Research Highlights

Scrubbing out acid rain — Unique liquid can remove and store harmful gases from power plant emissions

Results:

Researchers at Pacific Northwest National Laboratory (PNNL) have developed a unique reusable organic liquid that can absorb harmful combustion gases such as sulfur dioxide (SO₂), a primary cause of acid rain. This new process could allow power plants to capture harmful gases using less energy than current treatment methods. The team has successfully demonstrated reversible capture of all four acid gases found in industrial gas streams. "Our system is completely reversible with almost every component of combustion," said David Heldebrant, who leads the research team.

Why it matters:

According to our research, acid-gas-binding organic liquids promise a more effective and less energy-intensive method to capture gases such as carbon dioxide (CO₂) and SO₂. Current methods used to capture and release harmful emissions such as SO₂ from power plants use a lot of energy because they pump and heat water during the process. These power plants also use large amounts of caustic waste to capture the SO₂, which is then buried in the ground. The SO₂ is not stripped or reused and is extremely toxic and corrosive. Heldebrant explained that with this new organic liquid, the corrosion problem is avoided because they are not using a water-based system. The SO₂ that is separated can be recycled and reused in other industries that use SO₂ commercially, such as the cement and wine industries.

The liquids have shown promise with CO₂ as well. They can capture twice as much CO₂ by weight as the monothanolamine (MEA) system now widely used for CO₂ removal, yet have half the specific heat, explains Heldebrant. "We are hoping the same energy efficiency and lack of corrosion are going to be impacted by the different gases as well."

Methods:

Each acid-gas-binding liquid consists of a mixture of an alcohol and a base. The liquid binds to the gas to form a colored ionic liquid. The system is selective because it chemically binds CO₂ as liquid alkylcarbonate salts, and the other gases as liquid alkyl-sulfite, alkylthio-carbonate and alkyldithiocarbonate salts. Heating the liquid to a temperature between ambient and its boiling point (about 125ºC) releases the gas and returns the liquid to its initial state.

Researchers have run the liquids for CO₂ and SO₂ removal through five regeneration cycles. "There's been minimal evaporative loss and no absorption performance decline," said Heldebrant.

What's next?

The next step for the researchers is to run gas-specific trials in a wetted-wall column. PNNL engineers are now designing a unit, which probably will have a 100-ml to 1-L capacity, notes Heldebrant. Tests should begin early 2010 and should yield process information and a full economic analysis by the end of the year. Optimistically, this will lead to pilot-scale deployments in 2011. The key challenges are to cut the cost of the liquids. Researchers plan to look at new ways to make them more cost effectively and how to deal with any water that gets into the system.

Researchers also are working on second-generation systems that use a single compound instead of a mixture. This won't pose any tradeoffs in tailoring, will make modeling easier, and may offer higher absorption capacity, explains Heldebrant. Such a liquid for CO2 capture already has been developed and tested.

Acknowledgements:

Researchers at Pacific Northwest National Laboratory are working in collaboration with Dr. Philip Jessop and his research group at Queen's University in Ontario, Canada.

The research is funded through a variety of programs including PNNL internal investments made through its Laboratory-Directed Research and Development program and Energy Conversion Initiative.

Research team: David Heldebrant, Phillip Koech, and Clement Yonker of Pacific Northwest National Laboratory.

PNNL-SA-70620