EP4689363A1

EP4689363A1 - An expander with a pre-heating system and method

Info

Publication number: EP4689363A1
Application number: EP24717590.4A
Authority: EP
Inventors: Marco Fabbrini; Giacomo RAGNI; Leonardo Tognarelli
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2023-03-31
Filing date: 2024-03-27
Publication date: 2026-02-11
Also published as: IT202300006360A1; KR20250159272A; WO2024199735A1; JP2026509609A; AU2024247455A1; CN120882956A; MX2025011646A

Abstract

The expander comprises a casing including an outer casing and an inner casing. The inner casing is arranged in the outer casing and houses at least one set of annularly arranged stationary blades. The expander further includes at least a first annular fluid chamber between the inner casing and the outer casing. The inner casing has a peripheral wall having an outer surface facing the annular fluid chamber and an inner surface. A rotor is at least partly housed in the inner casing and is provided with at least one set of annularly arranged rotor blades, and preferably a plurality of sets of annularly arranged rotor blades, for instance between four and twelve sets of rotor blades, each arranged downstream of a respective set of annularly arranged stationary blades. The expander further includes a pre-heating arrangement, adapted to preheat the inner casing and cause thermal expansion thereof at expander start-up

Description

AN EXPANDER WITH A PRE-HEATING SYSTEM AND METHOD

DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure concerns expanders for generating mechanical power through a thermodynamic cycle. Embodiments disclosed herein specifically concern oxy-fuel expanders and expanders specifically intended for supercritical CO2 cycles.

BACKGROUND ART

[0002] Expanders, sometimes referred to as turboexpanders, are turbomachines including sequentially arranged stationary blades, aka stationary vanes, housed in a casing, and rotor blades forming part of a rotor supported for rotation in the casing. The stationary blades and rotor blades define an expansion flow path for a process gas, which flows through the expander. The enthalpy drop of the process gas is converted into mechanical power available on the expander shaft.

[0003] The expander comprises at least one annular row of stationary blades and one annular row of rotor blades mounted on a rotor for co-rotation therewith in the expander casing. More frequently, the expander includes a plurality of sequentially arranged annular rows of stationary blades and a plurality of sequentially arranged annular rows of rotor blades, the stationary blades and the rotor blades being arranged in an alternate manner, a row of rotor blades following a row of stationary blades in the direction of flow of the process gas. Each pair of stationary blades row and rotor blades row forms a stage of the expander.

[0004] In all operating conditions of the expander, there must be a clearance between the stationary components and the rotary components along the flow path, sufficient to prevent a rubbing contact therebetween. The mutual distance must however be as small as possible when the expander operates in steady state conditions, to reduce gas leakages from the flow path, as the leakages reduce the efficiency of the expander.

[0005] In some cases, for instance in oxy-fuel expanders, and expanders operating with supercritical carbon dioxide flow, the rotor may have a higher heat transfer coefficient compared to the stator parts. This may cause accidental rubbing contact between stationary and rotating components during start-up of the expander, since the thermal expansion of the rotor is faster than the thermal expansion of the stator. To prevent rubbing between stator components and rotor components during transient conditions at start-up, a larger clearance between these components must be considered, which negatively affects the efficiency of the expander at steady state operating conditions.

[0006] An expander adapted to avoid or alleviate the above drawbacks of the expanders of the current art would be welcomed.

SUMMARY

[0007] According to one aspect, disclosed herein is an expander comprising a casing including an outer casing and an inner casing. The inner casing is arranged in the outer casing and houses at least one set of annularly arranged stationary blades forming an annular row of stationary blades. The expander further includes at least a first annular fluid chamber between the inner casing and the outer casing. The inner casing has a peripheral wall having an outer surface facing the annular fluid chamber and an inner surface. A rotor is housed at least partly in the inner casing and is provided with at least one set of annularly arranged rotor blades, and preferably a plurality of sets of annularly arranged rotor blades, for instance between four and fifteen sets of rotor blades, each arranged downstream of a respective set of annularly arranged stationary blades. The expander further includes a pre-heating arrangement, adapted to preheat the inner casing and cause thermal expansion thereof at expander start-up. The preheating increases the gap or clearance between stationary components and rotary components during start-up transients, preventing accidental contact therebetween. Once a steady state operating condition is achieved, the initially enlarged gap or clearance narrows again to the a steady-state clearance of minimum dimension, reducing the process gas leakages from the expansion flow path.

