peak coal coming, peak oil in 2010, page-8

I understand that UCG has advantages over standard coalmines plus power station on both environmental and thermal efficiency grounds;
-doing away with digging up coal, washing, transporting and grinding it for use in a standard power station probably saves around 10% of the thermal value of the coal
- UCG burns more of the coal resource than is recovered by either opencut or underground mining
- UCG causes relatively little above ground damage (some slumping might occur)
- UCG leaves the waste (silica ash, some sulphur and heavy metals)essentially in place although likely to be chemically changed and perhaps with increased or decreased mobility whereas power stations need coal washing plants, ash ponds and volatilize significant amounts of sulphur and mercury
- UCG is economically more efficient due to savings on developing and equipping mines and UCG plants are initially gas turbine plants which are cheaper to build in small units than big power stations
- the UCG syngas can potentially be stored either in the reaction space or above ground in gasometers to be used to run the gas turbine as a 'peaking' plant or to cover dips in solar/wind or other renewables, both of which are potentially high price electricity supply options and options which large thermal steam power plants do not work well at
- UCG gas substitutes in low value use (burning in big plants for electricity) for expensive natural gas (methane) which can then be freed-up for export at double or triple the price of the syngas feed to local power stations
- UCG syngas is a feedstock for chemical industries including plastics/fertilizers/fuel

On the thermal side, Carnot efficiencies are potentially higher with a combined cycle UCG (or even aboveground gasification) plant than for even a supercritical power station, partly because UCG gas makes better use of the higher temperature of combustion of gas (in a gas turbine), which subsequent temperature drops between heat supply and working fluid initial temperature (combustion gas through gas turbine --> steam through steam turbine --> ammonia through Kalina cycle turbine) being smaller than in a standard power station which loses significant effieciency in the temperature drop between buring coal and heated steam.

Above ground and UCG plants can both use Kalina cycle but fuel costs are not yet high enough to warrant their installation.
- UCG plants gasify in a confined space with little heat loss to the surrounding medium so more might go into the product stream (be interested to know more on this; could heat recovery pipes be fitted into the reaction space to capture residual heat?)

And you can grow a few trees around a UCG plant and hardly know it is there.