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High-Temperature Corrosion of Calcium Hexaaluminate with Biomass Slag

Incin­erators optimised for fossil fuels are commonly lined with mullite, Al2O3–Cr2O3­ or SiC­based refractory products, however those materials are not always suitable for the use of organic combustible. Calcium hexaaluminate (CaO · 6Al2O3) based refractory products have shown promising performance because of their high resistance against alkali attack. Indeed, previous works have shown that the reaction between CaO · 6Al2O3 and alkali does not lead to the strong volume expansion observed with other mineral phases like corundum or andalusite.
The present work aims to assess the reaction mechanisms between CaO · 6Al2O3 materials and selected biomass ashes compositions. Therefore, the main oxides (CaO, SiO2 and K2O) contained in an average bio­mass composition were selected to examine the high­temperature reactions with CaO · 6Al2O3. The analy­ses used mainly Scanning Electron Microscope (SEM) and X­Ray Diffraction (XRD) analysis supported by thermodynamic calculations. The experimental trials were carried out on powders, matrices and full range castables, with a strong focus on the results obtained for the matrix components. The corrosion resistance was compared with the performance of an alumina matrix to determine the suitability of CaO · 6Al2O3 to replace aluminosilicate. The CaO · 6Al2O3 matrix showed less contact surface to react with the slag with lower permeability and lower average pore size. This resulted in lower extent of reaction for CaO · 6Al2O3 than for the alumina matrix, with slower dissolution kinetics. Moreover, the microstructure of the alumina matrix is strongly damaged after the corrosion due to structural spalling, while the CaO · 6Al2O3 micro­structure remained sound. This work contributes to propose CaO · 6Al2O3 as innovative raw material for new energy applications.

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