Mar 2008

Carbon dioxide emissions per barrel of crude

Thanks to a disastrous decision to rely upon data published by BP, I royally cocked up a calculation involving their Miller Field carbon capture project (see previous post). However, that calculation was preceeded by — in my view — a more important one; an estimate of the amount of atmospheric CO2 emitted by a single barrel of crude oil. Thanks to the kerfuffle surrounding the error in scaling (an error, let me again stress, based upon some bad data from BP), the original calculation is being rather obscured.

CO2 emissions

1 litre: 2.331kg of CO2
1 US gallon: 8.824kg of CO2

1 litre: 2.772kg of CO2
1 US gallon: 10.493kg of CO2

Crude oil:
1 barrel: 317kg of CO2 (min.)

1 tonne of CO2 is:

429 litres / 113.33 gal of petrol*
360.75 litres / 95.3 gal of diesel
3.15 barrels of crude oil

* less than 8 fills of an average-sized car with a 55 litre tank

In the (probably vain) hope of rescuing it from that obscurity, and due in no small part to a comment on the previous entry which suggests to me that it’s still a useful piece of information, I’ve decided to reproduce it here in isolation. I invite comment and correction, as always.

How much carbon per barrel?

First up, it’s important to realise that crude oil is (almost) never used directly. Instead it’s refined into a wide range of products, most of which we burn in various engines, but some of which never get converted into CO2 (lubricant oils, plastics, asphalt, etc.). Different grades of crude oil will produce significantly different amounts of each. So a barrel of light / sweet crude might produce lots of petrol and kerosene but only a small amount of asphalt (as a very simple example). But a barrel of heavy / sour crude would produce more asphalt (still less than the amount of petrol produced, but more in comparison with the sweeter oil). This means that, ironically, less of the heavier and more sulphuric stuff, although it’s called sour (and sometimes “dirty”) oil tends to end up as atmospheric CO2 (we coat our roads with it instead).

So while we could, no doubt, work out a figure for the CO2 emitted by burning a given barrel of crude oil, it would be very much a red-herring. To get any meaningful figure for CO2 per barrel we’re going to need to do our calculations on the products of crude oil.

It makes sense to perform this calculation on oil that is of average quality (i.e. not some kind of heavy sulphuric sludge or tar-sand) to make it more generally useful. So taking Riegel’s Handbook of Industrial Chemistry as our guide, we know that the average barrel (~159 litres) of crude oil to pass through U.S. refineries in 1995* yielded the following products:

1. Gasoline: 44.1% (70.12 litres)
2. Distillate fuel oil: 20.8% (33.07 litres)
3. Kerosene-type jet fuel: 9.3% (14.79 litres)
4. Residual fuel oil: 5.2% (8.27 litres)**

Percentage values from Riegel’s Handbook of Industrial Chemistry, 2003 edition (Page 515, Fig. 15.6). Litre values based upon conversion rate of 159 litres per barrel.

All of the other products*** of refined crude have sufficient alternative uses to make it possible (even if not entirely probable) that they will not end up as atmospheric CO2. Of the four grades of fuel listed above, however, it’s fair to say all of it is destined to be burnt. It’s worth noting, therefore, that our final result will represent a minimum CO2 per barrel.

Now, the litre values are no good to us by themselves. Each of the fuels has a different specific gravity (a different weight per litre), and it’s the weight of carbon we’re looking for, not the volume. Once we’ve multiplied the volume of each fuel by the relevant specific gravity we’ll have a rough “kilogram per barrel” number for each fuel. So:

1. Gasoline: 70.12 litres x 0.74 = 51.89kg
2. Distillate fuel oil: 33.07 litres x 0.88 = 29.10kg
3. Kerosene-type jet fuel: 14.79 litres x 0.82 = 12.13kg
4. Residual fuel oil: 8.27 litres x 0.92 = 7.61kg****

Overall, this suggests that the average barrel of crude refined in the United States in 1995 yielded a shade over 100kg of liquid fuels (that’s an uncannily round number… 100.73kg to be exact). Now, we know that a carbon-based fuel will emit 3.15 times its own weight in CO2 when burnt.

When fuel oil is burned, it is converted to carbon dioxide and water vapour. Combustion of one kilogram of fuel oil yields 3.15 kilograms of carbon dioxide gas. Carbon dioxide emissions are therefore 3.15 times the mass of fuel burned.

Calculating the Environmental Impact of Aviation Emissions, Oxford University Study (PDF file)

This may seem anti-intuitive at first glance, but it’s a result of each atom of carbon reacting with two atoms of oxygen to produce CO2. The “extra” weight is being drawn from the air (hence why a fuel fire will die out if deprived of oxygen).

Using the 3.15 multiplier, we see that the combined liquid fuels from an average barrel of crude oil will produce a minimum of 317kg of CO2 when consumed.

* I don’t have more recent numbers, but there’s no reason to assume 1995 wasn’t a representative year.

** 1: automobile grade fuel. 2: includes home heating oil and transportation diesel. 4: industrial grade fuel oils; used in ships and oil-burning power plants.

*** Still gas, coke, asphalt, road oil, petrochemical feed stocks, lubricants, etc.

**** Specific gravities taken from this list. The value of 0.92 is an educated guess for what is a mixture of heavy oils with a range of specific gravities. I will gladly accept correction if someone can point me towards a more accurate number.

Posted in: Opinion