Abstract: During the operation of newly installed SNCR denitrification system in a power plant, the problem of water wall corrosion is serious, which causes many times of non-stop accidents in the boiler. By analyzing the characteristics of SNCR equipment operation, the paper analyzes the causes of water wall erosion caused by SNCR operation, And put forward effective control measures. 1 SNCR brief introduction Due to the increasing social concern for environmental protection, the issue of environmental protection in power plants has become the focus of attention. Therefore, researches and application of technologies such as SNCR and SCR on the flue gas side to control NOX have been carried out. SNCR is a technique of Selective Non-Catalytic Reduction (abbreviated as SNCR). The SNCR method is mainly carried out by injecting NH3-based reducing agent such as ammonia or urea solution into the flue gas at a high temperature (900 to 1100 ° C.) (Without catalyst), NOx is reduced to nitrogen and water by a chemical reaction. 2 water wall corrosion problems  A factory SNCR system began pilot operation in 2007, with obvious NOx reduction during operation (NOx emission reduced from 350mg / Nm3 to 200mg / Nm3), ammonia escape rate controlled within less than 5PPm, and other items Indicators were found no abnormalities parameters. However, about 4 months after the pilot operation, it was found that the corrosion of the water wall near the SNCR nozzle caused several leaks of the water wall of the boiler, resulting in the tension of the passive shutdown and seriously affecting the safety in production. Through observation, several water wall corrosion leaks showed the following characteristics: no corrosion products on the metal surface, but showed large or small ulcer state, corrosion of pipe sections often appear irregular corrosion pits, and some were shell-like, and some oval Shape and so on. Another notable feature is the corrosion occurred in the orifice of the water wall elbow position. 3 corrosion mechanism  In the SNCR injection system, the gun design uses the high pressure side of the furnace steam as the atomization medium, atomizing steam pressure of 0.6-0.9Mpa, to meet the needs of atomization pressure. In the process of testing, it is found that the spray gun is dripping near the spray nozzle and is a droplet formed by the urea solution near the spray hole during the atomization process. At the same time, because of the need to adjust the injection amount of the urea solution during the test, , In the adjustment process will produce dripping droplets. As before SNCR put into operation, the boiler water wall has not occurred in a similar state of corrosion, it is concluded that the corrosion of water wall tubes around the nozzle hole with the dropping of urea solution. According to the experience of the urea industry, urea solution has strong corrosiveness under certain conditions. SNCR nozzle hole around the ash, flue gas, air and water vapor, leakage of water droplets and urea role, resulting in a series of chemical reactions. According to the mechanism of the corrosion process, the corrosion can be divided into two categories: chemical corrosion and electrochemical corrosion. Chemical corrosion is the absence of current corrosion process; electrochemical corrosion is the current generated corrosion process. However, sometimes it is difficult to distinguish between the two types of corrosion, which are based on the corrosion of the SNCR waterwall, but are dominated by electrochemical corrosion. 3.1 The basic principles of corrosion  Corrosion mechanism: Electrochemical corrosion is due to metal interaction with the electrolyte as a conductor, causing the current flow from one part of the metal to another, and metal damage occurs. The boiler water wall tube (20G) comes in contact with the urea solution dripped by the spray gun (the urea solution is an electrolyte and has a polarity). A portion of the iron atoms on the surface of the steel is attracted by the polar molecules of the water and starts to migrate into the solution The formation of positively charged iron ions, while the steel retains excess electrons, and with a negative charge. If the iron ions continue to drip into the urea solution, water wall tube will gradually appear potholes, causing corrosion damage. However, under normal circumstances, after a period of time on the steel surface will appear double layer phenomenon. Due to the negative charge on the surface of the steel, the negative charge on the steel attracts positively charged iron ions in the urea solution, forming an electric double layer on the steel surface. At this time can hinder the further dissolution of iron corrosion, is conducive to preventing further corrosion. However, as the urea solution continuously drips and evaporates (and other anions that dissolve iron ions are dissolved in the urea solution), the electrons are continuously attracted from the cathode, so that the anode constantly has electrons moving toward the cathode, and the iron ions at the anode Continue to move into the droplet, increasing corrosion, resulting in depolarization. 3.2 SNCR spray holes around the water wall corrosion analysis  2.1 dissolved salt: steel corrosion rate in water and water  Salt concentration, the lower concentration within a certain range, the corrosion rate increases with increasing concentration. Because the higher the salt water, the smaller the resistance of water, higher conductivity, corrosion will speed up. If some water in the anion, such as chloride and sulfate ions, they will destroy the metal surface protective film. Chloride (Cl-): Among them, the harm of chloride is the most serious. Ferric oxide turns into soluble ferric chloride because it is easy to replace oxygen ions in the metal surface protection film. Oxide film damage, the metal will be further electrochemical corrosion. The use of SNCR urea is boiler water-repellent, but due to the presence of urea-containing impurities in urea itself and the dissolution of chloride in flue gas and fly ash, the chloride content in the urea solution dripping onto the waterwall Rise, causing corrosion of steel. Sulfate and sulfite ions (SO4 2 +, SO3 2+): Although the urea solution itself does not contain sulfate and sulfite ions, urea droplets still absorb SO2 in the flue gas in the furnace and SO3 forms ions , Resulting in sulfate corrosion, destroy the protective film on the water wall steel to further aggravate the electrochemical corrosion. 3.2.2 The stress to which the material is subjected exceeds the yield limit of the material  Later, in the local exceed the yield point of the site, prone to electrochemical corrosion. Water wall elbow bend parts in the production process to produce deformation, the number will always cause some stress concentration, likely to cause potential difference. Steel deformation degree, the internal stress of the site, the electrode potential is lower than the deformation, the internal stress of small parts, as the anode, corrosion occurred. In the shutdown inspection found that water wall leaks mainly occurred in the bend at the corrosion phenomenon is also close to the phenomenon of stress corrosion: Often there are irregular corrosion pits, and some were shell-like, and some were triangular, and some were Oval and so on. Visible, due to the presence of residual stress at the bend, the urea solution found a breakthrough in corrosion, accelerated the occurrence of water wall corrosion. Can determine the residual stress corrosion is one of the main reasons SNCR water wall corrosion. 3.2.3 Free carbon dioxide corrosion: urea solution at high temperature  Disintegration of carbon dioxide gas, when there is free water in the presence of CO2, will make the water acidic reaction: CO2 + H2O → H ++ HCO3-