may be the key to capturing carbon dioxide from fossil fuel combustion.
ORNL researchers have developed a promising new technology called carbon
fiber composite molecular sieves (CFCMS) that can be designed to capture
carbon dioxide emitted from coal-fired power plants and gas turbines.
The recovered carbon gas could be collected in a vessel for transport
to a carbon sequestration site. It could then be injected into an underground
coal bed, depleted oil reservoir, or the ocean.
Charlie Weaver, and Bill Chilcoat, all of ORNL's Metals and Ceramics
Division, developed CFCMS technology in collaboration with researchers
at the University of Kentucky. The work was grounded in ORNL's previous
experience of developing carbon-bonded, carbon-fiber insulation for
thermoelectric cells in space probes.
has several advantages over conventional granular activated-carbon beds
for removing carbon dioxide from gas streams," says ORNL Fossil Energy
Program Manager Rod Judkins. "It not only adsorbs more carbon but it
also takes it up 5 to 10 times faster. And 2 to 10 times less energy
is required to recover the adsorbed carbon dioxide and regenerate the
filter so it can be used again."
Because the CFCMS
filter is electrically conductive, carbon can be removed from the saturated
sieve by running an electrical current through it at low voltage. "There
are several possible explanations of how electrical desorption works,"
Judkins says. "Maybe it's a surface heating effect." In a conventional
activated-carbon bed, the carbon is recovered by a more energy-intensive
process, such as heating or depressurizing the bed.
can make monolithic CFCMS structures in various shapes, such as a rectangular
slab or a cylinder. "The structure is very porous and very low in density,"
Judkins says. "It's 80% void space."
The secret to
adsorbing a specific gas is to create a structure with numerous pores
of the right sizewidth and volumeto trap the gas molecules,
which are naturally attracted to the carbon. Burchell, Judkins, and
their colleagues used chopped-up carbon fibers made from petroleum pitch,
bonded them together with phenolic resin, and then "activated" the structure
to create micropores as adsorption sites for gas molecules.
"We activate the
structure by flowing in steam, oxygen, or carbon dioxide at 850°C
to gasify its carbon and carry much of it off," Judkins says. "We control
this process to get a large enough surface area and pore volume and
width to optimize the capture of carbon dioxide."
The ORNL technology
has attracted the interest of many large industrial companies looking
for better ways to remove or recover carbon dioxide and other gases
from process streams. An international consortium of oil companies and
a fuel cell manufacturer want to use the technology to remove carbon
dioxide from natural gas. Another fuel cell manufacturer is interested
in removing sulfur compounds from natural gas to make it a better hydrogen
source. Sulfur compounds are added as odorants so people can smell leaking
natural gas and take precautions. The international consortium plans
to use the ORNL technology to remove carbon dioxide from the exhaust
stream of a gas turbine to mitigate its emissions.
"We are negotiating
with a major carbon company to license the CFCMS technology," Judkins
was used in Burchell, Judkins, and Kirk Wilson's development of a self-cleaning
carbon air filter that received an R&D 100 Award in 1999. When this
filter becomes dirty, it doesn't have to be replaced like filters containing
loose, granular, activated carbon. Instead, it uses an automatic reverse-air-cleaning
cycle in which an electric current is passed through the filter, releasing
the adsorbed contaminants into a purge air stream that exhausts harmful
Burchell (left), Kirk Wilson, and Rod Judkins developed a self-cleaning
carbon air filter that uses carbon fiber composite molecular sieves.
carbon air filter could be used to reduce cooking odors in kitchens,
filter formaldehyde and other airborne toxic gases from home and office
air, and preserve air quality aboard airplanes and submarines. The British
Ministry of Defense and the U.S. Army are testing the ORNL technology
for removing chemical agents from air. It is likely that ORNL's gas
capture technology will continue to capture industrial and military
Metals & Ceramics Division