Catalytic effect of metal salts on deoxygenation and aromatization reaction during pressurized pyrolysis of corncob waste at mild temperatures

Converting biomass to high quality bio-char with high carbon solidification at low energy consumption can greatly promote the application of biomass to alternative coal. Metal salts have been confirmed to enhance carbonization and dehydration during traditional pyrolysis and hydrothermal conversion, whereas the catalytic deoxygenation and aromatization mechanisms during dry mild pyrolysis in the presence of pressure are currently unclear. A mild gas-pressurized pyrolysis assisted by metal salts catalysts at 200–300 °C was proposed in this work. The effects of catalysts on three-phase products as well as synergistic catalytic reaction mechanism were studied. Results indicated that CaCl2, ZnCl2 promoted carbon fixation rates by 19.77 % and 26.89 %, and ZnCl2, KCl could remove oxygen content by 11.45 % and 6.56 % (wt.%, dry ash free basis). ZnCl2 exhibited the best deoxygenation and aromatization effect. With ZnCl2 catalysis at 280 °C, the volatile content reduced to 37.25 %, and O/C, H/C values decreased by 19.05 % and 11.25 %, achieving coking coal-like quality. FTIR and NMR results implied that oxygen-containing functional groups were removed even more rapidly under the catalysis of ZnCl2, facilitating the rearrangement of O-alkyl C and aliphatic C into aromatic C, exhibiting a higher aromatization degree in semi-char. Moreover, CH4 and H2 increased in gas products catalyzed by ZnCl2, while C2H4 and C2H6 decreased. Pressure inhibited the escape of small molecule gas volatiles, making them easily adsorbed on surfaces of molecular structures for secondary reactions. Under the action of the [ZnOZn]2+ acid sites, alkanes underwent dehydrogenation activation to yield H2, and demethylation activation to generate CH4, followed by cyclization reaction. The upgrading promotions of biomass were caused by the synergistic effect of pressure and ZnCl2 catalysis.