Wednesday, July 29, 2009

Toward Better CO2 Capture

I have posted on the problems related to CO2 separation from flue gas in the past while to be fair the literature has been silent. My interest was initiated by the discovery that one can produce acid rain in a pipe and thus separate out everything except the CO2. That struck me as the most likely practical method available to us.

This discovery or advance allows CO2 capture by potentially a wide range of ionic solvents, should that prove to be a good thing. More likely it will lead to dozens of research projects that sort of do the job but continue to have the difficulties of the conventional system.

During the Acid rain panic, over 160 research projects attempted to eliminate SOx and NOx and all were inherently flawed by the slow process velocity of the targeted reagents.

An acquaintance had the insight that the obvious reagent was chlorine gas injected with moisture directly into the hot flue gas stream and then into a water quench to absorb remaining chlorine and heat. We discovered in bench tests that the Sox and NOx were preferentially reduced and the resultant acids attacked available heavy metals in the quench. The CO2 was partially reduced to carbonic acid but most escaped.

The bottom line is that present flue gas is still a toxic brew escaping into the atmosphere and I have every reason to think that the above method solves the problem.

Livermore CA (SPX) Jul 28, 2009

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.

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