In-situ observations of devolatilisation of coal

Coal devolatilisation phenomena were observed in-situ during heating. A single particle of coal of approximately 1 mm diameter was placed on a stainless steel strip heated by an alternative current source. The behaviour of the coal particle was recorded at 100 times magnification through an arrangement of a long focal distance microscope attached to a CCD camera. The two different coal types examined (thermal and coking coal) exhibited significant differences during heating. At about 460°C, during the plastic stage, the coking coal showed rapid swelling and formation of bubbles within the liquid. Unlike the coking coal, the thermal coal showed very little swelling, although devolatilisation (predominantly tars for both coals) was taking place. The volatiles were diffusing to the surface of the particle through the pores in the char, without any significant swelling. At temperatures above 600°C, significant shrinking was observed with the coking coal, while in both samples, evolution of hydrogen was monitored. These observations were compared to the measured specific heats for these coals. The specific heats showed significant differences between the samples associated with a very complex behaviour following the onset of softening. While the coking coal exhibited predominantly exothermic reactions coinciding with the observed swelling phenomena, the thermal coal showed a sequence of endothermic and exothermic reactions. The changes in the specific heats also correlated well with the observed evolution of hydrogen (and shrinking for the coking coal) at higher temperatures.