Waste heat recovery system

Abstract

A method of operating a waste heat recovery system (WHRS) for a supercritical carbon dioxide (sCO2) based waste heat to power system (SCO2WHTPS). The system comprising a waste heat recovering unit (WH

Description

Field of the invention The invention relates to a waste heat recovery system of a technical plant Background to the invention The technical background of the invention pertains to the significant amount of waste heat generated by industries such as cement and steel production, which can account for a substantial portion of the total heat consumed in the process. To reuse this thermal energy and to convert it to electrical power, it is known to apply a supercritical CO2 (sCO2) waste heat recovery system to enhance power generation efficiency. The efficiency of the sCO2 Brayton cycles can be improved by optimizing temperature through heat recovery methods, with a focus on maximizing heat extraction through waste heat recovery . Conventional methods involve direct utilization of flue gases in heat exchangers by transferring the thermal energy of the flue gas directly into the supercritical CO2. Such a system for a dust laden or overloaded flue gas is known from the scientific paper from Dario Alfani, Marco Binotti, Paolo Silva, Giacomo Persico, "Part Load Analysis OF A Constant Inventory Supercritical CO2 Power Plant For Waste Heat Recovery In Cement Industry", 2023-SCO2.eu-125. Because of the dust challenges like dust accumulation on the heat exchanger pipes arises which lead to known drawbacks like lower heat recovery as the operation progresses. The higher pressure of CO2 working fluid increases the thickness of heat exchanger pipes / tubes increasing the weight and cost of the heat exchanger. The therefore needed dust removing systems, like mechanical hammering system, reduces drastically dust separa tion efficiency and effectiveness which in turn reduces the heat recovery. The material resistance to high pressure and temperature conditions have led to increased costs and reduced unit availability. An alternative known approach known from Olumide Olumayegun, Matteo Marchionni, Muhammad Usman, Savvas A Tassou, "Influence Of Variations Of Flue Gas And Ambient Temperature On The Dynamics And Performance Of A Mw Scale Supercritical CO2 Waste Heat To Power Unit", 2023-SCO2.eu-139. They propose to mitigate the beforementioned challenges by involving a secondary loop utilizing a working fluid, as an intermediate heat transfer carrier is before the thermal energy is transferred into the supercritical CO2, although it may negatively impact the overall system efficiency and increase the lev-elized cost of electricity (LCOE) of the WHR system (WRHS). Hence, the object of the present invention is to provide an alternative solution, i.e., a method and an apparatus for providing a supercritical CO2 based waste heat recovery system having a higher mean temperature of heat addition, lower levelized cost of electricity and / or a higher availability. Summary of the invention In order to solve the afore-mentioned problems, the present invention regarding the method has the features of claim 1 and regarding the apparatus has the features of claim 6. Further preferable embodiments are mentioned in the dependent claims, whereby their features can be combined arbitrarily. The method for operating a waste heat recovery system of a supercritical carbon dioxide-based waste heat to power system for recovering or utilizing waste heat energy from a waste heat source comprises the steps of - transferring a first heat portion from the waste heat source to a first working fluid in a first heat exchanger and - transferring a second heat portion from the waste heat source to an intermediate working fluid in a second heat exchanger, wherein the first working fluid is a supercritical carbon dioxide based working fluid of the waste heat recovery system, and the intermediate working fluid is a thermal working fluid, which is different from the first working fluid, and - transferring in an intermediate heat exchanger thermal energy from the intermediate working fluid into the first working fluid. The inventive apparatus comprises the waste heat recovery system for a sCO2 based waste heat recovery system for recovering or utilizing waste heat energy from a waste heat source of an industry process solving the beforementioned objective comprises, comprising a waste heat recovering unit with a first heat exchanger embodied, i.e., configured and arranged for a transfer of a first heat portion from the waste heat source into a first working fluid, and with a second heat exchanger embodied for a transfer of a second heat portion from the waste heat source into an