Effect of CaO/Fe2O3 on fusion behaviors of coal ash at high temperatures

The ash fusibility, usually investigated and evaluated by the ash chemical composition, is widely used for guiding the coal utilization in boiler and gasifier. The ratio of basic and acidic oxides (B/A) in coal ash is the most important parameter for ash fusibility evaluation. While the coal ash has the identical B/A ratio, SiO2/Al2O3 and CaO/Fe2O3 ratio also show significant influence on the ash fusibility. However, the researches on CaO/Fe2O3 on ash fusibility are rare. In order to quantify the influence of CaO/Fe2O3 on the ash fusibility, thermal mechanical analyzer (TMA) is applied to investigate the ash fusion behavior of coal ash with various CaO/Fe2O3 ratios. XRD, FactSage and DSC are combined to explore the ash fusion characteristics and mechanisms. The results showed that AFTs increased with the increasing of CaO/Fe2O3 ratio, which was attributed to variation of Fe2O3 content and iron valence. However, the ash with low CaO/Fe2O3 ratio trended to form mullite, increasing AFTs. Hence, the optimal CaO/Fe2O3 ratio for fluxing effect existed to balance the mineral component and iron valence. The different shrinkage curves were attributed to the different fusion events and different mechanism. The fusion process of ash (CaO/Fe2O3 = 0.25) exhibited as the behavior of a pure substance, which has the narrow fusion range. And the fusion range was dominated by the low temperature eutectics including spinel, quartz, corundum, and anorthite, which were beneficial for preventing ash sintering. Meanwhile, the fusion process of ash sample (CaO/Fe2O3 = 4) occurred step by step, and it had a wide fusion range. The low-melting point minerals such as wollastonite, clinopyroxene, calcium silicate induced and manipulated the fusion process. These ash samples with the wide fusion range were favored for slagging process. Besides, the shrinkage rate of ash was mainly determined by the slag formation rate when viscosity was low, but it was dependent on slag formation rate as well as viscosity for the high viscosity slag. Two correlated relations between shrinkage rate and ratio of liquid phase formation rate and viscosity for ash with CaO/Fe2O3 < 1 and CaO/Fe2O3 ≥ 1 were established to prevent ash sintering at high temperatures.