The Future for Ethanol


Looking at the Chemistry

Some of the advantages and disadvantages of ethanol as a fuel are apparent when we compare the equations for the combustion of ethanol and for octane, a typical hydrocarbon one is likely to find in petrol.

One mole of ethanol burns to give three moles of water, two moles of carbon dioxide, and 1409.4 kJ of heat.

The combustion scheme for ethanol

One mole of octane burns to give nine moles of water, eight moles of carbon dioxide, and 5470.5 kJ of heat.

The combustion scheme for octane

So octane generates 683.8 kJ of energy per mole of carbon dioxide generated, while ethanol generates 704.7 kJ. In fact, you may have noticed in the description of fermentation we presented earlier that the process of fermentation itself gives rise to carbon dioxide; this is captured by the ethanol manufacturers and used to prepare dry-ice for use in other industrial applications. If this is taken into account, ethanol produces only 469.8 kJ of energy for each mole of CO2 produced, which makes ethanol slightly worse off...

This may not seem like a huge difference, and is unlikely to make much of an impact on CO2 emissions, even with the huge amounts of automobile fuels that are used. Ethanol's chief benefits lie elsewhere.

Proponents of ethanol as a fuel like to point out that it is 'CO2 neutral'. Instead of taking carbon from deep underground and adding it to the total amount in circulation in the atmosphere, burning ethanol generates CO2 that was itself only removed from the atmosphere months before, by the plant that used it to make starch or sucrose.

One downside to ethanol is slightly less apparent - a mole of ethanol weighs about 46 g, while a mole of octane weighs about 130 g. So per gram, octane generates 42.0 kJ of energy, and ethanol only 30.6 kJ. This means that a much larger amount of ethanol needs to be produced and transported to achieve the same energy - and transporting 40% more fuel makes a big impact, both economically and environmentally.

This is without trying to take into account the energy costs of producing the two fuels. Each fuel takes an enormous amount of energy to produce: oil (petrol) must be drilled for, collected, transported, refined, transported again and then finally consumed; corn or other starch must be planted, fertilised or sprayed, harvested, trasnported, fermented, refined and transported again before it can be consumed. Even if waste products are used as the source of starch, it is not immediately obvious that ethanol will be ahead at the end of the race...

That's about all we can tell from the chemical equations themselves. There are a few other pros and cons that are not so obvious that are worth mentioning...