About DOE Button Organization Button News Button Contact Us Button
Search  
US Department of Energy Seal and Header Photo
Science and Technology Button Energy Sources Button Energy Efficiency Button The Environment Button Prices and Trends Button National Security Button Safety and Health Button
_DOE Office of Fossil Energy Web Site
You are here:  Educational Activities > Energy Lessons > Coal-Introduction > 2-Clean Coal Technology

An Energy Lesson

Cleaning Up Coal

The Clean Coal Technology Program

The Clean Coal Technology Program began in 1985 when the United States and Canada decided that something had to be done about the "acid rain" that was believed to be damaging rivers, lakes, forests, and buildings in both countries. Since many of the pollutants that formed "acid rain" were coming from big coal-burning power plants in the United States, the U.S. Government took the lead in finding a solution.

One of the steps taken by the U.S. Department of Energy was to create a partnership program between the Government, several States, and private companies to test new methods developed by scientists to make coal burning much cleaner. This became the "Clean Coal Technology Program."

How do you make coal cleaner?

Actually there are several ways.

Take sulfur, for example. Sulfur is a yellowish substance that exists in tiny amounts in coal. In some coals found in Ohio, Pennsylvania, West Virginia and other eastern states, sulfur makes up from 3 to 10 percent of the weight of coal.

For some coals found in Wyoming, Montana and other western states (as well as some places in the East), the sulfur can be only 1/100ths (or less than 1 percent) of the weight of the coal. Still, it is important that most of this sulfur be removed before it goes up a power plant's smokestack.

- Coal Molecule
-
Although coal is primarily a mixture of carbon (black) and hydrogen (red) atoms, sulfur atoms (yellow) are also trapped in coal, primarily in two forms. In one form, the sulfur is a separate particle often linked with iron (green) with no connection to the carbon atoms, as in the center of the drawing. In the second form, sulfur is chemically bound to the carbon atoms, such as in the upper left.

One way is to clean the coal before it arrives at the power plant. One of the ways this is done is by simply crushing the coal into small chunks and washing it. Some of the sulfur that exists in tiny specks in coal (called "pyritic sulfur " because it is combined with iron to form iron pyrite, otherwise known as "fool's gold) can be washed out of the coal in this manner. Typically, in one washing process, the coal chunks are fed into a large water-filled tank. The coal floats to the surface while the sulfur impurities sink. There are facilities around the country called "coal preparation plants" that clean coal this way.

Not all of coal's sulfur can be removed like this, however. Some of the sulfur in coal is actually chemically connected to coal's carbon molecules instead of existing as separate particles. This type of sulfur is called "organic sulfur," and washing won't remove it. Several process have been tested to mix the coal with chemicals that break the sulfur away from the coal molecules, but most of these processes have proven too expensive. Scientists are still working to reduce the cost of these chemical cleaning processes.

Most modern power plants and all plants built after 1978 are required to have special devices installed that clean the sulfur from the coal's combustion gases before the gases go up the smokestack. The technical name for these devices is "flue gas desulfurization units," but most people just call them "scrubbers" because they "scrub" the sulfur out of the smoke released by coal-burning boilers.

How do scrubbers work?

Most scrubbers rely on a very common substance found in nature called "limestone." We literally have mountains of limestone throughout this country. When crushed and processed, limestone can be made into a white powder. Limestone can be made to absorb sulfur gases under the right conditions much like a sponge absorbs water.

In most scrubbers, limestone (or another similar material called lime) is mixed with water and sprayed into the coal combustion gases (called "flue gases"). The limestone captures the sulfur and "pulls" it out of the gases. The limestone and sulfur combine with each other to form either a wet paste (it looks like toothpaste!), or in some newer scrubbers, a dry powder. In either case, the sulfur is trapped and prevented from escaping into the air.  

The Clean Coal Technology Program tested several new types of scrubbers that proved to be more effective, lower cost, and more reliable than older scrubbers. The program also tested other types of devices that sprayed limestone inside the tubing (or "ductwork') of a power plant to absorb sulfur pollutants.

But what about nitrogen pollutants? That's another part of the Clean Coal story.  

  BackGO BACK

READ ON  

 

Dr. H. Carbon asks:  How much of the sulfur in coal can Clean Coal Technologies remove?
LESS THAN HALF

Less than half

HALF

Half

THREE FOURTHS

3/4ths

ALMOST ALL OF IT

Almost all of it

Click on your choice

 Page owner:  Fossil Energy Office of Communications
Page updated on: February 12, 2013 

The White House USA.gov E-gov IQ FOIA Privacy Program
U.S. Department of Energy | 1000 Independence Ave., SW | Washington, DC 20585
1-800-dial-DOE | f/202-586-4403 | e/General Contact

Web Policies | No Fear Act | Privacy | Phone Book | Accessibility