Cellulosic ethanol: Fuel of the future?
In his Jan. 23 State of the Union address, President George Bush outlined his plan to reduce the nation's dependency on foreign oil by requiring the production of 35 billion gallons a year of renewable and alternative fuels by 2017, roughly five times the current target set by Congress of 7.5 billion gallons by 2012.
Among the most promising alternatives are fuels derived from biological material. Currently, the main biofuel used in the United States is ethanol distilled from kernels of corn. There are about 140 corn ethanol refineries nationwide, which produce more than 5 billion gallons a year. But critics say that corn ethanol alone won't meet the president's goal of 35 billion gallons of alternative fuels in 10 years, because cultivating corn to use only its grain would take up too much land. According to the National Environmental Trust, producing 35 billion gallons of ethanol annually would require putting an additional 129,000 square miles of farmland—an area roughly the combined size of Kansas and Iowa—into corn production.
One way to reach the president's objective is offered by Chris Somerville, professor of biological sciences at Stanford and director of the Carnegie Institution's Department of Plant Biology. Somerville advocates increasing the production of cellulosic ethanol, which is distilled from the fermentation of sugars from the entire plant, not just the grains.
"To expand beyond 12 billion gallons, we need to use the body of the plants rather than use the seeds," Somerville said. He discussed his research at a Feb. 17 symposium titled "Domestic Bioenergy: Weaning Ourselves from Foreign Oil Addiction" during the annual meeting of the American Association for the Advancement of Science in San Francisco.
The ideal plant: A perennial grass
The body of a plant is composed of polysaccharides, such as cellulose, which can be converted to ethanol by fermentation. Using the entire plant body as a starting raw material will result in a higher yield of fermentable sugar per unit of land, Somerville said.
The ideal plant for producing cellulosic ethanol, he added, is Miscanthus, a perennial grass native to subtropical and tropical regions of Africa and southern Asia, which is used as an ornamental plant in the United States.
What makes Miscanthus so special?
"It uses less water per gram of biomass produced than other plants," he said. "For example, to make a pound of alfalfa or spinach requires about 600 pounds of water, while to grow a pound of Miscanthus requires only about 200 pounds of water."
According to Somerville, Miscanthus produces about twice as much biomass per acre without irrigation than other grasses, and reaching the president's target of 35 billion gallons of biofuels annually would require putting about 17.5 million acres of fields into Miscanthus production. "[This] is less than what we pay farmers in the United States not to farm," Somerville said. "Currently we pay the farmers not to farm 40 million acres."
The main reason behind the call for increased biofuel production is to reduce emissions of greenhouse gases, not because we are running out of fossil fuels, he added. "There are reserves of coal for 200 years at least, and coal can be liquefied into fuel, but it produces an awful lot of CO2," he explained.
Biofuels, on the other hand, are carbon-neutral sources of energy, Somerville said, noting that plants absorb atmospheric carbon dioxide during photosynthesis, which compensates for the CO2 that is released when biofuels burn.
Some environmentalists criticize the use of biofuels by arguing that planting large quantities of corn or grass to produce ethanol will require widespread deforestation, which threatens biodiversity. "It depends on what acres of land one uses [to plant Miscanthus]," said Somerville, who advocates growing biofuel crops on land currently used for food production.
"There's a lot of deforestation certainly going to take place in tropical regions, because those countries are going to develop biofuel businesses," he said. "Already in Malaysia, Indonesia, the Philippines, the acreage of palm oil is extending very rapidly because palm oil can be converted to biodiesel with a quite high efficiency and very low capital investment. But is it worse for the environment than climate change? That's the question."
According to Somerville, "Climate change threatens biodiversity more than anything that I know. For example, in British Columbia they are losing each year forests the size of Rhode Island because of beetle infestation, because it is not cold enough in the winter to kill the beetles, and they are killing the forest."
The president's target of 35 billion gallons of alternative fuels by 2017 "is very substantive, but Bush did not provide any insights into what he is going to do to make that happen," Somerville said, adding that it will take seven to 10 years to produce cellulosic ethanol at competitive prices.
"It is certainly possible to achieve Bush's goals technically," he said. "The question in my mind is whether investors are ready to put up the money required to make it happen."
Maria José Viñas is a science-writing intern with the Stanford News Service.