113. THE DECLINING EROEI OF COAL
If you go over to eroei.com, and look at their coal page, you'll find the following blurb, sitting in the middle of the page, with no further explanation:
While it is true that deeper mines have higher costs, it is typical of peak oilers to ignore exactly how much higher the costs are. The impression you get from this soundbite is that the EROEI of coal is declining (because the coal is getting deeper and deeper), and that at same point soon, we will reach a point where coal mining will no longer be feasible because EROEI<1.
Let's calculate, and find this point. A short-ton of coal weighs 2000pounds=907kg. To lift one ton of coal one meter requires an energy expenditure of 8,889joules (8.4btu). Also, a short-ton of coal contains about 20 million-btu (MMbtu).
So, how deep would that ton of coal have to be buried in order for EROEI to be 1 (i.e. for the energy required to lift the coal to be equal to the energy in the coal)? It would have to be so deep that raising it would require 20MMbtu.
Setting up the equation:
(8.4)D=2 x 10^7
Solving, the depth is 2.4 x 10^6 meters = 2,400km. For comparison the deepest mine on earth has a depth around 3.7km (Source).
Also, the radius of the earth is 6,378km. Thus the coal would be in a deposit 1/6 of the way through the earth, deep in the lower mantle.
Grades of coal are irrelevant to this argument. Even if we calculate with lignite (the lowest grade of coal, with a heat value of around 15MMbtu/ton), the deposit would still have to be 14% of the way through the earth, deep in the mantle.
Increasing depth does affect the decision of whether or not to strip mine. But the fact that strip mining requires too much energy does not mean that mining the coal requires too much energy. If the overburden is so high that removing it will make EROEI<1, then the answer is simple. Don't remove it. Dig a deep mine, and that will bring the EROEI back up again.
The deepest Coal Mine in the world is over 5000 feet below the ground in the UK. Many in the United States are over 1200 feet deep, most of those are closed and now few are remaining. There is a mine in Alabama, which is the deepest vertical shaft coalmine in North America, with operations at 2,140 feet beneath the surface."SourceClearly depth (and the costs imposed by it) are not a serious limiting factor. The heat produced inside the earth (and its effects on human beings) will become a limiting factor way before the energy required to lift the coal to the mine mouth. The increasing depth of coal will not decrease the EROEI of coal mining to any meaningful degree.
In the comments, anon brings up a common but invalid objection to this calculation:
This is actually not very well thought out. The calculation you perform doesn't have anything to do with finding the coal (although that is not a problem for the present), digging the mine, actually mining the coal, transporting it to the surface as you say (I don't see any friction coefficients in there, although those'd probably have to be averaged), processing it, and transporting it. Those costs are going to be gargantuan compared to the energy cost to lift it.Nice try, anon, but virtually all the costs you mention (finding the coal, actually mining the coal, processing, transporting) are costs whether the coal is 10 feet deep or 1000 feet deep. Therefore, the increasing depth of the mine does not add anything to those costs.
What you call "digging the mine" is avoided as far as possible by coal miners because it is unproductive work. The mine is dug by following the coal seam -- i.e. digging the mine and mining the coal are, for the most part, the same process.
Finally, yes, friction is a factor, but the fact remains: even including friction, the amount of energy in the coal is vastly larger than the energy needed to lift it from any real-world coal mine depth.