SELECTED OBSERVATIONS ON THE COMBUSTION OF CHARCOAL BRIQUETS UTILIZED FOR THE OUTDOOR PREPARATION OF FOOD. VII. ANALYSES OF THE SHRINKING CORE MODEL IN THE COMBUSTION PROCESS

摘要

This work represents a continuation of our investigation of charcoal briquet combustion, the first six parts of which were reported upon at the last two International Pyrotechnics Seminars held in the United States. A Weber-brand covered cooking kettle served as an assumed semi-batch reactor in which the chemical reaction was a batch of "Kingsford" charcoal briquets combusting in a stream of fresh air entering through inlet ports on the bottom of the kettle. We observed that a grey ash layer covers the surface of the briquets after they have become fully ignited, and at the termination of the combustion there is a significant amount of ash residue remaining. The glowing surface of the core gradually moves inward and the ash clings to the briquets in such a way that the original size remains rather constant. If not disturbed, the shape of the ash residue resembles that of the original briquet. This manuscript thus represents the seventh part of our report, and examines the applicability of a "Shrinking Core Model" to the combustion process. We consider two possibilities, viz., (1) that diffusion of oxygen through the ash layer is the controlling resistance, and (2) that chemical reaction at the surface of the core is controlling resistance. For the case of oxygen diffusion, we plotted the data points representing the Diffusional Conversion Function versus time for six individual experiments. The slope of such a curve is related to the diffusion coefficient D_e. We observed that in each test the slope increased as time progressed, which indicated that the diffusion coefficient was increasing. But data represented in previous parts of our investigation showed that the temperature in the kettle decreased with time. This meant that the diffusion coefficient should have been decreasing because data in the literature reveal that diffusivities decrease with temperature (in an Arrhenius-type relationship). Thus, this contradiction resulted in our conclusion that the mathematical analysis did not support a situation wherein diffusion of oxygen would be the controlling resistance in the charcoal briquet combustion. For the surface chemical reaction case, the slope of the Surface Conversion Function versus time is proportional to the First Order rate constant k_s. Here, plots of this conversion function for the same six experiments as above yielded very smooth linear regression correlations. The average value of the First Order rate constant was 0.1387 m/min. These results supported the conclusion that a First Order chemical reaction at the surface of the core was the controlling resistance in the charcoal briquet combustion. This manuscript concludes with a recommendation for the direct measurement of the combustion temperature of the briquets, in order to provide more realistic kinetics correlations and to further clarify the mechanism of the reaction.