Study On The Interaction Mechanism Between Ammonium Bisulfate And Fly Ash In Denitration Process And Its Effect On Air Preheater Blockage

SCR denitration system is an efficient denitration technology.Although it effectively relieves the pressure of environmental protection,it also brings many problems for the safe and economic operation of coal-fired power plants.In the denitration process,the catalyst increases the probability of SO2 oxidation to SO3 in the flue gas,and the ammonium bisulfate(ABS)generated by the reaction of SO3 and unreacted ammonia(escaping ammonia)under certain conditions will adhere to the surface of heat exchange equipment,making the ash deposit thick.Wherein,the blockage of air preheater caused by ABS has always been the focus of scholars' research.The viscous liquid ABS adheres to the wall to absorb a large amount of fly ash,which reduces the flow cross-section area between the flue gas side walls of air preheater,forms the blockage,and increases the operation resistance of the unit.Therefore,it is of great significance to explore the adhesion mechanism of ABS and ash particles on the wall of air preheater to effectively prevent and reduce the blockage of air preheater of coal-fired boiler.In this project,the adhesion mechanism of ABS and fly ash particles on the wall of air preheater is selected as the research direction.An experimental platform is designed to study the adhesion of ABS and ash particles on the metal wall under different temperature,different particle size range and different mass ratio of ABS to ash(R).There is a temperature difference of 15?45? between the flue gas and wall.Thus,this paper studies the blocking problem of air preheater from two aspects:the direct condensation of ABS on the wall to absorb ash particles and the adhesion of the mixture of ABS and ash particles on the wall.It shows that the formation of liquid ABS is the fundamental cause of serious blockage of air preheater.The adhesion rate of premixed samples and unmixed samples increases with the growth of temperature at first.When they reach a certain temperature,the adhesion rate will decrease with the increase of temperature.However,the temperature point with the highest adhesion rate of fly ash of the two samples is different(premixed 220?,unmixed 180?),the adhesion rate of premixed samples is always higher than that of unmixed samples,and the adhesion strength of the former is lower than that of the latter.It suggests that the adhesion force of pure ABS in a certain area of air preheater is relatively large,which is difficult to be blown off by the soot blower with high wind speed,resulting in the continuous thickening of ash deposit layer.After passing through this area,the adhesion rate of the mixture of ash particles and ABS on the wall is higher,and the thickness of the ash deposit layer increases obviously in a short time.This conclusion provides guidance for the selection of soot blowing methods in different areas of the air preheater in the later stage.In order to analyze the coupling mechanism of ABS and ash particles at different temperatures,the ash samples adhered to the wall are taken for test.The samples at different temperatures(160,200,220,280?)are analyzed by Scanning Electron Microscopy(SEM),Energy Dispersive Spectroscopy(EDS)and X-ray Diffraction(XRD),respectively.It is found that the chemical reaction begins at a certain temperature between ABS and ash particles,and the increase in temperature promotes the chemical reaction.The original morphology of ash particles is changed by the existence of liquid ABS,and the agglomeration morphology of ash particles is different at different temperatures.Especially in 220?,there are floccules with strong adhesion between particles,making the adhesion between particles hard to be destroyed.Secondly,there are mainly C,O,Al,Si,Fe,Ca and other elements,mostly in the form of oxides in the components of fly ash.There exists a chemical reaction between ABS and ash particles.Moreover,the entire ash layer and ash particles exist pore structures and there must be physical adsorption.Therefore,in order to explore the degree of physical adsorption and chemical reaction between ABS and ash particles,the interaction experiments of different components of ash deposition and ABS are designed.In this experiment,CaO,A12O3,and SiO2,which are pure oxides with standard particle size,are utilized to represent the alkali metal oxides and common oxides in fly ash to simulate the reaction with ABS.The reaction products of CaO,AlO3,SiO2 and ash particles with ABS at different temperatures are tested by pH/conductivity and XRD.It shows that the pH of the mixture of different particles and ABS increases with the increase of temperature.The pH of CaO and ash particles changes relatively little,followed by the change of SiO2,and the pH of Al2O3 changes most significantly with the increase of temperature.Among them,CaO and ABS have strong chemical neutralization reaction,which consumes ABS.After reaction,CaO is alkaline,and the adhesion effect is not obvious.However,there is a little chemical reaction between Al2O3 and ABS,and the reaction intensifies after 200.There is no chemical reaction between SiO2 and ABS,only physical adsorption.Moreover,the mixture of SiO2,Al2O3 and ash particles with ABS is acid,and the adhesion effect between particles is obvious.The results suggest that there are different degrees of physical adsorption and chemical reaction between ash particles and ABS at different temperatures,which makes the ash adherent present different viscosity at different temperatures.In order to further explore the physical adsorption between ABS and ash particles,BET tests are carried out on the original ash and 200? experimental ash.The pore size distribution of the ash particles changes greatly before and after the experiment(the average pore size changed from 9.4561nm to 29.8231nm),the total pore volume of the particle pores decreases,and the specific surface area decreases from 1.3050mm2/g to 0.3127mm2/g,which confirms the physical adsorption capacity of the ash particles on the liquid ABSFinally,based on the results of theoretical and experimental studies and combined with the operation data of the power plant,its existing problems are analyzed.The methods to reduce the blockage of air preheater caused by liquid ABS are mainly explored from two aspects:adhesion mechanism and the source of ABS generation.Wherein,shortening the liquid range of ammonium bisulfate,reasonably adding alkaline substances to the boiler and changing the porosity of fly ash particles can effectively reduce the amount of liquid ABS,thus reducing the blocking phenomenon of air preheater.