Biodiesel, a diesel substitute made from biological materials, can be used directly in a diesel engine without clogging fuel injectors. It is the product of a chemical process that removes the sticky glycerins from vegetable and animal oils. Because it is made from biomass, biodiesel is considered to be a “carbon neutral” fuel. When burned, it releases the same volume of carbons into the atmosphere that were absorbed when the biomass source was growing. Controversy exists, however, in particular over the use of high-quality grains for biodiesel, because land is taken out of food production and devoted to the production of fuel.


I. Characteristics of Biodiesel

II. Applications of Biodiesel

III. Advantages of Biodiesel

Characteristics of Biodiesel

In comparison with diesel, biodiesel has reduced particulate, nitrous oxide, and other emissions and emits no sulfur. Biodiesel is used as a transportation fuel substitute, either at the rate of 100 percent or in smaller percentages mixed with diesel. It mixes completely with petroleum diesel and can be stored safely for long periods of time. Biodiesel is biodegradable and does not contain residuals that are toxic to life forms. It has a higher fl ash point than diesel and is safer to ship and store. Biodiesel is mixed with kerosene (#1 diesel) to heat homes in New England. It has been used as an additive in aircraft fuel, and because of its oil-dissolving properties, is effective as a nontoxic, biodegradable solvent that can be used to clean oil spills and remove graffiti, adhesive, asphalt, and paint; as a hand cleaner; and as a substitute for many other petroleum-derived industrial solvents. Biodiesel is appropriate as a renewable alternative to petrochemical diesel because it can be produced domestically, lowers emissions, and does not cause a net gain in atmospheric carbon. The overall ecological benefits of biodiesel however, depend on what kinds of oils are used to make it.

Biodiesel is made from the oils in seeds, nuts, and grains or animal fats. Oil sources for biodiesel production are called biomass “feedstock.” Agricultural crops are specifically grown to be utilized for biodiesel production. The crops vary according to region and climate; in the northern hemisphere, biodiesel is most often made from soybean, sunflower, corn, mustard, cottonseed, rapeseed (also known as canola), and occasionally, hempseed. In tropical and subtropical regions, biodiesel is made from palm and coconut oils. Experiments have been conducted in extracting oils from microorganisms such as algae to produce the fuel. These algae experiments have raised hopes of converting sunlight more directly into a renewable fuel to be used with existing diesel machinery. Biodiesel offers an efficient use for waste oils that have already been used for frying or otherwise manufacturing food for human consumption. Waste grease biodiesel is made from the oils left over in the fryer and the grease trap as well as from the animal fats and trims left over from the butchering process.

The fuel is manufactured from both fresh and waste oils in the same way, through a chemical reaction called transesterification, which involves the breaking up, or “cracking,” of triglycerides (fat/oil) with a catalytic agent (sodium methoxide) into constituent mono-alkyl esters (biodiesel) and raw glycerin. In this process, alcohol is used to react with the fatty acids to form the biodiesel. For the triglycerides to react with the alcohol, a catalyst is needed to trigger a reorganization of the chemical constituents. Most often, a strong base such as sodium hydroxide (lye, NaOH) is used as a catalyst to trigger the reaction between the triglycerides and the alcohol. Either methanol (CH3OH, or wood alcohol, derived from wood, coal, or natural gas) or ethanol (C2H6O, known as grain alcohol and produced from petrochemicals or grain) is used as the alcohol reactant. The chemical name of the completed biodiesel reflects the alcohol used; methanol makes methyl esters, whereas ethanol will produce ethyl esters. Most frequently, methanol is the alcohol used.

Triglycerides are composed of a glycerin molecule with three long-chain fatty acids attached. The fatty acid chains have different characteristics according to the kind of fat used, and these indicate the acid content of the oil. The acid content must be taken into account in order to get the most complete reaction and thus the highest yield of biodiesel. To calculate the correct proportions of lye and methanol needed to transesterify a sample of oil, the acid content of the oil is measured through a chemical procedure called titration. Waste vegetable oil has higher fatty acid content and requires higher proportions of lye catalyst than fresh oil. The titration results determine the proportion of lye to combine with the methanol or ethanol to form a catalytic agent that will complete the reaction fully.

To make biodiesel, the oil is placed in a noncorrosive vessel with a heating element and a method of agitation. The mixture of lye and alcohol is measured and mixed separately. Usually the amount of methanol or other alcohol needed amounts to 20 percent of the volume of oil. The amount of lye depends on the acidity of the oil and is determined by titration and calculation. When the oil reaches 120 to 30 degrees Fahrenheit (48 to 54 degrees Celsius), the premixed catalytic solution is added. The oil is maintained at the same heat while being gently stirred for the next 30 to 60 minutes. The stirring assists in producing a complete reaction. The mixture is then left to cool and settle; the light yellow methyl esters or ethyl esters fl oat to the top and the viscous brown glycerin sinks to the bottom. The esters (biodiesel) are decanted and washed free of remaining soaps and acids as a final step before being used as fuel.

