Moving beyond the steam cooling
After discussing the ways and means of integrating propane, gas and steam cycles in the previous part of this series, we now take a look at some of the designs that can conserve a considerable amount of water.
A final word of concern is about our resources, and that is water. Combined cycle power plants offer a way to save a considerable amount of water verses the standard steam power plant. The combined cycle power plant uses only about one third as much water because the steam power portion of the plant is only one third of the total power output and thus has only one-third as much steam to be condensed.
For every pound of steam condensed, a pound of cooling water has to be evaporated in cooling towers, cooling ponds or rivers. We are consuming more and more water for home uses, farms, factories and power plants and are running out of this resource, particularly in the Southwestern part of America. According to a report issued in December of 2003 by the National Renewable Energy Laboratory, 39 % of the raw water used is attributed to thermal power plants and is only surpassed by irrigation and livestock where the figure is 41%. The combined cycle power plant will conserve water.
The integrated cycle uses steam from the steam turbine to cool the blading. The cycle applies the same basic arrangement as the combined cycle, but the gas turbine cannot be operated without steam from the steam turbine or from some other sources such as a small packaged boiler. Steam cools the critical hot parts such as the blading, casing, combustor, transition ducts and discs. Steam offers twice the specific heat as air and has other superior heat transfer qualities. Such cooling is not as harsh or as hard to handle as water. Water cooling, although extensively tested, has never worked out.
Generally, the cold reheat steam from the steam turbine at about 600 o F and 600 psig is passed through the various hot gas parts requiring cooling and is then reintroduced to the steam turbine as reheat steam at about 1000 o F. The gas turbine hot gas expanding and passing around and through the turbine blades is not quenched and lowered in temperature to any great extent such as takes place with air film cooling.
More gas turbine power is produced because of the higher working temperature of the expanding gas. All of the air previously used for cooling is heated and expanded to produce more power. The 10 % pressure drop required for reheating of the steam in a normal reheat steam turbine and boiler is used to allow the steam to pass through the gas turbine hot parts. There is no steam pressure loss penalty. A net gain in power and efficiency is realized. A reheat gas turbine was included an ASME Journal paper and presented at the IGTI conference in 1986, in which the steam extraction point (cold reheat), the cooling areas and blading and the steam return to the steam turbine were clearly illustrated.
First application of steam cooling
Mitsubishi Heavy Industry (MHI) of Japan was the first company to apply steam cooling to cool the gas turbine hot parts. Steam was taken from the combined cycle steam turbine and passed through the combustion liners and transition ducts and then the heated steam was returned to the steam turbine. A partial "H" class gas turbine was thus formed. This turbine design ran very well and a number of units were sold internationally.
GE’s ‘H’ class gas turbine
After considerable DOE study work, GE decided to use steam cooling for its DOE effort. The GE “H” class gas turbine, using steam cooling, was developed through DOE funding as part of the Advanced Turbine System (ATS) program in the 1990 - 1995 era. Although gas turbine reheat was not used, a noticeable increase in combined cycle efficiency of several points in combined cycle efficiency was realized.
A combined cycle efficiency just short of 60 % was achieved by GE’s first 50 cycle Frame 9 “H” unit installed in Wales in 1995. This unit held the World record for efficiency until just recently when Siemens’ combined cycle in Germany obtained a cycle efficiency of 60.75 %. Reheat would have made both units even more efficient and will do so in the future, if and when used. Alstom could do so with their reheat gas turbines; it is up to the market and competition.
The GE “H” design passes the cooling steam through the rotating blades as well as the fixed vanes and then back to the steam turbine. Complicated steam shaft seals are required and extra cost is involved as well as the consideration of steam pressure drop. Apparently, GE has made it work, but the details are not known. Cooling the other parts is straight forward and seems to be a reasonable way to go. The future of steam blade cooling by others besides GE is not clear. Some companies still prefer air cooling and even GE is going back to air cooling on its new "FlexEfficiency" units.
The threat of peak temperatures
There is gas turbine reheat with steam cooling of the power turbine. There is more advanced air and steam cooling, more advanced TBCs, better metallurgy, and higher firing temperatures – all to achieve higher output and efficiencies, but today's peak temperatures are threatening to increase NOx. Also, there is the possibility of the fuel’s gas heating to increase, maybe 600 o F or preferably higher through low level heat sources and steam extraction. Such heating provides 100 % savings because this heat goes directly into the turbine.
There was an article in The ASME monthly magazine, Mechanical Engineering written by the author and published in April, 1982 titled, “The Reheat-Gas-Turbine Combined Cycle,” in which a target was set at 60 % efficiency. At the time, the efficiency level was about 48 %. This new high level of efficiency was scoffed at by some of his peers but Siemens has now made this target a reality and without reheat or steam cooling. The author is now setting a new target of 65 % combined cycle efficiency to aim at and achieve in the next five to ten years, and this efficiency level in five years seems realistic.
In the next part, the application of the steam injected cycle to gas turbines and its efficiency will be discussed.
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.
