Combustion of premixed gas and air in shallow fluidised beds is described and some of the limits set by gas velocity and particle size are discussed. There is a lower limit of gas velocity below which heat transfer back to the distributor plate causes excessive preheat leading to pre-ignition and to temperatures well above the equilibrium level. This condition can cause particle sintering and, at worst, distributor failure. A graph showing a solution of the problem is presented. It is shown that there is a lower limit to particle size, which depends upon the particle density, below which stable combustion will not occur. This limit is set when enough of the gas/air mixture bypasses the bed without burning to prevent the bed from reaching combustion temperatures. The experimental observations are explained in terms of the two-phase theory of fluidisation by postulating that the fuel which passes through the dense phase of the fluidisation is totally burnt, while that passing up through the bed in bubbles either does not react or reacts too late for its combustion heat to be transferred to the bed. A quantitative model based upon these two ad-hoc assumptions is shown to provide reasonable agreement with the observed results and possible refinements of the simplified theory, to make it more rigorously based, are indicated.
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