The effects of process conditions on the char reactivity: The experimental studies and the mathematical modeling
Doctor of Philosophy
The internal pore structure of chars derived from many bituminous and sub-bituminous coals can be best described by bi-modal pore size distribution. Their macropores, which account for nearly all the porosity, are interconnected spherical cavities with diameters as large 100 or 200 microns for 500--750 micron particles. Most of the internal surface area of these chars, however, is associated with micropores that are usually smaller than 30 A. In order to elucidate the effect of process conditions on the internal pore structure of coal-derived char particles and ultimately their overall reactivities, staged and sequential combustion algorithms are designed using thermogravimetric reactor equipped with in-situ video microscopy imaging (TGA/VMI). Our experiments showed that coal-derived char particles treated with higher pyrolysis heating rates have more open macropore structure than those treated with lower heating rates. At low combustion temperature where the reaction was kinetic controlled, the impact of the macropore structure on the overall reactivity of char particles was insignificant since all the internal surface area associated with micropores were accessible to the oxygen. At higher combustion temperature, however, the macropore structure of the char particle had a strong effect on its overall reactivity. Chars with more open macropore structure and larger macropore surface area exhibited higher reactivity. Also, combustion experiments showed that more open macropore structure and higher macropore surface area favored particle ignition, a transient phenomenon characterized by luminous flame engulfing the particle, which caused sharp increase of the overall reactivity of char particles. The development of the generalized grain model helped us to describe mathematically the mass and energy transfer during the char combustion process. The macropore structure was modeled as a network of interconnected spherical cavities. The walls separating these cavities consist of homogeneous assemblies of microporous grains made of carbonaceous material. The numerical analysis revealed that in the regime of diffusional limitation, a more open macropore structure could reduce the diffusional resistance inside the macropores. Therefore, it enhanced the accessibility of the micropores surface area to the oxygen and ultimately led to higher overall reactivity of the char particles.