Study On Heat Transfer Coefficient For Heating Surface In A Circulating Fluidized Bed Boiler

Circulating fluidized bed is one of main kinds of Power station boiler，the study onheat transfer coefficient for heating surface in a circulating fluidized bed boiler is verysignificant,it relates to design,manufacture,operation and maintenance,it relates to the safetyand economical operation of boiler. Heat transfer coefficient determines thedistribution,loaction,quantity,structure of the heating surface.If the choice of the heat transfercoefficient is improper,the purpose of steady combustion and the best economic benefitscann’t be gotten,even the heating surface can be destructed.Experiment is a method to choose the heat transfer coefficient in a circulating fluidizedbed,there are many factors to affect the heat transfer, it is impossible to do experiment foreach operating condition.It is necessary to establish a heat transfer model which fits to certainrequest in precision by theoretic analysis.This paper aims to establish an mathematical model of flow and heat transfer in acirculating fluidized bed on theoretic research and experimental data,so as to predict heattransfer coefficient accurately,the guidance can be given for design and optimizing operationof CFB boilers.Firstly, a hydrodynamic model based on the semi-empirical approach is proposed toestimate the local particle size distribution in CFB. The core/annulus flow structure issupposed in this CFB model.The furance can be divided into two regions along theheight:the thicky phase which is more thicky underside,the thin phase which is thinnerupside. Solid particles exhibit a wide particle size distribution in CFB boilers,it shows thatcoarser particles are gathered near the walls and the average particle diameter decreasesalong the boiler height.These resaults fit well to experimental data in reference.Secondly, The convective heat transfer model based on the cluster renewal theory isproposed,it is based on the hydrodynamic model,and the method which determines theparameters concerned the model.Therefore, the total heat transfer coefficient is simulated byconvective heat transfer coefficient and radiative heat transfer coefficient.Finally,the research on abrasion principle in CFB boilers is done,and thecountermeasures to prevent the abrasion is found.We have analysed the problem ofrefractory in CFB boilers,and the reason is found out,the method of choosing refractoryreasonably is proposed too.These two points are very importment in safty operation for CFBboilers,they are very significant to guide operation.