Ergo Exergy - UCG

εUCG & The Environment

εUCG™ has obvious environmental advantages as a coal recovery method: there is no scarring of the earth as there is with open-cast mining; no large tracts of land are buried under overburden rock and tailings damps; there is no acid mine drainage caused by reactions of the overburden rock with atmospheric water and air.

There are also obvious advantages compared to traditional coal use: in εUCG, coal energy is produced as an easily cleanable gas. This means its use as a fuel does not involve the handling and storage of large volumes of ash and slag in ash dams that are exposed to ash leaching, and have the potential to cause soil contamination. Air and surface water pollution is significantly reduced with εUCG, whereas coal mines cause a great deal of particulate emissions during mining, coal and overburden transportation. Also, self-ignition of coal tailings and coal stockpiles discharges toxic organics into the air; coal combustion is notoriously bad at emitting particulates, SOx, NOx, mercury, and even radioactive materials; and of course, coal mining creates much more noise than εUCG.

Surface subsidence in εUCG is similar to that of underground mining, with some mitigating factors:

  • εUCG normally extracts coal in a long-wall mining fashion; therefore, the subsidence occurs in a continuous manner without creating surface fractures or potholes.
  • εUCG leaves coal ash in the underground cavity, thereby reducing free volume and the degree of subsidence. Thermal alteration of the overburden rock caused by high gasification temperatures may lead to swelling and further reduce the free volume left after coal extraction.
  • The εUCG requirement of containing gas in the coal seam during the gasification leads to gasification of rather deep coal seams with high stripping ratio. Extraction of such coal seams would normally lead to fairly limited surface subsidence.
  • Surface subsidence during εUCG operations has proven to be more gradual and less disruptive than in underground mining.

The εUCG technology is designed and tested to prevent or minimize environmental impacts on air, soil, and water (including surface streams and groundwater). For instance, the main principle of groundwater protection during εUCG is illustrated in the process simulation above. The process is conducted in such a way that gasification pressure in the gasifier is always slightly less than the hydrostatic pressure of fluid in the coal seam and surrounding strata. This creates a pressure gradient directed towards the gasifier. As a result, no flow from the gasifier into the surroundings is allowed, thereby preventing the loss of valuable product gas and averting contamination of the underground environment. The thorough characterization of existing aquifers in the vicinity of the underground gasifier and careful monitoring of the hydrostatic pressure in the aquifers during operations, forms an integral part of the εUCG groundwater protection strategy.

An example of the environmental performance of εUCG is provided by the results to date of environmental monitoring in the Chinchilla UCG project in Australia:

  • No groundwater contamination.
  • No subsidence.
  • No surface contamination
  • Controlled shutdown completed.
  • No environmental issues.
  • Annual independent audit – full compliance with Environmental Management Plan and EPA regulations

Further environmental advantages of εUCG, as applied for power generation and chemical syntheses are discussed in the sections εUCG & Power Generation and εUCG & Greenhouse Effect.

© 2015 Ergo Exergy