Patent Abstract:
An improved combustor is disclosed in which conventional combustion is changed to “rich to quench to lean” by changing the air entry arrangement in the liner of the combustor to remove mixing holes, reduce liner cooling and admit dilution air into the combustor liner in place of mixing air. In an alternative embodiment, dilution air is admitted into the combustor liner with the help of a plurality of pipes arranged so that air comes into the liner as a swirling flow in a direction opposite to nozzle swirl, so as to thereby produce a large mixing of air with the combustion gases and a resulting quenching effect, i.e., a rapid cooling of the combustion gases by quenching air. As such, the requirement for cooling water to quench the combustion gases is significantly reduced, thereby helping turbine efficiency and reducing turbine emissions.

Full Description:
The present invention relates turbines, and more particularly to a method of introducing air into a gas turbine combustor to reduce combustor NOx emissions and water requirements in reducing such emissions. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases from the combustor flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power, often for powering an electrical generator. 
     Increased requirements for low emissions from turbine power plants now require low rates of emissions of NOx (mono-nitrogen oxides NO (nitric oxide) and NO 2  (nitrogen dioxide)), CO (carbon monoxide) and other pollutants from turbine combustors. 
     Conventional turbine combustors use non-premixed diffusion flames, where fuel and air freely enter the combustion chamber separately and mixing of the fuel and air occurs simultaneously with combustion, and where resulting flame temperatures typically exceed 4000° F. with NG, LF or syngas fuels, so as to produce relatively high levels of NOx emissions. Thus, temperatures in combustion chamber primary zones can get very high if water is not injected, although temperatures do drop along the length of the combustion chamber. Water is generally used because a diffusion flame is used in these combustors and primary zone temperatures are very high and produce NOx as much as approximately 250 ppm with syngas/LF fuels and approximately 120 ppm with NG fuel if water is not used. 
     Approximately 95% of the combustor exiting NOx, which is measured in ppmvd (parts per million, volumetric dry) @15% O2, has already been formed before the combustion gases reach the dilution holes in a conventional combustor liner. NOx formation rates are highest in a narrow zone of the combustion chamber, and become very much less so after the combustion gases reach the dilution holes in the conventional combustor liner. Thus, air introduced by dilution holes in a conventional combustor liner does not participate in a reduction of combustion gases&#39; temperatures and NOx production. 
     As is explained in the background section of U.S. Pat. No. 6,192,689, one method commonly used to reduce peak temperatures in conventional diffusion flame combustors, and thereby reduce NOx emissions, is to inject water or steam into the combustor. However, water or steam injection is a relatively expensive technique and can cause the undesirable side effect of quenching (i.e., rapid cooling) carbon monoxide (CO) burnout reactions, and which is limited in its ability to achieve low levels of pollutants. 
     Conventional diffusion flame combustors are effective for burning natural gas (NG), synthesis gas (syngas) and liquid fuels (LF) in low megawatt (MW) turbine machines. But conventional combustors use a very old liner cooling design that involves the use of water or steam injection, which is not desirable in gas turbine power plants from life of components, operability and cost of electricity perspectives. Sufficient efforts have not been made to reduce water consumption in these machines. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention seeks to reduce water requirements in conventional combustors to reduce temperatures and NOx emissions when operating on NG/LF or syngas fuels. In the present invention, combustion in a conventional combustor is changed from “rich to lean” to “rich to quench to lean” by changing the air entry arrangement in the liner of the conventional combustor. In this changed air entry arrangement, dilution holes are removed, liner cooling is reduced and dilution air is admitted into the combustor liner in place of mixing air admitted into the combustor liner through a third row of mixing holes. In an alternative embodiment, dilution air is admitted into the combustor liner with the help of a plurality of pipes arranged in such a manner so that such air comes into the liner as a swirling flow in a direction opposite to nozzle swirl, so as to thereby produce a large mixing of air with the combustion gases and a resulting quenching effect, i.e., a rapid cooling of the combustion gases by quenching air. As such, the requirement for cooling water to quench the combustion gases is significantly reduced, thereby helping in turbine efficiency and a reduction in turbine emissions. 
     The present invention reduces temperatures in the primary reaction zone of a combustor by moving dilution air upstream and providing swirl to incoming air to enhance mixing. Reduction in temperature leads to reduction in NOx generation which is very high in conventional liners before combustion gases reach the dilution holes in the combustor. The present invention also reduces the cooling water requirement in conventional liners, which is typically very high. 
