Thermodynamic analysis and optimization of the integrated system of pyrolysis and anaerobic digestion

Investigating suitable waste management processes is essential nowadays. Anaerobic digestion and pyrolysis are among waste treatment processes that have demonstrated promising potentials. The objective of this study is to evaluate the integration of pyrolysis and anaerobic digestion comprehensively in terms of energy/exergy analysis and comparing the integrated energy system with bare systems. To that end, novel pyrolysis and anaerobic digestion plants are designed and proposed. MATLAB was used for developing a code that simulated the plants and meanwhile, Aspen Plus provided thermodynamic properties. Results showed that the exergy efficiency of the integrated plant is 45.71%, while this parameter is 27.60% and 88.71% for the simple pyrolysis and anaerobic digestion plants, respectively. Furthermore, to make pyrolysis plant energy-independent and maximize bio-oil production, the optimum chemical composition of biomass feedstock is obtained. Seven samples were scrutinized, of which the sample with 46.00 wt% cellulose, 29.33 wt% hemicellulose, and 24.67 wt% lignin showed the optimal conditions. This composition could raise the exergy efficiency of the pyrolysis plant to 40.03%, while more interestingly exergy efficiency of the integrated system would reach 51.15%. Taken together, the findings suggested that the integration of pyrolysis and anaerobic digestion improves both exergy efficiency and methane production.