Modeling of low-temperature oxidation of steel tubes in biomass combustion

Corrosion scale morphologies typically show thick multi-layered oxides with chloride enrichments in and/or below the scale, irrespective of the often very low content of chloride species in flue gases from biomass combustion. Finding a plausible mechanistic explanation for this phenomenon would help to avoid high-risk operational conditions or to find suitable counteractive measures. The lack of thermodynamic data for various corrosion product species has been a serious hindrance for such analyses. To compensate for missing thermodynamic data, laboratory experiments and theoretical estimation models were applied in this study. Thermodynamic data for various alkali ferrites and ferrates and corresponding chloro-compounds in gaseous or molten phases were considered. Previous assumptions notwithstanding, preliminary modelling calculations indicated that the reaction between alkali chloride and iron oxide scale to form alkali ferrite of the type AFeC(2), and gaseous chlorine may not be the primary reaction for superheater tube wastage in biomass combustion. Subsequently, more complex corrosion reaction schemes including the involvement of molten alkaline salt melts and reactions of some important alloying elements like chromium, molybdenum and manganese were investigated. Primary alkaline deposit components (carbonate, hydroxide) or primary alkaline corrosion products such as alkali ferrites and ferrates were shown to be unstable even at low hydrogen chloride concentrations, eventually leading in actual deposition conditions to secondary chloride retention. Alkali ferrites or ferrates, carbonates and chlorides were found to be able to form aggressive melts in-situ, resulting in highly accelerated oxide growth, and eventually to corrosion morphologies typical of low-temperature oxidation. This proposition was supported by studies of corrosion scales from material exposures in the laboratory as well as in full-scale boilers. The results of calculations are discussed with reference to the so-called alkali index of biomass-based fuels. Proposals for further studies are suggested.