Separating carbon dioxide from its polluting source, such as the
flue gas from a coal-fired power plant, may soon become cleaner and
more efficient.
A Lawrence Livermore National Laboratory researcher has developed
a screening method that would use ionic liquids — a special type of
molten salt that becomes liquid under the boiling point of water
(100 degrees Celsius) — to separate carbon dioxide from its source,
making it a cleaner, more viable and stable method than what is
currently available.
There are major efforts to reduce CO2 emissions from
burning fossil fuel, but before it can be sequestered, it must first
be separated from its source, a step known as "capture." This new
technique could significantly enhance the efficiency of the CO2
capture process.
Currently, the few coal plants with commercial CO2
capture capability all use processes based on chemical absorption
with monoethanolamine (MEA), a general-purpose solvent developed by
chemists some 75 years ago. Unfortunately, it is non-selective,
corrosive, requires the use of large equipment, and effective only
under low to moderate partial pressures of CO2.
But the new system overcomes many of these shortcomings. Chemists
recently became interested in ionic liquids because they are
solvents with almost no vapor pressure, and do not evaporate, even
under high temperature conditions.
Using ionic liquids as a separation solvent has unique advantages
over traditional solvents, according to LLNL scientist Amitesh
Maiti, whose research appears as the cover article in a recent issue
of ChemSusChem, a new journal focused on chemistry and
sustainability.
Advantages include high chemical stability; low corrosion; almost
zero vapor pressure; supportable on membranes; and a huge library of
ion choices, which can be potentially optimized for CO2
solubility.
Maiti's work involved devising a computational strategy that can
reliably screen any solvent, including an ionic liquid, for high CO2
capture efficiency.
"It's a great advantage to have a method that can quickly and
accurately compute CO2 solubility in any solvent,
especially under the range of pressures and temperatures as would be
found in a coal-fired power plant," Maiti said. "With ionic liquids
serving as the solvent, the process could be a lot cleaner and more
accessible than what is used today."
Over the last few years several ionic liquids have been
experimentally tested to be efficient solvents for CO2,
providing data that could be useful in optimizing the choice of
ionic liquids for CO2 capture.
"But each new experiment costs time and money and is often
hindered because a specific ionic liquid may not be readily
available," Maiti said. "By creating a computational tool that can
decipher ahead of time which ionic liquids work best to separate CO2,
it can be a much more efficient process when field tests are
conducted."
Maiti developed a quantum-chemistry-based thermodynamic approach
to compute the chemical potential of a solute (CO2 in
this case) in any solvent at an arbitrary dilution. He found that
this result coupled with an experimentally fitted equation-of-state
data for CO2 can yield accurate solubility values in a
large number of solvents, including ionic liquids. He confirmed this
by directly comparing the computed solubility with experimental
values that have been gradually accumulating over the last few
years.
Next, he used this method to predict new solvent classes that
would possess CO2 solubility nearly two times as high as
the most efficient solvents experimentally demonstrated.
"With the vast choices of ions, we have barely scratched the
surface of possibilities," Maiti added.
His hope is that the accuracy of the computational method will
allow scientists to see useful trends, which could potentially lead
to the discovery of practical solvents with significantly higher CO2
capture efficiency.
About DOE/Lawrence Livermore National Laboratory
Founded in 1952, Lawrence Livermore National Laboratory is a
national security laboratory, with a mission to ensure national
security and apply science and technology to the important issues of
our time. Lawrence Livermore National Laboratory is managed by
Lawrence Livermore National Security, LLC for the U.S. Department of
Energy's National Nuclear Security Administration.
SOURCE: DOE/Lawrence Livermore National Laboratory