An expander with thermal shields

Abstract

The expander comprises a casing and a supporting structure arranged therein. The expander further comprises annular arrays of stationary blades attached to the radially inner surface of the supporting structure and a rotor rotatingly supported in the casing. The rotor comprises a plurality of annular arrays of rotor blades disposed inside the supporting structure. A plurality of shrouds are attached to the radially inner surface of the supporting structure and each shroud surrounds a respective one of said annular arrays of rotor blades. An annular thermal shield is arranged between the radially inner surface of the supporting structure and at least one of said shrouds, and / or between the radially inner surface of the supporting structure and at least one of said annular arrays of stationary blades.

Description

AN EXPANDER WITH THERMAL SHIELDSDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns power-generating turbomachines. Embodiments disclosed herein concern supercritical carbon dioxide (CO2) expanders, such as expanders adapted to be used in oxy-fuel cycles, for example Allam cycles.BACKGROUND ART

[0002] Fossil fuels are a major source of chemical energy used for the generation of mechanical power. Fossil fuels are mixed with air and combusted to generate a combustion gas at high pressure and temperature, which expands in a turbine or an expander. The expander or the turbine converts combustion gas enthalpy into mechanical power available at the output shaft of the expander or turbine and used to drive a load, such as a compressor or compressor train, or to rotate an electric generator and convert mechanical power into electric power.

[0003] A maj or concern regarding combustion of fossil fuels relates to the production of carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0004] To reduce the environmental impact of power generation through combustion of fossil fuels, the option of post combustion capture of carbon dioxide has been investigated. 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 in the environment. The cost of a carbon dioxide capturing facility is high, both in terms CAPEX, as well as in terms of energy required to run the facility, which reduces the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in flue gas is low. This requires large volumes of flue gas to be processed through the carbon dioxide capturing facility and renders the capturing process particularly inefficient.

[0005] In recent years, oxy-combustion cycles, also known as oxy-fuel cycles, havebeen developed, wherein fuel, such as natural gas, is blended into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2) at high pressure. The blend of fuel, oxygen and carbon dioxide bums in a combustor of an expander, thus producing a pressurized combustion gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0006] The combustion gas is expanded in the expander to generate mechanical power, which can eventually be converted into electric power by an electric generator driven into rotation by the expander. The exhausted flue gas discharged at the discharge side of the expander is cooled in a regenerative heat exchanger and further chilled to condensate 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. The concentration of carbon dioxide in the removed flue gas is high and therefore the carbon capture process becomes efficient.

[0007] Oxygen supplied to the combustor of the expander can be obtained by separation from ambient air, removing nitrogen therefrom, such that the working fluid supplied to the combustor mainly consists of oxygen and carbon dioxide and does not include nitrogen. The resulting flue gas does not include nitrogen and the percentage carbon dioxide content thereof is substantially higher than in flue gas from a standard gas turbine cycle. The higher carbon dioxide percentage in the flue gas renders carbon capture more efficient and less expensive.

[0008] Oxy -fuel cycles, such as those described above, are particularly interesting in terms of efficiency, reduction of noxious emissions, and more efficient carbon capture. However, they operate under CO2 supercritical conditions at the inlet of the expander. The high density and the high Reynolds number, as well as the high temperatures of the process fluid, pose serious difficulties in the design of the turbomachine, in particular in consideration of the high thermal stresses, resulting from the strong heat exchange, increased by the high density of the process fluid, in combination with mechanical load resulting from the high operating pressure of the combustion gas.

[0009] Thermal radiation and convection dominate heat transfer between the hotprocess fluid in the expansion flow path and the surrounding stationary components, such as stationary blades or vanes and shrouds, as well the relevant supporting structure. The high density of the process gas increases the heat transfer from the process gas to the stationary components and supporting structure thereof, which surrounds the expansion flow path.

[0010] The resulting high temperature of the components surrounding the expansion flow path leads to both increase of thermally-induced stresses and to reduction of the mechanical properties of such components.

[0011] The subject of the present disclosure are improvements aimed at reducing the above-mentioned drawbacks.SUMMARY

[0012] Disclosed herein is an expander comprising a casing and a supporting structure housed in said casing. A plurality of annular arrays of stationary blades are attached to the radially inner surface of the supporting structure. A rotor is housed in the casing and is supported therein for rotation around a rotation axis. The rotor is positioned inside the supporting structure.

