Residual moisture reduction of coarse coal using air purging. 1. Bench scale studies

Air purging is a new way of reducing the moisture content of coarse coal pr oduct from vibrating basket centrifuges. The process concept involves the i njection of a turbulent stream of high velocity air through the coal bed as it traverses the centrifuge basket. The study described in this paper show s that the technique works at the bench-scale. Simple theoretical treatment predicted that the main parameters controlling the removal of a moisture droplet of fired diameter should be air speed an d the degree of hydrophobicity of the surface. A series of air purge tests on surface-treated glass beads in static beds showed that this was the case . Air purge time was less important, since the moisture reduction appeared to occur in two stages, with an initial rapid removal of liquid water follo wed by a slower evaporation stage. Similar results were obtained on purging beds of coal particles. Encouragingly, the kinetics of the liquid removal stage appeared consistent with the 2-3 s mean particle residence times thou ght to pertain in vibrating basket centrifuges. A further study in a purpose-designed batch centrifuge basket, in which the sample was spun and purged at the same time, showed that air purging was e ffective on a rank range of coals, the higher rank coals responding the bes t. Under these idealised conditions, substantial moisture reductions of bet ween 1.5 and 2.7 wt% were achieved. The results were thought to warrant inv estigation of scale-up of the technique, the results of which will be descr ibed in a future paper. (C) 2001 Elsevier Science Ltd. All rights reserved.