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Baseload units: Generating units that are normally operated to provide all or part of the minimum energy requirements of customers. These units generally run continuously to produce electricity and are typically only shutdown for maintenance.
Btu (British thermal unit): A unit of energy, equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One kilowatt-hour is equivalent to 3,412 Btu.
Carbon dioxide (CO2): A molecule composed of one atom of carbon and two atoms of oxygen. It is a product of fossil fuel combustion and other processes.
Clarifier: A tank used to purify water, in which solids settle to the bottom by gravity and are subsequently removed as sludge.
Cold electrostatic precipitator: An electrostatic precipitator installed on the “cold side,” or downstream, of the air preheater.
Combustion emissions: The byproducts that result from burning fuels such as oil, coal, gas and wood. These emissions include but are not limited to sulfur dioxide (SO2), nitrogen oxides (NOx), carbon dioxide (CO2) and particulates.
Combustion tuning: Optimizing the fuel and air mixture in a boiler to improve generation efficiency and minimize emissions.
Condensate: Clean water that is the working fluid used in the boilers to produce steam. This water is recirculated through the boiler and turbine cycle, repeatedly heated into steam and cooled back into water in the process.
Condenser: A device that converts steam exhausted from the turbine into condensate; also removes air and other non-condensable gases from the steam.
Consent decree: A voluntary agreement between two parties that is approved by a judge.
Continuous emissions monitors (CEMs): A system consisting of all the equipment needed to measure and provide a permanent record of the emissions from an affected unit. The U.S. Environmental Protection Agency has established operational requirements for the continuous emissions monitoring that include equipment performance specifications, certification procedures, and recordkeeping and reporting.
Cycling units: Generating units that can be shutdown and restarted on a daily basis to respond to changes in electricity demand. They are started and stopped more frequently than baseload units.
Downwash: The deflection of a gas flow downward, brought on by eddies that form in the lee of a structure (the area protected from wind or airflow by the structure itself) when the wind is blowing.
Electrostatic precipitator (also called ESP): A device that removes fly ash particulate matter from combustion gases to prevent them from escaping through exhaust. It consists of vertical metal plates that use an electric charge to create static and capture the particles so they can be contained and removed. A “hot” electrostatic precipitator removes fly ash particulates before they enter the air preheater unit; a “cold” electrostatic precipitator removes fly ash particulates after they enter the air preheater unit. Each hot electrostatic precipitator at Potomac River has a particulate collecting area equivalent to six acres. This is approximately three times larger than typical units of this type.
Fly ash: Solid material that remains after fossil fuels are combusted. This material is carried by flue gases out of the boiler and is then captured (with 99.7% efficiency at Potomac River) by the electrostatic precipitator.
Fugitive dust: Particulate emissions that come from sources other than the generating unit stacks.
Hot electrostatic precipitator: An electrostatic precipitator installed on the “hot side,” or upstream, of the air preheater. Each hot electrostatic precipitator at Potomac River has a particulate collecting area equivalent to six acres. This is approximately three times larger than typical units of this type.
Low NOx burner: A modified design for coal and air nozzles (burners) that changes the fuel and air distribution in the boiler’s burner zone, producing smaller amounts of nitrogen oxides.
National Ambient Air Quality Standards (NAAQS): Standards set by the U.S. Environmental Protection Agency, under the auspices of the Clean Air Act, for six pollutants — ozone, carbon monoxide, total suspended particulates, sulfur dioxide, lead, and nitrogen oxides — that apply to outdoor air throughout the country.
National Pollutant Discharge Elimination System (NPDES): A provision of the Clean Water Act that prohibits discharge of pollutants into U.S. waters unless a special permit is issued by the Environmental Protection Agency, a state, or, where delegated, a tribal government on an Indian reservation. Mirant holds NPDES permits for all facilities that require them.
Neutralization system: A water treatment system that balances the pH of water collected, resulting in water that is neither acidic or alkaline.
Nitrogen oxides (NOx): A byproduct of burning fuels such as oil, coal, gas and wood in both stationary and transportation sources.
Oil/Water separator: A device that separates emulsified oil from water. The separated oil floats to the top and is drained off to a collection device. The clear water passes through.
Opacity: Applied to a smokestack, it is a gauge of the amount of moisture, vapor, and/or particulate matter being emitted. The higher the opacity, the more visible the stack emissions.
Outflow: Water discharged from a power plant.
Particulate matter: Tiny bits of solid material in the air, including smoke, liquid, mist, fumes, or smog, produced by a variety of sources.
pH: A measure of the acidity or alkalinity of water, expressed in terms of the concentration of hydrogen ions. The scale ranges from 0-14, with 0 being most acidic, 7 neutral, and 14 the most basic (alkaline). The pH of natural waters typically measures between 6.5 and 8.5. Examples of acidic substances are beer (pH 4.4) and lemon juice (pH 2.2); examples of basic substances are egg whites (pH 8.4) and household bleach (pH 12.6).
Reasonably available control technology (RACT): Control technology that is both technologically and economically feasible. The term usually applies to existing sources in nonattainment areas.
Selective autocatalytic reduction (SACR): A process (without a catalyst) wherein ammonia and natural gas are injected into the gas stream, causing a chemical reaction to reduce nitrogen oxides. (See selective catalytic reduction.)
Selective catalytic reduction (SCR): A process whereby ammonia (or an ammonia derivative such as urea), usually diluted with water, steam, or air, is injected into the exhaust stream of a power generating unit, and then that gas is passed over a catalyst. The ammonia reacts with NOx gases to produce molecular nitrogen (N2), a naturally occurring element that makes up 80 percent of the air we breathe, and water vapor. An SCR is “selective” in that the catalyst material promotes reactions specifically to remove NOx.
Separated Over-Fire Air (SOFA) system: A boiler retrofit designed to reduce NOx emissions. It works by “staging” combustion, where coal is ignited in a fuel-rich zone at reduced temperature, producing less NOx emissions. The combustion process is then completed in an air-rich zone prior to leaving the boiler.
Settling pond: A holding area for wastewater, where heavier particles sink to the bottom for removal and disposal.
Sulfur dioxide (SO2): A colorless gas consisting of sulfur and oxygen, produced primarily by the combustion of fossil fuel.
Total suspended solids (TSS): A measure of water quality. TSS is determined by pouring a specified volume of water (typically one liter) through a pre-weighed filter of a specified pore size, and then weighing the filter after it is dried. The increase in weight is a dry weight measure of the particulates present in the water sample, usually expressed in milligrams per liter or mg/l.
Trona: A naturally occurring mineral found in large quantities in the United States. It is known chemically as sodium sesquicarbonate and is similar to baking soda or soda ash. Trona is not volatile, flammable or combustible, or odorous.
Water intake: Water drawn into the plant for purposes of cooling plant equipment. |
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