Low-rank coals (LRCs), which include lignite and subbituminous coals, 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 and subbituminous coal 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.

This situation encouraged researchers at the EERC to investigate alternative methods for converting LRCs into economically viable liquid fuels. Their pioneering efforts led to the successful development of a direct LRC slurry liquefaction process at the EERC facility in Grand Forks, North Dakota, the world's largest research and development complex for investigating LRCs.

The EERC developed an integrated close-coupled multistep process to take advantage of the higher coal reactivity and moisture content of LRCs to produce a product that may be easier to upgrade than products from liquefaction of higher-rank coals. The process utilizes hydrogen-donating solvents, the water–gas shift reaction, and lower-severity reaction conditions. Coal liquid yields of 80 wt% of moisture- and ash-free (maf) coal fed with conversion rates of greater than 93% have been achieved. A multistep approach was taken to produce a tetrahydrofuran (THF)-soluble material that may be easier to upgrade in conventional catalytic hydrogenation. The process consists of three parts: 1) preconversion treatment to prepare the coal for solubilization, 2) solubilization of the coal in the solvent, and 3) polishing to complete solubilization of the remaining material. The product of these three steps can then be upgraded during a traditional hydrogenation step. Successful implementation of this process promises improved economic viability of low-rank coal liquefaction.

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.

As a result of earlier investigations, the EERC has developed an economical method of drying LRC based on a hot-water-drying process. The technical feasibility of this new, nonevaporative drying technique has been established at the EERC 7.5-ton-per-day pilot plant, and commercial demonstration of the technology is currently in the planning stage.