A mechanistic model of Coal-Water Fuel Combustion

Abstract

The various stages of combustion of coal-water fuels (CWF) (formulated from bituminous coals) such as drying, pyrolysis and char burning were studied in an experimental-theoretical investigation. Measurements were made in a 1.5 MW CWF turbulent diffusion flame to determine the spatial distributions of gas velocity, temperature, and gaseous species concentrations. Particulate samples, collected at several axial stations in the flame, were analyzed for their size distribution, density, and mass fractions of the residual volatile and ash. This formed the basis for the development of a mechanistic model to describe the transformations of a CWF droplet in the flame. A mathematical model which quantitatively predicts the changing properties of the CWF droplet was used to compute the progress of combustion of three particle size ranges, for the measured temperature and oxygen concentration distribution in the experimental flame. The model follows the regression of a drying interface in the droplet, and describes quantitatively the heating of the already dry outer shell while the interstitial water from the core is still evaporating. For calculations of particle heatup and pyrolysis, the particle specific heat is varied with temperature in the range 1200 to 2500 J /(kg K). The initial weight of the sampled particles is back-calculated from their physical and chemical analyses. Model predictions are compared with the progress of combustion of flame samples (adjusted for the varying initial weight) of the same size range in two regions. Close to the burner, where CWF particles are mainly evaporating and devolatilizing, attention is focussed on the smaller particle sizes (53–75 μm, 75–90 μm and 106–125 μm) which constitute the bulk of the sample in that zone. In the tail end region of the flame, char burnout of the larger particle sizes (150–180 μm, 180–212 μm and 212–250 μm) is considered.