Environmental Issues Related to Underground Coal Gasification Technology - Nikhil

Environmental Issues Environmental Issues related to related to

Underground Coal Gasification Underground Coal Gasification Technology Technology

By:Nikhil Ninad Sirdesai

Part V

Coal Gasification Technology Coal Gasification Technology

• Modern coal gasifiers convert coal into syngas via partial oxidation reactions with oxygen or with steam and oxygen under elevated pressures.

• Syngas is then cleaned and burned in place of coal to make electricity

• Coal gasification - A process that burns coal in an anaerobic atmosphere forming syngas –a mixture of CO, H2, CO2 and water vapour (H2O) from coal.

•Underground Coal Gasification (UCG) - An industrial in-situ gasification process.

• Carried out in non-mined coal seams using injection of oxidants and bringing the product gas to surface through production wells drilled from the surface.

• The product gas is used as a chemical feedstock or as fuel for power generation.

Coal Gasification Technology Coal Gasification Technology

Coal gas

• Coal gas, a combustible gas - traditionally used as a source of energy for municipal lighting and heat before the advent of industrial scale production of natural gas.

• H2 obtained from gasification was used for various purposes

eg. making ammonia, upgrading fossil fuels, wtc.

• Alternatively, the coal gas can be converted into transportation fuels eg. gasoline and diesel via the Fischer-Tropsch process.

• Earlier, coal was converted to make coal gas and piped to customers to burn for illumination, heating and cooking.

• High prices of oil and natural gas have led to increased interest in "BTU Conversion“ technologies eg. gasification, methanation and liquefaction.

Coal Gasification Technology : Process

Injection of Air or Oxygen into one well &

controlled combustion reaction in the seam

itself

Drilling two wells into a coal seam

Collection of gases from the production well & separation in a facility

at the surface

Coal Gasification Technology : Types of Reactors

CRIP REACTOR LATERAL EXTENSION REACTOR

Environmental Advantages of UCG

•No mining => Less Impact on Environment as compared to Open Cast and Underground•Smaller footprint for surface facilities•Fewer particulates, NOx, SOx

•Good coincidence between sites for carbon storage and UCG•Low dust and noise•No ash handling at power stations•No coal stocking and transportation•Larger coal resource exploitation

Environmental Issues In UCG

• Migration of VOCs (Volatile Organic Compounds) in vapour phase into potable groundwater

• Organic compounds derived from coal and solubilized metals from minerals contaminating coal seam groundwater

• Upward migration of contaminated groundwater to potable aquifers due to:

Thermally-driven flow away from burn chamber Buoyancy effects from fluid density gradients resulting

from changes in dissolved solids and temperature Changes in permeability of reservoir rock due to UCG

Groundwater Contamination:

• Subsidence of the overlying terrain etc.

• Potential concern with the spread of underground coal seam fires

• Global warming impacts

Environmental Issues In UCGcontd.

Other Problems:

Groundwater Contamination

Groundwater Chemistry Before and After Pilot UCG Burn

Groundwater Contamination: • Ground water contamination - A major problem in almost all

pilot UCG projects due to no need for above ground disposal of coal combustion wastes including coal ash.

• Pollutants are left behind in the coal seam and can leach out into surrounding groundwater.

• The combustion of coal traps a variety of toxic substances viz. Mercury, Phenol & Benzene into the former coal seam.

• Solubility of heavy metals in water increases if the coal seam is fully oxidized. Pollutants can leach out through the surrounding rock or water entering the chamber.

Groundwater Contamination: • Technology to prevent contamination is in its infancy and

groundwater pollution remains the biggest concern with UCG.

• Pollution source deep underground is inaccessible and permanent.

• Problems may crop up at any time during or after the project (including decades or centuries later) & clean up will be difficult or impossible.

• Waste remaining underground presents a problem into the indefinite future, making eventual leakage likely.

Factors leading to aquifer contamination:

•Shallow coal strata close to potable aquifers selected for pilot•Burn cavity over-pressured during operation•Organic compounds volatilized and migrated out of the cavity, then condensed as liquid phases outside the burn cavity, providing a persistent source for contamination•Contaminant migration potential poorly understood at the time of the tests.

