Ram G. Narula, Bechtel Fellow; James L. Ryan , Donald J. Koza, P.E. Bechtel Power Corp., USA
In August 1999, construction started on the Millmerran coal fired power plant in Australia. The 840 MW plant’s first unit achieved commercial operation in September 2002, while its second unit achieved substantial completion in February 2003.
Figure 1. Shop fabricated double-tee grating panel detail
Situated in southwest Queensland, Millmerran is Australia’s second supercritical power plant. The A$1.5 billion ($930 million) power plant, together with its adjacent open-cut coal mine, power transmission system and water supply pipeline, is owned by Millmerran Power Partners.
Modular design and erection and creative material sourcing were major contributors to the success of the 840 MW Millmerran supercritical facility in Australia.
A supercritical steam cycle was selected by Millmerran Power Partners for its energy efficiency and environmental performance. Higher steam cycle efficiencies result in lower quantities of fuel burned per megawatt of power produced, resulting in proportionately lower emissions to the atmosphere, including CO2. The project also incorporates low NOx burners and high efficiency baghouses that remove fine fly ash from the flue gas.
InterGen, a Shell-Bechtel joint venture, owns a 54 per cent interest in Millmerran Power Partners. Other partners include Japan’s Marubeni Corporation, GE Structured Finance – a division of General Electric Corporation, the EIF Group – part of the Allianz Group – and Tohoku Electric Power Co. Inc., a Japanese power utility.
InterGen is responsible for operating the power plant. The facility is expected to be operated in baseload mode; however, it is designed for load cycling operation ranging from 30 to 100 per cent load. For improved efficiency and operation, the unit will operate in sliding pressure from 95 to 30 percent load.
Bechtel Power, a global engineering/construction company based in the US, is the turnkey contractor with responsibility for all engineering, procurement, construction, and commissioning of the power plant.
When fully operational, Millmerran will produce approximately A$237 million worth of electricity each year for sale into Australia’s newly deregulated National Electricity Market on a merchant basis. There are no power purchase agreements for the sale of electricity from the plant.
Figure 2. The Millmerran plant during commissioning
Sinclair Knight Merz (SKM) acted as the bankers’ independent engineer during the construction of the facility. The consulting firm assisted the financial advisers and lenders in the negotiation of the Engineering, Procurement and Construction (EPC) contract and other project agreements and assisted in presenting the project to international lenders and equity participants to achieve financial closure.
“Millmerran is the largest project in the world to be funded on a non-recourse project finance basis,” said managing director of InterGen (Australia) Pty Ltd., David Nelson, “and is the second largest private investment project in Queensland.” According to Bechtel’s Asia Pacific general manager for power, Bob Clipper, “that is especially significant because such privately financed projects allow the state government to direct money to other public needs rather than having to invest in power generation facilities.”
In fact, Millmerran is a project of regional significance in several different respects. Although components for the plant were sourced globally, significant local manufacturing and services were also used. “This project brought a broad range of investment and opportunity to the local community,” says Bill Harper, Bechtel’s project manager. Local resources and labour accounted for greater than A$400 million during construction and commissioning. During peak construction activity, there were approximately 1200 craft and 170 staff. During the life of the plant, according to Nelson: “Employment and wages will account for about eight per cent of the annual operating expenditure with much of the local economy benefiting indirectly from the project.”
The Millmerran Power Project is a mine-mouth, steam-electric generating station firing low-rank coal in two pulverized coal fired 420 MW (gross) units. The plant’s units operate at 241 bar/566°C/593°C steam conditions at the turbine. These cycle conditions are well proven; however, the unit size is at the low end of the commercially available range for supercritical units. A grid stability study indicated the units needed to be restricted to less than 500 MW to protect the local 330 kV power transmission and distribution system. A secondary benefit of this size was faster equipment delivery and earlier commercial operation.
Babcock & Wilcox of the US supplied the two supercritical boilers. The pressure parts were fabricated in China and Indonesia. Each boiler is a two-pass design with tube spacing to accommodate the abrasiveness of the fuel ash. It has a spiral wound furnace with ribbed tubes, and an integral boiler startup system with two steam separators feeding into a single collection tank with a circulation pump. There are two PA, FD, and ID fans and a single tri-sector regenerative air heater. Five vertical roll and race MPS-type mills feed 218 t/hr fuel to low NOx burners.
Each unit emits less than 750 mg/Nm3 NOx without an SCR system. The fuel has less than 0.5 per cent sulphur resulting in less than 2000 mg/Nm3 SO2 emis-sions without an FGD system. Particulate is collected in a single pulse-jet baghouse for each unit with Ryton felted fabric bags, resulting in less than 50 mg/Nm3 particulate emissions. Ash from the power station will be buried in the mine with the mine overburden.
Ansaldo of Italy supplied the two supercritical steam turbines. Each steam turbine is tandem compound, single reheat, condensing with a 535 MVA, 3000 r/min generator. The turbine bypass system is sized for 40 per cent steam flow. Due to unavailability of water, cooling is by an air-cooled condenser.