[0013] The rotor comprises a plurality of annular arrays of rotor blades disposed inside the supporting structure. Each annular array of rotor blades is arranged downstream of a respective annular array of stationary blades and forms therewith a respective expander stage. The expander stages are sequentially arranged along an expansion flow path of the expander, along which, in use, process gas expands and transfers energy to the rotor blades. Shrouds are attached to the radially inner surface of the supporting structure, each shroud surrounding a respective one of said annular arrays of rotor blades.

[0014] To reduce the amount of heat transferred from the process gas to the supporting structure, an annular thermal shield is arranged in the expander. Specifically, the annular thermal shield can be disposed circumferentially between the radially inner surface of the supporting structure and at least one of the shrouds. Alternatively, the annular thermal shield can be disposed circumferentially between the radially inner surface of the supporting structure and at least one of said annular arrays of stationaryblades. In some embodiments, the annular thermal shield can be arranged between the radially inner surface of the supporting structure and at least one of said annular arrays of stationary blades, and additionally between the radially inner surface of the supporting structure and at least one of said shrouds. The thermal shield and the radially inner surface of the supporting structure whereto the thermal shield is connected form together an annular thermal insulation volume, which can be filled with a gaseous medium, preferably a stagnant gaseous medium. The gaseous medium forms a thermal insulation layer. In some embodiments, the radial dimension (i.e. the height) of the annular thermal insulation volume is sufficiently small to avoid the generation of a convective motion of the gaseous medium trapped therein.

[0015] In some embodiments, the distance between the radially outer surface of the thermal shield and the radially inner surface of the supporting structure is sufficiently small prevent the onset of convective motion in the gaseous medium entrapped therebetween.

[0016] The annular thermal shield arranged inside the supporting structure, between the supporting structure and components facing the expansion flow path is particularly beneficial in expanders, in which process gas having a high density is expanded. For instance, the expander can be a supercritical carbon dioxide expander, or an expander for an oxyfuel cycle. As understood herein, a supercritical carbon dioxide expander is an expander, wherein the process fluid consists of carbon dioxide or mainly of carbon dioxide, which is in a supercritical state at least in one section or stage of the expander.

[0017] As will be explained in more detail later, supercritical carbon dioxide or similar high-density process fluids, have a high heat transfer coefficient and therefore transfer by convection a large amount of heat to the mechanical structure surrounding the expansion flow path. The thermal shields disclosed herein provide a barrier against heat transfer and thus reduce the risk of thermal damages to the structure surrounding the flow path.

[0018] In some embodiments, the supporting structure can be directly housed in a single casing. In other embodiments, specifically when the expander is designed to process high-pressure gas, at a pressure of 50 barA or above, such as in supercritical CO2 expanders, the casing may comprise an inner casing and an outer casing, housedin the inner casing. The supporting structure can be in turn housed in the inner casing. In some embodiments, the outer casing is a vertically split casing, while the inner casing can be a horizontally split casing.

[0019] Further advantageous features and embodiments of the expander according to the present disclosure are described below, with reference to the attached drawings, and in are set forth in the attached claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Reference is now made briefly to the accompanying drawings, in which:Fig.l is a sectional view of an expander according to an embodiment of the present disclosure, taken along a plane containing the rotation axis;Fig. 2 is an enlargement of a detail of Fig. l;Fig.3 is a partial sectional view according to line III-III in Fig.2;Fig.4 is an enlargement of a further embodiment of the thermal shield;Fig.5 is an enlargement of a yet further embodiment of the thermal shield; and Fig.6 is an enlargement of a yet further embodiment of the thermal shield.DETAILED DESCRIPTION

[0021] A sectional view 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.

[0022] The expander 1 includes a casing. In some embodiments, the casing includes in turn an outer casing 3 and an inner casing 5. The outer casing 3 can include a main body 3 A and a closure 3B on the aft side of the expander. The main body 3 A and the closure 3B are coupled, through respective flanges, for instance, 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.

[0023] An annular chamber 6 is formed between the outer casing 3 and the inner casing 5.

[0024] A combustor, such as a can combustor including a plurality of combustion chambers 7, for instance, can be positioned at the forward side of the expander.

[0025] As used herein “forward” and “aft” are referred to the direction of flow of the process gas through the expander 1. Therefore, “forward” indicates a position on the side of the combustor chambers 7 and “aft” indicates a position on the side opposite the combustor chambers 7, i.e., the discharge side of the expander 1.

