Research On Growth And Exfoliation Mechanism Of Steam-side Oxide Scale On High Temperature Heating Surface Of Supercritical Boiler

High efficiency,clean,low carbon is the development trend of coal power,and the main way is to improve the steam parameters.The development of ultra supercritical thermal power units is the main method to improve energy utilization.However,frequent accidents such as tube plugging and tube failure caused by oxide scale exfoliation of high temperature heating surface of boiler cause huge economic losses and restrict the performance and reliability of the unit.Therefore,it is helpful to reveal the mechanism and law of oxide scale growth and spalling by deeply studying oxidation rate,tube wall temperature,stress and adhesion of steam side oxide scale on high temperature heating surface of supercritical boiler.Based on the study of the oxidation rate of ferritic martensitic steel,the tube wall temperature of superheater and the growth stress of oxide scale are predicted,and their laws are analyzed.Based on the density functional theory,the adhesion and fracture toughness of the interface between the inner and outer oxide scales are calculated from the atomic point of view.The main work is summarized as follows:(1)Taking ferritic martensitic steels NF616 and HCM12A as research objects,the Fe diffusion coefficient in magnetite layer at lower temperature is calculated according to the experimental data of Backhaus and Topfer.Two variables Rv and RI which are related to oxygen activities can be used to calculate the Fe diffusion coefficient in Fe-Cr spinel layer.The oxygen activities at alloy/Fe-Cr spinel interface and magnetite/supercritical water interface are estimated.Finally,the oxidation rate constants of NF616 and HCM12A in supercritical water at different temperatures are calculated by Wagner oxidation theory.The calculated results show that the simulated oxidation weight gain data is close to the experimental data in the relevant literature.The oxygen activity in the oxide scale is continuous,while the Fe diffusion coefficient at the interface of Fe-Cr spinel layer/magnetite layer is discontinuous.Voids may form at the position where the Fe diffusion coefficient is minimum and at the interface of Fe-Cr spinel layer/magnetite layer.(2)Based on the thermal deviation theory and the local energy and mass balance of superheater tubes,a new method for calculating the tube wall temperature is proposed.Considering the ash deposition on the flue gas side,the temperature distribution of superheater tube wall of supercritical boiler is calculated.At the same time,the thickness of steam-side oxide scale is estimated by using the oxidation rate calculation method of ferritic-martensitic steels in this paper and taking the interface temperature of oxide scale/supercritical water as the growth temperature of oxide scale.The calculated results of oxide scale thickness of boiler superheater tube for different service time are compared with the field measured data.Due to the high gas temperature and high heat transfer coefficient,the wall temperature of elbow(position N)rises suddenly.The uneven temperature distribution of flue gas along the height direction has a great influence on the final wall temperature calculation results.(3)According to the stress equilibrium relation of the generalized plane strain problem,the scale growth strain is introduced into Hooke’s law,and the calculation model of scale growth stress is derived considering the internal and external pressure of the tube.The influence of different tube diameter and different transverse oxidation strain on the stress distribution is calculated.The results show that the absolute value of circumferential stress is much larger than that of radial stress and axial stress of oxide scale.The closer to the magnetite/Fe-Cr spinel interface,the larger absolute value of circumferential stress is.The oxide scale is easy to expand and spall from this interface.The larger the tube diameter is,the smaller absolute values of circumferential stress and axial stress are.The circumferential stress of oxide scale increases with the increase of pressure.The lateral oxidation strain component of growth strain has a great influence on the scale stress.(4)The adhesion work,interfacial energy,interfacial fracture toughness,electronic structure and valence bond of Fe3O4(001)/FeCr2O4(001)interface are studied by first principles.When the number of atomic layers of(001)plane is more than 15 and 13,the Fe3O4 and FeCr2O4 slabs can reach the characteristics of their bulk phases respectively.The effects of different stacking positions on the interfacial bonding and fracture toughness of 12 kinds of interface models with different terminal combinations are studied.The Model E composed of Fe3O4(001)-FeO termination/FeCr2O4(001)-Fe termination has the maximum adhesion work and interface fracture toughness,which is more stable in thermodynamics.This configuration may be the actual atomic configuration in the process of oxide scale growth,The electronic structure indicates that there are ionic/covalent bonds and metal bonds between the atoms at the interface.