Conjugate heat transfer analysis in a radiatively/conductively-stabilized pulverized coal combustor

Pulverized coal combustion in a vertical, cylindrical, Radiatively/Conductively-Stabilized Combustor (RCSC) was successfully modeled by a comprehensive two-dimensional conjugate heat transfer model of the combustor interior and walls, combining radiation, convection, and conduction. The radiation intensity distribution in the combustor was computed from the Radiative Transfer Equation (RTE) by using the spherical harmonics method. The medium inside the RCSC is a mixture of gas and coal-derived particulates such as pulverized coal, char, ash, and soot, and participates in radiative heat transfer. The spectral absorption coefficient for the gas components was obtained by using the exponential wide-band model. Simplified approximations were used to compute the spectral extinction and absorption coefficients of the particles. The overall radiative properties of the gas-particles mixture was then found by adding the gas and particle contributions. Despite the great advance in computer and algorithm, the computation of radiative properties of the particles from the Mie equations still requires a significant computational effort. It is so especially when rigorous solution is used in models where these coefficients vary in space and time. A number of approximate expressions for calculating the coefficients for each of the particle types were examined in a wide range of size parameters. The results were compared with those obtained from the Mie equations which was computed by using Mathematica$\rm\sp{TM}$. A set of approximations, which typically produce errors of less than 8.9% and 5.1% in the spectral extinction and scattering coefficients, respectively, was selected and used in the modeling. The combined error introduced by the selected approximations was found to generate an error of less than 1.45% in the gas temperature and radiation intensity distribution. The coal combustion model was solved numerically, and the major findings are as follows. From the entrance of the combustor to the flame zone, the wall is heated by radiation, and transfers heat to the gas-particle mixture by convection. The direction of these heat flux components is reversed from the flame zone downstream. Ignition starts near the wall, and the shape of the flame front is convex in the flow direction. The flame zone is thin, resulting in coal residence times much smaller than those in commercial combustors using recirculation for flame stabilization, having thus the promise of significant reduction in NO$\rm\sb{x}$ production. In this thin-walled combustor and in the range of parameters considered, the thermal conductivity of the wall had insignificant effect on wall heat fluxes and flame position. Increasing, however, the wall emissivity and especially wall thickness were found to have a significant effect on wall and gas preheat and thus also on flame stabilization, resulting in an upstream-movement of the flame front. To validate the theoretical results, combustion experiments were conducted by burning pulverized coal in the RCSC, and the temperature distribution was measured in the combustor tube wall. The predicted temperature distribution was in good agreement with the measured data in the pre-flame region. The model also predicted the location of the flame front well. In the flame region, however, the model gave somewhat lower temperatures than those measured in the experiments, due to the significant conductive heat transfer through a slag layer which was observed to develop in the flame region