DFT and experimental study on the heterogeneous decomposition and steam/CO2 reforming of gasification tar over bio-char

Abstract

Tar decomposition/reforming over char surface is a hot topic for biomass gasification. In this work, heterogeneous decomposition and steam/CO2 reforming of gasification tar over bio-char were studied using experiment and density functional theory (DFT) approaches. The detailed mechanism focusing on the intrinsic char-tar interaction at molecular level was taken into consideration. Experimental results show that bio-char largely enhanced the decomposition of toluene. Meanwhile, the removal efficiency declined with duration time as a result of carbon deposition. Steam/CO2 reforming improved toluene decomposition efficiency by maintaining the activity of bio-char. At 900 °C, the toluene conversion efficiency maintained nearly 100 % throughout the process. DFT calculation indicates that the energy barriers for hydrogen dissociation reactions of toluene over char surface is about 235.52 kJ/mole, which is much lower than thermal decomposition. Char-toluene interaction benefits the dehydrogenation reaction rather than demethylation·H2O/CO2 could quick combine with char surface to inhibit carbon deposition. The energy barriers for formation of active phenoxy structure during steam and CO2 reforming are 261.04 kJ/mole (phO-H breakage) and 19.1 kJ/mole (O…CO breakage) respectively, which indicates that CO2 reforming of toluene over bio-char is more conducive to elimination of carbon deposition. Orbital electron properties results show that when adsorbed on char surface, the band gap of chemisorption system is reduced to 3.2622 eV, which is much lower than toluene (6.7179 eV). The orbital electron interaction greatly reduces the stability of tar molecules, which is the essence for the catalysis and activation effect of bio-char for toluene dehydrogenation and decomposition.