Deposit analysis of water-wall tubes in a municipal solid waste grate incinerator

• Deposition studies of water-wall tubes in a waste incinerator are novel.
• Comparison of characteristics between deposits and fly ash was investigated.
• Several mechanisms explain the existence of molten materials in deposits.
• Effects of deposits and steam temperature on the corrosion are discussed.
• Deposit corrosion mechanism of water-wall tubes is analyzed.

To understand the deposition mechanism and determine the factors influencing corrosion, a municipal solid waste grate incinerator with the steam parameter of 6.5 MPa, 450 °C was investigated. Deposits from the first pass as well as fly ash were characterized by SEM-EDS, XRF and XRD. Deposits contained more or less molten or semi-molten materials, due to the chemical reaction heat during sulfation, low melting point compounds and molecular cramming. The content of K, Na, Cl or Fe in the deposits was bigger than fly ash, but fly ash contained more Ca, S, Si and Al, due to their different source and deposition mechanism. Chlorides and alkali metal compounds were identified. The molten deposits increased the corrosion because liquid phase had faster chemical reactions and also provided an electrolyte for ionic transport or electrochemical attack. The chlorides and alkali metal compounds accelerated corrosion not only by lowering the melting temperature of the deposits, but also by attacking the metal surface with active oxidation mechanism. Moreover, entirety of deposits had a high strength, therefore, deposits were subject to stress impact and resulted in the metal defect on the heating tubes.