Low-cost reusable material could capture carbon dioxide
from power plants
Researchers have developed a new, low-cost material
for capturing carbon dioxide (CO2) from the smokestacks of coal-fired
power plants and other generators of the greenhouse gas. Produced with
a simple, one-step chemical process, the new material has a high capacity
for absorbing carbon dioxide — and can be reused many times. Combined
with improved heat management techniques, the new material could provide
a cost-effective way to capture large quantities of carbon dioxide.
Existing CO2 capture techniques involve the use of solid materials that
lack sufficient stability for repeated use — or liquid adsorbents that
are expensive and require significant amounts of energy.
“This is something that you could imagine scaling up for commercial use,”
said Christopher Jones, a professor in the School of Chemical and Biomolecular
Engineering at the Georgia Institute of Technology. “Our material has
the combination of high capacity, easy synthesis, low cost and a robust
ability to be recycled. All the key criteria for an adsorbent that would
be used on an industrial scale.”
Details of the new material, known as hyperbranched aluminosilica (HAS),
appeared in the March 19th issue of the Journal of the American Chemical
Society. The research was supported by the U.S. Department of Energy’s
National Energy Technology Laboratory.
Growing concern over increased levels of atmospheric carbon dioxide has
prompted new interest in techniques for removing the gas from the smokestacks
of such large-scale sources as coal-fired electric power plants. But
to minimize their economic impact, the cost of adding such controls must
be minimized so they don’t raise the price of electricity significantly.
Once removed from the stack gases, the CO2 might be sequestered in the
deep ocean, in mined-out coal seams or in depleted petroleum reservoirs.
If the CO2 capture and sequestration process can be made practical, America’s
large resources of coal could be used with less impact on global climate
Working with Department of Energy scientists Daniel Fauth and McMahan
Gray, Jones and graduate students Jason Hicks and Jeffrey Drese developed
a way to add CO2-adsorbing amine polymer groups to a solid silica substrate
using covalent bonding. The strong chemical bonds make the material robust
enough to be reused many times.
“Given the volumes involved, you must be able to recycle the adsorbent
material to be cost-effective,” said Jones. “Otherwise, you would be
creating expensive waste streams of adsorbent.”
Production of the HAS material is relatively simple, and requires only
the mixing of the silica substrate with a precursor of the amine polymer
in solution. The amine polymer is initiated on the silica surface, producing
a solid material that can be filtered out and dried.
To test the effectiveness of their new material, the Georgia Tech researchers
passed simulated flue gases through tubes containing a mixture of sand
and HAS. The CO2 was adsorbed at temperatures ranging from 50 to 75 degrees
Celsius. Then the HAS was heated to between 100 and 120 degrees Celsius
to drive off the gas so the adsorbent could be used again.
The researchers tested the material across 12 cycles of adsorption and
desorption, and did not measure a significant loss of capacity. The HAS
material can adsorb up to 5 times as much carbon dioxide as some of the
best existing reusable materials. The HAS material works in the presence
of moisture, an unavoidable by-product of the combustion process.
Adsorption of the CO2 generates considerable amounts of heat, which must
be managed and thermally recycled. Removal of the CO2 requires heating
“How to manage heat is one of the critical issues controlling the economics
of a potential large-scale process,” Jones added. “You must control the
production of heat by the adsorption step, and you don’t want to put
any more energy into the desorption process than necessary.”
Because of their chemical structure, the amine groups provide three different
classes of binding sites for carbon dioxide, each with a different binding
energy. Optimizing the production of binding sites is a goal for future
research, Jones said.
Beyond the material, other components of the separation and sequestration
process must also be improved and optimized before it can become a practical
technique for removing CO2 from flue gases. The best way to expose the
gases to the adsorbent material is also key issue.
“There are many pieces that must fit together to make the overall economics
of carbon dioxide capture and sequestration work,” Jones added. “The
biggest challenge for this research is to do this as inexpensively as
possible. We think that our class of materials, a hyperbranched amine
polymer bound to a solid support, is potentially ideal because it is
simple to make, reusable and has a high capacity.”