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Lessons From Lodi

New turbine technologies offer unprecedented flexibility.

Fortnightly Magazine - April 2010


GE, which also was in the running for the Lodi project, has introduced a new Frame 7FA gas turbine that will generate 211 MW, a 36-MW increase over the current 7FA in simple cycle operation. When configured in a rapid response two-unit combined-cycle arrangement, the company says the system will be able to deliver 150 MW to the grid in just 10 minutes.

The combined-cycle configuration, GE adds, will be able to achieve a fuel cost savings of more than $2 million a year with natural gas at $6 per MMBtu when compared to a similar plant with an earlier version of the 7FA, and avoid the annual emission of more than 19,000 metric tons of CO 2 compared to previous performance.

Key to the new turbine design, the company says, is: 1) a more efficient compressor that increases airflow; 2) enhanced fuel efficiency via three-dimensional aerodynamic airfoils; and 3) a hybrid radial compressor diffuser. Three variable stator vane stages support unit flexibility by allowing the control system to adjust compressor airflow to meet varying fuel and ambient conditions, or in response to changing operating conditions.

“Though we’re seeing slower sales overall due to the economy, the gas-fired power plant is going to be a strong segment going forward,” says Jim Donohue, gas turbine marketing manager at GE. “With the new 7FA, we’ve given customers a tool to respond to potential emissions legislation. Plus, it has a fast start capability, so it can deliver power quickly to the grid while striking a balance between efficient operations and emissions compliance.”

Alstom Power has introduced upgrades to its 193-MW, 60-Hz GT24 turbine as well. It too has focused on optimizing the unit’s compressor, increasing the airflows through it and facilitating an increase in combined- cycle performance of nearly 9 MW, by increasing the unit’s turbine inlet temperatures.

The turbine also is part of Alstom’s latest 60-Hz KA24 combined-cycle power plant offering, which is comprised of a three casing, double flow low-pressure steam turbine, air-cooled generator, and water-cooled condenser, and a three-stage reheat, horizontal type heat recovery steam generator (HRSG). The integrated system, Alstom says, will produce power with efficiencies exceeding 57 percent, with a turndown capability to near 20 percent and full start-up in less than 50 minutes.

“When we introduced the GT24 in the 1990s, the machine was ideally suited to serve the combined-cycle market,” says Mark Stevens, KA24 product manager. “The sequential combustion system provided the high exhaust temperatures needed to support combined-cycle operation at a variety of loads, which combined with three rows of inlet guide vanes supports high partial-load efficiency. When we first introduced it we were looking for good all-around performance, not just high baseload operation. From that viewpoint, it was a little ahead of its time.”

Flexible Tech

Today’s big issue, of course, is cycling. Due to changing market fundamentals, fewer gas turbine combined-cycle plants are running at full base load operation anymore. In many cases, cycling duty has become the new normal.

As a result, combined-cycle plants need to operate efficiently at low load and, when necessary,