Effect Of Alkaline Earth Metal Oxides On The Ash Fusibility Of Zhundong Coal

The high contents of alkali metal and alkaline earth metal in Zhundong coal ash induce severe slagging problems in boilers' heating surfaces durning coal combustion.The slagging of coal ash has important relationship with coal ash fusibility.In order to study the effect of alkaline earth metal oxides on the ash fusibility of Zhundong coal,high sodium synthetic coal ash samples with different CaO and MgO content were prepared,and the ash fusion temperatures(AFTs)were measured.Thermodynamic database FactSage 7.0 was applied to simulate the ash melting process,the mineral conversion of high sodium synthetic coal ash samples with different CaO and MgO content was analyzed.The minerals' composition and surface morphology of high temperature ash samples were investigated by X-ray diffraction(XRD)and scanning electronic microscope(SEM).The results show that the ash fusion temperatures drop first and rise later with the increase of CaO content,the variation of the ash fusion temperatures is consistent with the variation of minerals' melting points in ash.When the CaO content increases from 5%to 25%,the ash fusion temperatures were reduced by the eutectics formed by calcium-containing minerals and sodium-containing minerals.When the CaO content was further increased to 50%,a large quantity of larnite with high metling point was generated,thus improving the ash fusion temperatures.The ash fusion temperatures drop first and rise later with the increase of MgO content,the variation of the ash fusion temperatures has important relationship with the variation of minerals in ash.When the MgO content increase from 0%to 5%,a large quantity of diopside with low melting point was generated in ash at high temperature.Diopside could form eutectics with minerals such as nepheline,thus reducing the ash fusion temperatures.When the MgO content was further increased to 30%,minerals with high melting point such as akermanite,forsterite and merwinite were generated in ash at high temperature,thus improving the ash fusion temperatures.The results of binary phase diagram and analogous ternary phase diagram indicate that the variation of liquidus temperature is consistent with the ash fusion temperatures with the increase of CaO and MgO content.The addition of CaO and MgO could suppress the generation of molten slag in ash,thus reducing ash deposition.Futher studies should be focused on the effect of alkaline earths metal oxides CaO and MgO on the ash fusibility of Zhundong coal under different reaction atmospheres,to develop an effective addtive to suppress the slagging problems of Zhundong coal.