Patent Publication Number: US-5896738-A

Title: Thermal chemical recuperation method and system for use with gas turbine systems

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DE-FG21-95MC32071 awarded by Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an economical method and system for generating power. More specifically, the present invention relates to a method and system for efficiently recovering thermal energy from gas turbine exhaust. 
     2. Description of the Related Art 
     Conventionally, the thermal energy recovery from gas turbine exhaust is accomplished by a recuperator, a regenerator, or a heat recovery steam generator. The sensible heat of the gas turbine exhaust is thus recovered into the sensible heat or latent heat of the inlet stream of the gas turbine. In this form of thermal energy recovery, the efficiency is limited by the temperature approach, or driving force, between the exhaust and the inlet streams. 
     It is therefore desirable to provide a method and system for thermal energy recovery that is less dependant upon the temperature difference between the exhaust and the inlet streams of a gas turbine. 
     SUMMARY OF THE INVENTION 
     The claimed invention provides a system and method for efficiently generating power using a gas turbine, a steam generating system and a reformer. The gas turbine receives a reformed fuel stream and an air stream and produces shaft power and exhaust. Some of the thermal energy from the turbine exhaust is received by the reformer. The turbine exhaust is then directed to the steam generator system that recovers thermal energy from it and also produces a steam flow from a water stream. The steam flow and a fuel stream are directed to the reformer that reforms the fuel stream and produces the reformed fuel stream used in the gas turbine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart of the thermal chemical recuperation system according to the claimed invention. 
     FIG. 2 is a flow chart of the thermal chemical recuperation system incorporated into an electricity-steam cogeneration plant. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to FIG. 1, a thermal chemical recuperation power generation system 10 of the claimed invention comprises a gas turbine system 30, a steam generating system 32, and a reformer 18. The gas turbine system 30 generates power and a compressed air/turbine exhaust stream 60 from an air stream 40 and a reformed fuel stream 74. The steam generation system 32 generates a steam flow 70 and a system exhaust 64 from the compressed air/turbine exhaust stream 60 and a water stream 66. The steam flow 70 is used by a reformer 18 to reform a fuel stream 72 to produce the reformed fuel stream 74 used by the gas turbine system 30. 
     The gas turbine system 30 comprises a compressor 12 connected to a turbine 14 via a shaft 36 that is also connected to an electrical generator 28. The air stream 40 is directed into the compressor 12 and compressed to produce a compressed air stream 46. In a preferred embodiment of the invention, the compressor 12 may have a pressure ratio of 15. A first portion 48 of the compressed air 48 is directed to the turbine 14. A second portion 50 of the compressed air stream is directed to a combustor 16, where it is used to combust the reformed fuel stream 74 to produce a combustor exhaust stream 76. In a preferred embodiment of the invention, the oxygen concentration of the combustor exhaust stream 76 may be 6.7 mole percent. The combustor exhaust stream 76 is also directed to the turbine 14. 
     The turbine 14 expands the compressed air stream first portion 48 and the combustor exhaust stream 76, thus rotating the shaft 36 and driving the compressor 12 and an electrical generator 28. The expanded streams exit the turbine 14 as a turbine exhaust stream 58 and are combined with a third portion 52 of the compressed air stream 46 to form the compressed air/turbine exhaust stream 60 with thermal energy. Other embodiments of the invention may not mix the turbine exhaust stream with the third portion 52 of the compressed air stream 58. According to the embodiment of the invention shown in FIG. 1, the turbine 14 is cooled by a cooling compressed air stream 54 that splits off from the compress air stream third portion 52. Other embodiments of the invention may have other means for cooling the turbine 14. 
     The steam generation system 32 of the embodiment of the invention shown in FIG. 1 comprises an evaporator 20 and a economizer 22. The compressed air/turbine exhaust stream 60 is directed into the evaporator 20 where it heats a heated water stream 68 to produce the steam flow 70. The now cooled compressed air/turbine exhaust stream 62 is then directed from the evaporator 20 into the economizer 22 where it heats the water stream 66 to produce the heated water stream 68. The now much cooler compressed air/turbine exhaust stream exits the economizer 22 as the system exhaust 64. In a preferred embodiment of the invention, the flow rate of the water stream 66 may be adjusted with valve 82 in the line to generate a temperature difference of approximately 18° F. between the cooled compressed air/turbine exhaust stream 62 and the heated water stream 68. 
     As previously discussed, the reformer 18 receives the steam flow 70 and the fuel stream 72 to produce the reformed fuel stream 74 used by the gas turbine system 30. The fuel stream 72 comprises any fuel that is reformable and enables the reformer 18 to produce a reformed fuel stream 74 that is combustible in the combustor 16. In an embodiment of the invention, the fuel stream may be natural gas, liquefied natural gas, synthetically-derived hydrocarbon fuel, or a mixture thereof. In a preferred embodiment of the invention, the flow rates of the steam flow 70 and a fuel stream 72 of natural gas may be adjusted by valves 82 and 84 in the respective lines to maintain a steam-to-natural-gas mass ratio thereof of approximately 6.5 and a methane-to-carbon-monoxide conversion of approximately 59.6%. In an embodiment of the invention, the temperature of the reforming process may be between approximately 400° F. and 1100° F., however, a suitable catalyst for the reformer 18 and temperature range for reforming the fuel is determined based upon the fuel being reformed. 
     To achieve the requisite temperatures range to operate the reformer 18, the compressed air/turbine exhaust gas stream 60 passes through a closed heat exchange means in the reformer 18 to deliver thermal energy from the stream 60 to the, reformer 18. In the preferred embodiment of the invention, the compressed air/turbine exhaust gas stream 60 is approximately 36° F. hotter than the reformed fuel stream 74, which is a relatively low temperature approach or driving force. 
     Other embodiments of the invention may use other means to provide the steam 70 for reforming the fuel stream 72. In the embodiment of the invention shown in FIG. 2, the power generation system 10 is part of an electricity-steam cogeneration plant. The steam generation portion 78 of the cogeneration plant receives the compressed air/turbine exhaust stream 60 after some of its thermal energy has been removed by the reformer 18. The steam generation portion 78 recovers more thermal energy from the compressed air/turbine exhaust stream 60. The steam generation portion 78 also provides the steam flow 70 for reforming the fuel 72. 
     The claimed invention provides an efficient power generation system and device. The thermal chemical recuperation cycle 10 had a net cycle efficiency of 48.85% on an APSEN PLUS simulation thereof, compared to the efficiencies of 35.91% and 45.63% for a simple cycle gas turbine cycle and a steam injected turbine cycle respectively. Further, the thermal chemical recuperation cycle of the current invention has lower NO x  emissions. This is a result of the hydrogen-rich reformed fuel stream 74 having extended the flammability limits, and tolerating relatively large amounts of steam (not shown) to enter into the combustor 16 and lower the flame temperature. 
     Although the present invention has been discussed with reference a steam generation means comprising an evaporator/economizer system or a cogeneration plant, any means that provides steam to the reformer using thermal energy from the turbine exhaust is suitable for practicing the claimed invention. Further, any means that provides thermal energy from the turbine exhaust to the reformer is suitable for practicing the claimed invention. Consequently, the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.