Variation in Biodiesel Properties

As with petroleum-based fuels, the ASTM specifications for biodiesel allow for a variety of feedstocks and processes to be used to produce biodiesel. The specifications prescribe the amount of acceptable variability in the finished product. This variability is a compromise between maximizing the amount of fuel available for use and minimizing cost, while providing a minimum satisfactory level of engine performance.

Since biodiesel is produced mainly as a whole cut fuel, where the goal is to take all of the vegetable oil or animal fat and turn it into biodiesel, some of the properties of finished biodiesel depend heavily on the feedstock. These properties can include cetane, cold flow, bulk modulus (compressibility), and stability. In addition, testing has shown that differing biodiesel properties can also lead to different levels of NOx emissions from compression ignition (diesel) engines, although this does not appear to be the case with other regulated emissions (HC, CO, PM) or unregulated emissions (PAH, NPAH) or with open flame combustion in boilers or home heating applications.

Biodiesel can be produced commercially from a variety of oils and fats:
• Animal fats: edible tallow, inedible tallow, and all the other variations of tallow, lard, choice white grease, yellow grease, poultry fats and fish oils.
• Vegetable oils: soy, corn, canola, sunflower, rapeseed, cottonseed
• Recycled greases: used cooking oils and restaurant frying oils.

It is also possible to make biodiesel from other oils, fats and recycled oils such as mustard, palm, coconut, peanut, olive, sesame, and safflower oils, trap greases, and even oils produced from algae, fungi, bacteria, molds, and yeast.
Compared to the chemistry of diesel fuel, which contains hundreds of compounds, the chemistry of different fats and oils typically used for biodiesel are very similar. Each fat or oil molecule is made up of a glycerine backbone of three carbons, and on each of these carbons is attached a long chain fatty acid.

These long chain fatty acids are what react with methanol to make the methyl ester, or biodiesel. The glycerin backbone is turned into glycerin and sold as a byproduct of biodiesel manufacturing. The fats and oils listed above contain 10 common types of fatty acids which have between 12 and 22 carbons, with over 90% of them being between 16 and 18 carbons. Some of these fatty acid chains are saturated, while others are monounsaturated and others are polyunsaturated.

Within the limits of the specifications, the differing levels of saturation can affect some of the biodiesel fuel properties. This can be important when selecting the biodiesel for your application.
What makes each of these feedstocks different from the others is that they are made of different proportions of saturated, monounsaturated, and polyunsaturated fatty acids (Figure 2). A “perfect” biodiesel would be made only from monounsaturated fatty acids.

Under each category heading in Table 3 we show what types of fatty acids are considered saturated, monounsaturated, or polyunsaturated and their general impact on the fuel properties and emissions. The first number of the combination shows the number of carbons in the fatty acid chain; the second number is the level of saturation or unsaturation—0 for saturated, 1 for monounsaturated, and 2 or 3 for polyunsaturated. For example, 18:1 is a fatty acid containing 18 carbons and one point of unsaturation (monounsaturated). It should be noted that these are general trends only and that other factors such as the use of additives may modify the effects shown below.

As with conventional diesel fuel, the best type of biodiesel for your applications will be based on several factors. A No. 2 petrodiesel fuel with a cetane of 50 but that starts to freeze at 20°F may be perfectly suitable for December in Texas, while a No. 1 petrodiesel with a cetane of 42 and that starts to freeze at –40°F may be best for a December in Minnesota. The considerations and trade-offs made every day with petrodiesel fuel will also apply to the choice of biodiesel.