Abstract
Two methods were used to investigate the physical changes occurring in pulverised coal particles on rapid heating. The first involved the heat treatment of single captive particles, monitoring their behaviour with a, high speed cine camera and simultaneously recording their temperatures using a specially developed infra-red detector. The second concerned suspensions of particles which were treated in a laboratory tube furnace, in a pilot plant combustion chamber and in power station boilers. The two methods were complementary and permitted observations to be made of structural change in particles, together with measurements of their apparent and bulk densities, size and surface areas. When heated in air at temperatures above about 550°C, most bituminous coal particles formed hollow spheres which could be classified into three main types. A fourth type was solid and was associated with the fusain component of the coal. The hollow spheres possessed large holes in their surfaces through which gases could pass easily. Oxidation proceeded both internally and externally until the particles fragmented. Even so-called "high-swelling" coals underwent little swelling in air, but in nitrogen, the swelling was more pronounced and the structure of the particles was more open with fewer pores. Using the data gained in these investigations a mathematical model was developed to describe the combustion behaviour of single particles. The model was programmed for an analogue computer and the computer outputs were compared with experimental results obtained using the laboratory tube furnace. It was concluded that the combustion of pulverised coal is largely chemically controlled and that size changes undergone by the particles, especially fragmentation, are important in controlling their temperature through radiant heat exchange with the surroundings. There appeared to be no simple relationship between the surface areas theoretically required for combustion and those measured using a novel gas adsorption technique.