The effect of iron-bearing mineral melting behavior on ash deposition during coal combustion

Abstract

Mineral matter in coal is one of the most important sources of problems in coal combustion, including fouling, slagging and corrosion, etc. Mineral matter transformation and slag formation are specific properties of coal that provide more information on the suitability for coal combustion or gasification. In this research, the sintering behavior of Chinese pulverized coal (PC) ash as well as mineral transition mechanism under various atmospheres, especially iron-bearing minerals, was studied by heating microscopy, X-ray diffraction (XRD), Scanning Electron Microscope (SEM) combined with EDX (SEM-EDX). Furthermore, the molecular chemical characteristics of iron-bearing minerals, fayalite (2FeO·SiO 2), were also calculated using quantum chemistry theory to provide an insight into the mineral reaction mechanism at molecular level in detail. The results showed that the different states of iron (Fe 2+ or Fe 3+) are the dominant reasons for different sintering behaviors of coal ash under different conditions. The iron-bearing minerals in ash, such as wustite (FeO), almandite (3FeO·Al 2O 3·3SiO 2) and fayalite, etc., are the most important factors influencing the initial sintering behavior of coal ash in the temperature range from 1273 to 1373 K under reducing condition during coal combustion. In fayalite molecular cluster, it is because that the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) are mostly constituted by Fe atoms and the minimum energy difference between HOMO and LUMO in fayalite cluster (1.659 eV) is much lower than that of mullite (3Al 2O 3·2SiO 2, 6.8 eV) and kaolinite (Al 2O 3·2SiO 2·2H 2O, 3.49 eV), the thermal stability of fayalite is relatively low and Fe atoms (especially Fe(6), Fe(8) and Fe(3)) have relatively higher reactivity than any other atoms in fayalite molecular cluster. That is the reason why fayalite starts to melt at relatively low temperature during ash sintering behavior.