Biodiesel: cultivating alternative fuels

Environmental Health Perspectives, Feb, 2007 by Charles W. Schmidt

Back in the early 1990s, U.S. farmers took note of the first Gulf War, rising energy prices, and a huge glut of excess soybean oil sitting in tanks around the country, and they saw an opportunity. Soybean oil, they reasoned, could be refined to make biodiesel, an alternative fuel source. In Europe--where diesel fuel powers up to half the entire vehicle fleet--biodiesel was being produced in industrial quantities using rapeseed oil. Why not do the same with soybean oil, the farmers asked, and turn existing surpluses into an energy commodity?

The idea caught on; in 1992, the National SoyDiesel Development Board was formed to study biodiesel production based on the European model. In 1994, when the organization's name was changed to the National Biodiesel Board (NBB), fuels produced from soybean oil amounted to barely a few thousand gallons a year. But ten years later, that volume had grown to 25 million gallons, mainly due to the efforts of the NBB. The addition of a tax subsidy worth up to $1 per gallon, which took effect in January 2005, sent demand for the fuel soaring. Joe Jobe, the NBB's chief executive officer, says at least 200 million gallons were sold in 2006. Assuming existing and emerging facilities operate at full capacity, U.S. biodiesel production capacity could reach 1.5 billion gallons in 2007, he predicts.

Biodiesel, useable in any diesel engine, is now a key player in the alternative fuels market. Produced by industrial facilities that turn out millions of gallons annually, and also by smaller manufacturers that make it from used cooking grease, biodiesel could do much to reduce our reliance on foreign oil, experts say. "Long-term, we estimate it could produce a volume equal to about twenty-five percent of today's on-highway diesel fuel use," says Robert McCormick, a principal engineer at the DOE National Renewable Energy Laboratory (NREL).

McCormick stresses that biodiesel can't replace petroleum entirely. Although diesel powers most of the commercial trucks, ships, and farm equipment in the United States, roughly 95% of the passenger fleet here runs on gasoline. Even if the passenger fleet were to shift entirely to diesel, U.S. agriculture couldn't produce enough feedstock to meet its needs, he says. According to NREL's calculations, published in the June 2004 report Biomass Oil Analysis: Research Needs and Recommendations, agricultural capacity in the United States would probably limit production to at most 10 billion gallons of pure biodiesel a year, unless manufacturers used new higher-yield feedstocks, such as algae.

Still, according to Jonathan Cogan, a spokesman for the federal Energy Information Administration, the United States consumed more than 40 billion gallons of diesel fuel in 2005 alone. The possibility that biodiesel could substitute for up to a quarter of that amount is significant, McCormick emphasizes. "Biodiesel will be part of a multi-pronged approach to replacing imported petroleum," he says.

A Brief History

The diesel engine was invented in 1892 by engineer Rudolf Diesel. Diesel engines differ significantly from standard gas engines. Where gas engines ignite vaporized fuel in a cylinder using a spark plug, diesel engines compress air in a cylinder, making it so hot that when fuel hits the air, it explodes. That process converts fuel to energy more efficiently than spark plug designs, giving diesel engines greater fuel economy.

Early diesel engines ran exclusively on vegetable oil. But in the 1920s, the feedstock shifted to petroleum distillates refined from crude oil during gasoline production. But while so-called petrodiesel was cheaper and more plentiful than vegetable oil, it was also lighter and less viscous. Automakers had to modify engine designs accordingly, and vegetable oil as a fuel source was sidelined for decades.

Then in 1973, the Arab oil embargo sent crude oil prices through the roof. With gas and diesel suddenly four times more expensive than before, interest in biofuels returned. But there was a dilemma: pure vegetable oil was too thick for modern diesel engines; it plugged injection systems and didn't spray evenly into compression cylinders. Short of going back to older engine designs, two options remained: either heat the oil with an onboard system to make it less viscous (the method used by today's "Greasecars," which run on straight fryer grease), or make the oil's molecules smaller.

The latter option led to biodiesel. Most producers chose a manufacturing method called transesterification, which the South Africans used to make fuel from vegetable oil before World War II. With that process, refiners mix the oil with alcohol in the presence of a catalyst, usually sodium hydroxide. The alcohol and fatty acids react, creating biodiesel and a by-product of glycerin. The alcohol used is usually methanol, yielding a biodiesel consisting of fatty acid methyl esters.

Today, most biodiesel produced worldwide is made by transesterification. Soybean oil accounts for nearly 90% of the biodiesel produced in the United States, although any kind of vegetable oil or animal fat is suitable. Most scientists dismiss earlier suggestions that biodiesel requires more fossil fuel energy to make (in terms of chemical inputs, labor, transportation, and other factors) than it generates as fuel. What many consider to be the definitive analysis, described in a 1998 DOE/USDA report titled An Overview of Biodiesel and Petroleum Diesel Life Cycles, concluded that biodiesel generates 3.2 times more energy than is required to produce it.