Study On Iron Precipitation Mechanism In High-sulfur Coal Gasification Slag

The bulk slag from the operational obstacles in Shell pulverized coal gasification process had good crystallinity, and crystal metallic iron particles and troilite were precipitated in some slag. The modes of occurrence of Ferrum in coal were quantitatively studied by ultrasound-assisted sequential chemical extraction experiment (USCEE). The high-sulfur coals were selected for studying transformation of iron-bearing minerals. Meanwhile, iron phase was added into ZX coal to investigate further iron-bearing minerals transformation. X-ray diffraction (XRD) and scanning electron microscope combined with energy dispersive X-ray (SEM-EDX) were employed to observe compositions and microstructure of the slag. The equilibrium composition of the slag was calculated by FactSage software. The precipitation pattern of elemental iron was proposed and the iron precipitation mechanism in slag from entrained-flow gasification was concluded.The ferric element mainly occurred in the carbonate state, Fe-Mn oxides state, sulfide state, sialic state and organism-bonded state in coals. The ferric element in ZX coal occurred predominately in sulfide state and sialic state. The ferric element in sulfide state transformed partly to hematite in ashing process. The ferric ion doped in aluminosilicate minerals was exposed and reduced subsequently forming elemental iron in part after kaolin group minerals disintegrated.There were iron and troilite in ZX coal slag in the reducing atmosphere such as V(CO):V(N2)= 160:40 at temperature up to 1000℃, and the content of iron and troilite decreased after an initial increase with rising temperature. At high temperature of 1450℃, the crystallinity of the mineral matters decreased, and the ferriferous component in slag was mainly iron-bearing glass and minority iron and troilite.When CO concentration in the reducing atmosphere is 25% or higher, the reaction temperature is 1200℃ or higher, the content of Fe2O3 in ash is 8.82% or more, elemental iron can be produced in high-temperature slag. In entrained-flow gasification atmosphere elemental iron may be precipitated in slag through the following four reaction paths:(1) The hematite in coal ash is directly reduced. (2) The ferric ion doped in aluminosilicate minerals is exposed in gasification atmosphere and reduced directly or indirectly after kaolin group mineral disintegrates. (3) Pyrite in coal transforms into pyrrhotite(FeSx), and a series of FeSx undergo structural transfer releasing gaseous sulfur and producing FeS. The previously generated FeS becomes liquid with increasing temperature and the liquid FeS decomposes to liquid metallic iron when the temperature is over 1200℃. (4) Partial molten Fe-O-S releases sulfur at high temperature producing wustite (FeO) and FeO can be easily reduced. The sulfur-containing intermediates (FeS, Fe-O-S) can be easily formed when the residual sulfur in coal ash is sufficient. The two intermediates can postpone iron precipitation.Iron precipitation in slag is closely related to the crystallization of anorthite. The enrichment of ferric ion due to its strong polarization effect can induce anorthite crystallization from the melted. With the improvement of anorthite crystal lattice, the ferric ion is separated from the crystal texture and reduced producing elemental iron. On the other hand, in reducing atmosphere some reduzate(Fe+) enter into interstices of anorthite lattice and cause isomorphism substitute for partial Ca2+ since the radius of Fe2+is smaller than that of the Ca2+. With anorthite melting, Fe2+ escapes the melted and is reduced forming elemental iron.