Combustion of Coal in Shallow Fluidised Beds

Abstract

Some new information, which may enable coalfired shallow fluidized bed combustors to be designed with greater assurance, have been presented in this thesis. The mechanism of combustion of a single coal particle in a fluidized bed is studied. Experimental observations and published data suggest that carbon is oxidized to carbon dioxide and carbon monoxide. The carbon monoxide so formed is then oxidized to carbon dioxide in a diffusion flame surrounding the carbon sphere. Mathematical expression for the burning rate of carbon is derived. Account is taken of the fact that the void age, through which the diffusion of gas occurs, varies in space. This affects both the burning rate and particle/gas Sherwood number. The burning rate of carbon has been measured experimentally at a number of operating conditions and these results are compared with predicted values. The presence of burning coal particles is found to increase the heat transfer rate especially in very shallow beds. Experimental data on heat transfer in coalfired fluidized beds have been amassed and are in close agreement with theoretical predictions. The contribution of radiation to the total heat transfer has been determined through experiment and theory. It accounts for about 10 % of the total heat transfer. Lack of data on particle residence time prevents precise prediction of heat transfer coefficients by this method. Finally a simple model of chemical reaction in fluid bed is used to calculate the oxygen conversion for different carbon fractions in bed. This leads to the minimum carbon fraction ( which is associated with the maximum feed point spacing) to avoid partial quenching of the bed.