Abstract:
A method for recirculation of exhaust gas from a combustion chamber of a combustor of a gas turbine back to the supply side of the combustor provides a partial flow of the exhaust gas in the combustion chamber directly extracted from the combustion chamber and internally fed back to an entrance of the combustor through an internal channel of the combustor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European application 14172924.4 filed Jun. 18, 2014, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to the technology of gas turbines. It refers to a method for recirculation of exhaust gas from a combustion chamber of a combustor of a gas turbine according to the preamble of claim  1 . 
         [0003]    It further refers to a gas turbine for conducting said method. 
       BACKGROUND 
       [0004]    The reheat system (sequential combustion) or staged system that is for example incorporated in a gas turbine of the GT24 type of the applicant, proved to excel in achieving very low emissions at high firing temperature. 
         [0005]    Such combustion system fulfills also a number of stringent requirements such as: low pressure drop, low cooling air consumption, long lifetime and fuel flexibility. One of the key elements such systems rely on, is the mixing level achieved between hot gas and fuel or fuel/air mixture before the flame. 
         [0006]    The concepts developed up to now provide very good mixing levels and combustion performances. However they rely either in high momentum flux ratios, complicated geometries or complex sealing systems between the fuel injector and the hot gas path (see for example documents U.S. Pat. No. 5,645,410, WO 2011/037646 A1, U.S. Pat. No. 5,351,474, U.S. Pat. No. 6,192,688 B1, WO 2011/054757, WO 2011/054766, EP 2 211 109). 
         [0007]    Another way is flue gas recirculation. This way reduces the O 2  content in the air/flue gas mixture which results in lower NO x  emissions (see  FIG. 1 , which shows the NO x /CO content as a function of O 2  content). There are several documents (e.g. U.S. Pat. No. 3,949,548, U.S. Pat. No. 4,609,342, JP 11304151, EP 1 752 616 A2, WO 2008/155242 A1) dealing with flue gas recirculation. All these documents use external flue gas recirculation whereas the flue gas is taken from the exhaust flow and guided back to the compressor inlet or an additional compressor is used to compress the flue gas up to a certain pressure (fuel preheating, cooling of hot gas parts etc). 
         [0008]    Although the known flue gas recirculation scheme leads to better emission properties of the gas turbine, it is connected with substantial additional apparatus complexity and requires a substantial amount of investment. 
       SUMMARY 
       [0009]    It is an object of the present invention to provide a method for recirculation of exhaust gas from a combustion chamber of a combustor of a gas turbine, which has the advantage of flue gas recirculation in general but is less costly and requires a substantially reduced effort compared with know recirculation technology. 
         [0010]    It is a further object to provide a gas turbine for conducting said method. 
         [0011]    These and other objects are obtained by a method according to claim  1  and a gas turbine according to claim  11 . 
         [0012]    The inventive method for recirculation of exhaust gas from a combustion chamber of a combustor of a gas turbine back to the supply side of said combustor is characterized in that a partial flow of the exhaust gas in said combustion chamber is directly extracted from said combustion chamber and internally fed back to an entrance of the combustor through an internal channel of said combustor. 
         [0013]    According to an embodiment of the invention said partial flow of the exhaust gas is extracted by means of a Venturi arrangement, which is provided at said combustion chamber, and which is operated by a pressurized fluid. 
         [0014]    Specifically, said pressurized fluid is compressed air, said internal channel is a cooling air channel, and said exhaust gas is recirculated as part of a mixed exhaust gas/air recirculation flow. 
         [0015]    More specifically, said combustion chamber is confined by a combustor liner wall, said Venturi arrangement is provided at the outside of said combustor liner wall, and said partial flow of the exhaust gas is extracted through a suction hole in said combustor liner wall. 
         [0016]    Alternatively, said combustion chamber is confined by a combustor liner wall, said Venturi arrangement is integrated into said combustor liner wall, and said partial flow of the exhaust gas is extracted through a suction hole in said combustor liner wall. 
         [0017]    According to another embodiment of the invention said compressed air flows through a cooling air channel at the outside of and parallel to, said combustor liner wall, said Venturi arrangement is arranged within said cooling air channel, and most of the compressed air passing said Venturi arrangement is guided through said Venturi arrangement. 
         [0018]    Specifically, part of said compressed air passing said Venturi arrangement is guided outside along said Venturi arrangement through a separate cooling air flow path provided between said Venturi arrangement and said combustor liner wall. 
