Mathematical modeling of coal char reactivity with CO2 at high pressures and temperatures

Literature data on the gasification of coal chars with CO2 at moderate temperatures and high pressures has been reviewed, the focus being the factors affecting the reactivity. A model has been developed to extrapolate this reactivity data to high-temperature conditions. The intrinsic reactivity for various chars were predicted from the moderate temperature apparent reactivity data, assuming that reaction is in regime I, and the effects of pressure, char type and temperature on intrinsic reactivity were obtained. The apparent char reactivity was predicted at high temperature by incorporating the intrinsic reactivity with an effectiveness factor. It is shown that both apparent and intrinsic reaction rates at 1123 K increase continually with CO2 partial pressure. The char type has a more significant effect on the intrinsic reaction rate than the pressure. The char surface area is an important factor in obtaining char apparent reaction rate. Gasification temperature has the greatest influence on reactivity. The large variation in predicted intrinsic reaction rate for various chars at high temperatures is observed due to different activation energies. The activation energy for char-CO2 gasification generally decreases as coal char rank decreases. For char-CO2 gasification of 100 mu m particles in an entrained flow coal gasification system where the gasification temperature exceeds 1700 K, the char apparent gasification rate is limited by mass transfer in porous structure of char particles. The predicted apparent reactivity showed a reasonable agreement with experimental measurements that were obtained under high-temperature low-pressure gasification conditions. The extrapolated high-temperature gasification kinetics can be used in modeling the performance of an entrained flow gasifier. (C) 2000 Elsevier Science Ltd. All rights reserved.