Upgrading of Low-Rank Coals
Combined reserves of subbituminous coal and lignite (known as “brown coal” in the international coal trade) make up approximately one-half of the world coal reserves and about one-half of the coal resources of the United States. These coals are rarely processed before shipment or use. However, the oxygen and moisture contents of low-rank coals are greater than those of bituminous coals. This reduces the heating value of the coal as mined, which increases the transportation cost on a heating value basis and reduces the thermal efficiency of the steam boilers that use these coals. Most lignite mined in the United States is used in minemouth plants. Subbituminous coals, however, are generally transported considerable distances, so their high moisture content and low heating value add to the effective transportation cost and environmental impact. One way to offset these disadvantages is to dry the coal before transportation or utilization.
Numerous processes for drying low-rank coals to upgrade them have been proposed, demonstrated (e.g., Great River Energy Lignite Drying Process; Bullinger et al., 2006), and in a few cases, commercialized (e.g., K-Fuels Process; Kowalski, 2005). The characteristics of dried low-rank coal—it is friable, has a tendency to spontaneously heat, and readily reabsorbs moisture—constitute major obstacles that must be overcome to produce a saleable, transportable, dry coal product.
the severity of diseases, disasters, fatal accidents, and nonfatal accidents—the health and safety of miners remain a major concern for government, industry, and labor. As the coal mine disasters in early 2006 demonstrated (MSTTC, 2006), the safe operation of mines remains a major challenge—there needs to be constant monitoring and control of health and safety threats as well as continuous safety training and improvements in operating practices.
Past experience has shown that changes in mining operations or practices (e.g., introduction of new equipment and systems, mining of virgin areas, infusion of new workers) all have the potential to create a more hazardous environment. Similarly, experience has shown that adequate engineering controls and a knowledgeable workforce are the prerequisites for a safer work environment. Continued health and safety research is needed to identify new hazards and hazard sources as well as to improve the engineering controls for existing hazards, particularly through the development of reliable monitoring and intelligent control systems. The likelihood of deeper mines in the future means that there has to be increased attention to methane control (including methane capture before, during, or after mining), dust control, ignition sources, fires, and explosions. Recent disasters have shown that there are major knowledge gaps and technology needs in the areas of escape and survival, and emergency preparedness and rescue, emphasizing the need for research to develop systematic and comprehensive