Combustion behaviour and characteristics of a COM (Coal-Oil Mixture) droplet by forced ignition were investigated experimentally, considering time change of droplet diameter, droplet temperature and effect of ambient air temperature on the combustion rate coefficient.
The results were compared with those reported in the previous paper regarding self-ignited COM droplet. The results showed that the combustion behaviour of COM droplets were successive two process combustion, gasification combustion was followed by solid combustion of residual carbon as same as in the case of spontaneous (self-) ignition. During the gasification combustion period, it was observed that droplets swelled and deflated alternatively in consequence of the polymerisation reaction proceeding simultaneously with the thermal cracking of oil and coal. The gasification combustion rate coefficient of COM droplets was increased linearly with increasing of ambient air temperature and it's value was smaller than those of spontaneous ignition. It is confirmed that the solid combustion rate coefficient of residual carbon could be predicted roughly by the modified Essenhigh's equation. The reac-tion on the surface of solid residual carbon can be considered both C+1/2 O2→CO and C+O2→CO2 are proceeding competively.
Furthermore, overall combustion rate coefficient of COM droplets were about 1/2-1/4 comparing with that of base oil. It may be explained with the longer solid combustion period of COM droplets and this period are covering considerable part of overall combustion period.