00/00655 Enough and not enough—two truths about coal

Abstract

This study reports the chemical and petrographic characteristics of chars obtained at 1000 and 1300°C (within the range of temperatures reached by coal particles in the near burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are low and high volatile bituminous vitrinite-rich coals, and the third one has similar rank to the high volatile bituminous coal but a very different maceral composition (97% inertinite). The residual volatiles remaining after pyrolysis at the two temperature tested were determined by heating the chars in a thermobalance at 1100°C under nitrogen. Also, the morphology and optical texture of the chars were studied by optical and scanning electron microscopy. The increase in preparation temperature enhanced the plastic behaviour of the low volatile bituminous coal, resulting in an extensive development of secondary porosity. The drop in reactivity from low to high temperature char might be due to the enhanced fluidity of the metaplast, which increases the likelihood for polymerisation reactions to take place, with a parallel reduction in the number of active sites. Incomplete devolatilisation and poor plastic properties were achieved during low temperature pyrolysis of the inertinite-rich coal, whereas at 1300°C most of the material passed through a fairly plastic stage and was very efficiently pyrolysed. No significant differences were observed between the low and the high temperature chars from the high bituminous vitrinite-rich coal. This behaviour could be due to either extensive crosslinking reactions involving hydroxylic oxygen in an early stage of the pyrolysis process, or, which seems more likely, to the sheltering effect of soot particles formed in the vicinity of pyrolysing coal particles, which could minimise the effect of the increased flame temperature.