[0008] According to another aspect, disclosed herein is a method of operating an expander at start-up, the expander being configured as outlined above. The method comprises the step of pre-heating the inner casing at start-up of the expander and increasing a clearance between the rotor and components stationarily supported in the inner casing by thermal expansion of the inner casing. [0009] The expander can be pre-heated at start-up, such that the inner casing expands thermally and prevents rubbing of the components of the rotor against components of the stationary bundle of the expander. Once a steady state temperature condition is achieved, parts of the expander, such as the rotor or parts thereof, can be cooled to prevent overheating thereof during steady state operating conditions.

[0010] Further features and embodiments of the method and of the expander according to the present disclosure are set forth in the dependent claims and are described in the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Reference is now made briefly to the accompanying drawings, in which: Fig. l is a schematic of an oxy -fuel power circuit;

Fig.2 is a sectional view along of an expander according to the present disclosure in an embodiment;

Fig.3 is an enlargement of the expander of Fig.2.

DETAILED DESCRIPTION

[0012] In the following description reference will be specifically made to an oxyfuel expander, i.e., to an expander specifically designed for an oxy-fuel combustion cycle. However, features disclosed herein can be advantageously used in different thermodynamic cycles, such as closed supercritical or transcritical carbon dioxide cycles, for instance.

[0013] The schematic of Fig. 1 illustrates a simplified oxy-fuel cycle operating with supercritical carbon dioxide (shortly sCCh cycle), such as an Allam oxy-fuel cycle. As understood herein, a supercritical cycle is a cycle wherein the carbon dioxide is in supercritical conditions at least at the highest pressure point along the thermodynamic cycle, i.e., at the inlet of the expander rotor.

[0014] The power system 1 shown in Fig.l comprises an expander 3 that includes an expansion section 5 and a combustor 7. The combustor 7 can be an annular combustor, a can-type combustor, a can annular combustor, or the like, for instance. In currently preferred embodiments, the combustor is a can-type combustor comprising a plurality of combustor chambers arranged around the rotation axis of the expander 3, as shown in more detail in Fig.2. In some embodiments, the combustor chambers are housed in the outer casing of the expander, as will described in more detail below. In other embodiments the combustor can be combined with the casing but arranged on or around the casing. For instance, a transition piece can be provided to fluidly connect the combustor with the expansion flow path in the interior of the expander.

[0015] Reference number 7.1 in Fig.2 designates the combustion chamber of the combustor of an annular combustor, or each combustion chamber of a can-type or can annular combustor.

[0016] The combustor 7 is supplied with an oxidant flow delivered by an oxidant source. The oxidant may be oxygen (O2) or a blend of oxygen and carbon dioxide (CO2). The oxidant flow can be produced by an air separation unit 9, which features an oxidant source. The air separation unit 9 may remove nitrogen or nitrogen and carbon dioxide from ambient air to produce the required oxidant stream which is supplied through an oxidant line 11 to the combustor 7 of the expander 3. Carbon dioxide can be added to the oxygen such that an oxygen and carbon dioxide blend is delivered to the expander, wherein the blend contains a percentage of carbon dioxide sufficient to prevent damages to the piping.

[0017] Reference number 13 indicates a fuel supply line, for instance adapted to supply natural gas, such as methane, to the combustor 7. In the embodiment shown ni Fig.2, fuel is supplied to each combustor chamber 7.1. The oxidant and the fuel are supplied to the combustor 7 at high pressure, for instance around or above 50 barA, preferably at or above 100 barA, for instance at or above 150 bar, or at or above 200 barA, preferably at around or above 250 barA, or higher, for instance at or above 300 barA. In some embodiments, the pressure at the inlet of the expander is lower than 800 barA, preferably lower than 650BarA. The oxidant-fuel blend is burned in the combustor 7. Pressurized, hot combustion gas resulting from the combustion expands in the expansion section 5 of the expander 3.

[0018] The exhausted flue gas is discharged at a discharge side of the expander 3 in a discharge line 15. The flue gas in the discharge line 15 can be at around 600°C, for instance, and at a pressure which may range between 10 barA and 100 barA, for instance between 20 barA and 60 barA. [0019] The expander can be designed for a rated power value preferably higher than 50 MW, for instance in the order of 100MW o higher, for instance of 150MW or higher, e.g., 200MW or higher, or 300MW or higher. In embodiments, the rated power can be lower than 2000MW, preferably lower than 1500 MW, for instance lower than 1000 MW, or lower than 800 MW. For instance, the rated power can be comprised between 200 MW and 650 MW.

