Study On Water Wall Corrosion And Its Prevention Of 1000MW Ultra-supercritical Boiler After Low NO_x Combustion Retrofit

The applications of NOx emission control and low NOx combustion technology in pulverized coal-fired power paints have widely caused the serious corrosion of water walls in supercritical and ultra supercritical boiler units.In order to meet requirements of the NOx emission especially the ultra low emission control,the retrofit of the boilers with deep low-NOxcombustion has made the problem more prominent,whichseriously affects the safety and avaliability of the uints operation.Therefore,it is very important and necessary to explorethe main reasons and mechanisms of the corrosion and its fast ratedue to low NOxcombustion,and then put forward effective measures to control and even prevent the corrosion of the water walls.The study of this thesis aimed at a 1000MW ultra supercritical coal-fired boiler,which has undergone serious corrosion of the water walls after low NOx combustion retrofit.The mechanisms of the water wall corrosion of the boilers with low NOxcombustion were discussed in detail and reviewed.Based on the in-site tests of the boiler,the flue gas conditionsnear the water walls were measured,andthe historic operation data of the exhausted flue gas composition and the wall temperature of the boiler were analyzed.Combined with the numerical simulations of using the computational fluid dynamics method,the causes and factors of the corrosion were systematically analyzed and investigated to explore the main causes of the severe corrosion after deep low NOxcombustion retrofit.On the basis of these investigations,the technical measures to reduce and prevent the corrosion were explored pertinentlyby improving the near wall flue gas conditioninsthrough combustion opitimization.Based on the historical data of the holier operation and the results of the maintennace survey,the analyses on the water wall corrosion and the high CO emission after the retrofit showed that solving the problem of the high CO emissionis consistent with the requirement of solving thewater wall corrosion.The CO emission is mainly affected by the operating oxygen level and secondary air distribution modes and so on.So it can be controlled by the oxygen level control,reasonable air distribution and combustion optimization,which can also reduce the corrosion of the water walls.The analyses of the causes and influencing factors of the water wall corrosion showed that the wall temperature and the corrosive flue gas near the walls(reducing atmosphere or reducing/oxidizing alternating atmosphere)are the important factors,but the presence of FeS deposit fromtheflushing of the burning pulverized coal-air streams due to the factors related to the aerodynamic flow and combustion process characteristics is also a necessary condition for the occurrence of the corrosion.The in-site tests of the combustion tuning and optimization showed that,limited by the very low excess air coefficient of the lower furnace in the deep low NOx combustion boiler,the positive pod mode of secondary air distribution could reduce the emission of CO without affecting the NOx emission concentration,but have little influence on the near-wall corrosive atmosphere;Reducing the SOFA damper openningscan not only improve the near-wall atmosphere near to reduce the corrosion tendency and effecitively control the CO emission at the boiler outlet,but also improve the boiler combustion efficiency to a certain extent.However,it will lead to a rise in NOx emission to some extent,which is contrary to the use of a large amount ofthe SOFA to achieve the goal of the deep low NOxcombustion.Therefore,the use of this tuning measure to prevent the corrosion of water walls may require systematical technical and economic analysis and combustion optimization to justify its application.The combustion adjustment of changing the same level secondary air distribution not only can significantly improve the reducing atmosphere near the water walls,especially in the corrosion zones,but also is conducive to reducing and controling the boiler CO emission,and to some extent reducing NOx emission and improving combustion efficiency of the boiler.Increasingtheburner air of the upper three layers of pulverized coal burners can not improve the reducing atmosphere near the water walls.The water wall tube thickness checkingresults after long-time operationprovedthat the anti-corrosion measures of combustion tuningand combustion optimization are effective,but the corrosion rates of the corrosion zonesarestill hig,implyingthat further research and development forthe effective anti-corrosion technology and methods are still needed.