A power generating turbomachine with a discharge diffuser, and method

Abstract

The power-generating turbomachine comprises an inner casing (5) and an outer casing (3) surrounding the inner casing (5). A rotor (11) is positioned in the inner casing (5) for rotation therein around a rotation axis. The turbomachine further includes an expansion flow path extending in the inner casing (5) from the forward end to the aft end thereof. A discharge diffuser (21) is supported at the aft end of the inner casing (5) and is fluidly coupled with a discharge plenum (23) in the outer casing (3). The discharge diffuser (21) is supported by a diffuser support structure, which connects the discharge diffuser (21) to the aft end of the inner casing (5) with a clearance allowing displacement of the discharge diffuser (21) with respect to the inner casing (5).

Description

A power generating turbomachine with a discharge diffuser, and methodDESCRIPTIONTECHNICAL FIELD

[0001] Exemplary embodiments of the present disclosure pertain to turbomachines. Specifically, embodiments disclosed herein, pertain to power-generating turbomachines, i.e expanders or turbines. Some embodiments are particularly adapted to supercritical CO2 expanders. As understood herein, a supercritical CO2 expander is an expander wherein the working fluid contains mainly or almost exclusively carbon dioxide and wherein in at least one portion of the flow path the carbon dioxide is in supercritical conditions.BACKGROUND ART

[0002] Turbines or expanders include combustors which ignite a pressurized gaseous mixture containing fuel and oxidant. The resulting pressurized flow of hot combustion gas is expanded in an expansion flow path, including one or more expansion stages. Each expansion stage includes at least one annular row of stationary vanes, also known as stationary blades, and one annular row of rotor blades. These form part of a turbine rotor, arranged for rotation in a turbine casing.

[0003] One of the most significant issues associated with the combustion of fossil fuels is the production of carbon dioxide, a greenhouse gas that is widely recognized as a primary contributor to global warming and climate change.

[0004] In recent years, there has been a focus on developing thermodynamic cycles with the aim of reducing the environmental impact of power generation cycles using fossil fuels. To this end, the potential of post-combustion carbon dioxide capture has been explored. Carbon dioxide capture facilities have been developed to process flue gas exhausted from gas turbines and remove carbon dioxide therefrom prior to discharging the flue gas into the environment. The cost of a carbon dioxide capturing facility is significant, both in terms of capital expenditure (CAPEX) and in terms of the energy required to run the facility. This reduces the overall thermodynamic efficiency of the system. The concentration of carbon dioxide in the flue gas is relativelylow. This necessitates the processing of substantial volumes of flue gas through the carbon dioxide capturing facility, which renders the capturing process particularly inefficient.

[0005] In recent years, oxy-combustion cycles, also known as oxy-fuel cycles, have been developed. These cycles involve blending fuel, such as natural gas, into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2), at high pressure. The blend containing fuel, oxygen, and carbon dioxide is ignited in a combustor of an expander, resulting in a pressurized combustion gas comprising primarily, or consisting essentially of, carbon dioxide and water.

[0006] The combustion gas is expanded in the expander or turbine to generate mechanical power, which can eventually be converted into electric power by an electric generator driven into rotation by the expander. Alternatively, the mechanical power can be used for mechanical drive purposes, e.g. to drive a mechanical equipment such as a compressor or a compressor train into rotation. The exhausted combustion gas, i.e., the flue gas discharged at the discharge side of the expander, is cooled in a regenerative heat exchanger and further chilled to condense steam into water, which is removed from the chilled flue gas. A main part of the low-temperature flue gas, consisting mainly, or exclusively, of carbon dioxide, is pressurized and recycled through the regenerative heat exchanger towards the combustor of the expander. A remaining part of the flue gas is removed and carbon dioxide contained therein is captured.

[0007] The oxygen required for combustion in the expander combustor can be sourced from ambient air by removing nitrogen, creating a working fluid that consists primarily of oxygen and carbon dioxide, with minimal or no nitrogen content. The combustion gas produced has a percentage content of carbon dioxide which is significantly higher than that of combustion gas from a standard gas turbine cycle. The higher percentage of carbon dioxide in the combustion gas makes carbon capture more efficient and cost-effective.

