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Clean Coal
Technology
Coal is an abundant
resource in the world...It is imperative that we figure out a way to use coal as
cleanly as possible. "
Dr. Steven Chu,
Secretary of Energy
Senate Confirmation Hearing
January 13, 2009
"Rapid
commercial development and deployment of clean coal technologies,
particularly carbon capture and storage, will help position the United
States as a leader in the global clean energy race."
President
Barack Obama
Presidential Memorandum
February 3, 2010
With coal likely to remain one of
the nation's lowest-cost electric power sources for the foreseeable future, the
United States has pledged a new commitment to even more advanced clean coal
technologies.
COAL
is our most abundant fossil fuel. The United States has more coal than the
rest of the world has oil. There is still enough coal underground in this
country to provide energy for the next 200 to 300 years.
But coal is not a perfect
fuel.
Trapped inside coal are
traces of impurities like sulfur and nitrogen. When coal burns, these
impurities are released into the air.
While floating in the air,
these substances can combine with water vapor (for example, in clouds) and
form droplets that fall to earth as weak forms of sulfuric and nitric acid
- scientists call it "acid rain."
There are also tiny specks
of minerals - including common dirt - mixed in coal. These tiny particles
don't burn and make up the ash left behind in a coal combustor. Some of
the tiny particles also get caught up in the swirling combustion gases
and, along with water vapor, form the smoke that comes out of a coal
plant's smokestack. Some of these particles are so small that 30 of them
laid side-by-side would barely equal the width of a human hair!
Also, coal like all fossil
fuels is formed out of carbon. All living things - even people - are made
up of carbon. (Remember - coal started out as living plants.) But when
coal burns, its carbon combines with oxygen in the air and forms carbon
dioxide. Carbon dioxide is a colorless, odorless gas, but in the
atmosphere, it is one of several gases that can trap the earth's heat.
Many scientists believe this is causing the earth's temperature to rise,
and this warming could be altering the earth's climate (read more about
the "greenhouse effect").
Sounds like coal is a dirty
fuel to burn. Many years ago, it was. But things have changed. Especially
in the last 20 years, scientists have developed ways to capture the
pollutants trapped in coal before the impurities can escape into the
atmosphere. Today, we have technology that can filter out 99 percent of
the tiny particles and remove more than 95 percent of the acid rain
pollutants in coal.
We also have new
technologies that cut back on the release of carbon dioxide by burning
coal more efficiently.
Many of these technologies
belong to a family of energy systems called "clean coal
technologies." Since the mid-1980s, the U.S. Government has invested
more than $3 billion in developing and testing these processes in power
plants and factories around the country. Private companies and State
governments have been part of this program. In fact, they have contributed
more than several billion dollars to these projects.
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.
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.
Knocking the NOx Out of
Coal
Air
is mostly nitrogen molecules (green in the above diagram) and oxygen
molecules (purple). When heated hot enough (around 3000 degrees F), the
molecules break apart and oxygen atoms link with the nitrogen atoms to
form NOx, an air pollutant.
Nitrogen is the most common
part of the air we breathe. In fact, about 80% of the air is nitrogen.
Normally, nitrogen atoms float around joined to each other like chemical
couples. But when air is heated - in a coal boiler's flame, for example -
these nitrogen atoms break apart and join with oxygen. This forms
"nitrogen oxides" - or, as it is sometimes called, "NOx"
(rhymes with "socks"). NOx can also be formed from the atoms of
nitrogen that are trapped inside coal.
In the air, NOx is a
pollutant. It can cause smog, the brown haze you sometimes see around big
cities. It is also one of the pollutants that forms "acid rain."
And it can help form something called "groundlevel ozone,"
another type of pollutant that can make the air dingy.
NOx can be produced by any
fuel that burns hot enough. Automobiles, for example, produce NOx when
they burn gasoline. But a lot of NOx comes from coal-burning power plants,
so the Clean Coal Technology Program developed new ways to reduce this
pollutant.
