How Biodiesel is Produced

And How it Might be Produced in the Future

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Rapeseed, a current source of oil used for biodiesel in much of Europe. Picture courtesy of http://www.public-domain-image.com.

 

This article will explain what biodiesel is (including advantages and disadvantages), how it is produced, and how it can be produced if methods are improved.

 

Red, highlighted terms will be defined if you hover your cursor over them.

 

What is Biodiesel?

 

Biodiesel is an organically-produced fuel used as an alternative to petroleum-based automotive fuels such as gasoline or diesel. Like those fuels, it can also be used for purposes other than automotive fuel, but the main purpose of biodiesel production is to reduce or eliminate dependence on petroleum (a limited resource) for automotive fuel.

 

Depending on its purity (biodiesel is often mixed with regular diesel to decrease viscosity) and the oil source used to produce the fuel, biodiesel ranges from pale amber to dark brown and generally has a similar appearance to cooking oil.

 

Due to the increasing scarcity of fossil fuels, many labs are doing biodiesel research right now in an effort to improve on it and make it a viable replacement, both environmentally and economically, for fossil fuels.

 

Advantages of Biodiesel

 

Biodiesel has several advantages over petroeum-based fuels. Biodiesel:

 

• is renewable. A number of plant oils and animal fats can be converted to biodiesel, with more being researched. Plants (and animals, ultimately) get their energy from the sun, making biodiesel an infinitely renewable resource. Since it is much more efficient to raise plants than animals, this page will focus on non-animal sources of biodiesel.
• can be used in existing diesel engines. Some alterations may need to be made in cold climates, but most diesel engines can run biodiesel without a problem. In fact, biodiesel can help clean up corrosion left by high-sulfur diesel.

  

  Older Mercedes with diesel engines are a common vehicle choice for biodiesel users. Picture courtesy of wikimedia.

   

• burns cleaner than fossil fuels. This reduces emissions and helps the environment. Certain emissions are increased under some circumstances, but there are already processes in place to eliminate this problem.
• is safer to handle than fossil fuels. Gas and regular diesel are much more combustible than biodiesel and contain harmful compounds such as sulfur and benzene. Biodiesel is also biodegradable, an advantage in the case of spills.

 

Disadvantages of Biodiesel

 

Currently there are some disadvantages to using biodiesel, which helps explain why it has not been universally adopted. However, many of these problems can be resolved by improving technology and methods. The disadvantages are:

 

• Pure oils cannot be used directly. Most oils tend to be much more viscous than regular diesel, which combined with the relatively high melting temperature can make using natural oils in colder climates impossible without taking certain precautions – such as heating the fuel or mixing it with a less viscous fuel (i.e. regular diesel). A chemical process called transesterification is used to reduce oil's viscosity and convert it into biodiesel, but there are problems with the current methods used for this process. The methods and problems will be discussed in the Current Biodiesel Production Methods section.
• Some oil-yielding crops require large amounts of land to grow. Soybeans, for instance, have a relatively low yield of biodiesel per acre. They are also a food crop, which means that biodiesel production and food production can come into direct competition with each other. This leads to higher costs for both food and fuel.
• Biodiesel is a high-cost product. With current technology, only virgin oils can be processed into biodiesel without significant pre-processing. Waste oils are cheaper to obtain than virgin oils, but the cost of processing them is higher (see details under Current Biodiesel Production Methods).
• Most current methods used to process oil into biodiesel are not sustainable. Even though the source of the oil itself is renewable, non-renewable materials (such as methanol, the main source of which is natural gas) are being used in conversion of the oil to biodiesel. Read more about this under Current Biodiesel Production Methods.

 

Current Biodiesel Production Methods

 

Biodiesel production is composed of two main processes: producing oil and processing the oil into biodiesel.

 

Producing Oil

 

This step involves growing plants and harvesting the oil from them. Depending on the type of plant and the portion of the plant being used, the actual technique may vary, but it generally involves crushing the portion of the plant that contains the oil and filtering out the solid matter. Different plants yield different quantities and qualities of oil. 

