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Reducing Air Pollution

http://www.teachcoal.org/aboutcoal/articles/coalenv.html

When coal is burned, it releases impurities such as sulfur, nitrogen, and fly ash. These can pollute the air or contribute to conditions that can lead to "acid rain," precipitation that damages forests and pollutes rivers and lakes. Today, though, advances in technology mean that about 99 percent of the chemicals that can pollute the air and more than 95 percent of the chemicals that can cause acid rain are removed.

These technological advances are part of an effort referred to as the Clean Coal Technology (CCT) Program, which began in 1985. Since that time, the federal government has contributed more than $2 billion toward this program to make the burning of coal cleaner and safer. The coal industry has contributed more than $4 billion toward this goal.

How is coal made cleaner?

There are several ways. Coal can be crushed and washed before it is burned. The washing process often goes a long way in removing harmful sulfur, but it does not remove all of the sulfur.

While the coal burns, special combustion processes can remove more sulfur and nitrogen. Devices called flue gas desulfurization systems, or "scrubbers," remove more than 90 percent of the sulfur dioxide emissions from the burning process. The flue gas is sprayed with a mixture of water and lime or limestone, which reacts with the sulfur dioxide to form a wet sludge or, in some cases, a dry powder that can be disposed of or made into pellets for roadbeds or into plasterboard or concrete blocks.

Another method for tackling air pollution involves using devices called electrostatic precipitators, which give coal dust particles an electric charge so they can be attracted to a collector plate.

Other methods of removing pollutants involve the way the coal is actually burned. In fluidized bed combustion (FBC), coal is inserted into a bed of particles (including limestone) that are suspended in the air and react with the coal to heat the boiler more cleanly. In FBC, coal is burned at a slightly lower temperature, which helps prevent some nitrogen oxide gases from forming. The result is that FBC can remove more than 90 percent of the sulfur and nitrogen while the coal is burning. Through a chemical reaction, sulfur gases are changed into a dry powder called calcium sulfate, which can be used to make wallboard for building homes.

The coal gasification method changes coal into a gas that has the same heating value as natural gas. Coal gas burns cleaner than coal because so many pollutants have been removed during the transformation to a gas. This method can remove up to 99.9 percent of the sulfur and tiny dirt particles from burning coal. For more information about coal gasification, go to: http://www.eastman.com/Company/Gasification/Overview.htm.


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