Over a period of eight years (1989 through 1997) the Mineral Industry Resea
rch Laboratory (MIRL) at the University of Alaska, Fairbanks, AK, has inves
tigated the effects of temperature, residence time and particle size on the
degree of low-rank coal (LRC) upgrading for a nonevaporative, hydrothermal
drying process, also known as hot water drying (HWD). The following three
Alaskan LRCs were used in this study: Usibelli coal (Seam No. 4) from the N
enana Coal Field, Little Tonzona coal from near McGrath, and Beluga coal fr
om the Beluga-Yentna field on the west side of Cook Inlet. Replicated facto
rial tests were conducted. Experimental results indicate that most of the l
ow-rank coal upgrading via HWD occurs rapidly within the first 10 to 20 min
of residence time. Thereafter, the upgrading process slot-vs down, and mor
e than 100 min of additional residence time is required to produce percenta
ge increases equivalent to what was achieved in the first 10 to 20 min. The
properties of the hot water dried products were very sensitive to process
temperature. Calorific value, carbon and oxygen contents, equilibrium moist
ure levels and Hardgrove gindabilities all appear to change linearly with t
emperature within the 275 degrees C to 325 degrees C (525 degrees F to 615
degrees F) range.