SYSTEM AND METHOD FOR STACK HEAT RECOVERY

Abstract

A system and method are presented for recovering heat from flue gas produced by a recovery boiler in a pulp and paper mill, the recovery boiler having a flue stack. Flue gas is drawn from the flue stack and passed through a first and second condensing heat exchangers before exiting through a separate stack. The first heat exchanger is used to heat boiler feedwater whereas the second heat exchanger is used to produce process hot water for use in the pulp and paper mill. Steam previously used to heat the boiler feedwater and produce the process hot water can now be used to generate electricity that can be used for the pulp and paper mill operations or to export to the electrical power grid.

Claims

1. A heat recovery system for use with a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising: a) a first fan configured to draw flue gas away from the flue gas stack; b) at least one first heat exchanger operatively coupled to the first fan and configured to receive the drawn flue gas to pass therethrough; c) at least one second heat exchanger operatively coupled to the at least one first heat exchanger and configured to receive the drawn flue gas to pass therethrough after passing through the at least one first heat exchanger; and d) a second flue gas stack operatively coupled to the at least one second heat exchanger and configured to receive the drawn flue gas after passing through the at least one second heat exchanger.

2. The heat recovery system as set forth in claim 1, wherein the at least one first heat exchanger is configured to heat boiler feedwater.

3. The heat recovery system as set forth in claim 2, wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled together sequentially.

4. The heat recovery system as set forth in claim 1, wherein the at least one second heat exchanger is configured to heat process hot water for use in the pulp and paper mill.

5. The heat recovery system as set forth in claim 4, wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled together sequentially.

6. The heat recovery system as set forth in claim 1, wherein the at least one first exchanger is disposed above the at least one second heat exchanger, and wherein the drawn flue gas is directed downward through the at least one first heat exchanger and the at least one second heat exchanger.

7. The heat recovery system as set forth in claim 1, further comprising a wash system configured to wash off one or both of precipitate and the condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

8. The heat recovery system as set forth in claim 7, wherein the wash system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, the at least one spray nozzle configured to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger.

9. The heat recovery system as set forth in claim 1, wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger comprises a condensing heat exchanger.

10. The heat recovery system as set forth in claim 9, further comprising a condensation collector disposed beneath one or both of the at least one first heat exchanger and the at least one second heat exchanger, the condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

11. The heat recovery system as set forth in claim 10, further comprising a wash system configured to use at least some of the collected condensation to wash off one or both of precipitate and condensate off one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

12. The heat recovery system as set forth in claim 11, wherein the wash system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, the wash system comprising a first pump configured to pump the collected condensation through the at least one spray nozzle, thereby washing one or both of the at least one first heat exchanger and the at least one second heat exchanger.

13. The heat recovery system as set forth in claim 1, wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are comprised of one or both of stainless steel and titanium.

14. The heat recovery system as set forth in claim 13, wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are comprised of SAF 2205? stainless steel.

15. A method for recovering heat from a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the method comprising: a) drawing flue gas away from the flue gas stack; b) heating boiler feedwater with the drawn flue gas; c) then heating process hot water with the drawn flue gas; and d) then exiting the drawn flue gas to the atmosphere.

16. The method as set forth in claim 15, comprising heating the boiler feedwater with at least one first heat exchanger.

17. The method as set forth in claim 16, wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled together sequentially.

18. The method as set forth in claim 14, further comprising heating the process hot water with at least one second heat exchanger.

19. The method as set forth in claim 18, wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled together sequentially.

20. The method as set forth in claim 15, further comprising washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough.

21. The method as set forth in claim 20, further using at least one spray nozzle for washing one or both of the at least one first heat exchanger and the at least one second heat exchanger.

22. The method as set forth in claim 20, further comprising washing the at least one first heat exchanger and the at least one second heat exchanger sequentially.

23. The method as set forth in claim 15, further comprising collecting condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

24. The method as set forth in claim 22, further comprising washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger with at least some of the collected condensation, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough.

