Introduction to Coal Energy
Coal is a brownish-black to black combustible organic sedimentary rock formed by the decomposition of plant material, most often in a swampy or boggy environment. This organic material or peat is buried, compacted, and hardened over millions of years. This process is called coalification. During coalification, peat undergoes several changes as a result of bacterial decay, compaction, heat, and time. Peat deposits vary and contain everything from pristine plant parts like roots, bark, spores, etc. to decayed plants. In coalification, peat passes through four main phases of coal development: lignite, sub-bituminous coal, bituminous coal, and anthracite. These end products are composed primarily of carbon, hydrogen, oxygen, and some sulfur along with water moisture and non-combustible ash. The amount of carbon and volatiles (water and gas) as well as the amount of energy content of the coal determine its rank. The amount of energy in coal is expressed in British thermal units (Btu) per pound. The higher the rank, the greater the heating value.
Based on reliability of data, coal resources are classified into four classes that depend on standard-distances-from-points-of-thickness measurements. These are (1) measured, (2) indicated, (3) inferred, and (4) hypothetical. These four classes of coal resources are based on degree of geologic assurance that the rank and quantity of a coal seam (or seams) have been estimated from high (measured) to lowest (hypothetical). Identified coal resources include measured, indicated, and inferred coal resources.
Challenges of Coal Energy
Coal Energy in Alaska
Alaska's Coal Resources
|Summary of Total Alaskan Coal Resources|
|Resource Category||Total Resources (in millions of short tons)|
In general, hypothetical resources are located within broad areas of known coal fields where points of observation are absent and evidence is from distant outcrops, drill holes, or wells, and where coal may reasonably be expected to exist in known mining districts under known geologic conditions. Additionally, the classification of coal resources is based on the mineable thickness of the coal seam. For a more detailed discussion of coal-related terminology see this article on "Coal Terminology: Resource and Reserve".
The United States is estimated to contain 30% of the world’s coal resources. Alaska is believed to hold about half of that. Most coal resources in Alaska are in the hypothetical resource class because they have been poorly studied, there are few data points of measurement, and there has been little or no drilling to substantiate resource estimates. Identified resources are about 170 billion short tons; however, coal-bearing strata underlie about 9% of Alaska’s land. The state’s total hypothetical resources of coal are estimated to exceed 5.6 trillion tons.
Coal Development in Alaska
Coal was officially discovered in Alaska in 1786, with the first documented production in 1855. Small-scale mining is recorded at numerous sites throughout the state, with local mines being developed to fuel river steamboats, placer gold mines, and canneries before 1900. Significant production began in 1917 after extension of the Alaska Railroad into the Matanuska coalfield, and by 1968, more than 7 million short tons of bituminous coal had been mined from the Matanuska coalfield, most of it for electrical power generation. Significant mining in the Matanuska field ceased in 1968, when Cook Inlet natural gas replaced coal for electrical power generation in the Anchorage area.
Since the end of World War I, coal has been mined continuously in the Healy coalfield, which is within the Nenana coal province. Alaska’s only operational coal mine today, the Usibelli Coal Mine, produces sub-bituminous coal from its Two Bull Ridge mine site near Healy, with an output of 1.357 million short tons of coal in 2007. Usibelli shipped over 308,146 short tons of coal to Chile and supplied six power plants in interior Alaska with approximately 900,000 short tons of coal.
In 2007, BHP Billiton Ltd. drilled nine holes in the western Arctic coalfields on land owned by Arctic Slope Regional Corporation. The purpose was to test the thickness of the coal seams and evaluate the quality of the coal in the historic Kuchiak Mine area, first tested in 1994. Exploration continued in 2008, and BHP also began environmental baseline studies and initiated cleanup activities at the Kuchiak Mine. PacRim Coal LP has also been active with continued environmental, permitting, and engineering work on the Chuitna Coal project west of Anchorage, on the north side of Cook Inlet. The project is being designed to mine 3 to 12 million short tons of coal per year from proven reserves of over 770 million short tons. To learn more about the Chuitna coal project, go here.
Coal Working Group Conclusions
Alaska has vast resources of high quality, low sulfur coal that has great potential for providing energy locally and for export. Technology exists for extracting coal and for generating both electricity and space heating from it. The economics of coal mining through electrical power generation and space heating are well known for a given resource base. These economic models can be extrapolated for the economy-of-scale necessary for rural settings. Mine-mouth electrical power plants can greatly reduce the need to transport large volumes of coal, and electrons can be transmitted to a number of communities via power lines rather than by hauling coal over great distances. Because there is a lack of detailed information on bed thickness and lateral extent of coal seams in many of Alaska’s coal provinces, the total volume of identified coal resources suitable for mining remains much lower than what is likely present.
As coal technology advances and concern for the environmental impact of coal mining increases, there is a greater push for cleaner burning coal and more environmentally-friendly mining methods. There are 4 main types of mines: shaft mine, drift mine, slope mine, and surface mine.
