WO2014064334A1

WO2014064334A1 - Method and apparatus for thermal energy conversion

Info

Publication number: WO2014064334A1
Application number: PCT/FI2013/050996
Authority: WO
Inventors: Lasse Koskelainen
Original assignee: Ekogen Oy
Priority date: 2012-10-22
Filing date: 2013-10-22
Publication date: 2014-05-01

Abstract

A heat exchanger apparatus (200) comprises a first cell (MOD1), wherein the first cell (MOD1) comprises a first plate (10) and a second plate (20), the first plate (10) comprises a ceramic material and/or a ceramic-metal composite material, the second plate (20) comprises a ceramic material and/or a ceramic-metal composite material, the first plate (10) comprises a first sealing surface (SRF1), the second plate (20) comprises a second sealing surface (SRF2), at least one of the first plate (10) and the second plate (20) comprises one or more grooves (14), which define one or more internal channels when the first sealing surface (SRF1) is positioned against the second sealing surface (SRF2), the first cell (MOD1) comprises a first opening (11) and a second opening (12), and the first cell (MOD1) is arranged to guide at least a part of an inlet gas flow (CG1) from the first opening (11) to the second opening (12) via at least one of said internal channels.

Description

METHOD AND APPARATUS FOR THERMAL ENERGY CONVERSION

The present invention relates to transferring heat from a first gas to a second gas at high temperatures.

BACKGROUND Electric energy may be generated by combusting a fuel in a combustion chamber, and guiding the hot flue gas into a gas turbine. The gas turbine converts the thermal energy of the hot flue gas into mechanical energy. The gas turbine may be connected to a generator for producing electricity. However, guiding the hot flue gas directly into the gas turbine may cause fouling and severe erosion and corrosion problems in the turbine blades when the flue gas contains significant amounts of impurities, e.g. particles, chlorine and/or alkali compounds. The risk of damaging the gas turbine may be reduced by using an indirect process based on the use of a clean working fluid. The energy of the hot flue gas can be transferred to the working fluid by using a heat exchanger. The heated working fluid can be subsequently guided to the gas turbine.

SUMMARY

An object of the invention is to provide a heat exchanger. An object of the invention is to provide a method for transferring heat from a first gas to a second gas. An object of the invention is to provide an energy conversion system.

According to a first aspect of the invention, there is provided an apparatus according to claim 1 .

According to a second aspect of the invention, there is provided a power conversion system of claim 13. According to a third aspect of the invention, there is provided a method according to claim 14.

The apparatus may comprise a heat exchanger formed of ceramic plates and ceramic ducts. The use of the ceramic material may allow heating a working gas to a high temperature, which in turn may increase the efficiency of the energy conversion process.

The ceramic parts of the heat exchanger may be clamped together to form a substantially leak-proof assembly, wherein said parts may have a freedom to slightly move with respect to each other, in order to allow different thermal expansion of the parts without causing fracture. In an embodiment, the plates of the heat exchanger may have substantially planar ceramic sealing surfaces, which may form a leak-proof joint when held against each other by a clamping force. In an embodiment, the leak-proof joint may be implemented without using a gasket between the ceramic sealing surfaces of the plates. Thus, the properties of the gasket material do not limit the upper operating temperature of the heat exchanger. The use of the ceramic heat exchanger may provide one or more of the following advantages:

- mechanical creep strength of the plates and ducts may be maintained above a predetermined limit at the high operating temperature,

- the risk of scaling may be kept at a low level,

- corrosion rate may be kept at a low level when operating in hot flue gas, which contains impurities, e.g. alkali, sulfur compounds and/or chlorine,

- material costs may be kept at a low level,

- problems caused by thermal expansion may be handled,

- a high power-per-volume ratio may be attained

- the weight of the heat exchanger may be reduced (when compared e.g. with a metal tube heat exchanger),

- when used in combination with a heat engine, the overall efficiency may be increased thanks to the higher operating temperature. The heat exchanger may comprise an array of substantially similar cells. The modular construction of the heat exchanger may allow easy implementation in various different facilities having different thermal power.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings in which

Fig. 1 shows, in a cross-sectional side view, an array of heat exchanger cells,

Fig. 2 shows, in a three-dimensional view, the array of Fig. 1 ,

Fig. 3 shows, in a three-dimensional exploded view, a heat exchanger cell,

Fig. 4a shows, in a three-dimensional exploded view, a heat exchanger cell comprising internal baffles,

Fig. 4b shows, in an end view, the groove of a heat exchanger cell,

Fig. 5 shows, in a three-dimensional view, a heat exchanger cell supported by auxiliary clamping members,

Fig. 6a shows, in a cross-sectional side view, a joint between an inlet duct and a heat exchanger cell, Fig. 6b shows, in a cross-sectional side view, a joint between an inlet duct and a heat exchanger cell,

Fig. 6c shows, in a side view, bending of a ceramic plate, Fig. 7 shows, in a three-dimensional view, a circular heat exchanger cell, shows, in a cross-sectional side view, a heat exchanger comprising an array of heat exchanger cells, shows, in a cross-sectional side view, a heat exchanger comprising adjacent arrays of heat exchanger cells, shows, in a cross-sectional side view, a heat exchanger comprising an array of heat exchanger cells, shows, in a cross-sectional side view, a heat exchanger comprising adjacent arrays heat exchanger cells, shows an energy conversion system comprising the heat exchanger, shows, in a three-dimensional exploded view, a heat exchanger cell, shows, in a cross-sectional exploded side view, the heat exchanger cell of Fig. 12a, shows, in a cross-sectional side view, the heat exchanger cell of Fig. 12a, shows, in a cross-sectional side view, an array of heat exchanger cells, shows, in a cross-sectional side view, sliding of a second sealing surface with respect to a first sealing surface, and shows a fracture formed due to non-uniform thermal expansion.

