Near-Limit Combustion in Spouted and in Crater Beds

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Abstract
Combustion in spouted and in crater beds is studied under very fuel-lean and very fuel-rich conditions and compared with burning in an electrically heated fixed bed. Ultra-lean mixtures are relevant to burning low calorific value reactants, whilst burning beyond the rich limit allows self-sustaining partial oxidation reactions to proceed economically. The common aim is to employ heat recirculation by the bed in order to pre-vaporise or pre-gasify (in the case of solid or liquid fuels), pre-heat and pre-mix prior to combustion. The conclusions of this very extensive study include a model which successfully predicts flame stability limits, as well as a critical comparison of crater- with spouted-beds. The former, unlike the latter, can operate with the smallest of particles (dusts) and are not subject to minimum spouting depth limitations; however the fountain in the crater is not effective in recirculating heat. Nonetheless, by extending the heat transfer surface of the inlet tube, much leaner mixtures can be burned in crater beds. Means of increasing the residence time of coal particles in spouted beds in order to improve burn-out of the ungasified fractions, even in small laboratory systems, are presented.

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