In Amyris’s process, sugarcane stalks are crushed and the juices are then placed in 5,000-liter fermenters with the company’s engineered yeast, which makes a diesel-precursor molecule. (The company has tested the process in 60,000-liter fermenters, but the demonstration plant is not yet operating at this scale.) As the oily molecules are produced, they separate and float to the top of the solution, aided by a centrifuge. The low energy required for this separation, says Renninger, is one of the cost advantages of making hydrocarbons rather than ethanol. Centrifugation requires just one-ten-thousandth of the energy content of the diesel fuel; water-soluble ethanol, by contrast, must be distilled from fermentation solution, a process that takes one-third of its energy content. The hydrocarbons are then hydrogenated at low temperature and low pressure to make diesel or other compounds.
Sugarcane also comes out ahead of corn on environmental measures. Compared with petroleum fuels, the use of corn ethanol leads to a net 10 percent decrease in greenhouse-gas emissions. Burning sugarcane ethanol instead of petroleum leads to a 60 to 80 percent decrease in greenhouse gases, says Renninger. Relative to sugarcane ethanol, the company says, the Amyris fuels made from sugarcane release another 10 percent less.
However, many experts say that it will be far more environmentally beneficial to use biomass other than crops to make biofuels. Techniques for converting fast-growing, fibrous crops like poplar and switchgrass into fermentable sugars are still in development and are currently too expensive. “Cellulosic conversion has to come down in price,” says Helena Chum, a research fellow at the National Renewable Energy Laboratory. Amyris plans to expand into the United States once technologies for doing this conversion economically are further developed. The company’s business strategy–to start producing in Brazil, where costs are low, and then expand when cellulosic technologies are ready–is “very smart,” says Chum.