Kinetic Characteristics of Coal Char Combustion in Oxygen-Enriched Environment

Oxygen-enriched CFB combustion technology has the combined advantage of producing high CO₂ concentration flue gas which is suitable for direct compression and sequestration and allowing excellent fuel flexibility. Due to the nature of oxygen-enriched combustion, the combustion characteristic of fuel in CFB boilers would be significantly different from those in the conventional CFB systems. To accurately predict scale-up performance through CFD modeling, accurate rate expressions are needed to describe coal char combustion under this unconventional combustion condition. In the paper, the kinetic characteristic of coal char combustion in oxygen-enriched environment was studied using thermogravimetric method based on the requirement of CFB reactor with Huangling bituminous coal as samples. The mixture of O₂ and CO₂ were taken as the reacting gases to simulate flue gas recirculation, and a broad range of in-furnace conditions, i.e. four temperatures of 973, 1073, 1173, 1273K and five oxygen concentrations of 20, 40, 60, 80, 100 vol.%. were investigated. It was found that the combustion duration decreased with the increase of oxygen concentration and temperature. For further calculations, simplified Shrinking-Core Model was introduced. The apparent reaction rate increased when partial pressure of oxygen was raised. Finally, the fundamental kinetic parameters, i.e. reaction order, activation energy and pre-exponential factor, were established. Activation energy of 19.7 kcal/mol point to pore-diffusion controlling regime of coal char combustion under conditions mentioned above.