Coal Pyrolysis in a Rotary Kiln: Part I. Model of the Pyrolysis of a Single Grain

A mathematical model is presented which describes the pyrolysis of a single grain of coal and is designed to be incorporated into an overall model simulating the rotary kiln coal pyrolysis process. The grain model takes into account the principal physical phenomena occurring during the conversion of coal to coke, namely, heat transfer toward and within the grain, drying of the coal, and the evolution of volatile species. Particular care has been taken in the determination of the thermophysical and kinetic parameters necessary for the model. Thus, the drying kinetics for Lorraine coal were measured by thermogravimetry. The kinetics of pyrolysis were determined by both thermogravimetry and gas-phase chromatography, in order to separately monitor the evolution of the nine gaseous species considered. The true specific heat and the thermal conductivity of the solid were also mesured as a function of temperature. The numerical model, based on the finite-volume method, calculates the temperature, composition, and mass flow rates for the different gases evolved at each point in the grain at any instant of time. The model was, finally, validated by comparing the calculated and measured values of the overall conversion of the pyrolysis reaction and the temperature at the center of the grain.