| From the end of the 13th to the beginning of the 14th Five-Year Plan,the national environmental protection requirements for the atmosphere,water and soil have been increasing.In 2020,the Chinese government clearly put forward the goal of carbonpeak by 2030 and carbon-neutral by 2060,making the commitment to deal with global climate change,reflecting the responsibility of a major country.The white plume elimination system makes great contributions to smog mitigation and pollutant concentration decrease near ground.The saturated flue gas from desulfurization tower contains large amounts of water vapor and latent heat,effective recovery of which is of great significance to water and energy conservation of boiler system.Typical white plume elimination system contains MGGH(Media Gas Gas Heater)and flue gas condenser.According to the flue gas flow direction,the system is equipped with raw flue gas cooling device,desulfurization tower,flue gas condenser,clean flue gas heating device,chimney and other devices in series.The devices are connected with and interact with each other.The MGGH eliminating white plume is to increase the superheat degree of the steam and the temperature of flue gas after desulphurization.If the steam with certain degree is constantly in the superheated or saturated state in the process of mixing with the cold air,the flue gas is colorless.If there are droplets,the flue gas becomes white plume.The superheat degree depends on the environmental state,the definitive method is called the control criteria of white plume elimination.In order to obtain the criteria and accurately design of MGGH,it is necessary to clarify the flue gas characteristics,humidification law and process involved.At present,the principle of determining the boundary conditions of each equipment in the industry is not perfect,and there are few comprehensive studies on the flue gas characteristics,operation law and coupling effect between various equipment in the whole system,which often cause the inconsistent of design and operating parameters,thereby affect the safety and economy.In this dissertation,the flue gas characteristics in MGGH system of coal-fired boiler and the calculation criterion for eliminating white plume are systematically studied by theoretical analysis,laboratory experiment,field test and numerical simulation.Firstly,the volume and composition of flue gas,which is the inlet boundary conditions of MGGH,are studied.The calculation model of air volume and flue gas composition,as well as the real-time measurement of flue gas volume are established based on coal proximate analysis and operation data.laying foundation for accurate design of MGGH.Secondly,the limited corrosion characteristic by SO2 to carbon steel 20#(GB3087),which is widely used on medium and low pressure pipe of boiler,are studied.The concept of passivation dew point is defined based on the passivation characteristics of 20#in high concentration sulfuric acid(more than 60wt%),and its calculation formula is derived by fugacity equation,The limited corrosion characteristics of flue gas in the passivation dew point were studied by field test,and the reliability of which as the safe control parameter of 20#wall temperature was verified.Thirdly,the gas-liquid two-phase heat and mass transfer model and the slurry temperature calculation model were established and experimentally verified.The design parameters are provided for evaporation water consumption of desulfurization tower and flue gas cooler.The law of flue gas humidification is obtained,which provides parameters for water demand calculation and flue gas cooler design.Fourthly,the coupling variation mechanism of velocity and concentration distribution,the slurry carrying characteristics are studied,and the guide plate structure based on the gas-liquid coupling principle is proposed,which provides a new perspective for efficient heat and mass transfer of the desulfurization tower.Finally,based on the tangent theory in psychrometric chart,the net temperature rise of clean flue gas heating device is calculated.A MGGH system with flue gas condenser is closed-loop designed by the verified models in Chap 2,3 and 4.On this basis,the white plume elimination experiment was carried out to verify the calculation model.The contents of this paper are as follows:(1)Study and verification on flue gas volume and composition model based on coal proximate analysis data.The concept of Chemical Oxygen Demand on Combustible Basis(CODCB)is presented,and the relationship among CODCB,net calorific value(NCV)and moisture as received basis is discovered.Based on the CODCB,the calculation formulas of theoretical air volume and flue gas volume are established,considering the impact of moisture as received basis.The model can cover the whole range of coal,in which the average accuracy is 95%,for lignite with moisture 40%and higher,the accuracy can reach over 97%.The quantitative calculation method for the composition of flue gas is given.A convenient measurement with the accuracy over 90%of flue gas volume based on the velocity distribution is proposed.