Prediction and validation of ash sticking probability under fouling conditions in pulverized coal combustion

Abstract

Under the fouling conditions in stationary coal combustion systems, the sticking/rebound behavior of solid incident particles is a key issue in determining the ash deposition rate. From a dynamic point of view, the bulk fly ash, which dominants the deposited mass, successively interacts with the clean tube, the inner fine deposited layer and the bulk deposited layer during ash deposition. In this paper, we experimentally investigate the time-resolved evolution of ash fouling in a 25 kW coal combustor. The deposited mass flux rapidly reaches a stable state that fluctuates around a mean value of ∼3 g/(m2·s) for two kind of probe materials. The rapid initial stage only allows the formation of 1–2 layers of bulk deposited ash, revealing the dominant role of bulk deposit in capturing large incident particles. Inspired by the observation, we apply a 3D adhesive discrete element model (DEM) to fully describe the many-body evolving process subject to the incident events of a 30-µm particle. The simulation agrees well with the experiments when using a higher particle surface energy of 200 mJ/m2. The rapidly growing feature of ash sticking probability with increasing the bulk deposit layers can be reproduced in this case, and an empirical formula is proposed. It is also validated that, at the deposit growth stage, the newly-deposited particles stay just where they impact. The effectiveness of the DEM tool shall benefit a fully-validated sticking/rebound model under the fouling condition that is convenient for CFD use.