25. The method as set forth in claim 22, further comprising pumping the at least some of the collected condensation through at least one spray nozzle to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger.

26. The method as set forth in claim 22, further comprising washing the at least one first heat exchanger and the at least one second heat exchanger sequentially.

27. A heat recovery system for use with a recovery boiler system in a pulp and paper mill, the recovery boiler comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising: a) means for drawing flue gas away from the flue gas stack; b) means for heating boiler feedwater with the drawn flue gas; c) means for heating process hot water with the drawn flue gas; and d) means for exiting the drawn flue gas to the atmosphere.

28. The heat recovery system as set forth in claim 27, further comprising means for washing one or both of precipitate and condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough.

29. The heat recovery system as set forth in claim 27, further comprising means for collecting condensation produced by water vapour disposed in the drawn flue gas.

30. The heat recovery system as set forth in claim 29, further comprising means for washing one or both of precipitate condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water with the collected condensation, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 is a block diagram depicting a prior art pulp and paper mill recovery boiler system.

[0045] FIG. 2 is a block diagram depicting a simplified first embodiment of a system for recovering heat from flue gas from a pulp and paper mill recovery boiler system.

[0046] FIG. 3 is a block diagram depicting a simplified second embodiment of a system for recovering heat from flue gas from a pulp and paper mill recovery boiler system.

[0047] FIG. 4 is a block diagram depicting a third embodiment of a system for recovering heat from flue gas from a pulp and paper mill recovery boiler system.

[0048] FIG. 5 is a block diagram depicting a heat exchanger wash system for the heat recovery system of FIG. 4.

[0049] FIG. 6 is a block diagram depicting a fourth embodiment of a system for recovering heat from flue gas from a pulp and paper mill recovery boiler system.

[0050] FIG. 7 is a block diagram depicting a heat exchanger wash system for the heat recovery system of FIG. 6.

[0051] FIG. 8 is a block diagram depicting a control system for the heat recovery systems of FIGS. 4 to 7.

DETAILED DESCRIPTION OF EMBODIMENTS

[0052] In this description, references to one embodiment, an embodiment, or embodiments mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to one embodiment, an embodiment, or embodiments in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment can also be included in other embodiments but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein.

[0053] Referring to FIGS. 2 and 3, simplified embodiments of stack heat recovery system 10 is shown. In its simplest configuration, in some embodiments, flue gas can be drawn away from recovery boiler stack 102, via duct 12, by fan 14 to force the flue gas to flow through first heat exchanger 16 and then through second heat exchanger 18. After passing through heat exchangers 16 and 18, the flue gas can exit to the atmosphere through second stack 20.

[0054] In some embodiments, first heat exchanger 16 can be comprised of stainless steel and can be used to convert the sensible heat in the flue gas to heat boiler feedwater for use in recovery boiler 100. In some embodiments, first heat exchanger 16 can be comprised of SAF 2205? stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. SAF 2205? stainless steel is known to have high resistance to stress corrosion cracking chloride-bearing and hydrogen sulphide environments, and high resistance to general corrosion and corrosion fatigue. The steam previously used to heat the boiler feedwater in the prior art system, as shown in FIG. 1, can then be used to generate electrical power by running the steam through a steam turbine operatively coupled to an electrical generator (not shown), as well known to those skilled in the art. In a representative example, the steam previously used to heat the boiler feedwater can produce approximately 2.7 megawatts of power.

[0055] In some embodiments, second heat exchanger 18 can be comprised of stainless steel can be used to convert the sensible and latent heat in the flue gas to produce process hot water for use in operations in the pulp and paper mill. In some embodiments, second heat exchanger 18 can be comprised of SAF 2205? stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. The steam previously used to produce the process hot water in the prior art system, as shown in FIG. 1, can then be used to generate electrical power by running the steam through a steam turbine operatively coupled to an electrical generator (not shown), as well known to those skilled in the art. In a representative example, the steam previously used to produce the process hot water can produce approximately 6.0 megawatts of power. Thus, the implementation of this heat recovery system can then free up the steam previously used in the prior art system to produce approximately 8.7 megawatts of power, in the representative example. It would be obvious to those skilled in the art that amount of power produced can be scaled up or down depending on the size the recovery boiler and the amount of flue gas produced therefrom.

