Researchers have determined that the following properties characterize the chemical properties of biodiesel: fatty acid content, aromatics, olefins, paraffins, carbon, hydrogen, oxygen, and sulfur content, acid neutralization number, iodine number, and Conradson carbon residue number.
Note the chemical structure of a triglyceride (Fig. 2). R1, R2, and R3 represent the hydrocarbon chain of the fatty acid elements of the triglyceride. There is a three-carbon chain called the glycerol backbone that runs along the left side of the molecule. Extending away from this backbone are the three long fatty acid chains. The properties of the triglyceride and the biodiesel fuel will be determined by the amounts of each fatty acid that are present in the molecules.
Fatty acids are designated by two numbers: the first number denotes the total number of carbon
atoms in the fatty acid, and the second is the number of double bonds. For example, 18:1 designates oleic acid, which has 18 carbon atoms and one double bond. A typical sample of soybean oil based biodiesel would have the fatty acid profile shown in Fig. 3.
Biodiesel is essentially free of sulfur and aromatics. This is an advantage for biodiesel because sulfur poisons catalytic converter technology that is used to reduce engine exhaust emissions. The sulfur levels in biodiesel by ASTM D5453 are found to be as low as 0.00011% by mass (1ppm), where petroleum diesel is often no lower than 0.02% (200 ppm). The lack of aromatic hydrocarbons is also an advantage for biodiesel, as many of these compounds are believed to be carcinogenic. The test procedure normally used to measure aromatics in petroleum fuel (ASTM D1319) should not be used to determine the aromatics of biodiesel. This analytical procedure mistakenly identifies the double bonds commonly found in biodiesel as the resonance stabilized bonds normally associated with aromatics.
Paraffins are hydrocarbon compounds that are normally associated with petroleum diesel fuel. These compounds help increase the cetane value of the diesel fuel. However, they also typically increase cold flow problems of petroleum diesel fuel. Olefins are hydrocarbons that contain carbon-carbon double bonds. Molecules having these types of bonds are called unsaturated. Biodiesel from common feedstocks is usually 60 to 85% unsaturated. Some olefins are present in petroleum-based diesel fuel. However, the amount of olefins is usually small as they contribute to fuel oxidation.
The carbon content of biodiesel is nearly 15% lower than petroleum diesel fuel on a weight basis. Conversely, biodiesel has approximately 11% oxygen, on a weight basis, while petroleum diesel has almost no oxygen. Very little differences exist between biodiesel and petroleum diesel fuel concerning the weight percentage of hydrogen.
FIGURE 3 Fatty acid profile for soybean derived biodiesel.
The neutralization number is used to reflect the acidity or alkalinity of an oil. This number is the weight in milligrams of the amount of acid (hydrochloric acid [HCL]) or base (potassium hydroxide [KOH]) required to neutralize one gram of the oil, in accordance with ASTM test methods. If the neutralization number indicates increased acidity (i. e., high acid number) of an oil, this may indicate that the oil or biodiesel has oxidized or become rancid. Biodiesel is allowed to have a neutralization number up to 0.8mg KOH/g.
The iodine value is a measure of the unsaturated fatty acid content of biodiesel, and reflects the ease with which biodiesel will oxidize when exposed to air. The iodine value for petroleum diesel fuel is very low, but the iodine value of biodiesel will vary from 80 to 135.
A weighed quantity of fuel is placed in a crucible and heated to a high temperature for a fixed period to determine the Conradson carbon residue of a fuel. The crucible and the carbonaceous residue is cooled in a desiccator and weighed. The residue that remains is weighed and compared to the weight of the original sample. This percentage is reported as the Conradson carbon residue value. This procedure provides an indication of relative coke forming properties of petroleum oils. No real differences are to be expected when comparing biodiesel with petroleum diesel fuel (0.02 versus 0.01).