[0026] The expander 1 further comprises a rotor 11 housed in the inner casing 5 and adapted to rotate around the rotation axis A-A. In some embodiments, the rotor 11 comprises a rotor shaft 13 and a plurality of annular arrays, or sets, of rotor blades. In the exemplary embodiment of Fig.1, the rotor 11 comprises eight annular arrays of rotor blades. Each array of rotor blades extends around the rotation axis A-A of the rotor 11. The rotor blades are labeled 15.j, where j indicates the position of the array in the forward-to-aft direction. Specifically, the rotor blades of the first annular array are labeled 15.1, the rotor blades of the last annular array are labeled 15.8; the blades of the jtharray are labeled 15.j. As used herein, reference number 15 refers to rotor blades of a generic annular array of rotor blades.

[0027] In the inner casing 5, an annular array or set of stationary blades is positioned upstream of each annular array of rotor blades 15.j. The blades of the annular arrays of stationary blades are labeled 17.j. More specifically, the blades of the most upstream annular array of stationary blades are labeled 17.1, the blades of the most downstream array of stationary blades are labeled 17.8. In general, the blades of the jthannular array of stationary blades are labeled 17.j. As used herein reference number 17 refers to stationary blades of a generic array of stationary blades. As used herein “upstream” and “downstream” refer to the direction of flow of the flue gas flowing through the expander 1.

[0028] Each annular array of stationary blades 17.j and respective annular array of rotor blades 15.j, arranged downstream thereof, form together an expansion stage of the expander 1. The sequence of expansion stages forms the expansion flow path, wherethrough the process gas expands and transfers energy to the rotor blades, thus driving the rotor in rotation and generating mechanical power, which is available on the output shaft of the expander 1.

[0029] As will be described in more detail below, the stationary blades 17 of one, some, or each annular array of stationary blades are not mounted directly on the inner casing 5, but are rather mounted on a supporting structure 16 stationarily housed in the inner casing 5.

[0030] In some embodiments, the supporting structure 16 comprises a plurality of rings arranged in sequence in the forward-to-aft direction along the expansion flow path.

[0031] In some embodiments, each annular array of stationary blades 17 is supported by at least one respective ring, or by a pair of adjacent rings. In the embodiment of Fig. l, all stationary blades 17 of each annular array or set of stationary blades 17 are supported by a respective single ring, i.e. are mounted on the respective ring. The rings are labeled 18.

[0032] More specifically, each ring is labeled 18.j (wherein j=l -7). As used herein, reference number 18 refers to a generic ring. Each annular array of stationary blades 17 j (wherein j= 1-7) is mounted on a respective ring 18.j. The most downstream annular array of stationary blades 17.8 is mounted on a ring 18.8 which forms part of an aft portion 5B of the inner casing 5. The aft portion 5B of the inner casing 5 is coupled to a main body 5 A of the inner casing 5. The main body 5 A can in turn be formed by a plurality of casing portions. Each casing portion can be split along a plane containing the rotation axis A-A of the rotor 11, i.e., the inner casing 5 can be a so-called horizontally split casing.

[0033] In some embodiments, each annular array of rotor blades 15.j is surrounded by a respective shroud. The shrouds are labeled 19.1, ... 19.j, 19.8. In the embodiment of Fig.l, the shrouds 19.2 to 19.7 are supported each by the respective ring 18, on which the stationary blades positioned immediately upstream of the shroud are mounted. Thus, shroud 19.j is mounted on ring 18.j that supports the annular array of stationary blades 17 j, wherein j=2 to 7.

[0034] In other embodiments, one or more shrouds 19 can be supported by more than just one ring 18. For instance, one shroud 19 can be supported by two adjacent rings 18, which are part of the supporting structure 16.

[0035] Each shroud 19 comprises a plurality of annularly arranged shroud segments, mounted on the respective supporting ring(s).

[0036] In the embodiment of Fig.1, the first shroud 19.1 that surrounds the first, i.e., the most upstream annular array of rotor blades 15.1, is supported by an auxiliary ring 18.0, which does not support any stationary blade, and which is positioned between the first ring 18.1 and the second ring 18.2.

