RATE OF COMBUSTION OF SIZE-GRADED FRACTIONS OF CHAR FROM A LOW-RANK COAL BETWEEN 1200 DEGREES K AND 2000 DEGREES K

An apparatus to measure the reaction rate of coal char particles is described. The reaction rates of five size fractions of char have been measured to provide data for mathematical models of pulverized coal combustion. The gas temperature ranged from 1 200°K to 1 720°K giving a particle temperature ranging from 1 200°K to 2 000°K; two oxygen concentrations were employed, 5 per cent and 10 per cent; the particle sizee ranged from 28 μm to 105 μm. The measurements of burn-off have been analysed in terms of a diffusional reaction rate coefficient which depends on the rate of transport of oxygen to the esternal surface of the particle and a surface reaction rate coefficient which expresses the combined effects of chemical reactivity and internal diffusion. It has been found that the overall reaction rate of the two larger size fractions, 82 μm and 105 μm, reach the theoretical maximum limiting value for diffusion control, implying that the product transported away is carbon monoxide. For smaller particles the overall reaction rate is lower than this limit and from these measurements a surface reaction rate coefficient has been calculated. The surface reaction rate coefficient is found to be independent of oxygen concentration and particle size. A relation is given for the application of these results to calculations of the rate of combustion of pulverized coal in a flame. © 1969.