Claims
- 1. In a hybrid steam/gas turbine power plant of the type utilizing a direct fired steam generator supplying high pressure steam and combustion products at an outlet for operating a drive turbine, the improvement comprising:
- a direct fired steam generator having fuel, combustion air, and feed water inlets, and an outlet delivering combined steam and combustion gases as high pressure and temperature exhaust products;
- a drive turbine having an inlet and outlet, a fluid operated drive stage and a shaft coupled second compressor stage said compressor stage having an air inlet and an air outlet for supplying pressured combustion air to said generator;
- a first compressor having an air inlet and outlet, said outlet supplying pressurized combustion air to said second compressor inlet;
- means flow communicating said generator exhaust products to said turbine inlet for operating said drive stage;
- means in said drive turbine, extracting a plurality of turbine drive stage fluid discharge products at a plurality of first temperatures and pressures, respectively;
- means fluid communicating said drive turbine fluid discharge products to a plurality of predetermined locations;
- first heat exchange means in at least one of said locations having a drive turbine discharge product inlet and an outlet, a cooling fluid inlet and outlet, and fluid impermeable means therebetween; and,
- means supplying steam generator feedwater as cooling fluid to said first heat exchanger inlet, said cooling fluid inflow and outflow having second and third inlet and outlet temperatures respectively, and means limiting cooling fluid outflow at said third temperature and pressure corresponding to fluid saturation at said first drive turbine fluid discharge first temperature and pressure temperature;
- second heat exchange means intermediate said first compressor and second compressor means, for cooling said generator combustion air having an air inlet and outlet, said air inlet in fluid communication with said first compressor air outlet, a fluid inlet and outlet and fluid isolating means therebetween;
- means supplying said feedwater as cooling fluid to said second exchanger fluid inlet, for reducing said generator combustion air temperature thereby increasing generator exhaust product at increased pressure and temperature;
- whereby said generator exhaust product is increased and cooling fluid temperature does not exceed saturation providing turbine operation at increased efficiency.
- 2. The power plant of claim 1 further comprising:
- means controlling said cooling fluid flow through said first heat exchange means;
- a first fluid expansion turbine for extracting shaft work from said turbine discharge fluids;
- a fluid inlet and separate liquid and vapor outlets on said expansion turbine;
- means admitting at least one of said drive turbine discharge means to said expansion turbine inlet;
- means admitting said expansion turbine liquid exhaust to said first heat exchanger turbine discharge inlet; and
- a tertiary fluid loop thermally coupled to said cooling fluid, said tertiary loop fluid operating at a saturation temperature and pressure substantially lower than that of said cooling liquid;
- wherein heat recovered from said drive turbine exhaust liquid is transferred to said liquid cooling loop at temperatures below saturation of said tertiary liquid.
- 3. The power plant of claim 2 further comprising;
- means condensing said first expansion turbine vapor exhaust having tertiary fluid and drive turbine inlets and outlets, thereby recovering vapor exhaust heat and generator feedwater;
- means, in said condensing means, transferring said vapor exhaust heat to said tertiary fluid, said exhaust heat generating tertiary fluid vapor at a fourth temperature and pressure.
- 4. The power plant of claim 3 further comprising:
- a second combustion air compressor shaft coupled to said first fluid expansion turbine said compressor having an atmospheric air inlet and an outlet;
- a second fluid expansion turbine, for extracting shaft work from said tertiary fluid vapor;
- means on said second expansion turbine admitting said tertiary fluid at said fourth temperature and pressure and discharging tertiary fluid and vapor at a fifth temperature and pressure;
- a second heat exhanger intermediate said second combustion air compressor outlet and first combustion air compressor inlet, having a combustion air inlet and outlet tertiary fluid and vapor inlet and outlet and fluid isolating means therebetween;
- means fluid communicating said heat exchanger air inlet and second compressor air inlet;
- means fluid communicating said heat exchanger air outlet and first compressor air inlet;
- means fluid communicating said second expansion turbine discharge and second heat exchanger tertiary fluid/vapor inlet;
- means fluid communicating said exchanger tertiary fluid outlet and condensing means heat transferring means;
- whereby said tertiary fluid recovers first expansion turbine exhaust heat at temperatures below said turbine exhaust.