One of the best ways to
reduce NOx is to prevent it from forming in the first place. Scientists
have found ways to burn coal (and other fuels) in burners where there is
more fuel than air in the hottest combustion chambers. Under these
conditions, most of the oxygen in air combines with the fuel, rather than
with the nitrogen. The burning mixture is then sent into a second
combustion chamber where a similar process is repeated until all the fuel
is burned.
This concept is called
"staged combustion" because coal is burned in stages. A new
family of coal burners called "low-NOx burners" has been
developed using this way of burning coal. These burners can reduce the
amount of NOx released into the air by more than half. Today, because of
research and the Clean Coal Technology Program, approximately 75 percent
of all the large coal-burning boilers in the United States will be using
these types of burners.
A "Bed" for
Burning Coal?
It was a wet, chilly day in
Washington DC in 1979 when a few scientists and engineers joined with
government and college officials on the campus of Georgetown University to
celebrate the completion of one of the world's most advanced coal
combustors.
It was a small coal burner
by today's standards, but large enough to provide heat and steam for much
of the university campus. But the new boiler built beside the campus
tennis courts was unlike most other boilers in the world.
In a fluidized bed boiler, upward
blowing jets of air suspend burning coal, allowing it to mix with
limestone that absorbs sulfur pollutants.
It was called a "fluidized bed boiler." In a typical coal
boiler, coal would be crushed into very fine particles, blown into the
boiler, and ignited to form a long, lazy flame. Or in other types of
boilers, the burning coal would rest on grates. But in a "fluidized
bed boiler," crushed coal particles float inside the boiler,
suspended on upward-blowing jets of air. The red-hot mass of floating coal
— called the "bed" — would bubble and tumble around like
boiling lava inside a volcano. Scientists call this being
"fluidized." That's how the name "fluidized bed
boiler" came about.
Why does a
"fluidized bed boiler" burn coal cleaner?
There are two major reasons. One, the tumbling action allows limestone to
be mixed in with the coal. Remember limestone from a couple of pages ago
(> go back)? Limestone is a sulfur sponge — it absorbs sulfur
pollutants. As coal burns in a fluidized bed boiler, it releases sulfur.
But just as rapidly, the limestone tumbling around beside the coal
captures the sulfur. A chemical reaction occurs, and the sulfur gases are
changed into a dry powder that can be removed from the boiler. (This dry
powder — called calcium sulfate — can be processed into the wallboard
we use for building walls inside our houses.)
The second reason a
fluidized bed boiler burns cleaner is that it burns "cooler."
Now, cooler in this sense is still pretty hot — about 1400 degrees F.
But older coal boilers operate at temperatures nearly twice that (almost
3000 degrees F). Remember NOx from the page before (> go back)? NOx
forms when a fuel burns hot enough to break apart nitrogen molecules in
the air and cause the nitrogen atoms to join with oxygen atoms. But 1400
degrees isn't hot enough for that to happen, so very little NOx forms in a
fluidized bed boiler.
The result is that a
fluidized bed boiler can burn very dirty coal and remove 90% or more of
the sulfur and nitrogen pollutants while the coal is burning. Fluidized
bed boilers can also burn just about anything else — wood, ground-up
railroad ties, even soggy coffee grounds.
Today, fluidized bed
boilers are operating or being built that are 10 to 20 times larger than
the small unit built almost 20 years ago at Georgetown University. There
are more than 300 of these boilers around this country and the world. The
Clean Coal Technology Program helped test these boilers in Colorado, in
Ohio and most recently, in Florida.
A new type of fluidized bed
boiler makes a major improvement in the basic system. It encases the
entire boiler inside a large pressure vessel, much like the pressure
cooker used in homes for canning fruits and vegetables — except the ones
used in power plants are the size of a small house!
Burning coal in a
"pressurized fluidized bed boiler" produces a high-pressure
stream of combustion gases that can spin a gas turbine to make
electricity, then boil water for a steam turbine — two sources of
electricity from the same fuel!
A "pressurized
fluidized bed boiler" is a more efficient way to burn coal. In fact,
future boilers using this system will be able to generate 50% more
electricity from coal than a regular power plant from the same amount of
coal. That's like getting 3 units of power when you used to get only 2.
