Research on Generation and Spalling Mechanism of the Oxide Scale Located at Inner Wall of Boiler Heating Tubes
|School||North China Electric Power University|
|Course||Thermal Power Engineering|
|Keywords||Oxide scale component FAC(flow accelerated corrosion) feedwater oxygenated treatment heat transfer characteristics temperature heat flux stress spalling theory|
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.