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Show of the actual burning rate devided by the m a x i m u m burning rate possible under diffusion-controlled combustion. The m a x i m u m burning rate (represented by x = 1) 1S calculated from the 0 2 mass transfer equation assuming that the surface 0 2 concentration is zero. Chemical reaction dominates, rather than 0 2 diffusion through the boundary layer, when X is less than about 0.2. Our calculations show that x = 0.12-0.23 for 10-20 /zm particles under the conditions of this experiment, while \ = 0.36-0.42 for 40-60 /im particles. As particles swell above 60 /jm, the x factor increases, indicating that the char oxidation rate becomes more limited by the diffusion of the 0 2 to the particle surface and that chemical rates become less important. DISCUSSION This section is an overview of the influence of particle size distribution on devolatilization, ignition, flyash formation, deposition, and emission control. M a n y of the ideas presented here stem from our previous particle size measurements (Holve at al., 1985). Size Distribution Effects on Devolatilization For pulverized coal and coal/water slurries, carbon conversion is dependent on the extent of particle swelling, which affects particle size (and hence extent of diffusion control) as well as apparent density and porosity. Our calculations show that burnout times are strongly dependent on the extent of swelling for particles larger than 30 /zm, and that swelling increases char oxidation rates. Similar observations on the effects of swelling for fluidized bed combustion have been reported previously by Beer et al. (1980). It is particularly interesting to note that 100% swelling in the slurry gives the same total burnout time as for the finer constituent pulverized coal. Thus, sufficient swelling can compensate for a larger initial size distribution. Although the water evaporation and devolatilization stages are a small part of the total reaction time, the extent of devolatilization and particle swelling are sensitive to the particle heating rates, which in turn are a function of particle size. The brief devolatilization stage determines many of the physical properties and thus has a direct influence on the total reaction time of the resultant char. This observation shows that it is important to relate the properties of the char with the particle size and devolatilization history in order to properly characterize the char reactivity of a given coal. Size Distribution Effects on Ignition In previous work we have noted the increased ignition delay with increasing particle size, (Holve and Meyer, 1984). In experiments with near monosized particles, which have been obtained by aerodynamic classification, w e noted an ignition delay of 7-10 m s for the largest particles of 70-100 /xm. In contrast, for polydisperse coals, w e see immediate luminosity as the coal traverses the hot methane-air flame front, which w e attribute to ignition of the micron-sized carbon. Because the size distribution is continuous, the luminous zone continues until the largest particles have devolatilized. These observations suggest the importance of the continuous size distribution in initiating the devolatilization-ignition process for a self-stabilized flame. Consistent with these experimental observations, previous calculations (Holve et al., 1985) have shown a devolatilization time of less than 1 m s for a 15 /xm particle in 3 % 0 2 and 1700 K, 3 m s for a 40 //m particle and 5.5 m s for a 57 /jm particle. W e also showed that the residual char fraction increased with increasing particle size, which would have 5 |