Coal is a black sedimentary rock that burns. It contains more than 70 percent carbonaceous matter by volume. Since the 1880's, coal has been burned to produce electricity. The electric utility industry is the largest consumer of coal in the nation. Currently, 80 percent of the nation's coal production and 90 percent of Ohio's coal production are burned at power plants to produce electricity.
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Electric generation in the U.S. by source (Energy Information Administration, personal commun., 1996).
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The degree to which the nation and Ohioans are dependent upon coal for the generation of electricity is indicated by the following: (1) coal provided more than 55 percent of the electricity generated by utilities in the United States in 1995; (2) 89 percent of the total electricity generated in Ohio in 1995 was from coal, 0.75 percent from gas and oil, and 10.25 percent from nuclear generation; (3) Ohio utilities in 1995 consumed almost 50 million tons of coal, of which about 33 percent was produced from Ohio mines; and (4) of the 28.3 million tons of coal produced in Ohio in 1996, all but about 12.6 percent, or about 3.5 million tons, were consumed by electric utility companies; the remaining 3.5+ million tons were consumed by industry and domestic users. Coal consumption by electric utilities is likely to increase, as forecasters have predicted that the demand for electricity will increase by 2.3 percent per year through 2010.
CLEAN-COAL TECHNOLOGY
After coal is mined, it is either shipped directly to the user untreated, or it is cleaned before shipment because most of Ohio's coal contains impurities. These impurities include mine roof and/or floor rock, clay or shale partings, calcite (CaCO3), pyritic sulfur (FeS2), sulfate sulfur in the form of gypsum (CaSO4), and organic sulfur. When sulfur-bearing coal is burned, the sulfur is volatilized (converted into a gas) and combines with oxygen (O2) to produce sulfur dioxide (SO2). Although volcanoes, automobiles, and factories emit significant amounts of SO2, electric power plants are the primary sources of SO2 emissions. There are four types of technology for cleaning coal to reduce the amount of SO2 emissions: precombustion, combustion, postcombustion, and conversion.
Coal washing is a precombustion cleaning technology in which some of the impurities contained in coal are removed before the coal is burned. Modern coal-cleaning or coal-washing techniques are based on the principle that coal is lighter (less dense) than its associated rock and impurities. In coal-washing plants (also called preparation or prep plants), agitating liquids, high-velocity liquids, and magnetite-water suspensions in a variety of physical and chemical processes are used to separate impurities from crushed coal. This precombustion cleaning process can generally remove 30 to 50 percent of the pyritic sulfur and about 60 percent of the ash-forming minerals (residue left after coal has been burned). As a result of washing, the SO2 emissions from burned coal can be reduced by almost 50 percent under ideal conditions.
Combustion cleaning technology is used to clean coal inside the furnace where the coal is burned. Fluidized-bed combustion is an example of this type of technology. In fluidized-bed combustion, pulverized coal is mixed with finely crushed limestone and is injected with hot air into the boiler. The mixture of coal and limestone is suspended on a bed of hot, injected air and resembles a boiling liquid. Because the mixture resembles a liquid, the process is called "fluidized." As the coal burns, sulfur is released and limestone acts like a chemical sponge to soak up or capture the sulfur before it can escape from the boiler. This technology can reduce the amount of SO2 by more than 90 percent. In addition, the combustion temperature during this process remains between 1400°F and 1600°F, which is about half of the operational temperature of a conventional boiler. These lower temperatures are below the temperature threshold at which nitrogen pollutants form. As a result, fluidized-bed combustors can meet sulfur- and nitrogen-emission standards without additional pollution-control equipment. This process has been successfully demonstrated at the Tidd power plant in Brilliant, Ohio.
To generate electricity, most coal-fired power plants mix pulverized coal with hot air and inject the fine particles into a boiler (a furnace lined with water-filled tubes). A conventional boiler operates at temperatures of 2800°F to 3200°F. Water is heated and steam from the boiling water spins a steam-turbine generator to produce electricity, which is transmitted through distribution lines. The gases and particulates emitted from the burned coal are released into the atmosphere or are captured by flue-gas cleaning devices such as electrostatic precipitators or scrubbers.
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Flow diagram of flue-gas desulfurization technology (from Ohio Electric Utility Institute, p. 7). ESP = electrostatic precipitator; FGD = flue-gas desulfurization.
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Electrostatic precipitators are filtering devices that use static electricity to capture dust-sized ash called fly ash. The fly ash is disposed of in landfills or is used as an additive (filler) in cement, plastics, and a variety of other products.
technology located between the boiler and the smoke stack. Scrubber technology involves injecting a slurry of finely ground limestone or lime into the flue gas. The SO2 in the flue gas reacts chemically with the slurry to produce calcium sulfite and calcium sulfate precipitates.
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Combustor vessel of the pressurized fluidized-bed combustion boiler of the 70-megawatt Tidd demonstration plant of the American Electric Power Service Corporation near Brilliant, Jefferson County, Ohio (from Stith and others, 1997).
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Scrubber technology is currently capable of processing up to 1 million cubic feet of flue gas per minute and achieving 70 to 90 percent SO2 reduction. However, over its lifetime, a 500-megawatt coal-fired power plant will produce enough flue-gas-desulfurization (FGD) sludge to fill a 500-acre disposal pond to a depth of about 40 feet. Fortunately, there is growing interest in utilizing FGD materials in the manufacture of wallboard and in concrete products. Scrubbers are very expensive to install and operate. The scrubber facility at the General James M. Gavin generating station at Cheshire, Ohio, cost an estimated $800 million to build. However, because of its scrubbers, the Gavin station can burn high-sulfur coal, mined at the Meigs No. 2 and Meigs No. 31 mines in Vinton and Meigs Counties, and meet strict air-pollution-control standards.
