PARTICLE HEAT EXCHANGER FOR A SOLAR TOWER POWER PLANT

Field of the Invention [0001] The present invention relates to installations in the field of heat exchangers using the concept of falling particles as heat transfer medium. In particular the invention is associated to the area of Concentrated Solar Power (CSP) plants in which a heat transfer medium such as molten salt is generally used to convey heat from a solar receiver to a steam generator. Background and Prior Art

[0002] Currently, molten salt is used as heat transfer and storage medium in nearly all Solar Tower CSP plants due to its good thermal properties. For that reason, the applicant has commercialized a Molten Salt Solar Receiver (MSSR) designed to transmit heat from the incident solar power to the molten salt. The heat contained in the molten salt is exploited to generate superheated steam that will be used to produce electricity in a turbine.

[0003] However, using molten salt as heat transfer and storage medium also presents some disadvantages. Indeed, under a certain temperature (240°C for KN03-NaN03 molten salt), molten salt freezes and can further create some damages to the equipment such as pipe explosion. To avoid this drawback, heat tracing is used to maintain the equipment temperature always above the freezing point. Another limitation of the molten salt is that its operating temperature range is limited: the salt cannot exceed a certain level of temperature otherwise it can deteriorate. Since this operating temperature range is quite limited (usually from 290°C to 565°C for KNC -NaNOs), storage density of molten salt is relatively small.

[0004] Furthermore, it is known that ceramic particles are not subject to freezing since they are solid at any temperature and their operating temperature range is much broader than the one of molten salt (from 300°C to 1000°C for bauxite particles). Therefore their storage density is about 2.5 times higher than molten salt storage density. Moreover, due to their higher temperature operating level, ceramic particles can be coupled to high-efficiency thermodynamic cycles, such as supercritical CO2 cycles.

[0005] For these reasons, the use of ceramic particles has been investigated and these are considered good candidates for heat transfer/storage media for the next generation of Solar Tower CSP plants.

[0006] To anticipate this next generation of particles Solar Tower

CSP plants, the applicant has decided to design a particle steam generator, more particularly in the context of a European FI2020 Project called “HiFlex”, in which the applicant is partnering. This project consists in the development, the building and the demonstration of a complete commercial Solar Tower CSP plant using particles as heat transfer/storage medium.

[0007] The applicant is also involved in the “CompassC02” project, which consists, among others, in the demonstration of a particles-sCC heat exchanger.

[0008] Otherwise, the applicant already developed a molten salt steam generator comprising a shell-and-tube heat exchanger with helicoidal baffles. As solid particles in a fixed bed are intended to be used in the present invention, the concept will be very different.

[0009] So, the following points have to be taken into account:

- particles are solid: they cannot be set in motion as easily as a fluid (it should be reminded that the particles are not moving as a fluidized bed, and have a very low speed, e.g. 2 mm/s, so that the bed is considered stationary). Moreover, surfaces of the heat exchanger have to resist to the abrasive property of the solid particles, even if the velocity thereof is quite low ;

- particles at the inlet of the heat exchanger are typically at about 1000°C: the materials of the steam generator have to resist to these high temperatures ;

- particles have a diameter relatively high (typically about 1 mm): spacing inside the heat exchanger has to be designed such that there is no particles clogging between the tubes ;

- particles will be submitted to a spatial thermal gradient in the heat exchanger of about 700°C: tubesheets or other structures supporting the tubes will have to resist to such an important gradient ;

- particles heat transfer coefficient is not so good compared with usual heat transfer fluids: the heat exchange area shall be increased and the design must be such that heat transfer between particles and steam is optimal.

[0010] Document WO 2007/128104 A1 discloses a method and an apparatus for indirect-heat thermal processing of material, such as a dryer or evaporator for treatment of particulate material. A dryer for drying particulate material comprises a plurality of heat transfer plates arranged in spaced relationship for the flow of the material to be dried there between and downward. Each heat transfer plate is provided with an inlet and an outlet for the flow of the heating fluid through the plates, so that particles are heated thanks to the heating fluid passing through the plates. A purge fluid delivery system provides a flow of purge fluid, such as air, gas or steam between the plates in a direction across the direction of flow of the material to be dried, to remove evaporated volatiles generated by the indirect heating of the material.

