The need for biofuels
— By Daniel Gorelick, Science Planet, 22 April 2009
Chaitan Khosla and Harmit Vora
Fossil fuels account for 95 percent of world energy usage. Consumption of coal, petroleum, and natural gas has increased significantly over the last several decades, as have carbon dioxide emissions, the primary reason for global climate change.
The implications of climate change have stimulated significant efforts to discover and commercialize renewable sources of energy that have zero or reduced net carbon dioxide emissions. Finding replacements for gasoline has received significant attention in the United States, where the transportation sector consumes the most energy. Biofuels, liquid fuels derived from renewable plants, have been viewed as prime candidates to replace gasoline.
The two predominant biofuels on the U.S market today are corn ethanol and soybean biodiesel. Corn ethanol has drawbacks that might hurt its long-term chances in the biofuels market. It is not as energy-rich as gasoline – a gallon of ethanol contains less energy than a gallon of gasoline. Ethanol can’t be distributed using existing infrastructure because it has different chemical properties than gasoline. Unless significant modifications are made to current automobiles, ethanol can only be used in low percentage blends with gasoline.
The other major biofuel, biodiesel, is derived from lipids (fat) in plant seeds. Biodiesel’s biggest barrier to widespread use is the availability of raw material. A recent study showed that if all the plant (and even animal lipids) in the United States were dedicated to produce biofuels, the amount of biofuel produced would be less than five percent of the total volume of liquid fuels consumed each year.
The raw material for both corn ethanol and soybean biodiesel is food crops, so increasing production could create challenging impacts on global food markets.
Advanced Biofuels: cellulosic ethanol and algal biodiesel
Cellulosic ethanol is a well-publicized new biofuel that can be produced from non-food crops (cellulose is a carbohydrate found in all plants). Cellulosic ethanol produces 300 percent more energy than is used in its production, a significantly better energy yield than corn ethanol or soybean biodiesel, but it shares the inherent energy and distribution disadvantages of corn ethanol.
One of the most immediate challenges with commercialization of cellulosic ethanol is that cellulose and a related carbohydrate, hemicellulose, are difficult, and hence expensive, to break down into the simple sugars required for ethanol production. Thus, improving the efficiency of the initial cellulose processing steps is key to making this and other biofuels economically feasible.
There is interest in using microscopic algae to produce biodiesel. While providing the benefits in energy density and engine compatibility of biodiesel, it may not suffer from the same supply issues because simple sugars (and potentially cellulose) can be used as a starting material. Algae are also better stores of oils than plant seeds.
There have been increasing efforts to genetically engineer well-known organisms, such as the bacteria E. coli, to produce novel biofuels efficiently. Researchers have hijacked E. coli’s biosynthetic pathway for the amino acid valine to produce isobutanol, a more energy dense, less volatile alcohol than ethanol. Our own research has focused on theproduction of energy-dense fuels using the fatty acid biosynthetic pathway in E. coli.