Currently, Usibelli Coal Mine in Healy is the only operable coal mine in Alaska.
|TECHNOLOGY SNAPSHOT: COAL|
|Coal Resources (Worldwide)||Recoverable coal is approximately 998 billion short tons (2004 numbers)|
|Coal Resources (National)||Approximately 491 billion short tons of demonstrated reserves, estimated 275 billion tons recoverable (2007 numbers)|
|Coal Resources (Alaska)||170 billion short tons identified resources; approximately 5.6 trillion short tons of hypothetical coal resources|
|Resource Distribution (Alaska)||Distributed in eight major coal provinces and numerous smaller coal fields and occurrences|
|Number of communities with potential coal resources||Over 40|
|Technology Readiness||Proven technology for electrical power generation and space heating|
|Environmental Impact||Surface mining requires reclamation. Combustion for electrical power generation must meet EPA requirements|
|Economic Status||Economically mined in Interior Alaska (Healy) for power generation and some space heating, active coal exploration (see below) in Western Arctic, Cook Inlet and Alaska Peninsula|
Alaska's only operable coal mine, Usibelli, is a surface mine, using strip mining for all of its operations. In strip mining, overburden (soil, vegetation, rocks) are removed before coal is mined in open pits.
Because of the difficulty in excavating overburden and coal, Usibelli mine uses blasting to loosen the material before moving it. By using a blasting technique called "cast blasting," they are able to cast a portion of the overburden laterally into an already mined-out pit, reducing the amount that the dragline, bulldozers, shovels, and backhoes must remove from the site.1
Coal as a Fuel
Coal's main use is as a solid fuel to produce electricity and heat during combustion. Alaska has 6 small coal-fired power plants, located at Fort Wainwright, Eielson Air Force Base, Clear Air Force Station in Fairbanks (Aurora Energy LLC), on the University of Alaska Fairbanks campus, and near Usibelli Coal Mine.2
When used for electrical generation, coal is often pulverized before combustion in a furnace with a boiler; the steam from the boiler turns turbines which drive generators to produce electricity. There are other ways to use coal as a fuel, including gasification and liquefaction. The 6 coal-fired plants in Alaska use the pulverization technology.
Clean Coal Technology
Clean coal technology addresses atmospheric problems that occur as the result of burning coal on a large scale. Examples of clean coal technology are the use of chemicals to "scrub" impurities from the coal, treating flue gases with steam to remove sulfur dioxides ("SOx," pronounced "socks") and other particulate matter, and the capture and storage of carbon dioxide in the flue gases.
Golden Valley Electric Association has plans to reopen a coal-fired power plant near Healy that was built in the late-1990s. The State Department of Environmental Conservation filed a draft permit which must clear public and federal reviews before the plant can reopen. The 50 MW coal plant sits near the border of Denali National Park and was originally built with experimental technology designed to reduce emissions. GVEA says that the proposed reopening of the plant could save them $2 million a month in costs, allowing them to buy more coal and cut oil consumption.3
Because nitrogen oxides, or "NOx" (pronounced "knocks"), can form during combustion and be released into the air as a pollutant, low-NOx coal burners have been developed to reduce this pollution. Although these low-NOx burners are less effective then the devices that "scrub" the flue gases, they are far less expensive and can cut NOx pollution in half.
Low-NOx coal burners use staged combustion to reduce this pollution, where coal is burned in stages. In this process, Coal is burned in a chamber where there is more fuel than oxygen present, so that the majority of oxygen in the air combines with the fuel rather than with nitrogen. The remaining fuel is then moved to a second combustion chamber where the process is repeated.4
Many "scrubbers" that are used to treat flue gases at coal-fired power plants contain a mixture of crushed limestone and water which is sprayed into the coal combustion gases. The limestone absorbs the nitrogen gases and creates a paste that can be removed from the chamber; there are also "scrubbers" that use a dry power.5
Links and References
American Society of Testing Materials (ASTM), 1995, Standard classification of coal by rank: ASTM designation D388-82, Philadelphia, 1995 Book of Standards, v. 5.05, pages 168-171.
Clough, James, 2002, Coal Geology of Alaska, in 2002 Keystone Coal Industry manual, Chicago, Illinois, Primedia, p. 488-496.
How is Coal Formed? February 26, 2006, Kentucky Geological Survey, University of Kentucky.
Merritt, R.D., and Hawley, C.C., 1986, Map of Alaska’s coal resources: Alaska Division of Geological & Geophysical Surveys Special Report 37, 1 sheet, scale 1:2,500,000.
Wood, G. H., Jr., Kehn, T.M., Carter, M.D., and Culbertson, W.C., 1983, Coal Resource Classification System of the U.S. Geological Survey. U.S. Geol. Surv. Circ. 891, 65 pages.
- NETL: "Coal and Power Systems". This website is a valuable resource for information on the latest coal technology R&D from NETL.
NETL Coal and Power Systems News
Jim Clough, Steve Denton, Rajive Ganguli, Mike Harper, Jim Hemsath, Dave Hoffman, James Jensen, Paul Park, Dick Peterson, Karl Reiche, William Sackinger, Jim Strandberg, and Bob Gross
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