[0011] Document WO 2013/163752 A1 discloses a heat exchanger including a housing with an inlet for receiving bulk solids, and an outlet for discharging the bulk solids. A plurality of spaced apart, substantially parallel heat transfer plate assemblies are disposed in the housing between the inlet and the outlet for cooling the bulk solids that flow downwards under gravity from the inlet, between the adjacent heat transfer plate assemblies, to the outlet. In order to protect the parallel plates, a pipe extends along a top end of each heat transfer plate. Cooling fluid that is circulating in these pipes protects the plates from high- temperature particles.

[0012] Document WO 2017/024374 A1 discloses an indirect-heat thermal processor for processing bulk solids including a housing with an inlet for receiving the bulk solids and an outlet for discharging the bulk solids and a plurality of heat transfer plate assemblies disposed between the inlet and the outlet and arranged in spaced relationship for the flow of the bulk solids that flow downward from the inlet, between the heat transfer plate assemblies, to the outlet. The heat transfer plate assemblies include temperature detectors, heating elements and heat spreaders. These elements enable control and monitoring of the temperature of the solid bulk. There is no secondary heating/cooling fluid.

[0013] Document EP 2 352 962 B1 discloses a heat exchanger air/sand. Particles are flowing in a wall having a single-piece lattice body comprising straight channels. Particles exchange their heat with air that circulates across the particles channels. The sidewalls of the different particle channels are constituted of porous ceramic or metals. Air is heated/cooled by convective transfer with the particles.

[0014] Document WO 2021/030875 A1 discloses a vertical tube bundle heat exchanger with a particle moving bed. By moving downward by gravity the particles exchange heat with vertical tubes located in the housing of the heat exchanger. In these tubes is circulating a heating/cooling fluid. With this vertical tube bundle arrangement, there are no void or stagnant zones leading to poor heat transfer.

[0015] Document US 2016/076813 A1 discloses a heat exchanger including a housing with an inlet for receiving bulk solids, and an outlet for discharging the bulk solids, a plurality of spaced apart, substantially parallel heat transfer plate assemblies disposed within the housing, between the inlet and the outlet for heating the bulk solids that flow from the inlet, through spaces between the heat transfer plate assemblies, and at least two gas handling zones including a first gas handling zone and a second gas handling zone spaced from the first gas handling zone, the two gas handling zones disposed between the inlet and the outlet for entry of a pulsed flow of air into the housing and around the bulk solids and for exit of the pulsed flow of air from the housing.

[0016] Document US 2012/196239 A1 discloses a device for cooling the solid matter from a coal gasification. The device includes a container with a feed part, a cooling part and a venting part. Lines arranged transverse to the flow direction are located inside of the cooling part that are grouped in two kinds, the one carrying liquid and the other carrying gas. The liquid carrying lines are closed in the interior of the cooling part and are provided for the heat exchange. The gas carrying lines are gas permeable into the interior of the cooling part in such a way that solid matter comprising primarily cooled slag, ash and flue dust is cooled and the remaining gas present in and between the solid matter particles is exchanged. A method for cooling down the solid matter and for removing the remaining gas from the particles is also disclosed.

Aims of the Invention

[0017] The present invention aims to implement a new design for a compact, low-cost and highly efficient particle heat exchanger, whenever used with water steam, supercritical CO2 or other heat exchange media, in order to commercialize it in large-scale CSP power plants.

[0018] A particular goal of the invention is to provide an installation designed to serve high-efficiency power cycles that can withstand temperatures that can approach about 1000°C, i.e. much higher than what can be achieved using heat exchange fluids such as oil or molten salt and ordinary metals or alloys for tubing.

[0019] Another goal pursued by the invention is to operate the particle heat exchanger at least during 1000 h operation without breaking. However, in this design, an assumption of 10000 h lifetime will be made to be closer to the lifetime requested by large- scale power plants.

