| OCR Text |
Show reaction during the devolatilization stage of coal combustion previously observed. That is, the iron evolution process requires the presence of oxygen. After several variations of oxygen concentration, particle size, and coal type, the salient features of the experimental findings are: 1. The iron release depends on the presence of a high fraction of pyritic iron, as opposed to other iron forms. 2. The iron release depends on the presence of oxygen. 3. The extent of iron release measurably increases as the fractional penetration of oxygen into the particle interior increases. 4. The iron release occurs at temperatures as low as 1600 K. 5. The iron release occurs after, not during, the decomposition of pyrite. 6 A Postulated Mechanism for the Iron Release The most commonly cited mechanism of mineral evolution from coal particles is that of reduction followed by vaporization. The conditions of temperature, pressure, and residence time under which the iron release occurred preclude the possibility that it could have been a vaporization process, however. The partial pressure required to explain the data under these conditions is about four orders of magnitude larger than the vapor pressure of metallic iron, for example. The possibility of formation of volatile species of iron was extensively reviewed. All such species (carbonyls, chelate compounds, chlorides, etc.) were eliminated on the basis of chemical kinetic, thermodynamic, or mass balance constraints. Reduction or reaction followed by vaporization is not a satisfactory explanation of these data. A more satisfying explanation of the data than that of a vaporization process is related to the chemical reactivity of the iron-containing mineral species. Such a mechanism has been postulated for this data. It is illustrated in part in Figure 5. The figure shows an individual coal/ char particle at four stages of combustion. A mineral inclusion of pyrite is also illustrated, at a somewhat exaggerated size compared to the coal particle. The initial point represents the raw coal. The coal particles are entrained in a hot gas. As the coal devolatilizes , the volatiles consume the oxygen in the immediate vicinity of the particle. The pyrites also fracture, possibly liberating some fragments from the coal. The pyrite decomposition occurs in an oxygen-free environment. 14 |
