Patent Publication Number: US-6983605-B1

Title: Methods and apparatus for reducing gas turbine engine emissions

Description:
BACKGROUND OF THE INVENTION 
   This application relates generally to gas turbine engines and, more particularly, to combustors for gas turbine engine. 
   Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of high combustor flame temperatures. Making modifications to a gas turbine engine in an effort to reduce nitrous oxide emissions often has an adverse effect on operating performance levels of the associated gas turbine engine. 
   In gas turbine engines, nitrous oxide emissions can be reduced by increasing airflow through the gas turbine combustor during operating conditions. Gas turbine engines include preset operating parameters and any such airflow increases are limited by the preset operating parameters including turbine nozzle cooling parameters. As a result, to increase the airflow within the gas turbine combustor, the gas turbine engine and associated components should be modified to operate at new operating parameters. 
   Because such gas turbine engine modifications are labor-intensive and time-consuming, users are often limited to derating the operating power capability of the gas turbine engine and prevented from operating the gas turbine engine at full capacity. Such derates do not limit an amount of nitrous oxide formed as the engine operates at full capacity, but instead limit the operating capacity of the gas turbine engine. 
   BRIEF SUMMARY OF THE INVENTION 
   In an exemplary embodiment, a gas turbine engine includes a combustor system to reduce an amount of nitrous oxide emissions formed by the gas turbine engine. The combustor system includes a combustor and a fuel and water delivery system. The combustor is a lean premix combustor including a plurality of premixers and is operable with a fuel/air mixture equivalence ratio less than one. The water delivery system supplies at least one of water or steam to the gas turbine engine such that water or steam is injected into the combustor. 
   During normal gas turbine engine operations, fuel is supplied proportionally with airflow to the combustor such that the combustor operates with a fuel/air mixture equivalence ratio less than one. As gas turbine engine operating speeds increase and additional fuel and air are supplied to the combustor, the water delivery sub-system supplies either water or steam to the combustor. The increase in combustion zone flame temperatures generated as a result of additional fuel being burned within the combustor is minimized with the water or steam supplied to the combustor. As a result, nitrous oxide emissions generated are reduced. Alternatively, the gas turbine engine may achieve an increased operating power level for a specified nitrous oxide emission level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a gas turbine engine; and 
       FIG. 2  is a cross-sectional view of a combustor used with the gas turbine engine shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic illustration of a gas turbine engine  10  including a low pressure compressor  12 , a high pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high pressure turbine  18  and a low pressure turbine  20 . Combustor  16  is a lean premix combustor. Compressor  12  and turbine  20  are coupled by a first shaft  21 , and compressor  14  and turbine  18  are coupled by a second shaft  22 . A load (not shown) is also coupled to gas turbine engine  10  with first shaft  21 . In one embodiment, gas turbine engine  10  is an LM6000 available from General Electric Aircraft Engines, Cincinnati, Ohio. Alternatively, gas turbine engine  10  is an LM 2500 available from General Electric Aircraft Engines, Cincinnati, Ohio. 
   In operation, air flows through low pressure compressor  12  and compressed air is supplied from low pressure compressor  12  to high pressure compressor  14 . The highly compressed air is delivered to combustor  16 . Airflow from combustor  16  drives turbines  18  and  20  and exits gas turbine engine  10  through a nozzle  24 . 
     FIG. 2  is a cross-sectional view of combustor  16  used in gas turbine engine  10  (shown in  FIG. 1 ). Because combustor  16  is a lean premix combustor, a fuel/air mixture supplied to combustor  16  contains more air than is required to fully combust the fuel. Accordingly, a fuel/air mixture equivalence ratio for combustor  16  is less than one. Because combustor  16  premixes fuel with air, combustor  16  is a lean premix combustor. Combustor  16  includes an annular outer liner  40 , an annular inner liner  42 , and a domed end  44  extending between outer and inner liners  40  and  42 , respectively. Outer liner  40  and inner liner  42  are spaced radially inward from a combustor casing  136  and define a combustion chamber  46 . Combustor casing  136  is generally annular and extends downstream from a diffuser  48 . Combustion chamber  46  is generally annular in shape and is disposed radially inward from liners  40  and  42 . Outer liner  40  and combustor casing  136  define an outer passageway  52  and inner liner  42  and combustor casing  136  define an inner passageway  54 . Outer and inner liners  40  and  42  extend to a turbine nozzle  55  disposed downstream from diffuser  48 . 
