Research On The Effect Of Operation Conditionto The Heating Surface Safety In Supercritical Boiler

With the vigorous development of industrialization,Supercritical and ultra supercritical thermal power unit have gradually become the main unit for peaking regulation.Meanwhile,it will bring some safety problems to the heating surface of the boiler.Especially the high-temperature heating surface where the refrigerant is heated to the highest temperature.On the one hand,due to various thermal deviations caused by the design and operation of the boiler,the overheating problem may occur in some sections of the heating surface.One the other hand,the temperature and pressure of the steam have been constantly improved because of the more higher design demand make the steam oxidation process to be accelerated,and the lower thermal conductivity of the oxide film will drop the steam cooling rate in the pipe and cause over-temperature.In addition,the frequent variation load operation may lead to overheating of the heating surface due to the inconformity of the boiler's various adjustment and coordination systems.Therefore,it is important to study the influence of the operating parameters of the supercritical boiler units on the safety of the heating surface to the design of 700? units.The transient wall temperature model of the tube in boiler heating surface is established based on the finite volume method,which considering the oxidation conditions in the inner wall of tube.The secondary superheater with the highest working temperature is taken as the research object,and the non-steady wall temperature of the superheater is analyzed with the variation load.The boundary conditions under different working conditions are determined.The temperature distribution of-the single superheater tube was calculated under different constraint conditions such as temperature,mass flow,and oxide scale.The influence of temperature,flow rate,scale thickness and others on the wall temperature of the superheater was studied.The oxidation experiment of austenitic steel HR3C and nickel-alloy Haynes 282 were carried out by exposure to different temperature.Temperature,pressure,dissolved oxygen(DO)and velocity are 600-700 ?,25 MPa,0 ppb and 0-5 ml/min,respectively.The oxidation kinetics curve of the two kinds of alloys(HR3C and Haynes 282)under different temperature were obtained,the microstructure and the main oxide phase composition of oxide film were analyzed using scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),X-ray diffraction(XRD)and X-ray photoelectron spectroscope(XPS).By analyzing oxidation characteristics of austenitic steel HR3C and nickel-alloy Haynese 282 at different temperature,it can be found that the diffusion rate of iron along the inner oxide layer is the oxidation rate-controlling step to HR3C.And the diffusion rate of chromium ions is the oxidation rate-controlling step to Haynes 282.Meanwhile,the change of temperature will affect the oxidation kinetics of the two materials.The oxidation kinetics of HR3C performed at 600? under 25 MPa is following the parabolic law and obeying the cubic law at 650 and 700?.Unlike this,the oxidation kinetics of haynes 282 is following the parabolic law in 600? but following between the line law and parabolic law in 650 and 700 ?.The temperature fields of the superheater tubes in different spatial positions under three different working conditions was calculated,and the temperature variation rule of each tube under the working condition of the corresponding timetable dial was obtained.The results was shown that the highest temperature does not necessarily occur at the highest load.The effect of the oxide film on the temperature distribution of the tube wall was studied,and other influencing factors were analyzed.The most influence factor is the mass flow and next ones are oxide scales,steam temperature and gas temperature.Base on the grey relational analysis,the influencing factors of the superheater wall temperature in supercritical/ultra-supercritical boilers are analyzed.14 main influencing factors with the threshold more than 0.7 are used to predict the tube wall temperature under three operation conditions by using the BP neural network algorithm.The results of the largest relative error is about 1.42%so that it can be used to provide good guideline to avoid overheating.