The Change Law Of Ash Chemical Compositions And Its Effect On The Coal Ash Fusibility During Combustion

Ash fusibility has been one of wide and persistent research interests in order to predict ash deposition in coal combustion.However,the traditional ash fusion temperature results were frequently reported to be in poor agreement with practice.The paper studied the distribution of chemical and mineral compositions in the different density coal-ash samples and their change laws with ashing-temperatures,and influence laws on ash fusibility,combining characteristics of the ash agglomerating fluid bed,to put the slag build-up theory.Two typical Chinese coals were pulverized and separated into different density fractions with the float-sink test.The coals were then combusted in the muffle furnace and box-type furnace in air at 450 ^oC,815 ^oC,1000 ^oC,1300 ^oC and 1500 ^oC.X-ray fluorescence?XRF?and X-ray diffraction?XRD?were employed to study the change in chemical and mineral composition among the density fractions.Ash fusion temperature?AFT?test,pressure-drop temperatures?PDT?test,and scanning electron microscopy?SEM?were employed to study the ash melting behavior of the two whole coal samples and their density fractions.This paper combined STA with XRF and XRD to study the mechanism of the evaporation and evolution of the minerals.Mechanism study showed the segregation of chemical compositions,mainly SiO₂,Al₂O₃,Fe₂O₃,Na₂O and CaO,among samples of different densities caused the differences of melting and sintering behaviors.In the process of incessantly raising the ash temperature of coal samples,the rich SiO₂ and Al₂O₃ in Datong coal samples were transformed into melt-resistant minerals such as kaolin and mullite.The rich SiO₂ and Al₂O₃ in the coal samples of Zhundong were transformed into the form of fusible albite and calc-aluminopyrite,which is the reason that the melting temperature of Datong pulverized coal is higher than that of Zhundong coal ash.In addition,owing to the higher alkali and alkali earth contents in the forms of fluxing minerals,the sintering and fusion temperatures of the ash samples at 450 ^oC were correspondingly lower than those of the ashes at 815 ^oC.There are more Na,K or Na₂O and K₂O vapors in the vaporized flue gas of Zhundong coal ash,which can capture the components of SO₂/SO₃,CaO and Fe₂O₃,and form low melting and high viscosity substances.It is the reason that the coal of Zhundong is easy to slag on the water wall.The higher the tendency of coal ash deposition is,the greater the mineral evaporation and the total mineral evaporation are formed by alkali oxides such as Fe,K,Na,which affect slagging and fouling,and vice versa.