     In a first embodiment of the present invention, a combustor operating with a compressor to drive a gas turbine is comprised of an outer combustor wall having an upstream fuel entry end and a downstream turbine entry end; a plurality of mixing holes located proximal to the upstream fuel entry end of the outer combustor wall; and a plurality of dilution holes located proximal to the plurality of mixing holes to admit air into a combustion zone in the combustor for mixing of the admitted air with combustion gases in the combustion zone to thereby reduce NOx and carbon monoxide (CO) production in the combustion zone. 
     In another embodiment of the present invention, a combustor operating with a compressor to drive a gas turbine is comprised of an outer combustor wall having an upstream fuel entry end and a downstream turbine entry end; a plurality of mixing holes located proximal to the upstream fuel entry end of the outer combustor wall, the plurality of mixing holes being arranged in a plurality of rows which extend around a circumference of the outer combustor wall; and a plurality of dilution holes arranged in one or more rows which extend around the circumference of the outer combustor wall, the plurality of dilution holes being located proximal to the plurality of mixing holes; an outer shell; a nozzle from which compressed air and fuel are discharged into combustor; a flow sleeve located between the outer shell and the combustor wall so as to form a cavity between the outer shell and the combustor wall so that air from the compressor entering the combustor is divided between a first path by which a first part of the compressor air is admitted into the combustor by entering through the flow sleeve, and a second path by which a second part of the compressor air is admitted into the combustor through the cavity; and a plurality of pipes extending between the cavity and the plurality of dilution holes to admit the second part of the compressor air into the combustion zone for increased mixing of the admitted air with combustion gases in the combustion zone to thereby reduce NOx and carbon monoxide (CO) production in the combustion zone. 
     In a further embodiment of the present invention, a combustor operating with a compressor to drive a gas turbine is comprised of an outer combustor wall having an upstream fuel entry end and a downstream turbine entry end, the outer combustor wall having a length between 35 inches and 50 inches; a plurality of rows of liner louver cooling holes positioned longitudinally along the combustor wall; a plurality of mixing holes located proximal to the upstream fuel entry end of the outer combustor wall; the plurality of dilution holes being located proximal to the plurality of mixing holes; the plurality of mixing holes being arranged in first and second rows which extend around a circumference of the outer combustor wall rather than first, second and third rows which extend around the circumference of the outer combustor wall so that the plurality of dilution holes are arranged in the third row from the upstream fuel entry end extending around the circumference of the outer combustor wall so as to be located within a distance of five inches to forty inches from the fuel entry end of the combustor wall; an outer shell; a nozzle from which compressed air and fuel are discharged into combustor; a flow sleeve located between the outer shell and the combustor wall so as to form a cavity between the outer shell and the combustor wall so that air from the compressor entering the combustor is divided between a first path by which a first part of the compressor air is admitted into the combustor by entering through the flow sleeve, and a second path by which a second part of the compressor air is admitted into the combustor through the cavity; and a plurality of pipes extending between the cavity and the plurality of dilution holes at an angle to thereby tangentially admit the second part of the compressor air into the combustion zone for increased mixing of the admitted air with combustion gases in the combustion zone, the angle at which the pipes enter the combustor being achieved using an offset of the pipes of zero to seven inches from the center of the combustor, the diameters of the plurality of dilution holes though which air from the plurality of pipes is passed into the combustor being increased to a dimension that results in an increase in air flow into the combustor combustion chamber, and the diameters of the plurality of louver cooling holes though which louver cooling air passes being reduced to a dimension that results in a further increase in mixing of the admitted air with combustion gases in the combustion zone to thereby reduce NOx and carbon monoxide (CO) production in the combustion zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , which is a figure from U.S. Pat. No. 6,192,689, is a schematic representation of a portion of an industrial gas turbine engine having a low NOx combustor joined in flow communication with a compressor and turbine. 
         FIGS. 2A and 2B  are side elevational and perspective schematic representations, respectively, of a conventional combustor liner used in an industrial gas turbine engine. 
         FIG. 3  is a perspective schematic representation of a combustor liner according to the present invention. 
         FIGS. 4A to 4C  show a first embodiment of a Dry Low NOx (“DLN”) combustion system incorporating the combustor liner shown in  FIG. 5 . 
         FIG. 5A to 5C  show a second embodiment of a DLN combustion system incorporating the combustor liner shown in  FIG. 5 . 
         FIG. 6A to 6C  show a third embodiment of a DLN combustion system incorporating the combustor liner shown in  FIG. 5 . 
