The Physics Police

The Physics Police

Thursday, July 11, 2013

Methane vs. Coal

Josh Fox, creator of the film Gassland, opposes fracking for many reasons. One reason is the danger that leaked or "fugitive" methane emissions will contribute to global warming. He cites the fact that methane is a more potent greenhouse gas than is carbon dioxide. This is true, but he exaggerates to make his (incorrect) point:
Methane is a super greenhouse gas. It's 100 times more potent than CO2. (Source)
First of all, the ratio of the radiative efficiency of methane to that of carbon dioxide is:
(3.7 * 10-4) / (1.4 * 10-5) = 26.4
So, it's really only 26 and a half times as "potent" as carbon dioxide.

But that doesn't even take into account atmospheric lifetime. Methane leaves the atmosphere after just 12 years, compared to the century long lifetime of carbon dioxide. So, depending on how long of a view you take, the climate forcing from some mass of methane, compared with the same mass of carbon dioxide, changes.

As you can see, taking the long view of 500+ years, methane is only 7.6 times as strong a greenhouse gas. This is the number that the planet cares about. But the 100-year time horizon is more often used by climate scientists.

Fox himself admits that methane burns "50% cleaner" (18:40) than coal; and by that, he means it produces 50% less carbon dioxide per kilowatt.

Just for fun, let's calculate this ourselves!

The molar mass of methane is 16.04 grams per mole. Each molecule of methane has one carbon atom, which, after burning, becomes part of one carbon dioxide molecule. Carbon dioxide has a molar mass of 44.01 grams per mole.

So, 16 tonnes of methane burns to create 44 tonnes of carbon dioxide. Wow, how can the waste weigh more than the fuel? Because the carbon binds to two oxygen atoms, the resulting molecule is much heavier!

How about coal? 2,000 pounds of coal will generate about 5,720 pounds of carbon dioxide. (Source)

That leaves fuel efficiency. In current power plants, 1.07 pounds of coal produces the same amount of electricity as 226 liters of natural gas. (Source)

Annoyingly, the units aren't the same, so we have to convert to pounds of natural gas:
226 liters * (0.6556 grams per liter) = 0.327 pounds
So, the ratio of carbon dioxide output from methane compared with coal is:
(0.327 * (44.01 / 16.04)) / (1.07 * (5720 / 2000)) = 0.293
The next question is leakage. The highest estimate for methane leakage is 5.75%, even though the EPA estimate is only 2.3%.

If, like most climate scientists, we use the 100-year global warming potential for methane, equal to 25, we can now calculate the global warming potential of emissions from methane compared to coal:
(100% + (5.75% * 25)) * 0.293 = 71.4%
So, Josh Fox has no right to oppose fracking on the grounds that it contributes to global warming, when you compare it to the current alternative, coal.

I will be sure to respond to his other reasons for opposing fracking once I've watched Gassland.

In any case, fracking is the lesser of two evils, in terms of carbon dioxide emissions.

But remember kids, the only good carbon dioxide molecule is a sequestered carbon dioxide molecule.

Unless you're a plant.

1 comment:

  1. Thanks. It's always nice when someone does the math for me. I usually get bored after the first half of an equation... ;)