         [0019]    According to another embodiment of the invention said Venturi arrangement comprises a separate nozzle and a separate diffusor, and the nozzle is arranged within the combustion chamber and ejects compressed air through said suction hole into said diffusor, which is arranged outside said combustor liner wall. 
         [0020]    According to another embodiment of the invention said compressed air is supplied from a compressor of said gas turbine or from an external compressed air storage, and said compressed air flows along said combustion chamber in a direction opposite to said exhaust gas. 
         [0021]    According to another embodiment of the invention the mass flow of said pressurized fluid through said Venturi arrangement is controlled by a valve mechanism. 
         [0022]    The inventive gas turbine comprises at least one combustion chamber for generating hot gas by burning a fuel/air mixture, which combustion chamber opens with a transition section into a subsequent turbine section, whereby said at least one combustion chamber is surrounded by a cooling air channel, through which compressed cooling air flows opposite to said hot gas in said combustion chamber 
         [0023]    It is characterized in that said combustion chamber and said adjacent cooling air channel are separated by a combustor liner wall, and that there a suction holes provided in said combustor liner wall at places, where the static pressure is low enough that exhaust gas is sucked through said suction holes into said cooling air channel. 
         [0024]    According to an embodiment of the invention Venturi arrangements are provided at said suction holes in order to generate the necessary low static pressure. 
         [0025]    Specifically, said Venturi arrangements are provided at Venturi arrangement locations in said transition section of said combustion chamber. 
         [0026]    More specifically, said gas turbine is provided with sequential combustion and comprises first and second combustion chambers and related first and second subsequent turbine sections, whereby Venturi arrangements are provided at first and/or second combustion chambers at respective Venturi arrangement locations. 
         [0027]    According to another embodiment of the invention said gas turbine has an annular combustion chamber with respect to a machine axis of said gas turbine. 
         [0028]    According to another embodiment of the invention said gas turbine has a plurality of combustion chambers in a circumferential arrangement around a machine axis of said gas turbine. 
         [0029]    According to another embodiment of the invention said gas turbine has a silo-type combustion chambers arranged perpendicular to a machine axis of said gas turbine. 
         [0030]    According to another embodiment of the invention said gas turbine comprises a compressor, and that said compressed cooling air flowing through said cooling air channel is supplied by said compressor. 
         [0031]    According to another embodiment of the invention said Venturi arrangements are provided at the outside of said combustor liner wall. 
         [0032]    According to another embodiment of the invention said Venturi arrangements are integrated into said combustor liner wall. 
         [0033]    Specifically, said Venturi arrangements each comprise a separate nozzle and a separate diffusor, and that the nozzle is arranged within the combustion chamber and ejects compressed air through a respective suction hole into said diffusor, which is arranged outside said combustor liner wall. 
         [0034]    According to another embodiment of the invention said Venturi arrangements each comprise a separate nozzle and a separate diffusor being arranged one after the other along a common axis. 
         [0035]    According to another embodiment of the invention the mass flow of said pressurized fluid through said Venturi arrangements is controlled by a valve mechanism. 
         [0036]    Venturi nozzles can have a circular cross section, as for example in a constriction of a pipe. However, other cross sections are conceivable. For example a constriction between two parallel plates in which the flow area is defined by the height of a channel between the two plates be also used to accelerate the inflow of pressurized by a reduction in channel height, which leads to a reduction in static pressure which in turn can be used to intake exhaust gases from a combustor. By subsequent increase of the channel height the dynamic pressure can be recovered to deliver a mixture of exhaust gas and inflow of pressurized fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings. 
