Catalytic Mechanism of K and Ca on the Volatile–Biochar Interaction for Rapid Pyrolysis of Biomass: Experimental and Simulation Studies

A good understanding of the catalytic mechanism of alkali and alkaline earth metal species during pyrolysis is required for the development of biomass utilization, to take advantage of its special thermochemical properties. The mechanism of the volatile–biochar interaction, especially between H radical/CO2/H2O and K/Ca in biochar, was systematically investigated by combining experimental analyses and density functional theory calculation. The results indicate that, at 500–900 °C, the yield of biochar from rice husk pyrolysis is basically constant, which mainly shows the mutual conversion between gas and liquid products. With the increase of loading alkali and alkaline earth metal species (AAEMs), the tar yield gradually decreased, and with it reaching a certain content, a significant catalytic effect of AAEMs can be observed. K is more beneficial to maintain the stability of the biochar structure than Ca. The intensities of C═O and C–O–C functional groups would appear with a certain increase with the increasing concentration of bond-linking AAEMs. At 700–900 °C, the precipitation ratio of K is almost twice that of Ca and even more. The presence of AAEMs can largely inhibit the formation of three-ring tar components, and more converted to one-ring tar components. The H radicals in the volatile can be bonded to C in the C–O–K structure, thereby causing the K element to decouple with the functional group, and tend to be adsorbed on the carbon matrix. For the Ca-loaded structure, it is manifested in strong adsorption of H radicals by Ca, resulting in the weakening of C–O bonds and the looseness of the biochar structure. The interaction between K/Ca in biochar and H2O/CO2 is reflected in the mutual attraction of AAEMs and the O atom in the H2O/CO2 molecule.