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Show The principal product of pyrite decomposition is pyrrhotite. Pyrrhotite is both a porous and chemically reactive material. The porosity is so great that the material could be thought of as a structure of submicron nodules connected by small bridges in a three dimensional lattice. When this lattice is exposed to oxygen after coal devolatilization, it has been heated to a temperature such that it would react vigorously. We postulate that the bridges in the pyrrhotite structure would collapse, allowing the structure to disintegrate and form many submicron-sized particles. All of the essential features of this mechanism have been experimentally observed in the coalj char samples. The SEMjEDS analyses indicate rupturing and fragmentation of the pyrite grains during the heat-up and devolatilization stages of coal combustion. The particle size of the iron-containing minerals decreases by a factor of approximately 2 between the first and second stag·es of combustion, indicating fragmentation into approximately eight pieces. The same analyses indicate that thermal decomposition of the pyrite is essentially complete by the time the first samples are drawn from the reactor. Only a trace amount of pyrite still exists at the first sample point taken from the large particles. The pyrites have reacted to form pyrrhotite primarily, with some iron oxide already formed at this stage. The subsequent sample points indicated no pyrite whatsoever. Examination of the pyrrhotite structure under the SEM shows that it is composed of ~ O.lJLm nodules. At later stages of combustion, large pyrrhotite and iron-oxide structures appear to disintegrate, leaving many small particles both in the cavities they create and scattered over the particle surface. The EDS probe used to measure these is not sensitive to either carbon or oxygen, therefore, siderite, all oxides of iron, and several other iron minerals are indistinguishable to the probe, and they all appear iron rich. The small particles are predominately iron rich, presumably iron oxides, with small amounts of sulfur present. The formation of submicron-sized particles from the combustion of iron minerals common to coal has been observed previously. Raask [1984], for example, shows illustrations of such small particulate. Such fumes were formed from both pyrite and siderite, sometimes comprising up to 80 % of the original mass of the mineral species. Observations in our laboratory also have shown submicron-sized particles. These were found under circumstances suggesting they originate in a fume. For example, some were found imbedded in lacy soot structures, strongly suggesting that they were scavenged from the gas phase. Although the specific mechanism of their formation has yet to be verified, such submicron-sized particles appear to be playing a major role in the evolution of iron from pyritic iron minerals during combustion. 16 |
