Transport Of Corrosion Products In The Steam-water Cycle Of Supercritical Power Plant

The operation of the coal-fired units has to face new requirements to keep the ultra-low carbon emission standards under the constraint of carbon.The supercritical power plant usually improved the efficiency of the unit by improving the steam parameters,from 538? and 572? to 600? and 620?,even 700?,the increasing steam parameters brings new challenges to the high temperature segment.The operational reliability is significantly influenced by corrosion of the steam-water cycle.In recent years,the research of corrosion of the steam-water cycle is concentrated in the metal side,including flow accelerated corrosion in low temperature feedwater train and high temperature corrosion in boiler tube.The corrosion product in the steam-water cycle is a direct expression of metal corrosion,the generation and deposition of corrosion is significantly influenced by the water parameters.So it is important to study the transport of corrosion product in the steam-water cycle.Most of corrosion products are derived from carbon steels in feedwater train due to flow accelerated corrosion.The effect of main factors on flow accelerated corrosion is analysed.The generation model of corrosion product is built based on the mass balance,the data shows the porosity and the diffusivity is proportional to the corrosion rate,and pH is inversely proportional to the corrosion rate.The oxidation experiment of ferritic-martensitic steel T92 and T22 were carried out by exposure to different pressure 0.1MPa,8MPa and 25 MPa at 600?,respectively.The oxidation kinetics are similar in three different environments and they showed near parabolic behaviour.It appears that the level of benefit arising from increased chromium content is not constant but is dependent on the exposure environment.Whilst it is evident that the increasing pressure does increase the oxidation kinetics,it is also true that the presence of high levels of Cr in the alloy does still impart some enhanced protection.The oxidation experiment of T92,T22 and austenitic steel TP347 HFG were carried out by exposure to different dissolved oxygen(0ppb and 2000ppb)at 600? under 25 MPa.The results show dissolved oxygen mainly changes the oxygen potential distribution through the oxide scale,and accelerate oxidation rate of metal.At the same time,the dissolved oxygen and supercritical water also participated in the oxidation.The deposition of corrosion product is discussed.The corrosion product of Fe2+ can be oxidized to ?-Fe2O3,?-FeOOH and ?-FeOOH in subcritical water.In the supercritical environment,it can be rapidly oxidized to ?-Fe2O3 and ?-Fe2O3,and then the corrosion product can be deposited on the outer oxide layer.The effect of the water parameters on the transport of corrosion products is analysed,including density,ionic product and dielectric constant.On the one hand,it has a direct impact on the corrosion mechanism of the steam-water cycle,from electrochemical oxidation to chemical oxidation.On the other hand,it affects the species and solubility of corrosion products,the higher dielectric constant in low temperature feedwater train allows the solvent to retain ionic species in solution whereas the supercritical water is dominated by uncharged,neutral species and complexes of the metal.The solubility of corrosion products in supercritical water is 3~5 order of magnitude lower than subercritical water.The results show about 85% of the corrosion products transferred in the fluid was derived from the feed-water train and about 90% of the corrosion products deposited on the boiler tubes,including the economizer,water-wall tubes and superheater.The different effect mechanics of dissolved oxygen on corrosion in different part of steam-water system are defined.Flow accelerated corrosion in low temperature feedwater system can be controlled by dissolved oxygen,the concentration of corrosion product in oxygenated treatment condition is decreased by about 80% compared with the reducing all-volatile treatment condition.But dissolved oxygen can accelerate failure of boiler tubes at high temperature from three aspects: corrosion rate,crack growth rate of SCC,evaporating consumption rate of chromium.Considering corrosion of the whole steam-water cycle,a more effective feedwater chemistry treatment method is proposed.The steps and methods of building a comprehensive corrosion monitoring system for power plant is dicussed.