Conversion is another coal-cleaning technology in which coal is turned into a combustible gaseous or liquified fuel that can be cleaned. Combined-cycle coal gasification is an example of this type of technology. In the combined-cycle coal-gasification process, pulverized coal is gasified by a mixture of steam and air heated to 1500°F to 3000°F. The gaseous products from coal gasification include hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), water (H2O), ammonia (NH3), hydrogen sulfide (H2S), nitrogen (N2), and methane (CH4). The gas is scrubbed of its impurities to consist primarily of H2, CO, CH4, and N2 and then is burned. The hot exhaust gas is routed through a gas turbine to generate electricity and the residual heat is used to boil water to power a conventional steam turbine to produce more electricity. The combined use of gas and steam to generate electricity accounts for the name "combined cycle." Combined-cycle gasification systems are among the cleanest clean-coal technologies available because the fly ash and most of the sulfur and nitrogen compounds are removed before the fuel is burned.
FEDERAL CLEAN AIR ACT
In 1990, Congress passed the Clean Air Act Amendments (CAAA90), a law that imposes strict emission standards on coal-fired utilities. Title IV of CAAA90 set a national cap on SO2 emissions. By the year 2000, total SO2 emissions from all electric power plants will be restricted to 9.9 million tons annually. This reduction will occur in two phases. Phase I began January 1, 1995, and affected 110 of the largest sulfur-emitting power plants (greater than 100 megawatts) in the United States, forcing them to reduce their emissions to an average of 2.5 pounds of SO2 per million Btu or less. Btu is the abbreviation for British thermal unit, the standard unit of measurement for heating value; 1 Btu equals the amount of heat required to raise the temperature of 1 pound of water 1°F. In Phase II, beginning January 1, 2000, all power plants larger than 25 megawatts will be required to reduce their SO2 emissions to no more than 1.2 pounds per million Btu.
Annual allowances, each permitting the emission of 1 ton of SO2, are allocated initially by the U.S. Environmental Protection Agency (USEPA). There are several methods by which utilities may meet the emissions restrictions if they do not initially hold sufficient allowances. (1) Utilities can purchase allowances from generating stations that greatly exceed the emission compliance standards. (2) Utilities may choose to reduce emissions by switching to low-sulfur coal, a coal blend containing low-sulfur coal, or an alternative fuel such as natural gas. (3) Utilities may reduce emissions by producing less of their electricity from plants that emit relatively high levels of SO2, while increasing generation from their less polluting plants. (4) Utilities can install flue-gas scrubbers and/or use other clean-coal technologies. Utilities may use any combination of these methods to meet the SO2 emission limitations of the CAAA90.
Ohio has 26 coal-fired power plants. These plants have 93 operating boilers. Forty-one of these boilers are affected by Phase I of CAAA90. The remainder will be affected by Phase II.
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Cross section of conventional coal-fired power plant (from Ohio Electric Utility Institute, p. 3).
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These boilers have various Phase I compliance plans: 19 boilers switched to low-sulfur coal, 5 boilers switched to medium-sulfur coal, 8 boilers switched to natural gas, 3 boilers continue to burn Ohio's high-sulfur coal, and 6 boilers were designated as transfer units. Transfer units are boilers for which emission-reduction requirements are offset by overcompliance by other boilers.
What is the future for Ohio's coal-fired electric utility industry? The demand for electricity is expected to increase annually through 2010. Coal is considered to be the primary fuel source to meet Ohio's increasing electricity needs because: (1) prices for natural gas are expected to double by 2010; (2) the nation's estimated reserves of natural gas are expected to be depleted in the next 30-70 years, so that domestic natural gas may be a doubtful long-term competitor of coal in the fuels market; (3) nuclear power plants are expensive to operate, extremely expensive to build, their 30-year licenses are very difficult to obtain, and no new nuclear power plants are currently scheduled for construction in Ohio; (4) deregulation of the electric utility industry will cause utilities to compete in an open market for electric consumers, and Ohio's coal is a least-cost fuel for the generation of electricity; (5) by 2010, more than half of Ohio's coal-fired boilers will be more than 50 years old and in need of replacement, and it is anticipated that these boilers will be replaced by boilers using clean-coal technologies; and (6) Ohio has about 400 years of mineable coal remaining.
FURTHER READING
Crowell, D. L., 1995, History of the coal-mining industry in Ohio: Ohio Division of Geological Survey Bulletin 72, 203 p.
_____ 1997, History of coal mining in Ohio: Ohio Division of Geological Survey GeoFacts 14.
_____ 1997, Coal mining and reclamation: Ohio Division of Geological Survey GeoFacts 15.
_____ 1997, Coal: Ohio Division of Geological Survey Educational Leaflet 8.
National Research Council, 1995, Coal, energy for the future: Washington, D. C., National Academy Press, 288 p.
The Ohio Electric Utility Institute, undated, To fuel our dreams, the promise of clean coal technology: 13 p.
Stith, D. A., Berg, T. M., Ault, C. H., Dever, G. R., Jr., Masters, J. M., Berkheiser, S. W., Jr., Simard, C. M., and Hester, N. C., 1997, Limestone and dolomite availability in the Ohio River Valley for sulfur sorbent use, with observations on obtaining reliable chemical analyses: Ohio Division of Geological Survey Information Circular 59, 16 p.
Weisgarber, S. L., 1997, 1996 Report on Ohio mineral industries: Ohio Division of Geological Survey.