A new concept for enhancing energy recovery from agricultural residues by coupling anaerobic digestion and pyrolysis process

In a full-scale anaerobic digestion plant, agricultural residues are generally converted into biogas and digestate, the latter usually produced in large amount. Generally, biogas is converted into heat, often lost, and electricity, which is completely valorized or it is sold to the public grid. In this context, the aim of this study was to investigate the feasibility to combine anaerobic digestion and pyrolysis processes in order to increase the energy recovery from agricultural residues and the sustainability of the anaerobic digestion plant. Results revealed that heat excess produced during anaerobic digestion could cover the drying needs for the solid digestate, while pyrolysis of digestate at 500 °C resulted in 8.8 wt.%, 58.4 wt.% and 32.8 wt.% of syngas, oil and char, respectively. The LHV of syngas was 15.7 MJ N m−3, whereas pyrolysis oil exhibited a HHV of 23.5 MJ kg−1 after water extraction. The hybrid system operating in symbiosis could increase the production of electricity from 9896 kWhel day−1 to 14,066 kWhel day−1 corresponding to an increase of 42% compared to AD stand-alone plant.