| The research aim consisted of characterizing the rate-controlling mechanisms of the iron pyrite combustion transformation FeS2 → Fe3O4, in order to reduce slagging during pulverized coal combustion. Techniques were developed for the preparation of homogeneous samples of extraneous pyrite, synthetic model coal with pyrite inclusions, and synthetic model coal without pyrite inclusions. A drop tube furnace and an up-fired flat flame burner were utilized for combustion. The intrinsic standard method of X-ray diffraction analysis was developed to analyze pyrite combustion products (FeS2, Fe1−xS, FeO, Fe2O3, and Fe3O4) down to 5 wt%, with errors less than 4% absolute.; Combustion of 20 μm extraneous pyrite in the drop tube furnace at 695 K–967 K average gas temperatures indicated that the rate of pyrite decomposition (FeS2 → Fe 1−xS) may be limited by oxygen diffusion or kinetics. Tests using non-macroporous model coal of 90–106 μm size class showed that carbon matrix effects control the oxidation rate of pyrite inclusions.; A monodisperse sample of 20 μm extraneous pyrite was burned in the flat flame burner (1% and 12% oxygen; 1550 K average gas temperature). Pyrite was found to transform according to the{09}pathway FeS 2 → FeS → FeO → Fe3O4, after a brief initiation step (FeS2 → Fe0.875 S). The FeO → Fe 3O4 step was seen to be rate-limiting for over 90% of post-heatup time. Intraparticle processes were determined to be rate-limiting during this step, from boundary layer diffusion rate calculations. Wavelength dispersive spectroscopy established that, specifically, intraparticle chemical kinetics processes are rate-limiting.; Pyrite-free model coals of 75–115 μm size class and three different macroporosities were burned in the flat flame burner at 1550 K and 12% oxygen. Modeling results indicated that a char particle experiences percolative fragmentation events, but not enough to reduce the particle size to a point where carbon matrix effects become negligible.; It was concluded that the rate-limiting mechanism for the oxidation of extraneous iron pyrite to magnetite is the intraparticle chemical kinetics of 3FeO+12O2&rarrr;Fe 3O4, for particle diameters less than 27 μm diameter, temperatures of approximately 1550 K, and oxygen levels greater than 1 mol%. It was further concluded that carbon matrix effects are substantial. |
