Numerical Simulation Of Semi-coke Combustion Process In Circulating Fluidized Bed And Application Of Genetic Algorithm In Inverse Problem Of Bed Temperature

Circulating fluidized bed(CFB)is a mature clean and efficient combustion technology.However,due to the randomness of the internal flow,combustion and flow are coupled together,which makes numerical simulations more difficult.The method of simulating for calculating CFB combustion at home and abroad is summarized and analyzed.The Euler-Euler model is used to calculate the combustion process of semi-coke in the circulating fluidized bed.Bed material contains two kinds of particles.The coarse particle phase consists of semi-coke,coke,limestone and ash.Fine particle phase consists of coke,calcium oxide,calcium sulfate and ash during the first stage crushing process,the pyrolysis products of semi coke were classified into coarse coke and fine coke.The semi-coke combustion process is divided into three parts: pyrolysis of semi-coke particles,combustion of volatile reaction(homogeneous reaction),coke combustion(heterogeneous reaction).The contaminants of N and S were considered and described by corresponding reaction equations.Heat transfer between phases and radiative heat transfer are considered in the furnace chamber.In this paper,the Euler-Euler model and the established combustion model were used to carry out the numerical simulation experiment of a 116 MW circulating fluidized bed boiler.This article analyzes the flow behavior of coarse and fine particles.The distribution of particle concentration with the height of the furnace is that the particles have a higher concentration in the lower part of the furnace.The concentration of particles in the middle of the bed is smaller than that near the wall of the furnace.The particle concentration is parabolic along the width.The particle velocity is downward at the wall surface and upward in the central region of the furnace.The distribution of coarse and fine particles in the horizontal and vertical concentrations of the furnace is not uniform.The concentration distribution of particle and the distribution of pressure drop along the height are summarized.The process of combustion was calculated during the flow process.The furnace temperature distribution,gas phase composition distribution,and reaction rate distribution of combustion were obtained.The pyrolysis reaction takes place quickly after the particles enter the furnace.The temperature of furnace bottom is higher and gradually decreases along the furnace height.Explain the distribution characteristics of the components though the reaction rate.The combustion process of N and S components is considered during the combustion process.Analyze the relevant reaction rate of N component,the distribution rule of NO and N2 O concentration is presented.By changing the rate of secondary air and the rate of circulation,the trend of gas component concentration at the outlet was analyzed to study the effect of different operating conditions on the combustion.The study found that as the rate of secondary air increased,the concentration of NO at the outlet decreased.And the explanation was given based on the chemical reaction rates.The increase of the proportion of secondary air is beneficial to the reduction of NO emissions.But it also leads to low oxygen utilization and increased the concentration of CO at the exit.As the circulation magnification increases,the concentration of particles in the furnace increases,which also results in a decrease in the concentration of NO at the outlet.The object of calculation is a two-dimensional unsteady heat transfer inverse problem.The inversion calculation parameter is the heat flux at the boundary.Less error between inversion parameters and true value proves the accuracy of the established genetic algorithm.The inverse problem of the two-dimensional unsteady heat transfer equation of the furnace cross-section is established in the dense phase of the bed,the genetic algorithm is used to calculate the undetermined parameter values.The temperature field is reconstructed according to the positive problem in the dense phase of the circulating fluidized bed.The temperature field is compared with the measured temperature field which is from numerical simulations.The maximum error of the reconstructed bed temperature and numerical simulations is less than 50 oC in present study.The results prove the feasibility of the method.