 

• Soybean Oil. Currently this is the most popular oil choice for biodiesel production, because it is a natural byproduct of processing soybeans for use as food. However, increasing production of soybeans for the sole purpose of fuel is not practical due to the large amount of space soy requires for growth. As indicated by the name, the oil is contained in the beans.
• Rapeseed Oil. This is used as a major biodiesel component in Europe. Rapeseed is a flowering plant in the mustard family, and is the source of canola oil and other food products. The oil is extracted from the seeds. 
• Palm Oil. This is the main oil used for biodiesel production in many Asian countries. It is extracted from the crushed fruit of the oil palm. Palm oil is a food product as well, but since the oil palm can be used simultaneously to produce food and fuel, this is less of an issue with this crop.

 

 

Converting Oil into Biodiesel

 

Currently, most biodiesel production is done by reacting the vegetable oil with an alcohol in the presence of a base or acid. This section will outline both major production methods: transesterification by base catalysis and transesterification by acid catalysis. Other methods that are currently in small-scale production or under research are discussed under the Proposed Biodiesel Production Methods section.

 

Transesterification by base catalysis.

This is the most common method in use today. Virgin oil or oil that has been pre-treated to remove free fatty acids and water is required for this method.

 

As outlined by the National Biodiesel Board, alcohol (typically methanol) in molar excess and the base catalyst (sodium hydroxide or potassium hydroxide) are mixed together in the first step of transesterification. This mixture is put in a container, the oil is added, and the container is sealed. The mixture is then heated to the boiling point of the alcohol (which varies depending on the alcohol used; methanol’s boiling point is 64.7° Celsius) for 1-8 hours, depending on the composition of the source oil. This is the point in the reaction where the presence of free fatty acids or water in degraded (i.e. non-virgin) oils causes the formation of soap.

 

After the reaction has finished, the mixture is allowed to rest, or spun in a centrifuge. This causes the products of the reaction, glycerin and biodiesel, to separate (glycerin is more dense than biodiesel and will sink). Generally the glycerin is removed from the bottom of the container during settling; it can then be processed further and sold as a separate product.

 

Overview of transesterification by base catalysis. Oil is added to alcohol that has been mixed with a base, then heated for 1-8 hours. This yields biodiesel and glycerin; as it is more dense, the glycerin can be easily removed by draining it from the bottom of the container. Hourglass courtesy of http://www.wpclipart.com; flame brushes courtesy of obsidiandawn.com

 

Once the biodiesel has been separated, the alcohol needs to be removed from it. This is done by distilling it (heating the biodiesel to the alcohol’s boiling point in a container where the gaseous alcohol will be drawn from the top of the container).

 

Impurities such as residual catalyst and soap formed during production are often removed by washing the biodiesel with warm water. Once this has been done, the finished product is ready for testing, registration with the EPA (Environmental Protection Agency), and use.

 

The main problem with this production method is that it uses methanol, which is generally obtained from natural gas and is therefore a nonrenewable resource. Also, the base used in the reaction is strongly caustic and care must be taken in handling it properly.

 

Transesterification by acid catalysis.

Acid catalysis produces a lower yield than base catalysis, but can be performed even with oils that contain free fatty acids. However, the source oil still must be free of water.

 

Catalysts used in this process are normally sulfonic acid or sulfuric acid. The process is the same as with base catalysis, but generally takes longer and is done at about 100° Celsius instead of at the lower boiling temperature of the alcohol. 

 

Transesterification without catalyst.

While it is possible to complete these reactions without a catalyst, this must be done at a very high temperature and pressure, which raises the cost of production considerably. 

 

Proposed Biodiesel Production Methods

 

Due to problems with the existing procedures mentioned in the Current Biodiesel Production Methods section, such as the necessary use of harsh and/or nonrenewable chemicals, there has been much work done to develop new methods. Many of these are still in the testing phases, but show promise nonetheless. Improvements can be made in both the production of the source oil and in converting the oil into biodiesel, so both will be discussed here.

 

Producing Oil

 

Since the oils used for the majority of biodiesel today are either food products themselves or byproducts of food processing and/or production, they cannot be used to produce the vast volumes of oil that would be needed in order to meet the current demand for vehicle fuel. Another factor to consider is the amount of land required per unit of fuel produced; crops that can grow in minimal space (and, preferably, using minimal resources) are ideal.

 

Using Pongame Oil

This oil is taken from the seeds of Pongamia pinnata, a tree indigenous to many humid subtropical regions such as India and Pakistan. The oil is not used for food and has limited industrial use, making it an ideal choice for use in areas where food resources are already stretched. 

