| The development of next generation of coal fired power stations using integraed gasification-combined cycle addressed a number of problems concerning the increase of emissions of greenhouse gases, sulphur dioxide, nitrogen oxides and particulates from pulverised coal fired power stations. In entrained flow gasifiers the coal ash should be liquid and fluid enough to be tapped. The fusiblity, flow charactersitics and rheological properites of the molten coal ash are, therefore, very important factors affecting the gasification regimes and operating costs. It is, therefore, important to study the properties of coal ash at ultra high temperature for selection of coals, flux additions and coal blending to optimise industrial operations, and to reduce operating costs. In this work, we studied the fusibility, flow characteristic and rheological properties of various ashes derived from fifty-three Chinese coal ash at the ultra high temperature. The Chemical analysis, mineralogical analysis, microstructural analysis, and the other physical properties at high temperature were ananlzed with X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), fusion temperature furance, the high temperature rotational viscometer, the high temperature rheometer, and thermodynamic software package FactSage. The effect of chemical composition on ash fusion temperature, the temperature of critical viscosity, and liquidus temperature of coal ash samples were systematically studied. The main contents are listed as follows:(1) The ash fusion temperatures (AFTs) of coal mineral matter at high temperature are important parameters for all gasifiers. Experiments have been conducted in which mixtures of selected coal ashes and SiO2, Al2O3, CaO, Fe2O3, MgO were subjected to the standard test for ash fusibility. The computer software package FactSage has been used to calculate the liquidus temperatures of coal ash samples and the proportions of the various phases present as a function of temperature. The results show that the AFTs of coal ash samples first decrease with increasing CaO, Fe2O3, and MgO contents, then reach a minimum value, before increasing again. However, for the effect of S/A ratio, its AFTs are always increased with increasing S/A ratios. The measured AFTs all show variations with mixture composition that correlated closely with liquidus temperatures for the appropriate pseudo-ternary phase diagrams. The liquidus and AFTs generally showed parallel compositional trends, but are displaced from each other because of the influence of additional basic components in the coal ash. The liquidus temperatures of coal ash samples are always higher than its AFTs.(2) The ash fusion temperatures of twenty-one typical Chinese coal ash samples were measured in Ar and H2 atmospheres. Since the iron oxides in coal ash samples fused under a H2 atmosphere are reduced to metallic iron, and lead to changes of mineral species and micro-morphology, the AFTs in a H2 atmosphere are always higher than those with an Ar atmosphere.(3) The computer software package FactSage was used to calculate the temperatures corresponding to different proportions of liquid phase and predict phase the equilibria of and sixty synthetic ash samples. Empirical liquidus models were derived to correlate the AFTs under both Ar and H2 atmospheres of sixty synthetic ash samples with their liquidus temperatures calculated by FactSage. These models were used to predict the AFTs of twenty-one Chinese coal ash samples in Ar and H2 atmospheres and then the AFT differences between the atmospheres was analyzed. The results show that for both atmospheres, there was an apparently linear correlation and good agreement between the AFTs of synthetic ash samples and the liquidus temperatures calculated by FactSage (R> 0.89,?<30?). These models predict the AFTs of coal ash samples with a high level of accuracy (SE<30?).(4) The viscosity and the temperature of critical viscosity values of eighteen synthetic slag samples formed from mixtures of five oxides (SiO2, Al2O3, CaO, Fe2O3, and MgO) have been measured. Moreover, the effects of these oxides on the temperature of critical viscosity values and the microstructure seen in metallurgical microscopy of synthetic slag samples have been analyzed. The computer software package FactSage has been used to calculate the temperatures corresponding to different proportions of liquid phase of synthetic slag samples. The results show that the temperature of critical viscosity values of synthetic slag samples decrease with increasing CaO, Fe2O3 and MgO contents and with increasing SiO2/Al2O3 ratio (named S/A ratio), then reach a minimum value, before increasing again. These curves are similar to the variations in the temperatures corresponding to liquid phase contents> 80.00 wt.% seen with increasing contents of these oxides and with an increase in the S/A ratio. Moreover, most of the temperature of critical viscosity values of the synthetic slag samples lie between the temperatures corresponding to liquid phase contents of 80.00 wt.