Because it uses less fuel
to produce the same amount of power, a more efficient "pressurized
fluidized bed boiler" will reduce the amount of carbon dioxide (a
greenhouse gas) released from coal-burning power plants.
"Pressurized fluidized
bed boilers" are one of the newest ways to burn coal cleanly. But
there is another new way that doesn't actually burn the coal at all.
The Cleanest Coal
Technology - a Real Gas!
Don't think of coal as a
solid black rock. Think of it as a mass of atoms. Most of the atoms are
carbon. A few are hydrogen. And there are some others, like sulfur and
nitrogen, mixed in. Chemists can take this mass of atoms, break it apart,
and make new substances - like gas!
How do you break apart the
atoms of coal? You may think it would take a sledgehammer, but actually
all it takes is water and heat. Heat coal hot enough inside a big metal
vessel, blast it with steam (the water), and it breaks apart. Into what?
The carbon atoms join with
oxygen that is in the air (or pure oxygen can be injected into the
vessel). The hydrogen atoms join with each other. The result is a mixture
of carbon monoxide and hydrogen - a gas.
Now, what do you do with
the gas?
You can burn it and uses
the hot combustion gases to spin a gas turbine to generate electricity.
The exhaust gases coming out of the gas turbine are hot enough to boil
water to make steam that can spin another type of turbine to generate even
more electricity. But why go to all the trouble to turn the coal into gas
if all you are going to do is burn it?
A major reason is that the
impurities in coal - like sulfur, nitrogen and many other trace elements -
can be almost entirely filtered out when coal is changed into a gas (a
process called gasification). In fact, scientists have ways to
remove 99.9% of the sulfur and small dirt particles from the coal gas.
Gasifying coal is one of the best ways to clean pollutants out of coal.
Another reason is that the
coal gases - carbon monoxide and hydrogen - don't have to be burned. They
can also be used as valuable chemicals. Scientists have developed chemical
reactions that turn carbon monoxide and hydrogen into everything from
liquid fuels for cars and trucks to plastic toothbrushes!
Today, outside of Tampa,
Florida (near the town of Lakeland), and in West Terre Haute, Indiana,
there are power plants generating electricity by gasifying coal, rather
than burning it.
Coal gasification could be
one of the most promising ways to use coal in the future to generate
electricity and other valuable products. Yet, it is only one of an
entirely new family of energy processes called "Clean Coal
Technologies" - technologies that can make fossil fuels future fuels.
OPTIONS
FOR CARBON CAPTURE AND STORAGE
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1.
CO2 pumped into disused coal fields displaces methane which can
be used as fuel
2. CO2 can be pumped into and stored safely in saline
aquifers
3. CO2 pumped into oil fields helps maintain pressure,
making extraction easier
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When coal burns,
it creates the greenhouse gas carbon dioxide (CO2), as well as other
air emissions.
Emissions
exit the boiler as "flue gas". CO2 in the flue gas
reacts with chemicals to remove most of it.
Flue gas
that has been stripped of CO2 is released into the atmosphere
through a stack (chimney).
Captured CO2
is compressed, dewatered and transported via pipeline to a geologic
storage site.
Captured CO2
is pumped into saline formations a mile or more below the Earth’s
surface where it can be safely stored.
Sometimes
captured CO2 can also be used to increase the flow of oil from
an otherwise depleted reserve, where the CO2 will eventually be
stored at least 2,500 feet below the surface.
WHY COAL?
We need it for our energy
security. It is America’s largest domestic energy resource — enough to last
250 years at current rates of use. It is a versatile energy resource that can be
used in solid, liquid, or gaseous form. It has traditionally been the largest
single source of U.S. electricity generation, currently providing more than 50
percent of the total, and will continue to be the primary generator of America’s
electric power for years to come. It can be used cleanly and affordably to drive
our economic prosperity.
CLEAN COAL TECHNOLOGY . . .
F ROM
RESEARCH
TO REALITY
Credit: U.S. Department of Energy,
World Coal Association, The American Coal Foundation, BP
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