All drawings are schematic. DETAILED DESCRIPTION

Referring to Fig. 1 , a heat exchanger 200 may comprise an array ARR1 of heat exchanger cells MOD1 , MOD2, MOD3. The array ARR1 may comprise two or more substantially similar cells MOD1 , MOD2. A heat exchanger cell MOD1 may comprise a first plate 10 and a second plate 20. At least one of plates 10 and 20 may comprise a groove 14, which forms a flow channel when the first plate 10 and the second plate 20 are clamped together. The sealing surface SRF1 of the first plate 10 may be arranged to match with the sealing surface SRF2 of the second plate such that a substantially leak-proof joint can be formed by clamping the plates 10, 20 together. During operation, the surface SRF1 may be in contact with the surface SRF2 such that the surfaces SRF1 , SRF2 may together form the leak-proof joint. The array ARR1 may optionally have an end cell MOD3. The cell MOD3 positioned at the end of the array ARR1 may have plate 30 which does not have any openings, in order to minimize leakage. The plate 30 may otherwise be substantially similar to the plate 20, for example. The outer surface of the cell MOD1 may be exposed to hot flue gas FG1 . A working gas CG1 may be arranged to flow in the groove 14 of the cell MOD1 . Heat may be transferred from the hot flue gas FG1 through the first plate 10 to the working gas CG1 to provide heated working gas CG2. Heat may be transferred from the hot flue gas FG1 also through the second plate 20 to the working gas CG1 .

The temperature T₂ of the heated working gas CG2 may be e.g. in the range of 900 to 1200°C. The maximum operating temperature of the plate 10 may be higher than the temperature of the heated working gas CG2. The maximum temperature of the plate 10 may be e.g. in the range of 1000 to 1300°C. Ti denotes the temperature of the working gas CG1 at the inlet 1 1 . To denotes the inlet temperature of the flue gas FG1 . The inlet temperature Ti is lower than the outlet temperature T₂. The outlet temperature T₂ is lower than the inlet temperature T₀ of the flue gas FG1 . For slight less demanding applications, the temperature T₂ of the heated working gas CG2 may be e.g. in the range of 900 to 1 100°C, and the maximum temperature of the plate 10 may be e.g. in the range of 1000 to 1200°C.

A first cell MOD1 may have plates 10, 20. The first plate 10 may comprise openings 1 1 , 12. The second plate 20 may comprise openings 21 , 22. In an embodiment, the opening 1 1 may be used as a first inlet, the opening 21 may be used as a second outlet, the opening 22 may be used as a second inlet, and the opening 12 may be used a first outlet.

The first inlet 1 1 may be arranged to transfer gas CG1 from an inlet tube 51 to the interior of the first cell MOD1 . The first outlet 12 may be arranged to transfer gas CG2 from the interior of the first cell MOD1 to an outlet tube 52. The second outlet 21 may be arranged to transfer gas CG1 from the interior of the first cell MOD1 to an adjacent second cell MOD2. The second inlet 22 may be arranged to transfer gas CG2 from the adjacent second cell MOD2 to the interior of the first cell MOD1 . The first inlet 1 1 may be in fluid communication with the second outlet 21 . The first outlet 12 may be in fluid communication with the second inlet 22.

The inlet working gas CG1 may be guided to the cell MOD1 via an inlet duct 51 . The heated gas CG2 may be guided from the cell MOD1 via an outlet duct 52. The working gas CG1 may be guided from the first cell MOD1 to the adjacent second cell MOD2 via a first duct 50. The heated working gas CG2 may be guided from the second cell MOD2 to the first cell MOD1 via a second duct 50. The inlet duct 51 may be clamped against a sealing surface of the inlet 1 1 so as to form a substantially leak-proof joint. The outlet duct 52 may be clamped against a sealing surface of the outlet 12. A first duct 50 may be clamped against a sealing surface of the outlet 21 . A second duct 50 may be clamped against a sealing surface of the inlet 22.

Thus, the first cell MOD1 may comprise at least four openings 1 1 , 12, 21 , 22, and the first cell MOD1 may be arranged to divide a flow of inlet gas CG1 at least into a first partial flow FLW1 and a second partial flow FLW2 such that the first partial flow FLW1 is guided from the first opening 1 1 to the second opening 12 via at least one internal channel 14, and such that the second partial flow FLW2 is guided from the first opening 1 1 to the second opening 12 via the third opening 21 , via the second cell MOD2, and via the fourth opening 22.

The second partial flow FLW2 may substantially directly pass through the first cell MOD1 , without passing through the channel 14 of the first cell MOD1 . The first cell MOD1 may be arranged to divide the gas flow CG1 into a first partial flow FLW1 and a second partial flow FLW2 such that the first partial flow FLW1 is guided to the second cell MOD2 via the channel 14 of the first cell MOD1 , and such that the second partial flow FLW2 is guided to the second cell MOD2 via a path, which is substantially shorter than the channel 14 of the first cell MOD1 . The direct path from the opening 1 1 of the cell MOD1 to the opening 21 of the cell MOD1 may be e.g. shorter than 30% of the length of the path Si2 defined by the groove 14 (Fig. 4b).

The material or materials of the plates 10, 20 may be selected such that the plates 10, 20 are substantially impermeable to the working gas, in order to minimize leakage through the plates 10, 20.

Metallic materials may exhibit scaling, oxidation, corrosion, and/or creep when exposed to high temperatures (scaling means herein that a part of the material is converted into small fragments). The plates 10, 20 may comprise ceramic material in order to provide reliable operation at high temperatures.

The plate 10 and/or 20 may comprise e.g. sintered aluminum oxide AI2O3 (alumina). The plate 10 and/or 20 may comprise e.g. more than 99% aluminum oxide AI2O3 (available e.g. under a trade name "Alsint"). The plate 10 and/or 20 may consist of aluminum oxide AI2O3.

The plate 10 and/or 20 may comprise e.g. silicon nitride Si3N . The plate 10 and/or 20 may consist of silicon nitride Si3N .