[0037] The supporting structure 16 formed by the rings 18 separates the inner casing 5 from the expansion flow path, formed by the alternately arranged annular arrays of stationary and rotor blades 17, 15. The supporting structure 16 thus supports stationary components (stationary blades 17 and shrouds 19), which are in direct contact with the process gas that expands through the expansion flow path of the expander 1.

[0038] The rings 18 can be structured such that they transfer to the inner casing 5 only axial reaction forces generated by the expanding hot gas which flows along the expansion flow path.

[0039] Moreover, the rings 18 decouple the expansion flow path (and the components in contact with the expansion flow path, namely shrouds 19 and stationery blades 17) from the inner casing 5. A cooling fluid gap 61 is formed between inner casing 5 and the support structure, i.e. the rings 18. The cooling fluid gap 61 is adapted to receive a cooling medium, for instance compressed process gas diverted from the process gas loop, upstream of the combustor or upstream of a recovery heat exchanger included in the thermodynamic loop of the expander, to recover heat from the exhausted process gas exiting the expander.

[0040] Calibrated flow passages can be provided in the supporting structure 16 formed by the rings 18, such that a controlled flow of cooling gas can flow from the cooling fluid gap 61 towards the expansion flow path. An exemplary flow passage is shown at 62. Flow passages can also be formed at the interface between mutually contacting rings 18, or between a ring 18 and the inner casing 5.

[0041] An enlargement of a detail of Fig.1, including the first shroud 19.1, which surrounds the first annular array of rotor blades 15.1, and the second annular arrays of stationary blades 17.2, as well as relevant rings 18.0 and 18.2, is shown in Fig.2.

[0042] In some embodiments, one, some or each shroud is associated with a thermal shield, positioned between the shroud and the supporting structure 16. Preferably, a thermal shield is combined with at least the most upstream shroud 19.1, where the process gas is at the highest temperature along the expansion flow path, downstream of the first set of stationary blades 17.

[0043] In the embodiment of Fig.2, a first thermal shield 51 is positioned between the ring 18.0 and the shroud 19.1. In some embodiments, the thermal shield 51 comprises a plurality of segments 51.1, which are arranged in sequence in an annular configuration around the rotation axis A-A of the expander. In some embodiments, each segment can be formed by a respective shaped sheet metal. Fig.3 shows a detail of mutually abutting segments 51.1 in a cross section along a plane orthogonal to the rotation axis A-A of the expander.

[0044] If the thermal shield 51 comprises a plurality of segments, each segment is attached to the supporting structure 16 with respective connection members. Specifically, in some embodiments, the thermal shield is attached to the radially inner surface of the supporting structure 16. In the embodiment of Fig.2, the thermal shield 51 is attached to the radially inner surface 18a of the ring 18.0, which forms a portion of the radially inner surface of the supporting structure 16.

[0045] In some embodiments, the thermal shield 51, and specifically each segment 51.1 thereof, is formed by a shaped sheet. In embodiments, the shaped sheet is made of metal. Each sheet has two lips 51.3, 51.4, respectively a forward lip 51.3 and an aft lip 51.4. The lips 51.3, 51.4 can be folded radially outwardly and can be in pressure contact with the radially inner surface 18a of the respective supporting ring 18 along two annular pressure-contact areas of the ring 18 on which the thermal shield 51 is attached. The two annular pressure-contact areas are distanced from one another in the axial direction, i.e. parallel to the rotation axis A-A.

[0046] In some embodiments, the contact pressure between the ring 18 and the thermal shield 51 is obtained by attaching the thermal sheet 51, i.e. to the supporting ring by a connection device 53, placed in an intermediate position between the lips 51.3, 51.4. The connection device can be adapted to press the respective sheet 51.1 radially outwardly against the radially inner surface 18a of the respective ring 18. The radiallyoutwardly oriented pressure elastically deforms the sheet 51.1 such that the lips 51.3, 51.4 thereof are maintained in pressure contact against the radially inner surface 18a of the ring 18.0.

[0047] In some embodiments, the connection device 53 can comprise one or more screws 55, each screwed in a threaded dead hole 57 formed in the ring 18.0. In this way the segments of the thermal shield can be easily removed and replaced, if required, by simply unscrewing and screwing the screws. Alternative reversible connection devices can be used with the same advantage of easy replacement of the thermal shield segments.

[0048] The thermal shield 51 and the radially inner surface 18a of the supporting structure 16 form together an annular thermal insulation volume 59.

