(27f) The Chemical Reaction - Pore Diffusion Model for Coal Char Gasification and Combustion Using the Random Pore Model

The random pore model with pore diffusion incorporating the properties of the char (porosity, ash content and structural properties) with equations accounting for the carbon removal were successfully solved and validated against experimental results. The char examined was derived from an unusual coal discard rich in ash and inertinites with a very dense structure. It was found for char particles with a mean diameter of 1mm, that the gasification reaction with carbon dioxide was controlled by the intrinsic chemical reaction for temperatures up to 900 C, whereas combustion with oxygen-nitrogen mixtures is controlled by the intrinsic kinetics and the pore diffusion for temperatures up to 600 C. The chemical reaction-pore diffusion model is characterised by a variable Thiele modulus which accounts for pore diffusion and undergoes a transition to a chemically controlled reaction model as a result of the depletion of carbon in the carbon/mineral particles. Intrinsic reaction rate parameters for gasification and combustion are reported and initial pore diffusion coefficients were found to compare very well with predictions with the Knudsen diffusion model which confirmed the presence of mainly micro-pores in the initial char structure.