The combustion characteristics of freely-falling and suspended mono-sized coal-oil mixture (COM) droplets in heated air were investigated by studying the quenched samples using scanning electron microscope. Parameters varied include the volatility of the oil, the concentration and size of the coal particles, the residence time, the intensity of blowing, and the effects of water and surfactant addition. Results substantiate the formation of coat particle agglomerates upon depletion of the volatile oil components, and emphasize the importance of oil volatility and external convective motion on COM combustion. In particular agglomeration is not expected to be a serious problem for COM with No. 6 oil, although addition of oils with higher volatility widens the volatility differentials between the oil and coal components such that the agglomerate formation, heating, ignition, and thereby the total burning times are significantly prolonged. Addition of water may cause droplet swelling and fragmentation due to internal bubbling which, however, can be inhibited by moderate amount of internal circulation induced by the external convective motion. The possibility that agglomeration may be prevented under high pressure combustion is also discussed.
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