Just how much oxygen is in our atmosphere? And is it imperilled by sequestering carbon dioxide from burning fossil fuels?
Firstly, how much atmosphere is there? Earth’s surface pressure is defined as 101,325 newtons/metre2 and Earth’s surface gravity is roughly 9.78 m/s2, so the mass per unit area of air is 10,332.3 kg. The average molar mass of air is 28.96 grams, so that’s 356,778 moles of air pressing down on every square metre of Earth. Earth’s surface area is 510.072 trillion m2 so there’s 182 examoles (1 examole is 1018 moles) of air, more or less. Oxygen occupies 20.9% of the volume of air, so there’s 38 examoles of it. At 380 parts per million (ppm) there’s 69 petamoles (1015) of CO2, so it’s a minor gas with a big effect.
Just how much has to be burnt to use up all the oxygen? Depends on the fuel. Some sample reactions:
C + O2 => CO2 …one mole O2 for every mole of carbon, releasing 401,155 J/mole (O2).
CH4 + 2O2 => CO2 + 2H2O …2 moles O2 for every mole of methane, releasing 393,520 J/mole (O2).
C8H18 + 12.5O2 => 8CO2 + 9H2O …one mole of octane uses 12.5 moles of O2 to fully burn, releasing 409,294.4 J/mole (O2).
H2 + 0.5O2 => H2O …half a mole of O2 for every mole of hydrogen, releasing 483,660 J/mole (O2).
Thus you can see that burning carbon is the worst performer, so let’s burn that in a worst case scenario. How much is there to burn is the really pressing question? According to new research there might only be 600 billion tons of coal left that can feasibly be extracted, though the total in the ground that’s known might be 3 trillion tons. Sounds like a lot, but how many moles is it? One mole of carbon masses 12 grams, thus 600 billion tons is 50 petamoles – all burnt up it would produce less carbon dioxide than what’s currently present in the atmosphere and would consume (50/38,000)x100% = 0.13% of the available oxygen. All the gas and oil might double that figure, thus consuming 0.26% of the world’s oxygen. But we’d be left without fuel to burn…
Oxygen supply is the least of our worries. Energy needs to come from something other than fossil fuels – carbon, methane or octane are all very finite resources. Nuclear power could replace the coal we burn for base-load power, solar and wind could provide peak-power, and cellulosic ethanol could provide liquid fuels… but they’re ALL needed in vastly higher proportions than the present in order to replace the fossil-fuels in time.
What about fusion? Here’s one hopeful news-bite…
Ultra-dense Deuterium May Be Nuclear Fuel Of The Future
…about a super-dense deuterium, 130,000 times denser than water, which might make practical laser-ignition fusion-fuel. Details about the new deuterium phase’s stability isn’t available so I don’t know if it can be stored indefinitely, or must be made continually. But it’s a hopeful development.
Another hopeful news-bite is from Richard Nebel and the team at EMC2, who are working on the fusion reactor design bequeathed to the world by the late Robert Bussard…
Interview Dr. Richard Nebel of IEC/Bussard Fusion Project by Sander Olson
…Dr. Nebel is expecting to be making commercial fusion reactors by 2020, which will be getting close to the “Peak Coal” period of c.2025 that’s expected. Might be a massive incentive to switch to fusion if coal is going to be getting more expensive as fusion starts to be available on the market. If fusion really is as cheap as hoped – no one is too sure yet – we might be able to use it to reform hydrocarbons straight from carbon dioxide and water extracted from the air. This would accelerate the downward trend on carbon dioxide in the air, hopefully returning the world to a pre-Greenhouse condition by 2100.
Now that’s a future to hope for!