   Combustor domed end  44  includes a plurality of domes  56  arranged in a triple annular configuration. Alternatively, combustor domed end  44  includes a double annular configuration. In another embodiment, combustor domed end  44  includes a single annular configuration. An outer dome  58  includes an outer end  60  fixedly attached to combustor outer liner  40  and an inner end  62  fixedly attached to a middle dome  64 . Middle dome  64  includes an outer end  66  attached to outer dome inner end  62  and an inner end  68  attached to an inner dome  70 . Accordingly, middle dome  64  is between outer and inner domes  58  and  70 , respectively. Inner dome  70  includes an inner end  72  attached to middle dome inner end  68  and an outer end  74  fixedly attached to combustor inner liner  42 . 
   Combustor domed end  44  also includes a outer dome heat shield  76 , a middle dome heat shield  78 , and an inner dome heat shield  80  to insulate each respective dome  58 ,  64 , and  70  from flames burning in combustion chamber  46 . Outer dome heat shield  76  includes an annular endbody  82  to insulate combustor outer liner  40  from flames burning in an outer primary combustion zone  84 . Middle dome heat shield  78  includes annular centerbodies  86  and  88  to segregate middle dome  64  from outer and inner domes  58  and  70 , respectively. Middle dome centerbodies  86  and  88  are disposed radially outward from a middle primary combustion zone  90 . Inner dome heat shield  80  includes an annular endbody  92  to insulate combustor inner liner  42  from flames burning in an inner primary combustion zone  94 . An igniter  96  extends through combustor casing  136  and is disposed downstream from outer dome heat shield endbody  82 . 
   Domes  58 ,  64 , and  70  are supplied fuel and air via a premixer and assembly manifold system (not shown). A plurality of fuel tubes  102  extend between a fuel source (not shown) and plurality of domes  56 . Specifically, an outer dome fuel tube  103  supplies fuel to a premixer cup  104  disposed within outer dome  58 , a middle dome fuel tube  106  supplies fuel to a premixer cup  108  disposed within middle dome  64 , and an inner dome fuel tube  110  supplies fuel to a premixer cup  112  disposed within inner dome  70 . 
   Combustor  16  also includes a water delivery system  130  to supply water to gas turbine engine  10  such that water is injected into combustor  16 . Water delivery system  130  includes a plurality of water injection nozzles  134  connected to a water source (not shown). Water injection nozzles  134  are in flow communication with premixer cups  104 ,  108 , and  112  and inject an atomized water spray into the fuel/air mixture created in premixer cups  104 ,  108 , and  112 . In an alternative embodiment, injection nozzles  134  are connected to a steam source (not shown) and steam is injected into the fuel/air mixture using nozzles  134 . 
   During operation of gas turbine engine  10 , air and fuel are mixed in premixer cups  104 ,  108 , and  112  and the fuel/air mixture is directed into domes  58 ,  64 , and  70 , respectively. The mixture burns in primary combustion zones  84 ,  90 , and  94  of domes  58 ,  64 , and  70  that are active. At high power gas turbine engine operations, fuel entering premixer cup  108  is increased, resulting in a higher fuel/air ratio within dome  64 . 
   Middle dome  64  is known as a pilot-dome and has fuel supplied thereto during all phases of operation of engine  10 . Domes  58  and  70  have fuel supplied thereto as demanded by operating power requirements of gas turbine engine  10 . As gas turbine engine operating power requirements are increased, water is also supplied to domes  58 ,  64 , and  70 , as demanded to meet nitrous oxide emission requirements. Gas turbine engine  10  has a rated engine operating capacity. To operate gas turbine engine  10  above 90% rated engine operating capacity, additional fuel is supplied only to combustor middle dome  64 . During such engine power operations, water delivery system  130  supplies additional water to middle dome  64  to minimize temperature increases as a result of additional fuel being burned within combustor middle dome  64 . 
