Cleaner air on the fly?The coal industry represents more than half of America's energy production, and DOE estimates place the recoverable reserve at more than 250 billion short tons. Coal is notorious for its drawbacks, however, including emissions of sulfur (which in the form of sulfur dioxide can react with atmospheric water to form sulfuric acid sulfuric acid, chemical compound, H2SO4, colorless, odorless, extremely corrosive, oily liquid. It is sometimes called oil of vitriol. Concentrated Sulfuric Acid
) and mercury (a known neurotoxicant). Now scientists from the Energy Research Center at Lehigh University, led by Carlos Romero, have shown that it may be possible to reduce mercury emissions by up to 70% without a lot of costly modifications, simply by optimizing boiler operation.
The USGS USGS United States Geological Survey (US Department of the Interior) report Mercury in U.S. Coal: Abundance, Distribution, and Modes of Occurrence states, "The mercury emitted directly from power plants is not considered harmful; however, in the natural environment, mercury can go through a series of chemical transformations that convert elemental mercury to a highly toxic highly toxic Occupational medicine adjective Referring to a chemical that 1. Has a median lethal dose–LD50 of ≤ 50 mg/kg when administered orally to 200-300 g albino rats 2. form [methylmercury] that is concentrated in fish and birds." In large doses, methylmercury can cause mental retardation mental retardation, below average level of intellectual functioning, usually defined by an IQ of below 70 to 75, combined with limitations in the skills necessary for daily living. , seizures, cerebral palsy cerebral palsy (sərē`brəl pôl`zē), disability caused by brain damage before or during birth or in the first years, resulting in a loss of voluntary muscular control and coordination. , and death in humans. Though some mercury is removed by cleaning the coal before burning, and more is recaptured in the stack, the EPA EPA eicosapentaenoic acid.
n.pr See acid, eicosapentaenoic.
n. estimates that coal-fired power plants release 40 to 52 tons of mercury each year.
Currently, according to Romero, the industry relies on techniques such as injecting activated carbon into the flue gas stream to adsorb adsorb /ad·sorb/ (ad-sorb´) to attract and retain other material on the surface; to conduct the process of adsorption.
To take up by adsorption. the mercury. One costly problem with this approach is that a typical 250-megawatt power plant can use significant amounts of activated carbon, at a cost of about 50 cents per pound.
The goal of Romero's optimization technique is to leave more unburned carbon in the fly ash, the residue left after combustion of pulverized pul·ver·ize
v. pul·ver·ized, pul·ver·iz·ing, pul·ver·iz·es
1. To pound, crush, or grind to a powder or dust.
2. To demolish.
v.intr. coal. The more carbon the fly ash contains, the better able it is to capture oxidized oxidized
having been modified by the process of oxidation.
see absorbable cellulose. mercury (formed when mercury combines with chlorine, also found in coal). It's not clearly understood why fly ash captures mercury, Romero admits, and more research is being done to explain this interaction.
"Our testing has shown that if you lower the amount of excess air in the boiler [and thus lower the flue gas temperature], you increase the level of unburned carbon," he explains. "You can also increase the level of unburned carbon by grinding the coal more coarsely." Results vary depending on the type of coal used and the boiler configuration.
Further tweaking will address a couple of potential drawbacks to the approach. Fly ash is used in Canada and the United States The United States and Canada share a unique legal relationship. U.S. law looks northward with a mixture of optimism and cooperation, viewing Canada as an integral part of U.S. economic and environmental policy. in the manufacture of cement, but due to the physical qualities of the unburned carbon, fly ash can contain only a certain amount (about 4-6%). Plus, flue gas temperatures must not be lowered too dramatically, says Romero, lest acids form in the gas, creating corrosion in the smokestack.
Under the Clean Air Interstate Rule of March 2005, the EPA has mandated a 23% reduction of mercury by 2010 and a 69% reduction by 2018. Romero thinks some boilers could achieve the first reduction through boiler optimization. "The sixty-nine percent [reduction] will be tough to achieve with combustion optimization," he says, "but I believe this approach can be a valuable tool in industry's efforts to reduce mercury emissions."
George Often, senior technical leader for air emissions and combustion product management at the Electric Power Research Institute, says that while this may be a low-cost approach to achieving moderate reductions in mercury emissions, larger plants will retrofit with other technologies to meet the requirements of the Clean Air Interstate Rule. "However," he adds, "many smaller plants, or those located far away from locations that use fly ash in concrete, could find this process very attractive."