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Solid Energy has plans for an underground coal gasification (UCG) pilot plant in the Waikato, while L&M Energy has applied for a permit to explore the potential for gasifying coal underground over a 2000 square kilometre area stretching north and south of Hamilton.
The UCG process gasifies coal very deep underground, producing syngas that can be used in electricity generation, the production of pure hydrogen and to make a range of high-value products such as methanol, synthetic transport fuel, fertilisers or waxes, plastics and detergents. Solid Energy says it will build and commission a $22 million UCG pilot plant by early next year that will operate for up to two years. The location is on private property within the company’s existing Huntly West coal mining licence area. Work will begin once consents are granted. Solid Energy’s general manager ‘New Energy’ Brett Gamble, says the launch of the pilot plant, which will employ up to 10 people, is the culmination of five years’ investigation into UCG’s suitability for New Zealand conditions. Solid Energy is working with Ergo Exergy, a proven technology supplier who has successfully developed UCG projects in several countries around the world, he adds. “UCG technology represents a huge opportunity to further ensure our energy security by using our most ample mineral resource, coal,” Gamble says. “This is a safe, credible technology which is carried out deep below the ground yet is fully controlled by operators on the surface. “An operator pumps air hundreds of metres deep into an ignited coal seam which, when combined with the huge pressure of the earth above it, causes a reaction that turns coal into syngas. By controlling the air flow into the seam, the operator regulates the speed of the gasification reaction and, if needed, can put it out rapidly and completely.” For the pilot plant, Solid Energy plans to drill up to seven wells about 25 to 50 metres apart into an underground coal seam 400 metres below the surface. It is also installing a number of other wells on site to gather process and environmental data. The surface plant, including wells, is expected to be contained within an area measuring just 300 metres long and 150 metres wide. Solid Energy says its pilot will gasify about 35–50 tonnes of coal per day for about 18 months before being shutdown. Throughout the trial it will test how UCG affects the environment, while measuring gas quality and quantity, in adherence to environmental standards set by local authorities. This technical information will help Solid Energy make a decision on whether or not to proceed to the next stage of a small commercial operation. The project builds on an earlier UCG trial undertaken in 1994 by the then Electricity Corporation of New Zealand (ECNZ) in partnership with Glencoal Energy and Energy International. The ECNZ trials were abandoned because UCG did not prove cost-effective at that time. Increasing energy prices and further advances in UCG technology, however, make UCG a viable energy option again. L&M Energy says that while it is mostly pursuing coal seam gas, it had earmarked areas with a potential for underground coal gasification south of Hamilton. Waikato is considered a good prospective for UCG technology because of the large resource of available coal and the gas pipeline system passing through the permit area to supply Auckland. UCG resurgenceUnderground coal gasification (UCG) offers an alternative to conventional coal mining for resources otherwise not commercially viable to extract. UCG and coal seam gas (CSG) technologies both recover energy from coal without digging it from the ground but UCG converts coal into gas underground while CSG only removes naturally occurring gas from within the coal deposit. The basic process of UCG consists of drilling one well into the coal for the injection of air or oxygen to initiate underground combustion and the conversion of coal into a gas hundreds of metres underground. Another well brings the resulting gas to surface. The technique has been used in more than 15 countries during the past 70 years, including commercial operations. Recent technical advances have led to the development of new projects around the world. The coal seams have to have special properties and be between 100 and 600 meters underground with a seam thickness of more than five meters. There must be minimal discontinuities in the seam, and no good water aquifers close by. The coal itself must have ash content less than 60 percent. If the natural permeability of the coal is too low to allow the gas to pass through it, then various methods must be used to fracture the coal. A variation on the method involves drilling dedicated inseam boreholes and a moveable injection point, using technology adapted from the oil and gas industry. The first coal gasification experiments were made in the UK some 100 years ago, and industrial-scale implementation started in the old USSR during the1950s and ‘60s, but declined after the discovery of cheap natural gas resources. Likewise there was renewed European interest in UCG in the late 1950s when there was an energy shortage, that dissipated when energy prices fell again. In recent decades, the technology has enjoyed a resurgence and Australia and New Zealand have been conducting UCG trials since the 1990s and state-owned Solid Energy has been working with experts in other countries, investigating UCG’s potential for New Zealand. More recently it has got into bed with UCG technology supplier, Ergo Exergy to capitalise on earlier successful UCG trial undertaken in 1994 by the then Electricity Corporation of New Zealand (ECNZ) in partnership with Glencoal Energy and Energy International. Burning the good coalOver 20 percent of the world’s primary energy demands are currently met by coal and almost 40 percent of all electricity is generated from it. Future coal burning energy will be a lot cleaner than it has been through new technologies producing very high thermal efficiencies and far less emissions. While CO2 capture technologies have been deployed for over six decades by the oil, gas and chemical industries (producing syngas, chemicals and liquid fuels) they are relatively new to the thermal power generation industry. Although ‘washing’ coal to reduce its emissions of ash and sulphur dioxide is standard practice, while electrostatic precipitators and fabric filters usually remove 99 percent of the fly ash from flue gases and these waste products from coal combustion are used in building materials such as concrete. Flue gas desulphurisation also reduces sulphur dioxide by up to 97 percent. Newer clean coal technology aimed at commercial-scale coal power stations comes under the general title of Carbon Capture and Storage (CCS). Post-combustion capture involves separating the CO2 from other exhaust gases after combustion of the fossil fuel, similar to the method used to remove pollutants. The two CO2 capture processes are post-combustion (including oxyfuel combustion), and pre-combustion (gasification). The capture of C02 from flue gas streams after combustion in air is an expensive option that can consume up to 25 percent of the plant’s total output. One of the cheaper of these options is using oxyfuel combustion (also called oxyfiring) where the coal is burned in pure oxygen to fuel a conventional steam generator. By avoiding nitrogen going into the combustion chamber, the amount of CO2 in the power station exhaust stream is greatly concentrated, making it easier to compress and captured by amine scrubbing. Amine chemicals are the most commonly used process for post-combustion CO2 capture. The power plant’s flue gas is ‘bubbled’ through an amine solution, which bonds with the CO2 while other gases continue up through the flue. The CO2-saturated amine solution is then removed from the amines, ‘captured’ and stored. The amines chemicals are then recycled. The advantage of post-combustion technologies such as Oxyfiring is that it can be retrofitted to older coal plants. The disadvantage is that the initial process of separating oxygen from air requires a lot of energy, pushing up the electricity costs of the plant.
Energy NZ Vol.4 No.4 July-August 2010 |
Underground coal gas (UCG) technology can access very deep coal deposits that are beyond the practical reach of mechanical mining methods. It is also different from coal seam gas (CSG), which only y removes naturally occurring gas from within the coal deposit, and typically less than one percent of the energy available.