[0020] The circuit further comprises a regenerative heat exchanger 17, wherein hot flue gas flowing through a hot side 17.1 of the regenerative heat exchanger 17 is cooled in heat exchange with a flow of chilled flue gas which flows in a cold side 17.2 of the regenerative heat exchanger 17. The flue gas discharged from the hot side 17.1 of the regenerative heat exchanger 17 is further chilled in a chilling heat exchanger 19 to a temperature which causes condensation of steam contained in the exhausted flue gas. Condensing water is removed from the exhausted flue gas in a water/gas separator 21.

[0021] The de-hydrated exhausted and chilled flue gas, consisting mainly or exclusively of carbon dioxide, is compressed in a flue gas compressor 23 to the pressure at the inlet side of the expander 3. While in the schematic of Fig.1 the flue gas compressor 23 is pictorially represented as a single compressor, in some embodiments a multiple compressor can be used. For instance, the flue gas compressor 23 can be a multi-stage compressor or a compressor train and can include one or more pumps in series.

[0022] The compressed flue gas delivered by the flue gas compressor 23 is partly removed from the cycle through a discharge line 25. The remaining compressed flue gas is delivered through the cold side 17.2 of the regenerative heat exchanger 17 and is heated by heat exchange with the hot flue gas flowing through the hot side 17.1 of the regenerative heat exchanger 17 and recycled to the expander 3 through a recycle line 25. The flue gas recycled through recycle line 25 is mixed with the combustion gas generated in the combustor 7, or with the oxidant stream from oxidant line 11.

[0023] A side stream of chilled flue gas is delivered through a cooling line 27, which bypasses the regenerative heat exchanger 17, towards components of the expander 3 which require cooling. A further side stream of chilled, dehydrated flue gas can be delivered through a line 28 to the air separator 9 and/or to the oxidant line 11 to blend carbon dioxide with oxygen from the air separator 9, such that a blend of oxygen and carbon dioxide is fed to the combustor 7. In embodiments, the blend of oxygen and carbon dioxide can include 20% by volume of oxygen and 80% by volume of carbon dioxide.

[0024] The expander 3 may include an output shaft 31, whereon mechanical power generated by the expansion of the combustion gas in the expansion section 5 of the expander 3 is available for mechanical drive or power generation purposes. In the exemplary embodiment of Fig. 1 the output shaft 31 is drivingly coupled to an electric generator 33, which is in turn electrically coupled to an electric power distribution grid 35. In Figs 1 and 2 the output shaft is shown at the aft side of the expander 3. In other embodiments, not shown, the output shaft 31 can be arranged at the forward side of the expander. In yet further embodiments, not shown, two output shafts can be provided, one at the forward side and one at the aft 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 3. Therefore, “forward” indicates a position on the side of the combustor 7 and “aft” indicates a position on the side opposite the combustor 7, i.e., the discharge side of the expander 3.

[0026] Fig. 2 illustrates a sectional view of the expander 3 in one embodiment. The expander 3 can comprise an outer casing 41, which can house the combustor 7. In some embodiments the outer casing 41 includes a high-pressure casing 41.1, in the form of a barrel, and a low-pressure exhaust casing 41.2. The high-pressure casing 41.1 can be monolithic, i.e., can consist of a single piece, for instance manufactured by forging, machining, casting, or combinations thereof. In some embodiments, the high-pressure casing 41.1 can be manufactured by combining annular components to one another, e.g. by welding. “Annular” as used herein means that the component extends around the axis of the expander without separations, as a single block or piece, to provide sufficient resistance to the high pressure values inside the high-pressure casing 41.1

[0027] The low-pressure exhaust casing 41.2 can be positioned on the discharge side, i.e., the aft side, of the expander 3, i.e., opposite the combustor 7. The low-pressure exhaust casing 41.2 can be monolithic as well, i.e. can consist of a single piece. In other embodiments, the low-pressure exhaust casing 41.2 can consist of a plurality of components connected to one another. For instance, the low-pressure exhaust casing 41.2 can be split along a plane containing the rotation axis of the rotor. Coupling of the components forming the low-pressure exhaust casing 41.2 can be by welding, or can be a reversible connection by screws or bolts.

[0028] The high-pressure casing 41.1 and the low-pressure exhaust casing 41.2 can be connected to one another along a plane P which is orthogonal to a rotation axis A- A of a rotor 43 supported for rotation in the outer casing 41. The outer casing 41 is therefore a so-called “vertically split” casing.

[0029] In some embodiments, the low-pressure exhaust casing 41.2 forms a discharge volute or discharge plenum 41.3, through which exhausted flue gas is discharged from the expander 3.

