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.