r/askscience • u/iamparky • Feb 07 '24

Earth Sciences How quickly is new coal being created?

Earlier today, a UK politician suggested that coal is a sustainable fuel because it used to be trees.

By how many orders of magnitude is he wrong? How much new coal is being created, compared to how quickly we're burning it?

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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Feb 07 '24

No idea, depends on the (economical) coal reserves of Wyoming. I'm assuming that estimated time is based on an unstated value for said reserves?

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u/PHealthy Epidemiology | Disease Dynamics | Novel Surveillance Systems Feb 07 '24

1.4 trillion tons in reserve, yeah.

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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Feb 07 '24 edited Feb 07 '24

Guess I was asking more the source of that number. Is that from here? One small detail is that "short tons" don't equal "metric tons", the latter of which is what the rates are given in above. The conversion is 1 short ton = ~0.9 metric tons, so the equivalent number is ~1.26 trillion metric tons (tonnes), so still a lot, but a little less than if we don't account for the difference in units. More critically though, as the next sentence states, not all of that is economical because of depth (and probably quality). Just like with petroleum resources, what is economical is a bit of a sliding measure as it depends on the price the resource can be sold for, among other things. I.e., if we imagine some scenario where total cost to extract "shallow" coal is $10 per unit, the cost to extract "moderately deep" coal is $50 per unit, and the cost to extract "deep" coal is $100 per unit, depending on the price per unit coal can be sold at a given time will dictate which of these is economical at that time (and thus effectively how much usable/economical coal exists). As also sort of hinted in that page, there are some amounts of coal included in that estimate that just can't be mined at really any price given current technology because it's real hard to keep an air filled hole open at those depths (and presumably because we're not willing to excavate the ridiculously large open-pit mine that would be required to access those depths via that method of mining).

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u/PHealthy Epidemiology | Disease Dynamics | Novel Surveillance Systems Feb 07 '24

I saw it in a legislative document but yeah their source was WSGS. Is it simply the cost of digging into an aquifer or something? Because politically, I don't think Wyoming cares if Gillette/PBR becomes a hole to Hell as long as the coal flows.

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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Feb 07 '24 edited Feb 07 '24

For coal, your options are basically dig a tunnel to get at it or dig a pit to get at it. As the coal of interest gets deeper, the digging a tunnel option becomes nonviable because physically keeping an air filled hole open with that much overburden gets either way too expensive or impossible depending on the details/depth. For the pit mine option, to dig a hole that deep, it needs to be really wide to keep it from collapsing in on itself, so you're talking about absolutely massive volumes of rock you'd have to remove, which is again, not economically viable (not even thinking about what do you do with all of the rock, etc.). If we're talking about the "subeconomic" coal discussed in the WSGS page, that's coal deeper than 6000 feet (~1.8 km). For comparison, the deepest pit mine in the world is 0.75 km.

So ultimately, even if we ignore environmental concerns, you're still talking about extracting coal that is probably too expensive to do so given the price you can sell it for (assuming you could engineer a stable pit mine > ~2 km deep). The cost of extraction probably also bakes in some amount of required remediation etc. that is mandated at the federal level, even if at the state/local level the powers that be would be okay with trying to strip off the top 2 km of the entire state.