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Underground coal gasification(UCG)/In-situ gasification (ISG)
Underground coal gasification (UCG) also known as in-situ gasification (ISG) of coal takes place underground, generally below approximately 370 m. The underground setting provides both the coal feedstock source as well as pressures comparable to that in an above-ground gasifier, and at a depth less attractive for mining owing to the cost to remove cover.
With most UCG facilities, two wells are drilled on either side of an underground coal seam. One well is used to inject air or oxygen (and sometimes steam) into the coal seam to initiate the gasification reactions. The second well is used to collect the syngas that is formed from the gasification reactions and to transport it to the surface for additional processing and use.
The UCG reactions are managed by controlling the rate of oxygen or air that is injected into the coal seam through the injection well. The gasification process can be halted by stopping the injection of the oxygen or air. After the coal is converted to syngas in a particular location, the remaining cavity (which will contain the left over ash and other solid residue from the coal) may be flushed and then flooded with water and the wells are capped. However, there is also growing interest in using these cavities to store fluids/gases captured from the above-ground syngas processing. Once a particular section of a coal seam is exhausted, new wells are drilled to initiate the gasification reaction in a different section of the coal seam.
The syngas that is produced from UCG is the same as that produced by above-ground gasification processes—it can be combusted in a gas turbine to produce electricity or further processed to produce chemicals, ultra clean transport fuels, or fertilizers.
UCG does face a number of challenges including but not limited to:
- Coal seams may not be suitable for UCG because of composition, thickness, geologic complexities or hydrologic conditions;
- Decommissioning will most usually entail injection of gases and/or fluids to minimise potential hazards of subsidence, and the sustainable and economic availability of water to flush the reaction cavity can be a limiting factor;
- Pending experience in profitable UCG projects in the context of competitive Australian energy markets, UCG project economics are somewhat uncertain; and
- Site selection needs to be done properly to avoid potentially harmful short-term and long-term impacts including but
not limited to: groundwater desiccation; groundwater contamination; surface subsidence; permanent damage to local and regional biota; and the sterilisation of land access for the multiple-use of land for other activities.
These issues can be mitigated through careful project design, site selection, and monitoring. UCG has enormouspotential for harnessing the energy of coal resources that would otherwise be too expensive or difficult to reach.
For more information on ISG see Chapter 1.2 of the "Roadmap for Unconventional Gas Projects in South Australia".
Mined coal gasification - gas to liquids
Mined coal can be gasified to create synthesis gas which is then converted to liquid hydrocarbons using the Fischer-Tropsch synthesis process. The success of the coal seam gas (CSG) industry in the US and now eastern Australia, and improvements in the economic feasibility of coal gasification have re-focused attention on the South Australian coal deposits. Assessment of the CSG and/or coal gasification potential of some deposits has commenced. In addition a Technical Feasibility Study for Coal to Methanol (CTM) plant based on the Arckaringa coal resources was completed in 2013. The Kingston lignite deposit in the south east of South Australia was assessed for use as a gasification plant feedstock, but surrendered the licence in 2014 after a review of their portfolio.
Extensive and thick Permian coal measures occur in the intracratonic Arckaringa, Pedirka and Cooper basins.
In the Arckaringa Basin seven coal deposits of lignite A/sub-bituminous C rank coal (American Society for Testing and Materials classification) aggregating more than 20 Gt of measured, indicated and inferred resource have been identified in the upper part of the Early Permian Mt Toondina Formation. These are multi-seam deposits with individual seams ranging up to 10m, with a cumulative thickness of up to 35m.
The Arckaringa Basin coals are low rank and therefore are not sufficiently mature to have generated significant thermogenic gas volumes. These coals are likely to have low gas contents unless large volumes of biogenic gas have been generated and trapped. However the coal deposits are suitable for coal conversion processing. Altona Energy has undertaken a Bankable Feasibility Study (BFS) for an integrated Coal to Liquid (CTL) plant with a co-generation power facility. The company’s pre-feasibility studies established a base case project comprising:
- An open cut mine at Wintinna producing 10 MTPA coal
- 10 MBPA liquid fuels (mainly ultra clean diesel)
- 1140 MW of power - 560 MW for delivery to the national grid
For more information on coal gasification projects see Chapter 4.2 of the "Roadmap for Conventional Gas Projects in South Australia".