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 product from vibrating basket centrifuges. The process concept involves the injection of a turbulent stream of high velocity air through the coal bed as it traverses the centrifuge basket. The study described in this paper shows that the technique works at the bench-scale. theoretical treatment predicted that the main parameters controlling the removal of a moisture droplet of fixed diameter should be air speed and 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 followed 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 thought to pertain in vibrating basket centrifuges. her 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 effective on a rank range of coals, the higher rank coals responding the best. Under these idealised conditions, substantial moisture reductions of between 1.5 and 2.7 wt% were achieved. The results were thought to warrant investigation of scale-up of the technique, the results of which will be described in a future paper.