Nanoadditives to mitigate jamming in cohesive granular media

Handling and conveying cohesive granular media are capital for many industries, which demand smooth and uninterrupted granular flows. This is especially critical at high temperatures when cohesion skyrockets, and flow patterns become intermittent; then, jamming, halts, and shutdowns.

Here, we focus on concentrated solar plants assisted by the calcium looping cycle, which are plagued with jamming vulnerabilities. Experiments analyze the use of nanoadditives to shield limestone particles and mitigate the side effects of cohesion. Our results dissect the evolution in flowability for samples coated with nanopowders of different materials: silica, titania, and alumina. To investigate the efficacy of these coatings, we subjected all the samples to conditions that soften the contacts and cause cohesion to escalate: high temperatures (up to 500 oC) and moderate consolidations (up to 2 kPa). The results showed how discrete layers of nanoadditives shape stiffer surfaces, preventing particles from softening at high temperatures. In this way, coatings modulate the contact area dilation that fuels cohesion. We distinguished two thermal regimes regarding contact deformation as the temperature rises. These observations are consistent with the onset of sintering induced at high temperatures, which predicts contact deformations triggered by the activation of a mass diffusion mechanism first at the surface (Hüttig temperature) and later at higher temperatures in the bulk (Tamman temperature).

Coatings were prepared considering the optimal amount of nanoadditives to introduce minimal inert material. In doing so, van der Waals forces were estimated by taking into account the effect of surface area coverage on the contact geometry. As predicted, concentrations below 1 wt % were enough to ensure uniform coatings and promote easy-flow regimes. Importantly, at high temperatures, only alumina coatings achieved flowability indexes within the easy-flowing regime. Moreover, samples coated with alumina exhibited cohesion profiles at 500 oC similar to those observed in bare limestone samples at 200 oC. This is an outstanding conclusion for a broad spectrum of chemical engineering processes that usually require transporting and storing cohesive granular media at high temperatures.