At the moment, New Zealand relies on fossil fuels for transport of both people and goods. Fossil fuels release fossil CO₂ into the atmosphere when burnt, which increases climate change.

World supplies of fossil fuels are falling, and their price is increasing. In response to these problems, many countries are starting to make biofuels from renewable resources such as plants. While the plants grow they remove CO₂ from the air to make starch or vegetable oils. These compounds are converted into biofuels such as biodiesel or ethanol. When these fuels are burnt they release CO₂ back into the atmosphere, but this process is simply ‘recycling’ CO₂ already in the atmosphere, rather than adding more CO₂.

To encourage the use of biofuels, the government is requiring fuel companies to sell minimum amounts of biofuels in New Zealand, starting in April 2008 at 0.53% of total petrol and diesel sold, and rising in following years. Almost certainly these biofuels will be blended at low levels with fossil fuels.

Most biodiesel around the world is made from low-grade vegetable oil, but we don’t have surplus vegetable oil in New Zealand: what we have is tallow from animal fat. The carbon chains in tallow are about the same length as those in diesel (14–18 carbon atoms in tallow, 14–21 carbon atoms in diesel).

We can’t run our trucks on bucket-loads of meat fat: even runny cooking oil is far too thick to use directly in a diesel engine. The fat molecules will need to be broken up first. However, the saponification process used to make soap does not produce a usable fuel because the glycerol makes the mixture sticky. If the glycerol formed is simply removed, the result is a solid fatty acid. Fatty acids make great candles, but you can’t run a truck engine on candles.

Carboxylic acids like stearic acid are solid at room temperature because the –OH group of one molecule hydrogen bonds with the lone pairs on the C=O of another molecule. If only we could stop them from forming hydrogen bonds.... Easy! Change the –OH into an –OCH₃ .

In biodiesel plants all over the world, chemists are taking fats or oils and reacting them with methanol (or ethanol) in a process called transesterification. The methanol breaks the COO groups in the oils in the same way that water does. The result is a mixture of esters such as methyl stearate, plus glycerol. Because the esters don’t form hydrogen bonds, they have lower melting points than their parent acids, so they are liquid at room temperature and they do not mix with the glycerol, so it is a simple matter to drain off the glycerol (which is more dense), leaving the esters behind.

As well as being a renewable fuel that does not increase the concentration of atmospheric CO₂, there are other benefits to using biodiesel.

• Most fossil diesel contains sulfur which produces acidic oxides when burnt. Biodiesel contains almost no sulfur.
• Biodiesel is a cleaner-burning fuel, because each molecule contains some oxygen. Quantities of carbon monoxide and particulates (ie particles of carbon) produced are roughly half that produced by fossil diesel. On the down side though, there is about a 10% increase in the amount of nitrogen oxides (which can cause lung damage and acid rain) produced.
• Fossil diesel contains significant amounts of aromatic hydrocarbons. These molecules are thought to cause cancer. Biodiesel contains no aromatic hydrocarbons.
• Because the ester molecules in biodiesel are polar, they are less volatile than the hydrocarbons in fossil diesel, making them less flammable and thus safer to use.
• Biodiesel is also biodegradable — in fact, fish will eat it — so spills of biodiesel will cause less damage to the environment. Even more impressive: blending biodiesel with fossil diesel makes the fossil diesel break down faster.

The very characteristics that make biodiesel biodegradable and less flammable than hydrocarbon fuels also bring the disadvantages:

• Although esters don’t hydrogen bond, they are polar molecules with stronger intermolecular forces than the equivalent hydrocarbons. In low temperatures biodiesel forms a gel which needs to be warmed before use.
• Being polar, the esters in biodiesel are more soluble in water than non-polar hydrocarbons. In theory, if the biodiesel absorbs enough water it will not perform well in an engine. This could, potentially, be a problem with biodiesel used in a marine environment (ferries for example), but trials suggest it is not likely to be an issue on land.
• A biodegradable fuel is great if it spills, but we don’t want it to degrade in a fuel tank! Fortunately, the fuel does not degrade as long as it remains in a sealed container.
• Esters are excellent solvents. In particular, they can dissolve away the rubber used in seals and tubing. Modern seals and tubes are made from solvent-resistant polymers, but this issue means that high concentrations of biodiesel cannot be used in older trucks and cars.

The biggest concerns about using biodiesel in New Zealand are to do with its performance in low temperatures. In 2007 Environment Canterbury ran 42 of its city buses on a mixture of fossil diesel and biodiesel from tallow. They found it helpful to keep the biodiesel above 20°C until it was mixed in with the fossil diesel, but other than that, had no difficulties, even with a 20% blend of biodiesel in fossil diesel.

At the moment, some New Zealand tallow is used to make soap, and the rest is exported. Converting this low-value by-product into biodiesel for use here makes much more sense than burning fuel to transport it off-shore. If all our tallow were to be converted into biodiesel, it could replace 7.5% of the fossil diesel currently used in New Zealand. 7.5% doesn’t seem like a lot, but it would be replacing 150 million litres of imported fossil fuel with a locally grown product made from waste.