| OCR Text |
Show pyrite particle arrives at a surface during this period, it is expected to stick, and therefore cause slagging. Comparison of Predictions with Experiments Partially oxidized samples of pyrite particles, obtained in a drop-tube furnace, were analyzed for the distribution of the iron among the different phases by Hossbauer spectroscopy.7 As shown in Figure 4, magnetite was the major iron oxide phase in most samples, and pyrrhotite constituted the principal sulfide phase. As expected from equilibrium calculations, no hematite was observed at the highest temperature. Significant quantities of hematite were noted only at the lOwest temperature and at long residence times, indicating that magnetite oxidation kinetics are slow. The corresponding calculated percentages of iron in each phase for the different gas temperatures and residence times, obtained from the model, are also shown in Figure 4. These data indicate that excluded pyrite particles of 40 microns or larger will be oxidized predominantly to magnetite in the residence time available under typical pulverized coal combustion conditions. Hematite, which is observed as the predominant oxide phase in boiler deposits, is therefore a result of oxidation of magnetite in the deposit. Evolution of Particle Structure During Pyrite Oxidation In our laboratory, laminar flow combustion experiments on pyrites yielded particles at different stages of burnout. Figure S shows scanning electron micrographs of partially oxidized pyrites, arranged in order of decreasing sulfur content. Pyrite particles in the feed are angular, with almost no porosity (Figure Sa). As decomposition begins, the sulfur escapes, resulting in a porous pyrrhotite particle (sulfur loss = 43 percent). Several fissures are visible on the surface of the pyrrhotite (Figure Sb), which could facilitate particle breakup in subsequent stages. Figure Sc shows a smooth ellipsoidal particle, demonstrating that melting had occurred subsequent to pyrrhotite formation. Helt formation was anticipated as the particle 8 |