           [0038]      FIG. 1  shows in a diagram the dependence of the NO x  emissions on the O2 content in an air/flue gas mixture of a flue gas recirculation scheme; 
           [0039]      FIG. 2  shows a simplified drawing of a Venturi exhaust gas recirculation arrangement in a cooling air channel according to an embodiment of the invention; 
           [0040]      FIG. 3  shows a simplified drawing of a Venturi exhaust gas recirculation arrangement integrated into the combustor liner wall according to another embodiment of the invention; 
           [0041]      FIG. 4  shows a simplified drawing of a Venturi exhaust gas recirculation arrangement in a cooling air channel with additional cooling means according to a further embodiment of the invention; 
           [0042]      FIG. 5  shows a section through a gas turbine of the GT24/26 type with sequential combustion and specific Venturi arrangement locations in accordance with a further embodiment of the invention; 
           [0043]      FIG. 6  shows a section through a gas turbine of the GT13E2 type with an annular combustor and specific Venturi arrangement locations in accordance with another embodiment of the invention; 
           [0044]      FIG. 7  shows a section through a different gas turbine (a) with a plurality of individual circumferentially arranged combustors (b) (can combustor configuration) and specific Venturi arrangement locations in accordance with another embodiment of the invention; and 
           [0045]      FIG. 8  shows in a perspective view a gas turbine of the GT11N2 type (a) with a silo-type combustor (b) and specific Venturi arrangement locations in accordance with a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    In the present invention a system is proposed, in which a novel passive flue gas recirculation is proposed. The recirculation is done inside the engine directly at the combustors. Such configuration could be well suited for sequential annular combustion systems (as in the GT24/GT26, see  FIG. 5 ), for annular combustion (GT13E2,see  FIG. 6 ), for can combustor configurations (see  FIG. 7 ) or silo combustors (GT11N2, see  FIG. 8 ). 
         [0047]    The system for the recirculation of exhaust gas should be designed to be able to recirculate 10-50%. That means the flue gas (or exhaust gas) content in the air/flue gas mixture is in a range of 10 to 50%. 
         [0048]    As shown in  FIG. 2 , a combustor liner wall  13  encloses the combustion chamber  11  of a combustor  10  of a gas turbine. Exhaust gas (flue gas)  18  flows in this example from right to left in order to leave the combustor and enter a subsequent turbine section with rotating blades (not shown). Outside the combustor liner wall  13  a cooling air channel  12  is arranged. Compressed air  19  flows opposite to exhaust gas flow  18  along cooling air channel  12  to cool the combustor liner wall  13  and later on to be mixed with fuel to enter the combustor through respective burners (see for example burners  24 ,  26  in  FIG. 5 ). 
         [0049]    A suction hole or flue gas exit  17  is provided in the combustor liner wall  13 . The suction hole or flue gas exit  17  is preferentially located in a place of the cooling air supply, where a low static pressure and high flow velocity of the cooling air is reached. A preferred construction option for generating such low static pressure is a Venturi arrangement (Venturi ejector)  14  comprising a nozzle  15  and a diffuser  16 , whereby the exit plane of the flue gas pipe coming from the suction hole  17  is located in the area of the Venturi arrangement  14  with the smallest cross section (see  FIG. 2 ). 
         [0050]    The compressed air  19  is moving along the combustor liner wall  13  towards the burner, where it enters the combustion chamber  11 . By guiding a part of the compressed air  18  into nozzles  15 , the air is accelerated and a surrounding low pressure region created. By this low pressure, exhaust gas  18   a  is sucked out of the combustor (combustion chamber  11 ) and entrained into the high velocity air jet resulting in a mixed exhaust gas/air flow  20 . 
         [0051]    By directing the air jet into a suitable Venturi nozzle  15 ,  16 , the process of exhaust gas extraction can be optimized for minimal pressure loss. A sufficient long path between the Venturi arrangement  14  and the burner at the upstream end of the combustor  10  will ensure good mixing between the ejected exhaust gas  18   a  and the compressed air  19 . 
         [0052]    As an option the air mass flow through the Venturi arrangement  14  can be regulated by a valve mechanism (not shown). 
         [0053]    As an alternative option ( FIG. 3 ) the nozzle  15  of a Venturi arrangement  14 ′ could be placed inside the combustor  10 ′ or combustion chamber  11 , as it will be cooled by the air flowing inside. The diffuser  16  is directly connected to suction hole  17 . The whole arrangement is oriented perpendicular to the direction of the compressed air flow  19  in cooling air channel  12 . The function will be the same as explained in  FIG. 2 . 
         [0054]    Large flue gas recirculation rates could result in higher temperature of the compressed air and flue gas mixture ( 20 ). In order to prevent overheating of the combustor, a separate channel flushed with compressed air could be used to separate the hot mixture (compressed air &amp; flue gas) from the combustor liner wall.  FIG. 4  shows a suitable embodiment in form of a Venturi arrangement  14  similar to the one of  FIG. 2 . In this embodiment, an improved cooling of combustion chamber  11  of combustor  10 ″ is achieved by providing a separate cooling air flow path  21 , wherein a partial flow  19   b  of compressed air is used to thermally separate the Venturi arrangement  14  and mixed exhaust gas/air flow  20  from the combustor liner wall  13 . 
