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Burnt vegetables

Jan 1, 2003

One of the advantages of a gas turbine engine is its ability to burn virtually anything. This could be valuable for the voyaging sailor who finds himself low on fuel and in a tight spot. No problem just dump some paint or perhaps a bottle of salad oil into the engine.

Turbines, however, are not the only type of internal combustion engine that can burn a variety of fuels. The trusty marine diesel can also use kerosene in a pinch. In fact, the first diesel engine, designed by Rudolf Diesel in 1892, was designed to run on coal dust. Diesel later switched to liquid fuel, and an early version of his engine ran on peanut oil.

The advent of petroleum oil led to its use as diesel fuel, pushing peanut and other vegetable oils aside. Now, due to heightened environmental awareness, vegetable fuels are attempting a comeback. One type of alternative fuel is based on processed soybean oil and is called Biodiesel. In July 1992, a customized Zodiac semi-rigid inflatable called Sunrider, began a circumnavigation from San Francisco. Piloted by Bryan Peterson, the vessel has been using 100% soy-based Biodiesel for fuel to run an unmodified 180-hp MerCruiser diesel sterndrive. Peterson is scheduled to return to San Francisco in August of this year. Sunrider has integral tanks with 300 gallons capacity and can use fuel bladders to boost total capacity to 600 gallons. (Even with added capacity, Sunrider's fuel storage was limited. One solution to this problem, used in the Indian Ocean, had Sunrider travel in tandem with a 55-foot sailboat that carried jugs of Biodiesel.)

Another example of Biodiesel being used in the marine world is in Santa Cruz, Calif. The port of Santa Cruz is running both a patrol boat and a dredge on the fuel.

Biodiesel has also been evaluated in test projects by more than 70 transit authorities around the country. City buses were run on a mix of petroleum and 20% Biodiesel, resulting in lower emissions of sulfur, aromatics (a class of toxic compounds like benzene that are formed in molecular rings), and particulates. Used as part of a mixture, soy-based diesel enables bus fleets to meet tougher emissions standards established by the EPA in 1993.

In Europe, many governments have decided to promote vegetable oil fuels by sharply reducing taxes at the pump. Thus, while vegetable oil fuels have a higher unit cost, the reduced taxes make petroleum diesel and vegetable oil diesel prices roughly equal for the end user. In Austria, Germany, and Switzerland, vegetable oil diesel is exempted from all sales and road taxes. Some corporations have also decided to move ahead with vegetable diesel. Italy's Novamont Corp. has a plant in Livorno with a capability of 19.5 million gallons per year. The vegetable oil most often used as a fuel source in Europe is rapeseed. And in addition to vegetable oils, animals fats can also be used for engine fuels.

Biodiesel, when used straight or "neat," yields no sulfur or aromatic compounds because soybeans don't contain these compounds. "There is a direct relationship between ring compounds and pollution," says Bill Ayres, executive vice president of Interchem Industries (Overland Park, Kansas, 913-599-0800), which sells Biodiesel (Interchem contracts out production of refined soybean oil to Proctor & Gamble, which manufactures it at plants in Kansas City and California.) Use of the fuel also produces very little carbon monoxide pollution. "Biodiesel is 11% oxygen by weight," says Ayres, "This results in more complete combustion and less carbon monoxide."

Biodiesel has 3 to 4% fewer BTUs per gallon compared to petroleum-based diesel. But, according to Ayres, the fuel makes up for this by its more complete combustability. On the safety side, even though it burns well when injected into a cylinder, Biodiesel is superior to diesel oil in its resistance to accidental ignition, having a flash point around 400° F.

Biodiesel also manages to eliminate one of the more unpleasant olfactory effects of diesel fuel. "One of the nice characteristics of the fuel is no diesel smell," says Ayres. "It literally smells like french fries cooking." In fact, the fuel is approved by the FDA for food uses, although it reportedly doesn't taste too good.

While Biodiesel can be run in a diesel without major modification, the fuel does tend to act as a solvent on natural rubber. Thus, rubber fuel lines must be replaced with synthetic materials like Viton. Also, the present major drawback to this fuel is a higher price. In small quantities, it can cost as much as $6 a gallon.

Before raw soybean oil can be used as a diesel fuel, it must go through a refining process. Pure vegetable oil doesn't work well in a diesel because it is too viscous and tends to clog the tight clearances inside a diesel injector. Soybean oil is refined in a process called transesterification. The oil is combined with methanol and a catalyst, such as sodium hydroxide (caustic soda). This breaks the oil down into two components: a linear monoester, similar to the straight hydrocarbons found in petroleum diesel, and glycerin, which is used as a moistener in soaps and cosmetics. The refined soy oil has a viscosity similar to No. 2 diesel (Diesel fuel comes in two main grades: No. 1a light distillate similar to kerosene, good for cold weather operation, and having a flash point around 100° F; and No. 2a medium distillate with a flash point around 125° F.)

An important characteristic of diesel fuel is something called "lubricity." This describes its abilities as a lubricant. Gasoline, for example, has low lubricity, is a powerful fuel for combustion, but it doesn't lubricate well. Diesel fuel has much higher lubricity than gasoline, and Biodiesel has a slightly higher lubricity. Lubrication is more important in a diesel engine because the passages inside an injector must be kept tight to maintain the tremendous pressure developed inside the cylinders. A gasoline engine will typically have compression ratios of 8:1 or 10:1. Diesels, however, will run up to 20:1 compression, producing pressures of 500 psi and temperatures in excess of 800° F within its cylinders. And, since the fuel has to be sprayed into the cylinder against this pressure, the injectors must not only resist the pressure inside the cylinder, but also operate at substantially higher internal pressure8,000 psi or morein order to force fuel into the highly pressurized cylinder. Injectors with bigger clearances couldn't hold these pressures as well. If a fuel like gasoline, which has low lubricity, were put into a diesel engine, not only would the gasoline burn faster and in a more uncontrolled fashion than diesel fuel, but the gasoline molecules would do a lousy job of lubricating the diesel's fuel injection pump and injectors.

Biodiesel is already being used by a group of about 50 sailors in the San Francisco area, who buy it in jerry cans from a company called CytoCulture (Point Richmond, Calif., 510-233-0102). According to John O'Bannon of CytoCulture, users have been pleased with the performance of the fuel. "Our customers have told us that their engines start easier when cold," says O'Bannon. "And the normal knocking of a diesel is reduced when burning soy diesel. Overall, it seems to improve engine running. Of course, you have the same problem with biological growth."

Will Biodiesel replace petroleum diesel? That isn't a likely possibility given the huge demand for diesel in the U.S. According to Bill Ayres at Interchem, 50 billion gallons of diesel are used in the U.S. per year and soy oil can't come close to meeting that target. A passage from an article on vegetable oil fuel in the February 1993 issue of Chemical Engineering indicates the nature of the supply shortfall: "If the 22 billion lb/yr of vegetable oils produced in the U.S. were used solely for biodiesel production, it would generate only 3.2 billion gallons of fuel, about 6.4% of U.S. demand."

While vegetable oil products aren't expected to sweep petroleum diesel from the field, there probably will be, however, significant market penetrations by products like Biodiesel. "We see it as a niche market fuel," says Ayres. "It will be used in areas where it makes environmental sense."

One of those niche markets could be green-conscious sailors. If one is sailing 20 miles offshore and smells a hamburger joint nearby, it might just be a boat burning Biodiesel.