[0030] Reference numbers 45, 47 indicate bearing arrangements, which rotatingly support the rotor 43. For instance, the bearing arrangement 45 on the aft side, i.e., opposite the combustor 7, may include an axial or thrust bearing in combination with a radial bearing, or a bearing having an axial-radial bearing capability. The bearing arrangement 47 on the combustor side, i.e., on the forward side, may include a radial bearing. A reversed arrangement is also possible, with a bearing having axial load capability arranged on the combustor side. The bearing arrangements 45, 47 can be arranged in bearing housings, not shown in detail. The rotor 43 can be drivingly coupled to a shaft 31 of a load, such as a compressor or an electric generator, thorough a joint 49.

[0031] The rotor 43 is surrounded by an inner casing 51, housed n the outer casing 41. The inner casing can be formed by plurality of sections arranged in sequence in a forward-to-aft direction. The inner casing 51 can be horizontally split, i.e., can include two portions which are coupled to one another along a plane containing the rotation axis of the rotor 43. If the inner casing 51 comprises a plurality of axially aligned sections, each section or some of them can be horizontally split.

[0032] One or more annular fluid chambers 42 are formed between the inner casing 51 and the outer casing 41. Specifically, in the exemplar embodiment shown in Fig.2 the annular chamber 42 includes two sequentially arranged annular fluid chambers 42.1 and 42.2 separated by a septum 44, which mechanically connects the inner casing 51 to the outer casing 41. The fluid pressure inside the two annular fluid chambers 42.1 and 42.2 can be different. For instance, the forward annular fluid chamber 42.1 can be at a higher pressure than the aft annular fluid chamber 42.2. In use, at steady state conditions, chilling or cooling fluid can be fed to the aft fluid chamber 42.2 and the forward fluid chamber 42.1. For instance, the chilling or cooling fluid can include dehydrated flue gas from cooling line 27, which may consist mainly or exclusively of carbon dioxide.

[0033] The inner casing 51 includes a wall 51.3 (see Fig. 3), having an outer surface

51.1 facing the annular fluid chamber 42, and an inner surface 51.2 facing the rotor 43.

[0034] The pressure drop across the expander 3 can be around 200 bar or higher. To expand the combustion gas generated in the combustor 7, a high number of expansion stages is preferred. In the exemplary embodiment of Figs.2 to 7, the expander 2 includes eight stages, each configured as an axial expansion stage. In other embodiments, a different number of expansion stages can be foreseen, preferably equal to or higher than four, more preferably equal to or higher than five. In some embodiments, the number of expansion stages can be higher than eight, for instance nine, ten, eleven, or more, and preferably less than fifteen.

[0035] Each expansion stage includes an annular row of stationary blades or stationary vanes 53 that are stationarily arranged in the inner casing 51. Each expansion stage further includes a respective annular row of rotor blades 55, arranged downstream the respective annular row of stationary blades 53 along a flow path which extends from the combustor 7 through the expansion section 5 to the discharge volute 41.3 in a for- ward-to-aft direction.

[0036] The rotor blades 55 form part of the rotor 43, i.e., they are connected thereto or manufactured as a single block therewith, for co-rotation with the rotor shaft. In embodiments, each annular row of rotor blades 55 is connected to a respective rotor disk, not shown in detail, or monolithically formed therewith as a single body. The structure of the rotor and of the rotor disks is not relevant and is not shown in detail.

[0037] In some embodiments, sequentially arranged annular rows of rotor blades are separated from one another by a respective seal runner 56 (see Fig.3). A respective annular row of stationary blades 53 is arranged around the seal runner 56 radially outwardly thereof.

[0038] In some embodiments, the rotor 43 further comprises a forward shaft portion 65 and an aft shaft portion 67. In some embodiments, the combustor 7 extends around the forward shaft portion 65. For instance, the combustor 7 comprises a plurality of combustion chambers 7.1, each of which can be housed, entirely or in part, in a seat formed in the high-pressure casing 41.1 and the combustion chambers 7.1 can be arranged around the rotation axis A-A.

[0039] In some embodiments, the discharge plenum 41.3 extends around the aft shaft portion 67.

[0040] Fig.3 illustrates an enlarged detail of one stage of the expander 3 of Fig.2. More specifically, Fig.3 shows the flow passage through one expander stage. The stage shown in Fig.3 includes an annular row of stationary blades 53 followed by an annular row of rotor blades 55. F indicates the flow of combustion gas expanding through the expander stage.

