Abstract
The energy transition from the incineration and gasification of fossil fuels to the incineration and gasification of biomass refractory linings is being held up by a severe corrosion issue, caused by high alkali contents and 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. Hibonite (CaO·6Al2O3)-based refractory products have shown promising performance because of their high resistance against alkali attacks. Indeed, previous works have shown that the reaction between calcium hexa-aluminate and an alkali does not lead to the strong volume expansion observed with other mineral phases, such as corundum or andalusite. The present work aims to describe the reactions kinetics occurring between hibonite-based raw materials and biomass ashes. Therefore, the three main oxides contained in an average biomass, namely, CaO, SiO2 and K2O, were selected to examine the high temperature reactions with a calcium hexa-aluminate matrix. The resulting phase composition and microstructure were compared with the performance of an alumina matrix through, respectively, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). The post-mortem observations show a higher extent of reaction for the alumina than for the calcium hexa-aluminate. Moreover, the microstructure of the alumina matrix suffered a strong chemical spalling, while the calcium hexa-aluminate microstructure remained undamaged after the corrosion.
Highlights
The use of biomass fuels challenges the refractory producer in two ways
The binding of the calcium hexa-aluminate matrix was provided by calcium aluminate cement and reactive alumina in order to react with CA6 during the sintering, and the weight ratios were adjusted to obtain the stoichiometry of the targeted phase
The resulting open porosity is slightly higher than for matrix A. This might be due to the poorer tendency for densification of calcium hexa-aluminate reported in the literature, leading to higher open porosity compared to alumina [5]
Summary
Products used for the incineration and gasification of biomass have to withstand a wide variety of chemical compositions due to the different origins of organic fuels. They have to be resistant to alkalis, since biomass ashes can contain up to 60 wt.-% of K2 O [1]. Andalusite and mullite are the oxide products currently used to line biomass incinerators [2]. Calcium hexa-aluminate has already proven to have high stability when exposed to an alkali, avoiding catastrophic volume expansion known as “alkali bursting.”. This property is due to its crystal structure being able to accommodate alkaline atoms. Calcium hexa-aluminate has a similar density to the products formed during the reactions with K2 O or Na2 O; no volume
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