Petroleum and other fossil fuels consist of organisms buried millions of years ago; burning these fuels releases their ancient carbon into the atmosphere as CO2. Biofuels are plant-based substitutes for fossil fuels. They have attracted attention recently, in part because they can help decrease reliance on foreign oil. The biofuel crops also carry out photosynthesis, temporarily removing CO2 from the atmosphere and helping to reduce global climate change.
Two types of biofuels are biodiesel and ethanol. Currently, most biodiesel comes from oil extracted from crushed soybeans or canola seeds. To avoid driving up the price of these food crops, researchers are looking for economical, nonfood sources of biodiesel. Examples include everything from green algae to the seeds of a plant called the jatropha tree. This long-lived tree uses little water and tolerates poor soil, so it does not compete with food plants for rich farmland.
Ethanol, the other main biofuel, is a gasoline substitute. Corn kernels are the main source of ethanol in the United States. Starch extracted from the corn kernel is enzymatically digested to sugar, which is fermented into ethanol. Sugarcane, the main source of ethanol in Brazil, is a more economical alternative in the tropics. Its tissues are high in sugar, not starch, so biofuel manufacturers can omit the costly enzymes from the ethanol production process.
Researchers are also searching for nonfood sources of sugar to use in ethanol production. The inedible stems of corn or of prairie grasses such as switchgrass would be ideal; bacterial and fungal enzymes easily break the cellulose in the plant cell walls into simple sugars. One problem, however, is that the stems also contain lignin, a complex molecule that interferes with cellulose extraction. So far, the heat and acid treatment needed to eliminate the lignin is too costly and inefficient to make cellulose-derived ethanol economical. Burning biodiesel or ethanol returns the CO2 absorbed in photosynthesis to the atmosphere, but it is important to realize that biofuels are not exactly “carbon-neutral.” Most biofuel crops require fertilizers and pesticides, both of which come from fossil fuels—and cause additional environmental problems of their own.