Research On Oxide Scale Failure Of High Temperature Tubes And Numerical Simulation In Supercritical Boilers

With increasing steam stress and temperature greatly, the supercritical power unit canimprove thermal efficiency substantially. It can also reduce generate electricity coalconsumption and reduce the discharge of greenhouse gases. However, the performancerequirement for high-temperature heating surface materials is much more severe. Thehigh-temperature tube inner wall could react with superheated steam to produce the oxidescale with a certain thickness. The generation and failure of oxide scale can lead tooverheating, blockage, and tubine erosion, etc. It does, however, bring some problems forcoal-fired power units'operation. So how to avoided or reduced steam oxide of the pipelineinside, control oxide scale growth and spallation in the supercriticle boiler have become aworldwide problem. This paper aimed at character of oxide sacle and boiler tubes' heattransfer on domestic supercriticle boiler, studyed the oxide scale failure by means ofnumerical simulation.Having analyzed the macro-morphology, micro-morphology of oxide scale and metalbase's metallographic structure, the mechanism and kinetics of high-temperature steamoxidation has been studied. Then, the failure mechanism of oxide scale was analyzed.Exfoliation failure mechanism of oxide scale is obtained in different tubing materials. Theoxide scale failure does not only just correlate with the oxidation kinetics, but also be relatedto oxide scale appearances, material mechanical properties and the tubes' geometriccharacteristics. The interaction of several factors should produce very big impact to thesubstrates-scales, resulted in oxide scale failure and spallation.Based on operation parameters and tubes heat transfer situation of final superheater in a600MW supercritical boiler, the temperature and thermal stress field of tubes were analyzedin its relationship with oxide scales failure by large-scale compute-software ANSYS. Thesimulation research shows that the temperature between different tube regions increases as thethickness of oxide scale increasing. The temperature in oxide scale-metal base interface,where heating rate was the biggest, was the most sensitive to the oxide scale thicknesschanged. The tuebe temperature in fireside is the higest; there accidents of overtemperatureoften occur. With the increase of fuel consumption, the temperature in oxide scale-metal baseinterface gradually increases. The scale of temperature variation is very small in cycling loadoperation.Steam pressure on the tube temperature in steamside is very small. As the thickness of oxide scale increases, thermal stress in oxide scale-metal base interface, equivalent thermalstress of fireside wall and equivalent stress in oxide scale show a liner increase. The higher aunit load, the larger the thermal stress in oxide scale and tubes. Oversize stresses will causethe oxide scale spalled or early failures. The result of tube temperature change according toscale thickness is similar with the actual measured values.Through the above research, the steam temperature must be carefully controlled in actuloperation of supercritical unit, so as to prevent local overtemperature in the tube and sloweddown oxide scale' growth rate. Due to decrease tubes' temperature gradie and maintain middleload operation, stress rupture of oxide scale should be avoided. So the large area of sacelspalling and tube early failures because of overstressing can be effectively reduced. Theallowable temperature and stresses of tubes must need comprehensive consideration inboiler's material selection,improving the security and reliability of power station boilers.