Coupling Interaction Mechanism Of Calcium And Magnesium On Coal Ash-fusibility In Reducing Atmosphere

The three-high-ash-fusion-temperature coals with different Si/Al ratio were selected to study the effect of coupling interaction of calcium and magnesium on decreasing coal ash fusion temperature by adding CaO.MgO and their compound flux under reducing atmosphere. The coal ash fusion temperature and coal ash fusion process were detected by intelligent ash-melting point measuring device and TG-DTA respactively. Meanwhile, the crystalline mineral composition, surface micrograph and micro-chemical composition, phase equilibrium composition of coal slag added with compound flux were invested by XRD, SEM-EDX and thermodynamics software, respactively. The coupling interaction forms of calcium and magnesium on mineral transformation expression and the coupling interaction mechanism of calcium and magnesium on coal ash-fusibility in reducing atmosphere were concluded. The main results were shown as follows:(1)The effects of calcium and magnesium compound flux on decreasing ash fusion temperature were better than the single flux CaO and MgO for different Si/Al ratio coal samples. With the proportion of CaO increasing in compound flux, the ash fusion temperatures of BLH and MDL are coming down. The decreasing AFTs trend curves of PC coal with the addition of CaO and MgO are nonlinear. When the addition dosage of flux is 6% and the mass ratio of CaO and MgO is 5:5,the flow temperature of PC coal ash could be reduced to 1297℃. which is less 69℃ and 110℃ than that of CaO and MgO supplementation alone respectively. Compound proportion of CaO and MgO that reflected notable coupling effect on decreasing ash fusion temperature changes with the amount of compound flux.(2)In the heating process, the original Si-O covalent bond was destroyed by the combined action of Ca2+ and Mg2+. The ionic bonds of O--Ca2 +-O- and O--Mg2+-O-are obtained by bonding Ca2+,Mg2+ and non-bridge oxygen ions O", which connect the broken SiO2 chains together. Ca2+ and Mg2+ have been involved in mineral transformation which leading to the framework silicate and phyllosilicate minerals being transformed into calcareous and magnesian aluminosilicate minerals with the structure of chain, group and island. The low temperature eutectic reaction generated between anorthite, wollastonite,forsterite, magnesium cordierite, enstatite and diopside, and then the ash mainly existed in the form of amorphous melt at high temperature, XRD spectrums show bread peak, coal ash fusion temperature decreasing significantly.(3) The forms of coupling interaction of calcium and magnesium on mineral transformation expression in low-Si/Al ratio LBH coal and middle-Si/Al ratio PC coal have two kinds, one is that coupling interaction between magnesium cordierite, anorthite and forsterite, and another is that coupling interaction between anorthite and forsterite. The reason calcium and magnesium do not exhibits coupling interaction on decreasing LBH coal ash fusion temperature is that a large number of sipnel were formed in coal ash slag. High content of Al2O3 in the BLH coal ash leading to coal ash fusion temperature controlled by the amount of spinel, and LBH coal ash fusion temperature remains high after adding compound flux. The low temperature eutectic reaction of anorthite and magnesium olivine is the main manifestation of the coupling interaction and their optimal proportion was 3.16 in PC coal.(4) The forms of coupling interaction of calcium and magnesium on mineral transformation expression in high-Si/Al ratio DML coal have three kinds. One is the formation of diopside, one is that coupling interaction between diopside, anorthite and enstatite, and the last is that coupling interaction between diopside, anorthite, and wollastonite. The formation of diopside is the main manifestation of the coupling interaction and then coupling interaction between diopside and calcareous, magnesian aluminosilicate minerals on decreasing coal ash fusion temperature remarkably. The amount of diopside raises with the CaO compound ratio increases, the DML coal ash fusion temperature decreased gradually.