In particular, the plate 10 and/or 20 may comprise silicon carbide (SiC). The plate 10 and/or 20 may comprise silicon carbide reinforced by silicon carbide whiskers. The plate 10 and/or 20 may consist of silicon carbide (SiC). Silicon carbide ceramics may exhibit sufficient creep resistance and/or a low tendency to scaling at high operating temperatures. Silicon carbide may have high resistance to corrosion when operating in hot flue gas, which contains slag-forming impurities. Silicon carbide ceramics may have high emissivity for infrared radiation, i.e. it may effectively absorb radiation emitted from hot flue gas and/or from a combustion flame.

In an embodiment, the first plate 10 and the second plate 20 may comprise the same ceramic material. This may e.g. minimize deformation due to thermal expansion.

In an embodiment, the first plate 10 may comprise a first ceramic material, and the second plate 20 may comprise a second ceramic material, which is different from the first ceramic material.

The plates 10, 20 may be implemented so that they do not comprise any metallic surfaces exposed to the working gas CG1 , CG2, in order to reduce the risk of scaling. The plates 10, 20 may be implemented so that the sealing surfaces SRF1 , SRF2 do not comprise any metallic surfaces, in order to reduce the risk of scaling.

In an embodiment, the plate 10 may comprise a ceramic metal composite (cermet), and/or the plate 20 may comprise a ceramic metal composite.

In an embodiment, the plates 10, 20 may comprise one or more metallic elements embedded within a ceramic material. The plates 10, 20 may be reinforced with the metalling elements. Also the ducts 50 51 , 52 may comprise ceramic material. For example, the ducts may consist of sintered aluminum oxide. In particular, the ducts may consist of silicon carbide (SiC).

The plates 10, 20 may be clamped together by using a clamping force F1 and a counter-force F2. pi denotes the pressure of the inlet gas CG1 inside the cell MOD1 , and po denotes the pressure of the flue gas FG1 outside the cell MOD1 . The internal pressure pi may cause a separating force, which pushes the plates apart from each other. The clamping force F1 may be selected high enough to create a leak-proof seal and to compensate the separating force caused by the pressure difference pi-po.

Thus, the flow rate of gas CG1 through the joint formed by the mating surfaces SRF1 , SRF2 may be kept at a low level even when the internal pressure pi is substantially higher than the external pressure po. The difference pi-po between the pressures pi and po may be e.g. in the range of 0.05 MPa to 1 MPa. In particular, the first plate 10 may have a substantially planar sealing surface SRF1 . Also the second plate 20 may have a substantially planar sealing surface SRF2.

The sealing surfaces SRF1 , SRF2 may be produced e.g. by mechanical machining. The sealing surfaces SRF1 , SRF2 may be polished such that the surface roughness value Ra is smaller than or equal to 5 μηη. In particular, the sealing surfaces SRF1 , SRF2 may be mirror-polished.

The clamping force F1 may be created e.g. by a mechanical or pneumatic spring, or by a (heavy) weight. The forces F1 , F2 may be exerted to the cell MOD1 e.g. by using pushing elements 60 and/or ducts 50, 51 , 52.

In an embodiment, a piece of thermal insulation material E1 may be used as a pushing element, which may be arranged to exert the clamping force F1 (or the counter force F2) on a cell (see Figs. 10a, 10b).

SX, SY, SZ denote orthogonal directions. The sealing surface SRF1 may be substantially parallel to a plane defined by the directions SY and SZ. Fig. 2 shows, in a three-dimensional view, the array ARR1 of Fig. 1 .

Fig. 3 shows an exploded view of a cell MOD1 . At least one of the plates 10, 20 may comprise a groove 14, which defines a fluid channel from the inlet 1 1 to the outlet 12 when the plates 10, 20 are clamped together. Only one of the plates 10, 20 may have the internal groove 14. This may allow minimizing manufacturing costs.

Alternatively, the first plate 10 may have a first groove 14, and the second plate 20 may have a second groove. The first groove 14 and the second groove may together define an internal channel for transferring the working gas CG1 from the inlet 1 1 to the outlet 12. In particular, the cell MOD1 may be substantially symmetrical with respect to a plane, which is parallel to the sealing surface SRF1 . The symmetrical structure may minimize asymmetric deformations caused by thermal expansion, in order to minimize leakage of gas out of the cell.

SRF5 denotes a sealing surface of the inlet duct 51 . Referring to Fig. 4a, the groove 14 may be curved e.g. in order to improve heat transfer from the plate 10 to the gas CG1 . The plate 10 may comprise one or more internal baffles BAF1 , BAF2, BAF3 to control the flow of the gas CG1 inside the cell MOD1 . Referring to Fig. 4b, the length Si₄ of a gas path defined by the groove 14 may be substantially longer than the distance diN,ouT between input opening and the output opening of a cell. In particular, the length Si₄ of a gas path defined by the groove 14 may be substantially longer than the distance diN.ouT between input opening 1 1 and the output opening 21 of a plate 10.

Referring to Fig. 5, the clamping forces F1 may be delivered e.g. to the plate 20 of a cell MOD1 by using two or more pushing elements 50, 50D. The clamping forces F1 may be delivered by a first duct 50 and a second duct 50. The first duct 50 and the second duct 50 may be active ducts, i.e. they are used for guiding the gas CG1 , CG2. The clamping forces F1 may be optionally delivered via one or more auxiliary elements 50D in order to improve mechanical stability and/or in order to provide a more leak-proof joint between the plates 10, 20. The auxiliary elements 50D do not need to be suitable for guiding gas inside them. The auxiliary elements 50D may be solid or hollow. The external shape of the cell MOD1 may be substantially rectangular, when viewed along the direction SX. The clamping forces F1 may be exerted on the cell MOD1 e.g. near the corners of said rectangular shape. The counter-forces F2 may be exerted on the plate 10 by using pushing members. In particular, the inlet duct 51 and the outlet duct 52 may be used as pushing members. Auxiliary pushing members 51 D, 52D may be optionally used to deliver the counter forces F2. Referring to Fig. 6a, the first plate 10 and/or the second plate 20 may have protrusions RIB1 to enhance heat transfer from the hot flue gas FG1 to the material of the plate 10, 20. Alternatively or in addition to the protrusions RIB1 , the material of the plates 10, 20 may be selected such that it has high absorbance for infrared radiation emitted from hot glue gas FG1 and/or from a combustion flame.

