Underground Coal Gasification
Underground Coal Gasification (UCG) avoids the need for surface mining, and leaves coal combustion wastes underground. However, in pilot studies it has often resulted in water contamination. Also, because the underground combustion uselessly heats rock and groundwater, more coal must be burned to get the same amount of energy, and as a result the process produces a lot of CO2. This diagram is intended to show the basic process and associated risks of pollution; it does not show mitigation measures. UCG proposals often suggest that carbon capture and sequestration could be used to reduce greenhouse gas impact, though this has never been demonstrated to be commercially viable. Also, by siting UCG to take advantage of impermeable rock layers, contamination might be reduced, though fractures from subsidence might create perforations allowing contaminants to flow upwards.