Abstract Plasma underground coal gasification (PUCG) facilitates targeted carbon reduction reactions, tackling CO 2 emissions and effectively generating syngas that is rich in H 2 and CO. The mineralogical properties are essential for determining gasification parameters, managing production, and assessing environmental stability. PUCG operates under high temperatures and a strong reducing atmosphere, which significantly affects mineral transformation, yet this field is still not extensively studied. In this work, PUCG ash slag from various coal types and ash compositions was created. The study investigated mineral transformation, melting behavior, and crystallization characteristics using X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and FactSage software. Findings reveal that the syngas concentration during PUCG can reach to 88% (volume fraction), with peak temperatures ranging from 3806 to 6569 K and average temperatures between 1391 and 1935 K. In high-silica-alumina coal ash slag, the predominant mineral is mullite, while high-calcium/iron coal ash slag contains gehlenite, hedenbergite, and wüstite. The strong reducing atmosphere results in a Fe + Fe 2+ /ΣFe ratio to ~ 1.0, compared to < 0.5 under weak reducing conditions, causing a 103 K difference between the complete melting temperature (CMT) of high-iron coal and the flow temperature (FT) under weak reducing conditions. An excess of SiO 2 in high-silica-alumina coal hinders the melting of mullite, resulting in a deviation of up to 340 K between CMT and FT. The minerals in high-calcium/iron coal undergo decomposition, transformation, complete melting, and recrystallization. Elemental iron serves as a stable nucleation site, while high energy density provides the necessary temperature and duration for molten minerals recrystallization. Graphical Abstract
Qi et al. (Wed,) studied this question.