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Hot-Water Drying

Low-rank coals (LRCs), which include lignite, constitute almost half of the vast reserves of coal that exist worldwide. 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

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.

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.