         FIG. 7  is an end elevational representation of the angle at which the pipes enter the combustor in the embodiments of  FIGS. 5A to 6C  using a range of offsets of the pipes from the center of the combustor. 
         FIG. 8  is a partial breakaway perspective view of part of a diffusion type combustor. 
         FIG. 9A  is a picture of a temperature field within the diffusion type combustor of  FIG. 8  during operation with a conventional type liner like that shown in  FIGS. 2A and 2B . 
         FIG. 9B  is a picture of a temperature field within the diffusion type combustor of  FIG. 8  during operation with a type liner according to the present invention like that shown in  FIG. 3 . 
         FIG. 10A  is a graph of the emissions inside and that exit a diffusion type combustor like that of  FIG. 8  during operation with a conventional type liner like that shown in  FIGS. 2A and 2B  and with a type liner according to the present invention like that shown in  FIG. 3 . 
         FIG. 10B  is a graph of the temperature inside and that exit a diffusion type combustor like that of  FIG. 8  during operation with a conventional type liner like that shown in  FIGS. 2A and 2B  and with a type liner according to the present invention like that shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 of U.S. Pat. No. 6,192,689 is a schematic representation of a portion of an exemplary industrial gas turbine engine  10  having a low NOx combustor  18  joined in flow communication with a compressor  12  and turbine  20 . The industrial gas turbine engine  10  includes a compressor  12  for compressing air  14  that is mixed with fuel  16  and ignited in at least one combustor  18 , as shown in  FIG. 1 . A turbine  20  is coupled to compressor  12  by a drive shaft  22 , a portion of which drive shaft  22  extends for powering, for example, an electrical generator (not shown) for generating electrical power. During operation, compressor  12  discharges compressed air  14  that is mixed with fuel  16  and ignited for generating combustion gases  24  from which energy is extracted by turbine  20  for rotating shaft  22  to power compressor  12 , as well as for producing output power for driving the generator or other external load. Combustor  18  comprises a cylindrical combustor wall  26 , which defines a combustion chamber  28  cylindrical combustor wall  26 . 
       FIGS. 2A and 2B  are side elevational and perspective schematic representations, respectively, of a conventional combustor liner  30  used in an industrial gas turbine engine  10 . The combustor  30  includes a cylindrical combustor wall  32  having a fuel entry end  34  and a turbine entry end  36 . The combustor liner  30  includes a plurality of rows of liner louvers cooling holes  38  positioned longitudinally along the liner  30  and having different diameters at different positions along the liner  30 . 
     The combustor liner  30  is also comprised of several sets of air holes disposed about its periphery. A first set of air holes  40 , referred to as mixing holes, supply a quantity of air to the reaction zone within combustion chamber  28 . The mixing holes  40  are disposed proximate to the fuel entry end  34  of combustor  30  to provide an entry for mixing air. The number of mixing holes  40  is variable, typically depending on the overall size of combustor  30 . A second set of air holes  42  are positioned at the downstream end of the combustion chamber to quench combustion gases  24  prior to entering a transition piece (not shown) or a turbine inlet (not shown). 
     A second set of air holes  42 , called dilution holes, are disposed in a central region of the combustor  30 , closer to the downstream end of the combustion chamber  28  within combustor  30 . The dilution holes  42  provide an entry area for dilution air into to combustor  30 . The dilution air is provided to lower the temperature of combustion gases  24  prior to entering a turbine inlet (not shown) or a transition piece (not shown). 
     The temperature field within combustor  30  during operation is such that temperatures are very high in the primary zone of combustor  30 , if water is not injected into combustor  30 , although it should be noted that temperatures drop along the length of combustor  30 . 
     The formation of NOx within combustor  30  during operation is such that approximately 95% of the ppmvd@15% NOx has already been formed before the combustion gases  24  reach the dilution holes  42 . NOx formation rates are highest in a narrow zone, with not much of the NOx being formed after the dilution holes  42  in combustor  30 . Thus, the dilution holes&#39; air does not participate in temperature and NOx reduction in conventional combustor  30 . 
     In the present invention, combustion in a conventional combustor is changed from “rich” to “lean” to “rich” to “quench” to “lean” by changing the air entry arrangement of the conventional combustor. In the air entry arrangement according to the present invention, dilution holes are removed from the region of the combustor closer to the downstream end of the combustion chamber within combustor, liner cooling is reduced and air is admitted into the combustor at the third row of the mixing holes with the help of a plurality pipes arranged in a manner that causes air coming from the pipes to enter the combustor  30  as swirling flow in a direction opposite to nozzle swirl, so as to therefore produce a large mixing and quenching effect. In a preferred embodiment of the modified combustor according to the present invention, the plurality of pipes comprises six pipes. 
