Influence Of Basic Oxides On Fluidity Of Coal Ash In Entrained Flow Gasification

The chemical composition of coal ash is an important parameter affecting the fluidity of slagging in entrained flow gasification.Basic oxides are important chemical components of coal ash and have long been using as fluxes in industry to improve the ash fusibility of highsilicon aluminum coal.Due to the diversity and difference of coal ash and slag system,along with the complexity of the influence of basic oxides on the crystallization behavior and viscosity change of high-temperature slag,a mature theoretical system is urgently needed to systematically explain the influence mechanism of basic oxides on the fluidity of coal ash and slag in the gasification condition.In this dissertation,devices and instruments such as ash fusion temperature determination meter,high temperature rotational viscometer,scanning electron microscopy coupled with energy dispersive spectrometry system,high-temperature stage coupled with optical microscope system,thermogravimetric analyzer,laser micro-Raman spectrometer,Fourier transform infrared spectrometer and FactSage thermodynamic simulation software are employed,the influence of CaO/MgO ratio,Fe content and Fe2O3/(CaO+MgO)ratio on the fusibility and viscosity-temperature characteristics of coal ash slag are systematically investigated based on mineral transition,slag structure and crystallization behavior.This dissertation would provide important reference for revealing the influence mechanism of basic oxides on the fluidity of coal ash slag in entrained flow gasification and promoting the gasification application of raw materials with high basic oxides contents.The main research contents and conclusions are as below:(1)The typical high silicon-aluminum coal and two kinds of biomass rich in basic oxides(rice straw and cotton straw)along with their leachates were chosen as samples.The feasibility of using biomass leachate to regulate the fusibility of coal ash was investigated,and the influence mechanism of basic oxides on coal ash fusibility was revealed.The results show that the addition of biomass or biomass leachate can reduce the ash fusion temperatures(AFTs),which is mainly owing to the increase of basic oxides and the formation of low-meltingtemperatures eutectic.Only water-soluble alkali and alkaline earth metals(AAEM)are contained in the biomass leachate,which means that the biomass leachate has stronger effects on regulating the AFTs of high silicon-aluminum coal ash than biomass.The content of watersoluble AAEM in biomass and the volatilization of alkali metal elements during ashing process are the main factors affecting the flux efficiency of biomass leachate.Moreover,rice straw leachate with a higher content of AAEM has a greater effect on the ash fusibility of high siliconaluminum coal than cotton straw leachate.(2)The influence of alkali earth metal oxides on the fluidity of coal ash was studied by preparing synthetic slags from pure oxides,and the synergistic and substitutive effects of Ca and Mg in coal ash slag system were investigated.The results show that the main minerals are found to be lime,anorthite and calcium silicate in slags with high CaO/MgO ratios,while periclase,forsterite and Mg-Al spinel are observed in slags with low CaO/MgO ratios.The existence of these minerals results in high AFTs of sample slags with both high and low CaO/MgO ratios.Therefore,the flow temperature(FT)of sample slags decreased first and then increased with the decrease of CaO/MgO ratio.When the sample slag had a high or low CaO/MgO ratio,or when the contents of CaO and MgO in the sample slag were similar,the homogeneous slag showed a lower viscosity.During cooling,sample slags with high or low CaO/MgO ratio show obvious tendency towards crystallization,and the formation of crystals will lead to a sharp increase in slag viscosity,so that the temperature of critical viscosity(Tcv)of the slags decreases first and then increases with the decrease of CaO/MgO ratio.The crystallization tendency of Ca is stronger than that of Mg.Anorthite precipitated from sample slags with high CaO/MgO ratio,while orthopyroxene precipitated from sample slags with low CaO/MgO ratio.(3)The influence mechanism of iron on the fluidity of synthetic slags was studied.The influence of iron content on the crystallization and viscosity transformation of slag was investigated based on the interface crystallization,temperature and operating atmosphere.The results show that the increase of iron content promotes the formation of low-meltingtemperatures eutectic to reduce the AFTs of slags,and increases the Fe2+content to decrease the viscosity of homogeneous slag.On the other hand,with the increase of iron content,Al changes from tetracoordinate[AlO4]5-to hexacoordinate[AlO6]9-,and Al3+,Si4+and Ca2+in melt network are replaced by Fe as amphoteric substance,which decreases the stability and polymerization degree of melt network,and lead to crystallization precipitation and growth of anorthite and gehlenite.Consequently,a sharp increase occurs in slag viscosity and the sample slag changes from glassy slag type to crystaline slag type.(4)Fe2O3,CaO and MgO,as three main basic oxides in coal ash were chosen as research objects to investigate synergies of basic oxides on the crystallization behavior and slag type conversion of ash slag by preparing synthetic slags of SiO2-Al2O3-Fe2O3-CaO-MgO slag system.The results show that with the decrease of Fe2O3/(CaO+MgO)ratio,the Tcv of samples decreases first and then increases under various CaO/MgO ratios.When the slag has a high Fe2O3/(CaO+MgO)ratio,the iron content in the slag was high.Iron can not only promote the formation and growth of anorthite,but also combine with Mg to form spinel.Therefore,the Tcv difference between the slags with high CaO/MgO ratio and slags with low CaO/MgO ratio is negligible under a high Fe2O3/(CaO+MgO)ratio.When the slag has a low Fe2O3/(CaO+MgO)ratio,due to the stronger crystallization tendency of Ca than Mg in the slags,slags with high CaO/MgO ratio have a higher Tcv and show more obvious characteristics of crystalline slag than the slags with low CaO/MgO ratio.Furthermore,based on the research conclusions,two Tcv prediction models were established by combining chemical composition of slag and thermodynamic calculation of FactSage software.(5)Based on the study of the influence of basic oxides on the fluidity of ash slag in this dissertation,the high-temperature fusibility and viscosity change mechanism of high-iron liquefied solid products and high-cacium Hami lignite co-gasification slags were investigated,and a reasonable blending scheme was proposed to realize the gasification application of liquefied solid products.The results show that the slag fluidity of the liquefied solid product and Hami lignite is poor and they can not be applied in entrained flow gasification alone.On the one hand,adding an appropriate amount of Hami lignite to the liquefied solid product can react with high-melting-temperature SiO2 in the liquefied solid product to form low-meltingtemperature eutectic so as to reduce the AFTs.On the other hand,a large amount of anorthite generated during cooling can be transformed into very liitle spinel,so as to decrease Tcv of the slags and change the slag type into glass slag with gentle viscosity growth.Futhermore,the size of crystals generated during cooling was inversely proportional to homogeneous slag viscosity.High viscosity makes ion migration in the slag become difficult and therefore inhibit crystal formation and growth.