Comparative Combustion Studies of Ultrafine Coal/Water Slurries and Pulverized Coal

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Abstract
The effects of the physical properties of slurried and pulverised fuels (specifically particle size, density, and phase composition) on combustion are described in this paper, For solid fuels there is considerable flexibility for controlling the particle size, which in turn has a significant impact on ignition, char burnout, and emissions. In contrast, the chemical properties and reaction temperature range of a given fuel are narrowly constrained in most applications. In this work we use a laminar flow reactor to study the initial stages of combustion of coal/water slurries and pulverized coal. In conjunction with the flow reactor, a laser-based in situ particle counter has been used to determine the size distributions of pulverized coal and various slurry mixtures prior to and during combustion. Initial size distributions and subsequent evolution of these distributions vary markedly depending on the fuel preparation, emphasizing the importance of the physical properties of the fuel in determining subsequent combustion performance. Two numerical models are used in conjunction with the experimental results to provide a preliminary analysis of the effects of water content on evaporation and devolatilization, and also of the effects of particle swelling on char burnout times. The effects of fuel type on atomization, devolatilization, and subsequent char burnout are described in the paper. Our experimental results show that particle agglomeration during atomization increases the largest particle size of the slurry by a factor of 2-3 over that of the constituent pulverized coal. Particle swelling also increases the particle size by 50-100 percent. These results are used as initial conditions in the numerical model, and computed results show that agglomeration increases char burnout times by a factor of 4-8 over that of the constituent pulverized coal, while the effect of swelling is a reduction in char burnout times. Fine grinding of the constituent coal does not appear to provide a benefit in reducing reaction times unles the largest size of the atomized slurry is equivalent to the largest size of the ground coal. Our experimental results show a high number density of fine material in the constituent pulverized coal which does not appear during atomization of the slurried material. Interpretations of these results suggest that coal/water slurries will have significantly longer char burnout time than pulverized coal, and will exhibit markedly different ash deposition features due to the production of larger and fewer flyash particles.