 

Growing and drying algae

Several species of algae have been discovered that have a high natural oil content (29% for one particular species, Neochloris oleabundans). The oil production of the algae can also be increased using techniques such as nitrogen starvation. Nitrogen starvation slows down cell growth, so two-stage mechanisms have been proposed to first produce many cells, then increase their oil content. The algae can then be crushed to release the oil, and the remainder (i.e. the non-oil content of the cell) can be utilized for other purposes such as ethanol production, fertilizer, and animal feed.

 

Converting Oil into Biodiesel

 

To avoid the use of harsh (and in some cases, nonrenewable) chemicals, high temperatures, and high pressures, and to make it possible to use non-virgin oils without extensive pre-processing, alternative ways to convert oil into biodiesel are being investigated. If these methods can be made more economically viable than the current methods outlined above, biodiesel can be propelled into the mainstream as a viable alternative to petroleum-based fuels.

 

Using harvested enzymes as catalysts

One method involves the use of enzymes instead of acid or base catalysts to convert oils to biodiesel. Lipases are able to work at low reaction temperatures and do not require high concentrations of methanol; in addition, since acid or base catalysts are not being used, extraction and cleanup are easier. The main drawback to this method is that lipases are expensive and, though they can be fixed to membranes to reduce loss, tend to need frequent replacement.

 

Using live bacteria to produce enzymes

Another method under research is using live bacteria (the sources of the lipases) instead of extracting the enzymes from them. If this method can be perfected, it will greatly reduce the cost of production since the bacteria can simply be allowed to reproduce (rather than continually harvesting additional lipase). Also, due to the mechanism of bacterial reproduction, fixing bacteria to a membrane is much less costly than fixing lipase to a membrane.

 

Using live bacteria or fungi to produce biodiesel from oil directly

Using modified E. coli bacteria in the presence of glucose, oleic acid, and air, scientists have been able to produce biodiesel. Other researchers have used fungi such as Rhizopus oryzae to convert various types of oil (canola oil, waste oil, and even grease) to biodiesel. These options are extremely promising as they would both reduce the costs and simplify the chain of production leading to biodiesel, since there would be little pre-processing needed.

 

Summary

 

Biodiesel is currently a promising alternative to petroleum-based fuels, but it is not yet a viable solution. With additional research and advances in technology, along with wise choices about which types of crops to use in which locations, the dependence on fossil fuels for transportation can be eliminated. This will help prevent problems with the fuel supply in areas that have no fossil fuel reserves, as well as lowering carbon footprints and reducing the emission of other hazardous byproducts. Renewability and sustainability are always worthwhile goals, and biodiesel can help achieve both in meaningful ways.

 

References

 

Ahmad, M., Zafar, M., Khan, M., & Sultana, S. (2009). Biodiesel from Pongamia pinnata L. Oil: A Promising Alternative Bioenergy Source. Energy Sources Part A: Recovery, Utilization & Environmental Effects, 31(16), 1436-1442. 

 

Balat, M. (2009). Biodiesel Fuel from Triglycerides via Transesterification—A Review. Energy Sources Part A: Recovery, Utilization & Environmental Effects, 31(14), 1300-1314. 

 

Chhetri, A., & Islam, M. (2008). Towards Producing a Truly Green Biodiesel. Energy Sources Part A: Recovery, Utilization & Environmental Effects, 30(8), 754-764. 

 

Du, W., Li, W., Sun, T., Chen, X., & Liu, D. (2008). Perspectives for biotechnological production of biodiesel and impacts. Applied Microbiology & Biotechnology, 79(3), 331-337. 

 

Gouveia, L., & Oliveira, A. (2009). Microalgae as a raw material for biofuels production. Journal of Industrial Microbiology & Biotechnology, 36(2), 269-274.

 

Jin, G., Bierma, T., Hamaker, C., Mucha, R., Schola, V., Stewart, J., et al. (2009). Use of a whole-cell biocatalyst to produce biodiesel in a water-containing system. Journal of Environmental Science & Health, Part A: Toxic/Hazardous Substances & Environmental Engineering, 44(1), 21-28. 

 

National Biodiesel Board. (2007). Biodiesel Production and Quality. Retrieved from http://www.biodiesel.org/pdf_files/fuelfactsheets/prod_quality.pdf. 

 

Vasudevan, P., & Briggs, M. (2008). Biodiesel production—current state of the art and challenges. Journal of Industrial Microbiology & Biotechnology, 35(5), 421-430.