That's the idea presented this morning by Stephen Jewell of Composite Energy Ltd, a UK firm. They are investigating whether it can be profitable to inject flue gases into the coal stratum underground, with the dual advantages of forcing out the methane that is found in coal deposits, which can then be burned, while depositing CO2 in its place.
He gave a number of positive points: "Coal has an affinity for CO2," he said, and can store 4-6 times as much CO2 as the methane it displaces. Generally, coal companies can only get 50 percent of the methane that occurs in coal, so this will help them get resources they couldn't previous obtain, at least not cost-effectively. He also said that there is plenty of unmineable coal that could now conceivably have value it didn't have before.
Meanwhile, he cited a couple of concerns for the notion, which is still under study. One is that this process would make the coal unusable in the future, since one would have to re-emit that CO2 into the atmosphere to burn it in the future (if mining technology made the coal more mineable, or if prices made it more cost-effective). To me, that's a bonus, though! Keep CO2 from the atmosphere now, keep coal from being burned later.
Now, I'm not a scientist, but the first question I had was, can they possible get enough of the flue gas into the ground to make that part of it cost effective? I would think they'd have much more effluent, and a perpetual supply of it, while the amount of methane they can get out of the ground is finite. In questioning afterward, he confirmed that at most, they could get 10-20 percent of the gas into the ground. I wish I had asked, 10-20 percent over what period of time, but I didn't.
Another drawback he cited is that the flue gases would have to be compressed to inject it into the ground, and the more they put in, the more the next portion would have to be compressed. Compression comes with an energy cost, of course. And, they don't yet know enough about coal's permeability, too, he said.
Altogether, the proposal had some interesting notions, but I'm skeptical how soon we'll see it in widespread use.