Biocombustibles Chris Somerville Science

In 1895, Swedish chemist Svante Arrhenius presented a paper to the stockholm Physical Society titled On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground, in which he argued that the combustion of fossil fuel would lead to global warming. He was right, so we must deal with the consequences of global climate change and somehow meet our expanding energy needs while limiting greenhouse gas emissions. Earth receives approximately 4000 times more energy from the Sun each year than humans are projected to use in 2050. Some of that energy can be captured through a variety of "renewable" sources, but the only form of solar energy harvesting that can contribute substantially to transportation fuel needs at costs competitive with fossil fuel is that captured by photosynthesis and stored in biomass.

Brazil now obtains a quarter of its ground transportation fuel from ethanol produced by the fermentation of sugarcane sugar, and in the United States, approximately 90 corn grain-to-ethanol refineries produce about 4.5 billion gallons of ethanol annually. The U.S. Energy Policy Act of 2005 would increase that production to 7.5 billion gallons by 2012, but the United States currently uses about 140 billion gallons of ground transportation fuel per year. To replace 30% of that amount with ethanol of equivalent energy content, as proposed recently by the Secretary of Energy, will require about 60 billion gallons of ethanol. A recent analysis concluded that the United States could produce about 1.3 billion dry tons of biomass each year in addition to present agricultural and forestry production. Because it is theoretically possible to obtain about 100 gallons of ethanol from a ton of cellulosic biomass (such as corn stover, the stalks remaining after corn has been harvested), the United States could sustainably produce about 130 billion gallons of fuel ethanol from biomass. In addition to a positive effect on the release of greenhouse gases, a biofuels program on this scale would have substantial economic and strategic advantages.

The creation of a new industry on that scale will require much basic and applied work on methods for converting plant lignocellulose to fuels, because several significant problems must be overcome to make the process ready for large-scale use. For example, cellulose is a recalcitrant substrate for bioconversion, and unacceptably large amounts of enzymes are required to produce sugar. Lignin occludes polysaccharides and inhibits enzymatic hydrolysis of these carbohydrates; energetically expensive and corrosive chemical pretreatments are required for its removal. The yeast currently used in large-scale ethanol production cannot efficiently ferment sugars other than glucose. And relatively low concentrations of ethanol kill microorganisms, requiring an expensive separation of the product from large volumes of yeast growth medium.