High temperature corrosion of calcium hexaaluminate with biomass slag

Loison, Lise Rebecca; Telle, Rainer (Thesis advisor); Poirier, Jacques (Thesis advisor)

Aachen (2018, 2020)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2018

Abstract

The energy transition from the incineration of fossil fuels to biomass confront refractory linings to severe corrosion issue, due to the high content of alkalis as well as the wide variety of biomass sources. Incinerators optimized for fossil fuels are commonly lined with mullite, Al2O3-Cr or SiC based refractory products, however those materials are not always suitable for the use of organic fuels. 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 calcium hexaaluminate and alkali does not lead to the strong volume expansion observed with other mineral phases like corundum or andalusite. The present work aims to determine the reaction mechanisms between calcium hexaaluminate materials and biomass ashes. Therefore the three main oxides contained in an average biomass composition, namely CaO, SiO2 and K2O, are selected to examine the high temperature reactions with calcium hexaaluminate. The analyses focus on the kinetic and microstructural aspects using mainly Scaning Electron Microscope (SEM) and X-Ray Diffraction (XRD) analysis supported by thermodynamic calculations. The corrosion resistance is compared with the performance of an alumina matrix to determine the suitability of calcium hexaaluminate to replace aluminosilicate. The calcium hexaaluminate show less contact surface to react with the slag with lower permeability and lower average pore size. This results in lower extent of reaction for the calcium hexaaluminate 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 calcium hexaaluminate microstructure remain sound. This work contributes to propose calcium hexaaluminate as innovative raw material for new energy applications.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Ceramics and Institute of Mineral Engineering [524110]