Experimental Study On Key Components Of CFB Gasifier

The loop seal and air distributor are the key components of the circulating fluidised bed?CFB?gasifier.,which have an important impact on the operation performance of the CFB gasifier.In recent years,the circulating fluidized bed coal gasification technology has developed towards the direction of large scale and pressurized operational condition,which puts forward higher applicable requirements for the operation of loop seal and air distributor.The conventional loop seal has the problems of insufficient adverse pressure differential under pressurized operational condition with flow dead zone at the bottom of the loop seal,which easily causes gas channeling and slagging in the loop seal.Present air distributors have problems including high carbon content in the furnace bottom ash and high resistance to aeration under pressurized operation.Therefore,how to optimize the structure design of loop seal and air distribuor for solving above problems is of great significance to improve the operation stability of CFB gasifier and the carbon conversion rate of coal gasification reaction.In this paper,cold test rigs for CFB coal gasification based on the studies of loop seals and air distributors had been built,respectively.New types of loop seals and air distributors were designed.The influence of the structural parameters of the recycle chamber,the inclination of the sloped passage under the supply chamber,the length of the horizontal passage and the air distribution form of the loop seal on the loop seal performance,including the circulating flow rate,the adverse pressure differential capacity and the flow dead zone,was studied under different operation parameters.Besides,the influence of nozzle structure,variable diameter structure and grading aeration form of conical air distributor on the particle size separation performance and the pressure distribution in the circulation was studied.The purposes of this paper were to develop a new type of loop seal technology which can undergo both high circulation flow rate and high adverse pressure differential with no flow dead zone;to develop a new type of air distributor technology which has stable operation,small air distribution resistance and can achieve particle size separation performance,so as to reduce the carbon content of bottom ash.The research results of the loop seals show that:under the normal operation condition,increasing the diameter ratio of the recycle chamber to the supply chamber could improve the circulation flow rate and the adverse pressure differential ability of the loop seal;when the insertion depth ratio of the recycle chamber is too large,it is not conducive to the stable operation of the circulation;by comparing the operation characteristics of the three kinds of loop seals,it is found that the loop seal with the diameter ratio of 1.4 and the insertion depth ratio of 1.0 possesses higher ability of returning material and adverse pressure differential;the rules of the loop seal performance with the air chamber or the air duct are basically the same.In addition,compared with the air duct distribution,the flow dead zone area of the loop seal is smaller under the air chamber distribution.At the same loop seal aeration velocity,the circulation flow rate and the pressure gradient provided by standpipe in the conventional loop seal?inclination?=0?is higher,and its adverse pressure differential performance is better;the fluidization quality of material at the bottom of loop seal with inclination?=1.5 is poor,which can not effectively reduce the flow dead zone,its circulation flow rate and adverse pressure differential performance are worst;the loop seal with inclination?=2.0possesses both the property of the loop seal with inclination?=0 and 1.5.In addition,although the adverse pressure differential performance of the loop seal with the inclination?=2.0 is not as good as that of the conventional loop seal,the flow dead zone area of the loop seal is obviously reduced,especially when the superficial velocity of the supply chamber is more than 0.4m/s,the flow dead zone of the sloped passage disappears completely.Because the return resistance of loop seals with L/D₁=2 and 3 is small,the reverse pressure difference resistance of the loop seal is insufficient.Only under the condition of high circulating material quantity can those loop seal be guaranteed to be normal without gas channeling.The loop seals with L/D₁=4 and 5 provide enough return resistance,which can realize stable operation under higher circulating flow rate and reverse pressure differential.Under the high loop seal aeration velocity,those two loop seals still maintains a high standpipe material level height,so as to provide enough adverse pressure differential.However,the return resistance of the loop seal with L/D₁=5 is too large,the material level in the standpipe is too high,so there is a risk of material piling up into the cyclone separator;within the range of structural parameters studied in this paper,the loop seal with L/D₁=4 has a high circulation flow rate and adverse pressure differential performance,its material level in the standpipe is moderate and its operation is more stable.The results of air distributor show that:compared with the radial nozzle,the conical air distributor with tangential nozzle structure is easier to establish the pressure balance in the circulation;the graded air distribution plays an important role in fluidising the gas-particle phase;besides the three-layer nozzle opening at the same time can avoid forming a flow dead zone,the fluidization quality of bed material is the best;but the conical air distributor of this structure has no separation ability for the wide or narrow size.The cone-shaped grading air distributor with variable diameter structure has obvious particle size separation ability for bed materials with wide or narrow particle size distribution;the cone-shaped air distributor with variable diameter structure has better particle size separation effect on coal and semi coke than quartz sand;when the bed materials are coal and semi coke,the mass ratio of particles with size less than1mm in the bubbling zone is only 18.97%,which is far lower than that in the conveying zone of 71.20%,this result is expected to provide a new design method of air distributor for reducing the carbon content of bottom ash.