Summary of the Invention

[0020] A first aspect of the present invention relates to a heat exchanger comprising a casing, said casing including a top inlet part for receiving hot solid particles and a bottom outlet part for discharging the solid particles after cooling ; and a plurality of spaced apart adjacent heat transfer tubes disposed in the casing between the top inlet part and the bottom outlet part, assembled in at least one tube bundle and intended for the circulation of a coolant fluid, for cooling the solid particles that flow downwards by gravity from the inlet, between the spaced apart adjacent heat transfer tubes, to the outlet ; wherein the spaced apart adjacent heat transfer tubes are arranged in rows substantially transverse to the flow direction of the solid particles, wherein the tube bundle is supported in the casing by at least two tubesheets disposed vertically and laterally near each of both ends of the tubes and wherein each tubesheet is composed of independent plates corresponding to the individual tube rows, adjacent plates being stacked on top of each other thanks to insulated sliding pieces.

[0021] According to preferred embodiments, the heat exchanger further comprises at least one of the following characteristics or a suitable combination thereof :

- each substantially transverse tube is connected at each of its ends to another substantially transverse tube, either in a same horizontal row or between two successive horizontal rows, thanks to a 180° pipe elbow ;

- dead chambers are at least partly separating the tubes and tubesheets (7) from the casing, so that in use no particles are present in the dead chambers ;

- the heat exchanger further contains an insulation located on the internal side of the casing and protecting the casing from the particles abrasion ;

- the insulation comprises a layer of refractory concrete in series with a microporous superinsulant, said concrete layer being disposed on a metallic mesh maintained with clips attached to pins welded on the casing ;

- the tubes are respectively connected to an inlet header located in the bottom of the casing and to an outlet header located in the top of the casing, so that the coolant fluid is flowing countercurrent in the heat exchanger ; - the connections between the substantially transverse tubes comprising a 180° pipe elbow are designed so as to allow horizontal spacing between the tubes of a same row as low as 9mm without exceeding a maximum allowable bending radius ;

- said horizontal spacing between the tubes is at least 2.5 times, preferably 10 times, the diameter of the particles ;

- the top inlet part has a rectangular cross-section and a height suitable to uniformly distribute the particles in the heat exchanger and the bottom outlet part is an outlet conical part for assuring suitable mass flow ;

- first thermocouples are disposed at the inlet and the outlet of the heat exchanger, respectively before and after the tube bundle, and second thermocouples are disposed at the tube bends of the last tube row before the outlet, said first thermocouples being also disposed in corresponding sliding drawers ;

- at least one slope is arranged at the bottom of the dead chambers so as to drain particles in case of sealing issue ;

- the connecting end of the tubesheet plates with the insulated sliding piece is of female design, while the connecting end of the insulated sliding piece is of male design, so as to limit thermal gradients ;

- the casing is provided with one or more rupture disks so as to evacuate toward the outside of the heat exchanger possible overpressure resulting from high-pressure fluid penetration into the casing due to tube break, said rupture disk being preferably installed with a torque resistance holder and made of a high-temperature resistant material.

[0022] A second aspect of the invention relates to a solar tower CST plant comprising :

- a solar tower with an energy direct absorption receiver located atop the solar tower ;

- a heliostat field made of mirrors reflecting and concentrating solar energy onto the direct absorption receiver ; - a closed circuit for conveying heat transfer particles from the direct absorption receiver to a particle heat exchanger as described above, and back to the direct absorption receiver, said circuit also comprising a particle hot storage tank, a particle cold storage tank and a particle transport system ; in which said heat exchanger is used to produce superheated steam in a one- through steam generator circuit connected to a turbine intended to generate electricity ; so that, successively, hot particles stored in the hot storage tank at a temperature of 950°C to 1000°C are directed to the particle heat exchanger inlet ; a fixed bed of particles goes downward very slowly in the heat exchanger under the action of gravity, the particles transferring their heat to the tube bundle of the particle heat exchanger, in which tube bundle water, as a coolant fluid, is flowing, superheated steam being then generated in the heat exchanger/steam generator circuit connected to the steam turbine, cooled and further directed to a condenser and back to the particle heat exchanger in liquid state, said heat exchanger being countercurrent with cold coolant fluid arriving in an inlet header located at the bottom of the heat exchanger and leaving, after heating, through an outlet header located at the top thereof, cold particles at the exit of the heat exchanger being directed to the cold storage tank and further to the direct absorption receiver thanks to the particle transport system, preferably a worm screw.

