Research On Interaction Mechanism Of Ash Particles And Condensed Acid Solution In Coal-fired Power Plants

In order to improve the heat efficiency of large coal-fired power plant,achieve the clean coal utilization,and for better use of the grid-connected power generation of renewable energy sources such as wind and solar energy,it is imperative to promote ultra-low emissions,reconstruction of energy-saving as well as the flexibility modification of coal-fired power plants.A large flue gas temperature drop is necessary for the above three reconstructions,while it will cause adhesive ash accumulation and low-temperature corrosion on the heating surface with the existence of acidic gas and fly ash particles in the flue gas.Therefore,research on mechanism of ash particles and condensed acid solution to prevent the adhesive ash accumulation and low-temperature corrosion is the key to deeply reduce the flue gas temperature and to ensure the safe and efficient operation of the heat exchange equipment in coal-fired power plants.In this dissertation,the mechanism of ash particles and condensed acid solution in the flue gas of coal-fired power plants is selected as the research direction.By means of theoretical analysis,field investigation,bench-scale experiment and numerical calculation,a comprehensive research on thermodynamic acid dew point,sulfuric acid condensation,mechanism of ash particles and condensed acid solution as well as effect of products on metal corrosion was conducted.Firstly,the theoretical analyses of thermodynamic acid dew point(ADP)and acid condensation are studied on the net flue gas without considering fly ash particles.It is proved theoretically that the low temperature corrosion mainly depends on the heating surface temperature rather than ADP.Secondly,based on the theoretical analysis,the field experimental study of acid condensation induced by variable heating temperatures in view of heat transfer characteristics of fouling layer is carried out with a special designed pipe.It is found that the drop of heat transfer characteristics was caused by the acid condensation on the low temperature heating surface.Thirdly,the characteristic ash samples are analyzed by microcharacterization test at the end of pulverized coal furnace and circulating fluidized bed boiler using the former experimental method.The concepts of three typical fouling layers are proposed according to the appearance characteristics and degree of adhesion to the heating surface,i.e.dry loose ash deposits,adhering ash deposits and viscous ash deposits.The formation mechanism of characteristic fouling layer under different acid-ash action is explored and the formation models of characteristic fouling layers are drawn.Fourthly,a test bed is designed and established for laboratory test to further study the mechanism between particles of different components and acid solution as well as the action of reaction products on metal corrosion.Finally,the mechanism between ash particles and condensed acid solution is studied by quantitative analysis according to the theoretical calculation model of acid condensation of net flue gas combing with the results of field test and laboratory test.The main research contents of this dissertation are as follows:(1)Considering the complexity of the actual flue gas composition in the coal-fired power plants and sulfuric acid steam condensation process,it is assumed that the flue gas is the net flue gas,and the influence of fly ash in the flue gas is ignored.Thus,the parameters of sulfuric acid condensation and flue gas are within the category of phase equilibrium,and the condensation system is simplified to H2SO4-H2O binary system.The physical model of sulfuric acid condensation is established with consideration of both the gas-liquid equilibrium effect and multi-component diffusion effect;the iterative calculation model of ADP taking account of the effect of local condensation mass transfer is firstly proposed,and the concentration of condensation acid is accurately predicted.It is found that condensation temperature is the key to determine acid condensation by analyzing the influencing factors of ADP and acid condensation.In addition,acid condensation solution on the heating surface is critical for the low temperature corrosion rather than ADP,and the safe operating temperature of heat exchanger in the coal-fired boilers is suggested above 80℃.(2)The field experimental study of acid condensation induced by variable heating temperature in view of heat transfer characteristics of fouling layer is carried out with a special designed pipe in a 300MW sub-critical unit.The effects of operating parameters of water and flue gas on the thermal characteristics of fouling layer are analyzed,as well as the flue gas components and heating surface temperature on acid condensation.It demonstrates that the transformation from dry loose ash to viscosity ash is vital for drastically reduction of heat transfer efficiency,which rest with the amount and concentration of acid condensation on the heating surface.(3)The characteristic ash samples are analyzed by microcharacterization test at the end of pulverized coal furnace and circulating fluidized bed boiler using the former experimental method to study the formation mechanism of characteristic fouling layer.The concept of three typical fouling layer are proposed according to the appearance characteristics and degree of adhesion to the heating surface,i.e.dry loose ash deposits,adhering ash deposits and viscous ash deposits.It can be inferred that the physical parameters of dry loose ash samples are similar to those of fly ash samples in flue gas.The submicron ash particles are gathered,the S element content is increased for adhering ash deposits obviously,and the increasing viscosity is caused by the quantity of condensed sulfuric acid solution exceeds the adsorption capacity of deposited ash particles on the heating surface.The viscous ash deposits can be divided into three layers:the innermost dense layer,the middle hard layer and the outermost adhesive layer.