The most widely used biofuel today is ethanol, a two-carbon alcohol whose carbon-oxygen bonds yield only about 70-percent of the energy per volume of gasoline’s hydrocarbon bonds. Advanced biofuels are oil-like hydrocarbons, such as the four-carbon alcohol butanol, that yield almost the same amount of energy per volume as gasoline. Unlike ethanol, advanced biofuels can replace gasoline, diesel and jet fuels on a gallon-for-gallon basis in today’s combustion engines with no loss of performance. Also unlike ethanol, which absorbs water that corrodes pipelines, advanced biofuels can be used in today’s supply and delivery infrastructures.
The idea behind advanced biofuels is to develop “fuel crops” that can thrive on lands not suitable for food crops with little fertilization or irrigation, extract sugars from the biomass of these fuel crops, and then ferment the sugars into liquid transportation fuels through a process similar to that used to make beer or wine.
There is an ample amount of land available that could be used to grow fuel crops with no impact on the food crops for humans or animals. In the United States alone, these lands total about 300 million acres, an area nearly three times the size of California. Given that transportation fuels are the largest end-use of energy by sector in the United States, meeting the bulk of our nation’s transportation energy needs with clean, green and renewable advanced biofuels makes good economic, environmental and national security sense.
Roadblocks to Advanced Biofuels
The cellulose in plant cell walls is not easily extracted. Cellulose itself consists of strong cable-like microfibrils harboring thousands of molecules of glucose, a monosaccharide, and hemicellulose, a polysaccharide made from a variety of five- and six-carbon sugars. As further protection, cellulose is wrapped inside lignin, a rigid non-carbohydrate polymer that forms a protective coating around cellulose and hemicellulose, shielding these sugars from attack by enzymes and microbes. Lignin is also a source of chemical by-products that can inhibit the conversion of sugars into biofuels. Commercialization of advanced biofuels requires cost-effective means of extracting cellulose from plant biomass.
Commercialization of advanced biofuels also requires cost-effective and energy-effcient means of fermenting fuels from complex sugars. While glucose can be readily fermented into biofuels, hemicellulose poses major difficulties. The common yeast, which is used to ferment simple starch-based sugars into ethanol, cannot digest the polysaccharide sugars in cellulosic biomass. New microbes will have to be either found or engineered with enzymes that do the job.
Finally, plants that will serve as the best feedstocks for the production of biofuels need to be identified and wherever possible improved upon to obtain the highest possible yield of biofuel per unit of land per year. For example, perennial grasses such as Miscanthus and switchgrass feature a special type of photosynthesis that makes them highly efficient in their use of water. Yields in excess of 25 tons of useable biomass per acre annually without irrigation have been reported. Other plants that have high potential as future fuel crops include trees such as poplar, hybrid poplar and willow, alfalfa, sorghum and algae.