[0049] Fig.3 is a cross section along line III-III of Fig.2, showing two sheets 51.1 or segments of the thermal shield 51, as well as respective connection devices 53. Two facing edges of the two sheets 51.1 are labeled 51.5. A joint therebetween is labeled G. To seal the thermal insulation volume 59, a joint cover 51.6 can be attached along one of the edges 51.5 and project therefrom in a cantilever fashion over the opposite edge 51.5, thus sealingly closing the joint G. The joint cover 51.6 can be formed by a metal strip brazed or welded along the edge 51.5, or can be obtained by deforming radially inwardly a portion of one of the two adjoining segments 51.1.

[0050] The annular thermal shield 51 and the thermal insulation volume 59 provide in combination a thermal insulation of the supporting structure 16 (and specifically of the ring 18), which reduces the heat transfer from the shroud 19, which is in direct contact with the expansion flow path, towards the supporting structure 16. This heat transfer reduction is beneficial in terms reduction of thermally-induced stresses and thermally-induced decay of mechanical properties of the supporting structure 16.

[0051] The thermal insulation volume 59 is preferably filled with a gaseous medium. In advantageous embodiments, the gaseous medium can be the same fluid processed through the expander. The fluid trapped in the thermal insulation volume 59 is stagnant, i.e. it does not participate to the flow motion of the process fluid which expands along the expansion flow path. If the same process fluid is used, this can besupercritical carbon dioxide, or another high-density fluid.

[0052] The stagnant, high-density fluid trapped in the thermal insulation volume provides an efficient thermal barrier against thermal transfer from the shroud 19.1 to the supporting structure 16.

[0053] The segmented structure of the thermal shield 51, including the plurality of segments 51.1, can be beneficial in terms of maintenance and repair in case of failure of the thermal shield 51. Individual segments 51.1 can be replaced, if needed, by simply releasing the connection device 53, removing the damaged thermal shield segment 51.1 and replacing the latter with a new thermal shield segment.

[0054] Fig.4 illustrates a further embodiment of a thermal shield according to the present disclosure. In this embodiment, a thermal shield 71 is used, which includes a honeycomb structure. In some embodiments, the honeycomb structure 71 comprises a radially outer surface 71.3 and a radially inner surface 71.4 and can comprise a plurality of annular honeycomb segments 71.1.

[0055] In some embodiments, the radially outer surface 71.3 is in contact with and connected to the radially inner surface 18a of the ring 18. In some embodiments, the radially outer surface 71.3 can be brazed or welded to the radially inner surface 18a of ring 18.

[0056] The honeycomb structure comprises empty cells which form a thermal insulation volume, which can be filled with a stagnant fluid, for instance high-density carbon dioxide. The thermal insulation volume formed by the cells of the honeycomb structure can be sealingly closed by a closure sheet 71.5. If the honeycomb structure 71 comprises a plurality of annular honeycomb segments 71.1, the radially inner surface of each annular honeycomb segment can be closed by a respective closure sheet 71.5. The closure sheet can be brazed or welded on the honeycomb structure 71 to seal the cells filled with insulating gaseous medium. Adjacent closure sheets of adjacent annular honeycomb segments can form joint or include joint covers similar joint cover 51.6 of Fig.3.

[0057] In all embodiments disclosed herein, the thermal insulation volume filled with stagnant fluid, preferably high-density stagnant fluid, does not necessarily require tobe sealingly closed. What matters, is that the fluid trapped therein is substantially nonmoving, thus acting as a thermal insulator, rather than promoting heat exchange by convection.

[0058] In some embodiments, one, some or all annular arrays of stationary blades 17 are combined with a thermal shield, like the thermal shield arranged around the shroud(s). In Figs.2 and 4 a thermal shield 81 is shown in combination with the annular array of stationary blades 17.2. The thermal shield 81 is positioned between the ring 18.0 and the base of the stationary blades 17.2.

[0059] In some embodiments, the thermal shield 81 comprises a plurality of segments 81.1, e.g., each formed by a respective shaped sheet, for instance a shaped metal sheet. The thermal shield segments 81.1 are annularly arranged around the rotation axis A- A.

[0060] In some embodiments, the thermal shield 81 is attached to the supporting structure 16. Specifically, in some embodiments, the thermal shield 81 is attached to the radially inner surface of the supporting structure 16. In the embodiment of Fig.2, the thermal shield 81 is attached to the radially inner surface 18a of the ring 18.0, which forms a portion of the radially inner surface of the supporting structure 16.

