April 14, 2005

From Plant to Fuel (and Back) 

Another point often brought up during discussions about alternative energy sources is the biodiesel. It is a fuel for diesel engines produced by chemical treatment of various vegetable oils, those less expensive and less suitable for food use.

Vegetable oils as such cannot really be used in engines, because their properties do not meet the rather strict specifications. Usually, their viscosity is too high, and their chemical stability too low. The use of raw vegetable oils in modern diesel engines will probably cause the formation of gums in the fuel injection circuit, that will need to be cleaned/replaced with considerable expenses. If not real damage to pistons and cylinders.

Vagetable oils are a mixture of triglycerides of unsaturated fatty acids, in which three long chains of carbon atoms are attached to a glycerol residue through an ester group. To produce biodiesel (on industrial scale), vegetable oils are treated with methanol at rather high temperature, eventually in presence of a basic catalyst (sodium methoxide): the products of this process are glycerine, a mixture of methyl esters of fatty acids (our biodiesel) and small amounts of impurities. The biodiesel then is washed with water and purified, and it's ready to use - many modern engines can use it without almost any modification. Almost, because there can be complications like the non-compatibility of this fuel with certain types of rubber used for gaskets. Research is also underway to improve the production methods of biodiesel. It can lso

You can even find instructions to make it at home, but I recommend against it: first, messing around with chemicals without proper knowledge, apparatus and facilities is always quite dangerous, even if you follow to the letter a safe procedure. Second, chemical processes like this are less efficient on a small scale, thus home production of biodiesel is likely to consume an awful lot of energy per liter of fuel. Also the disposal of waste from these activities must be considered.

What's interesting about biodiesel then? It is a clean fuel containing basically no sulphur (while petroleum diesel needs costly desulphurization), while it contains oxygen that can improve combustion in the engine (altough an angine needs to be properly tuned to take the full advantage of this) giving more power and less pollution - mainly, less particulate.
But the main advantage of biodiesel, at least for its fans, is that it's produced from biomass, and thus when burns it only gives back to atmosphere the carbon dioxide that was absorbed by plants to produce it. At least theoretically, because in practice one has to consider the energy spent to cultivate the plants, extract the oil and process it.

The cons of biodiesel are that, while it produces less particulate, it produces more oxygenated chemicals such as aldehydes and ketones, which can constitute a healt risk. But there are bigger cons: one is the cost, which is currently double or more than petroleum diesel - granted, if the oil price soars again, the gap will somewhat be narrower.
The main problem is that biodiesel (and any biomass source) has a low yield: a huge surface of oil crops is required to produce a modest amount of fuel. I spoke to a woman working in the field, and she told me that to replace all diesel used in Italy with biodiesel, the whole surface of Italy should be covered with oil crops... That is obviously not feasible.
The price of biodiesel is often kept low by governmental subsidies, but I think that subsidy policies are not a good idea.

All in all, I think that biodiesel is a quite good idea, but we must be realistic about it: it can cover only a small amount of the diesel oil consumption - maybe 10%, and at a price that is not that competitive even in the best cases. Let's not forget that the first task of agricolture is to provide food to humans, too.

Update: It is worth to add that any analysis of the environmental impact of biodiesel must take into account the whole lifecycle of this product. From the preparation of the fields, to growing and harvesting the plants; the oil extraction process and then the biodiesel production, distribution and use. I think that some research groups are working on processes using supercritical methanol to extract oil from oily seeds and perform the trans-esterification reaction at the same time, so it will be possible to obtain biodiesel in a single step.

I'm still convinced biodiesel (and othe biofuels) can have useful applications, especially in places with favourable local conditions, but these products will not Save the World. Only a commercially viable nuclear fusion process can come close to Save the World.

Update #2: I realized the update above does nothing more than repeating points already in the main post. Oh well, that's what happens when blogging after a couple of pints...


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