Offering such benefits as high efficiency, environmental compliance and operating flexibility, gas turbine combined-cycle technology is an extremely popular power generation option. In the past decade, gas turbine orders for combined-cycle projects worldwide have totaled more than 300 GW, while simple-cycle orders have represented less than 150 GW.
EL&P invited GE Power Systems to provide this month’s expert answers to commonly asked questions about gas turbine technology.
What is the difference between combined-cycle and simple-cycle gas turbine systems?
In a simple-cycle application, a gas turbine drives a generator and the exhaust from the turbine is discharged directly into the atmosphere. In a combined-cycle system, the gas turbine exhaust exits into a heat recovery steam generator where the exhaust heat is recovered by converting water into steam that is used to drive a steam turbine-generator to produce additional power. Efficiency is dramatically increased since this power can come without increasing the amount of fuel consumed.
There are two basic combined-cycle configurations: single-shaft and multi-shaft. In a single-shaft system, a gas turbine and a steam turbine are connected in a straight line and share a single generator. In a multi-shaft configuration, one or more gas turbines and the steam turbine each have their own generator.
How has the technology evolved over the past 50 years?
Most combined-cycle systems installed in the 1950s and 1960s included conventional, fully fired boilers. These systems basically were adapted from conventional steam plants, with the gas turbine exhaust gas serving as combustion air for the boiler, typically improving the total plant efficiency by 5 to 6 percent.
During the 1960s, the heat recovery type of combined cycle became dominant, beginning with power and heat applications where the power-to-heat ratio was well suited for many chemical and petrochemical processes. Power generation applications were few until gas turbines over 50 MW in capacity were introduced in the 1970s. Then, heat recovery combined cycle systems experienced rapid growth in electric utility applications.
Facing oil embargoes and rising oil and gas prices, power producers quickly recognized the value of the efficiency delivered by the combined-cycle alternative. With the 1980s and 1990s came a wave of natural gas-fired systems, including plants designed for power only, and those designed for simultaneous power and heat, or cogeneration.
As gas turbine and combined-cycle technology has evolved, its thermal efficiency has steadily increased. Early combined-cycle plants had efficiencies that were in the mid-30 percent range, then eased into the 40 percent levels during the 1980s, and in the past decade have surpassed 50 percent. The latest F-class gas turbines can reach 57 percent efficiency, while the H-class machines are capable of 60 percent.
How have the combined-cycle efficiency improvements been achieved?
Thermal efficiency gains have been led first by advances in gas turbine performance, primarily through higher firing temperatures. By incorporating advanced cooling techniques, enhanced materials and thermal barrier coatings, today’s F-class gas turbines are able to operate at firing temperatures in the 2,400 F (1,315 C) range, while H-technology machines can operate at 2,600 F (1,425 C).
Second, improvements have been made to steam cycle performance, achieved by advances in steam conditions, the use of longer last stage buckets in the steam turbine and improved system controls. Steam units for combined cycle have moved from simplistic recovery boiler cycles to today’s reheat cycles, which feature steam pressures above 1,800-2,000 psi, and operating temperatures of 1,000-1,050 F.
What are the key environmental features of combined-cycle?
Although today’s combined-cycle systems are capable of using a variety of fuels, natural gas is the dominant choice, and is one of the cleanest fuel options available. In addition, the high efficiency of combined cycle translates into better environmental performance, since more power is produced without increasing the amount of fuel that has to be burned.
Initially, water and steam injection was used for NOx control with combined-cycle systems. As efficiency has increased, so have firing temperatures that work against emissions control. However, the development of Dry Low NOx technology has enabled advanced gas turbines to operate at higher temperatures and efficiencies without sacrificing environmental performance.
How has combined-cycle technology been accepted by the industry?
Over the past decade, orders for combined-cycle plants have soared. Since 1990, more than 1,400 heavy duty and aeroderivative gas turbines have been ordered for combined-cycle projects in the U.S. and Europe, compared with approximately 500 coal-fired steam turbines and other fossil units for simple cycle.
Along with the increased efficiency, other factors driving the rapid market acceptance of combined-cycle technology include capital costs that are lower than large-scale coal-fired projects; shorter construction and installation cycles (three years vs. five for a typical coal plant); and the ability of combined-cycle systems, fueled by natural gas and using new emissions control techniques, to meet the most stringent environmental regulations.
What’s ahead for combined-cycle technology?
The “dash for gas” in countries such as the U.K., Italy, Spain and Argentina as well as the development of liquefied natural gas (LNG) have contributed to the global use of combined-cycles. The next logical application will be to provide increased fuel diversification. While most combined-cycle systems currently burn either liquid or gaseous fuels, through the Integrated Gasification Combined-Cycle (IGCC) process solid fuels such as coal, which is in great supply around the world, also can be used as a fuel source.
IGCC already has compiled considerable commercial experience. Conceived as a power generation technology to burn coal or other solid fuels in an economic and environmentally acceptable manner, IGCC today has the additional flexibility to cleanly burn heavy oils, petroleum coke, orimulsion, biomass and waste fuels.
Along with greater fuel diversity, combined cycle technology will continue to be more tolerant of cyclic operation. Future designs of combined-cycle power plants should also include greater peaking capability, through either power augmentation of the gas turbine or flexibility in the steam cycle.
Other developments on the horizon include even higher efficiency and greater power density-producing more kW per square foot of power plant space.
Edward Lowe, Manager, Gas Turbine-Combined Cycle Product Line, and Robert Fisk, Manager, Evaluation, Analysis and Pricing, both of GE Power Systems, provided this month’s Power Pointers. Lowe may be contacted at 518-385-5052 or firstname.lastname@example.org. Fisk may be contacted at 518-385-7798 or email@example.com.