The combustion of chars from pulverized bituminous coals was experimentally and theoretically investigated. The chars were made by pyrolyzing size-graded PSOC 1451 coal particles in nitrogen at temperatures of 1000-1600K. Sized char particles were then used for further experiments. Low temperature reactivities of such cenospheric chars were measured at 800K on a thermogravimetric analyzer. 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% oxygen ambients at wall temperatures of 1000-1500K in a drop tube laminar flow furnace. Particle temperatures were measured during the entire course of combustion. From the complete temperature-time histories of such burning particles, the apparent activation energy and pre-exponential factors were inferred, using numerical models and statistical modelling techniques. Questions of particle-particle variability were addressed. The ignition transients of single burning particles were studied and a model that predicted delay times observed experimentally was developed. Char samples were also partially oxidized at temperatures in the range 1200-1500K (particle temperatures) and physically characterized. Methods of characterization included optical and electron microscopy, gas adsorption methods for specific surface area and pore volume distributions, and mercury porosimetry for pore volume distribution measurements. The results of these characterizations were compared with those done on chars oxidized at 500°C.
The combustion of single char particles was numerically modelled. A continuum model for asymptotic shrinking-core combustion was developed using apparent reaction rates and temperature-dependent properties. Simplified assumptions were made regarding the gas-phase combustion. Parametric sensitivity of this model yielded significant insight into the combustion process. A more general continuum model was then developed. This model treated the internal pore structure 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 experiments was found. The model was extended to include the effects of some nonlinear kinetic reaction rate expressions. A discrete model for a cenospheric particle was also developed. This model consists of spherical voids randomly placed in a spherical particle. It simulates the combustion by taking into account the connectivity of the internal pore structure. 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.
