Abstract:
The invention pertains to a power plant including a gas turbine, a heat recovery boiler arrangement with at least a boiler inlet, and an outlet side with a first exit connected to a stack and a second exit connected to a flue gas recirculation, which connects the second exit to the compressor inlet of the gas turbine. The heat recovery boiler arrangement includes a first boiler flue gas path from the boiler inlet to the first boiler exit, and a separate second boiler flue gas path from the boiler inlet to the second boiler exit. Additionally, a supplementary firing and a subsequent catalytic NOx converter are arranged in the first boiler flue gas path. Besides the power plant a method to operate such a power plant is an object of the invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/EP2013/064765 filed Jul. 12, 2013, which claims priority to European application 12176258.7 filed Jul. 13, 2012, both of which are hereby incorporated in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to combined cycle power plant with a catalytic converter and a method for operating such a power plant. 
       BACKGROUND 
       [0003]    Selective catalytic reduction (SCR) with ammonia is a common means for NOx reduction in gas turbine power plants. It converts nitrogen oxides, also referred to as NOx, with the aid of a catalyst into N2, and water, H2O. The use of three-way catalytic converters has been proposed to avoid the use of ammonia. However they require a fuel to air ratio close to the stoichiometric point. To reduce the oxygen content in the combustion gases a gas turbine plant is proposed in the U.S. 2009/0284013 A1, which comprises a gas turbine, a heat recovery steam generator and a flue gas recirculation. The gas turbine comprises a compressor for air, and a compressor for recirculated flue gas, a burner and a turbine. The input side of heat recovery steam generator is connected to a turbine outlet of the gas turbine. The heat recovery steam generator comprises two boiler outlets. A chimney is connected to the first boiler outlet. The flue gas recirculation connects to the second boiler outlet with a compressor inlet of the compressor for recirculated flue gas. Further, a flue gas treatment device in the form of a three-way catalyst disposed upstream of the waste heat boiler is known from this gas turbine plant. 
         [0004]    The proposed plant allows combustion with a fuel to air ratio λ close to one by reducing the oxygen content in the combustor inlet gas. However, the complete stable combustion at close to stoichiometric conditions in the restricted space of a gas turbine combustor is difficult to realize. Additionally, the matching of two compressors (e.g. with respect to mass flows and pressures) on a single shaft is difficult to realize for different operation conditions. 
       SUMMARY 
       [0005]    One object of the disclosure is to provide a combined cycle power plant with low NOx emissions, which does not require ammonia for NOx reduction and has a good operational flexibility. 
         [0006]    One aspect of the present disclosure is to propose a combined cycle power plant with a gas turbine and the heat recovery boiler arrangement that comprises a first boiler flue gas path from the boiler inlet to a first boiler exit and a separate second boiler flue gas path from the boiler inlet to the second boiler exit with a supplementary firing and a subsequent three-way catalytic converter arranged in the first boiler flue gas path. 
         [0007]    The gas turbine comprises at least a compressor, at least one combustor and at least one turbine. The heat recovery boiler arrangement has a boiler inlet connected to a turbine outlet, and an outlet side with a first exit connected to a stack and a second exit connected to a flue gas recirculation, which connects the second exit to the compressor inlet of the gas turbine. 
         [0008]    According to one exemplary embodiment the catalytic converter is a three-way catalytic converter. According to another exemplary embodiment the catalytic converter comprises a NOx adsorbing catalyst. 
         [0009]    According to an exemplary embodiment the first boiler flue gas path comprises a first sector, which connects the diffusor inlet to the NOx adsorbing catalyst. When in operation the adsorbing catalyst in the first sector is loading with NOx from the gas turbine flue gas. Further, the first boiler flue gas path comprises a second sector, which connects the diffusor inlet to the NOx adsorbing catalyst. The supplementary firing is installed in the second sector between the diffusor inlet and NOx adsorbing catalyst. When in operation the NOx adsorbing catalyst is regenerated in the second sector of the NOx adsorbing catalyst. 
         [0010]    One possible arrangement with at least two sectors comprises an adsorbing catalyst, which is rotatable mounted in the first boiler flue gas path. In operation a section of the adsorbing catalyst rotates from the adsorbing sector to the regenerating sector and a regenerating section of the adsorbing catalyst rotates form the regenerating sector to the adsorbing sector. Thus in operation a cyclic loading and regeneration of the adsorbing catalyst can be achieved. 
