A range of pulverized coals were combusted in a laboratory drop-tube furnace at temperatures of 1573, 1723, and 1873 K under oxidizing and reducing conditions to determine the effect of combustion stoichiometry on ash formation mechanisms. As iron mineral transformations were expected to be most affected by combustion stoichiometry, two of the test coals chosen were of high pyrite (FeS2) content and two of high siderite (FeCO3) content. It was found that the ash formation mechanisms of excluded quartz, koalinite, and calcite were not affected by oxidizing or reducing combustion conditions. Excluded pyrite was found to decompose to pyrrhotite, which oxidized to produce an FeO−FeS melt phase which was stable under reducing conditions. Under oxidizing conditions oxidation continued, producing magnetite and hematite. Excluded siderite was found to decompose to wustite, which was stable under reducing conditions, but oxidized to produce magnetite under oxidizing conditions. Included pyrite and siderite were determined to behave as for excluded pyrite and siderite if there was no contact with alumino-silicates. Included pyrite that contacted alumino-silicate minerals was observed to form two-phase FeS/Fe-glass ash particles, with incorporation of iron into the glass proceeding as the FeS phase was oxidized. Included siderite that contacted alumino-silicate minerals was determined to directly form iron alumino-silicate glass ash particles. Iron alumino-silicate glass ash was determined to form with iron in the Fe2+ state, much of which subsequently transformed to the Fe3+ state in oxidizing conditions, but remained primarily as in the Fe2+ state under reducing conditions.

Four coals containing iron mineral pyrite (FeS2) and siderite (FeCO3) were combusted in a laboratory drop tube furnace at temperatures of 1300, 1450, and 1600 °C under oxidizing and reducing conditions. Results for the behavior of pyrite mineral were in agreement with the established literature. The behavior of siderite mineral was determined and comparisons made. Coals containing pyrite minerals were determined to have potential to produce ash deposition and slagging at lower temperatures than coals containing siderite mineral. Reducing conditions were determined to lower the temperature at which ash deposition and slagging may occur for coals containing iron minerals compared to oxidizing conditions. With respect to ash deposition and slagging, it was determined that the iron levels in a coal are not definitive, but rather the iron mineral type (pyrite or siderite), mineral association (included or excluded), degree of association of included minerals, and the type of included alumino−silicate minerals have important roles.

A model for the prediction of iron-based slagging precursors from the combustion of ironcontaining coals is detailed. The model accounts for the form of iron (pyrite or siderite), the distribution of iron within the pulverized coal, temperature, and oxidizing or reducing conditions. The input required for the model is a CCSEM analysis of the pulverized coal. For oxidizing conditions, the index predicts similar behavior for pyrite, and siderite-containing coals, with iron alumino-silicate ash particles becoming sticky at temperatures greater than 1400°C. This suggests that for oxidizing conditions, the extent of included iron minerals is the most important factor. For reducing conditions, the index predicts sticky ash particles are formed at lower temperatures, as low as 1000°C for pyrite-containing coals as a result of the decomposition and partial oxidation of pyrite-forming sticky particles, and 1100°C for siderite-containing coals. For reducing conditions, the level of excluded pyrite mineral for pyrite-containing coals and the level of included iron-containing minerals associated with clays for siderite-and pyrite-containing coals are the most important factors determining slagging.