Although transesterification is the most widely used process for producing biodiesel, more efficient processes for the production of biodiesel are under development. The fuel is most simply made in batches, although industrial engineers have developed ways to make biodiesel with continuous processing for larger-scale operations. Biodiesel production is unique in that it is manufactured on many different scales and by different entities. It is made and marketed by large corporations that have vertically integrated supplies of feedstocks to mesh with production, much the same way petrochemical supplies are controlled and processed. There are also independent biodiesel-production facilities operating on various scales, utilizing local feedstocks, including waste oil sources, and catering to specialty markets such as marine fuels. Many independent engineers and chemists, both professional and amateur, contribute their research into small-scale biodiesel production.

Biodiesel can be made in the backyard if proper precautions are taken. There are several patented and open source designs for biodiesel processors that can be built for little money and with recycled materials. Two popular designs include Mike Pelly’s Model A Processor for batches of waste oils from 50 to 400 gallons and the Appleseed Biodiesel Processor designed by Maria “Mark” Alovert. Plans for the Appleseed processor are available free on the Internet, and the unit itself is built from a repurposed hot water heater and valves and pumps obtainable from hardware stores. Such open-source plans and instructions available online have stimulated independent community-based biodiesel research and development. In some cases, a biodiesel processor is built to serve a small group of people who decide to cooperate on production within a region, taking advantage of waste grease from local restaurants. The biodiesel fuel is made locally and consumed locally, thus reducing the expenses of transporting fuel.

Applications of Biodiesel

The use of vegetable oil in diesel engines goes back to Rudolf Diesel, the engine’s inventor. The diesel engine demonstrated on two different occasions in Paris during the expositions of 1900 that it could run on peanut oil. (The use of peanut oil was attributed to the French government, which sought to develop an independent agriculturally derived power source for electricity and transportation fuel in its peanut-producing African colonies.)

As transportation fuel, biodiesel can be used only in diesel engines in which a fuel and air mixture is compressed under high pressure in a firing chamber. This pressure causes the air in the chamber to superheat to a temperature that ignites the injected fuel, causing the piston to fire. Biodiesel cannot be used in gasoline-powered internal combustion engines. Because its solvent action degrades rubber, older vehicles running biodiesel might need to have some hoses replaced with those made of more resistant materials. The high lubricating capacity of biodiesel has been credited with improving engine wear when blended at a 20 percent rate with petroleum diesel.

Advantages of Biodiesel

The benefits of burning biodiesel correspond to the percentage of biodiesel included in any formulation. The overall energy gains of biodiesel are also assessed according to the gross consumption of energy required to produce the oil processed into fuel. Biodiesel processed from waste grease that has already been utilized for human food consumption has a greater overall energy efficiency and gain than biodiesel produced from oils extracted from a virgin soybean crop grown with petrochemical-based fertilizers on land previously dedicated to food production.

Biodiesel’s emissions offer a vast improvement over petroleum-based diesel. Emissions of sulfur oxides and sulfates (the primary components of acid rain) are eliminated. Smog-forming precursors such as nitrogen oxide, unburned hydrocarbons, and particulate matter are mostly reduced, although nitrogen oxide reduction varies from engine to engine. The overall ozone-forming capacity of biodiesel is generally reduced by nearly 50 percent. When burned, biodiesel has a slightly sweet and pleasant smell, in contrast to the acrid black smoke of petroleum-based diesel.

Biodiesel has the additional and important advantage of carbon-neutrality in that it is produced from the energy stored in living organisms that have been harvested within 10 years of the fuel’s manufacture. During their growing cycle, plants use carbon dioxide to process and store the energy of the sun in the form of carbon within their mass. When plants are converted to fuel source and burned, they can release into the atmosphere only the amount of carbon consumed and stored (through photosynthesis) during their life cycle. When petroleum fuel is burned, carbons are released into the atmosphere at a much faster rate. The atmospheric release of the fossilized carbons of petroleum fuel places an impossible burden on existing living biomass (trees and plants) to absorb the massive quantities of carbons being released.

The mass production of biodiesel from biomass feedstock grown specifically for fuel has not been proven to produce a net energy gain because of the energy inputs needed in current industrial farming methods. These include the inputs of petroleum-derived fertilizers and herbicides, fuel for farm machinery, and the energy needed to pump water and transport the fuel. Concerns have also been expressed about taking land and other agricultural resources previously devoted to food production for the production of biomass for fuels such as biodiesel.

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  1. Earley, Jane, and Alice McKeown, Red, White, and Green: Transforming U.S. Biofuels. Washington, DC: Worldwatch Institute, 2009.
  2. Official Site of the National Biodiesel Board,
  3. Pahl, Greg, Biodiesel: Growing a New Energy Economy, 2d ed. Burlington, VT: Chelsea Green, 2008.
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