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Understand Fossil Fuels

By: Dr Gareth Evans - Updated: 19 Aug 2010 | comments*Discuss
 
Fossil Fuels Coal Gas Oil Prehistoric

We all know that fossil fuels have been largely cast as the villains in the environmental piece and that their carbon contribution in particular is something to avoid – but understanding why takes many of us onto rather less familiar ground. To answer that question calls for a journey into the past – around 300 million years, to be precise – and a little basic knowledge of some fairly simple chemistry.

Put those two together and that’s pretty much all there is to the fossil fuel story.

The ‘Fossil’ in Fossil Fuels

It’s hardly going to come as news that coal, oil and natural gas all derive from the remains of prehistoric life, chemically changed over millions of years to form the stuff we know – and rely on so heavily – today. However, what you might not know is that although we often tend to lump them together, their origins are different. Coal was formed principally from the trees and ferns that once grew in swampy tropical areas during the carboniferous period (the area which eventually formed the South Wales coal field, for instance, was – bizarrely enough – slightly south of the equator at the time!). Oil and natural gas, by contrast derives from the remains of marine creatures living in shallow, prehistoric seas which then became part of the sediment when they died and it takes roughly 20 tonnes of them – plus a bit of a wait – to make just one litre of petrol.

It’s All Sunlight Really

A couple of hundred million years after all that, give or take a while, and we’re left with a useful fuel – and the reason for that lies in the chemistry and the word ‘hydrocarbon’. The whole living world is made up of hydrocarbons – complex organic molecules with a carbon framework and a large number of attached hydrogen atoms at every point along the chain. You, me, the food we eat, the fuel we use and a whole lot of things besides, we’re all basically hydrocarbons – and when hydrocarbons burn, they release a great deal of heat, which is exactly why these very energy dense fossil fuels are so popular.

Unfortunately, that carbon framework is the reason for all those ‘fossil carbon’ worries – and it is easy to see why when you remember that all that coal, oil and gas was formed from the bodies of ancient living organisms. It’s all about sunlight, in a strange sort of way, and the clever trick that plants do during photosynthesis that can trap the sun’s energy in one particularly important hydrocarbon called sugar.

OK; a spot of school-kid chemistry coming up:

Carbon dioxide + Water (in the presence of sunlight) = Sugar + Oxygen + Water.

That’s it; just like the cute meerkat says, it’s “simples!”

So Where’s the Problem?

The problem comes, obviously, when you reverse the process, which is more or less what you do by burning coal, gas or oil. You get heat energy out, but you also release the carbon that the fuel contains too. Exactly the same thing happens when you burn wood, of course, but the big difference is that it’s modern carbon that is being added to the atmosphere – only a few decades old at most, which in geological timescales is pretty much an instant return. Dinosaur carbon from way, way back, however, doesn’t belong in our 21st Century atmosphere – and certainly not in the quantities we’re belching out; it’s an unwelcome addition, and definitely not a part of the natural flux.

It’s a simple enough tale and it makes one very clear point. If you think about it, there is only one source of energy on our planet – the sun; whether by photosynthesis, or by driving the tides and the winds, all energy is ultimately solar. Even nuclear power relies on elements thrown out across space as our sun was formed. The more directly you can use that energy for yourself, the better – although nuclear is one that’s probably best left to the experts – and it’s certainly worth trying to avoid too much dependence on those multi-million year old intermediaries which, for good reason, we call fossil fuels!

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