IN A GAS TURBINE

[0001] This invention relates generally to gas turbine combustion technology, and more specifically, to the injection of steam into a gas turbine combustor for combustion temperature reduction and resultant power output augmentation.

BACKGROUND

[0002] Steam injection is a technique in gas turbine combustion systems for augmenting the power output of the turbine. More specifically, the combustion process in a gas turbine typically results in the generation of oxides of nitrogen (NOx) which are considered an atmospheric pollutant. In the past, steam or water has been injected into the combustor for the purpose of reducing the flame temperature and, hence, the rate of NOx formation. In current steam injection arrangements for can-annular type combustors, the steam is injected directly through relatively large inlet holes or ports in the surrounding turbine shell, upstream of the annular array of combustor liners and in the general area of the transition pieces which funnel the hot combustion gases to the first turbine stage. The lack of steam flo guidance and resultant haphazard and uneven mixing of the injected steam with the compressor discharge air inside the turbine shell or casing has diminished the power- augmentation advantages of the steam injection techniques by decreasing flame stability and increasing unwanted emissions. This is especially evident with the advent of low emissions combustion systems which are more prevalent in modern gas turbines. Lower emissions, premixed combustion systems have less flame stability margin and more carbon monoxide sensitivity than the higher

emissions, or un-premixed, diffusion flame systems.

[0003] There remains a need therefore, for a steam injection arrangement that insures better air/steam mixing before the air/steam mixture enters the combustor and mixes with fuel separately supplied to the combustor. BRIEF SUMMARY OF THE INVENTION

[0004] Accordingly, in one exemplary but nonlimiting embodiment, the present invention relates to a gas turbine comprising a plurality of combustors within an outer turbine shell, arranged about a rotor of the gas turbine, each combustor having a combustion chamber defined by a liner with an end cover at a forward end supporting at least one fuel nozzle; a transition piece connected to an aft end of the liner for supplying hot combustion gases to a first stage of the gas turbine; and at least one steam injection rake projecting

substantially radially into the outer turbine shell and located circumferentially between each pair of adjacent transition pieces, the at least one steam injection rake provided with at least one steam injection nozzle for emitting steam into areas substantially between adjacent transition pieces for mixing with air flowing within the outer shell.

[0005] In another exemplary but nonlimiting aspect, the invention relates to a gas turbine comprising a plurality of combustors within an outer turbine shell, arranged about a rotor of the gas turbine, each combustor having a combustion chamber defined by a liner with an end cover at a forward end supporting at least on fuel nozzle; a transition piece connected to an aft end of the liner for supplying hot combustion gases to a first stage of the gas turbine; and at least two steam injection rakes projecting radially into the outer turbine shell and located circumferentially between each pair of adjacent transition pieces, each of the at least two steam injection rakes provided with a plurality of steam injection nozzles for emitting steam into areas

substantially between adjacent transition pieces for mixing with compressor discharge air flowing within said outer shell.

[0006] In still another exemplary but nonlimiting aspect, the invention provides a method of enhancing mixing of steam injected into a turbine shell enclosing plural combustors and associated transition pieces in an annular array about the turbine rotor with compressor discharge air within the turbine shell, the method comprising providing at least one steam injection rake projecting substantially radially into the turbine shell and into areas between each adjacent pair of the

associated transition pieces; providing the at least one steam injection rake with plural steam injection nozzles; and injecting steam through the plural steam injection nozzles into areas between the adjacent pairs of

transition pieces.

[0007] The invention will now be described in greater detail in connection with the drawings identified below. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a simplified cross-section of a can- annular type combustor, illustrating a steam injection port in the combustor casing; [0009] Fig. 2 is a partial cross-section similar to Fig. 1 but illustrating a steam injection configuration in accordance with an exemplary but nonlimiting

embodiment of this invention;

[0010] Fig. 3 is a partial aft-end view of the

transition piece shown in Fig. 1 and illustrating steam injection rakes on either side of the transition piece in accordance with the exemplary but nonlimiting embodiment;

[0011] Fig. 4 is a schematic detail of a steam

injection rake taken from Fig. 3; [0012] Fig. 5 is a side elevation of a gas turbine transition piece, similar to that shown in Fig. 1 but reversed in direction, and illustrating a variation of the steam injection rake shown in Figs. 3 and 4; and

