Particle motion and separation behavior of coal in gas–solid separation fluidized bed

Abstract

During coal separation using a gas–solid separation fluidized bed, the floating-sinking behavior of the coal particles plays a critical role. In this study, the float-sink coefficient, Xfs, and the fluctuation variance, Sp, were used to quantitatively describe the floating-sinking behavior of coal particles in a fluidized bed as well as the effect of the behavior on the separation performance of the bed. Further, theoretical force models were established for the coal particles under different conditions. The results showed that the addition of clean coal particles increases the pressure drop across the bed. However, this has a negative effect on the density fluctuations in the bed, with its float-sink coefficient remaining within a certain range. On the other hand, although coal gangue particles do not increase the pressure drop across the bed, the secondary air distribution of coal gangue stabilizes the bed density. Further, the float-sink coefficient curve for coal gangue with respect to the separation time can be divided into three stages: (i) a rapidly decreasing area at a low gas velocity (vf = 9.83 cm/s); (ii) a dynamic balance area for gas velocities of 10.81–12.78 cm/s; and (iii) an area with fluctuations under high-speed conditions (vf = 13.78 cm/s). During the process of coal separation, the bursting of air bubbles on the bed surface and the drag force related to the gas flow are responsible for the misplacement of the coal particles. When the gas velocity was 10.81–12.78 cm/s, the separation performance of the gas–solid separation fluidized bed was the best, and the probable error, E, value was only 0.03, with the yield of the clean coal product being 58.66% and that of ash being 9.55%. Further, the yield of the gangue product was 41.34% while that of ash was 78.21%.