- 5. A method extending the life of a turbine utilized in a hybrid steam/gas turbine power plant of the type utilizing a direct fired steam generator for supplying steam and combustion products to the inlet of a drive turbine having a shaft coupled compressor stage comprising the steps of:
- Operating a direct fired steam generator having feedwater, combustion air and fuel inlets, and an outlet delivering exhaust steam and combustion products for operating a drive turbine at a predetermined pressure;
- Providing a plurality of pressure and temperature staged exhaust discharges in said drive turbine;
- transferring heat, from said turbine exhausts to said generator feedwater; thereby heating said feedwater;
- limiting said feedwater heating to saturation temperatures and pressures of said feedwater;
- compressing atmospheric air in said compressor to a predetermined pressure and temperature for use as generator combustion air;
- supplying said combustion air, feedwater, and fuel to said generator inlets;
- cooling said combustion air by transferring heat to said steam generator feedwater;
- establishing a combination of said steam generator fuel, feedwater and combustion air flows such that said steam generator exhaust temperatures and pressure does not exceed a predetermined value.
- 6. The method of claim 5 wherein said establishing step includes the step of limiting the steam generator exhaust to a critical value of 1600.degree. F.
- 7. The method of claim 6 wherein said establishing step includes the step of limiting the steam generator exhaust pressure to a critical value of 3000 lbs. per square inch.
- 8. The method of claim 6 further comprising the steps of:
- operating a first expansion turbine from at least one of said drive turbine discharges;
- establishing a fluid discharge in said expansion turbine;
- condensing said discharged fluid thereby generating heat and condensed generator feedwater;
- establishing a tertiary fluid heat exchange loop, said tertiary fluid having saturation temperature and pressure substantially less than said expansion turbine discharge;
- transferring said condensed feedwater heat to said tertiary fluid, thereby generating tertiary fluid vapor;
- driving a second expansion turbine with said tertiary vapor, thereby generating shaft work.
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation in part of our application Ser. No. 06/701,767 filed on Feb. 14 1985, now abandoned.
It is well known thermodynamically that the Carnot cycle is the yardstick used to compare various practical power cycles. The thermal efficiency of a Carnot cycle is the maximum possible achievable efficiency and is given by ##EQU1## where T high is the maximum absolute temperature available for the cycle and T low is the absolute heat sink temperature the cycle rejects heat to. It is obvious that the above expression has been the guide to engineers and technologists striving to improve the thermal efficiencies of practical engines. It is obvious that the higher T high is, the more efficient the Carnot cycle becomes. However, T high is limited by the material used for the construction of any particular engine design. This explains why metallurgists are continuously searching for new materials that can withstand higher and higher temperatures. However, this can distract from finding useful alternative solutions as demonstrated by the teachings of this patent.
The regenerative-reheat steam Rankine cycle is the cycle used for power generation in most larger power plants today. At present the maximum temperature used for this steam power cycle is limited to approximately 1050.degree. F. which produces a maximum achievable thermal efficiency of approximately 40%. On the other hand gas turbine power plants operating on the Brayton cycle as well as large jet engines for aircraft propulsion have a maximum operating temperature in the range of 1600.degree. F. to 2000.degree. F. At 2000.degree. F. blade cooling is necessary. In spite of this high temperature, the thermal efficiency of such a power plant is approximately 28% which is considerably less than that of the steam power plant operating at 1050.degree. F. Of course, the most attractive characteristic of a jet engine for aircraft application is the power produced per unit weight of the power plant. However, once these power plants were developed, they were adapted for electric power generation use.
