Superheater corrosion in biomass and waste fired boilers : Characterisation, causes and prevention of chlorine-induced corrosion

Abstract: Biomass and waste fired boilers suffer severely from corrosion of critical components such as superheater tubes. In this work high temperature corrosion of superheater alloys, and methods to mitigate the problem, have been investigated by laboratory studies and controlled field exposures in commercial boilers.In Paper I, laboratory work investigated the detrimental effect of gaseous hydrochloric acid (HCl) on austenitic stainless steels at two different temperatures and two different surface treatments. At a lower temperature, a positive effect of preoxidation was apparent, effectively suppressing chlorine ingress and lowering the corrosion rate for all three materials. Chlorine accumulation at the metal/oxide interface was observed only on the ground surface specimens. At a higher temperature, the beneficial effect of preoxidation was lost and corrosion resistance depended on the alloying level. In this case, chloride evaporation contributed significantly to the material degradation. Based on the results, high temperature corrosion in the presence of HCl(g) is discussed in general terms.In Papers II and III, corrosion during waste incineration was investigated for a number of candidate superheater alloys. Laboratory and field exposures revealed that lowalloyed steels/carbon steels are more vulnerable to metal chloride formation and accelerated attack than candidate stainless steels. Boiler exposures showed unacceptably high corrosion rates for the lower alloyed ferritic steels and austenitic stainless steels. The corrosion attack for these alloys was manifested by the formation of mixed metal chloride/metal oxide scales with poor protective properties. Different behaviour was seen for the higher alloyed austenitic steels and nickel-base alloys, which developed a chromium-enriched oxide next to the metal and metal chloride formation was suppressed. However, these alloys suffered from localised pitting attack. Deposit analyses revealed substantial amounts of low melting salt mixtures such as zinc chloride-potassium chloride and lead chloride-potassium chloride. Molten mixtures of corrosion products and deposit compounds such as iron chloride-potassium chloride and sodium chloride-nickel chloride were also observed. It was evident that oxide dissolution in molten salts limits the performance of these alloys in waste-to-energy plants.In Papers IV-VI, the use of additives to avoid condensation of alkali chlorides on the tube surfaces was investigated and promising results were obtained by injecting ammonium sulphate into the flue gas stream. In more detail, the work investigated effects of the sulphate additive while firing the boiler at different air excess ratios (λ- values) showing a beneficial effect of increased air excess with faster sulphation reactions and less corrosion attack. Furthermore, in a comparison between ammonium sulphate and mono ammonium phosphate different behaviour was observed for the two additives. While ammonium sulphate captures alkali both in the gas phase and in the solid phase, mono ammonium phosphate reacts only in the gas phase. These findings explain why flue gas measurements and deposit measurements do not always correlate. Finally, in a study injecting ammonium sulphate in a waste-to-energy plant it was shown that the additive could also be used to significantly reduce alkali chlorides in the flue gas and deposit in waste fired boilers.