Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres

The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO₂) or an oxidizing (air) atmosphere was performed by Mössbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe₂SiO₄), hercynite (FeAl₂O₄), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al₂O₃-SiO₂ phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.