Optimization Of Corrosion-resistant Materials In Low-temperature Boiler Flue Gas

More than half of the coal resources are consumed by the coal-fired power plains in China, while their CO2 emissions account for over 40% of the national total amount. Thus researches on energy saving of coal-fired power plants are of great significance for national energy security and greenhouse gases reduction strategies. Usually, the temperature of the flue gas in coal-fired power plant is 110-140℃ or even higher, and the heat carried away by the flue gas could reach up to 3%-8% of input fuel heat value. In contrast to that, there are a lot of cold water or cold air to be heated, for example, make-up water, condensation water, backwater and make-up water in thermal power plant and air combusted in the boiler. Because of the huge energy in the power plant, if the waste heat of the boiler flue gas can be made full use of, we can play the effect of saving energy, reducing emissions and improving efficiency. It is necessary to recycling the waste heat of boiler flue gas. The part of boiler flue gas belong to the low-temperature heating surface of boiler, in order to absorb the boiler exhaust gas waste heat, it is necessary to break the limitations of the flue gas dew point, and now the choice of materials in the low-temperature flue gas has become the biggest factor constraining the utilization of the waste heat.In order to study the mechanism of dew point corrosion, amorphous composite coating and three kind of tube materials frequently used as anti-dew point corrosion materials were selected as research objects. The corrosion behaviors of materials were evaluated by immersion tests, electrochemical tests in sulfuric acid solution and simulated dew point corrosion tests. The results show that amorphous composite coating exhibits the most excellent corrosion resistance in each experiment. The sulfuric dew point corrosion resistance of the four test materials is as follows: amorphous composite coating>316 stainless steel coating>ND steel>20G steel. It can be concluded that the amorphous composite coating can prevent the matrix from dew point corrosion effectively in low temperature flue gas environment. Finally, the article describes the results of an investigation about the failure of air preheater tube on a pulverized-coal-fired power plant. In order to find out the failure causes and to suggest preventive measures, phase compositions and macrostructure of the tube metal surface were investigated by X-ray Diffraction and Scanning Electron Microscope equipped with energy dispersive X-ray spectroscopy micro-analysis. The results show that the dew point corrosion mainly is oxygen corrosion under the condition of the viscosity ash deposition, and the corrosion products are composed of ash and acid reaction products in the outer layer, iron sulfate in the middle layer, and iron oxide in the inner layer.In this paper, the methods and results concluded can be important theoretical basis for the optimization of corrosion-resistant materials in low-temperature boiler flue gas, and will popularizes the waste heat utilization system to a large extent.