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 temp eratures and high pressures has been reviewed, the focus being the factors affecting the reactivity. A model has been developed to extrapolate this re activity data to high-temperature conditions. The intrinsic reactivity for various chars were predicted from the moderate temperature apparent reactiv ity data, assuming that reaction is in regime I, and the effects of pressur e, char type and temperature on intrinsic reactivity were obtained. The app arent char reactivity was predicted at high temperature by incorporating th e intrinsic reactivity with an effectiveness factor. It is shown that both apparent and intrinsic reaction rates at 1123 K increase continually with C O2 partial pressure. The char type has a more significant effect on the int rinsic reaction rate than the pressure. The char surface area is an importa nt factor in obtaining char apparent reaction rate. Gasification temperatur e has the greatest influence on reactivity. The large variation in predicte d intrinsic reaction rate for various chars at high temperatures is observe d 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 s ystem where the gasification temperature exceeds 1700 K, the char apparent gasification rate is limited by mass transfer in porous structure of char p articles. The predicted apparent reactivity showed a reasonable agreement w ith experimental measurements that were obtained under high-temperature low -pressure gasification conditions. The extrapolated high-temperature gasifi cation kinetics can be used in modeling the performance of an entrained flo w gasifier. (C) 2000 Elsevier Science Ltd. All rights reserved.