[0023] Alternately, in the solar tower CST plant, the particle heat exchanger is replaced by a heat exchanger in which the tube bundle contains, instead of water, supercritical CO2 operated according to a recompression Brayton cycle.

Brief Description of the Drawings

[0024] FIG. 1 schematically represents a perspective view of a particle heat exchanger embodiment according to the present invention, including the casing panels. [0025] FIG. 2 schematically represents a cross-sectional view of the particle heat exchanger represented in FIG. 1.

[0026] FIG. 3 shows the piping and instrumentation diagram (P&ID) for a heat exchanger embodiment according to the present invention. [0027] FIG. 4 schematically represents a general view for specific tube elbows of the tubes in the heat exchanger according to the present invention (A), as well as a detailed view thereof (B).

[0028] FIG. 5 schematically represents an example of solar tower power plant in which the particle heat exchanger allows to generate superheated steam for feeding a steam turbine.

[0029] FIG. 6 schematically represents an embodiment of the tubesheet independent plates in the heat exchanger of the present invention.

[0030] FIG. 7A and FIG. 7B schematically represent an embodiment of the specific design of the sliding parts for attaching the tubesheets, according to the present invention.

[0031] FIG. 8 schematically represents the structure of the insulation of the casing from the hot abrasive particles in the heat exchanger according to the present invention.

[0032] FIG. 9 shows a front view and a cross-sectional view of the above-mentioned particle heat exchanger, where rupture disks protecting the shell have been represented.

Description of Preferred Embodiments of the Invention

[0033] A general structural arrangement of the particle heat exchanger according to the present invention is shown on FIG. 1, FIG. 2, FIG. 3 and FIG. 4. The particle heat exchanger 1 is composed, fully or partly, of the following parts:

- a tube bundle 2 in which a coolant fluid is circulated and heated. The tubes 3 in the bundle 2 are arranged in successive rows, i.e. horizontally, and are in staggered configuration. In order to increase coolant fluid speed, the tubes 3 can be multi-passes. The spacing 4 between the tubes 3 is very low in order to increase the compactness and the heat transfer between the particles and the tubes ;

- tubes 3 preferably made of nickel superalloy SB-444 N06625 Grade 2 ;

- a casing 5 inside which hot particles are flowing. This casing 5, preferably made of carbon steel, will be composed of removable screwed panels 50. Some stiffeners 51 will be disposed on the panels 50 ;

- insulation layers 6 to protect the casing 5 from the hot abrasive particles and to maintain it at the temperature of 60°C for example. The insulant will be composed of alumina refractory concrete in series with microporous superinsulant (see below). The concrete layer will be disposed on metallic meshes that will be maintained fixed thank to clips attached to pins welded to the casing (see below) ;

- tubesheets 7 to support the tube bundle 2 disposed on each lateral side along the heat exchanger longitudinal direction. Preferably such tubesheets 7 will be made of heat resistant stainless steel such as Avesta ^® SA 240 S30815 and specifically of SB-435 N06230 for the tubesheets exposed to the hottest particles. The tubesheets 7 will be composed of independent plates (one for each row) that will be deposited on the top of each other (not shown). These plates will be maintained thank to sliding pieces that are insulated and attached to the shell (not shown) ;

- dead chambers 8 between the tubesheets 7 and the casing 5 of the heat exchanger. These dead chambers 8 are empty of particles ;

- headers, among which a first one 10 is located at the inlet of the coolant fluid in the heat exchanger 1 and a second one 11 is located at the outlet of the coolant fluid. The tubes 3 will be connected to the headers 10, 11 and the coolant fluid’s flow will be evenly distributed in the tubes 3 ;

