Study On High Temperature Combustion And Melting Characteristics Of Fine Char From Fluidized Bed Gasifier

Fluidized bed gasification has been considered one of the promising technologies for low rank coal utilization,because of its wide fuel diversity,low-cost operation and minor pollutant emission.However,massive amounts of coal gasified fine char are entrained out by crude gas due to relatively lower operational temperatures of the gasifier,which reduces carbon conversion ratio of the gasification system.Generally,the fine char are characterized by fine size,low volatile content,high carbon and ash content and poor chemical reactivity,which brings great difficulties for its further transition.Therefore,it is crucial to achieve clean and efficient utilization of coal gasified fine char for the promotion and application of fluidized bed gasification technology.On the basis of existing utilization technologies of coal gasified fine char,the technique of fluidized bed gasification combined with fine char high temperature combustion melting process was proposed,which could effectively recover residual carbons in the fine char and avoid air pollution.Besides,volume reduction could be realized.In this study,we concentrated on the vital problems and crucial segments encountered during research and development of the technological process.We carried out fundamental studies on ash fusibility characteristics of coal gasified fine char by using ash fusion temperature determiner,heating stage microscopy and FactSage simulation method,etc.The combustion characteristics of coal gasified fine char were investigated in a 2.5 t/d preheating and high temperature combustion melting experimental platform,and the basic properties of molten slags and entrained fly ash were also analyzed.Furthermore,the performance characteristics of fluidized bed gasification combined with fine char high temperature combustion melting system was predicted by using an equilibrium model.The main conclusions are as follows:(1)When compared to Wucaiwan coal,the moisture and volatile content in the fine char after gasification are quite low,whereas the ash and carbon content are relatively high.Besides,the fine char is richer in Fe and Ca,but poorer in Al,Si and Na.The concentrations of mineral elements in the fine char are enormously influenced by its particle size.In comparison to pyrolyzed coal char,the fine char is characterized by more ordered crystalline structures,wider pore distribution,and richer pore structures,which results in its superior combustion and CO2 gasification reactivity.(2)The ash fusion temperatures(AFTs)of fine char from industrial circulating fluidized bed(CFB)gasifier are about 30-75? lower than the raw coal,mainly due to its higher contents in basic components.The main crystalline phases of fine char ash are quartz(SiO2),anorthite(CaAl2Si2O8),pyroxene(CaMgSi2O6)and hematite(Fe2O3)at high temperature.The formation and melting of anorthite are the primary mineral transformation process of fine char ash at high temperature,which dominates its fusion process.The melting of fine char ash is a gradual process,where low-melting-point minerals,e.g.,albite,first fuse at multiple locations and flow slowly between adjacent solid particles.As the temperature rises,high-melting-point minerals,such as pyroxene and anorthite,start to melt and the liquid phase content increases sharply,and the flow ability of fine char ash accelerates with increasing temperature.(3)As the particle size increases,the AFTs of Suqian fine char first increase rapidly,and then ramp up;whereas,minor disparities in the AFTs are found for Liaocheng fine char.With CaO addition,the AFTs of Suqian fine char first decrease slightly,and then increase;while a steady decline is found for Liaocheng fine char.When CaO is replaced by CaSO4,the reaction between CaO and mullite(Al6Si2O13)is promoted and more alumina(Al2O3)forms,resulting in increases in the AFTs;while an inverse trend is identified for Liaocheng fine char,which might be caused by the formation of Fe-O-S eutectic and FeS flux.The AFTs of both Suqian and Liaocheng fine char ashes appear in the order of oxidizing>inert>reducing,mainly related to the different valence states of iron under various atmospheres.A sharp increase in the AFTs is observed for both fine chars with the presence of residual carbon,which attributes to the formation of high melting point minerals,such as ferrosilicon(FeSi)and mossanite(SiC).The viscosity-temperature curves indicates that Liaocheng fine char ash tends to form glassy slag when completely melted,which turns out to be plastic slags for Suqian fine char ash.Dendrite particles are in situ observed for Suqian fine char ash during crystallization,which could strength the bindings inside the molten slags,promote the formation of solid phases,and accelerate the growth in viscosity.(4)The 2.5 t/d coal gasified fine char preheating and high temperature combustion melting experimental platform was designed and established,and the high temperature combustion and melting characteristics of Shenmu coal and Shenmu char were investigated.First,the plarform runs stably and continuously for 31.5 h,with Shenmu char as fuel and air as oxidizing agent.The carbon content in combusted fly ash is 2.7%and the combustion efficiency reaches 99.54%,which indicates that sufficient transition of Shemmu char has been achieved.Then,Shenmu coal is chosen as fuel material,and 44%oxygen-enriched air is adopted as oxidizing agent.The platform is set to operate at relatively higher temperature by adjusting total air volume,oxygen concentration and air distribution ratio.The platform runs stably for 40 h with successive liquid slag tapping,and the mean temperature of cyclone melting furnace is 1392?,and the technological route of coal gasified fine char preheating and high temperature combustion is proven feasible,which could provide valuable guidance for the selecetion of operational parameters.Compared to molten slag,the combusted fly ash is featured with finer sizes,which forms mainly by condensation of fine ash particles entrained out by flue gas.(5)The equilibrium model of fluidized bed gasification combined with fine char high temperature combustion melting system is establiashed.By taking a pratical case in an industrial CFB gasifier as example,the performance characteristics of the integrated system is predicted.Compared to fluidized bed gasification,the O/C molar ratio of the integrarted system increases from 0.62 to 1.02,the operational temperature of gasification unit is raised from 910.0? to 982.2?,and the carbon conversion ratio rises from 81.24%to 99.64%.