Ash Behaviour Of Huainan Coal In Reducing Conditions

To solve the ash related problems of Huainan coal during the commercial gasification process,it is essential and significantly important to know the proportion and particle size distribution of mineral matter in coal and to understand and predict the mechanism of mineral transformation and the ash behaviour in reducing conditions.Understanding the properties of mineral and ash behaviour of Huainan coals will help in providing a better understanding of the factors that control their performance in gasifier and a clean,effective and economic way to use Huainan coal resources.Huainan coal samples and three other coal samples from Shangdong, Henan,Gansu province of China,were analyzed by Computer-controlled scanning electron microscope(CCSEM),X-ray diffraction(XRD) and X-ray fluorescence (XRF) for their mineralogical phases,particle size distribution and elemental composition respectively.Effect of coal blending and flux addition on ash fusion temperature(AFT) and slag viscosity were studied.The mineral transformation and ash behaviour were characterized by XRD,Fourier transform infrared spectroscopy (FTIR) and CCSEM coupled with a modified ash fusion tester.The liquid phase formation,mineral phase change and conversion were calculated by FactSage thermodynamics software.The main conclusions are shown as below.1.The mineral composition and particle size distribution in Huainan coal samplesThe mineral phases,which vary with coal samples somewhat,generally include quartz,hematite,anhydrite,muscovite and anatase,implying the complicated behaviour of ashes though they were collected from Huainan coal deposits.The mineral matter was characterized by higher aluminosilicate clay minerals contents (more than 60%of the total mineral matter in coal) with quartz,which accounts for the higher ash flow temperatures(FT),frequently higher than1500℃.It is suggested that the higher the kaolinite in Huainan coals,the higher the AFT.Comparisons of the mineral contents of Huainan coal with that of G3,B1,and H coals(FT＜1350℃) showed the mineral phases are similar while the mineral contents are quite different. Low content of kaolinite-type clays and high content of calcite,dolomite and pyrite in coals are beneficial to ash melting.The ash melting behavior of Huainan coal in reducing condition is to be controlled by the mineral phases,mainly by that of kaolinite,illite,pyrite and calcite,rather than by the average chemical composition of coal ash.The mineral grains in Huainan coals have bimodal size distribution in six typical Huainan coals as a result of the high contents of kaolinite and quartz show the bimodal particle size distribution.The diameter of mineral grains reaches peak maximum at about 10μm and 100μm respectively.The particle size of the montmorillonite,quartz and pyrite is in a single size unimodal distribution.The minerals composition and grains size distribution are extremely important and have an effect on the ash melting behavior,minerals transformation and fly ash deposit in the coal gasification process.2.Effects of fluxes and coal blending on the ash fusion temperatureThe AFT of four Huainan coal samples,HN113,HN115,HN119 and KL1,could be reduced at certain degree by adding fluxes of ADN,ADC,ADF(wt%,ash base). The fluxing action rank for Huainan coal samples was listed as ADN＞ADF＞ADC. The addition of CaO at around 8%increased the melting temperature of HN115 and HN119 coal ashes somewhat.With the increase of its addition,the melting temperature of ash initially decreased gradually,reaching its minimum value at the amount of CaO around 25%,and however increasing again with the further addition of CaO,indicating the complicated behaviors of mineral phase change.The flux of ADN had the most significant effect on the AFT of four Huainan coal samples, especially for KL1 coal samples.The FT of KL1 coal ash could be reduced to 1380℃with the addition of 4.55%ADN,which fulfils the requirement of Texaco gasification process.With the addition of ADF,the ash fusion temperature of four coal samples showed a linear decrease tendency.The FT of Huainan coal ash samples could be decreased to less than 1380℃by blending with G3,B1 and H coals respectively.The effect for lowering the ash fusion temperature of coal blends was listed as the sequence B1＞H＞G3.With the exception of HN106 coal blended with B1,the ash flow temperature of blended coal showed linear decrease with the increase of blending ratio of G3,B1 and H coal.The mathematical model of ash FT with blending ratio and the adding amount of fluxes were established by the principle of multiple linear regressions respectively.3.Effects of fluxes and coal blending on the viscosity-temperature properties of Huainan coal ashThe slag viscosity of HN119,KL1 coal increased sharply with the decline of temperature,which implies a crystalline slag(short slag) at high temperature and is not suitable to be used in Texaco and Shell gasifier.The slag viscosity of G3 and B1 coal is much less than that of Huainan coal ash at the same temperature.The slag viscosity-temperature plot showed a typical glass slag(long slag) for G3 coal and a glass like slag for Blcoal,which is suitable for the operation of Texaco and Shell gasifier.The slag viscosity of Huainan coal could be effectively decreased by coal blending and flux addition.The slag viscosity of HN119 coal by adding 8.7%ADN (ash base,wt%) reached to 20Pa·s at 1380℃.Meanwhile the slag type changes from crystalline slag to glass like slag.The slag viscosity of 60%KL1 coal blended with 40%B1 coal reached to 25Pa·s at the temperature less 250℃than that of the parent KL1 coal.In the mean time,the slag type changes to glass like slag type.The slag viscosity was calculated through three empirical formulas for coal blending and fluxes addition.The fitness between the results calculated by empirical formulas and the tested results was also discussed.4.Ash behaviour and melting mechanism of Huainan coal at high temperatureThe minerals transformation and ash behaviour were characterized by XRD and FTIR.The results showed that the main components at 1350℃in the slag of Huainan coal were mullite and glass phase.The formed mullite phase,which caused the high ash melting temperature of Huainan coals,reached to maximum at 1250℃,but showed a tendency of slow decline when the temperature was over 1250℃. Differences in peak intensity of mullite and anorthite reflected differences in phase concentration of the quenched slag fractions,which contributed to the differences of ash melting temperature.The intensity of mullite formed at high temperature was declined by blended with low AFT coals.Meanwhile the formed anorthite(CaO·Al₂O₃·2SiO₂),hercynite (FeO·Al₂O₃) and hematite phases reacted with quartz to form a low-melting eutectic mixture.The structure of main aluminosilicate phases in Huainan coals was broken at high temperature by adding flux of ADC,ADN and ADF under reducing atmospheres. The formation of mullite was restrained while albite,anorthite,hercynite phases increased gradually and reacted with quartz to form a low-melting eutectic mixture which lowered the AFT of Huainan coals.5.Prediction of Ash behaviour of Huainan coal by FactSage Thermodynamics softwareFactSage has been an invaluable tool to predict the ash behavior and AFT at high temperature under reducing atmosphere.It had demonstrated a good agreement between the liquid phase formation of Huainan coal ashes as the function of temperature and the tested AFT.The predicted FT,at which the calculated weight percentage of liquid phase reaches to about 75～80%for Huainan coal ash samples, was quite fit for the tested FT.The maximum temperature difference between the tested FT and the predicted FT is less than 74℃,which is within acceptable range.It could be concluded that FactSage in combination with XRD is a new tool to predict the reactions occurring between minerals,as well as the mineral transformation and slag formation.This is probably an improved way to interpret melting properties of mineral matter in coal and assist in quantifying slag formation in gasifier operation.In a word,this paper will provide the fundamental knowledge of Huainan coal for the purpose of utilizing Huainan coal in a clean and efficiency way.It will help us to better understand the ash behaviour of coal in reducing condition and solve the ash related problems in coal utilization process.