[0008] Oxy-fuel cycles, such as those described above, offer significant potential for increased efficiency, reduced noxious emissions, and efficient carbon capture. However, the high carbon dioxide concentration, high temperature, and high pressure in the expander require several expander components to operate under severethermodynamic conditions. This is particularly true for the discharge diffuser, at the aft end of the expansion flow path. The discharge diffuser is subjected to high pressure and high temperature.

[0009] The thrust developed by the pressure variation over the discharge diffuser needs to be transferred to the adjacent inner casing. This load bearing capability necessitates a surface-to-surface contact resulting in a significant amount of heat inflow to the inner casing which is to be made of nickel-based alloy, due to the presence of high percentage of carbon dioxide in the expanding process gas flow. The high-strength nickel-based alloy used for this application has a limitation in terms of metal temperature which is to be kept below a certain value. This necessitates that the exterior surface of the discharge diffuser be cooled, while the inner surface thereof, which faces the expansion flow path is hot. The resulting temperature differential between opposed surfaces of the discharge diffuser and the thermal -induced stresses deriving therefrom may have a negative impact on the integrity of the discharge diffuser.

[0010] An object of embodiments disclosed herein is to provide a novel design which is capable of addressing these contradictory requirements.SUMMARY

[0011] Disclosed herein is a power-generating turbomachine comprising an inner casing having a forward end and an aft end, and an outer casing surrounding the inner casing. A rotor is positioned in the inner casing for rotation therein around a rotation axis. The turbomachine further includes an expansion flow path extending in the inner casing from the forward end to the aft end thereof. A discharge diffuser is supported at the aft end of the inner casing and is fluidly coupled with a discharge plenum in the outer casing. The discharge diffuser is supported by a diffuser support structure, which connects the discharge diffuser to the aft end of the inner casing with a clearance allowing displacement of the discharge diffuser with respect to the inner casing.

[0012] In some embodiments, the clearance is adapted to allow an axial displacement of the discharge diffuser with respect to the inner casing. The axial displacement of the discharge diffuser can generate form a gap for a cooling gas, which provides a thermal insulation between the discharge diffuser and the inner casing.

[0013] In some embodiments, the clearance is adapted to allow a radial displacement of the discharge diffuser with respect to the inner casing. A radial displacement of the discharge diffuser with respect to the inner casing reduces or avoids thermally induced stresses on the structural components of the turbomachine.

[0014] Further features and embodiments will be described below with reference to the drawings, and are set forth in the appended claims.

[0015] According to another aspect, disclosed herein is a method for reducing mechanical and thermal load on an inner casing of a power-generating turbomachine as outlined above. The method comprises the step of connecting the discharge diffuser to an aft end of the inner casing with a clearance allowing displacement of the discharge diffuser with respect to the inner casing.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Reference is now made briefly to the accompanying drawings, in which:Fig.l illustrates a sectional view of an expander including a combustor, according to one embodiment of the present disclosure, taken along a plane containing the rotation axis of the expander;Fig.2 illustrates an enlargement of the discharge diffuser of the expander shown in Fig.l;Fig.3 illustrates a front view of the discharge diffuser;Fig.4 illustrates a sectional view of the discharge diffuser along a plane orthogonal to the rotation axis, taken along line IV-IV of Fig.5;Fig.5 illustrates an enlarged sectional view, taken along a plane containing the rotation axis, of one the bushings which support the slidable discharge diffuser; and Fig.6 illustrates an axial view of the detail of Fig.5.DETAILED DESCRIPTION

[0017] The following detailed description of embodiments refers to an expander including a combustor and designed for supercritical CO2 thermodynamic cycles, particularly oxyfuel combustion cycles. This should not be taken to imply any limitation on the novel features described herein, which may be applied in other power-generating turbomachines, also based on different cycles and possibly without a combustor, where similar issues arise as those discussed above in connection with oxyfuel combustion cycles, such as those known as Allam cycles.

