Low-rank coals (LRCs), which includes lignite, constitute almost half of the vast reserves of
coal that exist worldwide. Typically, deposits of these LRCs occur in thick seams with relatively thin cover;
consequently, mining operations are generally inexpensive. Further, although at the bottom end of the quality range
in coal-ranking classification, LRCs are usually more homogeneous in their as-mined condition than high-rank coals.
This feature is especially characteristic of lignite. However, despite their excellent combustion characteristics
and other attractive properties, utilization of lignite has, to date, been largely confined
to generating electrical power at or close to the mine site. Their high moisture content and instability have, until
now, prevented their utilization in distant coal markets.
The widespread utilization of these LRCs in traditional coal markets is constrained by several factors:
- High inherent moisture content
- High degree of risk for spontaneous combustion
- Weathering and resultant dust nuisance
- Fouling and slagging problems caused by high sodium content
Although their high inherent moisture content lowers their heating value and thus increases the cost of transportation
and handling, their positive features, such as their low cost per Btu and excellent combustion characteristics, have, to
date, been ignored. No commercial drying process is currently available that can economically produce a conventional,
dried bulk coal product that will withstand the rigors of storage, handling, and transportation. Traditional thermal drying
methods used to dry bituminous coal are not effective on subbituminous coal and lignite because of the resulting
decrepitation of these coals.
The hot-water-drying process developed at the EERC is essentially pressure-cooking the coal in a water medium. It is
known that water separates from LRC under conditions similar to those encountered during natural metamorphism, but
metamorphism is achieved under extremely high pressure. It has now been found that, under suitable conditions of
elevated temperature and pressure, lignite not only loses chemically bound water, but undergoes such a change that
reabsorption of water does not occur when the coal is kept in a water phase at high pressure. This effect is a result
of a change in the LRC itself, whereby the tars that form tend to seal the pore entrances. In simple terms, the process
induces coalification in a condensed time scale of minutes rather than geological eras (millions of years), thus effecting
a permanent reduction in inherent moisture. In other words, the LRC is changed from hydrophilic to hydrophobic, thus making
it similar to some subbituminous coals. Hot-water drying also offers an added advantage: the removal of sodium from the
lignite during the drying process. Sodium removal is important since it reduces the risk of fouling and slagging in boilers.