Competition mechanism between Na release and Hg form evolution in high-temperature gasification condition revealed from bond distribution of coal slag

The entrained-flow coal gasification is a key technology to realize to the largescale and high-efficient utilization of coal in China. The Hg in the coal is readily released to the gas phase at high temperature, posing significant risks to human health and the environment. However, the evolution of the gaseous Hg in the entrained-flow coal gasification condition is scarcely investigated due to its extremely high operation temperature (>1500 °C) which failed the experimental characterization. Further, some alkali elements in coal slag are readily released to the gas phase, possibly influencing the Hg evolution mechanism. The relationship between these two processes remains unknown. In this study, the evolution behavior of the trace gaseous Hg in an entrained-flow coal gasifier was revealed by thermodynamic modelling. Results demonstrated that the HgCl (g) is the relatively thermodynamically stable form in the entrained-flow coal gasifer compared to the HgCl₂(g). The high water vapor content in the Texaco gasifier favored HgO (g) formation. The increases of the Cl content and H₂S content advantaged the generation of HgCl (g) or HgCl₂(g) and HgS (g), respectively. Further, the evolution mechanism of HgCl(g) was influenced by the Na release which is intrinsically determined by the slag structure. The increasing Na₂O content or the decreasing the SiO₂ content of coal ash improved the formation of the structure units Si-O-Al and Si-O-NaAl, inhibiting the Na release. Due to its stronger affinity for gaseous Cl compared to Na, Hg exhibits enhanced formation of both HgCl(g) and HgCl₂ (g) under such conditions. The findings of this study provided the theoretical guide for the adjuring the trace Hg evolution behavior based on the gasification parameter and the coal ash composition.