Prediction of medium-to-coal ratio effect in a dense medium cyclone by using both traditional and coarse-grained CFD-DEM models

Abstract

Dense medium cyclone (DMC) is the working horse in coal industry. In practice, it is usually operated under constant pressure and the operational conditions (mainly medium-to-coal (M:C) ratio and operational pressure) need to be adjusted according to coal washability data (mainly coal particle size and density distributions). Nonetheless, until now it is still not well understood how the M:C ratio would affect the performance of DMCs especially under the practical conditions. In this work, the effect of M:C ratio is for the first time numerically studied under conditions similar to plant operation by using both traditional and coarse-grained (CG) combined approach of computational fluid dynamics (CFD) and discrete element method (DEM), called as traditional CFD-DEM and CG CFD-DEM, in which the flow of coal particles is modelled by DEM or CG DEM which applies Newton’s laws of motion to individual particles and that of medium flow by the conventional CFD which solves the local-averaged Navier-Stokes equations, allowing consideration of particle–fluid mutual interaction and particle–particle collisions. Moreover, impulse and momentum connection law is used to achieve energy conservation between traditional CFD-DEM and CG CFD-DEM. It is found that under constant pressure, the M:C ratio affects DMC performance significantly. The specific effect depends on coal washability or coal type. Under extremely low M:C ratio, the phenomenon that high-quality coal product is misplaced to reject is successfully reproduced, which has been observed in plants in Australian coal industry and called as “low-density tail”. Moreover, strategies are proposed to mitigate the “low-density tail” phenomenon based on the model.