A FUNDAMENTAL INVESTIGATION OF THE FLAME KINETICS OF COAL PYRITE

The kinetics of the transformations which pyrite (FeS2) particles unde rgo in coal flames were studied in a laboratory burner. The size of th e particles leaving the burner was measured with an optical sizer and the composition of the sampled particles was analysed by computer-cont rolled scanning electron microscopy (CCSEM). The pyrite particles deco mposed into pyrrhotite (FeS) in a time corresponding to that required to transfer the necessary heat from the gas to the particles to sustai n the endothermic decomposition reaction. There were no further reacti ons in a reducing flame, only rapid melting. In an oxidizing flame, de composition and melting occurred prior to oxidation. The pyrrhotite dr oplets oxidized to solid iron at a rate corresponding to the rate of t ransport of oxygen from the gas to the droplets. The gas-side transpor t of oxygen is thus the rate-limiting step for the oxidation; the liqu id-side transport of oxygen in the droplets is apparently faster. This surprisingly rapid transport is explained by internal convective flow s induced by the friction of the droplets with the surrounding gas. Th is friction in turn is induced by the gravitational motion of the drop lets relative to the gas. In the final stage of oxidation, cenospheres were produced. The standard CCSEM method classified these cenospheres as small particles, which led to an initial erroneous conclusion that the particles had fragmented. (C) 1996 Elsevier Science Ltd.