【作者】 赵志渊；

【导师】 徐鸿；

【作者基本信息】 华北电力大学 ， 热能工程， 2012， 博士

【摘要】 我国在相当长的一段时间内能源结构主要以燃煤的火力发电为主。提高燃煤机组的能量转换效率,降低污染物排放是燃煤机组的主要任务。提高火电机组循环效率最有效的途径之一就是提高蒸汽的初参数。近年来,超(超)临界机组在我国已成为发展的主流,蒸汽温度已达600℃,压力达到30MPa,热效率达45%左右。近五年来,通过对国外超(超)临界机组技术引进和消化吸收,经过本土化的实践,已形成了自主设计、制造超(超)临界电站设备的能力。但对于高温材料特性和运行的技术还未完全理解,尤其一些运行操作规程的制定还主要是参照亚临界机组的运行经验,并未形成完整的成套技术。因而,氧化层导致的锅炉爆管事故时而发生,给电站锅炉、汽机以及热力系统的安全经济运行造成了重大影响。因此,对有必要对氧化层的成分、Fe离子的迁徙过程、给水加氧的可行性以及氧化层对管道的影响、氧化层的剥落机理进行深入系统的研究。本文首先运用热力学第二定律对金属元素和02的反应、Fe和H20(g)的反应及Cu和H20(g)的反应进行了计算,并将它们的吉布斯自由能与温度T的关系用Ellingham图表示出来。通过对Ellingham图的分析可得：Cr与氧的亲和力大于铁与氧的亲和力,这也就说明了,高温条件下,Cr含量越高,合金的抗氧化能力越强。并详细解释了过热器/再热器中,蒸汽的温度在540℃左右管道内壁氧化膜分层的原因,氧化膜由内向外的顺序是内层是FeO,中间是Fe3O4,外层是Fe2O3。电厂给水系统加氧处理的目的之一就是为了生成结构致密的Fe2O3,用以防止钢材的进一步腐蚀。而在汽水系统中生成致密的Fe2O3,必须保证给水中有充足的含氧量。由于Cu在有氧的环境下会发生明显的氧化反应,故冷凝器出口后面的铜管中不适合通过加氧水。以高温水和超临界水物理和化学性质的变化为切入点,从电站汽水系统中工质的电位和pH值两方面着手分析其对汽水系统中管材基体腐蚀的影响,并对电站汽水系统中Fe-H20系建立了升温条件下的电位-pH图。分别对固-液界面电位-pH图和液-液界面电位-pH图进行了详细而深入的分析,得到Fe-H20体系中Fe2+离子是最重要的腐蚀产物。在一定的pH和E的环境下,Fe转变成Fe2+,这些离子会随着工质流动,从而使管道基体中的[Fe]含量减少,使管壁减薄。本文从流动加速腐蚀和给水处理两方面进行了总结,得到以下结论：①流动加速腐蚀一般会发生在25-300℃之间,并且在150℃时,Fe转变为Fe2+,所需电位最低,此时最容易发生流动加速腐蚀。②为了阻碍管道基体中的[Fe]进一步转变成Fe2+,给水pH值应控制在9.6-12之间进行变化,使[Fe]转变为Fe30a和Fe203。从而得到给水加氧处理第二个目的就是同时从提高给水的pH值和提高反应电位,即氧化[Fe2+]；这两种途径均达到减小[Fe2+]的数量的目的。为了分析氧化层对过热器和再热器管道的影响,本文将L-M公式与传热学理论结合,建立了具有氧化膜的过热器、再热器的管道传热模型。分析它们在不同的管道的几何尺寸、蒸汽流量、蒸汽温度、烟气温度等因素下,管道基体外壁、基体／氧化层界面、蒸汽／氧化层界面三者的温度和热流密度的变化,以及氧化层厚度的变化。计算得到的氧化层厚度与实际测量值相比较,它们有很好的一致性,并在工程误差允许范围之内,所以这种计算内壁氧化膜厚度的方法是可行的。由于,管道运行过程中,管道的几何尺寸已定,对其他影响因素进行敏感性分析可以知道,它们对三个界面的温度和热流密度变化的影响顺序均为：蒸汽流量>蒸汽温度>烟气温度。所以,在电厂的实际运行中,重点监测蒸汽流量的变化。并且合理的证明了管道外壁的温升和氧化层／基体界面的温升是由于氧化膜的生长而引起的,它们之间呈现线性关系,并且它们的温升系数Aw1、Aw2与氧化层引起的温升系数B之间的关系为：B=α1Aw1+α2Aw2。并得到了各界面的热流密度与氧化膜厚度之间呈线性关系,并且是时间t和氧化膜厚度d的函数,其评估方程可表示为：q=C0+C1X+C2X2,其中X=logt-21og(0.467d)。本文运用有限元软件ANSYS对具有一定厚度氧化膜的过热器／再热器管道在蒸汽侧及烟气侧温度发生相同扰动时,在不同的蒸汽流量、初始温度和压力及管道的几何尺寸、烟气温度、氧化膜厚度等影响因素的情况下各个界面径向应力、周向应力、等效应力和壁面温度的变化进行了分析。相同温度扰动幅度下,蒸汽侧扰动对管基体及氧化膜内温度和应力的影响均比烟气侧扰动大得多。温度发生扰动,氧化膜内温度和应力在扰动初期有一个急剧变化的过程,且变化幅度很大,氧化膜易发生剥落。在各个影响因素中,蒸汽流量对三者的影响尤为突出,且远远高于其他因素,这是由于蒸汽对界面的冲刷所引起的。最后运用断裂力学理论,建立了由界面粗糙度参数ξ、弯曲指数∏和粘附比指数S组成的氧化膜屈曲剥落弯曲图,对氧化层的剥落形式进行了合理的解释。更多还原