[0041] In some embodiments, each rotor blade 55 has a shaft or foot 55.1, wherewith the blade is mechanically connected to the rotor, and a tip 55.2 facing radially outwardly. The tip 55.2 can be provided with fins or knives 55.3 arranged at a distance from a radially inwardly facing surface 71.1 of a stationary shroud 71, which is connected to the inner casing 51, or forms part thereof. The radially inwardly facing surface 71.1 of the stationary shroud 71 can be formed by an annular layer of abradable material 71.2. In some embodiments, the layer of abradable material can be omitted.

[0042] The distance between the knives 55.3 and the surface 71.1, i.e., the clearance between the rotating blades 55 and the stationary shroud 71 shall be as small as possible. This distance or gap is indicated herein as clearance C. In operation, the clearance C shall be small to prevent gas from flowing therethrough, or at least to reduce the flowrate of gas therethrough, since gas expanding through the clearance C will not contribute to the generation of mechanical power. At the same time, the clearance C shall be large enough to prevent mutual contact and accidental rubbing between the stationary shroud 71 and the rotating blades 55 in all operating conditions of the expander 3, i.e. both at steady state operating conditions, as well as during transients, such as at start-up. [0043] In the embodiment of Fig. 3, the wall 51.3 of the inner casing comprises a plurality of cooling ducts 73, 75, each of which has a respective duct inlet 73.1, 75.1 fluidly coupled with the annular fluid chamber 42 and a respective duct outlet 73.2, 75.2.

[0044] The cooling ducts 73 are adapted to establish a fluid connection between the annular fluid chamber 42 and an annular cooling plenum 77 positioned radially outwardly of the stationary shroud 71, opposite the abradable material 71.2. The cooling ducts 75 are arranged such as to establish a fluid connection between the annular fluid chamber 42 and an annular cooling plenum 79 arranged radially outwardly of the stationary blades 53. In the sectional view of Fig.3 only one cooling duct 73 and only one cooling duct 75 are visible, but it shall be understood that a plurality of cooling ducts 73 can be provided, for instance one or more for each annular cooling plenum 77 and a plurality of cooling ducts 75 can be provided, for instance one or more for each annular cooling plenum 79.

[0045] The arrangement disclosed above provides a cooling circuit which can be fed with chilled flue gas through the cooling line 27 (Fig.1).

[0046] During steady state operation of the expander 3, cooling carbon dioxide (or any other cooling fluid) is delivered to the annular fluid chamber 42, i.e., to each annular fluid chambers 42.1, 42.2, and flows inside the inner casing 51 to cool the stationary blades 53 and the stationary shroud 71. The clearance C is designed such that in steady state operating conditions, the temperatures of the rotor and of the stator are maintained at values such that the clearance C is minimized but is sufficient to prevent rubbing contact between the rotor blades 55 and the stationary shroud 71 in each stage of the expander 3.

[0047] In some embodiments, each annular row of stationary blades 53 can be provided with an inner stationary shroud 72, facing the respective seal runner 56. The inner stationary shroud 72 comprises a surface 72.1 facing radially inwardly, i.e. facing the respective seal runner 56. The surface 72.1 can be the outer surface of a layer of abradable material 72.2. In some embodiments, the abradable material 72.2 can be omitted. The radially inwardly facing surface 72.1 of the inner stationary shroud 72 is at a distance C2 (clearance C2) from fins or blades 56.1 of the respective seal runner 56. In quite the same way as clearance C, also clearance C2 shall be as small as possible, to reduce the flowrate of expanding gas escaping therethrough rather than flowing through the flow path (F) between stationary vanes 53 and rotor vanes 55.

[0048] To cool the stationary blades 53 and control the dimension of the clearance C2, cooling fluid is delivered to the annular cooling plenum 79.

[0049] At start-up, a risk of contact between the rotary components (seal runners 56 and rotor blades 55) and the stationary components (stationary shroud 71 and inner stationary shroud 72) may occur due to the temperature gradient, the geometry of the mechanical components, as well as to the heat transfer coefficients and the thermal expansion coefficients of the rotor and of the stationary components, respectively. This is particularly because the rotor heats and thermally expands faster than the casing.

[0050] To avoid the need to maintain larger clearances at steady state operating conditions and to still avoid mechanical rubbing contact between the shrouds 71 and the tips 55.2 of the rotor blades 55, and between the shroud 72 and the seal runner 56 during transient operating conditions, and specifically at start-up, the expander 3 includes a pre-heating arrangement. The pre-heating arrangement is adapted to pre-heat the inner casing 51 of the expander 3 at expander start-up, i.e., when the expander is at or around ambient temperature, for instance. The mechanical coupling between the inner casing 51 and the outer casing 41 can be such that a radial displacement of the inner casing 51 with respect to the outer casing 41 is allowed, to allow thermal expansion of the inner casing 51 when the outer casing 41 is still cold, e.g., at expander startup.

