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In a farm paddock behind a pine shelterbelt near Huntly a whole new gas energy industry may about to be born. By Lindsay Clark.
The Underground Coal Gasification pilot plant seven kilometres west of Huntly is due to be fired up next year and will be the first genuine test of advanced clean coal technology in the country. If results of the trial are technically successful and look commercially viable, it will offer the future possibility of a whole range of energy or chemical production. The synthetic gas or syngas that will first be produced from the Waikato coal could be used to generate electricity, produce liquid fuels, or make large qualities of hydrogen gas for use as a post-oil fuel in a future hydrogen based economy. The syngas could also be turned into various chemicals such as fertilisers, plastics, methanol, waxes and detergents. Steven Pearce, the manager of the underground coal gasification project for Solid Energy, confidently told the recent Petroleum Conference in Auckland that the coal-based UCG syngas could become a direct substitute for petroleum sourced products. And earlier at the conference Solid Energy chief executive Don Elder outlined target plans to lift production of UGC syngas to 18 petajoules by 2018 - energy equal to Kapuni natural gas field’s current production. Elder says underground coal gasification is one of a number of new energy technologies aimed at unlocking the country’s huge unconventional petroleum potential in the form of coal. Two years ago, Solid Energy produced another unconventional energy in the form of coal seam gas (CSG) at a pilot plant west of Huntly township and just a few kilometres away from the UGC project. As a technology UGC could make coal mining much more efficient.
Solid Energy’s UGC project is being developed in association with the Montreal-based company Ergo Exergy Technologies. Ergo Exergy says it is feasible to remove the carbon dioxide from the UCG gas and permanently store it in the highly-permeable zone left around a depleted underground gasifier. This would reduce CO2 emission rates from a combined UGC-Integrated Gaseous Combined Cycle electricity plant to less than that of a natural gas-fired combined-cycle generation plant. The resourceSolid Energy chose the Huntly west site was for its trial because it is in a known coal field where the company has a lot of underground mining experience. Once resource consents have been granted some six wells will be drilled on the small UGC site which is no bigger than two rugby fields. The wells will be drilled approximately 50 metres apart in a U-shaped pattern within one fault block. Because the UGC process effectively builds a gasification chamber in rock, the physical site has to have the highest environmental quality. Key issues relate to movement of groundwater and the strength of the rocks above the coal seam. UGC projects can only be undertaken well below any major aquifer to prevent unwanted chemicals from the coal combustion chamber getting into the groundwater. Solid Energy believes the Huntly West coal seam is well isolated from the major regional aquifer in the top 50 metres of rock by about 300 metre thick layer of siltstone, sandstone and claystone below. The rock above the gasification chamber – which will reach temperatures of 1200oC when operating and draw water out of the immediate rock around – is expected to heat up to 50 metres above the chamber but cause no subsidence at ground level. Six monitoring wells in and around the site have already been drilled to collect baseline data before, during, and after operation of the UGC pilot. Dr Pearce of Solid Energy says the company’s technology was originally developed in the former Soviet Union and has already been piloted in Queensland, South Africa and Canada, and is being looked at by a number of other countries. Pearce says he has visited the UGC trial project in South Africa, which is a couple of years ahead of Solid Energy’s project, that is run by the state-owned national electricity producer Eskom. The visit was “a fantastic opportunity”, he says, which enabled him to learn a lot of detailed information for application to the Kiwi project.
The UGC production process will work by igniting coal at the first well with a high temperature device after first injecting compressed air into the second well some 50 metres away. The fire will spread back towards the source of air which will flow through cleats or cracks in the coal. Over about 20 days the coal will burn back to the source of air, gasifying the coal as it goes. All the production syngas will go up the first well to the surface. The fire will gradually burn a channel three to four metre wide through the coal to the air-source well. Then the compressed air will be switched to the third well a further 50 metres away – attracting the fire to spread further toward the new air source, gasifying more coal and eventually extending the channel to all six wells. A larger width of coal will also be gasified that will test how the rock above the reaction chamber behaves when not supported by the coal which has all has burnt with only ash remaining. The primary chemical reactions that occur in the UGC reactor include oxidation, gasification and pyrolysis. Solid Energy plans to carry out some short tests using injections of pure oxygen and steam to alter chemical reactions and produce a different range of end products such as more hydrogen. For the test the syngas brought to the surface will be treated before being incinerated in a special combustion chamber which will burn the gas, hydrocarbons and any contaminated water. Tests will be carried out for about 18 months, after which the pilot plant will be shut down. Environmental monitoring of the site will carry on for some time afterwards to ensure no unexpected underground effects. If the UGC trials under New Zealand conditions prove successful, Solid Energy hopes it can move to a small-scale commercial operation.
Energy NZ Vol.4 No.6 November-December 2010 |
Some 350 metres below under thick layers of siltstone and sandstone rock Solid Energy technicians next year plan to deliberately set fire to a thick seam of coal in a pilot trial to turn the coal into energy filled gases which can be brought to the surface.
Up to 90 percent of a coalfield could be mined instead of the usual 50 percent, and most coal suitable for UGC extraction is actually not economical to mine with traditional methods, as is the case at the Huntly UGC site. As a comparatively ‘clean’ technology it is also safer and ideal for scoring greenhouse gas emission ‘brownie points’.
Once the six process wells at Huntly West well are drilled, production pipes will be placed into the holes and grouted into place to prevent any gas leakage.