         [0011]    For continuous operation a according to one embodiment the adsorbing catalyst is configured as regenerative rotary catalyst, comprising a generally circular disk arranged to rotate in a cylindrical casing. The casing comprises a connection to the first inlet sector inlet and a connection to a second inlet sector, and has a common outlet. 
         [0012]    To reduce the size and fuel consumption the supplementary firing can be minimized. The inlet area to regenerating second sector can be smaller than the inlet area of the adsorbing sector to reduce the size and fuel consumption of the supplementary firing. In an exemplary embodiment the inlet area of the first sector for loading the adsorbing catalyst is at least twice as large as the inlet area for the second sector for regenerating the adsorbing catalyst. 
         [0013]    In another exemplary embodiment the NOx adsorbing catalyst is stationary. To allow alternating loading and regeneration of different sections of the NOx adsorbing catalyst the flue gas path is divided in at least two sectors with at least two supplementary sector firings installed upstream of the NOx adsorbing catalyst. Each supplementary sector firings has an independent fuel supply control to allow independent operation of the supplementary firing in each sector for regeneration of the subsequent NOx adsorbing catalyst. 
         [0014]    In yet another embodiment of the power plant the heat recovery boiler arrangement comprises a control member arranged to control the mass flow split between of the flue gas entering the first boiler flue gas path and the separate second boiler flue gas path. This control member or control element can for example be a flap, a moveable baffle or a valve installed in one of the flow paths. 
         [0015]    According to one exemplary embodiment the compressor intake is split into sectors connected with a flow passage of the compressor, with a feed for fresh air leading through a first sector of the compressor intake, and with a feed for the first flue gas flow leading through a second sector of the compressor intake. In an exemplary arrangement the first and second zone are coaxial. This allows the connection of the line for recirculated flue gas to outer zone of the coaxial inlet zones. As a result flue gas is recirculated to the radially outer zone of the compressor intake and fed to the secondary air system of the gas turbine. This reduces or avoids bypassing oxygen from fresh air around the combustor and can therefore reduce the oxygen content of the flue gases. 
         [0016]    Besides the power plant a method for operation of a power plant, which comprises a gas turbine with at least a compressor, a combustor and a turbine, a heat recovery boiler arrangement with at least a boiler inlet connected to a turbine outlet, and an outlet side with a first exit connected to a stack and a second exit connected to a flue gas recirculation, which connects the second exit to the compressor inlet of the gas turbine, is a subject of the disclosure. 
         [0017]    According to an exemplary embodiment of the method for operating such a power plant the flue gas is split into two flows in the heat recovery boiler arrangement, with a first flow flowing from the boiler inlet to the first boiler exit and a second flow flowing from the boiler inlet to the second boiler exit. The second flow is recirculated from the second boiler exit into the compressor inlet flow of the gas turbine. The oxygen content in at least a fraction of the first flow is reduced continuously or at least for a period of time by a supplementary firing and NOx is removed from the first flow in a catalytic converter before the first flow is released from the first boiler exit. 
         [0018]    According to one configuration of the method NOx is removed in an adsorbing catalyst. 
         [0019]    According to an exemplary embodiment of the method the fraction of the first flow with reduced oxygen content is fed to a first sector in the adsorbing catalyst for regeneration of first sector of the adsorbing catalyst and the remaining first flow is fed to a second sector in the adsorbing catalyst. NOx is removed from the first flow in both: the first sector and second sector of the adsorbing catalyst. While adsorbing NOx the adsorbing catalyst in the second sector is loading. Depending on the capacity of the NOx adsorbing catalyst and the NOx emissions contained in the flue gas a periodic regeneration of the adsorbing catalyst is required. The sectors, which are regenerated are changed accordingly, e.g. periodically over time, as a function of at least one of NOx adsorbing capacity, time, NOx emissions and flow velocity. Alternatively or in combination the NOx loading of the catalyst can be measured. 
         [0020]    According to one embodiment of the method the NOx adsorbing catalyst is configured as regenerative rotary catalyst comprising a generally circular disk and is rotated to move the regenerated section of the adsorbing catalyst out of the first sector for loading with NOx in the second sector. Due to the rotation the at least partly loaded section of the adsorbing catalyst is moved out of the second sector for regeneration into the first sector at the same time. The rotational speed can be adjusted to assure that the adsorbing catalyst is moved back from the second sector to the first section before it is completely loaded. 
