Morphological changes and fragmentation of a coal particle during pyrolysis in a hot bed of sand fluidised by nitrogen

Six coals, differing in rank, were pyrolysed by rapidly immersing a sample particle into a bed of hot sand fluidised by pure nitrogen. The bed was electrically heated and held at a fixed temperature between 750 and 950 degrees C. The particles (of different sizes) were withdrawn from the fluidised bed after undergoing pyrolysis for a certain length of time and then quenched in nitrogen before examination. An anthracite quickly shattered into many small particles, a particle of lignite coal did not fragment at all, but all four bituminous coals broke into pieces just after they had lost all their volatile content. The effects of the temperature of the bed and the initial size of the coal particle were investigated. The masses of all the fragments were measured for coals pyrolysed for different times in beds at various temperatures. In order to fragment, a particle's size must exceed a threshold value. However, for anthracite and bituminous coal particles above a certain mass, the ratio of the final to initial particle diameters is small and independent of the initial mass of the particle. Thermal shock appears to be the main cause for the anthracite shattering rapidly into pieces immediately on entering one of these hot beds, whereas the bituminous coals developed large cracks in the layer of char around the kernel of raw coal, so that when all the volatile matter had been lost, the particle fragmented. Plastic coals produce char particles with sand embedded in their exterior. The lignite and bituminous coals lost their volatile matter according to the shrinking core model at a rate controlled by heat transfer.