The inlet duct 51 and/or an intermediate duct 50 may be positioned in a recess, which has a positioning surface SUP1 . The positioning surface SUP1 may stabilize the position of the duct 50, 51 in the direction SY with respect to the cell MOD1 .

The inlet duct 51 has a sealing surface SRF5, which may form a substantially leak-proof seal when clamped against the cell MOD1 . The sealing surface SRF5 may be e.g. substantially planar, substantially conical or substantially spherical.

Fig. 6b shows an inlet duct 51 having a substantially spherical sealing surface SRF5. The spherical sealing surface may provide a substantially leak-proof seal even when the angle γ1 between the direction AX51 of the duct 51 and the direction SY is different from 90 degrees.

The plate 10 may have a sealing surface SRF6, which may be arranged to be in contact with the sealing surface SRF5 of the duct 50. SPHE1 denotes a spherical reference surface. The form of the sealing surface SRF5 and/or the form of the sealing surface SRF6 may substantially match with the spherical reference surface SPHE1 . The sealing surface SRF5 may be substantially spherical and/or the sealing surface SRF6 may be substantially spherical.

One or more passive pushing members 50D, 51 D, 52D , which are not used for guiding the gas, may have a substantially planar ends in order to improve stability in the direction SY.

The first end of an intermediate duct 50 may have a substantially spherical sealing surface, and the second end of said duct 50 may have a substantially planar surface, in order to improve stability in the direction SY.

Fig. 6c shows bending of a ceramic plate 10, 20. The plate 10 or 20 may be positioned in a test set-up LAB1 where the edges of the plate are supported by supports SUP2, SUP3. A predetermined test force F_TEST may be directed on the center of the plate 10, 20. The force FTEST may cause a deflection d3 of the center with respect to the resting position REF1 . The plate 10, 20 may have a height d2 and a thickness d1 .

Non-uniform temperature distribution in a cell MOD1 may cause non-uniform thermal expansion, which in turn may cause a leaking gap between the sealing surfaces SRF1 , SRF2. The dimensions of the plate 10 and/or the plate 20 may be selected such that the ceramic plate is slightly flexible in the direction SX. Thus, the clamping force F1 may hold the entire perimeter of the second plate 20 in contact with the sealing surface SRF1 of the first plate 10 even in a situation where the non-uniform thermal expansion is present.

For example, the thickness d1 of the ceramic plate may be selected small enough such that the ratio d3/d2 can reach the value 0.0001 without fracturing the plate (0.0001 = 0.01 %). One or both plates 10, 20 may fulfill this criterion at the temperature of 25°C. One or both plates 10, 20 may fulfill this criterion at the temperature of 1000°C. Both plates may be flexible. One of the plates 10, 20 may be thin and flexible and the other plate 20, 10 may be thick and rigid. For example, the thickness d1 of a plate 10 having a height 300 mm may be selected such that the plate can be deflected at least 0.03 mm without fracturing the plate. Consequently, if geometrical deformations caused by non-uniform thermal expansion would lead to a gap of 0.03 mm between the sealing surfaces SRF1 , SRF2, the gap can be still be closed by pushing the plates 10, 20 together with the clamping force F1 .

However, the plates do not need to be flexible if a small leakage between the sealing surfaces can be tolerated and/or if the temperature expansion does not form the gap. Referring to Fig. 7, the shape of the cell MOD1 may be substantially circular, when viewed along the direction SX. This shape may minimize the length of the joint formed by the sealing surfaces SRF1 , SRF2, thereby minimizing the leakage. In this case, the sealing surfaces SRF1 , SRF2 of the plates 10, 20 may be e.g. substantially planar, substantially conical, or substantially spherical.

Referring to Fig. 8, a heat exchanger 200 may comprise two or more cells MOD1 , MOD2, MOD3, which may be in fluid communication with each other via the intermediate ducts 50. Working gas CG1 may be guided to the first cell MOD1 via the inlet duct 51 , and heated working gas CG2 may be guided from the first cell MOD1 via the outlet duct 52.

The cell MOD3 positioned at the end of the array ARR1 may have plate 30 which does not have any openings, in order to minimize leakage. The plate 30 may otherwise be substantially similar to the plate 20.

The clamping forces F1 may be generated by springs 70, and the clamping forces F1 may be delivered to the cell MOD1 by using pushing members 60. The pushing members 60 are not used for guiding gas inside them, and they may be solid or hollow.

One or more springs 70 may be used to create the clamping force F1 . The springs 70 may be e.g. metallic coil springs or metallic disk springs. A metallic spring may lose its elastic properties when heated to a temperature, which is too high. The springs 70 may be optionally positioned e.g. inside cooled chambers 140. The chambers 140 may optionally have removable covers 142, to allow removal and/or adjustment of the springs 70. Also e.g. a rubber element, a pneumatic spring, a hydraulic spring, or a weight may be used to create the clamping force F1 . A pneumatic spring may comprise e.g. a combination of a cylinder and a piston, wherein the cylinder comprises pressurized gas. A pneumatic spring may comprise e.g. bellows filled with pressurized gas. A hydraulic spring may comprise e.g. a combination of a cylinder and a piston, wherein the cylinder comprises pressurized hydraulic fluid. A hydraulic spring may comprise e.g. bellows filled with pressurized hydraulic fluid.

