Carbon Dioxide (CO2)

Wednesday 10th November, 2010
Back in the 80s, when the media started to become concerned about the environment, all concerns were directed at CFCs in fridges, acid rain and the nasty chemicals used in aerosol cans. Now we all buy happy fridge’/freezers, have catalytic converters on our cars and use stick deodorants, we have a new enemy...CO2.

CO2, aka Carbon Dioxide, is alleged to be a major cause of global warming and a major threat to the Earth. If you read the propaganda, you’d think this gas was as poisonous as cyanide and going to strike us all down dead but, of course, anybody who likes Coke will likely know that CO2 is used to make it fizzy - 2litres of Lemonade contains about 14g of CO2. All animals breathe CO2 out, we even use the stuff to put out fires but, regardless of all that, CO2 is now the enemy and high CO2 output acts as a penalty in numerous areas, particular car taxation and London’s congestion charge.

So where does it come from?

In the case of the human body, we digest our food (to produce glucose and other similar sugars) and then respire them, with oxygen, to produce CO2 and water. It’s a fairly simple formula, if you did degree level Physiology like me:

Glucose C6H12O6 + Oxygen 6O2 = Carbon Dioxide (6CO2)+ Water (6H20) + Energy

In simple terms, for every 180g of Glucose we eat, we output 264g of CO2 and 108g of water - the extra weight comes from the 192g of Oxygen we breathed in.

A plant does a similar process in reverse where it takes in 264g of CO2 and converts that, using water and UV light, to 162g of cellulose (C6H10O5) - losing weight due to the Oxygen it releases.

For petrol or diesel cars, it’s a very similar process to the human body. We burn hydrocarbon fuels, with oxygen, to liberate heat energy that makes the piston move. But which produces the most CO2? Petrol or diesel? It all comes down to which has the most carbon molecules. More carbon means that more CO2 is formed when it’s combusted.

We’ve all heard of Octane in petrol which is a standard hydrocarbon that gets it’s name from the latin for eight - it contains 8 carbon atoms. But it’s not alone in out petrol tank. The alkanes from pentane (C5H12) to octane (C8H18) are refined into petrol, the ones from nonane (C9H20) to hexadecane (C16H34) into diesel fuel. So diesel is heavier and, ultimately, contains more carbon.

Using figures from the EPA, we find that a litre of petrol contains 640g of Carbon and a litre of diesel contains around 734g. If we then completely combust these, we get respective CO2 emissions of approx 2346g & 2691g per litre or 10.666Kg & 12.233Kg per gallon. So CO2 emissions from diesel fuel are approximately 15% higher.

So why are CO2 figures for diesel cars so respectable? Partly due to better fuel consumption, simply going more miles per gallon of fuel, but there are other factors - a significant one being incomplete combustion. If you don’t burn all the fuel, then it isn’t all turned to CO2 and those emissions are lower. Uncombusted carbon is thrown out the exhaust pipe into the air in the form of soot, as a chimney sweep might call it, or particulates - as it’s now more widely known. I’ll cover diesel particulates in a future article.

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