On the products of the heterogeneous oxidation reaction at the surfaces of burning coal char particles

Abstract

The combustion of the char particles of two bituminous coals were investigated in a laminar flow reactor having gaseous environments containing 6 and 12 mole-% oxygen in the temperature range 1550 to 1700 K. Measurements of the size, temperature and velocity of individual particles were made at selected heights above the reactor inlet and hence, at different extents of burnoff. Samples of partially reacted chars, extracted from the reactor at these same heights, were analyzed in order to determine the fraction of the initial mass remaining, m/mo, and the char density. The data were used in the governing mass, momentum and energy balance equations in order to determine overall particle burning rates per unit external surface area. The burning rates determined indicate that account must be made of CO2 formation in the vicinity of the particle. Whether the CO2 is formed heterogeneously at the particle surface, or homogeneously in the particle's boundary layer cannot be established definitively. It is shown, however, that the single-film model of a burning carbon sphere with a fraction ψ of the carbon being converted to CO2 at the particle, surface adequately describes the burning behavior of coal char particles in the 75 to 125 μm size-range. Employing such a model, for the char particles of the bituminous coals investigated, as much as 15% of the carbon content of the particle can be converted to CO2 at temperatures in the range 1600 to 1700 K. At temperatures above 1800 K. CO is essentially the sole heterogeneous reaction product.