intermediate working fluid, wherein the first working fluid is a working fluid which is based on supercritical carbon dioxide, and the intermediate working fluid is a thermal working fluid which is a different medium from the first working fluid, wherein the waste heat recovery system comprises a intermediate heat exchanger, which is embodied for a transfer of a heat from the intermediate working fluid into the first working fluid. The terms "first" and "second" are used here simply to differentiate between various features. For example, the first heat portion and the second heat portion do not have to be transferred sequentially; they could also be transferred in parallel, or the second heat portion could be transferred into the working fluid before the first working fluid. Therefore, these terms should not be interpreted as limiting the scope of the claims. The invention is based on the knowledge, that to overcome the two drawbacks a hybrid system is most beneficial. In short, the hybrid system comprises in the conventional waste heat recovery unit two exchanger loops, the first based on thermal oil or other similar working fluid and the second based on supercritical CO2 . Having this, the heat transfer from the waste heat source is partially, i.e., the first part, directly transferred into the supercritical CO2 based working fluid by the first heat exchanger and partially, i.e., the second part, indirectly transferred into the supercritical CO2 based working fluid via the intermediate working fluid and two heat exchangers, the second heat exchanger and the intermediate heat exchanger. The first heat exchanger is embodied as a high temperature sCO2 heat exchanger and the second heat exchange is embodied as a conventional heat exchanger. The first heat exchanger recovers preferable the minor portion of waste heat and transfers it to the first working fluid which. The second heat exchanger will be employed to recover the major portion of waste heat from the flue gas where the waste heat will be first transferred to the intermediate working fluid, e.g., a thermic fluid like thermal oil, or the like, and then into first working fluid. The first heat exchanger will be used to increase the temperature of sCO2 working fluid directly from the waste heat source to maximum economically possible value closer to waste heat inlet temperature. Using the hybrid approach, it is possible to get the best of both prior art systems and at same time enhance the mean temperature of heat addition of the waste heat recovery system, which is very low in prior art relating to the indirect waste heat recovery. Additionally, this new approach not only solves the issue of utilizing a single, larger, and costly heat exchanger but also offers higher efficiency and availability of waste heat recovery system. The advantages of the new method and apparatus are the following : • Recovering waste heat at two temperature levels reduces the overall heat transfer area requirement thus reducing the overall cost. • Higher mean temperature of heat addition and thus higher efficiency and power output. The mean temperature of heat addition is way to approximate the average temperature at which heat is added during a thermodynamic process. It is calculated by taking the average of the temperatures at which heat is added at the boundaries of the process. • Lighter heat exchanger compared to direct heat exchange prior art and thus better dust removal efficiency and higher waste heat recovery unit availability. • Lower LCOE due to lower heat exchanger cost, higher system efficiency and waste heat recovery unit availability. The advantages which are given with the performance of the operating method relates in the same manner to the claimed apparatus . In accordance with a first preferred embodiment of the invention, the first working fluid is initially heated indirectly by the waste heat source through heat exchange via an intermediate working fluid before it is heated directly from the waste heat source. Therefore, in view of the flow direction of the first working fluid, the intermediate heat exchanger is located preferable directly upstream of the first heat exchanger. This multistage heating up of the supercritical CO2 based first working fluid leads to a finally higher temperature when the first working fluid leaves the waste heat recovery system. This increases the overall efficiency of the supercritical carbon dioxide-based waste heat to power system. According to another preferred embodiment during nominal operation of the first and second heat exchangers, the total of the first and second heat portion extracted from the waste heat source is normalized to 100%, wherein the first heat ex changer is configured