The prediction of coal char reactivity under combustion conditions

Abstract

The main objective of the current work was to develop a char reactivity submodel that could be used in a comprehensive code for entrained coal combustors and gasifiers. The requirement of a char reactivity model is to predict the reactivity of chars from a wide range of coals, over a wide range of temperatures, and for various degrees of burnoff. In order to predict intrinsic reactivity, correlations of reactivity with char hydrogen content, coal oxygen content and coal mineral content were used. A random pore model (high rank coals) and a volumetric model (low rank coals) were included to predict variations of intrinsic reactivity with burnoff. The correlations combined with those models gave good predictions of reactivity (within a factor of 2 to 4) and reactivity variations with burnoff (within 20%) for the range of chars studied. In the pore diffusion regime, the model uses the Thiele modulus to calculate the reaction rate as a function of the intrinsic rate (obtained using the correlations) and char properties such as porosity, tortuosity and mean pore radius. Predictions of reactivity required an estimate of the tortuosity and the mean pore radius. The tortuosity was kept constant at a value of 2. Using a value of 6(corresponding to the size of micropores) for the mean pore radius led to good predictions of the onset of diffusion limitations for low heating rate, fluid chars. For high heating rate chars, pore size distribution measurements showed that values of approximately 100were more appropriate. The corresponding predictions using this value gave a fairly good fit of the literature data investigated. This analysis shows that it may be possible to extend a low temperature reactivity model to high temperatures.