CO2 Removal Catalyst
There are several ways to remove CO2 from a stack gas. None have reached a commercial basis yet due to the expense of the processing. The current method of removing the greenhouse gas carbon dioxide (CO2) from the flues of coal-fired power plants uses so much energy that no one bothers to use it. So says Roger Aines, principal investigator for a team that has developed an entirely new catalyst for separating out and capturing CO2, one that mimics a naturally occurring catalyst operating in our lungs. With this success, the Laboratory has become a world leader in designing catalysts that mimic the behavior of natural enzymes.
The most commonly studied process for CO2 capture and removal are amines. In Norway for example, the CO2 Technology Center at Mongstad began construction in 2009, and was scheduled for completion early in 2012. It was to include two capture technology plants (one advanced amine and one chilled ammonia), both capturing flue gas from two sources. In addition, it would have included a gas-fired power plant and refinery cracker flue gas (similar to coal-fired power plant flue gas). Total capacity was to be 100,000 tons of CO2 per year. The project was delayed to 2014, 2018, and then indefinitely. At 80% completion, project cost rose to USD 985 million.
This small-molecule catalyst, dubbed "Cyclen," mimics carbonic anhydrase, which separates, captures, and transports CO2 out of our blood and other tissues as part of the normal respiration process. Carbonic anhydrase is the fastest operating natural enzyme known. For years, researchers have considered adapting it to capture carbon emitted in industrial operations. But carbonic anhydrase cannot take the heat in the intense conditions of industrial processes. Hot, high-pH flue gas quickly degrades it.
The Livermore team's best designer molecule behaves like carbonic anhydrase but has so far indicated that it is one tough cookie. "In fact," Aines said, "it has turned out to be thermodynamically stable. It is far more rugged than we had expected."
A team performing quantum molecular calculations led by computational biologist Felice Lightstone examined potential candidate molecules. They determined optimal designs to protect the essential zinc ion in the molecule that activates the catalyst. Synthetic chemist Carlos Valdez took the next step. Only about 2 percent of the computationally derived structures made it to the synthesis state. Newly synthesized molecules were tested by chemist Sarah Baker and her team to determine their kinetic behavior and stability. The team made nine catalysts in a year and a half. The name for the finalist comes from the chemical term for the ring around the zinc ion.
"Our tests effectively determined Cyclen's chemical kinetics," Aines said. "Pilot tests at the Babcock & Wilcox Power Generation Group in Ohio will push Cyclen to measure its industrial kinetics."
One challenge with Cyclen remains. The catalyst is designed to create a monolayer that clings to a gas-water interface much as mosquito larvae do. However, the Cyclen layer is too thin and some of the CO2 is able to pass through it without being captured.
For further information see Catalyst.
New Molecule image via Lawrence Livermore Lab.