Experimental Study On Melting Characteristics Of Fine Ash From Circulating Fluidized Bed Gasifier

Circulating fluidized bed gasification can operate at moderate temperatures,has high efficiency of heat and mass transfer and has a wider adaptability to a range of coal.However,the lower operating temperature can also result in the problem of high content of residual carbon in fine ash from coal gasification out of the tail of fluidized bed gasification boiler.New technology must be developed to dispose fine ash from circulating fluidized bed gasifier in order to improve carbon utilization and reduce the pollution to the environment.Using melting process not only can improve carbon utilization but also can effectively reduce ash volume by discharging fine ash in the form of dense glass slag.Therefore studing of fusion characteristics of fine ash will have important theoretical significance.In this study,work has been carried out to investigate the effect of atmosphere,carbon content and fluxes on fusion characteristics of two types of fine ash from industrial circulating fluidized bed gasifiers by using fusion point analyzer,muffle furnace,high-temperature graphite furnace,horizontal tube furnace,X-ray diffraction(XRD),scanning electron microscopy(SEM),and Superhigh temperature confocal microscope.The main conclusions are listed as follows:The effect of atmosphere on fusion characteristics of the fine ash from circulating fluidized bed gasifiers was studied.The results showed that the ash fusion temperatures of Suqian fine ash were much higher than that of Chiping fine ash.The presence of mullite at high temperature was the main reason leading to higher fusion temperatures of Suqian fine ash.The ash fusion temperatures of Suqian and Chiping fine ash under weak reducing atmosphere were lower than that under oxidizing atmosphere.The influence of atmosphere on the fusion temperatures of Suqian fine ash was bigger than that of Chiping fine ash.The formation of iron-bearing minerals which were easily to form low temperature eutectics with minerals such as anorthite and inhibited the formation of mullite under weak reducing atmosphere decreased fusion temperatures of fine ash.The effect of residual carbon on fusion characteristics of the fine ash from circulating fluidized bed gasifier was studied.The results showed that residual carbon had a significant influence on fusion characteristics of fine ash from industrial circulating fluidized bed gasifiers.Ash fusion temperatures of both fine ash increased with the increase of residual carbon content.When temperature was more than 1300? and the mass fraction of residual carbon of fine ash was above 3%,quartz reacted with carbon to form moissanite.When temperature was more than 1400?,mullite reacted with carbon to form moissanite and corundum.The existence of mullite and corundum caused the fusibility of fine ash from circulating fluidized bed gasifiers to become worse.The results of SEM and superhigh temperature confocal microscope were consistent with these conclusions.The effect of fluxes such as CaO and Fe2O3 on fusion characteristics of the fine ash from circulating fluidized bed gasifier was studied.The results showed that CaO and Fe2O3 can both effectively decrease the fusion temperatures of fine ash,and the effect of Fe2O3 was better.In this study,the fusion temperatures of fine ash decreased first and then increased with the increase of CaO content.When the mass fraction of CaO was 20%,the fusion temperatures of fine ash were lowest.With the increase of Fe2O3,the fusion temperatures of fine ash decreased monotonously.After adding CaO to Chiping fine ash,the main minerals in slag contained anorthite,wollastonite and gehlenite.These minerals transformed into eutectic which could decrease fusion temperatures of Chiping fine ash.Adding CaO to Suqian fine ash,the mullite in slag transformed into low fusion temperature minerals such as anorthite and gehlenite which could reduce fusion temperatures of Suqian fine ash.When the mass fraction of CaO was above 20%,a large amount of gehlenite was left in slag which increased the fusion temperatures of fine ash.After adding Fe2O3 to Chiping fine ash,the main minerals in slag were anorthite and fayalite.These minerals transformed into eutectic with other silicates and therefore the fusion temperatures of Chiping fine ash decreased.Adding Fe2O3 to Suqian fine ash,there were two reason that for the improvement of fusibility of Suqian fine ash.On the one hand,anorthite,fayalite and hercynite were main minerals in slag at high temperature.These minerals had low fusion temperatures and were easily transformed into eutectic.On the other hand,the formation of mullite was also inhibited.