[0061] In some embodiments, the thermal shield 81, and specifically each segment 81.1 thereof, is formed by a shaped sheet. In some embodiments, the shaped sheet is made of metal. Each sheet has two lips 81.3, 81.4, respectively a forward lip 81.3 and an aft lip 81.4. In some embodiments the lips 81.3, 81.4 are in pressure contact with the ring 18.0. In other embodiments, as shown in Figs.2 and 4, the forward lip 81.3 is in pressure contact with the shroud 19.1, and the aft lip 81.4 is in pressure contact with the radially inner surface 18a of the respective supporting ring 18.1.

[0062] In some embodiments, pressure contact is obtained by attaching the respective sheet to the supporting ring 18.0 by a connection device 83, positioned in an intermediate position between the lips 81.3, 81.4. The connection device 83 can be adapted to press the respective sheet 81.1 radially outwardly against the respective ring 18.0 and shroud 19.1. The radially outward pressure elastically deforms the sheet 81.1 such that the lips 81.3, 81.4 thereof are maintained in pressure contact against theshroud 19.1 and the radially inner surface 18a of the ring 18.0.

[0063] In some embodiments, the connection device 83 can comprise one or more screws 85, each screwed in a threaded dead hole 87 formed in the ring 18.0.

[0064] The thermal shield 81 and the radially inner surface 18a of the supporting structure 16 form together an annular thermal insulation volume 89. The insulation volume 89 is filled with stagnant gas, similarly to the thermal insulation volume 59.

[0065] In further embodiments, not shown, the thermal shield surrounding the annular array of stationary blades 17.2 can be in the form of a honeycomb structure with a sealing sheet on the radially inner surface thereof, in a manner similar to the structure described above in connection with Fig.4 with regard to the honeycomb structure 71.

[0066] A further embodiment of an expander including thermal shields according to the present disclosure is shown in Fig.5, which illustrates only a portion of the expansion flow path of the expander, with one array of rotor blades 15 and relevant shroud 19 surrounding the rotor blades 15. The thermal shield is labeled 51 and is formed by a plurality of thermal shield segments 51.1, one of which is shown in a sectional view in Fig.5. The thermal shield segments 51.1 are attached to the radially inner surface 18a of the respective ring 18 forming therewith an annular thermal insulation volume 59. In this embodiment, each thermal shield segment 51.1 comprises a forward lip 51.3 and an aft lip 51.4. The lips 51.3 and 51.4 are welded or brazed to the radially inner surface 18a of the ring 18, thus forming the annular thermal insulation volume 59, which contains a stagnant gas, forming a thermal barrier. The volume 59 can be seal- ingly closed or may allow a gas leakage from the expansion flow path into the volume 59. In any event, as in the previously described embodiments, the volume 59 is filled with stagnant or almost stagnant fluid, which can be the same process fluid expanded through the expander. The stagnant fluid forms a highly insulating thermal barrier, which reduces the thermal load on the supporting structure 16 comprising the rings 18.

[0067] A yet further embodiment of an expander including thermal shields according to the present disclosure is shown in Fig.6, which illustrates only a portion of the expansion flow path of the expander, with one array of rotor blades 15 and relevant shroud 19 surrounding the rotor blades 15. The thermal shield is labeled 51 and isformed by a plurality of thermal shield segments 51.1, one of which is shown in a cross-sectional view. The thermal shield segments 51.1 are attached to the shroud 19 and are distanced from the ring 18 such as to form therewith an annular thermal insulation volume 59. In this embodiment, the thermal shield segments 51.1 have each a forward lip 51.3 and an aft lip 51.4, which are in pressure contact with the radially outwardly oriented surface of the shroud. The pressure contact between the lips 51.3, 51.4 and the shroud 19 can be generated by resilient deformation of the lips and can be sufficient to maintain the thermal shield segments 51.1 in the correct position. The annular thermal insulation volume 59 contains a gas, forming a thermal barrier. The volume 59 can be sealingly closed, or may allow a gas leakage from the expansion flow path into the volume 59. In any event, as in the previously described embodiments, the volume 59 is filled with stagnant or almost stagnant fluid, which can be the same process fluid expanded through the expander. The stagnant fluid forms a highly insulating thermal barrier which reduces the thermal load on the supporting structure 16 comprising the rings 18.

