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Made from renewable feedstocks, biodiesel promises to supplement the limited supply of traditional petroleum-based diesel fuel. However, it is not meant to be a direct replacement. "Biodiesel is not intended to replace diesel fuel completely, and will not be the solution on its own," says Amber Thurlo Pearson, National Biodiesel Board.

One reason is capacity constraints. "Production capacity is increasing to the tune of just under 900 million gal. of pure biodiesel," says Pearson. "Our goal is currently '5 x 2015', or 5% of on-road diesel fuel needs being met by biodiesel by 2015. That would equate to about 2 billion gal. Our current production/consumption for 2006 was at 225 million gal."

Although still far from its long-term goal, biodiesel capacity is expanding at a rapid pace, tripling in both of the last two years. According to Pearson, there were 25 million gal. produced in 2004 and 75 million gal. produced in 2005.

Technical hurdles to biodiesel's adoption stem from differences in chemistry compared to petroleum-based diesel. "If we had started with biodiesel 70 years ago and we were transferring to petroleum diesel, we would be having some of the same conversations we are having today about biodiesel," says Don Borgman, director of industry relations - North America, Deere & Co.

These differences can be mitigated by using lower blend ratios of the fuels. The most common blend is B5, a mix of 5% biodiesel and 95% petroleum-based diesel. Likewise, B20 equals 20% biodiesel content, B30 equals 30% biodiesel and so on. B100 represents pure biodiesel.