Study On The Occurrence And Deposition Behavior Of Slagging Minerals In Xinjiang High-alkali Coal

Xinjiang high-alkali coals have many excellent characteristics,such as abundant reserves,high quality and low price.However,serious fouling,slagging and corrosion can occur in the combustion utilization of high-alkali coal because of the high content of alkali metal?Na?,alkaline earth metal?Ca?and transition metal?Fe?,which may threaten the safety and operation of the boiler.In this work,the occurrence,volatilization,deposition,melting and reaction of slagging minerals of typical Xinjiang coals were investigated by combining thermodynamic calculation and experimental methods,including high temperature calcination experiments and advanced analytical methods.The content and occurrence mode of slagging elements in high-alkali coal were comprehensively analyzed by means of sequential chemical extraction,floatation method,XRD,ICP-MS and XPS.The results show that the high-alkali coals possess a considerable amount of sodium while the potassium content is low.Overall,more than 50%of Na is water-soluble,including hydrated ionic state and discrete sodium nitrate,while insoluble sodium is mainly albite;K is mainly distributed in muscovite and illite;Ca mainly exists in the forms of calcite and gypsum;magnesium mainly exists in the forms of dolomite,magnesite and carboxylate;Fe is mainly composed of siderite and a small amount of pyrite;most of the sulfur is organic,in the form of sulfone?sulfoxide?type;Cl mainly exists in the form of hydrated ions in pore water.According to the fixed bed ashing experiment and the multiphase equilibrium thermodynamic analysis,the migration,transformation,deposition behaviors,together with the heterogeneous reaction between varying minerals were investigated during coal combustion.In addition,the adsorption experiment of kaolin on Na was carried out in a fixed bed.The results indicate that CaSO₄ and NaCl deposits probably generate when burning the coal of high content of Na and Cl with high ash melting point;N_a2SO₄ and CaSO₄ deposits mainly occur in high-sodium low-chlorine coals;for high sodium,high silicon and aluminium coals,the slag phase is formed by alkali metal eutectic at high temperature,and slagging is mainly caused by fused coal ash with low melting point;Fe-bearing minerals in high-sodium high-iron coals can produce augite,diopside,fayalite and other flux minerals,which can lower ash melting point and cause slagging.Blending kaolin can reduce the content of vapor phase Na,especially for low-chlorine coals,and a low temperature can favor the adsorption efficiency.The high-temperature melting and heterogeneous reaction process of deposits were investigated by pure mineral reagents simulation combined with high temperature calcination experiments and XRD,SEM-EDS and other characterization methods.The results indicate that the melting process of deposits undergoes four stages:liquid formation,liquid sintering,mass melting and free liquid,in which mass melting is mainly caused by co-melting of Ca-bearing minerals.Na₂SO₄ can be converted to aluminosilicates,which can easily form either eutectics?such as nepheline,albite and hauyne?or sulfate double salts.CaSO₄ is mainly converted to larnite with high melting point,and thus increase the melting temperature of deposits.Notably,Na₂SO₄ can react with CaSO₄ to form(Na_0.8Ca_0.1)₂SO₄ at low temperature,which can evolves to more stable Na₄Ca?SO₄?₃ at high temperature.Moreover,the reactions between Na₂SO₄ and Si-containing,Al-containing minerals can be inhibited when CaSO₄ is excessive.