Coal, Gas

Added Operational Flexibility Would Boost Combined-Cycle Profits

Issue 9 and Volume 107.

Gas turbines, especially in combined-cycle configurations, typically provide high efficiencies and low emissions, particularly under ideal temperature and humidity conditions. EPRI is currently investigating emerging CT technologies that have the ability to improve the operational flexibility of gas turbine power plants. The goal is to lower fuel consumption and operating costs. In light of rising natural gas prices, such flexibility improvements could provide significant competitive benefits to gas turbine facilities.

Among the technologies that will be investigated are inlet air conditioning, inter-stage cooling, advanced steam injection, supercharging, humid air injection, improved parts by after-market suppliers, and performance-based maintenance. Both peaking and combined-cycle technologies will be evaluated. The cost of a system for a peaking application is dependent on a number of variables but the lowest cost/benefit ratios found in experience and/or simulations are summarized for a large highly efficient combustion turbine such as the GE 7FA (Table). Similar results for combined cycles are under study and will be completed by the end of the year.

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Fern Engineering’s Philip Levine, principal investigator for the project, believes the study will address some of the biggest concerns gas turbine owners have today. “The issues are increased efficiency to deal with the volatility of fuel costs, the potential for greenhouse gas caps, and improvements in O&M to improve performance and reduce costs,” he says. “All of these issues are being addressed by looking at these technologies. We’re looking at the high efficiency F-class CT power plants.”

The dilemma facing combined-cycle plant operators is especially frustrating because when the ambient temperatures rise, gas turbine output goes down — just at the very time when the greatest demand and profit potential exists. If the flexibility of combined-cycle plants can be enhanced, the potential for high profitability can be increased.

At a site such as Miami, power output can drop as much as 20 percent between 30 and 100 F. That means a combined cycle generating 250 MW in cold weather can lose 50 MW in capacity in high summer temperatures. “So you have to look elsewhere for that 50 MW, which means you’re burning 50 percent more fuel if it has to come from a simple-cycle machine,” says Levine.

He says trying to recover that loss using only one approach in a method is not likely without overstretching the technology. “So we’re looking at the combinations — hybrids that are less dependent on weather conditions that you can count on to achieve high efficiency.”

The potential of hybrid combinations are site dependent. For example at a site such as Miami, units with inlet chilling might be supplemented with another technology using mass injection or supercharging. The operating flexibility needs at other sites with markedly different climatic conditions may be best suited for other technologies.

“Some of these technologies are mature but improvements would be helpful, while others have been demonstrated but are not in commercial use “says Leonard Angelo, EPRI’s manager of combustion turbine technology. “We want to demonstrate them and then place them into commercial operation. When the operational flexibility study is completed in December, we will be looking for a company to host a demonstration of these technologies or a combination of them.”