[0018] Turning now to the drawings, a sectional view of one embodiment of an expander 1 according to the present disclosure is shown in Fig.l. The section is taken along a plane containing a rotation axis A-A of the expander. The sectional view shows only half expander, which is substantially axial-symmetrical.

[0019] In some embodiments, the expander 1 includes an outer casing 3 and an inner casing 5. The outer casing 3 may comprise a main body 3 A and a closure 3B, the closure being arranged at the aft side of the expander. The main body 3 A and the closure 3B are connected to one another through respective flanges along a plane orthogonal to the rotation axis of the expander. In this embodiment, therefore, the outer casing 3 is a so-called vertically split casing. In some embodiments, the inner casing 5 can be a horizontally split casing.

[0020] This structure offers significant advantages for supercritical carbon dioxide expanders and other expanders that process gas under similar thermodynamic conditions involving high pressure drops across some or all the expansion stages. This structure is particularly well-suited to processes involving the use of expanding gas having a high heat transfer coefficient, which results in high temperature gradients. It should be noted, however, that the novel features disclosed herein can be used in a different expander structure such as, for example, a turbomachine including a vertically split inner casing and a horizontally split outer casing, or a vertically split inner casing and a vertically split outer casing, or a horizontally split inner casing and a horizontally split outer casing.

[0021] In some embodiments, a combustor 4, such as a can combustor comprising a plurality of combustor chambers 7, is positioned at the forward side of the expander 1, upstream of the first annular row of stationary blades 17.

[0022] As used herein “forward” and “aft” are referred to the direction of flow of the process gas through the expander 1. Accordingly, “forward” denotes a position on the side of the combustor chambers 7 and “aft” indicates a position on the side oppositethe combustor chambers 7, i.e., the discharge side of the expander 1.

[0023] The expander 1 further comprises a rotor 11 housed in the inner casing 5 and adapted to rotate around the rotation axis A-A. The rotor 11 comprises a rotor shaft 13 and a plurality of annular rows of rotor blades. Each row of rotor blades comprises a plurality of rotor blades 15, arranged circumferentially around the rotation axis A-A of the rotor 11.

[0024] A respective annular row of stationary blades 17 is positioned in the inner casing 5 upstream of each annular row of rotor blades 15. Pairs of adjacent stationary blades in a row define respective nozzles which orient the expanding gas in the correct orientation with respect to the downstream annular row of rotor blades 15. Each annular row of stationary blades 17 and the respective annular row of rotor blades 15 form together a stage of the expander 1.

[0025] In some embodiments, the stationary blades 17 of one, some or each annular row of stationary blades can be mounted on at least one respective ring, or on a pair of adjacent rings 18. The rings 18 form a mechanical structure which couples the stationary blades 17 to the inner casing 5. In some embodiments, the stationary blades 17 of one or some annular rows of stationary blades can be mounted directly on the inner casing 5.

[0026] The annular rows of rotor blades 15 can be surrounded by shrouds 19, which can be formed by shroud segments, mounted on the inner casing 5 directly or on rings 18.

[0027] An expansion flow path extends from a forward end 5.1 of the inner casing 5 to an opposite aft end 5.2 of the inner casing 5, through the annular rows of stationary blades 17 and rotor blades 15.

[0028] A discharge diffuser 21 (hereafter also referred to simply as “diffuser”) is attached to the aft end of the inner casing 5 as described in greater detail below. The discharge diffuser 21 encircles the last part of the expansion flow path and conveys the exhaust combustion gas to a discharge plenum 23, where the exhaust combustion gas is collected and wherefrom it is removed for further processing in a semi-closed cycle or discharged, according to the nature of the thermodynamic cycle.

[0029] A diffuser support structure connects the diffuser 21 to the aft end 5.2 of the inner casing 5. The diffuser support structure is configured such as to maintain a clearance that permits a displacement of the diffuser 21 with respect to the inner casing 5. As will be clarified below, the support structure is preferably adapted to permit a radial as well as an axial displacement of the discharge diffuser 21 with respect to the inner casing 5.

[0030] In some embodiments, the inner casing 5 terminates at the aft end thereof with an end flange 5.3. The end flange 5.3 may form a seat for the most downstream ring 18, which secures the shroud segments forming the most downstream shroud 19. This shroud encircles the last annular row of rotor blades 15 and supports the most downstream annular row of stationary blades 17.

