Numerical Study Of Gas-Solid Two-Phase Flow And Desulfurization Reactions In The Calcium-Based FGD System

In order to resolve the environmental problem of SO₂pollutants emitted by industrial productions,the flue gas desulfurization?FGD?technology,a leading method of SO₂ remove,is investigated and developed for decade years.The electricity,the petroleum processing and coking and et al.contribute most of the emissions and release more than thousand tons of SO₂per year.Based on the serious conditions,it is necessary to optimize desulfurization technologies to prevent the developing of pollutions.The reactors are the main desulfurization facilities and include three phases:the gas phase,the liquid phase and the solid phase.However,the multi-scale structures,such as clusters,and the anisotropy of absorbent particles are found which result in the heterogeneous profiles and affect the efficiency of desulfurization directly or indirectly.Thus,the flow behaviours of gas-solid two-phase flow in the processes of desulfurization need to be further investigated and optimised.According to previous study,the macro method instead of the meso and micro method was widely used that the explanations of the mechanism of desulfurization was insufficient.Especially,the wet particles found in the semi-FGD method that is unique with dry particles result in the agglomerates and their effect on desulfurization is still blurry.In present study,the Euler-Lagrange?E-L?numerical method by which the information of single particle can be obtained will be used to study the dry and semi-dry FGD technology.The mathematical models are established based on different technologies,and models of heat transfer between different phases are considered.Meanwhile,the reaction kinetic models and two-film theory are introduced in the simulation to meet the acquirement of the technology.The key factors affecting desulfurization efficiency are going to be analyzed.Above investigations are aimed to reveal the flow behaviours,the heat transfer and the desulfurization characteristics of absorbent particles in FGD technologies.For the simulations of three different kinds of FGD technologies,based on the Euler-Lagrange method,the mathematical models are established respectively.The coarse-grained MP-PIC method is used to save calculation resources due to the huge number of particles.Considering the non-uniform distributions of particles,the corrections of solids volume fraction are introduced on the base of the EMMS method to establish the model of dry desulfurization method.The momentum equations are corrected and the models for the semi-dry desulfurization method are proposed by incorporating the liquid bridge force.Moreover,the heat transfer models are established between the gas phase and solid phase,including contact heat transfer,the convective heat transfer and the radiant heat transfer.Furthermore,the models of the desulfurization reaction rate are introduced depended on the reaction kinetic model and the two-film theoretical model.The study on the low temperature FGD method is carried out based on the establishment of hydrodynamics and chemical reaction models.The effect of flow behaviours and the characteristics of heat transfer of particles on chemical reactions are analyzed.Moreover,the CO₂component of flue gas is taken into consideration and its effect on the desulfurization efficiency is studied.The results show that the distributions of the solids phase are non-uniform,and the volume fraction is low in the centre and high near the wall.The non-uniform distributions of solids volume fraction are weakened along with the increase of gas velocity.It is found that the smaller of the particles,the easier they are to be fluidized.Also,the gas transfers energy with particles rapidly at the bottom of the reactor and the dynamic equilibrium can be achieved within two seconds.The convective heat transfer rate of particles is improved by the increment of gas velocity.The desulfurization efficiency decreases by considering the reaction between CO₂ and Ca O,which verifies the need of considering the existence of CO₂ in desulfurization.Numerical simulations of flow behaviours of medium-temperature FGD circulating fluidized bed are performed by means of MP-PIC method and EMMS drag model.The effects of gas velocity,gas temperature,and particle diameter on the desulfurization efficiency are obtained.The results show that the solids volume fraction is much more uniform in the whole bed with a large-scale grid while it is non-uniform with small-scale grids.It shows that the concentration of particles is becoming more uniform as the increment of parcel weight,especially as the weight is higher than 500.It is clear that distributions of particles using the EMMS drag model are dense near the inlet and dilute near the outlet,which is more reasonable compared with the cases using a uniform drag force model.The reaction rate and desulfurization efficiency obviously decrease as the increment in gas velocity.Meanwhile,the rate increases along with the increase of flue gas temperature.By analysing the statistics of the second inlet,it is found that there is an increase of desulfurization efficiency by increasing the mass fractions of Ca O of absorbent particles.The flow behaviours and the characteristics of the desulfurization of semi-dry FGD technology are simulated based on the numerical models coupled with liquid bridge force model.The velocity of particles and the desulfurization efficiency are analyzed by considering the effect of liquid bridge force.From the perspective of flow behaviour,it is found that the agglomerates and free particles coexistence in the reactor which shows multi-scale flow structures as a result of the existence of liquid bridge.The concentrations of particles is higher than that of neglecting the liquid bridge force model.The diameter and the number of agglomerates increase as the humidity increases.From the perspective of the chemical reaction,the agglomerates have disadvantages on desulfurization.The utilization rate of the absorbents is lower for wet particles.The desulfurization efficiency increases first and decreases later as the increase of humidity.The aim of present paper is to study the flow behaviours of particles and their effect on desulfurization by establishing the corresponding numerical models for different FGD methods.The instability and heterogeneity of the flow during desulfurization are obtained and the mechanisms between flow behaviours and chemical reactions are explored.All the above conclusions can be used as theoretical and technical support for the design of the FGD reactors.