The cells MOD1 , MOD2, MOD3 may be surrounded by a shell 100. The shell 100 may operate as support for the clamping force F1 and for the counter force F2. The shell 100 may optionally prevent entrainment of external air into the hot flue gas FG1 . The shell 100 may optionally act as a support for thermal insulation E1 . The shell 100 may optionally act as a safety device, which protects instrumentation and/or human operators from hot gas in a situation where one of the ceramic parts fails. The shell 100 may have e.g. a rectangular or circular shape when viewed along the direction SY.

The number of cells MOD1 , MOD2 arranged in a linear array ARR1 may be e.g. in the range of 2 to 20. In particular, the number of cells MOD1 , MOD2 arranged in a linear array ARR1 may be e.g. in the range of 3 to 20. The heat exchanger 200 may have a removable cover 1 10 e.g. in order to allow installation of the cells through the side of the shell 100.

The counter-force F2 may be delivered to the duct 51 e.g. by a sleeve 80. The sleeve 80 may be supported e.g. by the shell 100 or by the cover 1 10. The operating temperature of the inlet duct 51 may be high, and the temperature of the cover 1 10 may be low. The sleeve 80 may operate as a thermally insulating element between the duct 51 and the cover 1 10, while also delivering the counter-force F2. The inlet duct 51 and the outlet duct 52 may be optionally covered with thermal insulation E2. The inlet duct 51 and/or the outlet duct 52 may be a ceramic duct. The inlet duct 51 and/or the outlet duct 52 may be optionally covered with a metallic exterior mantle in order to protect instrumentation and human operators from hot gas in case of fracture of the hot duct 51 , 52.

The shell 100 and the covers 1 10, 142 may be metallic. The covers 1 10, 142 may be attached to the shell 100 e.g. by screws (i.e. bolts).

Referring to Fig. 9, a heat exchanger 200 may comprise two or more linear arrays ARR1 , ARR2 of cells MOD1 , MOD2. The arrays ARR1 , ARR2 may be adjacent to each other e.g. in the direction SY and/or in the direction SZ.

The distance between adjacent cells MOD1 , MOD2 in a lower array ARR2 may be greater than the distance between adjacent cells MOD1 , MOD2 in an upper array ARR1 in order to facilitate cleaning of the cells MOD1 , MOD2 of the lower array ARR2.

The gas flow may be guided from a first array ARR1 to a second array ARR2 e.g. by using a coupling cell MOD4, which comprises a first part 91 and a second part 92. The coupling cell MOD4 may be e.g. substantially similar to the cell MOD3 shown in Fig. 8. However, the internal channel of the coupling cell MOD4 may be advantageously dimensioned so as to reduce or minimize flow resistance of working gas through the coupling cell MOD4.

The clamping forces may be delivered e.g. by using pushing members 62. Referring to Fig. 10a, an array ARR1 comprising two or more cells MOD1 , MOD2 may be arranged to guide gas CG1 from the inlet tube 51 to the outlet tube 52 such that each partial flow FLW1 , FLW2 is guided only once through each cell MOD1 , MOD2 of the array. This may minimize the flow resistance between the inlet tube 51 and the outlet tube 52. This may reduce a difference between the heat transfer power of the first cell MOD1 and the heat transfer power of the second cell MOD1 .

In particular, the array ARR1 may be arranged to guide gas CG1 from the inlet tube 51 to the outlet tube 52 such that a first partial flow FLW1 is guided only once through the first cell MOD1 and only once through the second cell MOD2. The first partial flow FLW1 may be guided through the channel 14 of the first cell MOD1 . The array ARR1 may be arranged to guide gas CG1 from the inlet tube 51 to the outlet tube 52 such that a second partial flow FLW2 is guided only once through the first cell MOD1 and only once through the second cell MOD2. The second partial flow FLW2 may be guided through the channel 14 of the second cell MOD2.

The first partial flow FLW1 may be guided from the inlet tube 51 via the cell MODO, via the channel 14 of cell MOD1 , and via the cells MOD2, MOD5 to the outlet tube 52. The second partial flow FLW2 may be guided from the inlet tube 51 via the cells MODO, MOD1 , via the channel 14 of cell MOD2, and via the cell MOD5 to the outlet tube 52.

The gas CG1 may be guided from the inlet tube 51 to the outlet tube 52 through the array ARR1 such the length of a path from the inlet tube 51 via the channel 14 of the first cell MOD1 to the outlet tube 52 is substantially equal to the length of a path from the inlet tube 51 via the channel 14 of the second cell MOD2 to the outlet tube 52.

The array ARR1 may comprise a first end cell MODO, which comprises a plate 31 and a plate 20. The plate 31 may comprise one input opening, and the plate 20 may comprise two output openings. The input opening may be connected to the inlet tube 51 . The output openings may be connected to the first cell MOD1 . The array ARR1 may comprise a second end cell MOD5, which comprises a plate 10 and a plate 32. The plate 10 may comprise two input openings, and the plate 32 may comprise one output opening. The input openings may be connected to the second cell MOD2. The output opening may be connected to the outlet tube 52. A heat exchanger 200 may comprise one or more arrays ARR1 .

The end cells MODO, MOD5 may also comprise e.g. four openings if the unused openings are closed, e.g. by a ceramic plug. In particular, the end cell MODO, MOD5 may be substantially similar to the first cell MOD1 . Referring to Fig. 10b, a heat exchanger 200 may comprise two or more arrays ARR1 , ARR2. An output of a first array ARR1 may be connected to an input of a second array ARR2. The output of the first array ARR1 may be connected to the input of the second array ARR2 e.g. by a coupling cell BLC1 , which defines a flow channel from the first array ARR1 to the second array ARR2. The coupling cell BLC1 may comprise e.g. plates 33, 34. The flow channel of the coupling cell BLC1 may be dimensioned such that the flow resistance of the coupling cell BLC1 is lower than or equal to the flow resistance of the array ARR1 .