[0031] In some embodiments, the end flange 5.3 comprises a plurality of radial appendages 5.4, which connect the end flange 5.3 of the inner casing to the outer casing 3.

[0032] In the illustrated embodiment, the diffuser 21 is divided along a plane P-P into two symmetrical portions 21.1 and 21.2, for facilitating the assembling process. As shown in the section of Fig.4, the two portions 21.1, 21.2 are connected to one another by tangential bolts 25 or other connection features.

[0033] The diffuser support structure may comprise a plurality of bushings 27, arranged along a circumference which extends around the axis of rotation A-A and can be coaxial thereto. The bushings 27 protrude in an axial direction from the aft end of the inner casing 5. In some embodiments, the bushings 27 protrude from a surface of the end flange 5.3 facing in the aft direction, i.e. facing the discharge plenum 23. As described in detail below, the discharge diffuser 21 is slidably supported on the bushings 27, and more precisely can be supported thereon with an axial clearance, thereby allowing the diffuser 21 to move in an axial direction with respect to the inner casing. The discharge diffuser 21 can be supported on the bushings 27 to with a radial clearance, allowing a radial displacement of the diffuser 21, i.e., a radial expansion thereof which can be thermally induced. In the illustrated embodiment, the discharge diffuser 21 is mounted to have both an axial as well as a radial clearance.

[0034] As best shown in Fig.5, each bushing 27 has a first end 27.1 engaging the aft end 5.2 of the inner casing 5, and more specifically the end flange 5.3 thereof. Each busing 27 further includes a body 27.2, which protrudes from the aft end of the inner casing 5 and which terminates with an annular abutment 27.3 at a second end of the bushing 27. In some embodiments, each bushing 27 engages a seat 29 formed in the inner casing 5, and more specifically in the end flange 5.3 thereof.

[0035] Each bushing 27 may be secured to the aft end 5.2 of the inner casing 5 by a respective clamping bolt 30, which extends axially in a through axial cavity of the bushing 27 and can be screwed in a threaded dead hole 31, which can be machined in the aft-facing side of the end flange 5.3.

[0036] Each bushing engages a respective seat 33 (see in particular Fig.5) of the diffuser 21. Each seat 33 is formed as a through hole in a peripheral portion of the diffuser 21. Each seat 33 extends between a forward flat and annular surface 35 of the diffuser 21 and a respective flat aft surface 37. Each flat surface 37 can be formed in a radial pocket 39 at the periphery of the diffuser 21. The dimension of each pocket 39 is sufficient to allow mounting of the bushing 27 and of the clamping bolt 30.

[0037] In the illustrated embodiment, each seat 33 has an axial length LI, i.e. a length in an axial direction parallel to the rotation axis A-A, which is shorter than an axial length L2 of the portion of the body 27.2 of the bushing 27 which protrudes from the seat 29 and ends at the annular abutment 27.3. The difference between L2 and LI defines an axial clearance which allows an axial displacement of the discharge diffuser 21 with respect to the inner casing 5 along the bushings 27.

[0038] Moreover, in the illustrated embodiment the diameter DI of each seat 33 is larger than the diameter D2 of the bushing 27, and specifically of the body 27.2 thereof. The difference D1-D2 defines a radial clearance which allows a radial displacement of the diffuser, i.e. allows the discharge diffuser 21 to expand radially with respect to the inner casing 5.

[0039] The radial clearance allows the discharge diffuser 21 to expand due to thermal expansion, for instance. The expansion may be different for the discharge diffuser 21 and the inner casing 5 due to thermal gradients and / or different thermal expansioncoefficients of the materials used for said different mechanical components. The radial clearance D1-D2 enables the discharge diffuser 21 to expand (or contract) radially to an extent different from the radial expansi on / contracti on of the flange 5.3 of the inner casing 5, thereby avoiding the generation of thermally induced stresses. The radial clearance also allows the diffuser to expand radially at a different rate or at a different time than the inner casing 5.

