Study On The Fusibility And Viscosity-Temperature Characteristics Of Typical Coal Ash And Blended Ash

The ash fusibility and viscosity-temperature characteristics at high temperature are important factors affecting the selection of gasification technology and operation parameter. It is necessary to investigate the ash properties at high temperature for solving the slagging problem, successful operation of gasifier, and the reduction of operating costs. The fusibility, viscosity-temperature characteristics and mineral composition of typical coal ash and blended ash were studied systematically in this paper. The physical properties of ash samples at high temperature were analysed by ash fusion determination meter, high-temperature rotational viscometer, heating stage microscope, X-ray diffraction and thermodynamic software FactSage. The influence of compositions on high-calcium coal ash properties and the mineral transformation at different temperatures were discussed. The flow behavior of high temperature ash and low temperature ash, the reason for the volume shrinkage and the process of slag formation were studied. The variation of viscosity, the temperature of critical viscosity and hysteresis with different straw content were investigated. The rheology of slag in reducing atmosphere was researched and the viscosity model was established. The main contents are as follows:1. The flowability study was conducted on the mixtures of high-calcium Shanxin coal and A12O3, SiO2, Na2O, K2O, TiO2. The results show that Al2O3, SiO2, Na2O, K2O and TiO2can reduce the fusion temperature of Shanxin coal and SiO2is the most effective. When the S/A(SiO2/Al2O3) and A/B is7.36and1.23, respectively, the flow temperature is192℃lower than the original value. The liquidus temperature, the minerals composition and the ternary phase diagram of the mixtures are obtained by FactSage. The effect of mineral matters on ash fusibility is illustrated. The viscosity-temperature characteristics of Shanxin coal and SiO2blended slag are improved. The temperature of critical viscosity of Shanxin blended slag is295℃lower than that of Shanxin slag. The lowest operation temperature of Shanxin blended slag is266℃lower than that of Shanxin slag. The blended slag is glassy type while the Shanxin slag has the crystalline type. Moreover, the operating flexibility of gasifier is enhanced. The mineral matters in slags are measured by XRD and the transformation of mineral matters at different temperature has been deduced. The particles in surface of blended slag are less than those of Shanxin slag and the flowability of blended slag is reinforced. In addition, the prediction of addition flux by FactSage is in good agreement with the experimental results.2. The flow behavior of high temperature ash (HTA) and low temperature ash (LTA) samples at high temperature were investigated by heating stage microscope. The volumes of Shenfu HTA and LTA samples shrink significantly in the heating process. Clinopyroxene forms in LTA and sodium feldspar forms in HTA in the first shrinkage. In the molten stage, LTA and HTA shrink with the SiO2as skeleton, and then the increase of molten matters enclose the refractory complex to form eutectic of augite and feldspar, respectively. The volumes of Yulin HTA and LTA samples show the same shrinkage behavior in the heating process. For the first shrinkage, gehlenite and srebrodolskite form in LTA and HTA samples. In the molten stage, the eutectic of akermarite and fassaite form in HTA and LTA samples. When the temperature increases further, the samples spread out and the volumes increase.3. The high-temperature flow behavior of biomass (straw) and coal (Guizhou coal) blended ash was studied. The variation of viscosity and the temperature of critical viscosity with different straw content were investigated. The fusion temperatures of straw ash and the Guizhou coal ash are higher than1350℃and a single feedstock of them is difficult for gasification. With the increased content of straw, the fusion temperature of blended samples decreases and then increases. The liquidus temperature calculated by FactSage and the measured ash fusion temperature show the parallel variation trend. The correlation between the liquidus temperature and flow temperature has been established. The20%straw content sample can effectively decrease the viscosity. Moreover, the temperature of critical viscosity and lowest operation temperature reach the minimum when the straw content is20%. The solid phase content and mineral matter variation calculated by FactSage demonstrates the change of viscosity. In addition, the network theory and variation of mineral matters detected by XRD well accounts for the change of viscosity. Hysteresis between heating and cooling process of the sample with20%straw content is more obvious than that of the samples with40%and80%straw content.4. The high-temperature rheology of Yulin slag in reducing atmosphere was investigated. The effects of temperature on the fluid model, the shear rate on the shear stress and thixotropy property of the slag were studied. The results show that the viscosity and shear stress of the slag increase with decreasing temperature at the constant shear rate. When the volume fraction of the crystallize particles is less than10%, the slag behaves as a Newtonian fluid. When the volume fraction of the crystallize particles is more than15%, the slag behaves as a non-Newtonian fluid. The non-Newtonian fluid behavior of slag is illustrated by shear thinning behavior and thixotropic behavior. In addition, the viscosity prediction model, base on S2model and Einstein-Roscoe model, has been established in the whole temperature range, where the R2is0.9947.