Recent research provided deep insight on the coal ash-to-slag transformation characterization during the entrained flow gasification process, with experimentation on a 40 kg/hr (dry basis) coal-fed opposed multi-burner (OMB) entrained flow gasifier and simulation via FactSage? software. A general mechanism is presented to relate the gasifier design temperature, ash fluid temperature, and operating temperature with the degree of the slag polymerization. The change of the high temperature zone, the corresponding particle residence time in the high temperature zone, and syngas composition have obvious effects on the slag mineral transformation behavior. Mineral types formed on the wall of the gasifier chamber were mainly anorthite (CaAl<
sub>
2<
/sub>
Si<
sub>
2<
/sub>
O<
sub>
8<
/sub>
), aluminum oxide (Al<
sub>
2<
/sub>
O<
sub>
3<
/sub>
), and calcium sulfide (CaS). These minerals transformed to anorthite and diopside (CaMgSi<
sub>
2<
/sub>
O<
sub>
6<
/sub>
) at the slag hole zone, while the minerals at the lock hopper were anorthite, orthoclase (KAlSi<
sub>
3<
/sub>
O<
sub>
8<
/sub>
), quartz (SiO<
sub>
2<
/sub>
), gypsum (CaSO<
sub>
4<
/sub>
), calcite (CaCO<
sub>
3<
/sub>
), and halite (NaCl). FactSage? predicted minerals as anorthite, diopside, orthoclase, and albite (NaAlSi<
sub>
3<
/sub>
O<
sub>
8<
/sub>
), etc., where the slag temperature was below the ash fluid temperature and when the ratios of CO/CO<
sub>
2<
/sub>
and (CO + H<
sub>
2<
/sub>
)/CO<
sub>
2<
/sub>
were lower than 1.0 and 2.0, respectively. By simulation, residual carbon was found to be the dominant factor over syngas composition to cause mineral transformation, and this was verified experimentally. The Ca-based crystals, typically anorthite, was shifted to diopside, near the slag hole zone, and a linear relationship was found between the content ratios of diopside/(anorthite + diopside), CaO/SiO<
sub>
2<
/sub>
, and (CaO + MgO)/SiO<
sub>
2<
/sub>
. A dimensionless number, ?, was defined to characterize the changing chemical composition and the degree of slag polymerization, with temperature deviation from the design condition. Three zones of ? were identified and related to the deviation between the actual gasification condition from the design condition. Finally, a low slag polymerization degree corresponded with a higher temperature deviation between the actual condition and design condition, and this proved that increased residual carbon content and changing iron valence state increased the mineral types when the slag temperature was below the ash fluid temperature.