   More specifically, when gas turbine engine  10  is operated above approximately 90% rated engine power capacity, additional fuel is supplied only to combustor middle dome  64  because outer and inner dome flame temperatures are limited by dynamic pressure or acoustic boundaries. When gas turbine engine  10  is operating at such a capacity, water delivery system  130  supplies water to combustor  16  to maintain flame temperatures generated within middle dome  64  approximately equal to flame temperatures generated within outer and inner domes  58  and  70 . Furthermore, nitrous oxide emissions generated within middle dome  64  are maintained at a level approximately equal to those levels generated within outer and inner domes  58  and  70 . Additionally, by supplying additional water to only middle dome  64  during such engine operations, the potential adverse effects of generating additional carbon monoxide emissions within combustor  16  are offset by the reduction in nitrous oxide emissions and the increase in operating capacity. Alternatively, the operating power level of gas turbine engine  10  may be increased for a specified nitrous oxide emission level. 
   Similarly, as engine performance degrades over time, additional fuel is required to produce similar engine output in comparison to engines that have not deteriorated. For the reasons discussed above, additional fuel is supplied to combustor middle dome  64 . During such engine operations, water delivery system  130  supplies water at an increased flow rate to middle dome  64  to maintain the middle dome flame temperatures and to control the generation of emissions resulting from increased fuel flow. 
   In a further embodiment, water delivery system  130  is selectively operable between a first mode of operation and a second mode of operation. The first operating mode of water delivery system  130  is activated during all phases of operation of gas turbine engine  10  above engine idle operations. Typically, in the first operation mode, water delivery system  130  supplies water proportionally to all three domes  58 ,  64 , and  70  at approximately the same rate. 
   The second operating mode of water delivery system  130  is activated when gas turbine engine  10  is operated above 90% rated engine operating capacity. When water delivery system  130  operates in the second operating mode, water is supplied to middle dome  64  at a higher flow rate than water supplied to dome  64  when water delivery system  130  is in the first operating mode. The increased rate of water supplied during the second operating mode reduces nitrous oxide emissions from gas turbine engine  10 . 
   In an alternative embodiment, when gas turbine engine  10  is operated above 90% rated engine operating capacity, steam is added to the fuel upstream from combustor  16 . In a further embodiment, steam is added to the fuel upstream from combustor  16  when the gas turbine engine is operated above idle power operations. The steam/fuel mixture is supplied only to combustor middle dome  64  because outer and inner dome flame temperatures are limited by dynamic pressure or acoustic boundaries. The steam/fuel mixture is heated prior to being introduced to middle dome  64  to prevent condensation from forming and is mixed thoroughly prior to be injected into combustor middle dome  64 . Additional steam permits flame temperatures generated within middle dome  64  to be maintained approximately equal that of flame temperatures generated within outer and inner domes  58  and  70 . As a result, nitrous oxide emissions generated within middle dome  64  are maintained at a level approximately equal to those levels generated within outer and inner domes  58  and  70 . Furthermore, because additional steam is supplied only to middle dome  64 , the potential adverse effects of additional carbon monoxide emissions generated within combustor  16  are offset by the reduction in nitrous oxide emissions and the increase in engine operating capacity. 
   The above-described combustor system for a gas turbine engine is cost-effective and reliable. The combustor system includes a combustor operable with a fuel/air mixture equivalence ratio less than one and a water delivery system that injects water and/or steam into the combustor to reduce nitrous oxide emissions generated during gas turbine engine operations. As a result, nitrous oxide emissions for specified turbine operating power levels are lowered. Alternatively, the operating power level of the gas turbine engine may be increased for a specified nitrous oxide emission level. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.