Early Brayton cycle gas turbine applications were used for peak electrical loads since they can be brought on line quickly and since their capital investment is low, thus compensating for the fuel expenses. In theory, applying regeneration to the Brayton cycle improves thermal efficiency. However, this efficiency improvement occurs at much lower turbine pressure ratios than that of the non-regeneration case. Even when an engine is designed with regeneration in mind, the low cycle pressure ratio makes it vulnerable to the effect of pressure losses in the various components and increases the components' size for any desired power output, thus increasing the capital cost of the plant.
With the availability of high operating temperature turbine technology, a logical question arises. Why not upgrade the steam power plants by raising their maximum operating temperature to 1600.degree. F. This would significantly raise their thermal efficiency. This also would not be unusual for the power industry since power plants in the past has been upgraded by raising both operating pressures and temperatures. However, with little investigation, it becomes clear that the steam producing boiler is the limiting component. Using carbon steel for the construction of the boiler limits the maximum steam temperature to about 1100.degree. F. To achieve higher temperature like 1600.degree. F. would require much more expensive material for the construction of the conventional boiler thus making it economically prohibitive.
Prior applications of gas turbines operating with steam injection are disclosed in U.S. Pat. No. 3,693,347 (Kydd and Day), U.S. Pat. No. 2,678,531 (Miller) and U.S. Pat. No. 3,353,360 (Gorzegno).
These systems indicate that it is well known that operation of gas turbine in the steam injection mode provides greater power output and produces improvements in thermal efficiency in the overall system.
In U.S. Pat. No. 3,978,661, Cheng teaches combining the gas turbine and the Rankine cycle steam turbine into a single operating turbine, thus raising the operating pressure of a Brayton cycle which is higher than the operating pressure of conventional Brayton cycle gas turbines. Cheng further teaches the use of a direct fired steam generator of the type disclosed in U.S. Pat. No. 4,490,542 (Eisenhawer) to inject steam and combustion products into the first stage of a conventional Brayton Cycle gas turbine.
However, an integral part of the Cheng patent, is the use of a waste heat boiler to produce steam by recovering part of the available heat energy in the exhaust gases. The steam produced is injected in the combustion chamber of the machine. By virtue of lowering the exhaust gas temperature, the Cheng machine achieves higher efficiency than conventional gas turbines.
When recovering heat as taught by Cheng, in the regenerative Rankine mode when the turbine exhaust temperatures are high, as required to achieve maximum internal efficiency, a heat recovery limitation exists. In this situation, the conventional heat recovery boiler operates at greatly reduced efficiency, since the recovery liquid approaches and exceeds it's saturation temperature and pressure causing the temperature differential between the exhaust gases and the recovery liquid to be minimized. This particular combination of exhaust gas temperature, and heat recovery liquid saturation temperature is known as the "pinch point".
The phenomenon of the "pinch point" limits the maximum achievable boiler pressure and the minimum achievable exhaust gas temperature putting a limit to the possible improvement in efficiency. Hence, Cheng does not solve the effect of "pinch point" limitation on maximum achievable thermal efficiency, since heat recovery is limited by exhaust gas temperature.
A combined Brayton-Rankine cycle power plant operating according to the Cheng patent is therefore limited in thermal efficiency by the "pinch point" phenomenon. This phenomenon further limits the maximum achievable pressure in the exhaust heat recovery boiler. It also limits the cooling of the exhaust gases to a relatively high temperature. The particular combination of low maximum boiler pressure and high exhaust temperature limits the maximum achievable thermal efficiency.
A system enclosed disclosed by Cheng shows a maximum achievable efficiency of 50% at a maximum turbine inlet temperature of 2200 degrees Fahrenheit. These high turbine inlet temperatures impose an additional limitation in that conventional technology limits turbine blade operating temperatures to approximately 1600 degrees Fahrenheit. Therefore, Cheng finds it necessary to utilize transpirational cooling of at least the first portion of the turbine inlet blades. As it is well known in the art that transpirational cooling is an expensive and sometimes unreliable process, the limitations is substantial and includes increased cost of a given installation.