- elbows 21 made by tube bendings to connect the tubes 3, in a same horizontal row or between two successive horizontal rows. To make the horizontal spacing possible, a special design of elbows between tubes of a same row has to be adopted ; - an inlet rectangular part 12, high enough to distribute the particles uniformly in the heat exchanger 1 at a speed of 2-3 mm/s ;

- an outlet conical part 13 designed such that the particle mass flow at the outlet of the heat exchanger is acceptable (mass flow instead of a funnel flow) ;

- thermocouples 14 at the inlet and at the outlet of the heat exchanger at particle side. These thermocouples will be disposed before and after the tube bundle and will be enclosed in a sliding drawer 15. Thanks to this removable part, in case of issue with a thermocouple 14, the latter could be easily replaced. Thermocouples 16 will also be disposed at the bends of tubes 2 of the last row (before leaving the heat exchanger) in order to capture their metal temperature ;

- a structure with beams 17 to withstand the whole heat exchanger weight ;

- slopes 18 foreseen at the bottom of the dead chambers 8 to drain particles in case of sealing issue in the heat exchanger. These slopes 18 are connected to the outlet part 13 of the heat exchanger 1 .

[0034] Functionally, the operation of the particle heat exchanger according to the invention in the example of a particle solar tower power plant 30 is shown on FIG. 5 and discussed here below. Solar energy is reflected by the mirrors of the heliostat field 31 and concentrated onto a direct absorption particle receiver 32 located on top of a solar tower 33. Hot particles are stored in a hot storage tank 34 at high-temperature (950-1000°C) and connected to the receiver 32. The particles are directed to the heat exchanger 35 inlet thanks to an entry pipe and the fixed bed of particles goes downward very slowly in the casing of the heat exchanger 35 under the action of gravity. By moving down in the heat exchanger, the particles transfer their heat to the tube bundle(s) of the heat exchanger 35, in which a coolant fluid, for example water, is flowing. Steam is then generated in the heat exchanger/steam generator circuit connected to a steam turbine 36. The superheated steam at the exit of the heat exchanger 35 is for example at a temperature of 620°C. After passing the steam turbine 36 and condenser 37, liquid water returns to the heat exchanger at a temperature of 175°C for example. The heat exchanger 35 is preferably countercurrent: the cold coolant fluid arrives in the header located at the bottom of the heat exchanger and leaves, after heating, through the outlet header located at the top thereof. After having crossed and contacted the tube bundle, the cold particles are directed to the cold storage tank 38 through the outlet conical piece of the heat exchanger and then further directed to the particle receiver 32 using a particle transport system 39, preferably a mechanical system such as a worm screw. [0035] In an alternate embodiment, the particle heat exchanger according to the invention contains supercritical CO2 operated according to a recompression Brayton cycle (not shown).

Specificities and advantages of the invention

[0036] Firstly, while presenting some similarities with the heat exchangers of prior art (WO 2013/163752 A1 , W02021/030875 A1 ), the heat exchanger of the present invention is based on a transverse tube design and not on a concept of plates or vertical tubes/channels, characterized by the absence of stagnant zones or slowing down zones for the particles.

[0037] One advantage of a heat exchanger designed with a tube bundle is that it is more suitable to resist to high-pressure fluid circulating inside the tubes, which would be the case for example if this heat exchanger is operated with supercritical CO2. In addition, the tube-in-shell design of the invention is superior with respect to severe thermal transients and temperature ramps. Further the SCO2 system does not use water, which is significant knowing that CSP plants are typically located in hot and dry climates where water is scarce.

[0038] Additionally, heating sC02 directly delivers benefits that are most pronounced with smaller systems that have a few hours of thermal storage, including the ability to place the power cycle in the receiver to minimize the length and cost of high pressure piping. Both novel power receiver approaches (particles and SCO2) have numerous potential advantages that void the current limitations of CSP systems using molten salt as a heat transfer fluid (FITF) and/or for thermal energy storage (TES).

[0039] Secondly, as shown on FIG. 4, the design of specific tubes with elbows is such that a small spacing between tubes (about 9 mm, corresponding to 10 times the diameter of bauxite particles) is possible without exceeding the allowable maximum elbow bending radius (according to manufacture constraints). This specificity enables high compactness of the tube bundle of the particle heat exchanger. The small spacing between tubes has a very positive impact on the heat transfer efficiency between the particles and the tubes.

