In-depth Research On The Corrosion Resistance Of Heat Transfer Tubes In Waste Incinerators By Using Adsorbents And Coatings

With the development of waste incineration technology,improving the main steam parameters and co-processing industrial organic solid waste are two dominant measurements to realize the economic benefits increase of waste-to-energy plants.However,the main limiting factor for the steam parameters improvement of waste incinerators is the high-temperature corrosion of the heating surface.When industrial organic solid waste containing plastics and rubber mixture is into the waste incinerator,there is an increase in the concentration of the corrosive substances such as HCl,Na Cl,and KCl in the flue gas.Therefore,the high-temperature corrosion on the heat transfer tubes will further deteriorate,which is a pivotal issue that needs consideration This paper systematically explored the corrosion resistance of modified kaolin and high-velocity oxy-fuel spraying coating,aiming to realize the corrosive medium control in flue gas and corrosion resistance enhancement of the heat transfer tubes,respectively.The scientific issues involved are:（1）whether the intercalation-exfoliation modification can effectively achieve the improvement of the adsorption capacity of kaolinite to alkali vapers,and the influence mechanism of the structural derivation on the adsorption capacity during thermal transformation is unknown.（2）the influence of components on the corrosion resistance of coatings,and the application performance of typical coatings in actual waste incinerators.To address these problems,detailed and in-depth work was carried out.The syringe pump and nebulizer were used to generate stable alkali metal vapor.And it was intuitively proved that the intercalation-exfoliation method could significantly improve the adsorption capacity of kaolin on alkali vapors.After intercalation-exfoliation modification,the sodium loading capacity and potassium loading capacity of kaolinite increased by 56%and 118%,respectively.Taking gaseous K as the representative of alkali vapors,the dynamic adsorption process of raw kaolinite and modified kaolinite can be mathematically described by the pseudo-first-order model and pseudo-second-order model,respectively.For the actual situation where multiple alkali vapors coexist,the adsorption capacity of kaolinite for alkali metal vapor was the largest when the Na/K molar ratio was1:1.Furthermore,the structural calculation results showed alkaline feldspar with a Na/K molar ratio of 1:1 had the shortest K-O bond length and the most stable structure.According to the adsorption performance and structural derivation behavior of the two kaolins at different temperatures,it was proved that modification can broaden the applicable temperature window of kaolin.Furthermore,the influence of structure derivation on the adsorption performance was revealed.The adsorption performance of raw kaolinite was relatively better in the range of 800-1000 ^oC,while the applicable temperature range of modified kaolinite was 450-1000 ^oC.Due to the pre-removal of partial hydroxyl groups during the modification process,the formation of metakaolin in the thermal conversion process was affected.During the primary dehydroxylation stage,the Al-O structure of modified kaolinite was easier to transform from saturated Al（VI）coordination structure to unsaturated Al（V）coordination structure,making the Al（V）content increased by 12%at 450^oC.With the increase of temperature,the Al（V）content derivation was consistent with the changing trend of adsorption capacity.The formation of the Al（V）coordination structure was the key factor affecting the adsorption performance of kaolin for alkali metals.Based on the optimized in-situ testing set-up of molten ash adhesion,the influence of components on the corrosion resistance of coatings was revealed.The corrosion resistance of the coating was quantitatively analyzed,and it was found that there was a strong positive correlation between the ash melting adhesion and the corrosion weight gain.The composition in coatings can be classified into three types,the main elements（Ni/Fe/Co）,the minor element-Cr and other elements,such as Si,B,Mo,W,etc.In the ternary composition distribution diagram,the ash adhesion force gradually increased from the“center point”outwards.The coating showed a linear or parabolic trend during the corrosion weight gain process,which was mainly affected by the contents of some elements such as Mo.When the coating contains some Mo,Mo O₂ oxide film can be formed,which significantly strengthened the microstructure and avoided the penetration of molten salts on the coating surface,thus slows down the corrosion weight gain rate.The effects of tube temperature and flue gas such as HCl on the anti-corrosion behavior of coatings were clarified.Both laboratory and field test results showed that the corrosion resistance of the coating,the corrosion degree of the Ni-based coating is only 10-20%of that of the boiler steel-12Cr1Mo V in the same corrosive environment.In the range of 400-600℃,the corrosion weight gain of 12Cr1Mo V increases about 10 times for every 100℃increase in temperature.The corrosion mass gain of Fe/Ni-based coating at 600 ^oC was only 10-20%of boiler steel,so coatings were suitable for waste incinerators with high steam parameters.HCl and SO₂ not only can corrode with metals directly but reduce the melting point of alkali metal salts in deposits to a large extent.When the corrosive gases and deposits coexist,the coupling effect significantly intensifies the corrosion of heat transfer tubes.In the actual waste incinerator,the wall temperature is higher in the high-temperature superheater zone,and there were more alkali chlorides with low melting points in deposits.As a result,the thinning loss of boiler steel in the high-temperature superheater was 1.9 times that of the low-temperature superheater.During the corrosion process,dense oxides of Ni,Mo and Cr were layered on the surface of Ni-based coating,which can prevent the diffusion of molten ions and corrosive gases,so the thinning loss of Ni-based coating was only 10%of boiler steel.In summary,the modified kaolin using the intercalation-exfoliation method and the coating can realize the corrosive species control and the strengthening of heat transfer tubes,respectively,thereby alleviating the fireside corrosion in waste incinerators.This study is of great significance to the stable operation and long-term development of waste incineration power plants.