High Temperature Coupling Corrosion Test And Mechanism Research Of Typical Heating Surface In MSW Incineration System

Waste-to-energy（WTE）is a sustainable technology for the treatment of municipal solid waste（MSW）.However,the development of this technology is limited by the high-temperature corrosion of the heating surfaces of boilers.The MSW incineration flue gas environment is very complex,and there are various corrosive media including chlorine,sulfur（HCl and SO₂ in the flue gas）,and alkali metals（salts like Na Cl and K₂SO₄ in the ash deposits on the heating surfaces）.This makes it difficult to determine the high-temperature corrosion mechanism of metals in this environment.Therefore,a series of experimental studies were carried out in this paper,aiming at clarifying the high-temperature corrosion mechanism of typical heating surface materials in MSW incineration systems（mainly focusing on high-temperature heating surfaces like reheaters and superheaters with pipe wall temperatures above 450°C in the actual site）under complex flue gas conditions where chlorine,sulfur and alkali metals coexist.Firstly,an on-site sampling study was carried out in a large-scale WTE plant in China.The salt compositions in the ash deposits were obtained for six actual heating surfaces,and the failure analysis of a corroded burst tube removed from the tertiary superheater was conducted.The results showed that sulfates were contained in the ash deposits of all heating surfaces,and the Na Cl in ash deposits was an important factor causing the corrosion of heating surfaces.The obtained data provide a reference for subsequent laboratory corrosion test research.Secondly,through the orthogonal pre-test,it was determined that the order of the significance of each factor on corrosion was pipe wall temperature,Na Cl content in ash deposits,HCl concentration in flue gas,and SO₂ concentration in the flue gas.On this basis,a single-factor corrosion test study was carried out.The effects of HCl and SO₂ in the flue gas in the typical concentration range on the high-temperature corrosion of the commonly used heating surface material 12Cr1Mo V were studied at four temperatures of 460,510,545,and 580°C.The results showed that the presence of 300-1200 ppm HCl in the flue gas can significantly accelerate the high-temperature corrosion of 12Cr1Mo V,and the corrosion rate showed a tendency to increase with the increase of HCl concentration,but there was a certain limit and delay.The corrosion mechanism under the influence of HCl belonged to“active oxidation”,and local corrosion morphology structures such as pitting pits or grain boundary cracks on the metal surface were the intrusion channels of Cl₂.On the contrary,50-200 ppm SO₂ did not accelerate the high-temperature corrosion of 12Cr1Mo V,and the corrosion rate was not affected by the change of SO₂ concentration.The corrosion mechanism under the influence of SO₂ belonged to high-temperature oxidation under dry conditions.After that,the high-temperature corrosion mechanism of 310S alloy under the multi-factor coupling effect of chlorine,sulfur,and alkali metals was studied.At 460°C,the mixed alkali salts were not molten,and the corrosion,in this case,was mainly attributed to the chemical reactions between the metal and solid alkali chloride salts,and the“active oxidation”reactions induced by Cl₂,which was generated by different reactions of HCl,SO₂,and O₂ in the atmosphere.At 510-580°C,the mixed alkali salts melted,and the corrosion mechanism,in this case,was“electrochemical”+“active oxidation”reactions,in which the electrochemical reaction arose from the oxygen concentration difference cell formed by the difference in oxygen partial pressure between the melt/gas phase interface and the matrix/melt interface.Finally,a comparison of the corrosion behavior of three alloy types,12Cr1Mo V,TP347H,and Inconel 625,in the coupled corrosion environment at 550°C was carried out.The results showed that the“electrochemical”+“active oxidation”reaction was also applicable to explain the corrosion behavior of the three alloy materials,which confirmed the coupled corrosion mechanism proposed in this paper.Combined with thermodynamic analysis,it was found that the formation of double-layer or multi-layer alloy oxidation products is crucial to protect metals from coupled corrosion,and the types and ratios of elements added to the alloy were the keys to the formation of multilayer protective oxide scales.