Research On Mechanism And Mitigation Scheme Of High-temperature Corrosion Of Waterwall In Large-scale Coal-fired Boiler

In recent years,under the guidance of the national policy of energy conservation and emission reduction,the ultra-low emission standard has been proposed in the coal power field to strictly control the pollution emission of the large-scale coal-fired boilers.Hence the pieces of equipment of coal-fired boilers have been retrofitted and upgraded by power plants.The low nitrogen combustion system which can reduce the NOx emission effectively has been applied widely.However,that system would lead to the presence of the strongly corrosive reducing-sulfidizing atmosphere in the primary combustion zone of the boiler,causing the high-temperature corrosion problem of the waterwall and severely affecting the operation safety of the boiler.Therefore,to ensure that the large-scale coal-fired boiler meets the ultra-low emission targets and operates safely and reliably,it is of great significance to study the high-temperature corrosion mechanism of waterwall and formulate relevant mitigation schemes.Firstly,the corrosive atmosphere near the waterwall was simulated to carry out the accelerated high-temperature corrosion experiments,and the corrosion mechanisms of H₂S and CO in the reducing atmosphere were studied.The results have shown that the corrosion degree of the metal in the mixed atmosphere with the coexistence of CO and H₂S is much higher than the corrosion degree of the metal corroded by the single H₂S or single CO.The conversion of sulfur-containing substances in the reducing atmosphere is the key factor affecting the corrosivity of the atmosphere.The intensively corrosive COS is generated in the mixed atmosphere with the coexistence of CO and H₂S.COS can more easily destroy metal oxide and metal matrix and can deepen the sulfurizing degree of corrosion products.Then,the functional mechanisms of water vapor and alkali metal chloride of coal ash in the reducing atmosphere were studied.The corrosion characteristics of steels in the reducing atmosphere were compared with those in the oxidizing atmosphere.The results have shown that although water vapor can corrode metals and the corrosion resistance of products is poor,water vapor inhibits the formation of strongly corrosive COS in the reducing atmosphere.So the corrosivity of the wet reducing atmosphere is lower than the dry reducing atmosphere.The alkali metal chloride can only be corrosive by interacting with the water vapor.However,the corrosion effect of alkali metal chloride in a short time（160h）in the wet reducing atmosphere does not exceed its blocking effect,and it takes a long time for the alkali metal chloride to show the influence of enhancing overall corrosion degree.Due to the small amount of alkali metal chloride in the coal ash,the corrosivity of coal ash is relatively inapparent.The corrosivity of SO₂ in the oxidizing atmosphere is weak,and the metal oxidation corrosion is dominant instead of the sulfur corrosion.Because the amount of alkali metal chloride in the coal ash is small and the corrosivity of alkali metal chloride is limited by the SO₂,the corrosivity of coal ash is weak in a short time（160h）.The overall corrosion degree of the deposited-free specimen and the deposited-coated specimen in the oxidizing atmosphere is much less than those in the reducing atmosphere.Furthermore,based on the in-situ test and numerical simulation,the combustion adjustment was adopted to study the high-temperature corrosion mitigation scheme of the wall-fired boiler.The results have shown that adjusting the inner and outer secondary air flowrate of the burners on sidewalls has the best effect on mitigating the reducing atmosphere near sidewalls in the wall-fired boilers.The increase range of NOx emission can be limited by reducing the OFA flowrate to supplement the secondary air flowrate of the burners and unchanging the central air flowrate.The decrease range of the inner and outer secondary air flowrate of middle burners needs to be restricted when using the U-mode air distribution method.If that decrease range is too high,the proportion of unburned carbon in ash would increase which is harmful for the combustion.For the 600MW working condition of the target boiler,decreasing the OFA flowrate（5.2%of the total air flowrate）to supplement the inner and outer secondary air flowrate of the burners on sidewalls is the optimum combustion adjustment scheme,which can achieve the balance among the low concentration of reducing atmosphere,low NOx concentration and high boiler efficiency.Finally,in order to further mitigate the high-temperature corrosion problem of the wall-fired boiler,the near-wall air（NA）technology was adopted,and the structural parameters and layout scheme of the targeted type near-wall air were designed.The mitigation scheme was studied based on the in-situ test and numerical simulation.The results have shown that,without adding fans,if the NA is generated from the pressure difference between the secondary airbox and the furnace,the film radius of a single NA can reach 0.8m to 1.2m.The fans must be extra added to ensure the normal operation of multi NAs at the same time in the actual retrofit.Adopting NA is more direct and effective for mitigating the reducing atmosphere than the combustion adjustment method.If arranging 30 NAs,the effect on mitigating the reducing atmosphere would exceed the optimum combustion adjustment scheme at the 2.17%NA ratio and the NOx emission concentration is lower.If arranging 20 NAs to significantly save the retrofit cost,the NA and combustion adjustment needs to be used together（1.74%NA ratio combined with two kinds of combustion adjustment methods）as the cooperative optimization,which can also obtain the better effect on mitigating reducing atmosphere than the optimum combustion adjustment scheme with the consideration of the NOx emission and boiler efficiency.