Ignition and combustion of single millimeter-sized coal particles

A technique for measuring the temperature gradients around a captive coal particle in a single-particle combustion test reactor (SCTR) was developed. The measurement technique employed a thermocouple array (TCA) which consists of a group of equally spaced extrafine thermocouples of 50 ;The temperature profile of the particle surface and surrounding gas revealed that the pyrolysis of single millimeter-sized coal particles was a highly nonisothermal process. Based on the spatial temperature gradients, the dimensions of the heat transfer boundary were also determined. The occurrence and location of homogeneous ignition in the gas phase and heterogeneous ignition on the particle surface were clearly distinguishable from the measurements of both temperature and luminosity. The mechanisms of propagation and extinction of volatiles combustion were discussed. Under the experimental conditions in this study, ignition for all the coals except the anthracite occurred homogeneously in the gas phase, rather than heterogeneously on the particle surface. Low rank coals exhibited a strong homogeneous ignition mode with a high intensity of volatiles combustion. Similar surface ignition behavior for the anthracite and char was observed due to their low volatile matter contents. The analysis of the measured temperature gradients verified the "flame liftoff" and the "thin flame sheet" theories.;The char samples collected from the SCTR for both pyrolysis and combustion were analyzed using different analytical techniques for compositional and morphological changes. Comparison of the data from these analyses revealed no significant structural changes in the char samples generated during the early stage of pyrolysis and combustion. This result is consistent with the conclusion of a homogeneous ignition mechanism.;A mathematical model was developed to investigate the degree of coupling between mass transport and chemical kinetics during char combustion. A mass transfer enhancement factor introduced to the combined mass diffusion and kinetic model accounted for Reynolds number effects and the effect of the diffusional resistance on the overall reaction rate. The combustion times of the coal particles predicted by using the model agreed with those measured experimentally and with published data in the literature for small scale fluidized-bed reactors.