What is biodiesel?

The definition I like is: Biodiesel is a renewable, non-toxic, biodegradable, safe fuel made from any plant oil or animal fat that can be used in any diesel engine.

Renewable:
Biodiesel is renewable because it's made from plants (though can also be made from animal fat). This aspect is great, because not only are plants a renewable resource, but also the process of producing biodiesel from them and burning it in our cars is carbon neutral. [See animation]. Remember your photosynthesis equation? Carbon dioxide plus water and sunlight yields starch (sugar) plus oxygen. (That's why some folks call trees solidified air and sunlight!). The reverse equation, using oxygen and starch (or food) and producing carbon dioxide and water is what we and other animals do. It is also what happens in the process of combustion. (Of course, combustion is never complete, and in actuality, a lot of other chemicals get produced in the burning reaction). It turns out that the absorption of carbon dioxide by the plants that we use for biodiesel pretty much exactly cancels out the production of carbon dioxide by burning it in our engines. This is very different from the petroleum cycle, where the stored carbon that was made millions of years ago is taken from underground and burned in engines to produce carbon dioxide in a one-way reaction. Yes, trees and other plants do soak up some of that carbon doxide too, but they can't keep pace with the continued burning of fossil fuels- it's just not a sustainable pathway. Plus, the burning of petroleum products produces sulfur dioxide which reacts in the atmosphere to produce acid rain that kills the trees that absorb all that carbon!

Non-toxic:
Biodiesel is less toxic than table salt. You can actually drink the stuff, but I wouldn't any more than I'd drink vegetable oil. I do recommend tasting it, just to get a sense of how innocuous it is. I do this routinely when I give talks about biodiesel.

Biodegradable:
Biodiesel degrades in the environment at about the same rate as sugar. This is especially important in regard to spills; if a tanker truck full of biodiesel spilled on the ground, you wouldn't need to call in the hazmat team- a little soap and water would clean up the area. If a boat spilled some, it wouldn't be a threat to marine life and would degrade quickly- might even be a food source for some critters! Diesel powered ships routinely spill diesel fuel which is a constant source of pollution to the marine environment. Just imagine the improvement if they all ran on biodiesel?

Safe:
Biodiesel is a safe fuel because it has a very high flash point- over 300 degrees F- the point at which it will ignite. The reason it works as a fuel in diesel engines is because it is vaporized and subjected to a lot of pressure inside the engines' cylinders. But liquid biodiesel isn't very flammable at all. You can try lighting it with a match, and nothing will happen (after a few seconds, you'll notice your match starting to burn a bit more- just as if you had dipped it in candle wax). Again, this is important when biodiesel is spilled accidentally- it's just not a fire hazard. Same thing for biodiesel being made in your garage- it's not hazardous. The main hazard in your garage would be from oil or biodiesel soaked rags laying around where they could get hot. And of course the methanol which is used to make biodiesel; this has to be stored properly in a cool place, because it is quite flammable.

Fuel:
Yes, biodiesel is a type of fuel. Chemically it is methyl esthers, chains consisting of carbon, hydrogen, and oxygen, that have been cleaved off of triglycerides (see wiki pedia biodiesel section under links). All plant oils and animal fats are triglycerides, which consist of a glycerin backbone, with three chains attached. The chains are formed from different types of fatty acids that are bonded to the glycerin. There are many fatty acids, but they are generally classified as either saturated or unsaturated. Saturated ones are "saturated" with hydrogen atoms, meaning every available bond on the carbon atoms is bonded to a hydrogen atom. Animal fats tend to be of the saturated type, and these fats are usually solid or semi-solid at room temperature. Tropical plant oils like coconut and palm are also high in saturated fats. When this type of fat is used to make biodiesel, the biodiesel is usable, but will cloud and gel at higher temperatures than biodiesel made from unsaturated fats such as most plant oils. Thus, this kind of biodiesel would only be usable in warm climates, because when it clouds or gels it will plug your fuel filter and fuel lines.

Unsaturated fatty acids do not have the maximum number of hydrogen atoms, because some of the carbon atoms are double-bonded to each other; each double bond takes up a bonding point that could be bound to a hydrogen atom. Oils containing mostly unsaturated fats, such as canola, safflower, or soy will produce biodiesel that has lower cloud and gel points than those made from tropical plant oils. Thus the biodiesel made from non-tropical plants is better to use in cooler climates- gee this seems to make sense doesn't it? The biodiesel made from high saturated fat oils may be slightly more stable however, plus the tropical plants usually yield much more oil per plant than other plants. Most of the biodiesel sold commercially in the US is made from soybean oil. It is not the best oil-yielding crop, but this is a whole 'nother topic that will be addressed later.

Diesel engines:
Diesel engines are different from gas engines. In a gas engine, gasoline and air are mixed together first and then injected into the cylinder where a spark emitted from a spark plug ignites the mixture. In diesel engines, the fuel is injected into the cylinder and subjected to high enough pressure to ignite the fuel with the need for a spark. Older diesels have glow plugs which add some heat and aid the combustion reaction. For more complete explanations, see this topic in Wiki pedia or in the How Stuff Works website (includes nice animation).

Benefits of Biodiesel!