The heat engine following the concepts of this invention utilize a direct fired high pressure steam generator and produces maximum efficiency at a much higher pressure ratio than conventional gas turbines. Because of the high pressure ratio it is necessary to use multi stage expansion and compression turbines. The water supply to the direct contact boiler is used to inter-cool the air between compressor stages. Also, as indicated above, thermal regeneration is utilized to improve the thermal efficiency of the power plant. Also, due to the presence of non-condensible gases in the exhaust of the turbine it is advantageous to condense steam in the driven turbine exhaust products at pressures above atmospheric thereby avoiding the use of an evacuator.
In keeping with the above described invention disclosed here, steam is condensed using a suitable secondary fluid such as Freon 11 in a bottoming Rankine cycle thus maximizing the power plant thermal efficiency. Through utilization of conventional heat recovery in regeneration, and a bottoming cycle in conjunction with the direct contact steam generator or boiler, the power output per unit weight or unit volume of plant is much higher than known power plants at this time.
It is therefore the object of this invention to provide an operating power plant having increased thermal efficiency while utilizing present day gas turbine technology while operating within maximum allowable turbine temperatures.
It is an additional object of this invention to replace the conventional combustion chamber of a gas turbine power plant and the conventional boiler of a Rankine cycle power plant by an inexpensive direct contact steam boiler or generator wherein a mixture of superheated steam and combustion gases is injected as the turbine operating fluid.
It is an additional object of this invention to burn a stoichiometric mixture of fuel-oxidizer in the direct contact boiler and to control maximum generator discharge temperature by controlling generator feed water input in relation to the combustion fuel and air.
It is yet an additional object of this invention to reduce the NO x pollutants by burning a stoichiometric mixture at substantially lower temperature than conventional direct fired generators.
It is further object of this invention to provide a steam injected Brayton cycle power plant wherein drive turbine blades do not require transpirational cooling.
It is a further object of this invention to provide a high efficiency steam injected Brayton cycle power plant where overall efficiency is greatly improved by heat recovery or regeneration wherein the thermal "pinch point" limitation is eliminated.
A particularly unique thermal advantage provided by the invention disclosed herein is the utilization of a bottoming Rankine cycle, utilizing intercoolers and regenerative feedwater heaters in place of heat recovery boilers to recover heat from the drive turbine exhaust. The disclosed heat recovery system overcomes the above mentioned "pinch point" limitations present in prior art systems, as will be further discussed in detail.
As indicated above, to achieve the best possible thermal efficiency operation of any thermodynamic system the highest possible temperature ratio is desirable. In the invention disclosed herein, the use of the direct fired steam generator provides substantially increased temperatures with attendant pressures at the drive turbine inlet. In order to maximize overall system efficiency, recovery of heat from the turbine discharge is absolutely necessary. This technique is termed the regenerative Rankine or Brayton cycle. However, heat recovery of regenerative Brayton type is a maximum at a pressure ratio of about 4:1 for present day operating temperatures. The system disclosed herein operates at an overall compression ratio greater than 200 making the system substantially less vulnerable to pressure losses and reduced power output. Typically, operation at maximum pressures of 200 atmospheres can be achieved.
The system disclosed herein provides substantially increased efficiency without the use of heat recovery boilers and thus avoids the "pinch point" phenomenon through the use of the above mentioned bottoming cycle, intercoolers, and regenerative heat water heaters. This combination greatly improves the Brayton cycle through the use of a high temperature, high pressure direct fired steam generator but does not suffer a loss of efficiency in recovering heat from the drive turbine exhaust although these temperatures and pressures fall within the "pinch point" region.
US Referenced Citations (6)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 81104 |
May 1982 |
JPX |
| 217708 |
Dec 1983 |
JPX |
Continuation in Parts (1)
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Number |
Date |
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| Parent |
701767 |
Feb 1985 |
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Patent Grant
4841721
