Redbank’s tailor-made fuel


Figure 1. Artist’s impression of the Redbank power project
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The Redbank IPP project is Australia’s first large-scale fluidized bed project. The plant will burn a fuel produced through the use of a unique process – used for the first time to help generate power – that will partially de-ash and de-water the coal tailings and turn them into fine coal particles.

In 1992, National Power Co., a California-based independent power developer was approached by a team consisting of Australia’s Ecogen and the Commonwealth Scientific and Industrial Research Organization (collectively known as the CSIRO team) about developing a project in Australia’s Hunter Valley that would use a common waste fuel – coal mining tailings – to generate power.

Hunter Valley is one of the world’s major producers of black coal. The coal is washed before it is shipped to export markets, which produces large volumes of tailings as a waste residue. Disposal methods have proven costly to local mining companies and sometimes prove controversial with environmentalists.


Figure 2. Aerial view of the power plant, Warkworth mine and associated fuel conveyor and the Hunter River water intake/discharge cross-country pipelines
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By using the tailings as a fuel, the CSIRO team felt it could convert a waste disposal problem into a useful product. Hence its decision to contact National Power Co (NP), which had an extensive background in developing power projects that burn waste fuels.

Now, roughly eight years on, the 150 MW, $200 million Redbank ‘mine mouth’ project is well under way. Located about 200 km north of Sydney at Peabody Resources’ Warkworth Mine, the plant will burn a fuel produced through the use of a unique process – used for the first time to help generate power – that will partially de-ash and de-water the coal tailings and turn them into fine coal particles.

The particles will then be burned in a circulating fluid bed boiler, making the plant Australia’s first large-scale application of fluid bed technology and one of the country’s cleanest burning solid fuel power plants.


Figure 3. Cyclone installation, Unit 2 – April 13, 2000
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The Redbank plant is being developed by independent power producer (IPP) Redbank Project Pty Ltd, a consortium of ABB Redbank Investments Pty Ltd, AIDC Power 2 Pty Ltd (a subsidiary of Babcock & Brown), and National Power Australia Holding L.P. It is scheduled to begin commercial operation in 2001 and will sell its power into Australia’s national electricity market under a long-term power purchase arrangement with EnergyAustralia, Australia’s largest electricity distributor. Power from the plant will be exported to EnergyAustralia through an interconnection facility at the existing 132 kV transmission line, which will be located on the east side of the property. In addition, the project will upgrade the 132 kV transmission line to Singleton Substation.

Alstom Power is supplying the plant’s two FI CIRC 70 MW CFB boilers, which will burn the coal washery tailings to readily meet the local emissions requirements by limiting the plant’s sulphur dioxide (SO2) emissions and nitrogen oxide (NOx) emissions. As the EPC contractor, Alstom Power will also supply the plant’s steam turbine, fuel storage and feed systems, back-up fuel systems and sand and limestone feed systems.

Upgrading the fuel


Figure 4. Redbank will inject over $10 million a year into the local economy
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The plant will be fueled by beneficiated, dewatered black coal tailings (BDT fuel) supplied by the Warkworth Mine, a large, open-pit mine managed by Peabody Resources Limited with an estimated two billion tons of coal resources. The mine currently produces more than four million tons per year of saleable coal for the Australian export market.

Almost 80 tonnes/hour of the 25 to 30 per cent moisture BDT fuel will be burned in the two boilers to produce 517 tonnes/hour of steam. The BDT fuel will be comprised of a combination of tailings coming directly from the Warkworth Mine’s coal washery and previously produced tailings that have been ponded in tailings dams (stored tailings produced from previous washery operations).

In 1995 a seam-by-seam projection of future washery tailings revealed 45 to 74 per cent ash (dry basis), resulting in HHVs of only 1600 to 4800 Btu/lb at 30 per cent moisture (CV of 4 to 11 GJ/t wet), with a mean seam HHV of 2900 Btu/lb wet (CV of 6.75 GJ/t wet) at 30 per cent moisture. The mean future washery tailings HHV were projected to be 17 per cent lower than a previous measurement made in 1993-1994.

