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Show INTRODUCTION It has recently been recognized that fly ash from coal combustion in utility boilers has a bimodal distribution (1-7). The larger particles are produced by fusion and coagulation of the mineral inclusions in coal; the smaller particles, in the submicron range, are produced by the vaporization and condensation of the mineral constituents. Several field and laboratory studies have been conducted to determine the factors that govern the physical and chemical state of submicron particles (4,5,8,9). These studies will be reviewed during the presentation. In this paper we will show, drawing partly on results in previous studies (3,4,10), that a detailed knowledge of the temperature history of coal particles during combustion is needed for the purpose of understanding ash vaporization. The combustion of pulverized coal is usually divided into four stages: preheating, devolatilization, ignition and volatiles combustion, and char burnout. When coal is first injected into a hot furnace it begins to decompose, giving off gaseous products composed of CO2, H2O, CO, light hydrocarbons and tars which burn in the host atmosphere. When the volatile content of the coal is exhausted, the remaining char will then ignite and burn to extinction. This final stage is usually longer in duration than the previous stages. Depending on the relative rates of oxygen diffusion and volatile evolution the volatiles may burn at the surface of a coal particle or in a diffusion flame displaced from the surface. For small particles and high oxygen partial pressures the rate of oxygen diffusion might be expected to be high enough to burn the volatiles at the particle surface. Heterogeneous combustion of the volatiles. Conversely for larger particles or smaller oxygen partial pressures the rate of the volatile evolution might be high enough to prevent oxygen form reaching the particle surface. The volatiles would then burn in a flame displaced from the particle surface. Such a flame would influence the temperature of the particle by feeding energy back to the surface. The rate of the energy feedback will be controlled by the distance of the flame front to the particle surface. The mode of volatile combustion will influence ash vaporization by its impact on particle temperature. If the volatile evolution rate is fast and 3-2 |