Abstract:
A combined Brayton cycle power plant ( 5 ). A combustion gas turbine engine ( 21 ) in the power plant uses a first Brayton cycle ( 20 ), and produces waste heat in an exhaust combustion gas ( 36 ). A heat exchanger ( 58 ) transfers the waste heat to a compressed working airflow ( 56 ) for a second Brayton cycle ( 50 ) in a heat recovery gas turbine engine ( 51 ). The heat transfer lowers the temperature of the combustion exhaust gas ( 36 ) to within an operating range of a conventional selective catalytic reduction unit ( 80 ), for efficient reduction of nitrogen oxide emissions to meet environmental regulations.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to electric power generation, especially to combined cycle power generation using a gas turbine engine in a first power cycle that produces waste exhaust heat, and a waste heat recovery system driving a second power cycle.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electric power plants commonly use F-class gas turbine technology, which is distinguished by firing temperatures of about 1,300° C. and exhaust temperatures of over 580° C. A strong demand exists for turbine power plants with nitrogen oxide (NOx) emissions low enough to meet increasingly strict environmental regulations. Since gas turbines themselves do not achieve the required low emissions, NOx removal technology must be applied to the combustion exhaust gas. There are currently two main commercial alternatives for this: 1) Hot selective catalytic reduction (SCR), which can operate at the gas turbine exhaust temperature; and 2) Conventional SCR, which must operate at temperatures far below the gas turbine exhaust temperature, such as 232° C. to 370° C. Conventional SCR is preferable, due to its higher efficiency, reliability, and lower cost. Thus, technologies have been developed to reduce exhaust gas temperature to the operating range of conventional SCR. These include mixing the exhaust with ambient air, or using the hot exhaust gas in a heat recovery system that powers a subsequent power cycle such as a steam turbine. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]     The invention is explained in following description in view of the drawings that show:  
         [0004]      FIG. 1  is a schematic view of a combined cycle power plant comprising two gas turbine generators and a conventional selective catalytic reduction unit. The second gas turbine is a heat recovery gas turbine that uses heated air for a working gas.  
         [0005]      FIG. 2  is a schematic view as in  FIG. 1  except the two gas turbines have a common power shaft and generator.  
         [0006]      FIG. 3  illustrates the volume and temperature envelopes of a illustrative prior art Brayton cycle.  
         [0007]      FIG. 4  is a graph of plant efficiency as a function of air compression ratio in the heat recovery gas turbine, and is based on thermodynamic modeling. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     Gas turbine engines operate on a thermodynamic Brayton cycle, in which ambient air is drawn into a compressor and pressurized. The compressed air is heated in a generally constant-pressure process in a heating chamber that is open to both inflow and outflow. This is normally done by burning fuel in the compressed air in a combustion chamber, producing a hot working gas comprising combustion gasses. The heated air is then expanded through a turbine to extract energy in the form of shaft power.  FIG. 3  illustrates aspects of an illustrative Brayton cycle comprising a series of transitions  1 ,  2 ,  3 , and  4  of a working gas, starting from atmospheric pressure  10 , then to compression  11 , combustion  12 , expansion through a turbine section  13 , and exhaust  14 .  
         [0009]     In accordance with an aspect of the invention  FIG. 1  schematically shows a combined cycle power generator  5  comprising two cooperating Brayton cycles  20  and  50 . The first Brayton cycle  20  may comprise a combustion turbine engine  21  with an air inlet  22 , an air compressor  24 , a compressed airflow  26 , a combustor  28 , a fuel supply  30 , a compressed combustion gas flow  32 , a combustion gas turbine  34 , and an exhaust combustion gas flow  36 . The combustion gas turbine  34  drives a power shaft  38  that drives the air compressor  24  and a generator  40 , supplying electrical power  41  to a plant load  72 , as known in the field of gas turbine generators.  
         [0010]     A second Brayton cycle  50  may comprise a heat recovery gas turbine engine (HRGT)  51  comprising an air inlet  52 , an air compressor  54 , a compressed airflow  56 , a heat exchanger  58 , a compressed heated airflow  62 , a hot air turbine  64 , and an exhaust airflow  66 . The hot air turbine  64  drives a power shaft  68  that drives the air compressor  54  and a generator  70 , producing electrical power  71 . The heat exchanger  58  transfers heat from the exhaust combustion gas flow  36  to the compressed airflow  56 , providing heat energy for the second Brayton cycle. This recovers waste heat from the first Brayton cycle, and reduces the temperature of the exhaust combustion gas flow  36  to the operating range of a conventional selective catalytic reduction unit  80 . The electrical power outputs  41  and  71  may be combined to supply the plant load  72 .  
         [0011]     In an aspect of the present invention, the heat recovery gas turbine  51  comprises a heat exchanger  58  instead of a combustion chamber  28  heating the compressed air in the generally constant-pressure process. The heat exchanger  58  transfers waste heat from the first Brayton cycle  20  to the second compressed airflow  56 , producing heated compressed air  62  as the working gas. The term “gas turbine” is used generically herein for gas turbine engines with either type of heating; i.e. combustion or heat exchange, while “combustion gas turbine” is used to denote a gas turbine engine in which combustion occurs in the working gas. In either case, the compressed and heated working gas, comprising either combustion gas or air, then transfers some of its energy to shaft power by expanding through a turbine or series of turbines. Some of the shaft power extracted by the turbine is used to drive the compressor.  
         [0012]     In accordance with another aspect of the invention,  FIG. 2  schematically shows a combustion gas turbine engine  21  and a heat recovery gas turbine engine  51  arranged to drive a common generator  40 , producing electrical power to supply a plant load  72 . Power shaft transmission gearing (not shown) may be used to match the speed of both engines  21 ,  51  to the same generator  40 , if necessary.  
         [0013]     An important factor in the efficiency of the present invention is the HRGT compression ratio; i.e. the ratio between the outlet and inlet pressures of the HRGT compressor  54 .  FIG. 4  illustrates exemplary optimization curves for plant power generation efficiency as a function of the HRGT compression ratio. The two curves represent results of thermodynamic modeling at two different air mass flow rates. Typical efficiencies for the HRGT components were used in the modeling, and were held constant. This analysis shows that an optimum HRGT compression ratio falls between 4 and 6 at both flow rates.  
         [0014]     A cost-effective means to produce an HRGT for the present invention is to use standard equipment wherever possible. An existing combustion gas turbine engine design can be modified for this purpose by replacing the combustor with a heat exchanger. Some combustion gas turbine engines have a combustion chamber in a silo connected by ducts to the gas flow of the engine. It is generally easier to replace this type of combustion chamber with a heat exchanger than to replace a can-style combustor. Typical commercially available combustion gas turbine engines have a compression ratio of over 10. One or more stages at the compressor outlet and one or more stages at the inlet of the turbine section may be removed to reduce the compression ratio of an existing gas turbine engine to a desired range for an HRGT application.  
         [0015]     As an illustrative example of this type of implementation of the invention, a primary combustion gas turbine generator such as Siemens SGT6-5000F may be enhanced by adding a heat recovery gas turbine made by modifying a second combustion gas turbine such as Siemens SGT5-2000F. The combustion chamber of the second gas turbine may be replaced with a heat exchanger. The last 4 stages of the compressor and the first stage of the turbine section of the secondary gas turbine may be removed to achieve a pressure ratio of approximately 6. Ducting the combustion exhaust from the primary gas turbine through the heat exchanger, and operating the second gas turbine as described herein, will bring the combustion exhaust within range of conventional SCR units.  
         [0016]     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.