Gasification kinetics during entrained flow gasification - Part I; Devolatilisation and char deactivation

The devolatilisation behaviour of solid fuels and the reactivity of the resulting char are important design parameters for entrained flow gasifiers. The volatile yield and devolatilisation rate of two coals are measured in a pressurised high temperature entrained flow reactor at up to 1600 °C and 4.0 MPa and in a wire mesh reactor. A pyrolysis model is derived that describes the influences of temperature, pressure, and residence time on devolatilisation. During the entrained flow pyrolysis experiments char samples are collected from the hot reaction zone and their reactivity and surface area are measured. A significant char deactivation is observed when the chars are exposed to a high temperature for a longer time during pyrolysis. This loss of reactivity is caused by a loss of specific surface area, but also by a decrease in intrinsic reactivity. Based on the experimental data set, a char deactivation model is derived that describes the loss of active sites by the continuous transformation of fresh reaction sites to deactivated sites. The experimental data and the models indicate that the devolatilisation rate is significantly faster than the char deactivation rate. Therefore char deactivation also occurs during the heterogeneous char reactions and should be considered in a char conversion model for entrained flow gasification.