Effect of Five Different Additives on the Sintering Behavior of Coal Ash Rich in Sodium under an Oxy-fuel Combustion Atmosphere

This paper was addressed to investigate the influence of different additives on the sintering behavior of coal ash under an oxy-fuel combustion (O-2/CO2 combustion) atmosphere. A lignite (Zhundong coal) rich in sodium has been used as the fuel material. The raw ash mixed with additive was heat-treated at 1350 degrees C under an oxy-firing (30 vol % O-2/70 vol % CO2) atmosphere. In this study, five kinds of oxides have been applied as the additive: Al2O3, SiO2, kaolin, CaO, and Fe2O3, while the pure ash acts as a reference. At the same time, the shape shifting of the sintered ash samples during the sintering process was monitored by a charge-coupled device (CCD). The image processing system was used for analyzing the photos captured by the CCD to obtain the shrinkage of area and height of sintered samples. Moreover, the microstructure of the sintered samples was observed by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDX), which can analyze the chemical compositions. In addition, the distribution of mineral phases in the sintered samples was identified by X-ray diffraction (XRD). It turned out that SiO2 obviously promoted the melting process, but CaO suppressed the sintering and melting processes greatly. Furthermore, Al2O3, Fe2O3, and kaolin also affected the quantity of molten substances to some extent. Meanwhile, all of the samples are characterized by a layer structure with different colors. The results also show that the mineral phases and microstructure changed dramatically with different additives from the XRD and SEM analyses.