By Nancy Spring, Senior Editor
Pulverized coal combustion comes in two flavors: subcritical and supercritical.
 The coal-handling transfer tower at We Energies’ Elm Road power project. Photo, Bechtel. |
Supercritical units operate at higher temperatures and pressures than subcritical units. Higher pressures increase turbine efficiency and power output so less coal is used to produce the same amount of electricity.
Supercritical steam cycle technology has been around for decades. The first large power units with supercritical main steam pressure were built in the U.S. in the early 1960s.
Back then American Electric Power (AEP) built a series of 800 MW supercritical units—Amos, Mitchell, Big Sandy and Conesville—while Tennessee Valley Authority (TVA) commissioned a 915 MW unit at power plant Bull Run. Consolidated Edison in New York built a 1,000 MW unit.
In the early 1970s, the first 1,300 MW supercritical units were commissioned—TVA’s Cumberland and AEP’s Amos. The rated steam conditions of these units are 3,580 psi, 1,000 F. At that time, they were the largest power units in the world. Another AEP supercritical unit—Mountaineer—demonstrated a world record for continuous operation in 1987.
The 1,300 MW units were designed for baseload operation but by the 1970s had to be able to handle some load cycling. Design modifications were made to do that and also keep efficiency high, with a net result of improved unit heat rate at half-load.
By the end of 1989, 164 supercritical units were in operation for a total of 108.4 GW. Only seven supercritical units came online between 1980 and 1989, when the trend in the U.S. was once again to build subcritical units.
It’s too early to tell if the tide has turned again, but three supercritical coal plants will be ready to come online this year:
- We Energies’ Elm Road Generating Station in Wisconsin is a two-unit, 1,230 MW supercritical plant that will burn bituminous coal.
- Xcel Energy is building a 750 MW supercritical coal-fired power plant at the Comanche Generating Station in Pueblo, Colo., the third unit at the site.
- Luminant’s Oak Grove plant in Texas will consist of two supercritical, lignite-fueled power generation units. When complete, the plant will deliver about 1,600 MW.
“While most units were subcritical, we are now dominated by supercritical,” said Tom Hewson, principal, Energy Ventures Analysis (EVA). “They may cost more to build but fuel costs are less and they are more efficient. They are even more efficient than integrated gasification combined cycle (IGCC) plants.”
Elm Road Generating Station
Working for Wisconsin Energy, Bechtel is building two coal-fired steam turbine generating units and related civil works next to an existing power plant on the west shore of Lake Michigan.
The two-unit, 1,230 MW supercritical Elm Road Generating Station project is sited at Oak Creek, a brownfield site with access to cooling water, transmission and rail networks. The existing Oak Creek plant continues to operate.
Bechtel broke ground in June 2005 on the project with a scope of work that includes engineering, procurement, construction (EPC) and startup of new boilers, steam turbines, air quality control systems, new coal-handling facilities and a coal storage building. Plans also call for water treatment facilities, a deep-rock water intake tunnel under Lake Michigan and a new cooling system to supply both the existing power facility and the new power units. The first unit is on track to begin operation in 2009 and the second will follow in 2010.
“Work continues apace to complete Unit 1 of the 1,230 MW Elm Road coal-fired supercritical power plant this year,” said Alasdair Cathcart, Bechtel Project Director, Elm Road Generating Station. The project has completed the majority of the common systems required to support the start-up of the first unit, including coal, limestone and gypsum handling, raw water and waste water treatment and the various lift and pumping stations required for the once-through circulating water system, which is fed from a 27-foot-diameter tunnel that is 9,200 feet long.
Cathcart said the balanced draft boiler successfully passed its hydrostatic test last fall and chemical cleaning was completed in May. Both are precursors to first-fire on gas this summer. Lube oil flushing is complete and the condensing steam turbine and generator were placed on turning gear in early May.
