Mineral transformation during rapid heating and cooling of Zhundong coal ash

Abstract

Zhundong (ZD) coalfield has a large number of coal reserves. However, the severe fouling and slagging problems that occurred in ZD coal-fired utility boilers have seriously hindered the large-scale utilization of ZD coal. Deep understanding of the mineral transformation of ZD coal during combustion and its impact on fouling and slagging can provide technical support for fouling and slagging prevention in utility boilers. In this paper, a novel temperature and atmosphere controlling method was proposed to study ash deposition. Wucaiwan (WCW) coal ash samples in different atmospheres and temperatures were collected efficiently by the established single thermocouple high-temperature online microscope observation test rig. The influence of rapid heating and cooling on the mineral transformation of WCW coal ash samples was systematically investigated using XRD and SEM-EDS, respectively. The effects of SO2 in the flue gas on the mineral transformation and the sulphates generation of the coal ash were analysed by comparing the experimental results in oxidizing atmosphere and simulated flue gas atmosphere. The experimental results showed that the microscopic morphology, mineral and element conversion of coal ash samples during heating and cooling were quite different. Compared with oxidizing atmosphere, SO2 in the flue gas significantly increase the S content of ash at 600 °C−1200 °C and the amount of molten minerals on the quenched ash sample surface during cooling. These results indicate that SO2 can promote the sulfation reactions of alkali/alkaline earth metal (AAEM) on the quenched ash sample surface and restrain the decomposition of S-containing minerals in ash cooling. The investigation also reveals that the molten AAEM sulphate on the ash surface is mainly generated at 800 °C−900 °C. The present study not only provides the fundamental understandings of mineral transformation but also guides industrial applications to reduce the risk of ash deposition.