Referring to Fig. 1 1 , the heat exchanger 200 may be used e.g. in an energy conversion system 500. The system 500 may be e.g. arranged to convert chemical energy of solid fuel into electrical energy. The system 500 may comprise a heat engine TRB1 , which is arranged to convert thermal energy of the heated working gas CG2 into mechanical energy. The engine TRB1 may be e.g. gas turbine, a Stirling motor, or an ORC engine (organic Rankine cycle engine). The working gas CG2 may be e.g. air, CO2, or steam. The system 500 may comprise a generator GEN1 , which is arranged to convert the mechanical energy into electricity.

The system 500 may comprise a combustion chamber CMB1 , where fuel FUEL1 may be combusted. One or more oxidizing gas flows AIR1 , AIR2 may be guided to the combustion chamber CMB1 . An oxidizing gas AIR1 , AIR2 may be e.g. air or a mixture of air and recirculated flue gas. The oxidizing gas AIR1 may be oxygen-enriched air, wherein the concentration of oxygen may be e.g. higher than 30%. The oxidizing gas AIR1 may be a mixture of oxygen and recirculated flue gas. Combustion of the fuel FUEL1 may provide hot flue gas FG1 , which may be guided through the heat exchanger 200.

The system 500 may comprise a cleaning unit, which may be arranged to occasionally remove slag, ash and/or soot from the cells MOD1 by using a gas blast BLAST1 . The flue gas FG1 may be guided to a stack STACK1 , after it has been cooled to a sufficient degree. The flue gas FG1 may be optionally cleaned e.g. by a cyclone, bag filter and/or electrostatic filter before venting it to the atmosphere.

The fuel FUEL1 may be e.g. biomass fuel. The fuel may comprise e.g. wood, peat, forest residue, bark, straw, or municipal waste. The burning fuel particles P1 may be supported e.g. by a grate GRT1 . Alternatively, the fuel FUEL1 may be supplied to the combustion chamber CMB1 e.g. in pulverized or gasified form.

The thermal engine TRB1 may provide cooled working gas HOT1 , which may carry high thermal energy. The temperature of the cooled working gas HOT1 may be e.g. in the range of 400 to 800°C. The cooled working gas HOT1 may be e.g. guided into the combustion chamber CMB1 , to provide a hot air flow for the combustion. The cooled working gas HOT1 may be used e.g. for drying fuel FUEL1 , which has a high moisture content. The cooled working gas HOT1 may be used e.g. for pyrolysis of the fuel FUEL1 . The cooled working gas HOT1 may be used e.g. for some external purpose, e.g. for drying timber of for warming a building.

The system 500 may comprise a compressor COMP1 , which is arranged to pressurize the working gas CG1 prior to heating it with the heat exchanger 200. COOL1 denotes intake air of the compressor COMP1 . The temperature of the intake air may be e.g. in the range of -40°C to 100°C.

The velocity of the hot flue gas FG1 in the vicinity of the cells MOD1 may be e.g. in the range of 5 m/s to 30 m/s. The velocity of the working gas CG1 in the internal channel 14 of the cell MOD1 may be e.g. in the range of 10 m/s to 50 m/s.

The thermal power transferred from the flue gas FG1 to the working gas CG1 by a single cell MOD1 may be e.g. in the range of 100 W to 5 kW. The dimension of the plate 10 in the direction SY may be e.g. in the range of 0.1 m to 0.8 m, advantageously in the range of 0.2 m to 0.5 m. The dimension of the plate 10 in the direction SZ may be e.g. in the range of 0.1 m to 0.8 m, advantageously in the range of 0.2 m to 0.5 m. The projected area of the plate 10 on the SY-SZ-plane may be e.g. in the range of 0.01 m² to 0.6 m². Thanks to the protrusions RIB1 , the actual external area of the plate 10 exposed to the flue gas FG1 may be higher than the projected area. The external area of the plate 10 may be e.g. in the range of 1 to 5 times the projected area.

The number of cells MOD1 , MOD2 of a linear array ARR1 of cells may be e.g. in the range of 3 to 10. The length of the array ARR1 may be e.g. in the range of 0.1 m to 1 m. The heat transfer area per volume ratio for the array may be e.g. higher than 20 m²/m³. For example, a heat exchanger 200 having a volume of e.g. 2 m³ may be arranged to provide thermal power, which is higher than or equal to 400 kW.

The system 500 may also be arranged to process a material. The system 500 may be a material processing system. The system 500 may comprise e.g. a smelting furnace, a tempering furnace, a sintering furnace, or a lime kiln. A material processing system 500 may comprise one or more heat exchangers 200, which comprise the plates 10, 20. For example, the material processing system 500 may comprise one or more heat exchangers 200, which are arranged to recover residual heat from hot flue gas FG1 after the flue gas FG1 has been utilized for processing the material. Also in this case, the energy of the heated working gas CG2 may be utilized e.g. in a thermal engine TRB1 .

The cells MOD1 , MOD2 of an array ARR1 may be pulled downwards by gravity. The heat exchanger 200 may optionally comprise one or more supporting elements arranged to generate supporting forces. The supporting forces may be arranged reduce strain caused by the gravity. For example, one or more ceramic elements may be positioned under an array ARR1 in order to generate supporting forces. The sum of the supporting forces may be substantially equal to the weight of the array ARR1 . In an embodiment, the first plate 10 of a heat exchanger cell MOD1 may consist of a first monolithic structure, and the second plate 20 of the heat exchanger cell MOD1 may consist of a second monolithic structure. Thanks to the monolithic structures, leaking of gas through the joints formed by the sealing surfaces of the cell MOD1 may be minimized. Referring to Fig. 12a, the first plate 10 and/or the second plate 20 may also comprise several parts 10a, 10b. Forming the first plate 10 from several parts 10a, 10b may allow easier manufacturing when compared with a situation where the first plate 10 is formed as a monolithic structure. Forming the first plate 10 from several parts may also provide improved tolerance for non- uniform thermal expansion. The parts 10a, 10b may be separated from each other after the heat exchanger cell MOD1 has been used for containing pressurized gas.