[0013] Fig. 6 is a partial an aft-end view of plural can-annular combustors arranged within a turbine casing and illustrating an arrangement of steam injection rakes as shown in Fig. 5 about the interior of the casing.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Referring initially to Fig. 1, there is

illustrated a gas turbine combustor, generally designated 10, which includes an outer casing or shell 12, a flow sleeve 14 and a combustor liner 16 arranged substantially concentrically within the flow sleeve 14 and defining the combustion chamber 18. A transition piece or duct 20 is coupled to the aft or downstream end of the combustor liner 16 and supplies hot combustion gases to the first turbine section or stage, represented by a first stator blade or nozzle 22. At the forward end of the combustor 10, an end cover 24 supports a plurality of fuel nozzles 26 which project into the combustion chamber 18. The end cover 24 comprises the head or forward end of the

combustor, and supplies purged air and fuel to the fuel nozzles 26, while at the same time, supplying compressor discharge air into the forward end of the liner 16 to mix with the fuel supplied by the nozzles 26. As is well known, in certain gas turbine engines, a plurality of combustors 10 are arranged in an annular array about the turbine rotor axis, with combustion gases supplied to the turbine stages by the several associated transition pieces 20, also arranged in an annular array. In such known arrangements, a plurality of steam inlet ports or holes 28 are located about the periphery of the casing 12 and mount one or more steam injection heads 30 that supply steam from a source outside the casing or shell to the internal area within the casing or shell 12,

surrounding the array of combustors 10 and in general area of the transition pieces 20. As explained above, the current steam injection configurations result in uneven mixing of the injected steam with the compressor discharge air.

[0015] Turning now to Figs. 2-4, a generally similar can-annular type combustor 110 is shown, and similar reference numerals, but with the prefix "1" added, are used for convenience to identify corresponding combustor components, the general description of which need not be repeated. In an exemplary but non-limiting embodiment of this invention, the steam inlet holes or ports 128 (which may be already-existing ports formed in the turbine shell or casing) are fitted with elongated steam injection conduits 32 which project radially into the casing or shell 112 and which have lengths sufficient to place the outlet end of the attached steam injection rakes 34 into the areas between adjacent transition pieces 120.

[0016] More specifically, each steam injection conduit 32 mounts a steam injection rake 34 (Figs. 3 and 4), formed to include a plurality of substantially

perpendicularly-oriented discharge nozzles 36 at the remote or free end thereof as best seen in Fig. 4. Thus, the steam is injected through the plurality of small orifices or nozzles 36 distributed on the rakes 34, and the emitted steam is mixed with the compressor air flow around and about the transition pieces 120 (see Fig. 2) to thereby achieve more uniform mixing throughout.

Further in this regard, it is noted that the steam is injected at a temperature that is higher than steam saturation temperature so that the risk of injecting saturated moisture is reduced (especially at commencement of steam injection, when the steam manifold between the associated gate valve and the casing is cold. It should be noted, however, that the improved mixing capabilities of this system will improve the stability of the

combustion process with steam of near saturation

temperature. Well-mixed steam will not affect flame stability in such a negative sense as large regions o unmixed steam.

[0017] In addition, the individual nozzles 36 need not extend substantially perpendicularly from the

respective rakes 34 as shown in Fig. 4, but rather, are preferably adjusted or tuned to minimize or eliminate any direct impingement of the steam on the sides of the transition pieces 118, and to distribute the steam more evenly around the annular array of combustors 110. For example, utilizing conventional universal type nozzle mounts, it will be appreciated that the nozzles can be directed in various directions and can be angled radially downwardly (as in Fig. 3), or circumferentially to provide a circumferential swirl component to further enhance mixing with the compressor discharge air.

[0018] It will be appreciated that the number of nozzles 36 on the individual rakes 34 as well as the number of rakes 34 may be adjusted to obtain the desired degree of steam/air mixing. [0019] Figs. 5 and 6 illustrate a similar exemplary but nonlimiting embodiment. Again, similar reference numerals but with the prefix "2" added for convenience to identify corresponding combustor components. Here, the steam injection rakes 38 are shorter in length, but nevertheless sufficient to project radially inwardly of the radially-outermost surfaces of the transition pieces 220, enabling the adjustable nozzles 40 to provide the desired enhanced mixing of steam and compressor

discharger air. [0020] In addition, Fig. 6 illustrates an example embodiment where two steam injection rakes 38 are located between each pair of transition pieces 218, but as noted above, other arrangements are within the scope of this invention. The arrangement shown in Fig. 6 is, of course, equally applicable to the rake configurations of Figs. 2- 4.

[0021] The injection of steam utilizing extended- length steam injection rakes described herein provide more uniform mixing of fuel and air which, in turn, more effectively reduces the formation of carbon monoxide.

[0022] The steam injection system, while improving intra-can distribution of steam, will also provide sufficient flow restriction as to improve inter-can distribution of steam by ensuring even steam distribution to each steam injection rake. This represents an

improvement over current designs which consist of very little flow restriction at each injection location which can allow for very large location-to-location steam variance. Large differences in steam in location-to- location can cause unwanted high emissions or impact flame stability.

[0023] 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.