New, efficient industrial gas turbines coming
Technology advances in industry often proceed at a slow, conservative pace. This applies more so to large, complex systems like the new-generation stationary gas turbines coming online. These weigh up to 440 metric tons—nearly a million pounds of hardware! But at the heart of the latest combined-cycle gas turbine (CCGT) plants lies innovation, not just gigantic size.
For example, the newest gas turbines apply single-crystal materials, super alloys, special thermal coatings, and—in one design—closed-loop steam cooling to reach 60% thermal efficiency and beyond. Moreover, the turbine systems are said to produce fewer emissions than conventional CCGT plants. Even substantially higher efficiency is possible in a cogeneration plant, where excess generated heat is put to use for process or domestic heating, in addition electric power generation.
A key enabler of thermal efficiency is turbine design with higher temperature combustion and exhaust gases. However, this requires special cooling at the hottest section of the turbine. GE Energy uses steam cooling in its H system turbine that operates as hot as 1,430 °C (2,606 °F). Siemens Power Generation has opted for all air-cooling, because this method is considered simpler than steam cooling and offers more design flexibility by avoiding dependence on the steam cycle, according to Siemens.
Use of advanced materials also enables higher temperature operation. GE’s H turbine uses single-crystal materials in first-stage blades and vanes that endure extreme temperatures over a long service life. Similarly, row 1 turbine blades of Siemens’ SGT5-8000H machine are made of specialized high-temperature alloy material to combat long-term effects of high-temperature and stress (creep deformation).
Proof of the gas turbine’s viability comes after its incorporation into the combined-cycle system, notes Phillip Ratliff, director of next-generation gas turbines at Siemens. This is scheduled in phase 2 of development for the Irsching CCGT plant in Southern Germany (see Advancing Technology column, Control Engineering , August 2008). “The idea was to build a gas turbine for the most efficient and operationally flexible combined-cycle power plant,” says Ratliff. “However, the gas turbine is a major contributor to the eventual plant’s success; in fact it’s the driver for the high-temperature steam cycle.”
Ratliff is confident about the technological success of Siemens’ CCGT system, but has some concern about economic issues, such as cost of fuel and high-temperature materials used in the plant. In fact, fuel cost makes up the biggest expense to run a gas-fired power plant.
And that’s the main reason for developing CCGT power plants of the highest efficiency.