Reaction kinetics of char-O2/H2O combustion under high-temperature entrained flow conditions

The combustion kinetics of char-O-2/H2O reactions under high-temperature entrained flow conditions involving char-H2O and char-O-2 reactions were investigated experimentally and numerically. A numerical model was established based on the unsteady convection-diffusion equations, and the random pore model (RPM) was adopted locally to predict the change in intraparticle pore surface area. The Langmuir-Hinshelwood form rate equation was used to consider the possible competitive relationship between char-O-2 and char-H2O reactions, and the most probable intrinsic kinetics parameters were found by checking the correlation coefficient between the experimental and numerical results. For H2O gasification, the numerical results show that the enrichment of H-2 inside char particles decreases the intrinsic char-H2O gasification rate, especially at the outer layer regions. For O-2/H2O combustion, the experimental results show that H2O has a promotion effect on the total carbon conversion at low SRs (stoichiometric ratio of O-2 to fuel) but has an inhibition effect on the total carbon conversion at high SRs. Using the common active sites assumption, the numerical results match well with the experimental results. According to the numerical results, H2O penetrates deeper inside the char particle than does O-2, contributing more to the local carbon conversion at the inner layer region, while H2O inhibits the char-O-2 combustion rate at the outer layer regions due to H-2 formation. As a result, H2O pushes the kinetics-diffusion-controlled reaction towards the kinetics-controlled regime. As the SR increases, the char-O-2 combustion contributes more, magnifying the inhibition effects of H2O on the char-O-2 combustion rate; therefore, the total O-2/H2O combustion is gradually inhibited by H2O.