Pressurized Gasification of Beech Wood Char in Steam and Steam–CO ₂ Mixtures: Kinetics Considering Steam–CO ₂ Interaction

High-quality, tar-free syngas can be produced from lignocellulosic and low-grade biogenic feedstocks via pressurized entrained flow gasification. However, the gasification behavior of biomass char in typical atmospheres of mixtures of H2O and CO2 at partial pressures of about 10 bar each has not been investigated so far. The work at hand therefore presents experimental and kinetic modeling results of beech wood char gasification in H2O at partial pressures of up to 12.5 bar and in mixtures of H2O and CO2 at various partial pressure combinations at a total pressure of 20 bar. Two char qualities are investigated that vary with respect to their micropore surface area, degree of graphitization, and dispersion of calcium. The gasification kinetics in steam are described with a power law and a Langmuir–Hinshelwood model. In a H2O atmosphere, the power law model provides good fitting accuracy. Together with CO2 kinetics published in an earlier work, the theoretical expressions for additive and competitive behavior are formulated. While the additive mechanism adds the H2O and CO2 kinetics, the competitive mechanism combines the rate constants of the Langmuir–Hinshelwood kinetics in H2O and CO2 atmospheres in one expression. The theoretical approaches are compared to the experimentally derived kinetics in a mixture of H2O and CO2. For both chars, a transition from additive to competitive behavior is observed with increasing partial pressures. Experiments indicate that especially the degree of graphitization and therefore the severity of heat treatment during pyrolysis strongly impact the transition from additive to competitive behavior of H2O and CO2. With higher degrees of graphitization, the transition occurs at lower H2O and CO2 partial pressures. Interestingly, synergistic behavior can be observed in a small range of partial pressures and explained by the positive effect of steam on the dispersion of calcium, which, in turn, catalyzes the Boudouard reaction.