Underground Coal Gasification - Environmental and Social Impacts

Environmental and Social Impacts

Eliminating mining eliminates mine safety issues. Compared to traditional coal mining and processing, the underground coal gasification eliminates surface damage and solid waste discharge, and reduces sulfur dioxide (SO₂) and nitrogen oxide (NO_x) emissions. For comparison, the ash content of UCG syngas is estimated to be approximately 10 mg/m³ compared to smoke from traditional coal burning where ash content may be up to 70 mg/m³. However, UCG operations cannot be controlled as precisely as surface gasifiers. Variables include the rate of water influx, the distribution of reactants in the gasification zone, and the growth rate of the cavity. These can only be estimated from temperature measurements, and analyzing product gas quality and quantity.

Subsidence is a common issue with all forms of extractive industry. While UCG leaves the ash behind in the cavity, the depth of the void left after UCG is typically more than other methods of coal extraction.

Underground combustion produces NO_x and SO₂ and lowers emissions, including acid rain. The process has advantages for geologic carbon storage. Combining UCG with CCS technology allows re-injecting some of the CO₂ on-site into the highly permeable rock created during the burning process, i.e. where the coal used to be. Contaminants, such as ammonia and hydrogen sulfide, can be removed from product gas at a relatively low cost.

Aquifer contamination is a potential environmental concerns. Organic and often toxic materials (such as phenol) remain in the underground chamber after gasification and, therefore, are likely to leach into ground water, absent appropriate site selection. Phenol leachate is the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. Livermore conducted a burn at Hoe Creek, Wyoming, producing operating pressure in the burn cavity greater than the surrounding rock, forcing contaminants (including the carcinogen benzene) into potable groundwater. However, some research has shown that the persistence of such substances in the water is short and that ground water recovers within two years.