[0056] Referring to FIGS. 4 and 5, a third embodiment of stack heat recovery system 10 is shown. In the illustrated embodiment shown in FIG. 4, system 10 can draw flue gas away from recovery boiler stack 102 with VFD-operated fan 14. In some embodiments, system 10 can comprise damper inlet guillotine valve 13, which can be used to open and close the flow path from stack 102 to fan 14 to allow for operational and maintenance procedures on system 10. From fan 14, the flue gas can flow through ducting 15 to enter into stack structure 17 where heat exchangers 16 and 18 are disposed therein in a vertical configuration wherein first heat exchanger 16 is disposed above second heat exchanger 18 such that the flue gas flows downward from the top of first heat exchanger 16 through to the bottom of second heat exchanger 18. From there, the flue gas can flow through ducting 19 and exit to atmosphere 50 via second stack 20.

[0057] In some embodiments, system 10 can comprise two sets of heat exchangers 16 and 18 operatively configured in parallel vertical structures 17, as shown in FIGS. 4 and 5. Having multiple vertical structures 17 can be done to scale up the volume of flue gas processed by system 10 or can be done for practical reasons in terms of the logistics of shipping heat exchangers to a site where system 10 will be implemented. It can also be done for redundancy wherein one vertical structure 17 can be shut down for maintenance or repaid while the other vertical structure 17 remains in operation.

[0058] In some embodiments, boiler feedwater can enter first heat exchangers 16 via piping 22. Boiler feedwater to be heated can be pumped into inlet pipe 22a to enter heat exchanger inlets 16a, wherein heated boiler feedwater can exit via heat exchanger outlets 16b to be pumped via outlet pipe 22b to recovery boiler 100.

[0059] In some embodiments, process hot water can enter second heat exchangers 18 via piping 26. Process hot water to be heated can be pumped into inlet pipe 26a to enter heat exchanger inlets 18 in, wherein heated process hot water can exit via heat exchanger outlets 18out to be pumped via outlet pipe 26b for use in pulp and paper mill operations.

[0060] In the illustrated embodiment shown in FIG. 5, system 10 can comprise heat exchanger wash system 30 that can be configured to clean precipitate and condensate off of heat exchangers 16 and 18 that can accumulate thereon over time as flue gas passes therethrough. In some embodiments, wash system 30 can comprise of sump 34 that can be used to hold fluid, such as water or flue gas condensate that can form on heat exchangers 16 and 18. The flue gas condensate can accumulate in condensate collectors 32 disposed on the lower ends of vertical structures 17 due to gravity. The flue gas condensate can then be directed to sump 34 via piping 36. In some embodiments, fluid pump 40 can be used to draw flue gas condensate from sump 34 and pump it through piping 44 to be dispensed through spray nozzles 38a-38b disposed in heat exchangers 16 and 18. Valves 45a-45d can be opened and closed as needed to selectively operate one of spray nozzles 38a-38d in accordance with a predetermined wash sequence or protocol.

[0061] As flue gas flows downward through heat exchangers 16 and 18, the precipitate that can accumulate thereon can tend to accumulate more towards the top of heat exchangers 16 because of the first contact area and pressure drop as the flue gas contacts heat exchangers 16. In some embodiments, a wash sequence can include first pumping fluid, such as water or flue gas condensate, through spray nozzles 38a to first clean off heavy accumulation of condensate from the bottom of heat exchangers 18 so as to enable circulation therethrough again, and then spraying fluid sequentially through spray nozzles 38d, then spray nozzles 38c, then spray nozzles 38b and then spray nozzles 38a once again. In some embodiments, the wash sequence can occur as needed or on a pre-determined time schedule such as every 12 to 24 hours, or more, depending on how much soot and minerals are suspended in the flue gas as it exits from recovery boiler 100. Once a wash sequence is completed, drain valve 43 can be opened to allow fluid within piping 44 to drain back to sump 34.

