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Clean Coal Technology
What is clean coal
technology?
BY SARAH DOWDEY
Coal is the dirtiest of all fossil fuels. When
burned, it produces emissions that contribute to global warming, create acid rain and pollute
water.
With all of the hoopla surrounding nuclear energy, hydropower and biofuels, you might be forgiven for thinking that
grimy coal is finally on its way out.
But coal is no sooty remnant of the Industrial
Revolution -- it generates half of the electricity in the United States and will
likely continue to do so as long as it's cheap and plentiful [source: Energy
Information Administration].
Clean coal technology seeks to reduce harsh
environmental effects by using multiple technologies to clean coal and contain
its emissions.
Coal is a fossil fuel composed primarily of
carbons and hydrocarbons. Its ingredients help make plastics, tar and fertilizers.
A coal derivative, a solidified carbon
called coke, melts iron ore and reduces it to create steel.
But most coal -- 92 percent of the U.S. supply -- goes into power
production [source: Energy Information Administration].
Electric companies and businesses with power
plants burn coal to make the steam that turns turbines and generates
electricity.
When coal burns, it releases carbon dioxide and
other emissions in flue gas, the billowing clouds you see pouring
out of smoke stacks.
Some clean coal technologies purify the coal
before it burns.
One type of coal preparation, coal
washing, removes unwanted minerals by mixing crushed coal with a liquid and
allowing the impurities to separate and settle.
Other systems control the coal burn to minimize
emissions of sulfur dioxide, nitrogen oxides and particulates.
Wet scrubbers, or flue gas desulfurization systems, remove
sulfur dioxide, a major cause of acid rain, by spraying flue gas with limestone
and water. The mixture reacts with the sulfur dioxide to form synthetic gypsum,
a component of drywall.
Low-NOx (nitrogen oxide) burners reduce the creation of
nitrogen oxides, a cause of ground-level ozone, by restricting oxygen and manipulating the
combustion process.
Electrostatic precipitators remove particulates that
aggravate asthma and cause respiratory ailments by charging particles with an
electrical field and then capturing them on collection plates.
Gasification avoids burning coal altogether. With
integrated gasification combined cycle (IGCC) systems, steam and hot
pressurized air or oxygen combine with coal in a reaction that forces carbon
molecules apart.
The resulting syngas, a mixture of
carbon monoxide and hydrogen, is then cleaned and burned in a gas turbine to
make electricity.
The heat energy from the gas turbine also powers
a steam turbine. Since IGCC power plants create two forms of energy, they have
the potential to reach a fuel efficiency of 50 percent [source: U.S. Department of Energy].
Next, we'll learn about the most ambitious of
all clean coal technologies and what needs to happen before clean coal can
become commercially feasible.
Where do the emissions go?
Carbon capture and storage -- perhaps the most
promising clean coal technology -- catches and sequesters carbon dioxide (CO2)
emissions from stationary sources like power plants.
Since CO2 contributes to global warming, reducing its release into the
atmosphere has become a major international concern. In order to discover the
most efficient and economical means of carbon capture, researchers have developed
several technologies.
Flue-gas separation removes CO2 with a
solvent, strips off the CO2 with steam, and condenses the steam into a
concentrated stream.
Flue gas separation renders commercially usable
CO2, which helps offset its price.
Another process, oxy-fuel combustion,
burns the fuel in pure or enriched oxygen to create a flue gas composed
primarily of CO2 and water -- this sidesteps the energy-intensive process of
separating the CO2 from other flue gasses.
A third technology, pre-combustion
capture, removes the CO2 before it's burned as a part of a gasification
process.
After capture, secure containers sequester the
collected CO2 to prevent or stall its reentry into the atmosphere.
The two storage options, geologic and oceanic,
must contain the CO2 until peak emissions subside hundreds of years from now.
Geologic storage involves injecting CO2 into the
earth. Depleted oil or gas fields and deep saline aquifers
safely contain CO2 while unminable coal seams absorb it.
A process called enhanced oil recovery already
uses CO2 to maintain pressure and improve extraction in oil reservoirs.
Ocean storage, a technology still in its early
stages, involves injecting liquid CO2 into waters 500 to 3,000 meters deep,
where it dissolves under pressure.
However, this method would slightly decrease pH
and potentially harm marine habitats.
All forms of CO2 storage require careful
preparation and monitoring to avoid creating environmental problems that
outweigh the benefits of CO2 containment.
Since alternative forms of energy cannot yet
replace a power source as cheap and plentiful as coal, clean coal technology
promises to mitigate the increasingly severe climactic effects of coal
emissions.
Utility companies and businesses do not,
however, always accept technology purely for the sake of the environment -- the
technology must first make economic sense.
Cleaning coal and sequestering its emissions
significantly raises the per-BTU price of what would otherwise be an
inexpensive fuel.
While selling byproducts like gypsum or
commercial CO2 for sodas and dry ice can offset the price of clean coal
technologies, a charge on carbon could make emission-reduction financially
realistic.
Sarah
Dowdey
Contributor
— HowStuffWorks
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