The model developed in this paper can conveniently consider the thermal inertia of the treated biomass during the retention time. This model is subsequently linked to the TRNSYS environment, where the produced biogas is upgraded to biomethane. For this purpose, a membrane permeation model is considered. A concentrating photovoltaic/thermal collector is coupled to the digester to meet the plant electricity and thermal energy demands. In particular, the electricity demand is mainly due to the upgrading process while the thermal energy is necessary for the anaerobic digestion process. The results of the dynamic analysis show that neglecting the thermal inertia of the digester leads to an overestimation of the total biogas production by 20%. Furthermore, the integration of renewables with the anaerobic digestion process shows remarkable results. In this case study, the Primary Energy Saving is roughly 5% and the Simple-Pay Back is less than 10 years, despite the small scale selected for solar collectors.