Experimental and modeling studies on char combustion under pressurized O2/H2O conditions

Abstract

The desorption kinetic parameters for pressurized combustion and gasification reactions were determined based on a C++ program coupled with the Langmuir-Hinshelwood (L-H) kinetic model developed and experimental data from pressurized char combustion, and the established L-H kinetic model for pressurized char-O₂/H₂O combustion was refined in the current paper. The activation energy for desorption in reactions involving pressurized char-O₂ and char-H₂O was determined to be 250.8 kJ/mol, accompanied by a pre-exponential factor of 5.42 × 10¹⁰ g/(m² s). Using this foundation, the current research conducted simulations to investigate the impacts of temperature, pressure, and H₂O concentration on the oxidation adsorption rate (R_ads,oxi), desorption rate (R_des), gasification adsorption rate (R_ads,gas), and the competitive influences of kinetics and diffusion processes within the pressurized char-O₂/H₂O combustion. The simulation results indicate a gradual increase in R_des and R_ads,gas with char conversion to reach a peak, followed by a gradual decline. Conversely, the R_ads,oxi varies smoothly throughout the char conversion process. At 1673 K/1.0 MPa, the char-O₂/H₂O reaction rate is primarily constrained by R_ads,oxi and R_ads,gas, with the adsorption reaction serving as the rate-controlling step. Moreover, it was noted that a rise in pressure resulted in a linear increase in R_ads,oxi, R_des, and R_ads,gas. At elevated temperatures, the impact of pressure on them becomes more noticeable. However, the introduction of H₂O mitigates this effect. Elevated temperature and pressure facilitate the competition on the kinetics of char-O₂ combustion for O₂ diffusion, resulting in the conversion of char being more susceptible to O₂ diffusion rate limitation. With the addition of 20 % H₂O, the competition effect was weakened. In the case of pressurized combustion involving char and O₂/H₂O, the char conversion is primarily constrained by the O₂ diffusion rate and is scarcely influenced by the H₂O diffusion rate.