         [0021]    The sizes of the first and second sections are chosen depending on the thickness of the adsorbing catalyst in flow direction, the NOx emissions and the flow velocity. 
         [0022]    According to an alternative embodiment the first boiler flue gas path is divided into sectors, with at least one supplementary sector firing arranged in each sector. According to the method the supplementary sector firing are alternatingly turned on to regenerate the corresponding sector of the NOx adsorbing catalyst and turned off for loading the corresponding sector of the NOx adsorbing catalyst with NOx. 
         [0023]    According to a further exemplary embodiment the supplementary firing is alternatingly turned on for regeneration of the adsorbing catalyst and turned off to minimize fuel consumption for the supplementary firing while the adsorbing catalyst is filling with NOx. Turning on and off of the supplementary firing can be combined with sector-wise sector firing. 
         [0024]    The above described gas turbine can be a single combustion gas turbine or a sequential combustion gas turbine as known for example from EP0620363 B1 or EP0718470 A2. The disclosed method can be applied to single combustion gas turbine as well as to a sequential combustion gas turbine. 
         [0025]    It will be appreciated by those skilled in the art that the present invention can be embodied in other forms without departing from the spirit or essential characteristics thereof. For example a supplementary firing has been proposed for regeneration of the NOx adsorbing catalyst. The NOx adsorbing catalyst can also be regenerated by injecting or admixing CO or unburned hydrocarbon into the flue gas flow upstream of the NOx adsorbing catalyst or the section of NOx adsorbing catalyst. When regenerating the NOx adsorbing catalyst the speed of regeneration has to be controlled to avoid overheating of the NOx adsorbing catalyst or subsequent installations. Therefore a control in rate of injection of CO or unburned hydrocarbons can be foreseen. This control can for example control the flow of CO or unburned hydrocarbons as a function of the temperature in the NOx adsorbing catalyst or the flue gas flow downstream thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention, its nature as well as its advantages, shall be described in more detail below with the aid of the accompanying drawings. Referring to the drawings: 
           [0027]      FIG. 1  shows a first example of a gas turbine according to the present invention, 
           [0028]      FIG. 2  shows a second example of a gas turbine according to the present invention. 
           [0029]      FIG. 3  shows an example of a supplementary firing with sectors. 
           [0030]      FIG. 4  shows an example of a supplementary firing with subsequent rotating NOx adsorbing catalyst. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The same or functionally identical elements are provided with the same designations below. The values and dimensional specifications indicated are only exemplary values and do not constitute any restriction of the invention to such dimensions. 
         [0032]    According to the  FIGS. 1 and 2  an exemplary gas turbine power plant  1 , which can for example be applied in a power plant arrangement for electricity generation, comprises at least a gas turbine  2 , at least a heat recovery boiler arrangement  3  as well as at least a flue gas recirculation  4 . The respective gas turbine  2  comprises at least one compressor  5 , at least a combustor  6 ,  7  as well as at least one turbine  8 ,  9 . In the embodiments shown here the gas turbine  2  comprises two turbines  8  and  9 , namely a high pressure turbine  8  and a low pressure turbine  9 . Accordingly, two combustors  6  and  7  are also provided, namely a high pressure combustor  6  upstream of the high pressure turbine  8  and a low pressure combustor  7  upstream of the low pressure turbine  9 .  FIG. 1  shows a side view, and  FIG. 2  a top view of exemplary gas turbine power plants  1 . The steam generated in the boiler  3  can be used in a water-steam cycle or for co-generation (not shown). 
         [0033]    The heat recovery boiler arrangement  3  has a boiler inlet side  10  and a boiler exit side  11 . The boiler inlet side  10  is fluidically connected with a turbine outlet  12  of the low pressure turbine  9 . The boiler exit side  11  comprises a first boiler exit  13  and a second boiler exit  14 . The first boiler exit  13  is connected with a stack  15 . Between the first boiler exit  13  and the stack CO2 capture equipment can be arranged (not shown). The second boiler exit  14  is fluidically connected with an inlet  16  of the flue gas recirculation  4 . An outlet  17  of the flue gas recirculation  4  is connected with a compressor inlet  18  of the compressor  5 . Therefore the flue gas recirculation  4  connects the second boiler exit  14  with the compressor inlet  18 . In the examples a flue gas re-cooler  19  is arranged in the flue gas recirculation  4 , which can be designed as a DCC (direct contact cooler), so that the recirculated flue gas can be cooled and be washed at the same time. 