MITIGATION MEASURES

• Contamination can be prevented by proper siting - UCG plants at the place where there is no groundwater to be contaminated.

• Isolating the site from current or future groundwater sources and understanding how UCG affects the local hydrogeology.

• Regular monitoring of the groundwater around an UCG operation and take appropriate measures.

• Heating and subsidence from UCG might lead to fractures leading to change in groundwater flow

• As per the European Groundwater Directives, groundwater surrounding the UCG process is declared permanently unsuitable for other purposes like irrigation or animal consumption.

Pollutants/Contaminants• Coal tar was considered a waste and often disposed into the

environment in and around the plant locations and burnt as fuel for the boilers or dumped tar as waste.

• Commonly, waste tars were disposed of in old gas holders, adits or even mine shafts (if present).

• Waste tars degrades with phenols, benzene (and other mono- aromatics-BTEX - benzene, toluene, ethylbenzene, and xylenes) and polycyclic aromatic hydrocarbons (PAH) are released as pollutant plumes which escape into surrounding environment.

Following contaminants are commonly associated with former manufactured gas plants (MGPs): • BTEX - (Diffused out from deposits of coal/gas tars; Leaks

of carburetting oil/light oil; Leaks from drip pots used for collecting condensable hydrocarbons from the gas)

• Coal tar - (Waste/sludge typically found in sumps of gas holders/decanting ponds); Coal tar sludge has no resale value and so is always dumped.

• Volatile organic compounds • Polycyclic aromatic hydrocarbons (PAHs) - Present in coal

tar, gas tar and pitch at significant concentrations). • Heavy metals - Leaded solder, lead piping, coal ashes.• Cyanide - Purifier waste has large amounts of complex

ferrocyanides).

Pollutants/Contaminants

Pollutants/Contaminants• Lampblack - Only found where crude oil is used as

gasification feedstock.

• Tar emulsions - Coal tar and coal tar sludges are frequently denser than water and are present in the environment as a dense non-aqueous phase liquid.

Subsidence: • Subsidence occurs when the ground sinks or collapses over

an area where mining has occurred.• Subsidence is inevitable with UCG because the supporting

coal layer is being burned and removed as gases leaving only residual ash and a void.

• Most UCG projects gasify coal seams that are deep enough to be inaccessible to conventional mining and, therefore, the subsidence at the surface may be minimal and spread out.

• This deep subsidence may be subtle or invisible on the surface but could change patterns of groundwater flow.

Other Environmental Risks Even though contamination and subsidence are the most often cited problems, the other risks may involve:

•Gas Leakage: Gas leaks to the surface may occur through pre-existing faults or fractures. Risks include asphyxiation, vegetative die-off, or acidification of surface waters. Volatilization of metals and metalloids such as arsenic, mercury or selenium, if they occur, could also create toxic conditions if these volatile compounds migrate to the surface.

• Potential for Uncontrolled Reaction Rate: Potential environmental concern related to UCG is the relative lack of control on reaction rates in the subsurface. Rate of cavity growth or water influx to the burn zone cannot be controlled with existing technology.

• Spread of Underground Coal Fire: Nearby seams are susceptible to catch fire if the fracture carrying the hot air extends upto the seam.

References:• Wobus C., Ritter K., “Potential Environmental Impacts of the Proposed CIRI Underground

Coal Gasification Project, Western Cook Inlet, Alaska”, 2010, Centre for Science in Public Participation , pp1-13

• “Environmental Issues related to the Coal Gasification Technology”. A Presentation by : Dr. P. B. Rastogi, Director, MoEF at 3rd Annual International Summit–COAL ASIA-2012

• Burton E., Friedmann J., Upadhye R., “Environmental Issues related to the Coal Gasification Technology (Hoe Creek Example)”, Lawrence Livermore National Laboratory, pp 3-17

• Zamzow K., “Underground Coal Gasification“, 2010, , Centre for Science in Public Participation, pp 3-12

• Seminar Given by Sawhney P. at Meeting of Indo-US Working Group on Coal, 2006.• Ahern J., Frazier J., “Water Quality Changes at Underground Coal Gasification Sites”,

University of Wyoming• Bruining H., Wolf K., “Underground Coal Gasification”, 2007, Delft University, Dept. of

Geotechnology, pp 3-5

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