Fats and Biodiesel

Soybeans can be made into biodiesel.
Photo courtesy National Biodiesel Board
Soybeans can be made into biodiesel.

Part of what makes biodiesel so appealing and interesting is that it can be made from numerous natural sources. Although animal fat can be used, plant oil is the largest source of biodiesel. You've probably used some of these in the kitchen. Scientists and engineers can use oils from familiar crops such as soybean, rapeseed, canola, palm, cottonseed, sunflower and peanut to produce biodiesel. Biodiesel can even be made from recycled cooking grease!

The common thread shared by all biodiesel sources is that they all contain fat in some form. Oils are just fats that are liquid at room temperature. These fats, or triacylglycerols (sometimes called triglycerides) are made up of carbon, hydrogen, and oxygen atoms bound together and arranged into a specific pattern. These triacylglycerols are pretty prevalent. In addition to household vegetable oils, they're also in common things like butter and lard. You may have seen a triglyceride count listed if you've been to a doctor and had some blood work done.

One way to visualize these triacylglycerols is to think of a capital "E." Forming the vertical backbone of this E is a molecule known as glycerol. Glycerol is a common ingredient used in making such things as soap, pharmaceuticals and cosmetics. Attached to this glycerol backbone and forming the horizontal elements of the E are three long chains composed of carbon, hydrogen, and oxygen. These are called fatty acids.

 H   H         H         H   H
\ / | \ /
C-----------C-----------C
| | |
O O O
\ \ \
C=O C=O C=O
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H-C-H H-C-H H-C-H
| | |
H H H

So how do these triacylglycerols end up in a car, truck or boat? Biodiesel is not pure vegetable oil. Although raw vegetable oil has been used to fuel diesel engines in the past, it has usually caused problems. The raw fat or oil must first undergo a series of chemical reactions in order to become fuel. There are a few different ways to make biodiesel, but most manufacturing facilities produce industrial biodiesel through a process called transesterification. In this process, the fat or oil is first purified and then reacted with an alcohol, usually methanol (CH3OH) or ethanol (CH3CH2OH) in the presence of a catalyst such as potassium hydroxide (KOH) or sodium hydroxide (NaOH). When this happens, the triacylglycerol is transformed to form esters and glycerol. The esters that remain are what we then call biodiesel.

Is this old news? In the next section, we'll examine some of the history and motivation behind the biofuels movement.