% and 100.00 wt.%, respectively.(5) The temperatures of critical viscosity of eight coal ash samples and forty synthetic ash samples have been measured. The computer software package FactSage has been used to calculate the temperatures corresponding to different proportions of liquid phase of synthetic ash samples. Empirical liquidus model has been derived to correlate the temperature of critical viscosity of the forty synthetic ash samples with their liquidus temperatures calculated by FactSage. The liquidus model was then used to predict the temperature of critical viscosity of the eight coal ash samples. The results show that there was an apparently linear correlation and good agreement between the temperature of critical viscosity of the synthetic ash samples and the liquidus temperatures calculated by FactSage (R> 0.900,?< 30?). This model predicts the temperature of critical viscosity of coal ash samples with a high level of accuracy. Meanwhile, the temperature of critical viscosity is a result of many complex factors; hence, the adequacy of liquidus model in the paper is limited by a range of conditions.(6) The viscosities of forty-five coal ash samples at high temperature have been measured under different temperatures and shear rates. The computer thermodynamic software package FactSage has been used to predict liquidus temperatures, volume fractions of crystallized solid particles (?), and the compositions of remaining liquid phase for 45 coal ash samples. The flow properties of completely liquid and partly crystallized coal ash samples have been predicted by three viscosity models. The Urbain formalism has been modified to describe the viscosities of fully liquid slag and homogeneous remaining liquid phase in coal ash samples. The modified Einstein equation and Einstein-Roscoe equation have been used to describe the viscosities of heterogeneous coal ash samples of?<10.00 vol.% and??10.00 vol.%, respectively. These three models provided a good description of the experimental data of fully liquid and heterogeneous coal ashes samples. The new models also predicted flow properties of mixtures of coal ashes with CaO, Fe2O3, MgO, SiO2, and Al2O3.(7) The rheological characteristic of two coal ash from entrained gasifiers at high temperature was studied. Slag samples have been analyzed by X-ray fluorescence, X-ray diffraction, and scanning electron microscopy. The rheological behavior of the slag has been investigated experimentally using a high-temperature rheometer at temperatures. The effects of the shear rate and temperature on the rheological behavior of the slags have been explored. Moreover, the observed rheological behavior of the slag has been correlated with its solid-phase content, as calculated with the aid of the computer software package FactSage. The results show that the sensitivity of the slag viscosity to temperature decreases with increasing rotation speed. Above its liquidus temperature calculated by FactSage, the slags behave as a Newtonian fluid; below its liquidus temperature, however, the rheological behavior of the slags becomes non-Newtonian owing to its increased solid-phase content. Meanwhile, Slags containing a number of crystalline particles shows dramatic shear-thinning and thixotropic behavior. Moreover, the shear-thinning behavior of the slags becomes ever more distinct as the temperature is decreased. The yield stress values of the slag and the number and particle size of the crystalline particles in the slag increase with decreasing temperature.(8) The physical properties of ash and slag were analyzed with X-ray fluorescence, X-ray diffraction and scanning electron microscopy. The fusion temperature and the experiment viscosity were measured for the ash and slag with the addition of fluxing CaO. Ash and slag have properties that were approximated by the SiO2-Al2O3-FeO-CaO system. The computer software package FactSage was used to predict the proportion of solid phase and the mineral phase formed as a function of the composition and the temperature of the SiO2-Al2O3-FeO-CaO system. The results show that the fusion temperatures and the temperature of critical viscosity (Tcv) of ash are always higher than that of slag. Also, the viscosity of ash is always higher than that of slag at the slag tapping temperature range of 1400-1500?, and the hysteresis between the heating and cooling cycles for ash is more obvious than that of slag because of different physical properties. The fusion temperature and Tcv of ash and slag decrease with increasing CaO content, whereas those values increase rapidly with CaO content higher than 35%. Also, the sensitivity of the viscosity of the ash and slag with temperature decreases with increasing CaO content because the sensitivity of the phase equilibria of in the SiO2-Al2O3-FeO-CaO system to temperature excursions decreases with increasing CaO content. |
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