【Abstract】 Coal-fired power plant is the most important in the energy structure of China for a long period. They are the major tasks to improve energy conversion efficiency and reduce emissions of coal-fired power plant units. It is one of the most effective ways to improve the initial steam parameters for improving the cycle efficiency of power plants.In recent years,(ultra) supercritical units in China has become the main units for power plants, the steam temperature reached600℃, and the steam pressure reached30MPa, the thermal efficiency of units reached about45%. Past five years, through introducing and studying the foreign technology of (ultra) supercritical units and practicing in China, the (ultra) supercritical units can be designed and manufactured in China. But, for the characteristics of high-temperature material properties and its operating technology has not been yet fully understood. In particular, the operation technique of (ultra) critical thermal power units has not formed complete set of technology, mainly according to the operation experience of subcritical units. Therefore, the explosion of boiler tubes induced by the oxide scale frequently occurred caused a great influence on the safe and economic operation of boiler, turbine and the thermal system. It is necessary to in-depth and systematic study on the component of oxide scale, Fe ion migrationprocess, the feasibility of the WOT (water-supplied oxygen treatment) and the influence of oxide scale on tubes and its spalling mechanism.This paper first use the second law to thermodynamics to calculate the reaction of metal elements and oxygen, the reaction of Fe and H2O (g) and the reaction of Cu and H2O (g), then, the Ellingham graph was used to express the relationship of their Gibbs free energy and temperature T. Through the analysis of the Ellingham graph, it can be known thatthe affinity between the Cr and oxygen is stronger than that between the Fe and oxygen.In the other word, the content of Cr is higher; the alloy antioxidant capacity is stronger in the high temperature. The reason that the oxide scale of inner wall of tubes is layered when the steam temperature is540℃in the superheater or reheater, the component of oxide scale from inside to outside are FeO, Fe3O4, Fe2O3. One purpose of WOT in the power plant is to generate the Fe2O3whose structure is compact, used to prevent further corrosion of steel. It is necessary that the oxygen levels in the water-supply system are enough to generate the Fe2O3whose structure is compact in the steam-water system in the power plant. For the element Cu, it was oxidated obviously in the aerobic environment. So, the copper tube located at the export of condenser is not suitable for the water added oxygen.Taking the physical and chemical properties change of high temperature water and supercritical water as the breakthrough point, the influence of the potential and pH value of working substance on the corrosion of tube matrix were analysed and the potential-pH diagrams of Fe-H2O system under the different temperature were established. The potential-pH diagram of solid-liquid interface and liquid-liquid interface were detailed and in-depth analysed respectively, it can be known that Fe2+ion is the most important corrosion production in the Fe-H2O system. In certain pH and E, Fe was transformed to Fe2+ion; these ions were flowing with the working medium, the [Fe] content was deduced in the tube matrix, the tube thickness was redacted. This paper summarized from the flowing accelerated corrosion and feedwater treatment, the results are:①FAC (Flowing accelerated corrosion) generally occur in25-300℃, and in150℃the potential needed by that the Fe is transformed to Fe2+is minimum, the FAC is the most feastible to occur.②In order to prevent [Fe] in the tube matrix was transformed to Fe2+, the pH value of feedwater should be controlled between9.6-12to make the [Fe] transformed to Fe3O4and Fe2O3. The second purpose is that improve the pH value and reaction potential of feedwater, in other words, the [Fe2+] were oxidized; so the [Fe2+] content was reduced.In order to analyzing the influence of oxide scale on the tubes of superheater and reheater, this paper combined the L-M formula to the heat transfer theory, and the heat transfer models of tubes with oxide scales of superheater, reheater have been established. The change of temperatures and the heat flux of outer-wall, metal base-oxide scale interface and steam-oxide scale interface and the thickness of oxide scale have been analyzed under the different tube geometric, the different steam flow and temperature, the different gas temperature and so on. Compared the calculated value and measured value of oxide scale is feasible. When tubes are operating, their geometics have not been varied, it can be known by analyzing the sensitivity of the other factors that the order of the influence of them on the temperature and heat flux of each interface is steam flow> steam temperature> gas temperature. So, when the power plant is operating, the change of steam flow should be under the key monitoring. And it is reasonable proved that the temperature rising of outer-wall and metal base-oxide scale interface is caused by the growth of oxide scale, and there is a linear relationship between them. The relationship between temperature rising coefficient of outer-wall and metal base-oxide scale interface, Aw1,Aw2, and B that is temperature rising coefficient caused by oxide scale is B=α1Aw1+α2Aw2The relationship between the heat flux of each interface and the thickness of oxide scale is linear. It is a function about tuntime t and oxide scale thickness d. Its assessment formula can be expressed as q=C0+C1X+C2X2, where, X=logt-21og(0.467d)。The radial stress, circumferential stress, equivalent stress and temperature of each interface were analyzed by adopting softwear ANSYS for the tubes with oxide scale of superheater or reheater when the temperature of steam and gas changed the same under the different tubes geometrics, the different steam flow, the different steam initial temperature, the different steam press, the different gas temperature and the different oxide scale thickness. When the temperature of steam and gas changed samely, the influence of steam on the temperature and the stresses of tube is the bigger than the gas. The temperature and stresses of oxide scale changed rapidly in the initial period when the temperature of steam or gas changed. The bigger change, the more facile spalling of oxide scale. In all factors, the influence of steam flow on the stresses and temperature is the biggest; the reason is the washing away of interface by moving steam. Finally, the bulking map of oxide scale composed by the interface roughness parameter ζ, bulking index Ⅱ and adherency index S had been established by adopting the fracture mechanics theory, and the spalling form of oxide scale had been reasonable explained.更多还原