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The term “clean coal technologies” is used
to describe a wide range of processes that reduce or eliminate
emissions produced from coal-fired electricity
generation. Coal is typically used to produce electricity by pulverizing it
into a fine dust, which is then blown into a combustion chamber that heats
a boiler.
The boiler produces steam that propels a turbine, which, in turn, generates
electricity. Emissions found in the flue stack gases of these plants, which
are emitted to our atmosphere, include
•
Sulphur dioxide (SO2) – creates acid rain when combined with water vapour
•
Carbon dioxide (CO2) – the largest greenhouse gas by volume emitted by
Canada
•
Nitrous oxides (NOx) and particulate matter – creates smog and contributes
to the production of acid rain, and
•
Mercury – a neural toxin.
Some clean coal technologies include
• Supercritical boilers that employ higher temperatures and pressures to
increase combustion efficiency, which reduces the amount of all emissions released
per
unit of electricity produced.
• Scrubbers/solvent capture that removes SO2 and NOx from the flue stack
gases of a typical coal plant before the gas is run through a solvent that captures
CO2.
•
Oxygen/CO2 recycle combustion or oxyfuel that separates the nitrogen and oxygen
in the air, feeding only the oxygen into the combustion process of a conventional
pulverized coal operation. Since coal won’t burn using oxygen alone,
flue gas containing CO2 is recycled back to the burner and used to dilute the
oxygen, resulting in high concentration of CO2 in flue gas and making it more
economic to capture. This process reduces CO2 and other emissions.
• Integrated Gas Combined Cycle (IGCC) technology offers an entirely different
approach by turning coal into a gas. Gasification technologies differ widely
but share certain general production characteristics.
Coal is fed into the gasifier in either dry or slurried form. In the gasifier,
the coal is subjected to high temperatures and pressure, and low levels of
oxygen to create synthesis gas, or syngas, without burning the coal. Syngas
is a mixture of hydrogen and carbon monoxide that has about one-quarter of
the energy value of natural gas. The gas is then treated or cleaned to remove
trace elements or impurities such as sulphur that are either recirculated to
the gasifier or recovered and marketed.
The syngas is then burned in an advanced gas turbine (the first cycle). Waste
heat from the gas turbine is captured instead of being released to the atmosphere,
and used to create steam. The steam drives a steam turbine that generates more
electricity.
Since one fossil fuel source provides the energy to drive two turbines in this
combined cycle process, it is more efficient than pulverized coal plants and
fewer emissions are produced.
One of the biggest benefits of the Integrated Gas Combined Cycle process is
that it creates a pure CO2 stream that can be more easily captured and geologically
stored or used in other industrial processes.
Coal isn’t the only fuel source for IGCC technology. Any number of carbon-based
feedstocks can be used such as petroleum coke, the carbon residue from upgrading
oilsands bitumen to synthetic crude, and the high-sulphur bottoms produced
by refinery operations. Typically these products would be disposed of as
waste.
Some clean coal technologies are used in commercial applications
throughout the world. IGCC is gaining in popularity in
the United States and Asia for electricity generation but
it is still in the preliminary research and development
stage here. EPCOR’s Genesee Phase 3 plant addition,
southwest of Edmonton, will be the first operation in Canada
to use the supercritical process in conjunction with a
number of other emission reducing technologies.
-reprinted with permission from C3 Views, the monthly
newsletter of Climate Change Central.
On Genesee 3
All They Are Saying is Give Coal A Chance
September 2003 PEGG
www.apegga.org/whatsnew/peggs/Web09-03/coal.htm
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