EnviroSpin Watch

"Sir, what's biodiesel.....?"

I've been asked on a number of occasions recently to define 'biodiesel'. I thought a little 'EnviroSpin' 10-point Friday tutorial might therefore be in order. Thus: "Are you sitting comfortably? Then, I'll begin."

1. Biodiesel is a fuel that is derived primarily from plant oils, although some animal fats may be employed in warmer climates. To use it, you have either to fine-tune engines that burn petroleum-based diesel fuel (petrodiesel) or to modify the plant oil itself. The choice has been the latter, because, since the 1970s, it has been recognised that biodiesel is only likely to achieve c.10% of the petrodiesel market;

2. The basic problem with plant oils is their chemical structure, which comprises three fatty acid chains attached to a molecule of glycerin [hence the technical name for such oils: triglyceride = 3 + glycerin], which is a thick, viscous substance. The precise amount of glycerin varies by plant species, but averages around 20%;

3. The key need, therefore, is to remove the glycerin. This is achieved by a process known as transesterification [esters are simply natural compounds of oils and fats, or the organic compounds formed by mixing an acid and an alcohol]. In the process, every plant-oil molecule is broken down into three fatty acid chains and a free glycerin molecule;

4. For this to happen, an alcohol is added to the plant oil. The alcohol employed is either ethanol (made from plant grains or sugar-based crops) or methanol (made from wood, natural gas, or coal). Each of the three fatty acid chains attaches itself to one of the new alcohol molecules, forming what are termed three mono-alkyl esters. These alkyl ester chains comprise what is correctly termed biodiesel. They are thinner, and thus more usable as a diesel fuel, than the original glycerin-rich plant oil;

5. The biodiesel produced employing ethanol is referred to as ethyl esters; that produced with methanol, methyl esters. The more general term for any alcohol-derived plant-oil diesel is alkyl esters;

6. One additional factor is required in the process, however, namely a catalyst, in order to begin the reaction between the plant oil and the alcohol. The two chief catalysts chosen are sodium hydroxide (or caustic soda) (NaOH) and potassium hydroxide (KOH). [It may also prove necessary to add sulphuric acid when deriving biodiesel from waste and old cooking oils, which possess free fatty acids that result in excessive saponification, or soap-formation];

7. In general terms, the process proceeds as follows: add the correct amount of alcohol and catalyst to the plant oil. The exact amount will depend on the acidity (the pH) of the plant oil and on whether or not it contains, as with used-cooking oil, a lot of free fatty acids. At the small scale, biodiesel is then produced in batches; at the large-scale, by the continuous-flow process;

8. In a more detailed example, the correct quanitities of sodium hydroxide (the catalyst) and methanol (the alcohol) are mixed to produce sodium methoxide. This is then mixed with the chosen plant oil, and agitated (and, in some circumstances, heated). This procedure 'cracks' the oil molecules, allowing the resultant methyl esters (the biodiesel) to rise to the top of a settling tank. The heavier glycerin and remaining catalyst sink to the bottom. After a given length of time, the latter are drawn off, leaving only the biodiesel fuel. Normally, this is then washed (especially at the commercial scale) with water to remove remaining impurities;

9. Bingo: the biodiesel is ready.

10. The prime sources of plant oils are as follows:

(a) Tropical and warm climates: oil palm; coconut; Jatropha; peanuts;

(b) Temperate climates: maize (corn); mustard; oil-seed rape (canola); safflower (warm climates also); soybean; sunflower;

(c) All climates: used vegetable-based cooking oil;

(d) Future possibilities: algae (enormous potential; see also the excellent post on this by Back 40 over at Muck and Mystery: 'Bio-Fuelish');

(e) Alternative in warmer climates: animal fats may also be used, including: fish oil; poultry fat; and beef tallow. However, in cold climates, biodiesel from these sources does not work as well as plant-oil derived biodiesel.

To read more about biodiesel, I recommend: Greg Pahl 2005. Biodiesel. Growing a new energy economy. Chelsea Green: White River Junction, Vermont [ISBN 1-931498-65-2 (pbk)]. This is currently available from Amazon UK for £8.68; Amazon US for $12.24.

Philip, "Hm! Now, where can I get a nice 55-gallon drum?" Oh dear! The weather is too nice. Coffee in the garden is much better.