Adding carbon-capture technology to a conventional coal plant can nearly double the price of the electricity it produces. This fact represents one of the big obstacles to passing legislation to regulate carbon-dioxide emissions. Now researchers at Codexis, based in Redwood City, CA, are using genetically engineered enzymes to make carbon-dioxide capture less expensive--their method could increase electricity costs by less than a third.
Gene machine: A Codexis researcher operates a high-volume liquid-handling system used to make gene variants--part of a process for engineering new enzymes.
The new enzymes increase the efficiency, by a factor of 100, of a solvent used to capture carbon dioxide. This promises to decrease the energy needed to capture and store the greenhouse gas. The researchers developed new ways to engineer enzymes that can operate at the high temperatures inside a coal plant's smokestack.
The standard way to capture CO2 is to use a solvent called monoethanolamine (MEA). Carbon dioxide is absorbed by the solvent, which separates it from the other flue gases. To store the carbon dioxide it has to be freed by applying heat--this produces a pure stream of carbon dioxide that can be compressed and permanently sequestered. The energy required to do this decreases the power output of a coal plant by about 30 percent. Combined with the extra equipment and materials needed to capture the CO2, this increases the cost of the electricity produced by roughly 80 percent. Codexis's approach could limit this cost increase to 35 percent or less, says James Lalonde, the company's vice president of biochemistry and engineering R&D.
Researchers at Codexis genetically modified an enzyme, called carbonic anhydrase, involved with respiration in many organisms, including humans. Carbonic anhydrase helps a solvent called methyl diethanolamine (MDEA) bind with carbon dioxide. The most challenging problem was altering the enzymes so they could survive at the high temperatures found in smokestacks. The enzymes can survive at temperatures around 25 °C, but quickly stop working at temperatures higher than 55 °C to 65 °C.
Codexis's early results show that its modified enzymes can survive at temperatures above 85 °C for half an hour. This is high enough for the enzyme to survive in smokestacks, but not at the temperatures needed for freeing the carbon dioxide for storage (130 °C). Lalonde says the company has seen large improvements since these initial results were disclosed, but the company hasn't released the new figures yet.
The company has successfully engineered enzymes for drug development in the past. It has won two "green technology" awards from the U.S. Environmental Protection Agency for developing enzymes for making two drugs--atorvastatin, the active ingredient in the cholesterol-lowering drug Lipitor, and sitagliptin, the active ingredient in the diabetes drug Januvia. The enzymes simplified drug synthesis and reduced waste.
Codexis uses a proprietary version of directed evolution. In its simplest form, directed evolution involves making random changes to existing genes. These mutations alter one amino acid in the enzyme at a time. The genes that work best are then selected and changed to further increase performance. Codexis's researchers have developed a faster version of the process that involves swapping relatively large segments of the gene sequence--making multiple changes to amino acids each time. They've also developed computational techniques that allow them to determine what parts of the gene are most likely to lead to improvements in performance if they are modified. The changes make the process more efficient, and lead to big changes in performance in a relatively short amount of time.
"Codexis's technology has certainly proven itself quite powerful," says Stefan Lutz, associate professor of biomolecular chemistry at Emory University. He cautions that it may be more difficult to work with carbon dioxide than with pharmaceuticals. "If they do succeed, it would be a huge deal," he says.
By Kevin Bullis
From Technology Review