Numerical Model Development Of An Ultra/Supercritical Circulating Fluidized Bed Boiler

Supercritical circulating fluidized bed(CFB)boiler has become the important development direction of CFB boiler with high efficient,clean and low carbon,which takes the advantages of CFB combustion technology and supercritical steam generator.The combustion in furnace,heat transfer on heating surfaces and thermal hydrodynamics in membrane wall are the key issues in the design and optimization of supercritical CFB boilers.Based on these issues,this paper establishes a three-dimensional efficient integrated numerical model for ultra/supercritical CFB boiler combustion.The main work includes the following four parts:(1)Numerical modeling and study on the gas-solid flow,combustion and heat transfer process in the large-scale ultra/supercritical CFB boiler;(2)Numerical study on the heat transfer and thermal hydrodynamics characteristics in the membrane wall of ultra/supercritical CFB boiler;(3)Development of scaling-down method on supercritical fluid flow balance problem in the suspended surface's parallel channels.Firstly,this paper makes a review on the published CFB combustion mathematic models,which discusses the development process,status and direction of the combustion simulation on CFB;concludes the structures and characteristics of semi-empirical and computational fluid dynamics(CFD)combustion models;presents the detailed description of the combustion sub models.Moreover,it summarizes the key sub models and parameters of the fluidized bed combustion models,investigates the problems and key points in the design of large-scale ultra/supercritical CFB boiler.These works determine the theoretical basis and modeling method to build the numerical combustion model for ultra/supercritical CFB boilers.Based on literature review,this paper introduced a three-dimensional efficient integrated numerical boiler model for ultra/supercritical CFB boiler combustion.Based on the gas-solid hydrodynamics results computed by the Euler model and EMMS drag model,the coal combustion is taken into consideration,which consists of evaporation,devolatilization,char combustion,gas homogeneous reaction and pollutants emission.In the model,heat transfer process between heat-exchange surfaces and furnace is based on the cluster renewal model and the discrete ordinate radiation model.To consider the boiler operating under supercritical condition,the thermal hydraulics in the membrane wall tube is coupled with wall heat transfer.Based on the computing ability of the enterprise servers,the integrated boiler model can simulate the combustion process in a large ultra/super circulating fluidized bed boiler efficiently and accurately.This integrated numerical boiler model was successfully applied to the combustion simulation of a 330MWe subcritical and a 350MWe supercritical CFB boiler.The reasonable 3D distributions of gas-solid flow,combustion products including sulfur and nitrogen compounds,temperature profile in furnace and heat flux on membrane walls were obtained.The simulation results of oxygen,temperature and gaseous pollutants give good agreements with operation result in real CFB boilers.In addition,the integrated boiler model is verified through the comparison of the simulations result with actual operation results of these two large-scale CFB boilers,and some proposals are given on the design and operation.This paper proposes a calculation method to predict the heat transfer and thermal hydrodynamics of membrane wall in a supercritical CFB boiler,which is successfully applied to a lOOOMWe supercritical CFB boiler.The main results show that the fluid temperature profile is influenced by its specific heat capacity and wall heat flux,and is gentle in the tapered section and near pseudo-critical region;and the mass flux,inlet fluid temperature and furnace temperature have significantly impact on the fluid temperature profile,while the inlet pressure and superficial gas velocity has little impact.In order to study the flow balance of supercritical fluid in the parallel channels of suspended surfaces,a fluid-to-fluid scaling at supercritical pressure is carried out.According to the equational analysis method,the scaling factors and scaling rule are determined for ultra/supercritical CFB boiler.Based on this scaling rule,the design of modeling test-rig for superheater in a supercritical CFB boiler was obtained with R134a.