Biodiesel is a diesel fuel made from a non-petrochemical oil, whether animal or vegetable. Vegetable oils are by far the most common stock for biodiesel, with Soy leading the pack at least in the US, in spite of its general unsuitability.
Biodiesel has numerous advantages over ordinary petrochemical-based diesel fuel; for one, it's not a fossil fuel, meaning that we're not polluting the environment and going to war to come up with the feedstocks. Second, it burns much cleaner than petrodiesel (up to 75% cleaner1) and even improves fuel lubricity, reducing engine wear, and improves cetane rating, improving power. Actually making biodiesel is outside the scope of this article, but it can be done in your own home, and numerous companies are now selling completely automated units which make biodiesel for you.
In fact, Rudolf Diesel's vision for his Diesel engine was that it would be run on locally available fuels, making power available in remote locations and to a wide cross-section of the population. When he demonstrated it at the 1900 world's fair, he ran it on straight peanut oil, and in fact many people do run their diesel engines on straight vegetable oil (SVO) or, more commonly, waste vegetable oil (WVO). However, while biodiesel has about the same energy density (available energy per gram) as petrochemical diesel fuel, vegetable oil is down around 85% of the same energy as diesel. As a result, you get less power, and less mileage, but with a minor expenditure of energy and effort you can turn that oil into biodiesel and get all of the power you so richly deserve.
While I won't go into detail, the process for making biodiesel is fairly simple. The oil, if not already clean, is heated to cook off water, and deacidified. Most oil has some acid content, but used oil has far more than fresh oil, and the acids must be eliminated to avoid corrosion. The catalyst for the conversion is lye, which is fairly readily available at fine hardware stores everywhere. If your hardware store doesn't have it, then it's not very fine - mine doesn't either, but the one near me growing up had it, and that wasn't all that long ago. Lye is also known as "sodium hydroxide" and besides biodiesel, it's a necessary component in soapmaking, and is great at getting grease clogs out of drains (it turns them into soap.) Basically, methoxide is made from methanol and lye, then you mix that into your (clean) oil, and let the resulting mixture settle. Glycerine will settle out of the bottom, and biodiesel will float on top.
Besides the glycerine, some of the methanol comes back out of the mixture. This methanol can be reused when making the next batch of methoxide. Glycerine can be fed to plants, made into soap, or what have you, but this stuff is contaminated with methanol so the only way to get it to be very pure is to subject it to some combination of heat and agitation, preferrably while under vacuum. Putting it under a partial vacuum will lower the boiling point of the methanol, making it easier to pull out of the glycerine.
Amusingly, home-made biodiesel is usually of a higher quality than commercial biodiesel for two main reasons. The first is that most commercial biodiesel is made with "surplus" soybean oil. In the US, this surplus is generated by farm subsidies. Soy oil has a very high acid content and is among the worst oils to use for this process. Rapeseed (also known as "canola", although there is no such thing as a canola plant) is one of the best, but is seldom used because people actually want this oil, so they can cook with it. Relatively few people cook with soy oil, although it is often added to cooking oil blends purchased by the unwary or unawares.
Now, the biodiesel industry competes with several others for waste oil, including (to a very large degree) the cosmetic industry which - no lie - uses the waste fat from McDonalds and similar institutions to make lipstick and other gooey makeup. Remember that next time you smear some of that junk on your face. So, the vast majority of the oil that goes into biodiesel is fresh. What this means is that a certain amount of farming is done simply to sustain biodiesel production. This, however, is a horrible, terrible mistake. Egypt was once a green place, but agriculture turned it into desert. While modern methods of farming are less destructive than those, the simple fact is that uncovered soil blows and washes away, but it takes literally hundreds of years to create topsoil. Our quest for fuel has already led us to do serious damage to the biosphere in the name of convenience, but shifting from fossil fuels to biofuels will actually do more damage, if those biofuels are topsoil-based. Thus, if biodiesel is to be successful, it should be made only out of waste oil and from hydroponic sources, such as algae with high oil content.
Interestingly, the US government performed extensive research along these lines. A Look Back at the U.S. Department of Energy’s Aquatic Species Program details the results of this research and provides some insight into the methodology, but I can provide a quick summary for you here. The US National Renewable Energy Laboratory ("NREL"), under the US Department of Energy ("DOE") performed an eighteen year study concluding in 1996 intended to "produce renewable transportation fuels from algae" (see the report itself for this and more.) They made the following conclusions:
- That biodiesel can effectively be produced from algae;
- That the algae which should be used is that which colonizes open bodies of water naturally, and not any purpose-bred algae;
- That the most convenient and ecologically friendly way to produce biodiesel (so far) is to do it with algae;
- That it is essentially irrelevant whether you use fresh or salt water;
- That it is possible to capture up to 80% of the CO2 output of a coal or oil-fired power plant by bubbling it through algae ponds being used to produce biodiesel;
- And, finally, that this should all be cost-effective by the time diesel fuel reaches three US dollars per gallon.
The report has a great deal more to say, but the above is most definitely the meat of it. It is possible and even feasible (Especially at this point, with diesel fuel prices over four and a half US dollars per gallon!) to produce algae feedstocks for biodiesel production in the desert with seawater; the amount of the former on the planet is only growing, and likewise the amount of the planet covered with the latter is also increasing as land-bound ice melts in Alaska and Greenland.
For another fuel comparable to biodiesel, but for gasoline engines, take a look at Butanol - it can be produced from any organic material by bacteria.