Assembling a super team
In 2002, MidAmerican Energy chose a competitively bid, turnkey approach to building Unit 4. It awarded the project's engineering/procurement/construction (EPC) contract to a team led by Mitsui and Co. Energy Development Inc. on February 2, 2003. The MidAmerican contract is reported to be the largest U.S. power plant deal ever struck by Japanese companies. Mitsui then assembled its team (Figure 2), led by Hitachi America Ltd. (HAL), which subsequently hired Sargent & Lundy as its subcontractor with responsibility for overall plant design, detailed engineering, and balance-of-plant (BOP) equipment procurement support. Hitachi Ltd. supplied the steam turbine, generator, boiler, and air quality control systems. Hitachi Ltd.'s Thermal Engineering Department provided high-level thermal design along with the power block's general arrangement.
Hitachi's supercritical boiler design experience seems to have begun when supercritical installations in the U.S. waned in the 1970s. Over the past 30-plus years, the company has refined its designs and pushed steam generator pressures and temperatures steadily upward. Hitachi manufactures the boiler at its BHK subsidiary in Kure, Japan, and the steam turbine-generator at Hitachi Works in Hitachi City, Japan.
Boiler wars
Hitachi's experience with supercritical boilers dates back to the 1970s and has been refined over the years to result in a very reliable design, as witnessed by a large network of similar operating units in Japan (Figure 3). The first 700-MW coal-fired supercritical boiler plant with turbine inlet conditions comparable to current levels began commercial operation in 1983. Steady increases in unit temperature, pressure, and efficiency over the ensuing two decades culminated in the 1995 commissioning of a supercritical (3,625 psia/1,058F/1,105F) boiler to power the 500-MW Unit 1 of Hokuriku Electric's Nanao-Ohta power plant. By 2002 this plant was operating at 100% boiler reliability on a 24-month turnaround schedule, despite firing (primarily) high-slagging imported coals.
Like EPCOR's G3, the WSEC Unit 4 derives its design from a 1,050-MW unit that Hitachi supplied for Tokyo Electric Power Co.'s Hitachi Naka plant near Hitachi City. WSEC Unit 4 has steam conditions of 3,675 psia and 1,057F/1,103F and delivers 5.5 million pounds per hour (Table 1).
The Benson sliding-pressure boiler includes a spiral-wound waterwall furnace and a double backpass convection section (Figure 4), the first of its kind in the U.S. The tubes are rifled to increase heat transfer by suppressing DNB (departure from nucleate boiling) in the subcritical-pressure region and pseudo-film boiling in the supercritical-pressure region. The lower part of the furnace has an opposed firing system. The boiler design minimizes imbalances of fluid temperatures at the furnace waterwall tube outlet, improving reliability.
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