to extract 5% - 30% of the total heat energy extracted from the waste heat source and the second heat exchanger is configured to extract 70% - 95% of the total heat energy extracted from the waste heat source. Hence, the first heat exchanger is embodied, i.e., configures and arranged to extract 5% - 30% of the total heat energy ex tracted of the waste heat source and the second heat exchanger is embodied to extract 70% - 95% of the total heat energy extracted of the flue gas. Such a split of indirect and di rect heat transfers enables higher efficiency and lower lev- elized costs of electricity. In accordance with another preferred embodiment of the invention the pressure of the second working fluid when leaving the second heat exchanger is in the range of 5 bar (0.5 MPa) - 15 bar (1,5 MPa). Hence, the intermediate heat exchanger is embodied to pass the intermediate working fluid, when leaving the intermediate heat exchanger, having a pressure in the range of 5 bar (0.5 MPa) - 15 bar (1,5 MPa). This pressure range defines the most efficient and economical operation ranges and limits the structural and mechanical requirements of the piping and heat exchanger pipe thickness to achieve higher efficiency, lower heat exchanger cost, higher dust removal efficiency &effectiveness and lower levelized costs of electricity. According to another preferred embodiment the pressure of the first working fluid when leaving the first heat exchanger is in the range of 150 bar (15 MPa) - 200 bar (20 MPa) and / or the temperature difference between the first working fluid when leaving the first heat exchanger (FHE) and the waste heat source temperature is in the range of 15°C - 100 °C corresponding to different temperatures of waste heat source. Hence, the first heat exchanger is embodied to pass the first working fluid, when leaving the first heat exchanger, having a pressure in the range of 150 bar (15 MPa) - 200 bar (20 MPa) and / or a temperature in the range of 250°C - 400°C. This pressure range and / or temperature range define the most efficient operation ranges and limits the structural and mechani cal requirements of the piping and heat exchanger pipe thick-ness to achieve higher efficiency, lower heat exchanger cost, higher dust removal efficiency &effectiveness and lower lev-elized costs of electricity. Most preferable the invention comprises a supercritical carbon dioxide waste heat recovery system., comprising a Brayton-cycle based waste heat to power system combined with a waste heat recovery system as described above. Brief Description of the Drawings Further advantages and features of the invention will be apparent from the following description based on the drawings. Thereby showing: Fig. 1 a first exemplary embodiment of a waste heat recovery system as part of the supercritical carbon dioxide-based waste heat to power system, Fig. 2 a second exemplary embodiment of a waste heat recovery system as part of the supercritical carbon dioxide-based waste heat to power system and Fig. 3 an alternative exemplary embodiment of the first exemplary embodiment. Detailed description of the various embodiments Multiple exemplary embodiments according to the present invention will be described below with reference to the drawings. In all drawings, the same features are provided with same reference signs. Each of the Figs. 1, 2 and 3, depicts schematically a super critical carbon dioxide (sCO2) based waste heat to power system SCO2WHTPS. The supercritical carbon dioxide sCO2 based waste heat to power system SCO2WHTE;S comprises different el ements: first, a waste heat to power unit WHTPU, second a heat rejection system HR and third a waste heat recovery system WHRS . The waste heat to power unit WHTPU comprises conventional elements like a first turbomachine having a high-pressure turbine HPT, which in operation drives via a common rotor ROT a compressor COM. It further comprises a low-pressure turbine LPT which in operation drives a generator GN for producing electrical energy. Next, the waste heat to power unit WHTPU comprises a recuperator REC and a cooler CLR. The different turbo-machineries and elements of the waste heat to power unit WHTPU are fluidly connected via different conduits A - G to expand, compress, cool, and to receive and forward a first working fluid FWF from and to the turbo-machineries of this loop. The first working fluid is based on supercritical carbon dioxide (sCO2). Hence, its essential ingredient for transferring the thermal energy is mainly sCO2 or pure sCO2 with unavoidable impurities. The detailed explanation of which of the different turbo-machineries of the waste heat to power unit WHTPU the different conduits A - G connect to will be provided further