“We are nearing completion on the back-end emission control systems,” said Cathcart. “Work is complete in the wet flue gas desulfurization and wet precipitator systems with our remaining effort focused on installing the catalyst in the SCR and the fabric filter bags in the baghouse.” The goal is to fire on coal early in the fall.
Hitachi provided two supercritical, pulverized-coal once-through Benson Boilers and steam turbine generators for the two 677 MW units. The steam conditions are 3,800 psia/1,050 F/1,050 F.
Elm Road is the third supercritical project awarded to Hitachi in North America, following Genesee Unit 3 in Alberta, Canada, and the Walter Scott Jr. Energy Center Unit 4 in Council Bluffs, Iowa. Hitachi is currently supplying equipment for two other supercritical coal plants, Keep Hills, a 495 MW duplicate of Genesee, and a plant in the southeastern U.S.
Elm Road will burn bituminous coal. At the station’s peak burn rate, conveyors will deliver nearly 3,000 tons of coal an hour.
Comanche Generating Station Unit 3
Construction on Xcel Energy’s first new coal-fired project in 30 years began in January 2006. The new plant is scheduled to be completed in the fall.
 Mitsubishi steam turbine at Xcel Energy’s Comanche Generating Station. Commercial operation is slated for the end of the year. Photo, Mitsubishi. |
Comanche Generating Station Unit 3, a 750 MW supercritical pulverized coal-generating unit, joins two existing sub-critical units that generate 660 MW on a site near Pueblo, Colo.
The Shaw Stone & Webster unit of The Shaw Group Inc. was awarded the EPC contract that covers balance-of-plant facilities, including the erection of the steam turbine and associated systems such as piping and pumps.
Comanche Unit 3 will burn low-sulphur Powder River Basin coal. Alstom’s TFS 2000 firing system together with a selective catalytic reduction system should keep nitrogen oxide emissions low.
Alstom will design, supply, erect and commission a high-efficiency, supercritical boiler for the unit and Mitsubishi Heavy Industries (MHI) will supply the supercritical steam turbine for the plant. The Mitsubishi TC4F-36 features one of Mitsubishi’s characteristic designs: three casings in one steam turbine, one on the high pressure side and two on the low pressure side. (The company plans to build even larger supercritical steam turbines with four casings.) High chromium grade material is used to withstand higher supercritical temperatures.
Comanche Unit 3 is Mitsubishi’s first order in the U.S. for a supercritical steam turbine, but Tetsuya (Terry) Fujino, manager, boiler engineering, Mitsubishi Power Systems Americas Inc., said Mitsubishi hopes to provide more supercritical technology to U.S. projects in the future.
Fujino said that because fuel is abundant and fairly low cost in the U.S., it hasn’t been as critical to pursue high efficiency as in Japan. “But now we’re living in a different world, so we’re hoping that we can use our high efficiency experience to make similar units in the U.S.”
First firing of the boiler is expected in July or August. Completion of commissioning and commercial operation is slated for the end of the year.
The Oak Grove Plant
Luminant’s Oak Grove supercritical coal-fired power plant is under construction about 100 miles northwest of Houston. Comprising two supercritical lignite-fueled power generation units, the plant will deliver 1,600 MW.
Oak Grove is being built at the site of a previously planned power plant where significant infrastructure is already in place, including a dedicated rail line to the lignite mine.
“The power plant is approximately 12 miles from the mine, which is owned by the power generator,” said James Brown, engineering manager on the project for Fluor, the EPC company at Oak Grove.
Brown said the main steam conditions on the 2x800 MW net lignite coal-fired supercritical cycle units are 3,550 psia with 1,000 F on the main steam heat and 1,000 F on the reheat.
The units at Oak Grove use different steam generator boiler technologies. Unit 1 includes an Alstom tangentially-fired supercritical steam generator boiler and a General Electric single reheat steam turbine. The second unit is a wall-fired Babcock & Wilcox steam generator boiler that also has a GE single reheat steam turbine.