(2)Study on passivation dew point of 20#(PDW)and limited corrosion characteristics of flue gas.Based on the corrosion characteristics of 20#,that is,the passivation effect of sulfuric acid over 60wt%on ordinary carbon steel,the concept of PDW was proposed.As the degree of freedom of phase equilibrium of binary system is 2,PDW was deduced based on fugacity equation with 60wt%sulfuric acid and water vapor content as the intensive variable parameters of phase equilibrium of H2SO4-H2O binary system,The functional relationship between PDW and partial pressure of steam is obtained.Through field test,the heat transfer characteristics and ash deposition characteristics of heat exchanger under different wall temperatures are studied.The heat transfer test results show that when the wall temperature is above PDW,the heat transfer coefficient is higher and in a stable state,and the heat transfer coefficient decreases obviously in this range,which indicates that the ash deposition gradually increases and affects the heat transfer resistance.The characteristics of ash deposition are as follows:when the wall temperature is above PDW,the ash deposition is fluffy without viscosity,the micro structure is small particles without agglomeration,and the S element is at a low level.In the PDW range,the ash gradually becomes sticky,and the micro particles agglomerate with large particles,and the S element is obviously increased by electron microscopy.Based on the above tests,the feasibility of PDW as the limited corrosion dew point of 20#is determined.(3)Study on the determination method of slurry temperature and humidification characteristics of flue gas in desulfurization tower.The concept of adiabatic saturation temperature of flue gas is proposed,and the mathematical model for determining the slurry temperature of desulfurization tower is established.Based on the spray mode,the Lewis relation is re-derived,and the Merkel formula is re-derived based on the enthalpy analysis.Based on the Lewis relation and Merkel formula,the heat and mass transfer equations of water vapor are established.For one-dimensional flow,the velocity of slurry particles,flue gas temperature and humidity are obtained.Based on the determined slurry temperature,the one-dimensional coupling model of slurry particle velocity,flue gas temperature,humidity and pressure difference is iteratively calculated,and the numerical solutions of parameter distribution are obtained.The effects of slurry particle diameter,flue gas temperature and liquid gas ratio on flow,heat and mass transfer are discussed.In order to verify the correctness of the model,field and physical model tests are carried out to determine the accuracy of the model.(4)Study on numerical simulation of flow field,concentration field and slurry carrying characteristics in desulfurization tower.Based on the actual size of the desulfurization tower of a 600 MW unit,the geometric modeling of the desulfurization tower is carried out.The physical model of the flow field and concentration field is completed by using the CFD software,and the accuracy of the model is verified.The influence factors of different outlet positions on the flow field of desulfurization tower without spray are analyzed,the reasons for the change of internal flow field of desulfurization tower are analyzed,and the flow pattern distribution characteristics of pressure induced flow and circular cross-section converging flow are put forward.The coupling effect of flow field and particle concentration field is studied.The flow pattern distribution characteristics of particle induced flow,rebound flow and uplift flow under coupling effect are proposed.The flow field,concentration field and pressure difference distribution under different load and liquid gas ratio are discussed.The dynamic variation characteristics and reasons of flow field and concentration field are studied,The change law of slurry carrying characteristics entering the demister is discussed.Based on the coupled field characteristics,the guide plate structure is designed.(5)Study on the mechanism of white plume generation at chimney outlet and control criterion of white plume elimination.The formation mechanism of white plume is explained by the enthalpy-humidity chart.Taking the tangent principle of enthalpy-humidity chart as the calculation criterion of white plume elimination,the tangent equation and the calculation are realized.The clean flue gas temperature increase under different ambient temperature and humidity and saturated flue gas temperature at the outlet of desulfurization tower are listed in table.The distribution characteristics of tangent points at enthalpy-humidity chart and the trend of flue gas temperature increase under different ambient temperature and humidity are studied.Based on the calculation model of Chap2,3,a MGGH system is designed,and its correctness is verified by actual measurement.The calculation results of clean flue gas temperature increase are verified by field test,and the deviation between the test data and the predicted value of the calculation criterion is analyzed systematically,which analyses the errors of the calculation criterion.In this dissertation,the flue gas volume and composition characteristics,limited corrosion characteristics of flue gas,humidification characteristics and slurry carrying characteristics of desulfurization tower are systematically studied.The closed-loop verification is realized for the calculation and measurement method of flue gas volume,which widens the lower limit of safe wall temperature of 20#,greatly reduces the cost of MGGH system,and improves the safety and economy of equipment.The research on the heat and mass transfer inside the desulfurization tower can provide guidance for the operation of the desulfurization tower,and provide boundary conditions for the subsequent design of flue gas cooler.The research on the three-dimensional flow field,particle concentration field and slurry carrying characteristics in the desulfurization tower provides a new idea for optimizing the heat and mass transfer of water vapor,reducing the slurry carrying,and the layout of guide plate.Based on the tangent theory of enthalpy-humidity chart,the calculation criterion of flue gas temperature rise has guiding significance for the design of flue gas heater.The conclusion of this dissertation has high theoretical value and engineering application value. |
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