Greenhouse gas emissions per kg of fossil fuel

January 21, 2008 10:49 PM

Given that, say, 100kg of fossil fuel is used in the making of a certain consumer product, I am interested in knowing - roughly - how much greenhouse gas emissions this might produce. Anyone know off-hand?

Specifically I am interested in these two metrics:
- Emissions of greenhouse gases (e.g. carbon dioxide, methane etc.) [tonnes]
- Emissions of other air pollutants (e.g. sulphur dioxide, nitrous oxide, etc.) [tonnes]

This is for an eco-centric grant application for a non-profit I volunteer with which promotes re-use of this product (computers, if you're curious) in order to extend its lifetime. I'm trying to come up with a rough estimate of how much GHG emission reduction we might reasonably be able to take credit for.

That 100kg figure is an estimate based on a bunch of contributing factors including direct energy usage in manufacturing as well as "embodied" energy. The original source says:
"Although the amount of fossil fuels needed to generate electricity varies from nation to nation acording to different mix of energy sources used (e.g., more hydropower and less coal implies reduced fossil fuel use), throughout this analysis the global average is used (IEA 2002). " The source is the IEA Key World Energy Statistics, which I can find online.

I've found this pdf from the EPA that gives CO2 emissions in lbs per ton of coal, or barrel of oil, or cubic foot of natural gas, etc. I don't have a breakdown of how much of which type of fossil fuel is used. I could probably reconstruct it from the original data or from the IEA report but that may be time-consuming and I'm worried I might screw it up - this is my next step if no-one here has any ideas - but it also seems kind of like overkill. I don't need a precise figure for overall emissions; one or two significant figures is fine. The original data is not very precise in any case.

Thanks. I can provide more info if it would help.

posted by PercussivePaul to science & nature (6 comments total)

The reason they can do that for coal and natural gas is because the ratio of carbon and hydrogen are known. Anthracite is nearly pure carbon. Natural gas is methane. (Though some places it's actually a mix of methane, ethane, butane, and propane, but the chart is dealing with methane.)

But for arbitrary "fossil fuel", in particular for petroleum, it's more problematic. Gasoline has a different hydrogen fraction than diesel, which is different from jet fuel, which is different from heating oil. So it's not easy to say for generic "fossile fuel" how much carbon gets burned per unit fuel. Which fossil fuel?

When it comes to other air pollutants, it's even worse. Some coal and petroleum has a lot of sulphur; some has nearly none. But some power plants have scrubbers that take sulphur gases out of the exhaust; others don't.

The extent to which nitrogen oxides are created depends a lot on the exact nature of the combustion plant, but also on pollution controls. One of the things a catalytic converter in a car does is to break down NOx.

Likewise the emission of uncombusted hydrocarbons. That's another thing that a catalytic converter gets rid of in a car, and big power plants try to burn completely, because exhausted hydrocarbons are wasted fuel.

So unfortunately, the only real answer to your question is, "It depends." What fuel? Burned how? In what kind of facility?
posted by Steven C. Den Beste at 11:11 PM on January 21

well, which fossil fuel = the global average mix for energy production, as stated in my data source. I know the correct answer is "it depends" but even an order of magnitude guess for C02 alone would be helpful.
posted by PercussivePaul at 11:19 PM on January 21

for CO₂:

i think you should just do an average of the emissions for each fuel type, weighted by their fraction of the total electric power generation, e.g. the percentages here. however, for power generation, coal and natural gas are far and away the main fuels used, the use of petroleum etc is basically down there in the noise (2% or less). so, just add up 49% + 20% = 79%.

the weighting factor for coal is then 49/79 = .62 and for gas .38. so, of your 100 kg of fuel, roughly 62 kg is coal and 38 is methane. methane is CH₄; hydrogen weighs 1 AMU and carbon weighs 12 AMU, so of those 38 kg of methane only 12/16 or 75% is carbon = 28.5 kg. so, of your 100kg of "fossil fuel" you have 62 + 28.5 = 90.5 kg of carbon atoms. at 12 g/mol that is 7541 moles of carbon.

assuming complete combustion, each carbon atom turns into a CO₂ molecule. CO₂ has molar mass 44, so 44 * 7541 = 331 kg = 729 lbs of CO₂.

as far as other pollutants are concerned, that is, as steven says, highly dependent on the particular fuel used and the way it is converted.
posted by sergeant sandwich at 3:17 AM on January 22

Awesome sgt sandwich, thanks.
I tried out these numbers (62kg coal/38kg gas) with the EPA conversion factors I linked in that .pdf. It came out slightly less - 251kg of CO2 instead of 331kg - but I think theirs is a measurement whereas yours is an upper bound. Given that they are in the same ballpark I am confident enough to use this number. (oh also, the IEA statistic has a similar breakdown for electricity generation - not quite equivalent since it is global and not just US - but again in the ballpark).

So incidentally, would you believe manufacturing one PC accounts for about 600kg of CO2 emissions?
posted by PercussivePaul at 1:26 PM on January 22

Which PC? The tower workstation I used to own certainly must have been quite different than the budget laptop I later bought.

"Measure with a micrometer, mark with a pencil, cut with an axe." Ever hear that? If you're trying to do calculations, you should try to use the same kind of resolution all the way through. If you're using rough estimates for the amount of fuel consumed (e.g. 100 kg of undefined fossil fuels, which is a suspiciously round number) then it's unimportant whether that is 75% or 73% carbon. You're already operating in a realm where at best you're going to be within a factor of three or four of the true answer, so there's no point in trying to be more accurate than that.
posted by Steven C. Den Beste at 8:36 PM on January 22

100kg was just an example. The actual figure comes from a study in this book. It's for a desktop PC. Unfortunately the figures are a little old -- for laptops they have certainly changed -- but they are the best ones I have been able to find, and believe me I have looked very hard for them. In any case, you will notice that my answer (600kg) has only one significant figure. I appreciate the advice Steven, but it's really not necessary.
posted by PercussivePaul at 12:25 PM on January 23