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Background
The nation's first steam-electric station was opened by Edison Electric Light Company in New York City in 1882. Its six generators supplied a total of 600 kilowatts (kW) of electricity and served 500 customers in approximately six square miles. Since that time, coal has become the most common fuel source used in generating steam to produce power. Coal fired power plants currently account for 56% of the electricity generated in the US and about 36% worldwide. Because coal is an abundant and inexpensive fuel, a considerable amount of new coal fired power plant capacity is planned worldwide in the next 15 to 20 years. As smokestack emissions of sulfur dioxide, nitrogen oxides, and fine particles are an ongoing concern, emissions control equipment is an integral part of new plant construction.

System Design and Operation
Most modern coal fired power plants generate from 125 megawatts (MW) to 1000 MW. One MW is equal to one million watts, and one MW hour can supply electricity to approximately 330 homes for one hour on a typical hot afternoon. A conventional coal fired plant consists of a coal handling system, boiler, turbine, generator, transformer, water handling, and emission control system.

Coal is fed into a boiler where it is burned in order to heat water to produce high-pressure steam. Depending on many factors including the size of the boiler and the type of coal burned (energy content, ash content, etc.), the amount of coal used will vary. For instance, one of our customers operates a 500 MW plant. The boiler servicing the plant consumes 225 tons of coal per hour when operating at full load. This equates to 5,400 tons of coal per day, enough to fill 54 rail cars. Another boiler located at a different 500 MW plant uses 160 tons of coal per hour, which equates to 3,840 tons per day. Less coal is consumed in the second case, since the coal used has a higher energy content.

Inside the boiler, water rises to temperatures of 1000°F (540°C) to 1200°F (650°C) to produce steam. As it exits the boiler and enters the turbine, the steam is at a pressure between 1800 to 3500 pounds per square inch. The steam expands through the turbine, which consists of rows of blades attached to a shaft. The steam turns the blades of the turbine, which then turns a generator made up of magnets rotating between coils. The rotation of the generator induces alternating current in the coils to produce electricity. To ensure that the alternating current is kept constant at the standard frequency (60 hertz in the US), the turbine and generator must rotate at a constant speed. Once generated, the electricity passes through a transformer, which steps up the voltage to ensure efficient transmission over long distances. Modern transmission systems operate at 66,000 to 765,000 volts.

A critical part of the power generation system is water handling. After the steam passes through the turbine, it enters a condenser. The condenser converts the low pressure steam to liquid water. The water is then pumped back to the boiler to repeat the heating/condensing cycle. Proper operation of the condenser requires access to a large amount of water. Cooling water from rivers or large lakes is generally used. If the rivers or lakes are distant, cooling towers are constructed.

Emissions Control System
The emission control system operates in parallel with power generation. The byproduct of coal combustion, ash, is composed of both bottom ash (~ 20%) and flyash (~ 80%). (These percentages vary based on the type of coal burned and the boiler in service). For our customer's 500 MW plant burning 225 tons of coal per hour, approximately 108 tons of bottom ash and 432 tons of flyash are produced per day. The bottom ash settles to the bottom of the boiler and is collected by a hopper for disposal. Meanwhile the flyash is carried with the flue gas toward the electrostatic precipitator (ESP) where collection efficiencies are in excess of 99.5%. Electrodes in the ESP negatively charge the flyash, which is then attracted to collector electrodes of opposing charge. The flyash is next removed from the collecting electrodes into a hopper for disposal and, in some cases, is sold to cement companies to manufacture high quality concrete.

In addition to fine particles, coal fired power plants generate sulfur dioxides (SO2) and nitrogen oxides (NOx). The 500 MW plant burning 225 tons of coal per hour generates 12 tons per hour of SO2 and 1.7 tons per hour of NOx. This equates to 288 tons of SO2 and 20 tons of NOx per day. Due to environmental regulations restricting SO2 and NOx emissions, many coal fired power plants control SO2 emissions in one of two ways: 1) by burning coal that has a lower sulfur content; or 2) by removing the sulfur from the flue gas before it passes to the smokestack. To control NOx emissions, most plants use low NOx burners that lower the temperature at which coal combustion occurs. At the lower temperature, less NOx is formed.

Coal's Future
Although coal has been a reliable, abundant, and relatively inexpensive fuel source for most of the 20th century, its future in electric power generation is under increasing pressure as environmental regulations become more stringent worldwide. More than ever, affordable air pollution control will be critical to coal's continued viability as a primary fuel source for electricity generation.