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Lignite is a low rank, brownish-black coal that produces less than 8,300 British thermal units (Btu) per pound on a moist, mineral-matter-free basis (1). Lignite has a high content of volatile matter which makes it more convertible into gas and liquid petroleum products than the higher ranking coals. However, its high moisture content and susceptibility to spontaneous combustion may cause problems in transportation and storage.
Lignite is typically mined by stripping methods and burned for electric power generation. Arkansas's lignite deposits are suitable for this purpose and were tested as fuel source for power generation. In 1988, a 221,799 ton sample of Arkansas lignite mined in Saline County was successfully test-burned in a local power plant as a blend of up to 20 percent lignite with coal from the western United States. The Arkansas Geological Commission (now the Arkansas Geological Survey) has estimated total lignite resources at approximately 9 billion tons, at depths of less than 150 feet (2). Proven reserves announced by private companies are approximately 2 billion tons (3).
History of Mining
Lignite was mined in south Arkansas by underground methods and used before the Civil War. It was first used for steam-boiler fuel and later as fuel for small locomotives near the mines in Ouachita County. In 1907, two small oil-distilling plants were operating in Ouachita County. The plants produced oils from lignite mined by open pit methods in the Camden field in Ouachita County. In 1913, lignite from the Camden area was yielding up to 38 gallons of oil per ton, although the average oil recovery was about 25 gallons per ton (4). In 1938, a plant was constructed in El Dorado, Union County, to extract Vandyke brown dye as a product of lignite processing, followed in 1943 by production of the dye from a plant at Malvern, in Hot Springs County. The dye was used in staining ammunition boxes their characteristic brown color during World War II. Annual output during the war years was small – about 1,000 tons per year (5). Another company in Malvern extracted Montan wax from lignite for a few years (6). Montan wax is used in some polishes, carbon paper, and insulating materials.
During the 1970's and early 1980's, various companies implemented exploration programs and discovered a number of major lignite deposits in Arkansas. The lignite was examined for use as a boiler fuel for electric-power generation, as a source of chemical feedstock, and as a source of petroleum-type products through gasification and liquefaction.
Geology of Arkansas Lignite Deposits and Resources
Arkansas lignite has an average heat of combustion value of 6932 Btu/1b on a moist, mineral-matter-free basis. This value makes the lignite rank (A) which has a Btu/1b of between 6300 and 8300 (1). Lignite is present at shallow depths in sedimentary strata of Eocene age in the West Gulf Coastal Plain region of Arkansas and along Crowley's Ridge in the Mississippi River Alluvial Plain. Potentially economic deposits of lignite occur within certain stratigraphic sections of the Wilcox and Claiborne Groups. No significant lignite deposits are known within the Jackson Group sediments or in Quaternary age sediments.
The resource potential of lignite in the Wilcox is estimated to be about 4.3 billion tons (2).The Wilcox was deposited in a fluvial, non-marine environment. The Claiborne Group is estimated to contain 4.7 billion tons of lignite resource (2). The lignite in the Claiborne Group was deposited in deltaic to near-shore shallow marine environments.
|Approximately 2 ½ feet of Tertiary lignite from the Wilcox Formation is exposed in the highwall of this abandoned clay pit southeast of Malvern, Arkansas. Note that the lignite is the dark colored bed at the top of the Jacob's staff.|
(1) American Society for Testing Materials, 1976, Standard specifications for classification of coals by rank: ASTM Standards, Part 26, p. 211-215.
(2) Prior, W. L., Clardy, B. F., and Baber, Q. M., III, 1985, Arkansas lignite investigations: Arkansas Geological Commission Information Circular 28-C, 214 p.
(3) Ozark Regional Commission, 1978, Lignite conference proceeding, 140 p.
(4) White, D, and Thiessen, R, 1913, The origin of coal: U. S. Bureau of Mines Bulletin 38, 390 p.
(5) Selvig, W.A., Ode, W.H., Parks, B.C., and O'Donnell, H.J., 1950, American lignites: geological occurrence, petrographic composition, and extractable waxes: U.S. Bureau of Mines Bulletin 482.
(6) McEntire, J.A. III, 1963, Arkansas lignite, Arkansas Geological Commission Open File Memorandum.