The mechanism of combustion of coal-water slurries

Abstract

The combustion of coal-water slurries has been investigated using suspended and falling droplet techniques. Measurements have eeen made of ignition delays, mass loss and temperature rise for a range of coals with differing coal particle sizes and slurry preheat temperatures. Ignition delays have been shown to be a function of water content and its initial temperature, coal type and particle size. It was observed that in the majority of cases ignition was followed by agglomeration of coal particles so that its subsequent behaviour was determined by combustion of the agglomerated particle. Char combustion rates were determined and the results have been experessed in terms of burning rate constants suitable for use in practical situations. A simple theoretical model has been developed which represents the behaviour of a coal-water slurry of a bituminous non-swelling coal to a reasonable degree. The early stages involving heat-up, ignition and burning of the volatiles of the droplet are well reproduced by the model. Ignition occurs when sufficient water has been evaporated so that a layer of coal particles become immobile on the droplet surface. The subsequent char burning stage is moderately described by the model but an exact mechanism is difficult to define because of uncertainties arising from the open structure of the char produced from agglomerated coal particles.