[0051] Pre-heating of the inner casing 51 causes a thermal expansion thereof in radial direction. The thermal expansion of the inner casing 51 causes a displacement in a radial outward direction of the shrouds 71 and 72 and specifically of the surface 71.1 and of the surface 72.1. Pre-heating of the inner casing 51 can be performed fully before starting rotation of the rotor 43, or when the rotor 43 has started to rotate, or partly before and partly after the rotor has started to rotate. In any event, pre-heating is controlled such that the radial thermal expansion of the stationary components will prevent any contact between the rotary component of the rotor 43 and the stationary components housed in the inner casing 51. Thus, pre-heating expands the stationary components before a rubbing contact between stationary and rotary parts of the expander occurs, due to thermal expansion of the rotor.

[0052] Pre-heating of the inner casing 51 increases the clearance C between the knives 55.3 of the blade tips 55.2 and the inner surface 71.1 of the shrouds 71, and the clearance C2 between the knives or fins 56.1 of the seal runners 56 and the inner surface 72.1 of the inner shrouds 72.

[0053] The clearances increase obtained by pre-heating prevents accidental rubbing of the rotor blades 55 against the respective shrouds 71 and of the shrouds 72 against the seal runners 56 even if initial (cold) and final (hot) clearances C and C2 are very small.

[0054] Pre-heating of the inner casing 51 can be performed by feeding a pre-heating fluid in the annular fluid chamber 42, i.e., in the annular fluid chambers 42.1 and 42.2. The pre-heating fluid enters the inner casing 51 through the cooling ducts 73, 75 and heats the wall 51 of the inner casing as well as the shrouds 71, 72 of each expansion stage of the expander 3. Pre-heating causes a radial outwards thermal expansion and a temporary increase of the clearances C and C2. This increase will subsequently be compensated by an expansion of the rotor, i.e., the clearances C, C2 will be reduced due to gradual radial expansion of the rotor 43.

[0055] The expander 3 can thus start operating without a risk of rubbing contact between rotary components (rotor blades 55, seal runners 56) and stationary components (shrouds 71, 72), as thermal radial expansion of the casing 51 is anticipated with respect to the thermal radial expansion of the rotor 43.

[0056] After start-up, the compressed hot combustion gas flowing in the flow path between stationary blades 53 and rotor blades 55 will gradually heat the rotor causing thermal expansion thereof, and further heating the inner casing causing further thermal expansion thereof. Since the inner casing 51 has been pre-heated, the clearances C, C2 will be sufficiently large to prevent rubbing contact between rotary and stationary components, even if the radial expansion of the rotor 43 is faster than the radial expansion of the inner casing 51.

[0057] Circulation of pre-heating fluid will be stopped when steady-state temperature conditions are achieved, or in any event when the radial expansions of non-rotating and rotating components has achieved such values that mutual contact thereof is avoided. At this stage, the cooling circuit, which has been used for pre-heating the inner casing 51, can be used to circulate a cooling fluid to maintain the inner casing at the requested steady state temperature. As a matter of fact, the same fluid which acts as a pre-heating fluid at start-up, can act as a cooling fluid at steady state conditions.

[0058] In Fig.2 a schematic arrangement 80 is shown, which can be used to feed selectively pre-heating and cooling fluid in the annular fluid chamber 42 (chambers 42.1, 42.2) and therefrom in the cooling ducts 73, 75. The arrangement can include a heat exchanger 81 with a hot side 81.1 and a cold side 81.2. The hot side 81.1 can be fluidly coupled with a source of heat transfer fluid 83 through a heat-transfer fluid duct 84. The heat transfer fluid, which circulates in the hot side 81.1 of the heat exchanger 81, can transfer heat to a flow of recycled carbon dioxide fed through the cooling line 27.

[0059] A valve arrangement 85 including a valve 85.1 in parallel to the cold side 81.2 of the heat exchanger 81, and a valve 85.2 in series with the cold side 81.2 of the heat exchanger 81, can direct the flow of carbon dioxide from the cooling line 27 selectively through the heat exchanger 81 (valve 85.1 closed, valve 85.2 open) or directly to the annular fluid chamber 42 (valve 85.1 open, valve 85.2 closed). In the first condition, the carbon dioxide from the cooling line 27 will be pre-heated, and used as a pre-heat fluid in the expander 3, during the transient pre-heating phase. In the second condition, the heat exchanger 81 is inoperative and carbon dioxide enters as a cooling fluid directly into the annular cooling chamber 42.