InterGen considered the environment to be a key consideration from the start on this project. SKM was involved in the Environmental Impact Assessment necessary for permitting the Project, which InterGen and Bechtel used in the design development. Given that the local area was suffering from a prolonged drought, the decision was made to incorporate air-cooled condensers rather than wet cooling towers. With this significant reduction in water requirements, the facility could rely solely on sewage treatment plant effluent (gray water) for plant cycle makeup and not consume other valuable water resources. This 90 km pipeline carries treated effluent to the power plant from the Wetalla Sewage Treatment Plant north of the town of Toowoomba where the water had previously been discharged into the local river system.
The plant therefore incorporates a zero discharge water treatment system with up to 5500 ML/annum makeup water consisting of treated sewage effluent supplied through a 100 km pipeline from the city of Toowoomba. This water purchase contract is worth approximately A$500 000 per year to the city. Since the site is arid most of the year, power plant wastewater and any rainwater runoff are detained for use in dust suppression and for watering the more than 9000 new trees planted to date.
Figure 3. Erection of shop fabricated stair tower
Water treatment includes an RO/EDI demineralizer system and a full-flow pre-coat type condensate polisher. An on-line condensate polisher is included to control the feedwater cationic conductivity. The boiler will operate under a combined treatment mode, with oxygen injected into the feedwater stream to control oxygen levels and ammonium hydroxide to control pH. A deaerator is included in the cycle to remove oxygen when the unit is operated under AVT chemistry with no chemical oxygen scavenging during startup/cleanup mode.
Each unit has a single 100 per cent steam turbine driven boiler feed pump with a steam turbine driven booster pump and is also provided with a 50 per cent motor driven startup pump. An 18 t/hr auxiliary boiler is provided to facilitate startup of the power plant.
The supercritical once-through boiler design eliminates the need for the thick-walled steam drum and generally uses smaller diameter tubes than does the natural circulation boiler design. This significantly redistributes loads from the typically natural circulation boiler support structure, allowing for some optimization in structural steel design. Advanced tube and pipe material has reduced wall thickness and weight of supercritical boilers and has also favourably impacted structural steel design.
For most solid fuel power projects, the boiler building structural steel erection activities lie on the critical path to boiler erection. Therefore, each project team works long hours to design, procure, detail, fabricate, and erect the structural steel in the most expeditious manner.
To mitigate the schedule, while staying within the established quantity estimates in a competitively bid, lump-sum turnkey project, the Millmerran steel design and erection team implemented modular techniques throughout the structural steel framing. The use of trusses and built-up columns eliminated the need for expensive jumbo steel shapes.
The modular approaches employed on the Millmerran power project significantly improved material cost, along with speed and safety of construction. Among the essential elements guiding design development were the frequent communication and brainstorming sessions with field personnel who would be charged with the implementation. It was realized that an awareness of local and global commercial markets, fabrication capabilities, labour practices, and so on, is required to optimize the design opportunities in any given project. Further, an environment that brings designers together with builders so that both can identify and exceed their mutual objectives is key to producing the most favourable results.
In addition, to take advantage of competitive international markets, components for the project were sourced globally, from throughout Europe, India, China, South Africa, Korea, and the US. This was possible as financing was not tied to credit from any particular country.
Market drivers behind supercritical technology
Coal is a strategic fuel for many coal-producing countries. Because it is also a major source of greenhouse gas emissions, efficient use of coal is very prudent. Supercritical boiler technology has matured and meets this challenge.
Three major drivers therefore influence the growing popularity of supercritical boiler technology:
- Fuel supply and diversity
- Plant life cycle cost
- Climate change
Fuel supply and diversity: Coal reserves are abundant and widely distributed around the world, providing an accessible and affordable energy source. Coal plays an important role in the economic and social development of many coal-producing countries, such as the US, China, India, Indonesia, Australia, South Africa, and Colombia. The abundant supply of coal maintains price stability. In comparison, natural gas reserves are smaller (about 20 per cent of coal reserves), and natural gas is not as easily transportable, making natural gas prices more volatile than those of coal. Major power producers generally deem it prudent to have a balanced portfolio of their fuel and power generation technology mix, so coal will continue to play an important role in power generation.
Plant life cycle cost: Supercritical and ultrasupercritical designs require the use of superior materials and water chemistry and can increase the plant initial cost. However, the associated improvement in plant efficiency and reduced fuel consumption more than offset the initial cost increase (except where the fuel is inexpensive). Monetary value, if assigned, to reduced emissions can further improve the economics of supercritical/ultrasupercritical design.
Climate change: While the Kyoto Treaty on global warming is controversial in terms of CO2 emissions reduction quotas for various nations, the underlying need for reduction in carbon dioxide emissions has gained widespread acceptance. Currently, direct sequestration of CO2, which includes removal of CO2 from the plant’s flue gas and permanent disposal in either underground aquifers or deep seas, is expensive. It can add as much as 3-5 ¢/kWh to the cost of electricity. So the best way to reduce CO2 emissions from burning fossil fuels is to increase plant overall efficiency. Advanced ultrasupercritical double reheat steam cycles, coupled with one or two additional regenerative feedwater heating stages, will improve plant efficiency by as much as ten percentage points and thereby reduce coal consumption and all associated plant air emissions, including CO2, by 25 to 30 per cent.
Since coal is an abundant domestic fuel source in Australia and the owners of the 840 MW Millmerran plant were committed to reducing CO2 emissions, supercritical technology was chosen for this project.