Experimental Research On Oxidation Of Power Plant Material In Supercritical Water

"The coal as the main body, power as the center" is China’s future energy development strategy. Under the pressure of the energy shortage and environmental protection, ultra (ultra) supercritical power generation technology has become the main development direction of China’s clean coal power generation technology. The water in high temperature part of the unit will change to supercritical water with the rising of temperature and pressure in ultra (ultra) supercritical power plant. The change of supercritical water properties including density, hydrogen bonding, diffusion coefficient, viscosity and solubility lead to the transport process and chemical reaction of metal in supercritical water oxidation corrosion environment change significantly. With the improvement of unit parameters and the change of supercritical water properties, the oxidation of materials in supercritical water has become the key technology which restricts the development of ultra (ultra) supercritical power plant. At present, a lot of power plants using oxygenated treatment as chemistry water treatment method, therefore, study the oxidation behavior of steels using in power plants exposed to supercritical water with different dissolved oxygen concentration has important significance.The oxidation of low alloy steel T24, ferritic-martensitic steel P92and austenitic steel Super304H exposed to supercritical water at550℃under25MPa with the dissolved oxygen contents of100ppb,300ppb and2000ppb was investigated. The exposed time was200h,400h,600h,800h and1000h. Weight gain measurements, surface morphologies observation(SEM), cross-sectional morphology observation and elements analysis (SEM-EDS), X ray diffraction (XRD) analysis were done after the test. The results indicated that the weight gain of T24and P92was close and the oxide scale with a typical dual-layered structure including a Fe-rich outer magnetite layer and a Cr-rich inner layer was formed on all samples with the dissolved oxygen(DO) of100ppb and300ppb. However, a distinct gap between inner oxide layer and substrate alloy was observed in some regions when the T24and P92was exposed to2000ppb, a multi-layer might be appear in these regions. The weight gain of Super304H was significantly less than T24and P92, which also form a double-layered structure including a herpes-shaped loose outer magnetite layer and a spinel ((Fe, Cr, Ni)3O4) structure inner layer, the inner layer and the substrate divided irregular, and the intergranular corrosion was obserded. Supercritical water can promote oxidation corrosion. The Cr content of the alloy improve oxidation resistance, but the oxidation resistance didn’t get fundamental change when the Cr centent increased from2.51%to8.63%in this test. DO have a significant influence on weight gain and the crack. At the beginning, the oxidation process was controlled by the reaction of the supercritical water, oxygen and alloy, after that, the process was controlled by the diffusion of supercritical water carrying oxygen and the metal ion. The chemical composition and crystal structure of alloy and the DO have effect on the reaction process. In the test, Super304H have a best oxidation resistance in550℃/25MPa supercritical water with dissolved oxygen, then is P92, T24is the last. In550℃(and above) supercritical water with dissolved oxygen environment T24can not be applied, P92can be applied to550℃environment with low dissolved oxygen, Super304H have a good oxidation resistance in550℃supercritical water with dissolved oxygen.