Biodiesel

Another biofuel that has found some application, especially in the United States and Europe, is the mixture of fatty acid methyl esters, R—COOCH3, called biodiesel. This material usually corresponds to an oil—usually derived from a plant source such as soybeans or rapeseed (canola)—that has been esterified and can be used in diesel engines. The rapid rise in annual global biodiesel production began in the late 1990s, as illustrated by the dark green curve in Figure 8-9; note the difference by a factor of 10 in the scales for bioethanol and biodiesel.

In principle, the raw vegetable oils could be blended with diesel oil—or even used neat—as a fuel. Indeed, when diesel engines were introduced in the early twentieth century, they were fueled with pure peanut oil. However, due to its high viscosity and impurities—such as free fatty acids, water, and odorous substances—the raw oil cannot be used in modern diesel engines. Even refined vegetable oil cannot be used as a general fuel, because of its viscosity and because the polymerization of the unsaturated hydrocarbon components of the oils that occurs during combustion produces gums that result in carbon deposits and thickening of lubricating oil in the engine. One solution to the viscosity problem, employed by so-called Grease cars that are fueled with grease from deep-frying, is to heat the oil onboard the vehicle.

More commonly, the virgin vegetable oils are transformed into a less viscous, less corrosive liquid that is used as the fuel. The main component in the original oil is triglycerides, which are triesters of various fatty acids with glycerin, CH2OH—CHOH—CH2OH. The transformation converts each triglyceride molecule into three methyl esters of long-chain fatty acid molecules, which then constitute the fuel, and a molecule of glycerin (also called glycerol), which is removed from the mixture of fatty acids and sold separately for other uses. To accomplish the transformation, the triglyceride is reacted using base or acid catalysis with methanol obtained from natural gas, as described in detail in the green chemistry section that follows. The use of methanol, most commercial supplies of which involve its synthesis from natural gas, makes biodiesel less than 100% renewable, though the great majority of the carbon atoms in the fuel esters—and hence in its fuel value— originate with the vegetable oil.

Overall, soybean-derived biodiesel generates over 90% more energy than is used to produce it, compared to about 25% for corn-based ethanol. Biodiesel blends produce less carbon monoxide, particulate matter (PM10), and sulfur dioxide emissions when combusted than does the 100% diesel fuel that they replace; the reduction in soot and CO arises because it is an oxygen-containing fuel. There is controversy as to whether biodiesel blends produce more or less NOx than does pure diesel. Although energy and fossil-fuel-derived methanol are used in its production, and nitrous oxide emissions are associated with fertilizers to grow the plants, biodiesel produced from soybeans on existing agricultural land overall reduces the C02 equivalent emissions by about 40%. The much greater decrease in C02 from biodiesel, compared to corn-based ethanol, is due primarily to the much lower amount of energy required: Soybeans create oil that can be obtained readily from seed by physical methods, whereas ethanol requires fuel-intensive distillation. Soybean production also uses much less fertilizer and releases much less nitrogen, phosphorus, and harmful pesticides into the environment than does corn production for ethanol. Of course, the two biofuels are used in different types of vehicles, so a comparison between them is of limited significance.

The fraction of biodiesel in diesel fuel is designated by a system analogous to that used for alcohols in gasoline. Thus B5 symbolizes diesel fuel containing 5% biodiesel by volume, and B100 is pure biodiesel. In the past, the most common blend was B2C—indeed, the U.S. Navy, the largest single user of biodiesel in the world, uses that blend in all nontactical vehicles—but more dilute blends such as B- and B5 are becoming popular. Currently, the largest manufacturer of biodiesel is Germany, which produced more in 2005 than the rest of the world combined. The European Union has mandated that all fuels contain 5.75% biofuels by 2010, which will mean a tripling of their consumption there compared to 2005 levels.

Almost all the biodiesel produced in the United States uses domestic soybeans (which are about 20% oil) as its raw material. The oil yield, about 40%, is even higher for rapeseed (canola). In tropical areas, massive plantations of palm trees are being planted to produce palm oil for biodiesel, since the yield of oil per square kilometer greatly exceeds that from soybean or rapeseed crops. Unfortunately, in the rush to produce more palm oil destined to become biodiesel in Europe, huge areas of tropical rain forest in Malaysia, Brazil, and Borneo and peatland in Indonesia have been burned and cleared and thereby destroyed; the result is large amounts of greenhouse gas emissions, totaling one-twelfth of all global C02 in the case of Indonesia. The advantage of biofuels in producing lower greenhouse gases than conventional gasoline or diesel will be overcome by these emissions for many years.

Anther concern about biofuels is the effect they have on the price of food. In 2007, the Food and Agriculture Organization of the United Nations noted that the rapidly increasing demand for biofuels is transforming agriculture worldwide and contributing to the increase in food prices. The inflation in prices applies not only to the corn, sugar, and vegetable oil sources but also indirectly to prices for the livestock that are usually fed these crops.

In the future, species of algae that contain as much as 50% oil may be grown as raw material from which biodiesel fuel would be obtained, since the yield per square kilometer could greatly exceed that of even tropical palm oil. However, the practical mass production of single-species algae has not yet been achieved.

Green Chemistry: Valuable Chemical Feedstocks ^Sr from Glycerin, a Waste By-Product in the Production of Biodiesel

As mentioned above, biodiesel is produced from a transesterification reaction of the triglycerides that make up animal fats and vegetable oils (Figure 8-11). This reaction not only produces the methyl esters of the fatty acids, which comprise biodiesel, but also yields glycerin as a by-product. For every 9 L of biodiesel produced, about 1 L of glycerin is formed. The market for the glycerin by-product has not kept pace with biodiesel production; consequently, there is now a glut of glycerin on the market. Chemists and engineers have been searching for new uses for glycerin and for processes to convert glycerin to other valuable and useful chemicals.

Professor Galen Suppes and his group at the University of Missouri won a 2006 Presidential Green Chemistry Challenge Award for their discovery of a process to convert glycerin to propylene glycol and acetol (Figure 8-12). Although other methods exist for the conversion of glycerin to propylene glycol, they require high temperatures (200-400°C) and high pressures (1450—4700 psi). As a result of the stringent reaction conditions, these other

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