         [0034]    In the embodiments shown the heat recovery boiler arrangement  3  comprises a first boiler flue gas path  20 , which is indicated in the  FIGS. 1 and 2  by an arrow. The first boiler flue gas path  20  starts at the boiler inlet side  10  and leads to the first boiler exit  13 . Further, the heat recovery boiler arrangement  3  comprises a second boiler flue gas path  21 , which is also indicated by an arrow. The second boiler flue gas path  21  also starts at the boiler inlet side  10  and leads to the second boiler exit  14 . Both boiler flue gas paths  20 ,  21  are separated and lead to the respective boiler exits  13 ,  14 . For the realization of the separated boiler flue gas paths  20 ,  21  within the heat recovery boiler arrangement  3  a boiler partition  22  can be arranged in the heat recovery boiler arrangement  3 , which fluidically separates both boiler flue gas paths  20 ,  21 . 
         [0035]    In the embodiments shown in here a diffuser  23  is arranged upstream of the boiler inlet side  10 . The diffuser inlet  24  is connected with the turbine outlet  12 . In each case the diffuser  23  comprises a diffuser inlet  24  and at least a diffuser exit  25 ,  26 . In the embodiment of  FIG. 1  two diffuser exits, namely the first diffuser exit  25  and the second diffuser exit  26  are shown. In contrast only a single, common diffuser exit  25  is shown in the embodiment of the  FIG. 2 . 
         [0036]    In the embodiment of  FIG. 2  a common diffuser exit  25  is fluidically connected with the boiler inlet side  10 . In the embodiment of  FIG. 1  the first diffuser exit  25  is fluidically connected with the first boiler inlet  27 , while the second diffuser exit  26  is fluidically connected with the second boiler inlet  28 . Both boiler inlets  27 ,  28  are arranged at the boiler inlet side  10 . According to the embodiment of  FIG. 1  the first boiler flue gas path  20  leads from the first boiler inlet  2  to the first boiler exit  13 . In parallel and separately the second boiler flue gas path  21  leads from of the second boiler inlet  28  to the second boiler exit  14 . 
         [0037]    In the diffuser  23  of  FIG. 1  a common diffuser main path  29 , which is indicated by an arrow, as well as the first diffuser flue gas path  30  which is indicated by an arrow, and the second diffuser flue gas path  31 , which is also indicated by an arrow, are arranged. The common diffuser main path  29  is split into the separated diffuser flue gas paths  30 ,  31  at a diffusor branching point  32 . To separate the diffuser flue gas paths  30 ,  31  a diffuser partition  33  is arranged in a diffuser housing  58  of the diffuser  23 . A leading edge  34  of the diffuser partition  33  defines the diffusor branching point  32 . The diffuser partition  33  separates both diffuser flue gas paths  30 ,  31  from the diffusor branching point  32  up to both diffuser exits  25 ,  26 . In the example of the  FIG. 1  the diffuser partition  33  and the boiler partition  22  are arranged such that trailing edge  35  of the diffuser partition  33  and a leading edge  36  of the boiler partition  22  adjoin. 
         [0038]    By the adjoining the partitions  22 ,  33  the first diffuser flue gas path  30  passes directly on to the first boiler flue gas path  20 , while at the second diffuser flue gas path  31  passes on to the second boiler flue gas path  21 . 
         [0039]    In the exemplary embodiment of  FIG. 1  a control member  37  is arranged at the diffusor branching point  32 , which is pivotable around a swivel axis  39  as indicated by the arrow  38 . With the help of the control member  37  the split of the flue gas flow to both diffuser flue gas paths  30 ,  31  can be controlled. 
         [0040]    In the exemplary embodiment of  FIG. 2  is a control member  45  is arranged at the boiler branching point  44 , which is pivotable around a swivel axis  47  as indicated by the arrow  46 . With the help of the control member  45  the split of the flue gas flow to both boiler flue gas paths  20 ,  21  can be controlled. 