       FIG. 3  is a perspective schematic representation of a combustor liner  50  according to the present invention. The combustor  50  includes a cylindrical combustor wall  52  having a fuel entry end  54  and a turbine entry end  56 . In the combustor  50 , the air entry arrangement has been changed so that dilution air is admitted into the combustor  50  closer to a fuel entry end  54 . The combustor wall  52  also has a plurality of rows of liner louver cooling holes  58  positioned longitudinally along the combustor  50  and having different diameters at different positions along the combustor  50 . 
     Like the combustor shown in  FIGS. 2A and 2B , the combustor  50  includes several sets of air holes disposed about its periphery. Here again, the combustor  50  includes a set of mixing holes  60  which are disposed proximate to the fuel entry end  54  of combustor  50  to provide an entry for a quantity of mixing air to be supplied to the reaction zone within the combustion chamber  28 . The combustor  50  also includes a set of dilution holes  62 . Again, the number of mixing holes  60  and the number of dilution holes  62  vary according to the overall size of combustor  50 . 
     Like the combustor disclosed in U.S. Pat. No. 6,192,689, a preferred embodiment of the combustor wall  52  has a preferred nominal diameter (d) in the range between about 9 inches to about 15 inches and a preferred nominal length (L) in the range between about 35 inches to about 50 inches. In addition, the mixing holes  60  have a preferred diameter in the range between about 0.5 inches to about 1 inch, and the dilution holes  62  have a preferred diameter in the range between about 1.25 inches to about 4.0 inches. 
       FIGS. 4A to 4C  show a first embodiment of a Dry Low NOx (“DLN”) combustion system incorporating the combustor liner  50  shown in  FIG. 3 . The DLN combustion system includes combustor liner  50 , a nozzle  51  from which compressed air  14  and fuel  16  that is mixed with the compressed air  14  is discharged into combustor  50  and a diverging cone  53  positioned between nozzle  51  and combustor  50 . An endplate  55  holds the body of the combustor  50  together. 
     In the preferred embodiment shown in  FIGS. 4A to 4C , the mixing holes  60  are preferably arranged in two rows, which extend around the circumference of the cylindrical combustor wall  52 , and which are proximate to the fuel entry end  54  of the cylindrical combustor wall  52 . The dilution holes  52  are arranged in a single row, which replaces a third row of mixing holes that would typically be present in a conventional combustor. The row of dilution holes  52  preferably extends around the circumference of the cylindrical combustor wall  52 , and is proximate to the two rows of mixing holes  60  in cylindrical combustor wall  52  so that dilution air is admitted into the combustor  50  proximate to the fuel entry end  54  of combustor  50 . In a preferred embodiment of the claimed combustor  50 , the dilution holes  62  are located within a range of 5 inches to 40 inches from the fuel entry end  54  of the combustor wall  52 . Thus, in the preferred embodiment shown in  FIGS. 4A to 4C , part of the mixing holes  60 , i.e., those typically located in the third row of mixing holes are removed, and the number of dilution holes  62  is increased. Preferably, eight of the  24  mixing holes  60  in a conventional combustor, i.e., those holes in the third row of mixing holes, are removed, and the number of dilution holes  62  is increased from four typically in a conventional combustor to six to maintain jet penetration for mixing air to be supplied to the reaction zone within the combustion chamber  28 . Mid-frame air  64  from the compressor  12  continues to be admitted into the combustor  50  by entering through flow sleeve  66  within a shell  74  containing combustor  50 . 
       FIG. 5A to 5C  show a second embodiment of a DLN combustion system incorporating the combustor  50  shown in  FIG. 3 . In the embodiment shown in  FIG. 5A to 5C , the modified liner shown in the embodiment of  FIGS. 4A to 4C  is maintained. However, the embodiment shown in  FIG. 5A to 5C  also includes a modified cavity arrangement for much larger mixing of air with the combustion gases within the combustion chamber  28 . Thus, as in the embodiment of  FIGS. 4A to 4C , the dilution holes  52  are again moved to the third row of mixing holes  50  in combustor wall  62  so that dilution air is admitted into the combustor  50  at the third row of mixing holes  50 , and, as such, the mixing holes  50  in the third row are removed. In the modified cavity arrangement, the mid-frame air  64  is divided into two paths, i.e., one path for a part of the mid-frame air  64  to continue to be admitted into the combustor  50  by entering through flow sleeve  66 , and another path for another part  68  of the mid-frame air  64  to flow through a cavity  70  between the flow sleeve  66  and the outer shell  74 , whereupon air flowing through the cavity  70  will tangentially enter the combustor  50  through a plurality of pipes  72  extending at an angle between the cavity  70  and the third row dilution holes  62  into the combustor  50 . The air  68  entering the combustor  50  tangentially through pipes  72  results in an increase in air mixing with combustion gases  24  in the combustor primary zone. The angle at which the pipes  72  enter the combustor  50  in a preferred embodiment is achieved using a range of offsets of zero to seven inches of the pipes from the center of the combustor  50 , as shown in  FIG. 7 . The mixing is improved because air flowing from the pipes  72  flows counterclockwise to the air flowing from the nozzle  51 . 