[0068] While in the enclosed drawings a thermal shield is shown in combination with the first shroud 19.1 and with the second annular array of stationary blades 17.2, it shall be understood that similar thermal shields can be foreseen for one, some or all the remaining stages of the expander 1.

[0069] The thermal shields reduce the heat transfer from one or more of the expander components, which face the expansion flow path, towards the supporting structure 16, thus reducing the thermal load and consequent thermally-induced stresses and thermally-induced decay of mechanical properties of the supporting structure 16.

[0070] Thermal shields as disclosed herein are particularly beneficial in combination with expanders adapted to expand a process gas having high density, such as for instance and in particular expanders for oxyfuel cycles. Specifically, expanders for Al- lam cycles often use a process fluid consisting mainly of carbon dioxide which can be in a supercritical state, in particular in the first, i.e. most upstream stages of the expander.

[0071] Supercritical fluids, such as supercritical carbon dioxide, have a high density, roughly in the same order of magnitude as a liquid. This leads to very high Reynoldsnumbers and, consequently, very high heat transfer coefficients (HTC) even when the fluid velocity is low. Indeed, HTC can be expressed as:HTC = f(Pr, Re wherein cp-pPrandtl number: Pr = kReynolds number:Cp: constant pressure specific heat; m dynamic viscosity; k'. thermal conductivity; r: density;V reference velocity;L'. reference length.

[0072] HTC values of supercritical fluids are from one to two orders of magnitude greater than the typical ranges of HTC values in standard gas turbines. Supercritical carbon dioxide expanders typically process a fluid with an HTC ranging between 2,000 and 50,000 W / m2K, while in gas turbine engines the expanding flue gas has an HTC typically values can vary within the range 200-5,000 W / m2K).

[0073] Moreover, in supercritical carbon dioxide expanders the purging or cooling fluid flowrate must be small, to avoid loss of efficiency of the thermal cycle. Expanding process fluid leaks from the expansion flow path towards the stationary structure which surrounds the expansion flow path, typically through joints between adjoining roots of the stationary blades and through components of the shroud. The thermal shield disclosed herein is particularly efficient in preventing or limiting damages induced on the stationary structure, which surrounds the expansion flow path and which can be induced thermally by the high-density process gas.

[0074] Stagnant, high density fluid which remains trapped in the honeycomb structure or in the annular thermal insulation volume 59 provides an efficient thermal barrier against convective heat transfer from the high density to the stationary structure arranged around the shrouds and the stationary blades. Specifically, the layer of stagnant supercritical carbon dioxide (sCCh) acts as a thermal resistance towards theexternal rings facing the shrouds and, consequently, reduces the resulting HTC to values that are closer to levels that are typical of the gas turbine engines.

[0075] The thermal shield structures disclosed herein are therefore particularly beneficial in combination with expanders adapted to expand process gas having a high HTC, above 2,000 W / m2K, preferably equal to, or higher than 5,000 W / m2K, more preferably equal to or higher than 10,000 W / m2K and typically equal to, or lower than 50,000W / m2K.

[0076] In certain embodiments disclosed herein, the annular thermal insulation volume 89 is sealed with respect to the expansion flow path by contact pressure established between the thermal shields 51, 81 and the adjacent supporting structure. During standard operation of the expander, for example at pressures equal to or exceeding 50 barA, the volume defined between the radially inner surface of the supporting structure 16 and the radially outer surface of the thermal shields becomes filled with stagnant process gas, i.e., the same working fluid undergoing expansion within the expansion flow path. When the process gas primarily comprises supercritical carbon dioxide, the fluid exhibits extremely high density, particularly within the initial expansion stages, and is characterized by a correspondingly high transfer coefficients, as noted above. Nonetheless, due to the absence of bulk fluid motion within the sealed annular gap, convective heat transfer mechanisms that affect the surface of the supporting structure are effectively suppressed. As a result, the stagnant process fluid trapped in the volume between the supporting structure and the thermal shield acts as a thermal insulation medium by limiting heat transfer predominantly to conduction.

[0077] In other embodiments, for example if the thermal shields are soldered or welded on the supporting structure, the gaseous medium trapped between the supporting structure and the thermal shield can be selected during assembling and can e.g. be air or an inert gas at ambient pressure, or more in general at a pressure lower than the pressure of the process gas during use of the expander. This may result in an even more efficient heat barrier.