(4)The sulfuric acid vapor condenses around the submicron ash particles firstly;then the condensation solution reacts with ash particles leading to dissolved ash particles and sticky ash particles,which can be served as "adhesion agent" during the formation of the fouling layer,afterwards forms adhering ash deposits and viscous ash deposits gradually.With the decrease of surface temperature,the amount of sulfuric acid condensation increases,the condensed acid contains HCl and HF in addition to H2SO4.When the quantity of condensed sulfuric acid solution exceeds the adsorption capacity of deposited ash particles on the heating surface,the condensed acid solution penetrates into the heating surface,the viscosity of the fouling layer increases and the ash particles are difficult to spin off,leading to thicker viscous fouling layer.Therefore,the characteristic fouling layer is determined by the action degree of the deposited ash particles and condensed acid solution on the surface.The dry loose fouling layer corresponds to the unsaturated acid-ash action fouling layer,the adhesive fouling layer to the supersaturated acid-ash action fouling layer,and the viscous fouling layer to the mixed acid-ash action fouling layer.(5)With the simplification of the ash deposition and acid condensation process,a laboratory test is designed to study the mechanism between SiO2 particles,MgO/CaO particles,fly ash particles and acid solution,respectively.The pH value,conductivity value and SO42-mass content of reaction products are tested to reveal that the mechanism of particles and acid solution is physical adsorption and chemical reaction,and that the maximum reaction amount of sulfuric acid solution with ash particles is 2.52g/g.The SEM&EDS,XRF and XRD results of reaction products are found to be consistent with those of the ash samples tested in the field experiments.The mental corrosion is studied by the SEM&EDS test of metal surface after reaction.The corrosion test shows that the corrosion rates of different pipes under 60%sulfuric solution from high to low is 316L steel>20#steel>Corten steel>ND steel;and that the corrosion rate can be reduced by particulate matters,which is followed as:MgO/CaO particles>fly ash particles>SiO2 particles.Thus the mechanism between particles of different components and acid solution as well as the effect of reaction products on metal corrosion are obtained,which lay a foundation for quantitative analysis of the acid-ash action.(6)The impact of ash particle size on ADP and acid condensation covering ash particles is analyzed combing the droplet formation and acid condensation calculation.The ash particle size has a significant effect on the condensation rate,the critical radius r0 can be used as a criterion to determine the fine ash particles whose radiuses are smaller than the critical radius(rash<r0).The acid condensation process is more prone to occurrence since these fine particles can be performed as condensation nucleus with low subcooling.With the reduction of fine ash particle radius,the condensation rate around fine ash particles,especially acid condensation rate,is markedly increased at ADP.That is to say,the acid condensation is easier to occur with the smaller ash particle radius.However,when the supercooling degree exceeds 30℃,the effect of fine ash particles on the acid condensation of heating surface can be ignored.The amount of acid solution condensed around submicron fly ash particles,which can not be caught on the heating surface,is small;then the reduction effect of the fly ash particles on the acid vapor of flue gas can be negligible,which is also consistent with the hypothesis in the theoretical calculation of acid condensation in chapter 2.(7)The condensed acid solution concentration and the adsorption of deposited ash particles on acid solution should be considered in the mental corrosion of low temperature surface in coal-fired power plants.There is no obvious low temperature corrosion,even if the condensed acid solution concentration is lower than 60%.When the quantity of condensed sulfuric acid solution can be absorpted by the ash deposits,the fouling layer is kept dry and loose,no serious acid corrosion will appear.Nevertheless,with less ash particles on the surface,the acid solution would exceeds the adsorption capacity of deposited ash particles,and the viscous fouling layer would be formed.The metal corrosion on the surface with condensed acid solution is more likely to occur.The corrosion products increase the surface roughness and cause the ash particles easier to get captured,which makes the fouling layer thicker and thicker.The lower temperature,the more viscous fouling,the more remarkable mental corrosion.The mechanism of ash particles and condensed acid solution in the flue gas of coal-fired power plants are studied qualitatively and quantitatively for the first time.The research method of the interaction of ash particles and acid condensation is established,and a better theory of acid-ash action is developed based on the typical fouling layer.The results found a technical route for reducing and preventing the fouling and corrosion of flue gas in coal-fired boilers.This study can provide useful guidance and practical methodology for the safe and efficient operation of the heat exchange equipment in the coal-fired boiler.