Study On The Effect Of Na/Fe Compound Flux On The Melt Structure Of High Ash Melting Point Coal

The high ash fusion temperature coal is abundant in China accounting for more than half of the coal storage and production.The coal ash has high ash fusion temperature and high slag viscosity lead to the difficult application in entrained-bed gasification.Therefore,the addition of flux to regulate coal ash can improve the ash chemical behaviour of high ash fusion temperature coals so that it can be effectively used in the coal gasification process.In this paper,two high ash fusion temperature coals,HN coal with high Si O₂ and ZGR coal with high Al₂O₃,were selected to study the effect of Na/Fe compound flux on the melting temperature of high ash fusion temperature coal by adding different proportions of Na/Fe compound flux.The changes of crystal mineral composition with the addition of flux was analyzed by XRD.The changes of mineral and phase equilibrium of coal ash at high temperature were calculated by Fact Sage simulation.The melt structure of coal ash and the element occurrence forms were studied by FT-IR and XPS,further discussing the mechanism on the change of the melt structure of high ash fusion temperature coal by flux,and the following conclusions were obtained:With the addition of different proportions of Na/Fe compound flux at 6%of coal base,the effect of flux on HN coal is significantly better than that of ZGR coal,and the fusion temperature can be reduced to 1225℃.The refractory minerals of HN coal ash are mainly mullite,and the refractory minerals of ZGR coal ash are mainly mullite and corundum at high temperature.By adding Na/Fe compound flux,Na₂O will react with Si O₂ and Al₂O₃ inherent in coal ash to convert quartz into low melting point mineral nepheline and prevent the formation of mullite.The hematite will first be converted into spinel,and then form low melting point minerals such as fayalite,almandine and sekaninaite,thus reducing the fusion temperature of coal ash.Na elements are mainly endowed in nepheline and sodium feldspar;Fe-based minerals are mainly spinel,cordierite,olivine,etc.The results of Fact Sage simulations showed that the liquid phase production of coal ash increased significantly after the addition of fluxes,and some corundum were still present in ZGR coal ash at high temperatures in agreement with the results of XRD analysis.The silicate structure in HN and ZGR coal ash is mainly shelf-like,followed by lamellar,and the shelf-like structure is initially transformed into lamellar with only a small amount of depolymerization into chain-like structural units when the temperature rises;after adding flux,the shelf-like structural units in coal ash are obviously reduced,and a large amount of depolymerization forms lamellar and chain-like silica-oxygen backbone,and more dimeric and monomer appear in the melt.The maximum R value at higher Na content in the flux corresponds to the lowest degree of polymerization of silicate network structure,when a large amount of Na⁺becomes the modifying component of the silicate network structure,destroying the bridge oxygen bond in the melt and generating more non-bridge oxygen bonds,thus reducing the degree of polymerization of the melt.The destructive effect of Na on the network structure is stronger than Fe for high-silica-alumina coal species.The Si in the melt of coal ash exists in the form of bridging oxygen silicon and non-bridging oxygen silicon,the Al exists in the form of[Al O₄]tetrahedra and[Al O₆]octahedra,the O consists of non-bridging oxygen,bridging oxygen and inorganic oxygen in three states,the quartz in the original coal ash is transformed into mullite as the temperature rises,forming a double chain with the structure of silica oxygen tetrahedra and aluminum oxygen tetrahedra,and the double chain is connected by aluminum oxygen octahedra;the addition of After adding flux Na⁺will combine with[Al O₄]tetrahedral structure to form nepheline,Fe²⁺will fill in the tetrahedral gaps to form spinel structure,Al³⁺exists in the form of six-coordinated octahedra,Na⁺and Fe²⁺effectively promote the breakage of bridging oxygen bonds in the melt,leading to an obvious increase in non-bridging oxygen bonds and the deconcentration of the silicate network-like structure of the melt,thus reducing the melting temperature of coal ash.Figure[63]Table[18]Reference[79]...