High Operational Flexibility and Low Emissions

Gissi plant

A power plant’s operational flexibility is a crucial factor for its commercial success in a dynamic market environment. There is a much higher demand for flexibility regarding fuel composition, and plants are expected to manage operations between the two extremes of daily start and stop and base-load operation. Alstom tapped into this need by introducing a new concept in 2007 called low-load operation. The low-load operation concept (LLOC) enables operators to run a gas turbine power plant at reasonable plant efficiency levels with very low power output levels, while maintaining low exhaust emissions and keeping the water-steam cycle running.

The basis for the concept was developed almost 60 years earlier, when the sequential combustion principle entered the market using the reheat principle for gas turbines to increase efficiency at low turbine inlet temperature levels. In the mid-1990s, Alstom introduced two similar sequential combustion gas turbines: the GT24 for the 60 Hz market and the GT26 for the 50 Hz market. Since their launch in 1995, these advanced class GT24/GT26 engines have demonstrated significant advantages for a plant operator.

“Our GT24 and GT26 advanced technology platform offers superior operational flexibility, low emissions, and high part-load efficiency combined with world-class levels of reliability and availability,” said Jürg Schmidli, vice president of Gas Turbines, Alstom Power. “We integrate sequential combustion in a single shaft engine. A low NOx emission environmental (EV) burner in an annular combustor is followed by the sequential environmental (SEV) burner in the second combustion stage. Our GT24/GT26 machines therefore have a high power density.”

Using GT24/GT26 technology, Alstom offers increased plant operational flexibility for times of low electricity demand, where operation at a high partial load is either not possible or not commercially viable due to high fuel cost and low power tariffs. “With our LLOC, a plant operator can overcome the typical dilemma of needing to operate the plant at relatively high partial loads in order to fulfill emissions requirements, or shutting down the plant,” said

The LLOC gives operators the possibility to shut down the second combustor at low partial loads while keeping the first combustor on nominal conditions. This avoids the need for a plant shutdown in the evening and start-up in the morning. The LLOC thus allows plant operations in combined-cycle mode at a very low load factor with the combustor operating in lean premix mode. In addition, the plant gross power output at the low-load operation point is less than 20% of the base load gross power production. Power plants can therefore operate at very
low load during times of low spark spreads (typical for overnight operation), and operators can make quick use of readily available, spinning plant power trains once spark spreads increase or in case of emergency power demand by the power grid (i.e., a form of minute reserve).

“This is a unique feature of our GT24/ GT26 technology. Operators can keep the engine’s water-steam cycle up and running and still operate the gas turbine at low emission levels with a homogenous turbine inlet temperature distribution,” Schmidli explained. The integration of the GT26 low- load feature into Alstom’s KA26 combined- cycle power plants focused on keeping basic cycle design, as well as operation and protection concepts ofthe plant as close to standard as possible. Alstom particularly looked into ensuring uncomplicated and reliable
de-loading and reloading procedures to minimize the risk of forced shutdowns. At any time during a low-loadcycle, the plant is ready for reloading with normal plant load gradients from the low-load operation point to a newdispatch load. Hence, the spinning gas turbine and steam turbine power trains offer an operation reserve that is immediately available.

“Our cycle simulation models supported concept development,” concluded Schmidli. “They provide feedbackon selected functional and control concepts before the application in a live plant. As a result, Alstom’sLLOC allows for highly flexible operation in combined-cycle mode at lessthan 20% load without exceeding emission limits.”

Part-load efficiency
Part-load efficiencyof GT24/GT26 incomparison with single-combustion gas turbine.

The ability of the Alstom GT24/GT26 to operate at the LLOC point was tested and validated at the GT26 test powerplant in Birr, Switzerland. Measurements of emissions and pulsation behavior showed that the concept is reliableand robust. As a result, LLOC will be implemented on two KA26-1 modules at the Gissi KA26 combined-cycle power plant in Italy, which began commercial operation in March 2009


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