Numerical Simulation Of Gas Solid Flow And NO_x Generation In A New Type Of Dual Circulating Fluidized Bed Boiler

To meet the needs of environmental protection,various low NO_x emission technical approaches such as optimization of the primary air/secondary air,elevation of the position of the secondary air outlet,and installation of SNCR have been adopted in CFB boilers,those approaches can achieve good NO_x reduction results.However,it is practically very difficult to control the emissions of NO_x and SO₂ at the same time meanwhile maintaining the combustion efficiency of coal under the ultra-low emission standard.This is because the low NO_x emission requires reducing atmosphere,yet the low SO₂ emission for the furnace desulfurization requires oxidizing atmosphere.To solve this problem,a new type of low emission Dual Circulating Fluidized Bed?DCFB?boiler consisting of bubbling fluidized bed?BFB?and circulating fluidized bed?CFB?was proposed.Through the pretreatment of BFB,the combustion processes of volatiles and char were separated,then the NO_x and SO₂ control zones were decoupled to achieve the goal of low NO_x emission.In this paper,the effect of particle size distribution?PSD?on the gas-solid flow in a fluidized bed was investigated by Euler multiphase flow and DDPM-KTGF model,respectively.The calculation results show that the Euler multiphase flow model with the mean particle size can obtain good simulation results.On the other hand,DDPM-KTGF model with the widely PSD can obtain better simulation results than Euler multiphase flow model.Compared with the Euler multiphase flow model,DDPM-KTGF model is more representative for the real particles motion in both dense and dilute phase region.The PSD of real ash in real dense phase zone was used as boundary condition to simulate the gas-solid flow characteristics of the fluidized bed.The results show that the CFB dense phase particles have a similar surge motion.In the dilute phase,the granular flow shows a typical "ring-core" structure.As expected,the particles can move to CFB part through the BFB overflow port.At the same time,the vertical section should be designed in the DCFB connecting passage to prevent the accidental particles reverse from CFB part.In the case of circulation ash injection to BFB part,the particle smaller than 0.1mm would pass through the channel above the BFB part and enter the dilute phase region of CFB part,meanwhile,the particle bigger than 0.1mm would go to the CFB part dense phase through the overflow port.The key factors of influencing the fuel-NO_x formation were studied with the detailed chemical reaction mechanism of coal combustion and fuel-NO_x mechanism.A one-dimensional chemical reactor network was built.Based on the calculation results of the CFB/DCFB matched group under the same condition,the CFB emission of NO_x is 224.48mg/Nm3,while the NO_x of DCFB is 97.29mg/Nm3,and the reduction rate of DCFB for fuel-NO_x is 56.66%.For CFB validation group,the oxygen concentration in the reaction zone is the key to the formation of NO_x.Low oxygen concentration can inhibit the conversion of N to NO_x.High H2 concentration in reaction zone can promote the conversion of fuel-N to NO_x.The pyrolysis and combustion region is the main region of fuel-N conversion to NO_x.The oxygen concentration in the region is the key to controlling the final NO_x emissions.Through the numerical simulation of gas solid flow,the effectiveness of DCFB structure design was verified.With the calculation of DCFB combustion and NO_x generation,the key factors affecting the generation of fuel-NO_x were obtained and the emission reduction effect of DCFB on NO_x was verified.