TP347h、T91 Boiler Steel Oxidation And Regulation In Supercritical Steam

ultra-supercritical unit of the unit pipe steam side oxidation corrosion is serious,not only reduces the boiler life and heat transfer efficiency,more seriously is the corrosion generated by the oxide skin easy to shed,shed the oxide skin accumulation,blocking the pipe caused by local overheating of the boiler tube leads to boiler burst,seriously affecting the safety and economic operation of thermal power plants.The National Huaneng Corporation has set up a research project on "prevention of oxidation skin in supercritical boilers",of which this study is a part.The paper takes the two boiler steels TP347H and T91,which are commonly used in industry,as the research objects.Starting from the corrosion and oxidation rates and mechanisms of the two boiler steels,the paper explores the feasibility and the best process parameters of oxygen removal and pH adjustment to reduce corrosion,which are easily implemented in industry;through the analysis of the mechanism of oxide film peeling,the paper explores the controlled peeling of the oxide film and proposes the method of adding EDTA complex to the boiler feed water to thin the oxide layer midway through boiler operation by passing oxygen into the feed water steam before boiler shutdown to thicken the oxide layer on the surface of boiler steel to produce cracks,coupled with the disturbance of boiler steam,and using the descaling idea of oxide layer blowing before shutdown.The process parameters of supercritical boiler operation and the working history of restarting after shutdown of thermal power plants were further simulated to investigate the regrowth pattern of oxide skin on the flaking surface of two types of boiler steel to provide a basis for safe operation of thermal power plants.The following main conclusions were obtained from the study:(1)Boiler steel TP347H,T91 in supercritical steam occurred faster corrosion,the beginning of the 24-hour corrosion rate reached 0.4729mm/a,0.9522mm/a.With the extension of corrosion time,the rate of oxidation corrosion gradually reduced.Corrosion products for three layers of oxide,the upper layer is mainly iron oxide is relatively loose,the surface layer and the middle layer of oxide combined poorly,the lowest layer is thin,mainly Cr oxide,the upper layer is easy to break and fall off.Removing boiler steel TP347H from supercritical water steam and leaving it in air will increase its corrosion and will result in the rupture of the oxide layer.Removing boiler steel T91 from supercritical water steam and leaving it in air does not corrode further and cracks in the oxide layer do not extend further.(2)TP347H steel surface oxide film layer in a circular defect,the oxide film layer at the defect growth,so that the stress around the defect becomes large,in the vertical cross-section of the film layer produced a tensile stress,and finally make the film off,off the shape of the round;T91 surface film under stress cracks,with the growth of cracks,the formation of the film off,off the part of the irregular polygon.In the boiler steel oxide shedding surface corrosion and oxidation again,the corrosion rate of boiler steel is lower than the corrosion rate of the new steel surface,but the corrosion rate after shedding is higher than the corrosion rate at the place of nonshedding,repeated shedding after the surface residual oxide more,get with the increase in the number of shedding,the corrosion rate decreases.Film off and then grow the oxide film is relatively dense,which is conducive to the corrosion protection of boiler steel.(3)Boiler steel TP347H,T91 at 650℃ feed water through Ar deoxygenation of water steam corrosion rate is greatly reduced,with ammonia to adjust the pH of the feed water to 9 and feed water through Ar deoxygenation conditions under the lowest corrosion rate,compared with the corrosion rate under the same conditions without deoxygenation,respectively,decreased by 85%and 42%.(4)The use of feed water through N2 deoxygenation,boiler steel corrosion rate reduction effect is not obvious,especially the use of ammonia to adjust the pH,feed water through N2 deoxygenation conditions,N2 impedes the decomposition of NH3,increasing the NH3 complexation reaction,so that the T91 steel supercritical water steam corrosion rate increased instead.NaOH to adjust the feed water pH,boiler steel T91 corrosion rate is greatly reduced,lower than that of ammonia to adjust the pH pH adjustment,the corrosion rate of boiler steel T91 is greatly reduced,lower than the corrosion rate of pH adjustment with ammonia,but also less than the corrosion rate of feed water through Ar deoxygenation.(5)The oxidation rate of the boiler can be increased by passing oxygen into the feedwater 18 hours before the boiler is shut down.powder oxide peeling occurs spontaneously on the surface of TP347H steel,while the surface of T91 steel does not appear to be peeling,but bulging occurs,and a lumpy peeling occurs when the bulge is lightly touched.After boiler shutdown,steam is used to purge the pipe so that the oxide skin is blown off from the surface of the inner wall of the boiler tube and brought out by steam.The addition of EDTA to the boiler feed water can partially dissolve the Fe ions in the oxide layer,so that the generated oxide is partially dissolved to achieve the effect of oxide thinning,which can prevent the continued operation of the process of oxide too thick to produce cracks,shedding and other accidents.