Written by Suzanne Heibel
|Tuesday, 11 November 2008|
Clean Coal: Can Coal Really be “Clean”?
The first method is to strip coal of any sulfur it may contain before it is actually burned. Although sulfur makes up anywhere from 10 to less than 1 percent of coal, it is important to get rid of before combustion because in the external environment sulfur is the main ingredient of acid rain. There are two ways in which sulfur is attached: 1) it can be chemically bound to coal's hydrogen and carbon atoms or, 2) it can be an impurity, attached by extraneous physical properties. The latter is what is extracted when coal is washed.
To separate the sulfur, coal is ground into small pieces and then ran through a water filter. Sulfur is more dense than coal, so the sulfur sinks to the bottom while the coal remains floating at the top. Coal can then be burned with no fears of acid rain.
Unfortunately no cost effect method has yet been discovered to remove the organic sulfur that is part of coal's molecular structure, in other words, it cannot be simply washed.
Integrated Gasification Combined Cycle (IGCC)
IGCC is a method employed in many power plants which aspires to squeeze the most amount of energy out of the combustion of coal. In the presence of oxygen and steam, coal reacts and is converted into a gas composed of carbon monoxide, hydrogen, and carbon dioxide commonly known as a syngas. Coincidently, this reaction ends up filtering out nearly all of the pollution-causing impurities like sulfur and nitrogen oxides.
The most prominent use of syngas is to burn it inside of a turbine to create an enormous amount of heat that is used to spin that turbine so it can generate electricity. This process is a doubly whammy because this first turbine will also bring about additional waste heat that can be used to boil water, create steam, and run a second turbine that produces electricity.
The key environmental factors you should remember about an IGCC system is its ability to prevent pollution (by erasing the imperfections through gasification) and using and reusing waste heat for actual energy instead of just letting it dissipate and be, well, wasted.
Officially titled flue gas desulferization units, scrubbers work to collect sulfur dioxide—a major pollutant—before they exit into the outside environment. Wet scrubbers utilize a mixture of powdered limestone and water that is sprayed through high pressure nozzles onto gases leaving through the flues of the coal plant. The limestone blend then traps emissions creating a gooey paste on the inside of the flues that eventually has to be processed. But none the less, scrubbers are very affective at what they are designed to do, the newest ones are able to catch up to 99% of all sulfur dioxide!
Dry scrubbers have a circulating fluid bed that is composed of hydrated lime and water, which is used to regulate temperature. The flue gases are forced through this bed of liquid, the sulfur dioxide then binds with the lime and is prevented from external release.
Put very simply, electrostatic precipitators are electromagnetic fields that are attached to filters in flues called collection plates. The charged electromagnetic field attracts particles and therefore prevents the particles (like ash and dust) from escaping out the flue. This keeps the junk that causes allergies and smog out of the skies.
Carbon Capture and Sequestration (CCS)
If you haven't yet noticed, there has been no explanation of how exactly coal plants would prevent carbon dioxide release. I mean, isn't that what all this global warming hype is all about? Yeah, that's one part, but scientists report that coal is the leading contributor to global warming, acid rain, toxic airborne particulates, and smog. So don't be so upset we haven't discussed the big C-O-2 yet, coal has a lot on its plate right now as far as pollution goes.
The World Coal Institute has a plan to lasso carbon dioxide, they want to stick it in the ground, just bury it. As an environmentalist, this at first seems like a stupid idea. It mimics our genius “lets just hide it underground and then build stuff on top of it” idea for dealing with garbage disposal. But the world is running out of natural carbon sinks. As the ocean warms, its ability to hold carbon diminishes; as forests are cut down, trees that once took in carbon dioxide are now releasing it. And to be quite honest, we are producing so much carbon dioxide these days that we really do need some place to put it besides the atmosphere.
Carbon capture involves pumping coal at least 800 meters below ground where the pressure of the earth will keep it in a liquid form for thousands to millions of years. The CO2 is pumped down into pockets of the earth that have already been vacated due to previous oil or coal excavation. Ironically, the oil industry has already performed this act of injecting the greenhouse gas into oil fields that have been producing less and less fuel. The carbon is actually proven to boost petroleum extracted from a dry oil well.
Many environmentalists argue that this idea is dangerous. Scientists worry about deep sea creatures' sensitivity to change in levels of carbon dioxide in their environment and the pH, which is already shifting due to warming waters. Is it safe? No one is yet completely sure but be aware that carbon storage is a very real and an already implemented method for combating global warming.
Coal Extraction and Mountain Top Mining
Emission removal is the most crucial aspect in curving coal's negative environmental effects and through the topics discussed above, the industry is doing a great job at keeping pollutants out of the atmosphere while still providing the world with ample amounts of energy. But to “green” the burning of coal, the industry should also examine how they extract this precious resource. Chopping the heads off mountains and leaving the excavation site as a wasteland is not very eco-friendly. The coal industry resorted to this ecologically devastating practice after underground mining proved too dangerous for miners. However, there are no solutions to mountain top mining. If coal should be deemed “clean” then an alternative to this method needs to be explored.
It will be very interesting to see how the next United States administration develops and deploys “clean coal” technology as part of a larger national energy plan!
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