       FIG. 6A to 6C  show a third embodiment of a DLN combustion system incorporating the combustor  50  shown in  FIG. 5 . In the embodiment shown in  FIG. 6A to 6C , the modified liner with relocated dilution holes, as shown in the embodiment of  FIGS. 4A to 4C , is again used. In addition, the modified cavity arrangement for much larger mixing of air and combustion gases in the embodiment shown in  FIG. 5A to 5C  is again used. However, increased air flow of 10-15% is added to increase the penetration of air into the hot temperature zones in the combustion chamber  28 . This is achieved by increasing the size/diameter of the dilution holes  62  though which air from pipes  72  is passed into combustor  50 . Also, louver cooling air passing through the plurality of rows of louver cooling holes  58  in the combustor liner  50  is reduced by half from 25-35% of the mid-frame air flow  64  to 10-15% of the mid-frame air flow  64  by decreasing the size/diameter of the cooling holes  58 . It is noted that 25-35% louver cooling is an old design, in which the liner temperature can reach to 800° F. to 1000° F. in temperature. 
     It should be noted that one alternative arrangement in which the larger dilution holes are used is one in which the mixing holes and larger dilution holes are arranged a single row located a distance from the fuel entry end  54  of the combustor liner  52  as would be the row of dilution holes  62  in the embodiment of  FIGS. 3A to 3C  would be. 
       FIG. 8  is a partial breakaway perspective view of part of a diffusion type combustor  80 . The combustor includes an inlet nozzle  81 , a combustor liner  82  with a cylindrical combustor wall, and a flow sleeve  84  through which mid-frame air enters the combustor  80 . 
       FIG. 9A  is a picture of a temperature field within the diffusion type combustor  80  of  FIG. 8  during operation with the liner  82  being a conventional type liner like that shown in  FIGS. 2A and 2B .  FIG. 9B  is a picture of a temperature field within the diffusion type combustor  80  of  FIG. 8  during operation with the liner  82  being of a type like that shown in  FIG. 3 . As can be seen in  FIG. 9B  where a type of liner like that shown in  FIG. 3  is used, the temperatures in the combustor  80  are less than those shown in  FIG. 9A  where a conventional type liner like that shown in  FIGS. 2A and 2B  is used. It can be seen from  FIGS. 9A and 9B  that the high temperature reaction zone within the combustion chamber  28  is reduced significantly after the dilution holes  62  have been moved closer to the fuel entry end  54  of the cylindrical combustor wall  52 , even though the exit profile of the combustor did not change much. 
       FIG. 10A  is a graph of the emissions inside and that exit a diffusion type combustor  80  like that of  FIG. 8  during operation with a conventional type liner like that shown in  FIGS. 2A and 2B  and with a type liner according to the present invention like that shown in  FIG. 3 .  FIG. 10B  is a graph of the temperature inside and that exit a diffusion type combustor like that of  FIG. 8  during operation with a conventional type liner like that shown in  FIGS. 2A and 2B  and with a type liner according to the present invention like that shown in  FIG. 3 . 
     It can be seen from  FIGS. 9A and 9B  and the graph of  FIG. 10B  that the high temperature reaction zone within the combustion chamber  28  is reduced significantly after the dilution holes  62  have been moved closer to the fuel entry end  54  of the cylindrical combustor wall  52 , so that the diffusion type combustor  80  of  FIG. 8  was operated a liner  82  of a type like that shown in  FIG. 3 , even though the exit profile of the combustor did not change much. It can also be seen from the graph of  FIG. 10A  that the combustor  80  of  FIG. 8 , when operated with a liner  82  of a type like that shown in  FIG. 3 , reduces NOx emissions by approximately 65% and CO emissions by approximately 50% at the exit of combustor  80 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Technology Classification (CPC): 5