Porosity-Based Corrosion Model for Alkali Halide Ash Deposits during Biomass Co-firing

This paper presents a physics-based model to describe accelerated corrosion because of alkali-halide-containing deposits, which form on superheater tube walls during biomass co-firing. Increased rates of corrosion during the co-firing of peat with biomass have been identified as a limiting factor on the level of biomass, which is viable to use at elevated temperatures. In the present work, a synthetic salt, representative of a 70:30 peat/biomass mix, has been applied to pure iron samples in air at 540 and 600 °C. The corrosion layers have been examined using scanning electron microscopy (SEM), optical microscopy (OM), and energy-dispersive X-ray (EDX) spectroscopy elemental mapping to provide insight into the material degradation and structure of the corrosion layer. Two distinct types of oxides are found to form on the iron substrate. Initially, a compact, uniform oxide layer forms over the substrate. As the process continues, this oxide layer degrades, leading to spalling, which sees the broken oxide pieces mix with the salt layer. Additional test samples were examined without deposits as controls to highlight the accelerated rate of corrosion. Two modeling techniques are examined: the widely used labyrinth factor method (LFM) and the newly proposed porosity-based corrosion method (PCM). The PCM uses measurements of porosity and pore radius, coupled with a physically based corrosion mechanism, to predict corrosion rates. Results from the two modeling techniques are compared, and both agree satisfactorily with experimental measurements for times of up to 28 days.