In addition, stored dam tailings were projected to contain approximately 8.5 million tons (dry basis) of higher quality tailings from past operations at HHV of 3700 Btu/lb at 30 per cent moisture (mean CV of 8.6 GJ/t, wet) when full in mid-1997. With more, but lower quality, Warkworth washery tailings available in the future, several options to upgrade the tailing to an acceptable fuel were evaluated in late 1995 and early 1996.

Ultimately, “beneficiation” of the tailings through froth flotation utilizing a device known as the Jameson Cell – developed by Professor Graeme Jameson at the University of Newcastle – was selected as the prime candidate for removing rock and mineral content followed by a centrifuge for de-watering. This process was selected due to its simplicity, its state of development and growing acceptance by the Australian coal industry, especially in the Hunter Valley region of New South Wales.

To measure the BDT fuel’s potential as a primary fuel source, a fuel preparation test programme was defined in mid-1996 and initiated at the Warkworth washery. The test system was an integrated package of Jameson Cells, thickeners, and a horizontal belt filter. From the sub-scale tests with a horizontal belt filter for dewatering, it was determined that:

  • BTD fuel could be produced with 10 to 20 per cent ash (dry basis) and 25 to 35 per cent moisture.
  • The HHV of the produced moist BDT fuel would be 8100 to 9650 Btu/lb (CV of 18.9 to 22.4 GJ/t, wet).
  • Sulphur content of the BDT fuel was below 0.3 per cent on a wet, as-produced basis (typically below 0.4 per cent on a dry basis).
  • Nitrogen content of the BDT fuel was below 1.2 per cent wet basis (typically below 1.6 per cent on a dry basis).
  • BDT fuel yield was 27 per cent to 41 per cent from the tailings (dry solids basis), when impacts of floc over-dosage in the thickener were controlled or eliminated.
  • Estimated coal fines recovery from tailings to the BDT fuel was 75 per cent to 87 per cent (dry basis).

Fuel supply

The coal washery tailings will be beneficiated and de-watered by the mine. Tailings from the dams will also be used when the washery tailings are not available. When the project is commissioned in 2001, there will be an estimated eight million dry tons of tailings in the dams, representing a five- to seven-year fuel supply.

Thus, in addition to producing the 130 MWe of electrical power for the NSW grid, the project will mitigate the existing land-disposal environmental problem. High-ash coal will be used as backup fuel when BDT fuel is not available. The Warkworth Mine will supply both fuels.

The BDT fuel is a black, moist cake comprised of fine coal and will be transported by overland conveyor from the mine, a distance of approximately 3 km. It will be fed by variable speed progressive cavity pumps and will be combusted in the CFB boilers at a rate of nominally 57 dry (80 wet) t/h.

The fuel will be fed within the combustors through a system of spray nozzles and high-pressure progressive cavity pumps. Limestone will be added to control sulphur oxide emissions in-situ. The boilers are designed for ash loading totaling 8-10 tons/hour from the BDT fuel and up to 30 tons per hour from the backup fuel. The ash will be transported to the Warkworth Mine and blended with the barren tailings for emplacement.

The FI CIRC boilers will produce, nominally, 517 t/h of steam at 10 700 kPa(g) and 510à‚°C from 190à‚°C feedwater. This will be used to drive the single steam turbine, producing nominally, 145 MWh/h gross power. The boilers will also be designed to achieve 100 per cent Maximum Continuous Rating (MCR) when firing the backup fuel.

The balance of the plant will be typical of power plants of this size with a condensing steam turbine, three stages of regenerative heating and a surface condenser. Cooling water for the condenser and other plant auxiliaries will be provided by a wet cooling tower and associated circulating water system. Make-up water for the cooling tower will come from an intake structure at the Hunter River. The plant make-up water will be mixed with recycled water and then treated before being used in the cooling tower and other plant services.

Fuel preparation

The fuel preparation facility will have a capacity to process 1600 kdt/yr of tailings from either the washery or the new dredge on the dam. The BDT fuel production is expected to be approximately 400 kdt/yr, with a capacity of 500 kdt/yr, depending on operating time and actual fuel-from-tailings yields.

