Full steam injection in early gas turbines
In the previous part of this series, the first application of steam cooling and its combined cycle efficiency were discussed. In this article, the author writes about some of the early steam injected gas turbines, their overall cycle efficiency and their applications.
Steam injection has been applied to gas turbines since the mid l950s. Westinghouse, about this time, proposed to use full steam injection in one of its early gas turbines to generate considerably more power, but an objection was raised as to the large plumb of vapor to be formed at the exhaust exit as reported to the author by J. O. Stephen, Manager of Engineering, Westinghouse. The proposed plant was never built.
Then, Cheng came up with a similar idea to use steam injection to obtain more gas turbine power. He proposed to cool the exhaust and thus reclaim most of the water from the exhaust. The cycle became known as the Cheng Cycle. Jim Strothers of Allison, a division of General Motors, arranged to have a 4000 HP 501 K single shaft gas turbine adapted for steam injection. The unit was so designed and tests were run by Allison and considerably more power was obtained. Several of the units were installed with moderate success, but the efficiency was rather low. The cycle did not catch on to any great extent.
Steam injection to obtain more power
In the 1960s, GE and Westinghouse offered steam injection up to about 3 % of the air flow to obtain about 10 % more power without changing the nozzle areas of the turbine section for this small amount of steam flow. A number of gas turbines in the refineries and chemical plants were equipped for steam to be injected directly into the combustion chambers for summer time high temperature periods to produce the needed power output, but of course the efficiency was quite low and the steam and water was lost to the atmosphere. Therefore, steam injection was only used for peaking so to speak. However, the experience gained was favorable with no harm done to the gas turbines.
About the mid l970s along came environmental concerns of NOx formation and both water injection and steam injection were used to control NOx, which incidentally are still used today. However, dry NOx combustors have been developed to control NOx where the peak flame temperatures reach a limit of about 3500 o F, the point where NOx becomes objectionable.
GE’s early units
GE offered in the early ’80s the steam injected LM 5000 gas turbine with an IHI power turbine called the "STIG" unit. The power output was increased from 33 MW to 45 MW. A number of these units were sold. They performed quite well and were efficiency wise acceptable because of the high pressure ratio of 30 and the higher firing temperature compared to the heavy duty machines. These units were packaged by Stewart and Stevenson of Houston, Texas. Simpson Paper installed the first unit in California. It was packaged by IHI of Japan. A number of ASME papers were written on the early units.
Advancing to the 1990s, the Humidified Air Turbine cycle (HAT) was extensively studied. This cycle showed some early possibilities but the cycle required several very large and costly heat exchangers. After a few years of study and promotion nothing came to be of this cycle.
Injection steam rates and efficiencies
Today, steam injection is still an option to obtain additional power and to control NOx for a limited number of applications. However, there appears to be a preference for the dry NOx control for large gas turbines and combined cycles. Steam injection has been a popular subject to explore. Extensive studies have been made on steam injection over the years. A long three-part ASME paper in 1993 was presented by the author at the Cincinnati ASME conference giving a complete analysis of injection steam rates and efficiencies for various gas turbine types (paper numbers 93-GT-132, 420 and 421). These papers were accepted to be Transaction publications.
These studies, made eighteen years ago, were based on the then current firing temperature levels of 2200 to 2300 o F and lower pressure ratios of no more than 30. Now that the firing temperatures have reached a high plateau of 3000 o F and pressure ratios are 40 to 44 such as for the LMS100, maybe a new look should be taken at steam injection based on these new conditions. The picture has changed an unbelievable amount the past few years.
Applying new GE FlexEfficiency
A careful review of Part II of the above mentioned paper indicates that the reheat gas turbine with a 40 to 50 pressure ratio having firing temperatures of 3000/2600 o F has the greatest cycle efficiency improvement potential for a finite amount of steam injection of 5 to 10 % of the air flow. Applying the new GE FlexEfficiency 1100 o F HRSG and steam turbine and extracting the steam at the appropriate pressure level for injection can give an injection efficiency of 56 to 57 % when credit is given for the extra heat in the exhaust.
This level of efficiency should overcome the extra fuel burned to heat the injected steam versus the no heat added for the bottoming portion of the steam turbine with its 41% efficiency level. The overall cycle efficiency should improve. Less steam will be condensed in the condenser. Much more power will be produced in the process. Make up water will have to be provided, however, to account for the injected steam lost to the atmosphere.
Low cost solar heat collectors
The Press recently announced that Israel and India are looking into steam injection whereby water is heated at low pressure and low temperature to produce steam by the sun through lower cost sun heat collectors and less costly piping before being injected into the combined cycle gas turbine. A large amount of latent heat is thus saved without burning any fuel for evaporation. Extra fuel is burned to heat the vapor. Left over exhaust steam heat is recovered in the HRSG to produce incremental power.
This hybrid combined steam injected cycle is reported to offer a cost effective way to use solar heat without using a complex and high cost direct solar steam to power system. Also, it is reported in this same Press release that GE has announced a similar hybrid plant to be installed in Turkey. According to GE, considering free heat delivery of the solar steam, the incremental power is produced at over 70 % efficiency.
Is the steam injected cycle really dead considering today's gas turbine operating parameters? Get an answer to this question, and know a lot more about gas turbines from the next part of this series.
Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.