         [0055]    Optionally, the air mass flow and subsequently the exhaust gas recirculation could be adjusted by valves or flaps. 
         [0056]    Further, the process could be driven by steam instead of compressed air. 
         [0057]    Finally, as shown in  FIG. 4 , an improvement of the cooling of the combustor liner wall can be reached with a separate flow path for cooling air. 
         [0058]    As already mentioned before, Venturi arrangements of the kind shown in  FIGS. 2-4  can be used in various types of gas turbines at respective Venturi arrangement locations A-E ( FIGS. 5-8 ). 
         [0059]      FIG. 5  shows a section through a gas turbine  22  of the well-known GT24/26 type with sequential combustion comprising first and second combustion chambers  25 ,  27 , first and second burners  24 ,  26 , and first and second turbine sections  28 ,  28 ′. Venturi arrangements are located at specific Venturi arrangement locations A and B at the transition between combustion chambers  25 ,  27  and turbine sections  28 ,  28 ′. The compressed air is generated in this case by a compressor  23  of the gas turbine  22  and supplied through a plenum  29 . 
         [0060]      FIG. 6  shows a section through a gas turbine  30  of the well-known GT13E2 type with an annular combustion chamber  25  and specific Venturi arrangement locations C between the combustor with its combustion chamber  25  and burners  24  and the turbine section  28 . Again, a compressor  23  supplies compressed air via a plenum  29 . 
         [0061]      FIG. 7  shows a section through a different gas turbine  31  (a) with a plurality of individual, circumferentially arranged combustion chambers  25 ′ (b) (can combustor configuration) and specific Venturi arrangement locations D at the transition sections  35  between combustion chambers  25 ′ and a subsequent turbine section  28 . Again, a compressor  23  supplies compressed air via a plenum  29 . 
         [0062]      FIG. 8  shows in a perspective view a gas turbine  32  of the well-known GT11N2type with a silo-type combustor with a combustion chamber  34  and burners  33  and specific Venturi arrangement locations E in the transition section  35  between combustion chamber  34  and turbine section  28 . A compressor  23  supplies compressed air. 
         [0063]    The cooling air (or compressed air) driving the Venturi ejector can be taken from a compressor plenum ( 29 ) or cooling air pipes (e.g. for the GT 24/26 type of  FIG. 5  compressor exit air or high pressure cooling air can be used to drive the recirculation of the second combustor  26 ,  27 ). In the cooling air supply line a control valve and/or booster/blower can be arranged to control the cooling air flow and thereby the amount of recirculated hot gas. 
         [0064]    A possible operating concept can for example have a high recirculation rate at low part load (e.g. up to 50 or 60% rel. load) to increase the combustor inlet temperature due to the recirculation of hot flue gas. 
         [0065]    For higher loads circulation can be reduced (completely switching off of compressed air or minimized flow to avoid hot gas ingestion without cooling air flow). 
         [0066]    At high load (for example above 80 to 90% rel. load) and base load the recirculation can be increased again to reduce NO x  emission due to the reduction of oxygen concentration in the combustor inlet gas with high recirculation rate. 
         [0067]    When using compressor exit air to drive the ejector a mass flow control can be achieved due to the change of temperature: At part load the compressor exit pressure is reduced while the hot gas temperature remains high. Thus the ratio of density of the compressed air driving the injector to the density of the flue gas is higher than at base load. This increased density ratio leads to a higher recirculation rate (ratio of total combustor flue gas to recirculated flue gas). 
         [0068]    In another embodiment re-cooled compressed air can be used to drive the Venturi injector. The temperature can be controlled to control the recirculation rate. 
         [0069]    In special cases of a gas turbine with CAES (compressed air energy storage, see for example document DE 34 114 44 A1) the application of flue gas recirculation around the combustor is in particular advantageous for such a system. In a CAES system, in particular during start up, the combustor inlet temperature is very low since the compressed air is taken from the storage and has not compressor exit temperature. 
         [0070]    The advantages of the invention are:
       Lower NO x  emissions   Higher air temperatures at part load
           Better part load operation behavior   Better flame stabilization   Better burn out and with it lower CO emissions   
           Less costly than active flue gas recirculation
           Less piping   No heat exchanger required   No water separator   Compact design   
           Adjustability of recirculated mass possible