A Ton of CO2

Galen Bowen – Senior Mechanical Engineer

I just got back from a great vacation.  I was out all of last week (plus the weekends on either end), driving, hiking, and biking through and camping in the canyons and plateaus of southern Utah.
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We drove my 27 year old Land Cruiser, which spends most of its time burning biodiesel, but which had to be fed a lot of petroleum diesel on this trip since we were so far from home. I did manage to stuff 22 gallons of biodiesel in the tank before I left, as well as bring another 5 gallons in a gas can, but by the time I got back home, I had purchased an burned an additional 134 gallons of petroleum diesel. I do get a tad better fuel mileage with petroleum diesel, and so I was quite pleased with the fuel mileage I did attain. My rig, loaded down like the Joad-mobile from the Grapes of Wrath and with coefficient of drag of a cinder block, averaged a hair under 20 mpg, and hit 21 mpg from Cedar City to Tonopah on the way home.

But after patting myself on the back for this achievement (not real great for a Prius, but it’s all relative), I wondered just how much CO2 I had actually emancipated into nature during our 10-day outing.

134 gallons of petroleum diesel weighs, on average 422 kg, or 930 lbs., a bit under a half of ton.  But by burning up this fuel, it turns out my beast managed to spew way over a ton of CO2 into the atmosphere.  How so?

Well, again on average, each gallon of diesel fuel contains about 226 moles of carbon.  (For those whose recollection of chemistry class is a bit foggy, a mole is a huge number of atoms, specifically 6.02 x 10^23, which is, incidentally, referred to as Avagadro’s number. Since a carbon is so tiny to begin with, speaking in terms of moles rather than in terms of numbers of actual atoms brings quantities of these atoms into a realm that we can actually relate to.)  Each mole of carbon weighs roughly 12 grams. So each gallon of diesel fuel has 226 moles X 12 grams/mole = 2712 grams of carbon.

CO2 is formed in the combustion process, when each carbon atom joins forces with two oxygen atoms.  A mole of oxygen weighs around 16 grams.  So a mole of these CO2 atoms weighs 12+16+16 grams = 44 grams.

So then, if each gallon of fuel has 226 moles of carbon that are burned and converted into CO2, you wind up with 226 x 44 grams/mole = 9944 grams (21.91 lbs) of CO2 produced per gallon of fuel. So for my little excursion which consumed 134 gallons of diesel fuel, I pumped a grant total of about 2936 lbs of CO2 into our atmosphere, almost 1.5 tons.

Now, this is a simplified calculation.  The actually carbon content of diesel can vary a bit, a some carbon is emitted as carbon monoxide, unburned fuel, aromatics, and VOC’s, and thus isn’t converted into CO2.  These factors, though are relatively minor, in fact my calculation is conservative compared to the EPA’s number of 22.3 lbs of CO2 per gallon of fuel burned.

I don’t care who you are, how much concern you do or don’t have for the environment, or how concerned you are or aren’t about global warming, that’s an awful lot of CO2.

On average, burning biodiesel that is made from recycled feedstocks reduces net CO2 emissions by about 19.3 lbs per gallon.  (Even if I’d have burned soybean oil based biodiesel, it’d still be a reduction of nearly 17.3 lbs per gallon).  So if I’d burned 134 gallons of our RVO based biodiesel, rather than the petroleum diesel I bought at the pump, I’d have reduced my net CO2 emissions by 2586 pounds, well over a ton.  (I would’ve also saved about $300 as well).
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Then again … I suppose it could’ve been worse: