Numerical investigation on ash deposition on the surface of tube bundle

Abstract

Ash deposition is a major challenge for boilers fired with coal, biomass as well as other solid fuels. It can cause fouling and slagging along the flow path of flue gas, resulting in significant efficiency drop and safety problem. For these reasons, ash deposition needs to be further studied. Computational Fluid Dynamics (CFD) is an effective tool for predicting ash deposition. But there is still a big space for this technology to improve the prediction accuracy due to the complexity of ash deposition problem. Ash deposition is actually a dynamic process, some important parameters such as surface temperature and the shape of ash deposit will change with the going on of deposition time, which have significant influences on ash deposition. In this paper, an effective tool based on CFD was developed to simulate the process of ash deposition with some sub-models implemented into User Defined Function (UDF) codes. Ash deposition on a bundle of tubes was investigated in the unsteady mode. Both inertia and thermophoresis deposition mechanisms are considered in ash deposition model. Fine particles (<10µm) play an important role at the initial stage of ash deposit formation. It is because that the deposition of these fine particles is largely dependent on the temperature gradient between flue gas and surface of ash deposit. The variation of surface temperature and shape of ash deposit with time were predicted. The differences of deposition properties on the tubes at different positions were found. The modelling results indicate that the proposed method is an effective tool to predict the formation and growth of ash deposit.