Evaluation on energetic and economic benefits of the coupling anaerobic digestion and gasification from agricultural wastes

Abstract

The sustainability of the anaerobic digestion (AD) plant was closely related to the disposal of digestate and enhancement of energy recovery. Aiming to promote this, coupling AD and digestate gasification (AD-gasification) was proposed and evaluated for a large-scale AD plant. Firstly, characterization of the solid digestate was measured. Secondly, air gasification of the solid digestate was performed. Finally, two scenarios aiming at enhancing energy recovery as electricity or gas were investigated.Superior C content (50.14 wt%), H/C (1.57) and O/C (0.62) molar ratios and lower heating value (LHV) of solid digestate (18.01 MJ/kg) made it propitious to gasification. Air gasification of digestate at 800 °C with ER = 0.26 resulted in the optimal LHV of producer gas (6.35 MJ/Nm3) and cold gas efficiency (CGE, 66.99%). In the biomass-to-electricity scenario, the excess of heat from the CHP system in the AD plant was sufficient to dry the solid digestate. Compared with the stand-alone AD plant, the total electricity from the coupling system increased about 11%. In the biomass-to-gas scenario, the gas income increased by 14% when the initial solid digestate was naturally air-dried to a water content of 50%. This research would provide technical guidance for the sustainability of the AD industry.