Study On The Bi-spouted-bed Bottom Ash Cooler Of CFB Boilers

Circulating fluidized bed (CFB) boiler technology has got a tremendousdevelopment worldwide because of its unique advantages, such as excellent fueladaptability, high combustion efficiency and low pollutants emission. As a key auxiliarydevice for CFB boilers, the operation performance of the bottom ash cooler (BAC)directly affect the operational stablility, security and economicalefficiency of CFBboilers. Compared with the rapid development of domestic CFB boilers towards largercapacity and higher parameters, the development of BACs has fallen behind relatively,and the existing BACs cannot guarantee the operational stablility, security andeconomical efficiency of CFB boilers already. Under the circumstance thatthe coal typeand quality supplied to CFB boilers cannot be improved effectively, how to prevent theoperational failures of the existing BACs, to recover heat from ash efficiently, and toimprove the operational performance of BACs have become critical problems needed tobe solved imminently for the development of the domestic CFB boilers.In order to overcome these problems, a novel BAC called Bi-spouted-bed bottomash cooler has been proposed in this work based on the requirements of BAC.It isappropriate to treat the bottom ash of domestic CFB boilers and has obtained aninvention patent in China.Besides, the hydrodynamic characteristics, particle mixingcharacteristics, and heat transfer characteristics on the ash-side of the water-cooled tubebank of quasi-rectangular spouted bed, which is the basic component of Bi-spouted-bedbottom ash cooler, were systematically studied by experimental measurement,theoretical calculation, and numerical simulation in the present work.Considering thatthe operational performances of ash discharge device directly affect the operationalstablility and security of CFB boilers and BACs, the bottom ash conveyingcharacteristicsof the L-valve for ash conveying, which is applied in the bottom ashtreatment system of a domestic CFB boiler, has also been studied.The main contents ofthis paper are listed as follows:â‘ Study on the hydrodynamic characteristics of quasi-rectangular spouted bed:A small cold model of quasi-rectangular spouted bed was designed and built. Thehydrodynamic characteristics of the quasi-rectangular spouted bed, and the effects ofstatic bed height, taper angle, particle size of bed material and air inlet section width onthe hydrodynamic characteristics were experimentally studied. Based on the results, theoretical models and empirical correlations have been proposed to predict therelationship between the superficial gas velocity and bed pressure drop.Additionally, anumerical simulation for the hydrodynamics of the gas-solid two-phase flow in thequasi-rectangular spouted bed was also conducted using the software Barracuda. Thesimulation results were validated by comparison with the experimental results.â‘¡Study on particle mixing characteristics in quasi-rectangular spouted bed:Experimental study on vertical and lateral mixings of a binary mixture of differentsizes was conducted to investigate the adaptability of Bi-spouted-bed ash cooler to thebottom ash granularity. The effects of static bed height, particle size of bed material,superficial gas velocity,and air inlet section width on the vertical mixing index andlateral dispersion coefficient of tracers were investigated.Furthermore, a comparison ofthe vertical and lateral mixing characteristics betweenthe quasi-rectangular spouted bedand a bubbling fluidized bed with the same upper cross-section was also conducted.â‘¢Heat transfer characteristics of the water-cooled tube bank in the freeboard regionof the quasi-rectangular spouted bed:To reveal and master the heat transfer characteristics on the ash-side of thewater-cooled tube bank in the Bi-spouted-bed bottom ash cooler, an experimental studyon heat transfer characteristics on the ash-side of the water-cooled tube bank in thefreeboard of quasi-rectangular spouted bed was conducted on a big model of the bed.And the effects of static bed height, particle size of bed material, superficial gas velocity,and elevation of tube in the freeboard on the local and average heat transfer coefficientsof each tube in different regions of the freeboard were systematically studied.â‘£Experimental study on the bottom ash conveying characteristics of the L-valve forash conveying:In order to provide theoretical basis and operation guidance for guaranteeing theoperational performance of the L-valve for ash conveying, a semi-industrial cold modelwas built. The effects of air flowrate, air tube insert ratio, and the mean ash size on theflux control characteristics of the L-valve for ash conveying, as well as these effects onthe distribution of ash in the following equipment, were studied.