Underground Combustion of Coal
In lower school science we are taught that convection currents have been used for centuries to provide ventilation in coal mining.
There are many disused coal mines that have two main points of access at different levels – one could be used for access of fresh air and the higher level for exit of combustion gases.
The hot gases can be used to drive a turbine to produce electricity. There is then no cost of mining to extract the coal and no solid waste product to dispose. The combustion gases can be cleaned on the surface.
Drift mines as in Betws, Cwmgwili and Carway Fawr locally could have the incoming air pre-heated by exhaust gases allowing higher energy efficiency:
The incoming air in conveyed to the lowest point in the mine. Combustion is initiated at this level. The hot gases will rise into the upper section of the drift tunnel where on the surface they can drive a turbine. They would then be cleaned before discharge into the atmosphere. It is envisaged that all of the above would operate at normal pressure. The level of the exit pipe for the flue gases would need to be well above the entry level for fresh air. This "head" of air of perhaps 10 metres would drive the system.
As regards the capacity of the above plant this would depend on the availability of fresh air. As quoted earlier, modern industrial combustion equipment can achieve an energy density of 600 kilowatts per square metre of air flow under normal pressure. A drift of 4 metre diameter would allow sufficient air flow for 3-4 Megawatt generation capacity. It would require a drift of about 20 metres diameter to provide 100MW capacity. The entry point of the fresh air at the bottom of the delivery pipe would mark the lowest level of combustion and the level of the hot gases collection pipe would mark the highest level of combustion.
There are hundreds of disused coal mines in Britain with an intricate network of underground roads where combustion could spread easily – the rate of combustion would be controlled simply by access of air. Initial start up would of course be problematic because of methane but this could be flushed out using nitrogen or carbon dioxide prior to ignition.
There could be some problem of very high temperatures of combustion gases because of the restricted air supply. If this was a problem, water could be added to the incoming air. It would react with the coal producing water gas –
C H² O = CO H²
This reaction is endothermic and so would reduce the temperature of the flue gases whilst generating synthetic natural gas. A more sophisticated possible design is drawn below
The above operates at atmospheric pressure. The incoming gas is fresh air with added water or steam. It is preheated in a high capacity heat exchanger by the combustion products. The pre-heated incoming air is used to drive a turbine, as it will be clean and non-corrosive. This is the main electricity producing unit. The fresh air passes down the inner pipe to the lowest combustion level. The combustion products carbon dioxide and nitrogen with carbon monoxide, hydrogen and methane will rise through the outer pipe. This will pre-heat the incoming gases. The combustion products then pass through the heat exchanger and a scrub/ clean up unit. The SNG can be used to generate electricity or sold commercially. The rate of underground combustion can be reduced by cutting the access of fresh air or can be extinguished by adding water. If there are safety concerns about mainland usage then there are coal seams up to 50 feet thick under the Southern North Sea that cannot presently be exploited.
The principles described above may also apply to secondary/tertiary recovery of crude oil and the exploitation of oil shale and tar sands. Since the rate of underground combustion will be controlled by the air supply, air will always be in short supply and the combustion temperature will be up to 2000° C if there is an abundant hydrocarbon supply.
The high combustion temperature will vaporise hydrocarbons from the rock – the exhaust gases could be fed directly into a fractionating column or into a combustion unit/ heat exchanger to produce electricity.