Study On The Characteristics And Regulation Of Ammonium Bisulfate As Byproduct Of SCR Denitrfication In Coal-fired Power Plants

In the flue gas operation system of coal-fired power plants,the damage to key equipment caused by ammonium bisulfate(ABS)has attracted widespread attention,among which SCR denitrification and air preheater are the most affected by ABS,especially the air preheater is the most seriously harmed.The industry generally agrees that the existence of ABS in flue gas is the main cause of air preheater fouling,and many scholars and power plants have carried out research on ABS and related problems,but there are still no systematic results on the characteristics of ABS.The analysis of ABS formation,volatile decomposition,deposition and corrosion process characteristics,and the analysis of ABS control measures on this basis,have important theoretical significance and application value for revealing ABS characteristics and guiding ABS regulation.The root cause of air preheater fouling is the formation of ABS.Firstly,an ABS formation experimental system was designed,and the ABS formation law under different concentrations and ratios of NH3/SO3 was studied at 130-350℃.The ABS generation rate is significantly higher in the temperature range of 130-250℃ than that of the temperature range above 270℃.In the temperature range of 130-250℃,the generation rate of ABS increases significantly when NH3:SO3 is 1:1 and below.The increase of NH3/SO3 concentration product increased the ABS formation rate in particular.The ABS formation characteristics were analyzed by FT-IR and XRD,and reaction product in any NH3/SO3 ratio in the high temperature section(>230℃)was ABS.In the medium and low temperature section(190℃,150℃),when NH3:SO3 is 1:1 and 1.5:1,the NH3/SO3 reaction products are mainly ABS/AS mixtures.Due to the different ratio of NH3/SO3,the proportion of ABS and AS in the mixed sample was different.In general,the NH3/SO3 reaction occurred in the operating temperature range of the air preheater,which reduced the concentration of NH3/SO3 and helped to inhibit the formation of ABS in the mid-bottom temperature range(<230℃).When the escape NH3 concentration is constant,reducing the SO3 concentration can also inhibit the formation of ABS.While the concentration of NH3/SO3 decreases,the temperature also decreases,and the NH3/SO3 reaction products have a tendency to transfer from ABS to AS.The ABS volatile decomposition system was used to study the law and mechanism of ABS volatile decomposition by ion chromatography,SEM,flue gas analyzer,thermogravimetry analyzer,XRD and DRIFTS detection methods,and the ABS decomposition reaction path was analyzed.The results show that for every 50℃increase in temperature,the volatile decomposition rate of ABS shows an order of magnitude increase trend.When the temperature of ABS volatilization is lower than 300℃,the product of ABS volatilization and re-condensation is mainly ABS,When the volatile temperature of ABS is higher than 300℃,the products of ABS volatilization and re-condensation are mainly AS,ABS.H2SO4 and ammonia.In the temperature range of 140-206℃,the ABS decomposition reaction is mainly based on the reaction path Ⅱ of direct decomposition to generate NH3,H2O,and SO3.Above 206℃,ABS decomposition is dominated by reaction path I.ABS decomposition is divided into two steps,ABS dehydration to generate(NH4)2S2O7 in the temperature range of 206-297℃,and(NH4)2S2O7 decomposition to produce NH3,N2,SO2 and H2O in the temperature range of 297-390℃.During the ABS decomposition process,a secondary reaction occurs in the decomposition product,and the main product is(NH4)3H(SO4)2.Air preheater fouling is a manifestation of the ABS/fly ash deposition process.Through XRF analysis of the bottom sediment of the cold end of the air preheater of coal-fired power plants,a total of 8 kinds of mixed sediments of ABS/AS/H2SO4 and fly ash and ground fly ash(small particle size fly ash)were prepared according to the ratio of N:S:Si element molar ratio of 0.357:0.475:0.127.XRD and DRIFTS analysis were used to reveal the mechanism of ABS and NH4Al(SO4)2·12H2O affecting the deposition process in ABS/AS/H2SO4/fly ash mixed sediments.Overall,the presence of AS attenuates fly ash deposits caused by ABS and H2SO4.The deposition of ABS and NH4Al(SO4)2·12H2O on small particle size fly ash was more significant at 250℃,and its deposition performance was greatly affected by temperature.With the increase of temperature,the deposition performance of ABS and NH4Al(SO4)2·12H2O was weakened by decomposition and viscosity reduction.The results show that increasing the temperature of the fly ash deposition site helps to reduce the accumulation of sediment.Using ABS corrosion experiments,combined with SEM and XRD analysis,the corrosion law of ABS/AS/H2SO4 on 20#carbon steel and weathering steel was studied,and the corrosion rates of the two metal materials were H2SO4,ABS and AS from high to low,which were the most serious corrosion below 190℃.XRD and DRIFTS were used to explore the corrosion characteristics of ABS/AS/H2SO4/fly ash mixed sediments.The corrosion performance of H2SO4/fly ash was still higher than that of ABS/fly ash.AS promoted the corrosion performance of ABS/fly ash,but reduced the corrosion performance of H2SO4/fly ash.Since the corrosion performance of(NH4)3H(SO4)2 is stronger than that of ABS,the corrosion of ABS/AS/fly ash to metal materials is higher than that of H2SO4/AS/fly ash.Finally,the ABS process mechanism and regulation method are studied.Based on the ABS formation characteristics,a countermeasure for the cooling process of precision ammonia injection combined with air preheater is proposed.On the basis of controlling ammonia escape,the process reduces the cold junction outlet temperature of the air preheater to about 150℃,which can effectively inhibit the formation of ABS and promote the transfer of ABS to AS.Based on the decomposition and deposition characteristics of ABS,a bypass flue gas warming process is proposed.The process promotes ABS decomposition and reduces sediment viscosity by intermittently heating the cold end outlet of the air preheater at a high temperature of 250℃,which can effectively alleviate and eliminate the deposition at the bottom of the cold end of the air preheater and ensure the normal operation of the air preheater for a long time.From the perspective of coordinated control of flue gas pollutants in coal-fired power plants,the concept of integrated SO3 removal process of desulfurization wastewater was proposed.In this process,the concentrated desulfurization wastewater is alkaline tempered and sprayed in front of the air preheater to achieve the coordinated control of SO3 while the saline wastewater evaporates.In this paper,the formation mechanism,volatile decomposition path,sedimentation process,corrosion characteristics and regulatory strategies of ABS are studied.The pathways that inhibit the formation of ABS were discovered,two reaction paths of the volatile decomposition process of ABS were analyzed,the secondary reaction mechanism of the decomposition products was discovered.The mechanism of ABS in the deposition of fly ash in the air preheater was revealed,and the influence of flue gas temperature on deposition was clarified.It provides a theoretical basis for the formation of ABS regulation and industrial application.