The first plate 10 of a heat exchanger cell MOD1 may comprise a first part 10a and a second part 10b. The first part 10a may be called e.g. as a spacer part. The second part 10b may be called e.g. as a cover part. The spacer part 10a may have a first auxiliary sealing surface SRFXa in addition to the sealing surface SRF1 . The cover part 10b may have a second auxiliary sealing surface SRFXb. The spacer part 10a and the cover part 10b may together form the first plate 10 when the first auxiliary sealing surface SRFXa is positioned against the second auxiliary sealing surface SRFXb. In an embodiment, the cover part 10b may be separated from the spacer part 10a without damaging one or both of auxiliary sealing surfaces SRFXa, SRFXb. The first auxiliary sealing surface SRFXa and the second auxiliary sealing surface SRFXb may together form a substantially pressure tight joint when the first plate 10 and the second plate 20 of the heat exchanger cell MOD1 are clamped together by a clamping force F1 . The second auxiliary sealing surface SRFXb may slide against the first auxiliary sealing surface SRFXa when the first plate 10 and the second plate 20 are clamped together by a clamping force F1 . The second auxiliary sealing surface SRFXb may at least partly movable with respect to the first auxiliary sealing surface SRFXa when the first auxiliary sealing surface SRFXa and the second auxiliary sealing surface SRFXb together form a substantially pressure tight joint. The second auxiliary sealing surface SRFXb may be substantially parallel to a plane defined by the directions SY, SZ, and the second auxiliary sealing surface SRFXb may slide e.g. in the direction SY or in the direction SZ.

The sealing surface SRF1 and the first auxiliary sealing surface SRFXa may be located on opposite sides of the spacer part 10a. The first auxiliary sealing surface SRFXa may be substantially parallel to the sealing surface SRF1 . The spacer part 10a may have an opening 14, which extends through the spacer part 10a. The opening 14 may extend from the sealing surface SRF1 to the first auxiliary sealing surface SRFXa. The spacer part 10a may have an inner surface 15, which defines the opening 14.

The opening 14 of the spacer part 10a may provide a space, which contains one or more internal channels for guiding a partial flow FLW1 . In an embodiment, the cover part 10b and the inner surface 15 may define one or more grooves 14 when the first auxiliary sealing surface SRFXa is positioned against the second auxiliary sealing surface SRFXb. In an embodiment, one or more baffles may also be inserted into the opening 14 of the spacer part 10a such that the baffles may at least partly define one or more walls for one or more grooves. The grooves may form one or more internal channels when the plates are clamped together.

The cover part 10b may have one or more openings 1 1 , 12 for guiding working gas CG1 to the internal channel. The cover part 10b may have one or more openings 1 1 , 12 for guiding working gas CG2 from the internal channel.

The spacer part 10a may comprise ceramic material and/or a ceramic-metal composite material, and the cover part 10b may comprise a ceramic material and/or a ceramic-metal composite material. The spacer part 10a may consist of a ceramic material and/or a ceramic-metal composite material, and the cover part 10b may consist of a ceramic material and/or a ceramic-metal composite material.

Fig. 12b shows, in an exploded cross-sectional view, the heat exchanger cell MOD1 of Fig. 12a. The cover part 10b may have a thickness d iot The spacer part 10a may have a thickness d io_a- The second plate 10b may have a thickness c o- The surfaces SRF1 , SRF2 may form a first substantially pressure tight joint, and the surfaces SRFXa, SRFXb may form a second substantially pressure tight joint when the plates 10, 20 are clamped together. The surfaces SRF1 , SRF2 may form a first substantially pressure tight joint, and the surfaces SRFXa, SRFXb may form a second substantially pressure tight joint when the plates 10, 20 are removably clamped together. The thickness dio_b of the cover part 10b may be substantially equal to the thickness d2o of the second plate 20 in order to minimize leaking of working gas through the joints in case of non-uniform thermal expansion.

Fig. 12c shows the heat exchanger cell MOD1 , which comprises the parts of Fig 12a. The heat exchanger cell MOD1 may comprise the spacer part 10a, the cover part 10b, and the second plate 20. In this case, the second plate 20 may also be called e.g. as a second cover part. The heat exchanger cell MOD1 may comprise one or more internal channels for guiding a partial flow FLW1 .

Fig. 13 shows an array ARR1 of heat exchanger cells MOD1 , MOD2. di2 denotes a distance between a first heat exchanger cell MOD1 and a second heat exchanger cell MOD2, which is adjacent to the first heat exchanger cell MOD1 . The distance di2 may be greater than or equal to a predetermined limit e.g. in order to provide sufficient radiative transfer of heat from the hot flue gas FG1 to the surfaces of the heat exchanger cells MOD1 , MOD2. For example, the distance di2 between adjacent cells MOD1 , MOD2 may be e.g. greater than or equal to 50 mm. The first cell MOD1 may comprise at least one internal channel for guiding a first partial flow FLW1 . The second cell MOD2 may comprise at least one internal channel for guiding a second partial flow FLW2. Referring to Fig. 14a, the second plate 20 may expand and/or shrink with respect the first plate 10 due to non-uniform thermal expansion. The second sealing surface SRF2 may at least partly movable with respect to the first sealing surface SRF1 when the first sealing surface SRF1 and the second sealing surface SRF2 together form a substantially pressure tight joint. The first sealing surface SRF1 and the second sealing surface SRF2 of a heat exchanger cell MOD1 may together form a substantially pressure tight joint when the first plate 10 and the second plate 20 of the heat exchanger cell MOD1 are clamped together by the clamping force F1 . The relative movement between the sealing surfaces SRF1 , SRF2 may allow non-uniform thermal expansion of the plates 10, 20 without damaging one or both of the plates 10, 20. The second sealing surface SRF2 may slide along the first sealing surface SRF1 when the first plate 10 and the second plate 20 are clamped together by the clamping force F1 . The second sealing surface SRF2 may slide along the first sealing surface SRF1 when the first sealing surface SRF1 and the second sealing surface SRF2 together form a substantially pressure tight joint. The plate 20 may have spatially separate reference points REF2, REF3. The distance dR23 between the reference points REF2, REF3 may be changed due to thermal expansion.

