Evaluation of char reactivity data by different shrinking-core models

Carbon reactivity was measured in two entrained flow reactors, two drop tube furnaces and one fluidized bed. The results were interpreted by using three shrinking-core models. A number of minor differences between the models were identified and the resulting deviations could mostly be ascribed to non-measurable parameters such as particle specific heat capacity as a function of temperature, boundary layer gas composition and Nusselt and Sherwood numbers, together with bulk flow equations and numerical procedure. Using all experimental data, the calculations gave different overall activation energies ranging from 107 to 130 kJ mol^-1. The differences between the model calculations with respect to overall kinetics were small but significant under realistic conditions for combustion and for the experiments performed. A comparison between individual experiments showed that for experiments performed with small coal particles, long residence times or preheated particles, good agreement between model results was obtained. This was not so for larger particles, short residence time and no preheating, since data taken under transient conditions impair the results. Guidelines are given for experimental design for making reactivity measurements on solid fuels in experimental reactors.