Mineral transformation during combustion of coal blends

Mineral behaviour for two individual coals (I, J) and their two‐component coal blends and 800°C ash blends heating were studied. Ash samples were heated progressively from 800°C to IT (initial deformation temperature) at 100°C intervals under different conditions. Coal samples were heated from room temperature to the corresponding temperature. Mineral transformation at each temperature was determined by X‐ray diffraction and SEM measurements. The results show that Si, Al, Fe and Ca compounds have a great form variation during heating. Their forms at different temperatures depend on the chemical composition of the ash, the blending ratio and the atmosphere. For different coal ashes, the main mineral matters at 800°C were quartz, anhydrite, hematite, calcite and feldspar. As the temperature increased, oxidation, thermal decomposition, transformation and reaction occurred between the components. Comparing a 40% I+60% J ash blend with individual ashes, fayalite was formed at 1100°C for the blend; the reaction product existed in a glassy phase at 1300°C. For a coal blend having the same ash ratio as the ash blend, FeO reacted with amorphous SiO₂ or Al₂O₃ to form fayalite and hercynite at 1000°C. As the temperature increased to 1100°C, fayalite and hercynite increased obviously. At 1200°C, some iron inclusion compounds melted to become glassy phase matter. Compared with the ash blend, iron species undergo a different change during coal blend heating: fayalite and hercynite formed earlier, iron compounds melted to form a glassy phase at lower temperature. This may be caused by early combustion of the more reactive coal (J coal) in the blend inducing local variation in oxygen concentration gradients around the less reactive coal and consequently affecting the reaction atmosphere and Fe mineral behaviour and interaction. That is to say, for coal blends, the mineral transformation was affected by both the mineral species interaction and the combustion behaviour. The calculations were performed to examine the fate of mineral matter under different combustion conditions using a thermodynamic chemical equilibrium calculation program. Calculations from coal blends were comparable with experiments from ash blends, this is because the calculation program only considers the interaction among the mineral species but does not consider the combustion reaction. It indicates that combustion and the relative volatiles also affected the mineral behaviour and slagging during coal blend combustion. Meanwhile, the mineral species evaporations were measured at high temperature: the main evaporated species were Na, K pure species and compounds, Fe, FeO, SiO and SiO₂. The evaporation of Fe has an important effect on initial deposition. Calculations were comparable with the experiments. Copyright © 1999 John Wiley & Sons, Ltd.