Current physical cleaning processes are used primarily to reduce the ash content of as-delivered coal, although some sulfur reduction (typically 20 to 30 percent) is also achievable in coals with high pyrite content. Because coal is an abundant and relatively low-cost fuel, the incremental cost of coal cleaning is a major factor limiting the degree of impurity reductions that are economically feasible.
DOE research in recent years has focused on advanced processes to clean fine coal fractions to achieve a relatively low ash, low-sulfur product suitable primarily for premium applications, such as the production of coal-liquid mixtures that can be substituted for petroleum-based fuels. More recently, attention has also focused on the potential for coal cleaning to remove trace species as a means of reducing power plant emissions of air toxics. A series of RD&D goals has been defined (DOE, 1993a).
Coal-liquid mixtures or slurries—primarily coal-oil and coal-water fuels—are another commercial technology that allows coal to be substituted for liquid fuels in combustion applications. R&D in this area peaked during the late 1970s and early 1980s when oil prices were high and coal-based substitutes were attractive. Commercial interest waned, however, as oil prices declined and oil price projections remained stable. Nonetheless, DOE has continued to fund basic and applied research related to CWSs (coal-water slurries), primarily at universities.
Finally, interest in recovery of coalbed methane has been stimulated by concern about greenhouse gases and EPACT requirements. Methane recovery technology for high methane concentrations is commercially available, and recovery is practiced by the gas and coal mining industries where local conditions justify the investment. However, systems for the capture and use of dilute coalbed methane streams, which are found in many coal mining operations, are not sufficiently mature for commercial implementation. As noted in Chapter 3, increased efforts will likely be needed to reduce coalbed methane released from underground mining, in accordance with the Climate Change Action Plan (Clinton and Gore, 1993). The research challenge is to economically recover coalbed methane from very dilute gas streams.