“Basically, we’ve seen the design of two technologies and the construction and installation and now commissioning of two technologies side by side,” said Brown. “They are very different steam generator technologies with similar results.”
Oak Grove is equipped with the latest technology in air quality control systems. Luminant says the new plant will have among the lowest sulfur dioxide, nitrogen oxide and mercury emissions in the nation and will be 75 percent cleaner than the average U.S. coal plant.
The once-through cooling system uses water from a nearby reservoir. The heat sink for the turbine cycle is the Twin Oak Reservoir while any makeup water for the cycle is supplied from wells. The chemistry of the water going into the boiler is critical, said Brown. Oak Grove was designed to use an oxygenated treatment approach for the cycle water.
“It’s a once-through flow through the steam generator boiler so the water quality has to be very pure because any water you put in the steam generator boiler will go to the turbine,” Brown said. “Water quality requirements are more stringent compared to subcritical drum boiler systems.”
Unit 1 is nearing substantial completion and Unit 2 will be completed by the middle of next year, said Jim Mackey, vice president of Fluor’s power group. “We’ve hydrotested both boilers, Unit 1 and Unit 2, and we have fired on gas for Unit 1.” Plans call for Unit 1 coal firing in the next couple of months.
Research and development is underway on ultra-supercritical units that operate at even higher efficiencies, potentially up to around 50 percent. But EVA’s Tom Hewson said more work needs to be done. “At 5,500 psig, we don’t yet have the right materials,” he said.
Fluor has been involved in ultra-supercritical technology projects and one is in the design phase, said Mackey. “Ultra-supercritical technology and associated metallurgical requirements are ready for implementation in the next coal-fired units.” Whether the business opportunity exists to build them in the U.S. remains to be seen.
Defining Supercritical Steam
When the pressure in a boiler exceeds the “critical” pressure of 3,208 psi, water and steam have the same density and no longer exist as separate phase states. Steam conditions above the critical pressure are referred to as “supercritical.” Modern supercritical units operate at steam conditions above 3,900 psi/1,075 F/1,110 F, while typical steam parameters in subcritical reheat steam plants are 2,400 psi/1,000 F/1,000 F.
Supercritical steam conditions result in a net plant efficiency of up to 45 percent, an improvement of 3.5 percentage points over a subcritical unit. Less coal is used to generate the same amount of electricity. Higher plant efficiency also reduces the production of pollutants by up to 8 percent, leading to lower CO2 and mercury emissions.
Drum-type boilers are typically used in subcritical plants, where water is recirculated through the drum. But because water and steam don’t have to be separated to generate supercritical steam, once-through boilers of a Benson or Sulzer design are usually used. Water is completely evaporated in a single pass through the boiler.
Once-through boilers on supercritical units:
- Use high temperature materials such as T-91 to provide high temperature strength and improved corrosion resistance and rotor and cylinder components are upgraded to enhanced 10 percent to 12 percent chrome steels.
- Use less coal and water than drum-type boilers.
- Produce smaller volumes of fly ash and scrubber waste.
- Have a faster start-time and a higher degree of operational flexibility than a drum-type boiler.
- Use a spiral-wound furnace or incorporate vertical tubing with rifled inner surfaces to assure even heat distribution across the tubes.
- Go through more stages in the turbine and the cylinder walls are thickened.
- Require a condensate polisher and use oxygenated water chemistry to maintain steam purity. (On a subcritical boiler the drum concentrates and removes impurities.)
System processes and plant arrangements for balance of plant are the same, but a supercritical unit uses smaller diameter pipes with thicker pipe walls than a subcritical unit. Coal-handling and emissions control equipment on supercritical plants can be smaller.
Operation and maintenance costs are about the same for the two designs. Supercritical units have added costs for the condensate polisher and more sophisticated maintenance requirements but have lower costs for consumables such as limestone, ammonia and fuel due to higher efficiency levels.