down. The second element, the heat rejection HR, consists of a cooling tower CT and suitable conduits M, N for establishing an open loop in which water can circulate towards and from the cooler CLR. In other embodiment, the heat rejection HR. may consist of fin fan cooler to cool the cooling water instead of cooling tower CT. The third element, the waste heat recovery system WHRS, which may differ slightly in accordance with the following exemplary embodiments, is depicted in the various figures. According to the Figs. 1-3, the waste heat recovery system WHRS includes a waste heat recovery unit WHRU. The waste heat recovery unit WHRU is designed to receive a fluid from a waste heat source to extract thermal energy, i.e., heat, from the fluid, transfer the heat to different working fluids, and exhaust the fluid of the waste heat source WHS through a suitable outlet as flue gas FG. For clarity, the fluid of the waste heat source and with that, the waste heat source are represented in the Figs. 1, 2 and 3by arrows WHS. The waste heat recovery unit WHRU also includes an intermediate heat exchanger IHE. Both are interconnected by conduits 0, P, through which an intermediate working fluid IWF can circulate in a closed loop to transfer the thermal energy extracted from the waste heat source WHS in the waste heat recovery unit WHRU to the intermediate heat exchanger IHE. The intermediate working fluid IWF is preferably thermal oil, but not based on sCO2. Finally, conduit G connects the recuperator REC and the intermediate heat exchanger IHE and conduit H connects the intermediate heat exchanger IHE and the waste heat recovery unit WHRU for transporting the first working fluid, the sCO2-based working fluid, from recuperator REC and the intermediate heat exchanger IHE to the waste heat recovery unit WHRU. It should be noted that, for the sake of clarity, any auxiliary pumps, valves, and other auxiliary parts and / or devices required for circulation and operation, particularly for starting and stopping the operation of the supercritical carbon dioxide-based waste heat to power system SCO2WHTPS, and / or the operation of the different working fluids, are not shown in all figures. Additionally, the terms heat and thermal energy are used exchangeable. During the normal operation of the supercritical carbon diox-ide-based waste heat to power system SCO2WHTPS in accordance with the first and second exemplary embodiment the first working fluid with low thermal energy enters at the downstream end of conduit G the intermediate heat exchanger IHE. In the intermediate heat exchanger IHE the first working fluid is heat up for the first time by thermal energy, which was extracted from the waste heat source WHS and transferred via the intermediate working fluid IWF into the intermediate heat exchanger IHE. In other words, the first working fluid FWF received indirectly the thermal energy from the waste heat 5 source WHS via the intermediate working fluid IWF. Thus, could be understood as a of the first working fluid to an intermediate temperature level. After leaving the intermediate heat exchanger IHE, the heated 10 first working fluid FWF is forward via conduit H to the waste heat recovery unit WHRU, in which thermal energy from the waste heat source WHS is directly transferred into the first working fluid FWF. With that, the first working fluid is directly substantially heated up again by thermal energy of the 15 waste heat source WHS. Latest here the first working fluid continues to be in supercritical condition having its highest temperature. In the next step the supercritical first working fluid FWF is 20 feed via conduit A into the high-pressure turbine HPT. By partially expanding the first working fluid in the high-pressure turbine the compressor COM is driven. The partially expanded first working fluid FWF is then forwarded through conduit B to the low-pressure turbine LPT, in which the first 25 working fluid is expanded again, for driving the electrical generator GN and generating power. After this, the first working fluid FWF is feed via conduit C into a recuperator REC, in which the remaining thermal energy 30 of the first working fluid is transferred into the first working fluid again as a kind of pre-heating, to increase the thermal energy of that part of the first working fluid, which has left the compressor COM. From the recuperator the cooled first working fluid FWF is feed via conduit D to the cooler 35 CLR, in which the first working fluid is cooled down to its lowest temperature of this cycle. From here, the first working fluid FWF is forwarded via conduit E to the compressor COM for increasing the pressure of the first working fluid. Then, the first working fluid FWF is