[0060] When the arrangement 80 is used, pre-heating of the inner casing 51 may require the expander 3 to start operating before pre-heating, in order to have sufficient carbon dioxide recirculation through cooling line 27. Timing may become a critical aspect in this embodiment, as pre-heating shall start before the thermal expansion of the rotor causes rubbing between stationary and rotary components.

[0061] In some embodiments, a thermal transfer fluid from an external source, for instance source 83, can be fed directly into the annular fluid chamber 42 to start pre- heating of the inner casing 51 before ignition of the combustor 7.

[0062] In other embodiments, the heating fluid can be provided by the combustor 7, through a suitable fluid coupling schematically shown at 8 in Fig.2, that is selectively opened only during the pre-heating phase. The rotor 43 can be maintained stationary, i.e. rotation thereof can be prevented, during a first time interval following combustor ignition 7, such that combustion gas can pre-heat the inner casing 51 and cause thermal expansion thereof, before the rotor starts rotating. Rotation of the rotor 43 can start for instance when a suitable temperature of the inner casing 51 has been achieved and the clearances C and C2 are sufficiently large to ensure smooth rotation of the rotor 43 without rubbing while the rotor expands at a faster rate than the stationary components of the expander.

[0063] 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.

[0064] For instance, as mentioned above the combustor 7 can be arranged outside the expander 3 and can be connected thereto e.g. by means of a transition piece. In other embodiments, the expander 3 can be used in a close loop cycle, wherein the process gas may heated, e.g., by heat exchange with a heat transfer fluid in a heat exchanger.

Claims

1. An expander comprising: a casing comprising an outer casing and an inner casing; wherein the inner casing is arranged in the outer casing; and wherein the inner casing houses at least one set of annularly arranged stationary blades; at least a first annular fluid chamber between the inner casing and the outer casing; and wherein the inner casing has a peripheral wall having an outer surface facing the annular fluid chamber and an inner surface; a rotor at least partly housed in the inner casing for rotation therein; wherein the rotor is provided with at least one set of annularly arranged rotor blades, downstream of the at least one set of annularly arranged stationary blades; a pre-heating arrangement, adapted to preheat the inner casing and cause thermal expansion thereof at expander start-up; wherein the pre-heating arrangement comprises a circulation system adapted to circulate a pre-heating fluid in heat exchange with the inner casing;

2. a cooling system adapted to cool the at least one set of annularly arranged stationary blades during operation of the expander; wherein the cooling system comprises cooling ducts extending through the peripheral wall of the inner casing and are fluidly coupled to the at least one annular fluid chamber and are adapted to feed a fluid from the at least one annular fluid chamber toward the inner surface of the peripheral wall of the inner casing; wherein the pre-heating arrangement is adapted to feed a pre-heating fluid in the at least one annular fluid chamber and through the cooling ducts at expander start-up, such that the inner casing is pre-heated by said heating fluid. The expander of claim 2, comprising at least one stationary shroud housed in the inner casing and surrounding the at least one set of annularly arranged rotor blades; and wherein the cooling ducts are adapted to feed said heating fluid from the annular fluid chamber into an annular plenum between the inner surface of the peripheral wall of the inner casing and the at least one stationary shroud, at expander start-up

3. The expander of claim 1 or 2, comprising a further annular plenum between the inner surface of the peripheral wall of the inner casing and the at least one set of annularly arranged stationary blades; wherein the cooling ducts are adapted to feed said heating fluid from the annular fluid chamber into said further annular plenum at expander start-up.

4. The expander of any one of the preceding claims, wherein the stationary blades comprise respective inner platforms and outer platforms; wherein an inner sealing shroud facing a seal runner of the rotor is connected to the inner platforms; and wherein the cooling ducts are adapted to feed a fluid from the annular fluid chamber towards the outer platforms of the at least one set of annularly arranged stationary blades.

5. The expander of any one of the preceding claims, wherein the expander comprises at least one combustor housed in the outer casing.

6. The expander of claim 5, wherein the pre-heating arrangement comprises a fluid connection between the at least one combustor and the at least one annular fluid chamber; and wherein the fluid connection is adapted to be selectively open at expander start-up to feed hot combustion gas from the combustor to the annular fluid chamber.

