Effect of surface roughness and surface energy on molten fly ash deposition

Abstract

One of the ways to mitigate fouling and slagging in downstream sections of boilers, power plants, and gas turbines employing coal combustion is to coat the boiler tubes with low surface energy materials. Such a coating not only reduces the surface energy but can potentially offer a rough surface as well. Hence, the simultaneous effects of surface energy and roughness on fly ash deposition need to be understood. Slag-layer deposition involves drop dynamics associated with arrival of molten ash droplets. While recoiling after reaching maximum spread diameter, the drop may bounce or stick depending on the retraction energy. Bouncing potential of the recoiling droplet decides its fate—to stick or to bounce. In order to quantify the bouncing potential, the spread ratio—maximum spread diameter to initial drop diameter—needs to be evaluated. In this study, models are presented to calculate the spread ratio and bouncing potential of impacting drops on rough surfaces; these incorporate the phenomenon of air/gas pockets present beneath the spreading and recoiling splat. The velocity profiles of the spreading droplet, derived by solving axisymmetric creeping flow, are employed to theoretically estimate the dissipation energy while spreading. The predictions of the proposed spread ratio model—free from fitting parameters—are compared with experimental data in literature and are shown to be within ±5 % of the reported values. With the capability to predict whether the droplet bounces or sticks, the bouncing potential model is exploited to suggest choices of roughness and surface energy to mitigate slagging deposits.