Lab-Scale Study on Fireside Superheaters Corrosion in MSWI Plants

Abstract

Combustion of the municipal waste generates highly corrosive gases (HCl, SO 2 , NaCl, KCl and heavy metals chlorides) and ashes containing alkaline chlorides and sulphates. Currently, corrosion phenomena are particularly observed on superheater's tubes. Corrosion rates depend mainly on installation design, operating conditions i.e. gas and steam temperature and velocity of the flue gas containing ashes. This paper presents the results obtained using an innovative laboratory-scale corrosion pilot, which simulates MSWI boilers conditions characterized by a temperature gradient at metal tube on the presence of corrosive gases and ashes. The presented corrosion tests were realized on carbon steel at fixed metal temperature (400°C). The influence of the flue gas temperature, synthetic ashes composition and flue gas flow pattern were investigated. After corrosion test, cross section of tube samples were characterised to evaluate thickness loss and estimate corrosion rate while the elements present in corrosion layers were analysed.Corrosion tests were carried out twice in order to validate the accuracy and reproducibility of results. First results highlight the key role of molten phase related to the ash composition and flue gas temperature as well as the deposit morphology, related to the flue gas flow pattern, on the mechanisms and corrosion rates.

Temperature-Gradient-Driven Aging Mechanisms in Alkali-Bromide- and Sulfate-Containing Ash Deposits

“…This implies that, in boiler design, in addition to the absolute temperature of the heat exchanger material, the temperature gradient from the flue gas to the steel also affects the corrosion risk. Others have reported results, which support this view. , …”

“…Others have reported results, which support this view. 17,18 The temperature-gradient-induced gas phase migration model has been shown to qualitatively work with alkali chlorides and alkali bromides. The model relies on Fick's first law of diffusion and on the kinetic theory of gases, which makes it also easily applicable for other volatile species within deposits and for other applications where temperature gradients are present.…”

“…In addition, temperature gradients have been shown to affect corrosion of steel and the chemistry and morphology of ash deposits. Corrosion rates of the heat exchanger tube have been shown to increase with higher flue gas temperature even if the tube temperature is kept constant. , Temperature gradients have been shown to induce diffusion within oxide layers and to induce penetration of corrosive chloride species through unstable oxide layers to the unaffected steel surface …”

“…Corrosion rates of the heat exchanger tube have been shown to increase with higher flue gas temperature even if the tube temperature is kept constant. 17,18 Temperature gradients have been shown to induce diffusion within oxide layers 19 and to induce penetration of corrosive chloride species through unstable oxide layers to the unaffected steel surface. 20 The effects of temperature gradients on ash deposit chemistry and morphology have been studied mainly in laboratory scale.…”