MINERAL BEHAVIOR DURING COAL COMBUSTION .1. PYRITE TRANSFORMATIONS

The transformations and deposition characteristics of pyrite (FeS2) under pulverized coal combustion conditions were examined in an entrained flow reactor. Mössbauer spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis, were used to monitor the evolution of particle composition and morphology. Pyrite initially decomposed to form pyrrhotite (Fe0.877S). Fissures resulting within the particle during this step led to limited bulk fragmentation. Subsequently, the pyrrhotite particles melted to form an iron oxysulfide droplet. Magnetite (Fe3O4) crystallized out of the melt once the melt oxide content exceeded 85%. Hematite (Fe2O3) was formed at longer residence times. A comparison of kinetic model calculations with experimental data revealed the controlling resistances for each of the stages during the transformation process. Deposition experiments established the iron oxysulfide phase to be responsible for particle adhesion, and the duration of this melt phase was determined to be a function of pyrite particle size, gas temperature, oxygen concentration, and the extent of fragmentation. Combustion experiments were also conducted with two coals, one with predominantly included pyrite grains and the other with a significant fraction of excluded pyrite, to discern the limiting behavior of pyrite in forming the final ash. © 1990.