Measurements of coal particle shape, mass and temperature histories: Impact of particle irregularity on temperature predictions and measurements

摘要

Individual coal and carbon particles were levitated in an electrodynamic balance (EDB) and characterized using high-speed diode array and video based imaging systems to determine particle surface area, volume, drag, mass and density. These same particles were then heated bidirectionally using a long pulsed Nd:YAG laser to simulate combustion level heating fluxes (heating rates on order of 10(sup 4) to 10(sup 5) K/s). Measurements of particle surface temperature, size and laser temporal power variation were made and recorded during each heating experiment. Measured temperature histories were compared with a heat transfer analysis that accounted for variations in particle shape, mass, density, and laser heating power. Results of this study indicate that with well characterized materials of known properties agreement between measurement and model of within 20 K is typical throughout an entire heating and cooling profile. Large particle to particle variations are observed in coal particle temperature histories during rapid heating. These variations can be explained in large part by accounting for particle to particle property (shape, mass and density) variations. Even when accounting for particle to particle shape and density variation, however, model predictions greatly underestimate observed temperature histories. It is concluded that these discrepancies are largely due to uncertainties in the thermal properties (heat capacity and thermal conductivity) typically used to model coal combustion behavior.