Effects of particle size and heating rate on swelling characteristics of a bituminous coal

A size-classified bituminous coal was pyrolyzed in a laboratory drop tube furnace at different heating rates. The effects of coal particle size and heating rate on particle swelling properties were investigated. The results show that coal particles undergo obvious swelling during pyrolysis, leading to the formation of a large number of char cenospheres with a large central void surrounded by a thin shell. Analyses indicate that this is caused by high concentrations of vitrinite present in coal samples. At the same heating rate, the extent of swelling increases with decreasing particle size and the difference in swelling decreases with increasing particle size. Since finer coal samples contain higher content of vitrinite, the observed phenomena are considered to be the result of the different content of vitrinite in these samples. The reason is that coal particles containing more vitrinite easily undergo a softening and deformation stage and swell significantly during pyrolysis. When the heating rate increases from 0.5 × 104 K/s to 4 × 104 K/s, the swelling of coal particles in the same size range firstly increases and then decreases, which implies that an optimum heating rate at which coal particles swell most must exist between 0.5 × 104 K/s and 4 × 104 K/s. Reasonable explanations for this effect of heating rate on particle swelling are provided in the present study