[0040] The axial clearance L2-L1 allows the discharge diffuser 21 to expand axially without generating thermally induced stresses. In addition, the axial clearance enables the discharge diffuser 21 to shift axially in the aft direction under the load applied by the combustion gas flowing through the expansion flow path toward the discharge plenum 23. The axial aft movement of the discharge diffuser 21 along the bushings 27 increases the distance between the forward flat surface 35 of the discharge diffuser 21 and a corresponding flat surface 5.31 of the end flange 5.3 facing in the aft direction. The gap formed between the surfaces 35 and 5.31 thereby completes a cooling gap 51 (see Fig.2), along which a pressurized cooling fluid can flow and which thermally insulates the discharge diffuser 21 from the end flange 5.3 of the inner casing 5. The cooling fluid may originate from an annular chamber 6 (Fig.1) between the inner casing 5 and the outer casing 3 and may have sufficient pressure to flow along the cooling gap 51 and to be discharged in the expansion flow path through an annular one-way seal 53 (Fig.2).

[0041] More specifically, the cooling gap 51 is fluidly coupled through a cooling gas inlet 51.1 (Fig.2) at a radial outer side thereof with the annular chamber 6. The cooling gap 51 is further fluidly coupled through the annular one-way seal 53 at a radial inner side thereof with the expansion flow path, such that pressurized cooling gas from the annular chamber 6 flows through the cooling gas inlet 51.1, through the cooling gap 51 and finally into the expansion flow through the annular one-way seal 53. The annular one-way seal 53 allows a cooling gas leakage from the cooling gap 51 toward the expansion flow path and prevents ingress of expanding process gas from the expansion flow path into the cooling gap 51.

[0042] In some embodiments, the discharge diffuser 21 comprises an annular protrusion 21.3 (Figs. 2, 5) which extends in an annular recess formed between the endflange 5.3 of the inner casing 5, and the outer casing 3. An annular one-way seal 55 can be arranged between the annular protrusion 21.3 of the discharge diffuser 21 and a forward surface of the annular recess, formed by the outer casing 3. The cooling gap 51 can extend around the annular protrusion 21.3, allowing cooling gas to flow between the annular protrusion 21.3, and the inner casing 5 and and outer casings 3. The annular one-way seal 55 allows cooling gas to leak from the gap 51 into the discharge plenum 23.

[0043] While during normal operation the process gas expanding in the expansion flow path pushes the diffuser 21 in an aft direction, in some circumstances, such as during transients, the diffuser 21 may be pushed in the opposite direction. A damper can be positioned in the cooling gap, between the aft-oriented surface 5.31 of the inner casing 5 and the forward-oriented surface 35 of the diffuser 21. The damper can be comprised of a plurality of damper pins 55 (Fig.5) arranged around the rotation axis A-A.

[0044] The diffuser support structure disclosed herein makes it possible to mitigate the disadvantages specific to the prior art mentioned above. In particular, the ability of the discharge diffuser 21 to expand radially allows the reduction of mechanical stresses induced by differential thermal expansion. The ability of the diffuser to slide axially away from the end flange 5.3 of the inner casing 5 allows the formation of the cooling gap 51 which provides thermal insulation between the discharge diffuser 21, which is exposed to high temperatures as it is in direct contact with the exhaust gas flow, and the inner casing 5 and more particularly the end flange 5.3 thereof. This allows the use of less expensive and more suitable materials for the manufacture of the components of the inner case 5.

[0045] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A power-generating turbomachine comprising:an inner casing having a forward end and an aft end;an outer casing surrounding the inner casing;a rotor arranged in the inner casing for rotation therein around a rotation axis; an expansion flow path extending in the inner casing from the forward end to the aft end thereof;a discharge diffuser supported at the aft end of the inner casing and fluidly coupled with a discharge plenum in the outer casing;a diffuser support structure connecting the discharge diffuser to the aft end of the inner casing with a clearance allowing displacement of the discharge diffuser with respect to the inner casing.

2. The power-generating turbomachine of claim 1, wherein the clearance is adapted to allow an axial displacement of the discharge diffuser with respect to the inner casing.

