Double impact: gas and steam turbines

In the previous part of this series, we saw how energy efficiencies of aircraft jet engines jumped, and how significant advancements were made further. In this article, we will discuss combined cycles.

Looking back at the development of the gas turbine, Whittle and Ohain only had the slide rule to use, to design their engines. Then came the gas tables, and then the hand-held calculators, the computers, and the computer programs. What changes have taken place since 1935! We have single crystal blades, DS blades, active blade tip clearance control, advanced metallurgy, TBCs,  unthinkable manufacturing techniques, three dimensional flow analysis and accurate heat transfer and gas flow programs. The author, an old slide rule engineer himself, has seen all of these remarkable things happen, some of which are discussed here.

The combined gas and steam cycle                            

Three major components must work together to form the combined cycle. These are: (1) the simple cycle gas turbine, (2) the HRSG and (3) the steam turbine. Each element is dependent on the other but they all work together in unison. The gas turbine exhausts into a HRSG where steam is generated. The steam is piped to a steam turbine, is expanded part way, is reheated in the HRSG and then expanded to a condenser to produce bottoming power.

There are four gas turbine considerations that need to be addressed to explain how the combined cycle can be optimized. These are: the specific power output per pound of air flow, the pressure ratio temperature, the firing temperature, and the pressure ratio and the exhaust temperature.

Power output of modern aircraft jets

In general, the specific power output per pound of air flow should be as high as possible and thereby obtain a low exhaust flow and thus a low HRSG stack loss.  Maximum specific output is accomplished by maximizing the firing temperature and raising the cycle pressure ratio accordingly. However, the higher the cycle pressure ratio, the lower the exhaust temperature will be. Therefore, the modern aircraft jets that operate at 30 to 40 pressure ratios and fire at elevated temperatures have rather low exhaust temperatures of around 800 o F because of their high pressure ratios.

The steam turbine throttle temperature for the non supplementary fired HRSGs are limited to about 750 o F whereas the best temperature should be 1000 o F or higher. Therefore, gas turbines such as the Aero GE LM 6000 40 MW unit do not lend themselves to efficient combined cycles.

Similarly, the GE LMS 100 100 MW intercooled gas turbine with an even higher pressure ratio makes a rather poor combined cycle (the core part of this turbine is the GE LM 6000). These units have a high simple cycle efficiency level of 40 to 45 % and are used for peaking and intermittent duty. They have found a place to back up the growing windmill farms. The 30 pressure ratio reheat Alstom GT24 and 26 models have high exhaust temperatures of about 1200 o F because of the reheat feature. Steam throttle temperatures can be obtained at 1000 o F and higher. Also, these combined cycles offer very good part load efficiencies because of the reheat function.

High steam throttle temperatures

There is one way to obtain high steam throttle temperatures for high pressures ratio Aero gas turbines. The exhaust flow can be split and only a small part of the exhaust stream supplementary fired to achieve a high exhaust temperature of this portion only. Only a small amount of additional steam is generated to minimize the degrading effect of the bottoming cycle. The higher throttle steam temperature far overcomes the degrading effect by greatly improved the bottoming steam cycle efficiency. However, this scheme, although quite workable, has never been developed or used.

Industrial heavy duty gas turbines have lower compression ratios of 20 to 24 and at today’s firing temperatures of 2700 to 2900 o F have exhaust temperatures of 1050 to 1150 o F needed for 1000 o F or higher throttle steam. The HRSG must convert the exhaust heat to as high a pressure and superheated steam temperature as permissible with a minimum of exhaust back pressure. Back pressure reduces gas turbine power by reducing its expansion ratio and the HRSG design has to be such that it obtains these high superheat temperatures with a minimum of gas side pressure loss. 

The ‘hockey stick’ heat rate curve

The combined cycle efficiency utilizing most gas turbines (except for the GT24 and 26s) falls off  noticeably at part load and  during winter time low ambient temperature conditions even while utilizing variable inlet guide vanes to reduce air flow. There is the so called “hockey stick” heat rate curve that is manifested. The heat rate increases noticeably as the power is reduced. Therefore, the best place to run combined cycles is at full load. More work needs to be done to improve part load efficiency because most combined cycles run at 75  to 80 % load according to load dispatching requirements and the need for having spinning reserve. Having a high-peak load efficiency is one thing, but operating a plant is another.

A way to improve part load efficiency of combined cycles is to recirculate HRSG stack gas at its 300 o F level back into the gas turbine inlet to be mixed and modulated with the incoming air. This procedure fools the gas turbine into thinking it is operating at a much higher inlet air temperature. The exhaust temperature rises. The scheme has been offered, but not used to date. 

Transcontinental’s 7000 HP gas turbine

As an item of interest regarding combined cycles, Transcontinental Gas Pipeline Company purchased a 7000 HP simple cycle gas turbine from GE in the early 60's.  It was installed in one of two Louisiana gas pipeline stations, each consisting of three condensing 5000 HP steam turbines, quite unique for pipelines. The gas turbine exhausted into a Struthers -Wells forced circulation 600 psig - 750 o F waste heat boiler with an oversized feed water heater for all three of the existing steam turbines to produce incremental steam power. This was an innovation by the author to lower the stack temperature to about 300 o F and produce incremental station power in the process. The application was at first a great success. GE was forced by Transco to furnish the boiler in order to sell the gas turbine.

After a few months, the forced circulating pump started leaking badly and a new pump with a better seal had to be installed. Then the horizontal finned tubes (about 3" in diameter) started leaking half way up the horizontal sides. Caustic corrosion took place. These tubes had to be replaced and a larger circulating pump along with piping changes had to be made. The boiler then worked satisfactorily. GE was heavily involved in making the boiler work. An ASME GT paper was presented on the installation by George McIntosh, Chief Engineer for Transco. Pipelines in this way helped push combined cycles along the way.

GE’s waste heat boiler business

This episode led GE into the waste heat boiler business headed by Paul LaHaye and accompanied by Douglas Todd, a young engineering salesman who later became GE's Manager of Marketing for Combined Cycles. At the beginning a few units were sold. They had horizontal extended surface tubes and forced circulation like the Struthers-Wells units. GE called the boilers "Heat Recovery Steam Generators" (HRSG), a name that stuck to this day for the new industry about ready to evolve. 

There were some supplementary duct firing HRSGs sold by GE and troubles developed. GE had to rethink the idea of the HRSG business. In a few years, GE decided to go out of the business. Only the name "HRSG" remained, and is used today. Doug Todd survived, and this was a bump in the road for HRSGs and a lesson on what not to do.

Combined cycle power plants have been around for 50 plus years. A more or less steam turbine is applied but with extended surface tubing especially for combined cycles. The exhaust temperatures have risen. The  HRSG includes a large economizer to lower the exhaust temperature to about 300 o F, the threshold of  SO3 formation and corrosion concerns where sulfur in the fuel forms sulfurous acid through the condensation of water on the tubes. The feed water is introduced to the economizer at about 250o F to allow for a 25 psia deaerator / heater.


In the next part of this series, we will discuss the latest combined cycles. 

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.