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Lessons From Lodi

New turbine technologies offer unprecedented flexibility.

Fortnightly Magazine - April 2010

ramp up quickly without negatively impacting emissions, fuel or O&M costs. That needn’t be a problem, according to the OEMs, because of these new integrated offerings.

With a traditional gas turbine-powered combined-cycle plant, the startup always, by necessity, has been gradual in order to protect the downstream steam cycle piping and components within the HRSG and the gas turbine. Start up too quickly and the components will be thermally stressed, which degrades and weakens the metals and drives up O&M costs.

Under its combined-cycle configuration, GE’s Donohue says the gas turbine is decoupled from the HRSG and steam turbine to enable a rapid response to changing load demand. Instead of bringing the entire system up slowly to avoid over-stressing metal piping and other components, the bottoming cycle hardware associated with the HRSG and steam turbine is warmed up separately.

A twin gas turbine set is brought up to full output quickly, delivering some 150 MW of electricity to the grid in about 10 minutes, compared to nearly an hour under previous combined-cycle configurations. Equally important, Donohue says, is that bringing a turbine to full output quickly cuts startup emissions by half. Therefore, he says, the configuration is ideally suited to backup duty for RPS-mandated intermittent renewable sources.

“With the HRSG and steam turbine cycle, you’ve got a lot of steel,” Donohue says. “With the system’s rapid response capability, you let the HRSG and steam turbine warm up to about 30 percent, while the GT goes right to full load. It’s done through a combination of new controls, piping and valves. The overall system efficiency remains the same.”

GE will demonstrate the new combined-cycle plant at the 586-MW Oakley Generating Station in Oakley, Calif., being developed by Radback Energy Inc. When the plant is finished, Radback will transfer ownership to PG&E.

The Siemens plant at Lodi also integrates a number of features to facilitate fast startups, including a three pressure reheat HRSG with Benson once-through technology, high capacity steam attemperation and full capacity steam bypass systems. Simply put, steam initially produced in the HRSG is bypassed to the condenser until the steam turbine is ready to accept it, and then the steam turbine is monitored for faster ramp rates. Full combined-cycle plant load, Siemens says, could be reached in as little as 45 minutes.

“It’s not that the turbine design is radically different. The difference is in the steam cycle.” says John M. Wilson, vice president of new unit sales at Siemens North America. “If you cut the gas turbine startup time, you can cut the startup emissions by well over 50 percent of a non Flex-plant, plus you have the associated fuel savings. But the real design changes are to the steam cycle in the HRSG components, like the steam valving and our T-3000 integrated control system.”

The generator is motored to bring the gas turbine up to speed ( i.e., less firing of the gas turbine than is required when using a starting motor) and the unit can generate power with a low NOx combustor to deliver NOx emissions of