Titania Coatings: A Mechanical Shield for Cohesive Granular Media at High Temperatures

Abstract

Fine granular media are pivotal in thermochemical energy storage technology. Reactors based on granular materials store the heat using reversible reactions at high temperatures. Yet, powders become increasingly cohesive in those conditions. The rise of powder cohesion at high temperatures is one of the most irksome phenomena still limiting the scalability of this technology. We found titania coatings comprise an excellent solution to control cohesion in fine limestone powders at high temperatures. Limestone is the main component in granular flows running solid-based storage circuits based on the calcium looping process. It is also involved in many other industrial applications. Titania layers were used to shape stiffer carbonate surfaces at high temperatures (close to the Tamman point). The experiments conducted in this work investigated the benefits of these layers, examining the powder cohesion as the contact between particles evolved from rigid to plastic surfaces. In doing so, samples were subjected to different temperatures varying from 25 oC to 500 oC and preconsolidations up to 2 kPa. The results revealed that titania coatings shield (mechanically) carbonate particles, making surfaces more resilient to deformation while particles interact. The efficiency of titania layers was compared with samples coated with nanosilica, which is a solution broadly accepted nowadays for limestone powders. The experiments tackled one of the weaknesses of nanosilica coatings, namely their efficiency when particles are barely coated. Interestingly, at high temperatures, samples treated with titania outperformed those layered with nanosilica for surface coverages around 9 %. Moreover, despite such a moderate amount of coverage, samples coated with titania reached an easy-flow regime even at high temperatures. However, samples treated with nanosilica fluidized less uniformly, and their flowability fell into a cohesive-flow regime in similar conditions. In conclusion, titania coatings represent an excellent alternative to deal with those flowability issues that still limit the scalability of solid-based storage technology.

Versión aceptada del artículo. La versión final puede consultarse en: https://doi.org/10.1016/j.cej.2022.138123