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Evaluation of chlorine accelerated superheater corrosion during biomass and waste combustion

To explain excessive steel tube wastage occurring at metal temperatures below the first melting point of the deposits, it has been suggested that accelerated corrosion may be due that alkali chlorides react with metal oxides and oxygen to form complex alkali-transition metla oxides, such as alkali ferrites, and chlorine. The main objective of the present study was to evaluate in detail to which extent this postulation may apply in biomass and waste combustion. Modelling experiments were performed exposing metallic specimens to synthetic alkali chlorides, carbonates and sulphates or to their mixtures, or to ashes from biomass combustion in different atmospheres. Laboratory studies were also performed using cooled probes allowing condensation and subsequent evaporation of the salts. Thermodynamic and corrosion mechanistic analyses were performed to evaluate the role of alkalis and chlorine in the corrosion.

- Themodynamic analysis performed predicted that alkali chloride-iron oxide scale reaction to form alkali ferrites (NaFeO2, KFeO2), as is commonly postulated when applying "active oxidation" theory, may not occur and do not explain the accelerated corrosion phenomena in the presence of alkali chorides.

- Instead, complex mixture phases of alkali alkali oxides and iron oxides may play an important role in the corrosion mechanism. Accordingly, compositional features of experimentally produced corrosion products on the low alloy steel specimens in the presence of salts of alkali metals (Na, K) could be crossly explained using available experimental phase diagrams and other published material data for Na-Fe-O and K-Fe-O systems in support to theoretic thermodynamic calculations performed.

- High alkali metal activity together with the presence of chloride in the superheater deposits, as may be the case in biomass and refuse co-combustion, is especially harmful. Calsium chemistry may play a special role in "active oxidation" of heat transfer surfaces at the superheater area in waste and biomass combustion when water vapour concentration in flue gases is high and that of sulphur dioxide low.

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