Cane bagasse largely consists of bundles of cellulose that are surrounded by hemicellulose. Cellulose is made of long chains of glucose, a six-carbon sugar of the type usually fermented to make ethanol from sources such as corn. Hemicellulose, however, is made of five-carbon sugars, which typically can’t be fermented using the same organisms as glucose. One of the things that makes Verenium’s process novel, says John Malloy, the company’s executive vice president, is its ability to ferment sugars from both cellulose and hemicellulose.
The process begins when the cane is ground up and cooked under high pressure with a mild acid to hydrolyze the hemicellulose and separate it from the cellulose. The five-carbon sugars in hemicellulose are then fermented using genetically modified E. coli. The cellulose is broken down with enzymes and fermented with another type of bacteria called Klebsiella oxytoca. This bacteria does double duty, since it also produces enzymes that break down cellulose, reducing the amount of enzymes from outside sources by 50 percent. The dilute ethanol produced from fermentation of both types of sugar is then distilled to make fuel.
In addition to opening the demonstration plant, Verenium is also starting to grow energy cane and to work with local farmers to ensure a steady stream of material for its planned commercial plants. Short term, the company says that it can rely on leftover bagasse from sugar production, but eventually it will draw on energy cane grown specifically to make ethanol. Provisions in the Farm Bill, which was recently passed by the United States Congress, will help by providing farmers with incentives to plant energy crops, says Carlos Riva, Verenium’s CEO. The incentives are important because it takes two to three years for energy cane, a perennial plant, to become established and reach ideal production levels. As a result, farmers will need to start planting the crops next year, before commercial plants are built and there is a market for these crops.
The opening of the demonstration plant, and the current construction of a number of other demonstration- and commercial-scale cellulosic ethanol plants, marks a turning point for the industry, Riva says. The development of improved enzymes and fermentation organisms means that no further scientific breakthroughs are needed to make cellulosic ethanol commercially successful, he says. “There’s been a tremendous amount of background work in science and technology development,” he says. “We’ve learned so much about the process that the really important thing now is to start to deploy the technology at a commercial scale.”