[0040] Thirdly, a tubesheet design is originally provided to support the tubes, under the form of a set of plates 7 (one for each tube row) that are stacked on top of each other without connection (FIG. 6) and further maintained by an insulated sliding pieces (see FIG. 7A and FIG. 7B). The tubesheet plates 7 are supported by the heat exchanger bottom and the sliding pieces are welded to the casing 5. This design enables to break the important thermal gradient between the top and the bottom of the heat exchanger in smaller ones seen by each part. As a result, thermal stress in strongly reduced. In order to resist to particles at the highest temperatures, nickel superalloy (e.g. FI230) is advantageously used for a few plates located at the top of the heat exchanger, the other plates being in refractory alloy. Furthermore sliding pieces are made for example of nickel superalloy FI230 in the case of the two sliding pieces 44 exposed to the highest temperatures while the sliding pieces 45 exposed to lower temperatures are made of 304 stainless steel (see FIG. 7B).

[0041] Fourthly, in order to reduce the large thermal gradient (and therefore of additional thermal stresses) induced by the insulant 43 that covers the sliding part in current design, a sliding piece design will be a male part 42 with retracted insulation, while tubesheet plates will be a female part 41 (FIG. 7A).

[0042] Fifthly, innovative heat exchanger insulation solutions are chosen to be resistant to high-temperature abrasive particles. For example, a first layer 52 of 20mm alumina refractory concrete protects the heat exchanger wall from the abrasion of the particles and an insulant layer 53 up to 90mm thick, such as microporous insulant (silica) is further provided to obtain an external temperature of about 60°C for the metal casing 54 (FIG. 8). The concrete layer will be applied on welded metallic meshes (not shown) that will be maintained fixed thank to clips 56 (with spacers, not shown) attached to equidistant pins 55 welded to the casing 54. [0043] Finally, the fluid flowing in the tubes of the particles heat exchanger is, most of the time, pressurized and can reach very high pressure (of the order of magnitude of 250 bar for supercritical CO2). If, for some reasons (weld defects, manufacturing issues,...), a tube breaks, the high-pressure fluid will be released in the shell and will expand due to the fact that the inside of the steam generator is at much lower pressure. Therefore, the fluid will vaporize in the shell 5. Since the heat exchanger is full of particles, the room available for the fluid expansion is quite reduced and the pressure inside the shell will increase. Since the shell is not designed for sustaining such high pressures, its aim being simply to resist to the hydrostatic pressure induced by the particles, it may easily break due to this overpressure.

[0044] According to an embodiment of the invention, a solution to protect the shell from rupture is to install on it some rupture discs that are going to burst if a fixed setpoint pressure is exceeded (FIG. 9). In this case, the vaporized fluid will have an available path and will be directed outside the heat exchanger thanks to pipes connected to the outside of the rupture disc. This release of the fluid will allow to reduce the pressure inside the heat exchanger and will protect the casing from rupture due to overpressure. One or several rupture discs can be placed on the casing of the heat exchanger according to its size.

[0045] The rupture discs are installed with a torque resistance holder

(not shown). Since they are in contact with particles at high-temperature, it is preferable that they are composed of high-temperature resistant material such as Inconel superalloy.

List of reference symbols

1 heat exchanger

2 tube bundle

3 tube

4 spacing

5 casing (shell)

6 insulation layer

7 tubesheet

8 dead chamber 10 inlet header 11 outlet header 12 inlet rectangular part

13 outlet conical part

14 inlet/outlet thermocouple

15 sliding drawer

16 bend-located thermocouple

17 structure beam

18 draining slope 21 tube elbow

30 particle solar tower power plant

31 heliostat field

32 direct absorption particle receiver

33 solar tower

34 hot storage tank

35 heat exchanger (per se) steam turbine condenser cold storage tank particle transport system female part male part insulant sliding piece in nickel superalloy H230 sliding piece in 304 stainless steel casing panel stiffener refractory concrete insulant insulant casing metal pin clip rupture disk