7. The expander of any one of the preceding claims, wherein the heating arrangement comprises a heat exchanger with a hot side and a cold side; wherein the hot side is fluidly coupled to a heat source through a heat transfer fluid duct; and wherein the cold side is fluidly coupled to a pre-heating circuit adapted to feed heating fluid to the inner casing.

8. The expander of claim 7, wherein the cold side of the heat exchanger is arranged in parallel to a cooling fluid feed duct, adapted to feed a cooling fluid to the inner casing; and wherein a valve system is adapted to selectively divert cooling fluid fed by the cooling fluid feed duct through the cold side of the heat exchanger, such that the cooling fluid is heated by heat exchange with the hot side of the heat exchanger before being delivered to the inner casing.

9. The expander of any one of the preceding claims, wherein the expander is an oxy -fuel expander.

10. A supercritical carbon dioxide thermodynamic circuit comprising:

- an oxidant source;

- an expander with an inlet side and a discharge side, wherein the inlet side is fluidly coupled to the oxidant source;

- a flue gas recycling line, adapted to recycle flue gas from the discharge side of the expander to a combustor of the expander;

- in the flue gas recycling line, a cooler adapted to cool the flue gas from the discharge side of the expander and condense water contained in the flue gas;

- a regenerative heat exchanger, wherein flue gas from the expander flows in heat exchange with chilled flue gas from the cooler; wherein the expander is an expander according to any one of the preceding claims.

11. A method of operating an expander at start-up, the expander comprising: a casing comprising an outer casing and an inner casing; wherein the inner casing is arranged in the outer casing; and wherein the inner casing houses at least one set of annularly arranged stationary blades; at least a first annular fluid chamber between the inner casing and the outer casing; and wherein the inner casing has a peripheral wall having an outer surface facing the annular fluid chamber and an inner surface; a rotor at least partly housed in the inner casing for rotation therein; wherein the rotor is provided with at least one set of annularly arranged rotor blades, downstream of the at least one set of annularly arranged stationary blades; a pre-heating arrangement, adapted to preheat the inner casing and cause thermal expansion thereof at expander start-up; wherein the pre-heating arrangement comprises a circulation system adapted to circulate a pre-heating fluid in heat exchange with the inner casing; a cooling system adapted to cool the at least one set of annularly arranged stationary blades during operation of the expander; wherein the cooling system comprises cooling ducts extending through the peripheral wall of the inner casing and are fluidly coupled to the at least one annular fluid chamber and are adapted to feed a fluid from the at least one annular fluid chamber toward the inner surface of the peripheral wall of the inner casing; wherein the method comprises the step of feeding a pre-heating fluid in the at least one annular fluid chamber and through the cooling ducts towards the inner surface of the peripheral wall of the inner casing, at expander start-up, such that the inner casing is pre-heated by said heating fluid to increase a clearance between the rotor and components stationarily supported in the inner casing by thermal expansion of the inner casing.

12. The method of claim 11, wherein the step of pre-heating the inner casing is performed at least in part while the rotor of the expander is maintained stationary.

13. The method of claim 11 or 12, wherein the step of pre-heating the inner casing is performed at least in part with the rotor in a rotating condition.

14. The method of any one of claims 11 to 13, wherein the expander comprises at least one combustor housed in the outer casing; and wherein the step of pre-heating the inner casing comprises the step of feeding hot combustion gas from the combustor towards the inner casing.

15. The method of any one of claims 11 to 14, wherein the heating arrangement comprises a heat exchanger with a hot side and a cold side; wherein the hot side is fluidly coupled to a heat source through a heat-transfer fluid duct; wherein the cold side is fluidly coupled to a pre-heating circuit adapted to feed heating fluid to the inner casing; and wherein the step of pre-heating the inner casing comprises the steps of: feeding a heating fluid through the cold side of the heat exchanger; heating the heating fluid through heat exchange with the heat transfer fluid circulating in the hot side of the heat exchanger; and feeding the heated heating fluid to the inner casing.

16. The method of claim 15, wherein the heating fluid circulating in the cold side of the heat exchanger is recycled from a discharge side of the expander.

17. The method of any one of the claims 11 to 16, wherein after a stationary temperature condition of the inner casing and of the rotor is achieved, heating of the inner casing is interrupted, and circulation of a cooling fluid in heat exchange with the inner casing is started.

18. The method of claim 17, wherein the step of circulating a cooling fluid in heat exchange with the inner casing comprises the step of removing heat from the at least one set of annularly arranged stationary blades through said cooling fluid.