[0062] Referring to FIGS. 6 and 7, a fourth embodiment of stack heat recovery system 10 is shown. In the illustrated embodiment shown in FIG. 5, system 10 can draw flue gas away from recovery boiler stack 102 with VFD-operated fan 14. In some embodiments, system 10 can comprise damper inlet guillotine valve 13, which can be used to open and close the flow path from stack 102 to fan 14 to allow for operational and maintenance procedures on system 10. From fan 14, the flue gas can flow through ducting 15 to enter into stack structure 17 where heat exchangers 16 and 18 are disposed therein in a vertical configuration wherein first heat exchanger 16 is disposed above second heat exchanger 18 such that the flue gas flows downward from the top of first heat exchangers 16 through to the bottom of second heat exchangers 18. From there, the flue gas can flow through ducting 19 and exit to atmosphere 50 via second stack 20.

[0063] In some embodiments, first heat exchangers 16 can comprise condensing heat exchangers comprised of material that is resistant to corrosion. In some embodiments, first heat exchangers 16 can be comprised of stainless steel, titanium or other corrosion-resistant materials as well known to those skilled in the art. In some embodiments, first heat exchangers 16 can be comprised of SAF 2205? stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. In the illustrated embodiment shown in FIG. 6, second heat exchanger 18 can comprise of three separate heat exchangers operatively coupled together sequentially, labelled as 18a, 18b and 18c. In some embodiments, second heat exchangers 18 can comprise condensing heat exchangers comprised of material that is resistant to corrosion. In some embodiments, second heat exchangers 18 can be comprised of stainless steel, titanium or other corrosion-resistant materials as well known to those skilled in the art. In some embodiments, second heat exchangers 18 can be comprised of SAF 2205? stainless steel, as manufactured by Sandvik AB, of Stockholm, Sweden.

[0064] In some embodiments, system 10 can comprise two sets of heat exchangers 16 and 18 operatively configured in parallel vertical structures 17, as shown in FIGS. 6 and 7. Having multiple vertical structures 17 can be done to scale up the volume of flue gas processed by system 10 or can be done for practical reasons in terms of the logistics of shipping heat exchangers to a site where system 10 will be implemented. It can also be done for redundancy wherein one vertical structure 17 can be shut down for maintenance or repaid while the other vertical structure 17 remains in operation.

[0065] In some embodiments, boiler feedwater can enter first heat exchangers 16 via piping 22. Boiler feedwater to be heated can be pumped into inlet pipe 22a to enter heat exchanger inlets 16a, wherein heated boiler feedwater can exit via heat exchanger outlets 16b to be pumped via outlet pipe 22b to recovery boiler 100.

[0066] In some embodiments, process hot water can enter second heat exchangers 18 via piping 26. Process hot water to be heated can be pumped into inlet pipe 26a to enter heat exchanger inlets 18in of heat exchangers 18a, wherein process hot process can flow sequentially through heat exchangers 18a to 18c to be heated and can then exit via heat exchanger outlets 18out to be pumped via outlet pipe 26b for use in pulp and paper mill operations.

[0067] In the illustrated embodiment shown in FIG. 7, system 10 can comprise heat exchanger wash system 30 that can be configured to clean precipitate and condensate off of heat exchangers 16 and 18a to 18c that can accumulate thereon over time as flue gas passes therethrough. In some embodiments, wash system 30 can comprise of sump 34 that can be used to hold fluid, such as water or flue gas condensate that can form on heat exchangers 16 and 18a to 18c. The flue gas condensate can accumulate in condensate collectors 32 disposed on the lower ends of vertical structures 17 due to gravity. The flue gas condensate can then be directed to sump 34 via piping 36. In some embodiments, fluid pump 40 can be used to draw flue gas condensate from sump 34 and pump it through piping 44 to be dispensed through spray nozzles 38a-38b disposed in heat exchangers 16 and 18a to 18c, one set of spray nozzles per heat exchanger as an example. Valves 45a-45d can be opened and closed as needed to selectively operate one of spray nozzles 38a-38d in accordance with a predetermined wash sequence or protocol.

