VAULT // 1961
Using the Gas Turbine’s Heat
A piece published in Mechanical Engineering more than 60 years ago explores how the gas turbine offers possibilities of greater economies when its heat is recovered.
Written by Werner P. Auer
THE COMBINED processes of the four plants described in this paper are not identical, but in all cases the utilization of the exhaust heat of gas turbines results in considerable improvement of the efficiencies. Operating experience, where already available, confirms the expected results and proves that the decision to install a combined plant was correct.
Two 25/30-Mw Turbines
The first system is a thermal power plant in Korneuburg, Vienna. Basic equipment is two 25/30-mw sets of double-shaft open-cycle gas turbines burning natural gas. Although these turbines have a relatively high efficiency, a means was sought to improve performance further by utilizing the exhaust gas. To do this, each turbine sends its exhaust gas to a waste heat boiler which in turn supplies steam to a turbine.
This arrangement, however, could only produce low live-steam conditions which would have resulted in a small heat drop with a low efficiency. It was therefore decided to increase the temperature and pressure level of the steam by supplying additional heat through a superheat boiler. The steam could then effectively operate the exhaust-heat steam turbine, a reaction-type machine with a rated output of 25,700 kw.
Overall efficiency of the combined plant at an ambient temperature of 32 °F is expected to show an improvement of 18.5 percent over the gas turbines alone.
The first thermal power plant in Korneuburg, Vienna. Photo: Getty
Blast furnace Cornigliano Steel Works in Genoa, Italy. Photo: Getty
Burns Blast Furnace Gas
The second example is the Cornigliano Steel Works in a suburb of Genoa, Italy, which has a 16,000-kw single-shaft gas turboset burning blast furnace gas.
There is no heat recuperator on this machine; instead, it exhausts into a waste heat boiler. The necessary live-steam temperature would not have required an additional heat supply. However, a burner will be provided to slightly increase the gas temperature. This will allow a smaller heat-transfer surface in the boiler.
Temperature of the feedwater returning from the steam system is too high to allow the cooling of the exhaust gases to the lowest possible temperature. Therefore, the waste heat boiler, in addition to the steam section, contains a water heater which supplies 23.8 × 10⁶ Btu/hr to an existing hot-water system.
Thermal efficiency of the gas turbine, referred to the generator terminals, is 18.6 percent under full load at an ambient air temperature of 59 °F and based on the lower heat content of the fuel. On the other hand, the steam generated in the waste heat boilers produces an additional output of 11,150 kw in the existing steam turbines.
Reduced Fuel Consumption
Example three comes from the Dudelange Steel Works, Luxembourg. Besides providing electric power, there is a high consumption of steam and heat at this plant. The existing power plant was a 5,400-kw gas-turbine set. This was combined with a system consisting of a 143,325-lb-per hr boiler and a 13,300-kw double-cylinder, double-pressure steam turbine.
Exhaust gases of the gas turbine are used as combustion air for the boiler, considerably reducing fuel consumption. About 17 percent of the gases leaving the recuperators of the gas turbine at a temperature of about 430 °F are consumed for this purpose. The remainder of the exhaust gases are used to heat feedwater. Increased efficiency of the gas turbine more than offsets the reduction in steam cycle efficiency caused by more steam flowing to the condenser.
An over-all performance of 27.4 percent for the combined plant was obtained in actual operation. The gas and the steam turbines were fully loaded but no steam was sent into the heating system or supplied to the turbines from the accumulators. Efficiency for the gas turbine was 20 percent and 24 percent for the steam turbine.
A large boiler photographed in 1922. Photo: Getty
A large heating boiler with vertical water pipes during the 1940s. Photo: Getty
Provides Central Heating
The last example is a public utility company supplying electric power and heat to a town of 40,000 people. It is located in a new suburb of Bremen, Germany.
There are two gas turbines in this system along with hot-water accumulators, four oil-fired boilers (24.2 × 10⁶ Btu/hr), and four electro boilers (10.3 × 10⁶ Btu/hr).
For best performance, as much heat as possible must be drawn from the gas turbosets. Therefore exhaust heat is recovered not only through waste boilers but through the intermediate coolers which are of a special divided design. They permit the extraction of heat from the first (hotter) section and the use of fresh cooling water for the second (colder) one. With this arrangement each intercooler produces about 47.6 × 10⁶ Btu per hr. In addition, valuable cooling water can be saved because only 40 percent of the heat to be extracted from the combustion air has to be removed with fresh water.
In exceptional cases where the gas turbines would run without producing heat for the homes, the heat extracted from the first intercooler section can be transferred to cooling water in an auxiliary cooler.
Heating water from the intercoolers is piped to the waste heat boilers where it picks up another 71.4 × 10⁶ Btu per hr and per gas-turbine set. It then flows back to the heating system at a temperature of 230 to 266 °F.
As mentioned, there are oil-fired and electro boilers in addition to the turbines. Separate heat exchangers transfer their heat to the hot-water system.
The two gas-turbine sets are rated at 25 mw at ambient conditions of 34.7 °F and 14.7 psia. Thermal efficiency at these conditions and at full load is 24.5 percent. Design temperature is 1,157 °F. Exhaust gases leave with a temperature of about 605 °F at full load and are cooled in the waste heat boiler to about 355 °F. The turbines are similar to those in the first example with the exception that they burn oil and have the divided intercooler.
It is hoped that the four examples of gas turbines combined with a steam or industrial cycle will promote the idea of the combined process and encourage the execution of more similar installations.
Werner P. Auer was chief engineer of gas turbine sales and application engineering at Brown Boveri and Co., Ltd., of Baden, Switzerland.
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