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Columnists
Sustainability
and Sustainable Development
Larry
Olson, PhD Professor ASU
Polytechnic
Larry
Olson, PhD., Professor, Arizona State University Environmental
Technology Management Program. Dr. Olson holds a Ph.D. in Chemistry from
the University of Pennsylvania, and is an environmental chemist with
interests in remediation technologies and international environmental
management. He can be reached at 480-727-1499 or by email at Larry.Olson@asu.edu.
Carbon
Capture and Storage
August
/ September 2009
Carbon Capture and Storage (CSS)
refers to the process of removing carbon dioxide from a gas stream and
then finding a permanent storage method that prevents the CO 2
from returning to the atmosphere. With climate change legislation moving
through the U.S. Congress and throughout the world, such technology is
going to become increasingly important. Of course, we have nature’s
perfect example of carbon capture in photosynthesis and there is a lot of
interest in adapting biological processes to industrial applications. But
what other non-biological technologies exist?
About 40% of the world’s
man-made carbon emissions are derived from fossil fuel power generation
and this represents a much more tractable target for carbon capture than
widely dispersed mobile sources. Direct combustion of fossil fuels
inevitably produces CO 2
and the basic problem with carbon capture is separating CO2
from other gases in a cost effective way when the gas volume is high and
the carbon dioxide concentration is low.
Among the methods being
considered are oxy-fuel combustion and pre- and post-combustion capture.
The oxyfuel process involves burning the fuel in an oxygen enriched
mixture. Removing most of the nitrogen from combustion air not only
reduces the flue gas volume by up to 75%, but changes the composition to
almost pure CO 2
and water vapor making it much easier to separate and capture the CO2.
The energy required to separate oxygen from air makes this process more
expensive than combustion with air, but new carbon taxes or limits on
emissions may change this calculation. There is also an added benefit of
reducing conventional air pollutants such as NOx.
Pre-combustion removal of CO 2
involves a partial combustion of the carbonaceous fuel to produce syngas,
a mixture of CO and H2.
This mixture can be burned directly or converted to a liquid fuel through
the Fisher-Tropsch process. Further reaction with steam can convert syngas
into CO2
and H2,
from which the carbon dioxide can be more easily separated because of its
high concentration. Burning the remaining hydrogen produces only water as
a product.
Both the oxyfuel and
pre-combustion processes are targeted towards new fossil fuel power
plants. However, the third option of post-combustion capture can be used
with existing conventional plants. The basic technology has been known for
decades and takes advantage of carbon dioxide’s acidity by scrubbing it
with an aqueous solution of ammonia or an alkanoamine. CO 2
can then be stripped from the solution by heating. All this, of course,
requires energy and a plant that could capture 90% of carbon dioxide
emissions might have to generate as much as 30% more power to capture and
compress the CO2
(Chemical and Engineering News, July 13, 2009, p. 19).
A number of pilot plants are
being constructed around the world to try to optimize this technology.
Innovative ideas include proprietary amine solvents, a chilled ammonia
process, and non-volatile ionic liquids instead of water which would
require less energy because the stripping process wouldn’t involve
evaporation of water.
Federal support for R & D has been crucial
to move CSS beyond the lab bench scale, but this is a brand new market and
it is expected to be huge. But once you have captured CO 2,
what do you do with it? We’ll take a look at the options being
considered in the next issue.
2008/1234
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