Experimental and theoretical investigation on the recovery of green chemicals and energy from mixed agricultural wastes by coupling anaerobic digestion and supercritical water gasification

Combining anaerobic digestion (AD) and thermochemical/hydrothermal processes is an interesting solution for the treatment and valorisation of agricultural waste. Most of the recent investigations focused on the solid digestate valorisation obtaining syngas and char, the latter for solutions different from land applications. However, there are few investigations based on hydrothermal processes aimed at enhancing the value of liquid digestate. In order to fill this gap, the study aims at verifying the technical feasibility of the combined treatment of AD and supercritical water gasification (SCWG). The experimentation was carried out at a full scale AD plant fed by 110 t/d of buffalo manure (81.8%) and maize silage (18.2%). Methodologically, the study first envisaged a bench-scale experimental phase concerning SCWG and then a theoretical phase aimed at quantifying the mass and energy balances with reference to the AD + SCWG and AD schemes. SCWG was examined using a plug flow reactor working at 550 °C and 250 bar for residence times of 20 min. For feed in the range of 4–14 mL/min, it was possible to produce a syngas with a higher heating value (22 MJ/kg organic content) and with a gas yield of about 13 mol/kg of dry-digestate. Carbon gasification and global gasification efficiency were of 35 and 45%, respectively. Results of the theoretical phase confirmed the energy sustainability of the AD + SCWG scheme although the added value was the production of green chemicals including syringaldehyde (7.6–11.6 kg/d), acetic acid (1.0–6.5 kg/d) and glucose (0.5–5.8 kg/d) with total estimated production in the range 10.2–33.5 kg/d.