[0068] As flue gas flows downward through heat exchangers 16 and 18a to 18c, the precipitate that can accumulate thereon can tend to accumulate more towards the top of heat exchangers 16 because of the first contact area and pressure drop as the flue gas contacts heat exchangers 16. In some embodiments, a wash sequence can include first pumping fluid, such as water or flue gas condensate, through spray nozzles 38a to first clean off heavy accumulation of condensate from the bottom of heat exchangers 18a so as to enable circulation therethrough again, and then spraying fluid sequentially through spray nozzles 38d, then spray nozzles 38c, then spray nozzles 38b and then spray nozzles 38a once again to cycle through all of heat exchanges 18a to 18c. In some embodiments, the wash sequence can occur as needed or on a pre-determined time schedule such as every 12 to 24 hours, or more, depending on how much soot and minerals are suspended in the flue gas as it exits from recovery boiler 100. Once a wash sequence is completed, drain valve 43 can be opened to allow fluid within piping 44 to drain back to sump 34.

[0069] Referring to FIG. 8, a simplified block diagram for control system 46 is shown for controlling the operation of system 10. In some embodiments, control system 46 can comprise computing control device 48 that can comprise of one or more of a programmable logic controller (PLC), a general purpose computer, a microcontroller and a microprocessor-based computing device configured for controlling the subcomponents let of system 10. In some embodiments, computing control device 48 can be operatively to one or more of guillotine valve 13, VFD-operated fan 14, fluid pump 40, drain valve 43 and nozzle control valves 45a to 45d. In some embodiments, computing control device 48 can be used to control the operation of guillotine valve 13 and VFD-operated fan 14 to regulate and control the volume of the flue gas flowing through system 10. In some embodiments, computing control device 48 can be used to control wash system 40 through the operation of fluid pump 40 and the operation of spray nozzle valve 45a to 45d in accordance with a pre-determined wash sequence of heat exchangers 16 and 18 to remove precipitate or condensate therefrom on a pre-determined time schedule for wash operations or in response to how much precipitate or condensate has accumulated on heat exchangers 16 and 18. In some embodiments, computing control device 48 can be used to open drain valve 43 to drain piping 44 after a wash operation of heat exchangers 16 and 18.

[0070] The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans can implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the embodiments described herein.

[0071] Embodiments implemented in computer software can be implemented in software, firmware, middleware, microcode, hardware description languages, or any combination thereof. A code segment or machine-executable instructions can represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment can be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. can be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc.

[0072] The actual software code or specialized control hardware used to implement these systems and methods is not limiting of the embodiments described herein. Thus, the operation and behavior of the systems and methods were described without reference to the specific software code being understood that software and control hardware can be designed to implement the systems and methods based on the description herein.

[0073] When implemented in software, the functions can be stored as one or more instructions or code on a non-transitory computer-readable or processor-readable storage medium. The steps of a method or algorithm disclosed herein can be embodied in a processor-executable software module, which can reside on a computer-readable or processor-readable storage medium. A non-transitory computer-readable or processor-readable media includes both computer storage media and tangible storage media that facilitate transfer of a computer program from one place to another. A non-transitory processor-readable storage media can be any available media that can be accessed by a computer. By way of example, and not limitation, such non-transitory processor-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible storage medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer or processor. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm can reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable medium and/or computer-readable medium, which can be incorporated into a computer program product.

[0074] Although a few embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, intent or functionality. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the invention is defined and limited only by the claims that follow.