Evaluation of the potential performance of hyphenated pyrolysis-anaerobic digestion (Py-AD) process for carbon negative fuels from woody biomass

A novel hyphenated Pyrolysis-Anaerobic Digestion prototype (Py-AD) was tested in order to evaluate the potential of hybrid thermochemical biological process to produce methane from woody biomass. An auger intermediate pyrolyzer was directly coupled to two biological reactors optimized for the digestion of residual condensable compounds and gas produced by pyrolysis of softwood. The Py-AD was monitored for 16 months and a detailed chemical analysis of the main fractions, gas (pyrobiogas), biochar, aqueous phase and pyrolytic lignin was performed under regime conditions. The results from Py-AD and those from experiments with bench-scale pyrolysis and fermentation reactors analysis provided information on the overall performance of the Py-AD and mass and energy balance based on chemical oxygen demand. Py-AD allowed to obtain, with acceptable volumetric productivity, a pyrobiogas with a composition approaching that of biogas (47 %v/v CH4 and 45 %v/v CO2). Pyrobiogas yield was about half of the theoretical value calculated from gas and liquid fractions. A preliminary technical evaluation of the process confirmed the feasibility of Py-AD and its value to produce carbon negative fuels with simple equipment and low waste generation. Important key constraints of the process were also evidenced in the study.