one-half of all electricity produced. The future of the industry depends not only on traditional market variables but also on coping with the environmental effects of mining and burning coal. Public perception about coal and technological advances in efficient utilization are likely to have major influences on the future of the industry.
Geoscientists contribute to the progress of the coal industry by addressing the origins of coal beds and the evolution of their genetic environments. For example, researchers have observed that although sulfur content varies greatly within regions of the country, there is a tendency for the younger coals in the western United States to be lower in sulfur than are the older coals in the Midwest and the Appalachians. The higher-sulfur coals are often associated with marine strata, suggesting that the sulfur originated as sulfate in seawater. The sulfate was reduced to sulfide by bacteria and then incorporated into the peat that eventually developed into coal. Coals that formed in river systems far removed from marine conditions tend to be lower in sulfur content.
Although researchers understand the origin of some sulfur-rich coal, they have not yet worked out how best to cope with the problems it presents. Much of the sulfur is contained as organic sulfur in very fine-grained iron sulfides, and to separate the coal from the sulfur-bearing minerals would require grinding it into very small particles. Such a procedure is not economical at present. Coal research is dominated by the quest to identify the coal-forming environment and trace its subsequent history. Characterizing the coal physically and chemically on scales ranging down to the submicroscopic is essential for interpretation of its origin and history as well as determination of its technological properties. Such characterization is vital to determine whether coal can be economically mined, cleaned, and used either in conventional combustion or in some other way that may prove economical.
Coal is composed largely of nonmarine plant material deposited in swamps. Understanding the depositional environments leading to peat formation and the subsequent diagenetic processes that determine coal type is important in predicting coal characteristics in advance of exploitation. The properties of coal are determined, in part, by the kinds of plants or parts of plants that dominated the original peat accumulation. As with petroleum source rocks, the study of coal distribution has been an important part of paleoclimatological research.
The salinity, pH, sulfate, and bacterial content of water in which peat accumulates largely determine the chemical characteristics of the resulting coal, including its sulfur and nitrogen content. Some of the mineral matter, the ash—which includes part of the sulfur—is derived from plants, but most consists of stream- or wind-transported material, and a large amount of it may enter the deposit long after the swamp has disappeared. Peat deposits in broad coastal freshwater swamps may result in thick coal beds of considerable areal extent. Swamps or marshes in an area of active delta deposition are episodically inundated by flood waters that deposit clay, silt, or sand on previously accumulated peat. The resulting coals contain splits—thin coal seams separated from the main seam by a layer of different sediments—that may reduce their minability. Abandoned stream channels sometimes accumulate thick peat deposits of elongate geometry.
Plant material is further altered chemically and physically by heat and pressure. Coals are ranked progressively upward, from lignite to anthracite, according to the degree of such metamorphism. Rank increases as the fixed carbon content increases and the percentage of volatile matter and moisture decreases.
Considerable research on the origin of coal involves study of the depositional environments of modern peat formation and is aimed at identifying physical and chemical changes during the coal-forming process. Discoveries of peatlands with incipient coal formation in Indonesia were detected through extensive mapping and coring. Studies are developing more sophisticated models for future investigations. Detailed outcrop and subsurface studies of individual coal basins are similar to the basin analysis methods used in the petroleum industry; they aim at defining depositional environments and subsequent coalification processes for model building.
Coal exploration does not have the search component inherent in mineral and petroleum exploration because the limits of most coal-bearing beds in U.S. fields have long been known. Nonetheless, enhanced time-stratigraphic depositional models aid in predicting the location, geometry, and quality of coals. Despite these advantages, the inherent heterogeneity of the material makes sampling of coal deposits a difficult task; thus, development of accurate sampling techniques and analysis of statistics are important in the mining of coal. Where coal beds are nearly horizontal and laterally continuous