Plants are recognizing that typical on-site cogeneration of heat or steam and electric power can yield fuel efficiencies of about 70%, compared to about 30% for electricity generated at regional powerplants and transmitted many miles to a facility. We’re seeing an increasing number of applications where turbines are being retrofitted for higher efficiency, increased capacity and lower emissions. “Industrial and power-generation facilities are being squeezed for output, and it’s hard to get permits for additional facilities,” says Anne McEntee, general manager, Power Services, GE Energy (www.ge.com).
At Dubai Aluminum, a 30-year-old plant on the coast, electric power represents 30% to 40% of the cost of production. “The plant was seeing tremendous growth and needed to increase capacity by 100 MW without increasing the facility footprint,” McEntee says.
GE modified Dubai’s turbines with tighter seals, higher firing temperature and improved airflow. The changes increased power output by 13% using less total fuel and lowered NOx per unit output by 15%. The same technologies can be used to decrease energy consumption by increasing efficiency and running slower:
- Advanced seals (ValPak technology): One significant way of improving the efficiency of a gas turbine is to decrease the amount of leakage flow through the machine and thereby maximize the amount of high-pressure gas directed through the turbine stages that produce the power. Advanced materials and designs such as brush seals, cloth seals, honeycomb shrouds and abradable coatings improve the sealing between the rotating and stationary surfaces. A favorable byproduct of this efficiency savings is an associated increase in power. Typical improvements for this package applied to a vintage GE E/B class unit is approximately a 3% to 5% decrease in heat rate and a 5% to 6% increase in output.
- "T-fire" upgrade: The output of the gas turbine can be increased by increasing the firing temperature of the unit. The hotter exhaust stream from the combustion system has higher energy content. However, the downstream components in the hot gas path have to accommodate these higher temperatures and not adversely affect the maintenance interval or costs. GE uses advanced technology such as creep-resistant, high-temperature alloys, thermal-barrier coatings and optimized cooling circuits that serve to protect the hot gas path components from the increased heat and thereby maintain component durability. Typical improvements for this package applied to a vintage GE E/B class unit can range from 3% to 10%, depending on unit age. An associated benefit of this package is an increase in exhaust energy, which is valuable to cogeneration and combined-cycle applications.
- Lean head end (LHE) liner: This modification reduces NOx emissions. The number, diameter and location of mixing and dilution holes in the LHE liner is different than in a standard liner, which introduces more air into the head end of the combustor to reduce NOx emissions.
“The turbine upgrade saved half the cost versus new capacity,” McEntee says, “$50 million versus $100 million.”