The role of residual char on ash flow behavior, Part 2: Effect of SiO2/Al2O3 on ash fusibility and carbothermal reaction

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SiO2/Al2O3 weight ratio (Si/Al) in the ash content is always regarded as the key factor on ash fouling and sintering in the gasifier, and residual char is often found in gasification slag. The effect of Si/Al on the fusibility of ash containing different contents of residual char (5–15%) was investigated in this work. The residual char was added into five ash samples with different Si/Al from 1.0 to 3.0. The results show that the residual char content shows a significant effect on the AFTs and the fusion temperature range (ΔT = FT-DT) when the Si/Al of the ash is higher than 1.0, especially for the ash with more than 10% residual carbon. Carbothermal reaction products (SiC and Fe3Si) have obvious influence on hemispherical temperature and flow temperature of ash with a high Si/Al. For the ash with Si/Al of 1.0, anorthite, corundum and mullite are the major minerals at high temperatures, and formation of SiC and Fe3Si shows a slight effect on the ash fusion temperatures. However, there are less anorthite and SiO2 at high temperatures in the ashes with Si/Al of 2.0 and 3.0. Besides, Si/Al mainly affects the formation of SiC, while it does not influence formation of Fe3Si. The formation rate of SiC in ash with Si/Al = 2.0 is slower than that of the ash with Si/Al = 1.0 and Si/Al = 3.0. Furthermore, it demonstrates that a high amorphous content and less SiO2 in the ash are detrimental to carbothermal reaction (3C + SiO2 = SiC + 2CO).