The pressure difference p po between the interior and the exterior of the cell MOD1 may be e.g. in the range of 0.05 MPa to 1 MPa. The flow rate of gas leaking through the pressure tight joint formed by the sealing surfaces SRF1 , SRF2 of the cell MOD1 may be e.g. smaller than 1 % of the flow rate of working gas guided into the cell MOD1 . In an embodiment, the cell MOD1 may be dismantled by separating the first plate 10 from the second plate 20 without damaging one or both of the sealing surfaces SRF1 , SRF2. The second sealing surface SRF2 of the cell MOD1 may be in contact with the first sealing surface SRF1 such that the second sealing surface SRF2 may be separated from the first sealing surface SRF1 without damaging one or both of the sealing surfaces SRF1 , SRF2. The sealing surfaces SRF1 , SRF2 may be removably joined to each other by the clamping force F1 so that the sealing surfaces SRF1 , SRF2 may form the substantially pressure-tight joint. Fig. 14b shows a comparative example where a monolithic cell may be fractured due to non-uniform thermal expansion. The monolithic cell may comprise a first portion POR1 and a second portion POR2. If the second portion POR2 expands more than the first portion POR1 , the resulting tension may cause a fracture. For the person skilled in the art, it will be clear that modifications and variations of the devices and methods according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1 . A heat exchanger apparatus (200) comprising a first cell (MOD1 ), wherein the first cell (MOD1 ) comprises a first plate (10) and a second plate (20), the first plate (10) comprises a ceramic material and/or a ceramic-metal composite material, the second plate (20) comprises a ceramic material and/or a ceramic-metal composite material, the first plate (10) comprises a first sealing surface (SRF1 ), the second plate (20) comprises a second sealing surface (SRF2), at least one of the first plate (10) and the second plate (20) comprises one or more grooves (14), which define one or more internal channels when the first sealing surface (SRF1 ) is positioned against the second sealing surface (SRF2), the first cell (MOD1 ) comprises a first opening (1 1 ) and a second opening (12), and the first cell (MOD1 ) is arranged to guide at least a part of an inlet gas flow (CG1 ) from the first opening (1 1 ) to the second opening (12) via at least one of said internal channels.

2. The apparatus of claim 1 wherein the first plate (10) comprises the first opening (1 1 ) and the second opening (12).

3. The apparatus of claim 1 wherein the first plate (10) comprises the first opening (1 1 ) and the second plate comprises the second opening (12, 22).

4. The apparatus according to any of the claims 1 to 3 wherein the first sealing surface (SRF1 ) consists of a ceramic material and the second sealing surface (SRF2) consists of a ceramic material.

5. The apparatus of claim 4 wherein the first sealing surface (SRF1 ) is positioned directly against the second sealing surface (SRF2) without using a gasket between said sealing surfaces (SRF1 , SRF2).

6. The apparatus according to any of the claims 1 to 5 wherein the sealing surface (SRF1 ) of the first plate (10) and the sealing surface (SRF2) of the second plate (20) are substantially planar.

7. The apparatus according to any of the claims 1 to 6 further comprising a second cell (MOD2), wherein an opening (21 ) of the first cell (MOD1 ) is connected to an opening (1 1 ) of the second cell (MOD2) so as to allow guiding of gas (CG1 ) from the first cell (MOD1 ) to the second cell (MOD2).

8. The apparatus according to any of the claims 1 to 4 wherein the first plate (10) and the second plate (20) are clamped together by a clamping force (F1 ) generated by using a spring (70).

9. The apparatus of claim 8 wherein gas (CG1 ) is arranged to be guided from the first cell (MOD1 ) to the second cell (MOD2) via a ceramic duct (50), the ceramic duct (50) being arranged to exert said clamping force (F1 ) to the second plate (20).

10. The apparatus according to any of the claims 1 to 7 wherein the external surface of the first cell (MOD1 ) has protrusions (RIB1 ) to enhance heat transfer from a flow of hot flue gas (FG1 ).

1 1 . The apparatus according to any of the claims 1 to 10 wherein said first plate (10) comprises a first part (10a) and a second part (10b), the first part (10a) comprises a first auxiliary sealing surface (SRFXa), the second part (10b) comprises a second auxiliary sealing surface (SRFXb) such that the first part (10a) and the second part (10b) together form the first plate (10) when the first auxiliary sealing surface (SRFXa) is positioned against the second auxiliary sealing surface (SRFXb), and the first part (10a) comprises an opening (14), which extends through the first part (10a).

12. The apparatus according to any of the claims 1 to 1 1 wherein the second sealing surface (SRF2) is at least partly movable with respect to the first sealing surface (SRF1 ) when the first sealing surface (SRF1 ) and the second sealing surface (SRF2) together form a substantially pressure tight joint.

13. A power conversion system (500) comprising:

- a combustion chamber (CMB1 ) for producing hot flue gas (FG1 ) by combusting a fuel (FUEL1 ),

- the heat exchanger apparatus (200) according to any of the claims 1 to 12, the heat exchanger (200) being arranged to transfer heat from the hot flue gas (FG1 ) to a working gas (CG1 ), and

- a heat engine (TRB1 ) arranged to convert thermal energy of the working gas (CG1 ) into mechanical energy.

14. A method for converting chemical energy of a fuel (FUEL1 ) to mechanical energy by using the system (500) of claim 13.

15. The method of claim 14 wherein the maximum temperature of the first cell (MOD1 ) is in the range of 1000°C to 1300 °C.

16. The method of claim 14 or 15, wherein the hot flue gas (FG1 ) comprises slag-forming compounds.

17. The method according to any of the claims 14 to 16 wherein the heat engine is a gas turbine or a Stirling engine.