Any fuel made to drive a diesel engine is called diesel fuel. Most people are familiar with petrodiesel, and don't even bother to add the prefix. But advances in physical and chemical biomass conversion and processing have made the term biodiesel a term that may not yet be commonplace but has probably been heard by most adults in developed nations. The usual sources for biodiesel are oils and fats, which are mixed with a solution of methanol that contains sodium hydroxide (lye, an extremely caustic substance). Amazingly, the eponymous Rudolf Diesel demonstrated biodiesel at the 1900 Paris World Exposition using an engine that ran on peanut oil. Gasoline engines rely on a spark to fire, and can be quite finicky about fuel, but diesel engines depend on high cylinder compression to heat and ignite the air/fuel mix, so many modern diesel engines can run on 100 percent biodiesel and others can run on petro-bio mixes. That's good news for the air: according to the Department of Energy, pure biodiesel emits 75 percent less CO2 than petrodiesel, and mixes by anywhere between 75 and 15 percent.

There are many potential biomass sources for making biodiesel. For example, the Industrial Agricultural Products Center, which is part of the University of Nebraska at Lincoln, recognized that its home state leads the nation in commercial cattle slaughter. That process yields not only the steaks and burgers in your grocery store but also 1 billion pounds a year of tallow. Accordingly, the IAPC has developed a biodiesel that makes use of this largely unused material.

Food industry giant Perdue Incorporated (the chicken people) actually formed a BioEnergy group dedicated to biofuels. Oddly enough, Perdue is the twelfth-largest grain company in the United States and has three soybean crushing plants and a deepwater port, so the company works with biodiesel and ethanol producers to make feedstock (any raw material fed into an industrial process in this case, for generating power).

Another food industry heavyweight, Tyson Foods, produces more leftover animal fat (from chickens, cattle, and hogs) than any other company in the U.S. The company recently announced a renewable energy division of its own to put to use the 2.3 billions pounds of chicken fat they create each year. That could make around 300 million gallons of pure biodiesel, or go into the most popular petro-bio mix, a B20 fuel 80 percent petrodiesel, and 20 percent biodiesel. Americans use almost 40 billion gallons of diesel a year.

Biodiesel currently has a good news/bad news story. The good news is that it exists, it works, and it's getting easier: in 2000 there were 88 plants in the U.S. producing 250 million gallons of biodiesel. The bad news is that most of the biodiesel (and other biofuels) comes not from industry leftover but from energy crops such as soybeans, which require significant farm acreage that could otherwise be used to produce vegetables and grains for human consumption.

The Defense Energy Support Center, which handles securing fuel for the Depart of Defense, is the single-largest consumer in the U.S. of biodiesel (5.2 million gallons in 2003-2004; more recent figures are unavailable). The U.S. began using B20 in its non-tactical vehicles in 2003. The military consumes between 120 and 145 million barrels of oil in a single year; according to the Department of Defense, every $10 increase in the price per barrel of oil means another $1.3 billion the military needs to keep its fleets operational.