Why couldn’t the coal fire in Centralia, PA be used to power a geothermal energy plant? ISTM that a lot of steam could be generated by flooding the tunnels, and that steam could be used to make electricity. (I suppose it’s barely possible that could put the fires out, but I doubt it. Even if it did, then the coal wouldn’t be going to waste.)
My guess is that it’s too dangerous. To run a power plant, you need more than just a power source - you need a reasonably stable power source. Would you build a ten million dollar power plant only to have it destroyed when the ground caves in five years later?
the fire moves and so would the heat exchanger.
it is producing deadly gases which people constructing the project would be exposed to.
It’s also burning very slowly.
Would the plant have to be built directly on top of a hot spot? Or could it be placed on stable ground and have the steam piped to it.
While the coal is burning slowly, isn’t it hot enough to boil water?
Just out of curiosity: is it burning like a candle, with visible flames? Or is it more like glowing charcoal?
Don’t you need a relatively contained source of heat to make high pressure steam? This particular coal fire has been going for so long because it’s close enough to the surface to get enough oxygen. Thus, if you dumped in a big quantity of water, the steam would seep out over a very wide area. If you managed to seal part of it enough to get useful pressure, the fires would burn out in relatively short order.
More like the latter. The heat of combustion is pyrolyzing the volatiles in the coal, which are slowly decomposing and then combusting as oxygen can get to it. Although there isn’t enough air flow to get an open flame, the collapse of damaged substrate can vent pyrolysis gas and expose the seam to oxygen, allowing it to virtual explode. Because of the combustion is removing and destabilizing material, the ground above is unstable, and although it isn’t likely to collapse akin to karst-initiated sinkholes, the ground is unstable and of course the environment is an unhealthy one for residents or workers.
Also, this is technically not geothermal energy, which is thermal energy released by compression of the crust and venting of the mantle via contact with steam or gas beds. This is just an underground fire. It should also be noted that while geothermal energy is “clean” (e.g. we don’t have to burn any fossil fuels to convert it into usable power) it is often unsustainable, as the conditions that create geothermal sources are rarely stable. For instance, the steam bed which serves as the working fluid for The Geysers, a complex of geothermal steam plants in the Northern Coastal Ranges, saw a peak production of 240 billion lbm of steam in 1987, but declined rapidly and has leveled off at 150 billion lbm of steam only with the injection of wastewater into the steam bed. Although this is probably sustainable for the foreseeable future, the need to deliver and inject wastewater detracts from net energy production, and at some point it may not be fiscally viable to maintain the steambed for energy production.
Stranger
If you created high pressure steam in the mine, couldn’t that run the risk of damaging rocks and increase the chance of collapse?
I agree with everything Stranger On A Train said, and also admire his correct use of lbm. 
double bow
Stranger
A little more ignorance fought today … thanks, Stranger.
Underground coal fires are not well understood. For example Stranger talks about volatiles in coal - however the coal being discussed is an anthracite and has very little volatiles to begin with.
Many coals (especially young coals) like lignite have their oxygen.
Water on a coal fire is tricky too - if you add too little - you just increase the thermal conductivity of coal and help spread the fire.
I drove through there a few weeks ago on our way to the Coal Mine tour in Ashland, just down the hill from Centralia. We didn’t bother to get out and look, but there was visible smoke pouring out in a few places. Besides any other issues of not working, the town/coal mine is on top of a ridge without much water so you would also have to pipe that in.