forwarded by conduit F to the recuperator REC for recuperation. Finally, the recuperated first working fluid is feed via conduit G to the waste heat recovery system WHRS and is intermediate heat ex changer IHE to close the loop. The difference of the first and second exemplary embodiments lies in the structure and / or construction of the waste heat recovery unit WHRU. In accordance with the first exemplary embodiment (Fig. 1) the intermediate working fluid and the first working fluid are heated up by the thermal energy of the waste heat source simultaneously, i.e., in parallel. Within the waste heat recovery unit WHRU a first heat exchanger FHE and a second heat exchanger SHE is located, which of course can spatially overlap (not shown). The first heat exchanger FHE is on its secondary side embedded into the loop of the intermediate working fluid IWF, whereas the second heat exchanger SHE is on its secondary side embedded into the closed loop of the first working fluid. In contrast thereto, the second exemplary embodiment (Fig. 2) features a waste heat recovery unit WHRU comprising two subelements: a high temperature heat exchanger HTHE and a low temperature heat exchanger LTHE, through which the fluid of the waste heat source WHS passes sequentially, first through the high temperature heat exchanger HTHEi and then through the low temperature heat exchanger LTHE. In this case, the high temperature heat exchanger HTHE can be understood as a first heat exchanger FHE and the low temperature heat exchanger LTHE can be understood as the second heat exchanger SHE. During nominal operation of both exemplary embodiments, the total of the first and second heat portion extracted from the waste heat source WHS in the waste heat recovery unit WHRU is normalized to 100%, wherein the first heat exchanger FHE is configured to extract 5% -30% of the total heat energy extracted from the waste heat source WHS and the second heat exchanger SHE is configured to extract 70% - 95% of the total heat energy extracted from the waste heat source WHS. E.g. for a cement plant it has been found out, that a split of 80% and 20% for the indirect and direct thermal energy transfer from the waste heat source WHS into the first working fluid FWF within a supercritical carbon dioxide-based waste heat to power system SCO2WHTPS is most beneficial . The typical pressure of the intermediate working fluid ISF when leaving the second heat exchanger SHE is in the range of 5 bar (0.5 MPa) - 15 bar (1,5 MPa). Higher pressure values are more beneficial as the higher the pressure at this location, the higher is the efficiency of this cycle. In the same manner, the pressure of the first working fluid FWF when leaving the first heat exchanger FHE is in the range of 150 bar - 200 bar (15 MPa - 20 MPa) and / or the temperature difference between the first working fluid FWF when leaving the first heat exchanger FHE and the waste heat source temperature is in the range of 15 °C - 100 °C. According to the alternative embodiment of the first exemplary embodiment the high-pressure turbine HPT and the low-pressure turbine LPT are in consideration of the flow direction of the sCO2 based working fluid not in direct sequence. Instead, the first working fluid FWF heated in the intermediate heat exchanger IHE is feed via conduit R to the high-pressure turbine HPT, before it is feed through conduit S into the high temperature, the first heat exchanger of the waste heat recovery unit WHRU, where it is heated up again. After this, the first working fluid FWF is feed via conduit A' into the low-pressure turbine LPT. In summary, the invention relates to a waste heat recovery system WHRS for a supercritical carbon dioxide sCO2 based waste heat to power system SCO2WHTPS for recovering or utilizing waste heat energy, comprising a waste heat recovering unit WHRU with a first heat exchanger FHE for transferring of a heat portion from the waste heat source WHS to a sCO2 based working fluid FWF, and with a second heat exchanger SHE for transferring of another heat por-5 tion from the waste heat source WHS to an intermediate working fluid ISE, wherein the intermediate working fluid ISE is a thermal working fluid different from the sCO2 based working fluid FWF. To provide an alternative solution, i.e., a method and an apparatus for providing a sCO2 based waste heat recov-10 ery system having a higher efficiency, higher mean temperature of heat addition, lower levelized cost of electricity and / or a higher availability, it is proposed that the waste heat recovery system WHRS comprises an intermediate heat exchanger THE, which is embodied for a transfer of a heat from 15 the intermediate working fluid ISF into the sCO2 based working fluid FWF.

