Part II: The Price of Biofuels

Do we really have any alternative to biofuels?

According to a 2005 report by the DOE and the U.S. Department of Agriculture, the country has enough available forest and agricultural land to produce 1.3 billion tons of biomass that could go toward biofuels. Beyond providing a vast supply of cheap feedstock, cellulosic biomass could greatly increase the energy and environmental benefits of biofuels. It takes far less energy to grow cellulosic materials than to grow corn, and portions of the biomass can be used to help power the production process. (The sugarcane-based ethanol produced in Brazil also offers improvements over corn-based ethanol, thanks to the crop’s large yields and high sugar content.)

But despite years of research and recent investment in scaling up production processes, no commercial facility yet makes cellulosic ethanol.
The economic explanation is simple: it costs far too much to build such a facility. Cellulose, a long-chain polysaccharide that makes up much of the mass of woody plants and crop residues such as cornstalks, is difficult–and thus expensive–to break down.

Several technologies for producing cellulosic ethanol do exist. The cellulose can be heated at high pressure in the presence of oxygen to form synthesis gas, a mixture of carbon monoxide and hydrogen that is readily turned into ethanol and other fuels. Alternatively, industrial enzymes can break the cellulose down into sugars. The sugars then feed fermentation reactors in which micro-organisms produce ethanol. But all these processes are still far too expensive to use commercially.

Even advocates of cellulosic ethanol put the capital costs of constructing a manufacturing plant at more than twice those for a corn-based facility, and other estimates range from three times the cost to five. “You can make cellulosic ethanol today, but at a price that is far from perfect,” says Christopher Somerville, a plant biologist at the University of California, Berkeley, who studies how cellulose is formed and used in the cell walls of plants.

“Cellulose has physical and chemical properties that make it difficult to access and difficult to break down,” explains Caltech’s Arnold, who has worked on and off on the biological approach to producing cellulosic ethanol since the 1970s. For one thing, cellulose fibers are held together by a substance called lignin, which is “a bit like asphalt,” Arnold says. Once the lignin is removed, the cellulose can be broken down by enzymes, but they are expensive, and existing enzymes are not ideal for the task.

Technology Review