3. The power-generating turbomachine of claim 1 or 2, wherein the clearance is adapted to allow a radial displacement of the discharge diffuser with respect to the inner casing.

4. The power-generating turbomachine of any one of the preceding claims, further comprising a cooling gap between an aft surface of the inner casing and a forward surface of the discharge diffuser.

5. The power-generating turbomachine of claim 4, wherein the cooling gap is fluidly coupled with a cooling gas inlet at a radial outer side thereof and with the expansion flow path at a radial inner side thereof, such that pressurized cooling gas from the cooling gas inlet flows through the cooling gap into the expansion flow path.

6. The power-generating turbomachine of claim 4 or 5, comprising anannular seal at the radial inner side of the cooling gap, the radial seal being adapted to allow a cooling gas leakage from the cooling gap toward the expansion flow path and to prevent a backflow from the expansion flow path toward the cooling gap.

7. The power-generating turbomachine of any one of claims 4 to 6, wherein the discharge diffuser comprises an annular protrusion extending in an annular recess, formed between the inner casing and the outer casing.

8. The power-generating turbomachine of claim 7, comprising an annular seal between the annular protrusion of the discharge diffuser and a forward surface of the annular recess, formed by the outer casing.

9. The power-generating turbomachine of any one of claims 4 to 8, further comprising at least one damper in the cooling gap, between the aft surface of the inner casing and the forward surface of the discharge diffuser.

10. The power-generating turbomachine of claim 9, wherein the damper comprises a plurality of damper pins arranged around the rotation axis.

11. The power-generating turbomachine of any one of claims 4 to 10, wherein the cooling gap is fluidly coupled at an annular inlet thereof with a peripheral cooling gas chamber extending around the rotation axis and positioned between the inner casing and the outer casing.

12. The power-generating turbomachine of claim 11 wherein the cooling gap has a first radial section extending radially inwardly from the peripheral cooling gas chamber towards the expansion flow path.

13. The power-generating turbomachine of any one of the preceding claims, wherein the diffuser support structure comprises a plurality of bushings, arranged along a circumference around the rotation axis and protruding in an axial direction from the aft end of the inner casing; and wherein the discharge diffuser is slidably supported on the bushings.

14. The power-generating turbomachine of claim 13, wherein the discharge diffuser is supported on the bushings with an axial clearance, thereby allowing the discharge diffuser to move in an axial direction with respect to the inner casing.

15. The power-generating turbomachine of claim 13 or 14, wherein each bushing has a first end engaging the aft end of the inner casing, a body protruding from the aft end of the inner casing and terminating with an annular abutment at a second end thereof; wherein each bushing engages a respective seat of the discharge diffuser, the seat having an axial length shorter than an axial length of a portion of the body of the bushing extending between the aft end of the inner casing and the annular abutment of the bushing, thereby allowing the discharge diffuser to slide axially along the bushings.

16. The power-generating turbomachine of claim 15, wherein each seat has a diameter larger than the diameter of the respective bushing, thereby allowing the discharge diffuser to expand radially with respect to the inner casing.

17. The power-generating turbomachine of any one of claims 13 to 16, wherein each bushing is secured to the aft end of the inner casing by a respective clamping bolt extending axially in a through axial cavity of the bushing.

18. A method for reducing mechanical and thermal load on an inner casing of a power-generating turbomachine, comprising: an outer casing surrounding the inner casing; a rotor arranged in the inner casing for rotation therein around a rotation axis; an expansion flow path extending in the inner casing from the forward end to the aft end thereof; and a discharge diffuser supported at the aft end of the inner casing and fluidly coupled with a discharge plenum in the outer casing; the method comprising connecting the discharge diffuser to an aft end of the inner casing with a clearance allowing displacement of the discharge diffuser with respect to the inner casing.

19. The method of claim 18, further comprising the step of radially expanding the discharge diffuser with respect to the inner casing.

20. The method of claim 18 or 19, further comprising the followingsteps: displacing the discharge diffuser in an axial direction under the action of process gas expanding in the expansion flow path and forming a cooling gap between the inner casing and the discharge diffuser; generating a cooling fluid flow in the cooling gap.

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