Claims

1. A method for operating a waste heat recovery system (WHRS) of a supercritical carbon dioxide (sCO2) based waste heat to power system (SCO2WHTPS) for recovering or utilizing waste heat energy from, a waste heat source (WHS), in particular flue gas (FG) of an industry process, comprising the steps of- transferring in a first heat exchanger (FHE) a first portion of heat from the waste heat source (WHS) into a first working fluid (FWF) and- transferring in a second heat exchanger (SHE) a second portion of heat from the waste heat source (WHS) into an intermediate working fluid (ISF),wherein the first working fluid (FWF) is a supercritical carbon dioxide (sCO2) based working fluid of the waste heat recovery system (WHRS), andthe intermediate working fluid (ISF) is a thermal working fluid, which is different from the first working fluid (FWF), and- transferring in an intermediate heat exchanger heat from the intermediate working fluid (ISF) into the first working fluid (FWF).

2. An operating method according to claim 1, wherein the first working fluid (FWF) is heated through heat exchange from, the intermediate working fluid (ISF) before it. is heated through heat, exchange from the waste heat source (WHS).

3. An operating method according to claim 1 or 2, during nominal operation of the first and second heat exchangers (FHE, SHE), the total of the first and second heat portion extracted from the waste heat source (WHS) is normalized, to 100%, wherein the first heat exchanger (FHE) is configured to extract 5% - 30% of the total heat energy extracted from the waste heat source (WHS) andthe second heat exchanger (SHE) is configured to extract 70% - 95% of the total heat energy extracted from the waste heat source (WHS).

4. An operating method according to one of claims 1 or 2, wherein the pressure of the intermediate working fluid (ISF) when leaving the second heat exchanger (SHE) is in the range of5 bar (0.5 MPa) - 15 bar (1,5 MPa).

5. An operating method according to one of the claims 1, 2 or 3, wherein the pressure of the first working fluid (FWF) when leaving the first heat exchanger (FHE) is in the range of 150 bar - 200 bar and / or the temperature difference between the first working fluid (FWF) when leaving the first heat exchanger (FHE) and the waste heat source temperature is in the range of 15°C - 100 °C corresponding to different temperatures of waste heat source.

6. A waste heat recovery system (WHRS) for a supercritical carbon dioxide (sCO2) based waste heat recovery system (SCO2WHTPS) for recovering or utilizing waste heat energy from a waste heat source (WHS) of an industry process, comprising a waste heat recovering unit (WHRU)with a first heat exchanger (FHE) embodied for a transfer of a first portion of heat from the waste heat source (WHS) into a first working fluid (FWF), andwith a second heat exchanger (SHE) embodied for a transfer of a second portion heat from the waste heat source (WHS) to an intermediate working fluid (ISF) ,wherein first working fluid (FWF) is the supercritical carbon dioxide of the waste heat recovery system (WHRS), andthe intermediate working fluid (ISF) is a thermal working fluid which is a different medium from the first working fluid (FWF), characterized in, that the waste heat recovery system (WHRS) comprises an intermediate heat exchanger (THE), which is embodied for a transfer of a heat from the intermediate working fluid (ISF) into the first working fluid (FWF).'7. A waste heat recovery system (WHRS) according to claim 6, wherein in view of the flow direction of the first working fluid (FWF), the intermediate heat exchanger (THE) is preferable directly upstream of the first heat exchanger (FHE).

8. A waste heat recovery system (WHRS) according to claim 6 or 7, wherein, during nominal operation of the waste heat recovery system (WHRS), the total of the first and second heat portion extracted from the waste heat source (WHS) is normalized to 100%, wherein the first heat exchanger (FHE) is embodied to extract 5% - 30% of the total heat energy extracted of the waste heat source (WHS) and the second heat exchanger (SHE) is embodied to extract 70% - 95% of the total heat energy extracted of the flue gas (FG).

9. A waste heat recovery system (WHRS) according to claim 6, 7 or 8, wherein the second heat exchanger (SHE) is embodied to pass the intermediate working fluid, when leaving the second heat exchanger (SHE), having a pressure in the range of5 bar (0.5 MPa) - 15 bar (1,5 MPa).

10. A waste heat recovery system (WHRS) to one of the claims 6 to 9, wherein the first heat exchanger (FHE) is embodied to pass the first working fluid (FWF), when leaving the5 first heat exchanger (FHE), having a pressure in therange of 150 bar (15MPa) - 200 bar (20 MPa) and / or a temperature in the range of 25 0°C --- 400°C.

11. A waste heat recovery system (WHRS) according to10 one of the claims 6 - 10, embodied to execute the operating method in accordance with one of the claims 1-5.12 . A supercritical carbon dioxide-based waste heat to power system (SCO2WHTPS), comprising a Brayton-cycle15 based waste heat to power unit (WHTPU) combined with awaste heat recovery system (WHRS) according to one of the claims 6 to 11.18

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