[0078] 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 disclosedherein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A supercritical carbon dioxide expander comprising: a casing; a supporting structure arranged in the casing and having a radially inner surface and a radially outer surface; a plurality of annular arrays of stationary blades attached to the radially inner surface of the supporting structure; a rotor housed in the casing and supported therein for rotation around a rotation axis; wherein the rotor comprises a plurality of annular arrays of rotor blades disposed inside the supporting structure; wherein each annular array of rotor blades is arranged downstream of a respective annular array of stationary blades and forms therewith an expander stage, the expander stages sequentially arranged along an expansion flow path of the expander; a plurality of shrouds attached to the radially inner surface of the supporting structure, each shroud surrounding a respective one of said annular arrays of rotor blades; an annular thermal shield circumferentially disposed: between the radially inner surface of the supporting structure and at least one of said shrouds; or between the radially inner surface of the supporting structure and at least one of said annular arrays of stationary blades; or between the radially inner surface of the supporting structure and at least one of said annular arrays of stationary blades; and between the radially inner surface of the supporting structure and at least one of said shrouds; wherein the annular thermal shield and the radially inner surface of the supporting structure form together an annular thermal insulation volume.

2. The expander of claim 1, wherein the annular thermal shield is attached to the radially inner surface of the supporting structure.

3. The expander of claim 1 or 2, wherein the annular thermal shield comprises a plurality of shield segments annularly arranged around the rotation axis.

4. The expander of any one of the preceding claims, wherein the casing comprises: an outer casing; and an inner casing housed in the outer casing; wherein the supporting structure is housed in the inner casing and connected thereto, between the inner casing and the rotor.

5. The expander of claim 4, wherein the supporting structure comprises a plurality of stationary rings housed in the inner casing.

6. The expander of claim 5, wherein each ring is in contact with two adjacent rings or with an adjacent ring and the inner casing.

7. The expander of claim 6, wherein the adjacent rings are configured to transfer only axial loads to the inner casing.

8. The expander of any one of the preceding claims, wherein, in use, the annular thermal insulation volume is filled with a substantially stagnant gaseous medium.

9. The expander of claim 8, wherein the stagnant gaseous medium is process gas of the expander.

10. The expander of any one of the preceding claims, wherein the annular thermal shield comprises: at least one honeycomb structure having a radially outer surface attached to the radially inner surface of the supporting structure and a radially inner surface facing the rotor; and a closure sheet attached to the radially inner surface of the honeycomb structure.

11. The expander of claim 10, wherein the honeycomb structure comprises a plurality of annular segments arranged around the rotation axis and in contact with one another.

12. The expander of any one of claims 1 to 9, wherein the annular thermal shield comprises a plurality of annularly arranged sheets; wherein a thermal insulation volume is formed between a radially outer surface of the annularly arranged sheets and the radially inner surface of the supporting structure, the thermal insulation volume being filled with a gaseous medium.

13. The expander of claim 12, wherein the sheets are metal sheets.

14. The expander of claim 12 or 13, wherein the sheets are attached to the radially inner surface of the supporting structure.

15. The expander of claim 12 or 13, wherein the sheets are attached to the shroud, between a radially outer surface of the shroud and the radially inner surface of the supporting structure.

16. The expander of claim 14, wherein each sheet is connected to the supporting structure by a removable connection device positioned in an intermediate position of the respective sheet, between opposite lips extending tangentially along the supporting structure and in pressure contact therewith.

17. The expander of claim 16, wherein each sheet is resiliently deformed by the connection device, such as to maintain contact pressure between the sheet and the supporting structure along the opposite lips of the sheet.

18. The expander of claim 16 or 17, wherein the connection device comprises at least one screw.

19. The expander of claim 14, wherein the sheets are welded or brazed to the radially inner surface of the supporting structure.

20. The expander of any one of the preceding claims, adapted to process a working fluid having, in at least one of the expander stages, a heat transfer coefficient higher than 2,000 W / m2K, preferably higher than 5,000 W / m2K, more preferably higher than 10,000 W / m2K.

21. The expander of any one of the preceding claims, wherein the annular thermal insulation volume is filled with an inert gas, preferably at a pressure lower than the pressure of the process gas during operation of the expander.

Images