Ignition of coal and char particles: Effects of pore structure and process conditions

This study reports the results from experimental and theoretical studies aimed at elucidating the effects of particle pore structure and process conditions on the ignition of coal and char particles.; A novel reactor combining thermogravimetric analysis and video microscopy imaging (TGA/VMI) was used for our combustion studies. By allowing simultaneous observation of light emissions from igniting particles and measurement of sample reactivity from weight-loss data, the TGA/VMI reactor was very effective in detecting and characterizing particle ignitions.; Investigations of ignition mechanism showed that the ignition of char particles typically occurs heterogeneously, while coal particles may ignite heterogeneously, homogeneously, or by a combination of both mechanisms. Homogeneous ignitions were favored by high oxygen concentrations and close particle interactions. Other transient phenomena such as multiple ignitions of a single particle were also observed.; A second hot stage reactor was also used for our studies to achieve heating rates as high as 1000{dollar}spcirc{dollar}C/sec. We found that chars prepared at higher pyrolysis heating rates ignited more frequently and exhibited higher reactivity when combusted at temperatures leading to diffusional limitations. Due to inherent differences in the two reactors, chars prepared in the hot stage at the same conditions as those prepared in the TGA/VMI exhibited lower ignition and reactivity behavior. Despite these differences, chars prepared at the highest heating rates in the hot stage reactor clearly exhibited higher reactivity and ignited more frequently.; Mathematical modeling efforts focused on the coupled transient mass and energy balances governing the diffusion-reaction problem in char combustion. Changes in particle size and pore structure with conversion were described using experimental data obtained in our laboratory. Combustion kinetics were also measured experimentally. Theoretical predictions agreed very well with experimentally observed trends. Important char structural properties which favored ignitions were larger particle radii, more open macropore structures, and larger macropore surface areas. Process conditions which increased particle ignitions were high pyrolysis heating rates, high combustion temperatures and increased oxygen concentrations during combustion.