When BDT fuel is not available, high-ash coal can be delivered via the overland conveyor at higher rates of 400-900 t/h. The backup fuel will be run of mine coal from seams selected by Warkworth or, on occasion, washed or semi-washed by-product from the washery.

Consumption of the two fuels by the boilers when operating at 100 per cent MCR will typically be 75-80 wet t/h for BDT fuel, and 70 to 75 wet t/h for backup fuel for expected total annual rates of 600 000 to 670 000 wet t/y (425 000 to 475 000 dry t/y).

Boiler description

The FI CIRC combustor design typically elutriates more than 50 per cent of its incoming solids to the recirculation cyclone for enhanced particle reprocessing. In a CFB, this reprocessing means improved carbon burnout and lime sulphation. At much higher gas velocities, complete entrainment of the bed occurs.

The FI CIRC approach was developed in the early 1980s as a design for a solid fuel fired boiler that could meet California’s stringent emissions standards. Key design features include:

  • A standardized modular design whereby each ‘module’ consists of a set of repeatable mechanical equipment, including one recycle cyclone system, fuel feed, evaporator surface and air distribution system.
  • Patented flat plate directional flow tuyeres, which continuously move unfluidized particles to the gravity bed drain. This ensures optimal air introduction, fuel/air mixing and elimination of gas jetting with subsequent solids impingement on metal surfaces.
  • Pneumatic fuel guns, which positively and accurately introduce very fine-sized solid fuels under-bed, thereby enhancing fuel/air contact.
  • Removable in-bed heat exchanger tube bundles, located in the upper region of the dense bed, ensure uniform oxidation and minimize erosion.

In-bed tube bundles provide about 60 per cent of the energy input into the steam. The remaining heat transfer occurs in convective superheaters, an evaporator if required, and an economizer external to the combustor. This allows for bottom support of all equipment and permits a low profile design with maximum boiler height of no more than 45.7 m (150 ft).

A key difference between the FI CIRC design and fast circulating beds is the use of multiple small-diameter, high-efficiency cyclones to recycle a larger cut of fine particles contained in the elutriated solids. Fast beds typically use one or two large diameter cyclones, which do not have as small a cutpoint as a small-diameter cyclone.

The FI CIRC CFB controls steam rate by pneumatically removing bed solids into an external bed material silo. By dropping the bed level and exposing tube surface, the steaming rate drops while operating temperature is maintained. It is thus possible to maintain SO2, CO and HC emissions levels as low as 40 per cent MCR, since these pollutants require a constant temperature and a minimum residence time (bed depth) in an oxidizing mode to be thoroughly abated.

The CFB boilers will have inside dimensions of 6.1 m by 21.1 m and have a 6.1 m high freeboard. This configuration will provide for a conservative heat release of 21 000 kcal/hr/m2 (3512 Btu/hr/ft2). The primary fuel, BDT fuel, will be fed through 24 above-bed feed guns (12 on each side of the furnace). The BDT fuel will be delivered to the feed guns using conventional progressive cavity pumps. Each boiler will have four pumps.

The secondary or backup fuel will be fed using six conventional wall-mounted feeder stoker spreaders per furnace located 3-4 m apart. The furnace will operate at a fluidizing velocity in the bed of 1.45 m/s and at 1.82 in the freeboard. The operating point will be at 3.5 to 4.9 per cent O2 (20 to 30 per cent excess air). Because of the ability to change the bed height by removing bed material and the wide range of secondary air that is needed, the in-bed heat release will be able to be varied from 95 to 75 per cent. There will be eight recycle cyclones per furnace.

Summary

The project is strongly supported locally since the Hunter Valley is in the midst of an economic slowdown due to the recent closing of some local industries. Over 70 per cent of the Redbank project’s engineering and construction expenditures are being made in Australia, with most coming from the Hunter Valley region and New South Wales. Construction employment will peak at about 200 jobs and up to 1000 more jobs are being supported in off-site fabrication and services.

Most on-site jobs are being drawn from local contractors, and the majority of off-site jobs are in New South Wales. In commercial operation, the plant will require up to 50 operating staff, hired locally and supported by some 200 others providing ancillary services. In total, the Redbank project will inject over $10 million a year into the local economy while simultaneously reducing a difficult solid waste disposal problem.

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