Mineralogical Controls on Heavy Metal Fate in Fine Particles from Coal Gasification Residues

Fine particles within coal utilization byproducts have high environmental and health risks because of their smaller size and enrichment in heavy metals (HMs). The modes of occurrence of inorganic particles and nine HMs (Cr, Co, Ni, Cu, Zn, As, Cd, Tl, and Pb) in the fine particles within the residues from two commercial entrained-flow coal gasification plants using General Electric (GE) and Gaskombinat Schwarze Pumpe (GSP) gasification technologies were investigated. The majority of mineral weight percentages in feed coals were in the 0.5–4.6 μm size range, whereas those in the fine particles were mainly in the <10.0 μm range. The significant depletion of iron oxide in coal led to an increase in the contents of Na–Fe silicate and Mg–Fe silicate in the GE gasification residues. The fractions of Mg–Fe silicate and Fe aluminosilicate increased significantly in the GSP gasification process. The low melting point and high volatility of alkali and alkaline earth metals in coal facilitated the formation of inorganic fine particles. The occurrence of inorganic fine particles in coal gasification residues was mainly controlled by the behavior of alkali and alkaline earth metals during the gasification processes, with finer inorganic particles having smaller particle sizes and better sphericity. Fe-bearing minerals, particularly Ca–Fe silicate, Fe aluminosilicate, and Fe oxide, were the main carriers in coal gasification residues. The bioavailability of Cd and Tl increased significantly in the fine particles within the coal gasification residues, whereas As, Cd, Tl, and Pb were less detectable, likely due to evaporation into the gas phase during gasification.