Research On Oxide Scale Growth Mechanism Of Boiler Tube In Power Plant Exposed To Supercritical Water With Dissolved Oxygen

Ultra supercritical power generation is one of the most advanced clean coal technologies. Operating the water medium in the supercritical state in ultra supercritical (USC) power plant improves the energy conversion efficiency and fuel usage, thus a reduction of pollutant emissions. In comparison to the subcritical state, the physical properties of the water medium including density, diffusion coefficient, kinematic viscosity and dielectric constant change more greatly when it works above the thermodynamic critical point. Oxygenated treatment (OT) is widely used in USC power plant. So the materials of boiler tube are working in supercritical water (SCW) with dissolved oxygen (DO).In recent years, the tube explosion resulting from the excessively growth rate and all-pervading exfoliation of oxide scale occurs frequently. The reasons for this appearance is rather complex. Consequently, the research on growth mechanism of oxide scale of boiler tube in supercritical water with dissolved oxygen has important significant and academic value。The corrosion experiments of the ferritic (T24), ferritic-mantensitic (P92) and austenitic steel (Super304H, TP347HFG, HR3C) were investigated in supercritical water at the temperature of550℃and600℃under a pressure of25MPa. To probe the influence of DO content on corrosion, the DO contents were controlled at0,100,300and2000ppb, respectively. The exposure time were200,400,600,800and1000h. After obtaining the weight gain curves under different conditions, the surface morphologies and composition were observed using SEM and EDS technologies. At the same time, the cross structures and element distribution of the oxide scales were analyzed.The influences of Cr concentration, temperature, DO and water on oxidation were also investigated. The results indicated DO can have an obvious effect on weight gain and exfoliation of oxide scale. FAC avoided in OT method proposed by EPRI was proved to be non-controlled in SCW. The growth and exfoliation mechanism of oxide scale was changed when the water was present. So the influence of diffusion coefficient and solubility on oxidation were also discussed in this paper.Based on the different effect of DO on corrosion of tubes under different operating conditions, a new treatment method of feed water was proposed. In this method, the oxygen was fed in the site of the main pipe after precision treatment or the vertical pipe after the deareator, and the diamide was fed in the site of the pipe after high pressure heater and before economizer. The oxygen and diamide were reacted each other in the pipe. The deareated water was fed into the tube of the boiler. The treatment method proposed in this study can avoid of the occurring of FAC in high and low pressure heaters, as well as can decrease the corrosion of boiler tubes enhanced by DO in SCW.Based on observations on the uniform dual-layered structure oxide scale formed on ferritic and ferritic-martenistic steels, a correction method of the thickness of the oxide scale on the boiler tube using supersonic measurement system was proposed because of the different propagation velocity of supersonic in the above two layers. The more accurate thickness of oxide scale on boiler tube can be measured by this method.