Some aspects of combustion of coal in fluidized beds

Abstract

This thesis presents an experimental study of the mechanism of combustion of carbon in shallow fluidized bed combustors. Burning rates and temp­ eratures of electrode carbon spheres and burn -out times of hatch charges of char particles were measured experimentally over a wider range of variables than reported hitherto. Bed temperature, carbon and inert particle size, superficial fluidizing velocity, oxygen concentration and static bed depth were the variables explored. Combustion was found to be controlled mainly by chemical kinetics at a bed temperature of 800 °C, whereas at a bed temperature of 900 °C rate of transfer of oxygen to the carbon influenced combustion progress­ively as the inert particle size was increased. When the inert particle size approaches about 1000 μm the rate of mass transfer of oxygen dominated the process entirely, Forced convection played an important role in the transfer of oxygen at high superficial fluidizing velocities. Proposals have been presented to modify the existing models of combustion of single carbon particles, taking into account the effects of mass transfer by forced convection and the oxidation of carbon to carbon dioxide at the surface by oxygen. Predictions of this modified model were found to be in good agreement with experimental data. Burning rates and particle temperatures increased with increase in bed temperature, inert particle size, superficial fluidizing velocity and oxygen concen­tration. Elutriation of unburnt carbon particles was minimal with large size of feed. The presence of a large number of burning carbon particles in a continuously-fed coal-fired fluidized bed combustor reduced burning rate and temperature of individual particles significantly by reducing local oxygen concentration within the bed. This helps to maintain the burning carbon particle temperatures to a value less than 100 K higher than the bed.The experiments showed that: (i) forced convection plays an important role to supply oxygen to burning carbon particles (ii) Higher intensity of combustion (MW/m3 ) could be achieved by larger size of carbon and inert particles, high bed temperature and high superficial fluidizing velocity (iii) Better control of bed temperature may be achieved by recycling a part of exhaust gas through the bed to reduce rate of combustion. lne work demonstrates the relative importance of some design and operating parameters and suggests what needs to be done to improve future shallow fluidized bed combustors and where future research is required.