As anybody who drives has noticed, the price of oil has increased continuously (except the major drop in late 2008) over the past 15 years, and almost continuously over the past 40 years. This price increase has increased interest in alternative fuels. Biofuels make up one category of these alternative fuels.
Biofuels are broadly defined as any fuel that is derived from a recently living organism. By this definition, biofuels are not new. People have been burning wood and animal fat for thousands of years. Of course, these are not the biofuels being discussed today–unless someone wants to build a car that runs on wood or lard! Usually, modern biofuels consist of alcohols, derived from sugars, starches, and cellulose, or as fatty acid esters (biodiesel), derived from plant lipids (and occasionally animal lipids–like lard!).
There are many advantages and disadvantages to using biofuels. They are cleaner, highly renewable, and come from relatively cheap sources. They also do not add carbon to the carbon cycle, although their use does not remove the carbon from the cycle that has been added by 150-200 years of fossil fuel use. On the other hand, biofuel production often increases food prices, and switching to the large scale use of biofuels would initially be costly. Also, the cost of producing the biofuels can equal or even exceed the value of the energy produced.
Biodiesel is usually more cost effective than bioethanol, and it has some unique advantages and disadvantages. Biodiesel is a mixture of esters (check wikipedia if you don’t remember your organic chemistry) made from fatty acids and a small alcohol, usually methanol that can be used as fuel for a diesel engine (but NOT a gasoline engine!). Biodiesel is chemically similar to petrodiesel, but it doesn’t have as many sulfur containing impurities that cause acid rain. It does have some physical and chemical differences from petrodiesel, including higher density, viscosity, flash point, heat of combustion and cloud point. These can effect engine performance, but these effects can be reduced by blending biodiesel and petrodiesel fuels together. B-20, 20% biodiesel and 80% petrodiesel by volume is commonly sold in Europe.
The reaction that produces biodiesel from vegetable oil is called transesterification. This reaction requires one of three catalysts to proceed: acid-catalyzed, base-catalyzed, and lipase-catalyzed. Acid-catalyzed transesterification doesn’t work very well; it takes a long time and produces many side reactions. Base-catalyzed is currently most commonly used at an industrial scale. It usually is fast and gives a good yield of biodiesel. Unfortunately, very specific amounts of the reagents must be mixed in a very specific way, or a side reaction called saponification will occur. “Saponification” roughly means “soap-making” in Latin, and a soap-like product (sodium fatty acid anion) is produced. This product will form an emulsion, a milky mixture of biodiesel, glycerol, water, and unreacted vegetable oil that is too difficult to separate to be useful. Lipase-catalyzed transesterification uses lipases, which are enzymes that break down fats and oils into glycerol and fatty acids. This process is safe, fast, and it eliminates the saponification risk, but it has not been optimized for industrial scale production of biodiesel. Isolating enough enzyme and keeping the enzyme functional long enough are factors that must be dealt with to make lipase catalyzed biodiesel production worthwhile.
A team in India tried to deal with the first problem. They tried to indentify soil (i. e. very common) bacteria that can produce enough lipase to carry out the reaction. They began by isolating fifteen strains of bacteria from soil samples in areas where lipolytic bacteria would be likely to thrive, such as an oil mill and a dairy processing plant. The strains were grown on agar media that contained tween 80, an emulsifier composed of oleic acid (a fatty acid) bound to a small sugar. If the bacterial strain produced lipase, this would be broken down, causing the fatty acid to be freed. Three strains produced lipase and one of these was studied further. The lipase was isolated on beads and was used to produce methyl oleate, a major component of most biodiesel.
A group from Spain recently tried to investigate ways to extend the life of lipase. Because these lipases must be exposed to methanol, the researchers hypothesized that methanol may denature the protein, inactivating it. For their experiments, the group used commercial lipases from the fungus Candida antarctica in both free enzyme and bead immobilized forms. They found that upon exposure to methanol, the bead immobilized enzyme became deactivated more quickly. This led them to conclude that methanol adsorbs to the beads, which causes faster deactivation.
In addition to the variety of catalysts available for biodiesel production, the source of the vegetable oil makes a difference in the utility and cost effectiveness of biodiesel. Frequently, “neat” vegetable oil (NVO) is used to make biodiesel. This is relatively easy because so much oil is already produced for food. Unfortunately, using this oil increases the price of food (a lot of our food is fried!), so it is not a cost effective method of fuel production. A much better source of vegetable oil is waste vegetable oil (WVO), oil that has been used for deep frying that has become too old to use.
WVO is not as pure as NVO; it contains semi-combusted fats, free fatty acids, hunks of burnt French fries etc., but it is an environmental hazard in its own right, and restaurants have to pay to have it disposed of. Making WVO into biodiesel is useful and is very cheap (if not messy).
WVO is cheaper than petrodiesel, but is it cleaner? How does engine efficiency compare? These questions were asked by a team of researchers from the New Mexico institute of Mining and Technology. Their methods were simple; they burned biodiesel, petrodiesel, and B-20 in two diesel generators and studied the efficiency and emissions. They found that WVO biodiesel is as efficient or more than petrodiesel, and produces the same amount or less of a variety of pollutants, including carbon monoxide, nitric oxide, and unburned hydrocarbons, which are poisonous, a source of acid rain, and carcinogenic respectively.
Some biodiesel research in recent years has involved in oils derived from algae. Algae grow fast, don’t take up much space, aren’t used for food, and can produce large amounts of oil. Unfortunately, according to a team of researchers from the Bulgarian Institute of Plant Physiology and Genetics (not affiliated with the American Petroleum Institute) calculated the cost of everything required to grow enough algae to make biodiesel and found that it is about equal to the price of an equal amount of petrodiesel. This means large scale production of algal biodiesel has long way to go to become economically viable.
Overall, WVO biodiesel is the most useful form. It is the cheapest, has the least effect on food prices and is as functional as petrodiesel. The issue is moot if you drive a car with a gasoline engine (as most Americans do), but if you drive a car with a diesel engine, use WVO biodiesel!
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