Variation in Biodiesel Properties

As with petroleum-based fuels, the ASTM specification for biodiesel allows for a variety of feedstocks and processes to be used in its production. In today’s market, biodiesel is most commonly a blend of B100 from two or more feedstocks. Many producers use feedstocks from a variety of sources to obtain B100 with desired properties and to improve production economics.
The specification prescribes a largely feedstock-neutral, performance-based set of requirements that ensure the B100 is fit to be used in diesel engines. Biodiesel can be produced commercially from a variety of oils and fats:

• Animal fats. Tallows, lard, choice white grease,yellow grease, poultry fats, and fish oils
• Plant oils. Soy, corn, canola, sunflower, rapeseed,cottonseed, corn
• Recycled greases. Used cooking oils and restaurant frying oils.

Biodiesel can also be made 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. Some properties of finished biodiesel such as cetane number, cloud point, and stability depend heavily on the feedstock.

Compared to the chemistry of diesel fuel, which contains hundreds of compounds, the chemistries of different fats and oils typically used for biodiesel are very similar. Each fat or oil molecule is made up of a glycerin backbone of three carbons, and on each carbon is attached a long-chain fatty acid that reacts with methanol to make the methyl ester, or biodiesel. The glycerin backbone is turned into glycerin and sold as a co-product of biodiesel manufacturing. Currently, the fats and oils used to make commercial biodiesel contain 10 common types of fatty acids that have 12 to 22 carbons, more than 90% of which are 16 to 18 carbons. Some of these chains are saturated, some are monounsaturated, and others are polyunsaturated. Within the limits of the specifications, the differing levels of saturation can affect some biodiesel fuel properties.

Each feedstock is set apart from the others because it is made of different proportions of saturated, monounsaturated, and polyunsaturated fatty acids (Figure 2).

In general, saturated FAME have high cetane numbers and cloud points and are more stable. As unsaturation increases (i.e., the number of double bonds increases), the cetane number and cloud point decrease, as does the natural stability of the FAME. The cetane number and stability are easily treated with conventional additives, while the cloud point is more difficult to treat. The length of the fatty acids also has an impact on the biodiesel properties. For example, coconut methyl esters are highly saturated, and the shorter chain length results in a cloud point of -5°C. While it is useful to understand the relationship between saturation and biodiesel properties, users are encouraged to base purchase decisions on measured fuel properties.

As with conventional diesel fuel, the best type of biodiesel for your applications will be based on several factors. A No. 2 petroleum diesel fuel with a cetane number of 50 and a cloud point of -3°C (26°F) may be suitable for December in Texas, but a No. 1 petroleum diesel with a cetane number of 42 and a cloud point of -29°C (-20°F) may be best for December in Minnesota. The considerations and tradeoffs for biodiesel use are like those made for petroleum diesel fuel. The following data provide more detail about B100 properties and considerations.