Tuesday, 31 May 2011

CO2 and fuels

A friend of mine commented to me about the previous blog article and mentioned that LPG was clearly the way to go on fuels.

Certainly I agreed that it burns cleaner, but I wasn't so sure about the amounts of CO2. A quick search reveals that (just like I uncovered in my Masters Thesis):
  • few know the facts
  • everyone likes to cite things poorly
  • different sources disagree
From my own experience of using an LPG car I get higher consumption of fuel, but the lower costs of the LPG compared to petrol offset this. Typically I use about 15L/100Km of LPG in my 6cyl car which then gets about 10L/100 when run on Petrol.

Now this NRMA site suggests that the mass of CO2 gas released of the exhaust pipe by the burning of one litre of fuel is:
  • 2.3 kg for Petrol
  • 1.5 kg for LPG.
however they refer to the Australian Greenhouse Office website where I am unable to verify such a figure easily (searching with this search query). None the less, if we run with that (as it seems reasonable) we would get (based on my above ratio) that I need to burn 1.5 times more LPG than I do Petrol which would result in:

1.5 x 1.5Kg = 2.25Kg or bloody close to petrol.

So, as Julius would ask "why is it so"??

Well energy stored in the chemistry is different. LPG is normally a simple hydrocarbon.

Energy is stored in the chemistry as the strengths of the bonds, Carbon can bond to Carbon in a single or a double bond, with something like 611 kJ/mol for C=C vs. 347 kJ/mol for C-C difference.

So without many Carbon Carbon double bonds LPG will not be able to store as much energy among the carbon and hydrogen in the compound as would something more complex (like petrol). Don't you wish you'd listened in chemistry more?

So to get the same power (you still want to drive at the same speed don't you?) then you simply must burn more LPG, thus the choice of LPG or Petrol would seem more or less CO2 neutral.

However, if you think about it, we burn petrol (or LPG) to get energy to power our engines. So if you have a choice to burn 2 butane molecules

or one benzene molecule

We would get more CO2 from the butane (because we'd have 8 carbon atoms) and we would get both more energy from the Benzene and less CO2, because it only has 6 carbon atoms and all the double bonds.

This is not an accident, chemists in the early 20th century poured over chemical understandings in order to get the most energy in the most compact form. So when you burn petrol you're burning stuff like:

and some of this

with a little of this tossed in to add to the energy per liter of fuel

The advantages however of LPG are not that its a compact energy storage, or that it gives us less CO2, rather its in other aspects. To quote from Wikipedia:
Two recent studies have examined LPG-fuel-oil fuel mixes and found that smoke emissions and fuel consumption are reduced
an advantage is that it is non-toxic, non-corrosive and free of tetra-ethyl lead or any additives
It burns more cleanly than petrol or fuel-oil and is especially free of the particulates from the latter

so, its not to say that I think LPG isn't a good fuel (in fact I've chosen it for my car) but just that I don't want people to be thinking that somehow its going to save the global CO2 problems (if you think we have one).


Charles Maclauchlan said...

well said.

Anonymous said...

I tought that the actual CO2 release may even be of little interest when comparing different fuels. Since the other greenhouse gases(CO, NOx, methane...) that burning fossil fuels creates are more harmful the comparison is normally done with the figures where all of them have been converted to CO2 equals.


obakesan said...

I would agree, and if not for the ignorance being expressed in the market place hailing LPG as CO2 neutral I'd not have bothered writing this.

the other points you raise are covered in the article