| Power generation technology has been developed very fast along with the economic growth in China.Environment protection is becoming a serious problem to obstruct the future development.In order to increase the power supply efficiency and control the emission,large coal-fired power plants have to upgrade to advanced(ultra)supercritical technology based on the power generation structure in China.China has become the largest country in the world for the number of operating and new building ultra-supercritical units.The technology development of(ultra)supercritical has strong support the power production improvement of more efficient and cleaner in China.But a lot of problems came up during the process of the advanced technology utilization.These problems have seriously effect on the reliable and safe operation of power equipment.Especially,boiler tube explosion and steam turbine solid particle erosion problems are caused by oxide exfoliation of(ultra)supercritical boilers.In order to solve the problem of power plant,study on spalling mechanism for its growth,research and practice of comprehensive control of oxide is significant to the power generation industry.High temperature steam oxidation behavior of high grade heat-resisting steels of(ultra)supercritical boiler was conducted an in-depth study based on the available research results collected in a wide range of domestic and international as well as B&W's leading(ultra)supercritical boiler technology.Research for the particular material characteristics and growth of oxide has been done for austenitic stainless steel TP347 HFG,Super304H,and HR3 C in this paper.Based on the supercritical water experimental facility,oxidation of austenite steel Super304 H was carried out by exposure to different temperature and dissolved oxygen(DO)content.The samples were weighed before and after exposure.The morphology and chemical compositions of the oxide films were analyzed with scanning electron microscope(SEM)equipped with energy dispersive X-ray spectrometer(EDS).A theta-2 theta X-ray diffraction(XRD)was employed to determine the crystal structure of the oxides.The influence of temperature and DO on oxidation rate and oxide morphology has been investigated.The weight gain increased with the increasing temperature and DO.A thress layer oxide scale developed on Super304 H steel,which was mainly composed of an outer magnetite layer,an inner a Cr-rich inner oxide layer and a diffusion layer.The porosity of outer layer increased with the increasing temperature and DO.It can be observed nodular oxides scale consisting of Fe-rich oxides distributes on the surface of samples at the initial oxidation stage.The nodular oxide constantly grows along the alloy surface and completely covers the surface of Super304 H steel with the increasing temperature and time.The main oxide phase exposed to deaerated supercritical water is Fe3O4and(Fe,Cr)3O4 while Fe2O3 was also observed at 2000 ppb DO.Leading oxide spalling stress factors are analyzed in this paper.There are wedge-type peeling under tensile stress and buckling spalling type for oxide exfoliation at present.The significant impact on exfoliation of temperature fluctuations and oxide thickness also has been discussed.According to the conclusion,replacing of existing austenitic stainless tube by higher grade material resolves the problem of oxide exfoliation for supercritical boilers in a specific power plant.After overall feasibility analysis of the technical and economic,all the TP347 H tube on boilers in power plant were upgraded to excellent high temperature anti-oxidation of Super304 H shot-peened tube to solve this problem which influenced the safe and reliable operation of power plants. |
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