THE REACTIVITY OF COAL CHARS GASIFIED IN A CARBON-DIOXIDE ENVIRONMENT

The rates of gasification by CO2 have been determined for three laboratory produced chars derived from U.K. coals with varying ash contents. Data for the temperature range 900 to 1233K were obtained using an isothermal thermogravimetric technique. Reaction rates were determined in atmospheres of either CO2, O2 or CO2/O2 mixtures for which the CO2 partial pressure varied between 10 to 101 kPa. Pre-exponential factors and activation energies for the elementary reaction steps were evaluated using the Langmuir-Hinshelwood expression. High temperature data for the range from 1400 to 1800K were obtained by projecting the char particles through a flat methane-air flame. Particle temperatures in the flame were measured using a two-colour pyrometer system and the rates of char gasification at various CO2 partial pressures were calculated from energy balance equations and then used to provide limited information on the char/CO2 reaction rate above 1400K. The activation energy obtained for the transfer of oxygen to an active carbon site varied between 170 and 187 kJ/mole. The equilibrium constant for oxygen exchange between CO2 and CO on carbon was calculated from the rate constants and the results compared with data from other investigators. The intrinsic reaction rate per unit internal surface area R(i) = 0.5 exp (-28700/T) g cm-2 s-1 was obtained from evaluation over the temperature range 900-1800Y.