【作者】 孙利；

【导师】 阎维平；

【作者基本信息】 华北电力大学(北京) ， 热能工程， 2021， 博士

【摘要】 高效、洁净、低碳是煤电发展的趋势,提高蒸汽参数是主要途径之一,因此发展超超临界火电机组是提高能源利用率的主要方向。但是,锅炉高温受热面蒸汽侧氧化皮剥落问题导致的堵管引发爆管等事故频发,造成巨大的经济损失,制约着机组的性能和可靠性。深入研究锅炉高温受热面合金材料的氧化速率、管壁温度、蒸汽侧氧化皮的应力和粘附性等问题,有助于揭示氧化皮生长与剥落的机理与规律,为锅炉高温受热面的设计和安全运行提供依据。本文在探索铁素体-马氏体钢氧化速率的基础上,对水平烟道高温末级过热器垂直管束管壁温度、氧化皮的生长应力进行了预测,分析了其变化规律;同时结合密度泛函理论计算方法从原子角度对蒸汽侧内外层氧化皮界面的粘附特性和断裂韧性进行计算。主要工作总结如下:(1)以铁素体-马氏体钢NF616和HCM12A为研究对象,根据Backhaus和Topfer实验数据,推算出较低温下磁铁矿层的铁扩散系数;讨论了用于计算铁铬尖晶石层铁扩散系数,且与氧活度相关的系数Rv、RI。估算基体/铁铬尖晶石层界面、磁铁矿层/超临界水蒸气界面处的氧活度。最后,由Wagner氧化理论,计算了不同温度超临界水蒸气环境下NF616和HCM12A的氧化速率常数。计算结果表明:模拟的氧化增重值与有关文献的实验值比较接近;氧化皮内氧活度是连续的,而在铁铬尖晶石层/磁铁矿层界面处铁扩散系数是不连续;空洞有可能在铁扩散系数最小的位置和铁铬尖晶石层/磁铁矿层界面位置形成。(2)基于热偏差理论和过热器管的局部能量和质量平衡,提出了一种计算管壁温度的方法。考虑烟气侧的积灰,计算了超临界锅炉高温过热器管壁温度分布;同时利用本文的铁素体-马氏体钢氧化速率计算方法,将氧化皮/超临界水蒸气界面温度做为氧化皮的生长温度,对蒸汽侧氧化皮厚度进行了估算。将不同服役时间下锅炉过热器管氧化皮厚度的计算结果与现场实测数据进行了比对,结果比较接近。由于烟气冲刷强烈,迎风弯管处壁温突然升高。烟气沿受热面高度方向的温度分布不均匀性对最终壁温计算结果影响很大。(3)根据广义平面应变问题的应力平衡关系,将氧化皮生长应变引入胡克定律,同时考虑管子所承受的内部压力,推导了氧化皮生长应力的计算表达式。分析了不同管径和不同横向氧化应变对应力分布的影响。结果表明:氧化皮环向应力值远大于径向应力值和轴向应力值;位置靠近磁铁矿/铁铬尖晶石界面时,环向应力值较大,在这个界面上容易出现氧化皮的膨胀和剥落;管径越小,氧化皮环向应力值和轴向应力值越大;氧化皮的环向应力随着受热管蒸汽侧压力的增大而增大;生长应变的横向分量对氧化皮应力有较大影响。(4)运用第一性原理研究了外氧化层/内氧化层界面,即Fe3O4(001)/FeCr2O4(001)界面的粘附功、界面能、界面断裂韧性以及电子结构和价键,为建立客观的蒸汽侧氧化皮断裂韧性指标提供依据。当(001)面的原子层数大于15层和13层,Fe3O4板和FeCr2O4板可达到各自体相的特征。研究了 12种不同终端组合的界面模型,不同堆积位置对界面结合强度和断裂韧性的影响。Fe3O4(001)-FeO终端/FeCr2O4(001)-Fe终端构成的Model E具有最大的粘附功和界面断裂韧性,热力学上更稳定,此构型可能是氧化皮生长过程中的实际原子构型。界面处电子结构表明界面原子之间存在离子/共价键和金属键。更多还原

【Abstract】 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.更多还原