         [0041]    In the first boiler flue gas path  20  a supplementary firing  49 , catalytic NOx converter  50  and a first heat exchanger array  52  are provided. The catalytic NOx converter  50  is arranged downstream of the supplementary firing  49 . In the examples shown here the first heat exchanger array  52  is arranged downstream of the catalytic NOx converter  50 . However, depending on the temperature after the supplementary firing and on the design of the catalytic NOx converter  50  a part of the first heat exchanger array  52  can be arranged upstream of the catalytic NOx converter  50  to reduce the flue gas temperature, and the remaining first heat exchanger array  52  can be arranged downstream of the catalytic NOx converter  50 . 
         [0042]    In the second boiler flue gas path  21  a second heat exchanger array  48  is provided. The first heat exchanger array  52  and second heat exchanger array  48  can be separated arrangements or integrated with at least part of the heat exchanger elements passing from the first to the second boiler flue gas path  21 . 
         [0043]    As shown in  FIG. 1  a control member  40 , which is pivotable around a swivel axis  40  as indicated by the arrow  41 , can be arranged at the downstream end of the heat recovery boiler arrangement  3 . This control member can be used as alternative or in combination with the control member  37  to control the split between recirculated flue gas and flue gas directed to the stack  15 . Further, it can be used to stop flue gas recirculation and to allow the second boiler flue gas path  21  to exit to the stack  15 . 
         [0044]    If the oxygen concentration of the flue gases in the first boiler flue gas path  20  can be controlled over the entire cross section with the help of the supplementary firing  49  a three-way catalytic converter  50  can be used. For measurement of the oxygen concentration at least a λ-sensor can be used. The measured oxygen concentration can be used to control the fuel flow to the supplementary firing  49 . 
         [0045]    Recirculated flue gas and fresh air  61  can be mixed upstream of the compressor inlet  18  as schematically shown in  FIG. 2 . 
         [0046]    In another exemplary embodiment of a power plant  1  the compressor intake is split into two sectors as shown in  FIG. 1 . In the depicted example, the compressor intake  66  is split by means of an intake baffle plate  67  into an outer fresh air intake sector  64  for fresh air  61  and into a flue gas intake sector  65  for recirculated flue gas  69 . This splitting of the compressor intake  66  leads to an essentially coaxial inflow of recirculated flue gas and fresh air  61  into the compressor  5 . A fresh air control element  68  allows the supply of fresh air to the flue gas intake sector  65  to allow operation with reduced or no flue gas recirculation. 
         [0047]    To minimize the fuel consumption of the supplementary firing  49 , the combination of a NOx adsorbing catalyst  50  with a supplementary sector firing  53 ,  54 ,  55 ,  56  is proposed.  FIG. 3  shows one exemplary variant of the cross section III-III of the  FIG. 2 . The cross section is divided into four sections I-IV. In each of the sectors a supplementary sector firing  53 ,  54 ,  55 ,  56  is arranged. Each of the supplementary sector firings  53 ,  54 ,  55 ,  56  can be individually controlled by sector control valve  57 . Thus the sector firings  53 ,  54 ,  55 ,  56  can be individually activated and the fuel flow controlled to a stoichiometric fuel ratio to assure that the flue gasses passing the activated sector firing  53 ,  54 ,  55 ,  56  have a λ close to one. Typically the fuel flow to an activated sector is controlled to keep λ in a range between 0.97 and 1.03 for regeneration of the NOx adsorbing catalyst  50 . The remaining supplementary sector firings  53 - 56  can be switched of and NOx adsorbing catalyst  50  is loading. 
         [0048]    Another exemplary embodiment with sectorwise loading and regeneration of the NOx adsorbing catalyst  50  is shown in  FIG. 4 . In this example the first boiler flue gas path  20  has a cylindrical shape in the region of the NOx adsorbing catalyst  50 . The NOx adsorbing catalyst  50  has the shape of a circular disk and is mounted rotatable around a rotating axis  59 . The supplementary firing  49  is arranged in a first sector I of a circle upstream of the NOx adsorbing catalyst  50 . When in operation part of the flue gas in the first boiler flue gas path  20  passes the supplementary firing  49 , which is controlled to a stoichiometric fuel ratio to assure that the flue gasses passing the activated sector firing have a λ close to one, typically with λ □in a range between 0.97 and 1.03 for regeneration of the NOx adsorbing catalyst  50 . The remaining second sector II of the NOx adsorbing catalyst  50  is loading with NOx. 
         [0049]    The sector of the supplementary firing  49  can be separated from the remaining flue gas by a partition wall  60 . 
         [0050]    It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted.