Effect Of SO₃ And Ammonium Bisulfate On Scale Formation Characteristics Of Boiler Downstream Components

The generation of SO3 in the flue gas of coal-fired boilers and the ammonia escape of the SCR denitration device together lead to the production of ammonium bisulfate(ABS)in the low-temperature flue gas.The ABS deposited on the heated surface of the downstream components will adhere to the fly ash in the flue gas and undergo chemical reactions to form dense scale,causing scaling and even blockage of the air preheater and downstream components,which seriously affects the safe and economic operation of the boiler.Existing studies pay more attention to the formation mechanism of SO3 and ABS,and the research on the scaling caused by ABS focuses on the physical adhesion.The influence of the chemical reactions between ABS and fly ash on the fouling characteristics of the downstream components cannot be ignored.The research on the related reaction characteristics can provide a theoretical reference for the prevention and treatment of the fouling and scale formation of the downstream components.In this paper,we combine thermodynamic simulation calculation and experiments to investigate the characteristics of reactions between H2SO4/ABS and fly ash,and analyze the components of scale product.The reaction process of H2SO4 and fly ash was studied by thermodynamic simulation.All solid phase products were divided into three categories as sulfate,aluminosilicate and free oxide as the scale component.The sulfate generated by the reaction will promote scaling.Calcium sulfate is the major sulfate phase in the limescale.In addition to magnesium sulfate,sodium sulfate,and potassium sulfate,potassium thenardite(K3Na(SO4)2)and potassium alum(KAl(SO4)2)are produced.Both potassium thenardite and potassium alum are products that promote scaling.The calculation results show that the flue gas temperature has little effect on the proportion of the three types of scale components and the amount of sulfate generated.Besides,magnesium sulfate and aluminum sulfate exist as sulfate hydrate in low-temperature flue gas.We define the dust-SO3 ratio(D/S)as the mass ratio of fly ash and H2SO4,and therefore the proportion of sulfate decreases as the dust-SO3 ratio increases;the proportion of aluminosilicate increases with the ash-SO3 ratio;as the degree of ash sulfation and aluminum silicification decreases,the proportion of free oxides that fail to participate in the reaction will increase.The relationship between the amount of sulfate produced and the composition of ash indicates that H2SO4 will preferentially react with calcium oxide in fly ash to produce calcium sulfate.By comparing the sulfate production characteristics of fly ash with different base-acid ratios(B/A),it is concluded that fly ash of high base-acid ratio generates more sulfate,which is more likely to cause scaling.By comparing the thermodynamic simulation results,we found that the reaction characteristics of ABS and fly ash are basically the same as those of H2SO4 and fly ash,though the influence of some ash components on the aluminum silicification is different,and the sulfate phase is different.We prepared ash scale with different amounts of H2SO4/ABS in the experiments.SEM and XRD analysis show that the formation of calcium sulfate in the ash scale enhanced the agglomeration of fly ash,and the dust-SO3 ratio will affect the form of calcium sulfate in the ash scale:as dust-SO3 ratio decrease,hemihydrate gypsum increases,and anhydrous gypsurn decreases.The reaction between ABS and fly ash produces not only calcium sulfate but also ammonium sulfate(AS)and potassium alum,during which fly ash forms tight agglomerates under the action of ABS,causing serious scaling.The lower the dust-SO3 ratio,the greater the degree of compaction.Thermogravimetric analysis of the experimentally prepared ABS ash scale showed that the maximum weight loss rate was reached at about 150℃,which validated the rationality and feasibility of online heating and gasifying air preheater for plugging control.Finally,we summarize the prevention methods and treatment of ABS scaling from the engineering perspective.