Development of a three-dimensional computational slag flow model for coal combustion and gasification

Abstract

Slagging and fouling are important phenomena associated with ash handling and discharge in coal combustion and gasification. Slagging may have strong impact on the heat transfer characteristics in the reactor, and may cause reliability and safety concerns. A novel, fully three-dimensional slag model has been developed for modeling the slag flow characteristics including char/ash deposition and molten slag flow on the reactor wall. The model couples two multiphase flow models, i.e., the Volume of Fluid (VOF) model and the Discrete Phase Model (DPM), to describe the gas phase, slag phase, and solid particle phase in slagging coal combustion. The model is implemented in the form of User Defined Functions (UDFs) in a three-dimensional Computational Fluid Dynamics (CFD) code, and applied for the simulation of slag flow in a full three-dimensional pilot scale coal slagging combustion facility. Including the particle dispersion model is necessary for accurate prediction of particle deposition rate near the wall. The simulation result shows that a thin molten slag layer up to 1–2mm is built up on the refractory wall in about 5h. The slag flows downward on the side wall, accumulates on the bottom and exits at the bottom end of the reactor, mainly driven by gravity. The average slag flow velocity is generally about 0.1mm/s. Experimental result in the test facility was compared with the simulation results, and the simulation results on ash particle deposition, slag thickness and flow velocity agree qualitatively well with the experimental observations.