The hybrid system makes it practical to use an alternative to the conventional distillation step used in ethanol production; the Coskata version uses only half as much energy. In this alternative process, called vapor permeation, water and ethanol vapor pass through a tubelike membrane. By the end, almost all the water has been removed, leaving behind ethanol that’s 99.7 percent pure. Ordinary fermentation processes produce a broth of water and ethanol full of processed biomass that would clog up such a membrane.
At least one other company has tried a hybrid approach to making ethanol: the biofuels company BRI Energy found similar bacteria that can process syngas. But Andy Aden, a senior researcher investigating cellulosic ethanol at the National Renewable Energy Laboratory in Golden, CO, says one problem with such approaches is that it’s been difficult to make the syngas accessible to the bacteria, since syngas doesn’t dissolve easily in water. Coskata has tackled this problem with a new bioreactor design in which bacteria grow in dense biofilms on the outside of hollow fibers. Syngas is pumped through the inside of these fibers and diffuses through them directly to the biofilm. Aden says the biofilm approach sounds promising, although he cautions that such systems have been difficult to scale up to the commercial scale.
While Coskata says its process can work with a very wide range of feedstocks, in practice it might be best suited for specific materials. “I think that it will work very well for woody materials and maybe almost uniquely well for municipal solid waste and some of these other high-carbon wastes, like tires,” says Bruce Dale, a professor of chemical engineering and materials science at Michigan State University. But he says biological approaches could work better with feedstocks such as switchgrass.
So far the company makes ethanol only a few drips at a time. The economics of the process at the commercial scale will depend on a number of factors, including how much the feedstock costs and whether the system works well in larger bioreactors.