Interphase heat/mass transfer and combustion of bubble phase in an elevated-temperature fluidized bed

Combustion of gaseous fuels within the bubble phase is expected to influence the arrangement of heat exchange surfaces as well as the formation of pollutant in a fluidized bed combustor. In this work, first of all, a lab-scale fluidized bed combustor was built. An isolated single bubble was injected into the fluidized bed at different bed temperatures. A high-sensitivity thermocouple was employed to measure the temperature of the fast-rising bubble, and a gas sample sucked in by a gas-sampling probe from the bubble was analyzed a GC/MS system on line. Second, interphase heat and mass transfer between the bubble and the emulsion phase, which are fundamental to the understanding of bubble phase combustion, were investigated experimentally and theoretically. The influence of bubble size, particle size and type, and bed temperature on the interphase heat and mass transfer were investigated. Experimental data show that the interphase heat transfer coefficient increases to a maximum, and then decreases as the bubble size increases in the bubble size range investigated, while the interphase mass transfer coefficient decreases with increase of the bubble size. Higher bed temperature leads to a decrease in both quantities. The corresponding models to predict the interphase heat and mass transfer coefficients were established or selected. Finally, experimental studies were conducted to find the effect of variation in bubble size, bed materials, and bed temperature on the combustion of methane in the bubble phase. Experimental data revealed that small size bubbles are favorable to bubble phase combustion. Enhancement of the bed temperature shortens the burn-out time of the bubble significantly, and the oxygen concentration in the emulsion phase plays an important role to the combustion of the fuel-rich bubble. A mathematical model was developed to predict the intermediate and final products of bubble combustion. The calculated results were in good agreement with experimental data.