Kinetics of Coal Char Gasification with Fe-Based Oxygen Carriers under Pressured Conditions

Under high-pressure conditions, coal chemical looping gasification (CLG) is a potential technology for industrial applications based on its ability to provide clean and efficient conversion of coal to produce syngas. Within the range of 0.1–1.2 MPa, simulations and experiments were carried out as a validation methodology to study the gasification of coal chars with H₂O in the presence of Fe₂O₃/Al₂O₃ as the oxygen carrier (OC) in a fixed-bed reactor. The characteristics of pressurized CLG and the crystalline phase of the reduced OC were investigated by combining the mass transfer model with intrinsic kinetic experiments. The results showed that H₂O replenishment and H₂ removal on the particle surface of coal char by causing the Fe-based OC to undergo reduction, within the mass transfer coefficient Kₘaₘ of 0.0015–0.0035 m³ mol–¹ min–¹, minimized the concentration gradient between the particle surface and the bulk caused by external diffusion as well as the H₂ inhibition effect on the surface of coal char particles. The maximum gasification rate of CLG was 2.14 times greater than the gasification rate without OC, and the influence of the coal type on the gasification rate decreased with increasing pressure. The reduced state of the Fe-based OC was Fe₃O₄ at atmospheric pressure, but FeAl₂O₄ appeared when the total pressure was greater than 0.3 MPa, while the average rate of oxygen release slowed significantly, and the maximum oxygen release increased. The deep reduction process of Fe₂O₃/Al₂O₃ to Fe–Al spinel could significantly enhance the gasification rate of coal char. In addition, an overall kinetic model was established that could describe the variations in the gasification rate with the temperature, pressure, carbon conversion, and lattice oxygen release, including the influence of external diffusion. The simulations reveal the pressured reaction mechanisms of the enhancement of char gasification during CLG with the kinetic model.