Kinetic rates of oxidation and gasification reactions of coal chars reacting in oxy-combustion environments

In this work, the kinetic rates of oxidation, and the oft-neglected CO₂ and steam gasification reactions for three pulverized coal chars were determined through experiments and analysis using a single-film model amenable for inclusion in CFD simulations of boilers. The three coals considered in this work were Black Thunder, Utah Skyline, and Illinois #6: one sub-bituminous and two high-volatile bituminous coals. Size-classified char particles were pre-generated at a high heating rate and then reacted in an entrained flow reactor with oxygen concentrations ranging from 24-60%, steam concentrations from 10-16%, and N₂ and CO₂ diluents. High oxygen concentrations were chosen to increase particle temperatures (from the exothermic oxidation reactions) and overcome the high activation energy barriers for the gasification reactions. Temperatures of individual particles were measured, and burnout measurements were made on collected chars. Best-fit activation energy and pre-exponential factors for the oxidation, CO₂ and steam gasification reactions were determined for each of the three chars. A transient single-film model demonstrates that the deduced kinetic rates effectively capture the experimentally observed trends, such as a higher temperature for the sub-bituminous char, the size dependence on peak char temperature, and the differences between CO₂ and N₂ diluent atmospheres.