| The combustion of pulverized bituminous coal chars was experimentally and theoretically investigated. The chars were made by pyrolyzing size-graded PSOC 1451 coal particles in nitrogen at 1000-1600K. Sized char particles were then used in subsequent experiments. Low temperature reactivities of such cenospheric chars were measured at 800K in a TGA. The effects of initial coal size, char size, pyrolysis temperature, and oxygen concentration were investigated. Single particle combustion experiments were done in both air and 50 percent oxygen ambients at 1000-1500K wall temperatures in a drop-tube laminar-flow reactor. Particle temperatures were measured throughout combustion by a two-color optical pyrometer. From such temperature-time histories, the apparent activation energy and pre-exponential factors were inferred using numerical models. Questions of particle-particle variability were addressed. The ignition transients of single burning particles were explained using a simple thermal model. Char samples were also partially oxidized at 1200-1500K and then physically characterized using optical and electron microscopy, gas adsorption methods, and mercury porosimetry. Results of characterization were compared to those done at 800K.;Single particle combustion was numerically modeled. At first, a continuum model for asymptotic shrinking-core combustion was developed using apparent reaction rates and temperature-dependent properties. Later, a more general continuum model was developed that treated the internal morphology of the particles more realistically, as inferred from experiments. The steady-state diffusion equation was solved inside the particle to determine its theoretical temperature-time history. Good agreement with experimental results was found. The model was extended to include the effect of nonlinear kinetics. A discrete model for a cenospheric char particle was also developed, in which spherical voids were randomly placed in a spherical particle. Connectivity of the internal pore structure was accounted for. This connectivity influences the access of reactant to the interior of the particle and, therefore, the extent of internal reaction. The changes in the internal connectivity led to a percolation type behavior in most particles. |
