Flare pilot with FLUE gas recirculation

Abstract

A method and apparatus for operating a flare stack pilot with a fuel having a high flame speed includes a structure which is operable to recirculate products of combustion to be mixed with the incoming fuel to dilute the incoming fuel to reduce the flame speed of the resulting mixture.

Description

FLARE PILOT WITH FLUE GAS RECIRCULATIONTECHNICAL BACKGROUND

[0001] This disclosure is directed to a flare pilot structure that utilizes a recycled stream of treated flue gas to reduce the flame speed of the pilot fuel to mitigate flashback. More specifically, this disclosure is related to a flare pilot structure where combustion products are taken from a pilot head of the flare and routed to a pilot mixer where the stream is mixed with combustion air and fuel.BACKGROUND

[0002] Flare pilot technology allows for a continuous source of ignition for emergency release of waste gases via industrial flares. It is advantageous for the pilot technology be compatible with a wide range of fuels. Modern flare pilot technology typically uses the fuel pressure energy of a gas fuel (e.g. natural gas, refinery gas, etc.) to entrain combustion air through a mixer. Fuel and air are premixed as they flow through a supply pipe and routed to a pilot head where the blend is ignited and stabilized to act as a flare pilot. The flare pilot acts a source of ignition for waste gases passing through the main flare. This method allows for the entrainment of air well below the main flare exhaust preventing the pilot mixer entraining other gases such as flare waste gases which could result in reliability issues.

[0003] Because the air and fuel are premixed, the supply pipe is susceptible to flashback that occurs when the fuel blend burns inside the pipe. This occurs when the flame speed is greater than the pipe velocity, and results in overheating of components. Fuel compatibility issues can occur when gas species with high flame speed are present in high concentrations in the gas fuel. Specifically, adding high hydrogen (H2) concentrations - which has a very high flame speed - can result in flashback. Due to this, many pilot technologies limit the maximum hydrogen volume fraction to avoid flashback.SUMMARY

[0004] The present disclosure includes one or more of the features recited in the appended claims and / or the following features which, alone or in any combination, may comprise patentable subject matter.

[0005] According to a first aspect of the present disclosure, a flare pilot for a flare stack includes a fuel line, an air inlet coupled to the fuel line and operable to entrain air in with the fuel line to form an air-fuel mixture, a fuel-air mixture inlet pipe coupled to the air inlet, a pilot head coupled to the inlet pipe and operable to combust the fuel-air mixture, a flue gas recirculation line coupled to the pilot head, and a pilot mixer coupled to a fuel line prior to the air inlet, the pilot mixer further coupled to the flue gas recirculation line and operable to draw from the flue gas recirculation line and mix flue gas with pilot fuel.

[0006] In some embodiments of the first aspect, the flue gas recirculation line is configured to collect inert gases from the combustion products of the pilot head.

[0007] In some embodiments of the first aspect, the flue gas recirculation line is configured to provide cooling of the flue gases passing through the flue gas recirculation line.

[0008] In some embodiments of the first aspect, the pilot mixer is configured to create a vacuum to draw the inert gases from the combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

[0009] In some embodiments of the first aspect, the pilot mixer comprises a venturi.

[0010] According to a second aspect of the present disclosure, a flare pilot for a flare stack includes a pilot head operable to combust a fuel-air mixture, a flue gas recirculation line coupled to the pilot head, and a pilot mixer operable to receive high pressure fuel, create a vacuum in the flue gas recirculation line to draw products of the combustion of the fuel-air mixture into the mixer, and mix the high pressure fuel and products of combustion form a diluted fuel gas to be used in the fuel-air mixture at the pilot head.

[0011] In some embodiments of the second aspect, the pilot mixer comprises a venturi.

[0012] In some embodiments of the second aspect, the flare pilot further includes an air mixer coupled to the pilot mixer, wherein the air mixer is operable to entrain air in the diluted fuel gas to form the fuel-air mixture.

[0013] In some embodiments of the second aspect, the flare pilot further includes a heat exchanger operable to cool the flue gas recirculation line.

[0014] According to a third aspect of the present disclosure, a method of mitigating flashback in a flare pilot includes the steps of operating a flare pilot head by combusting an air-fuel combustion mixture to form products of combustion, and recirculating a portion of the products of the combustion with an additional air-fuel combustion mixture being fed to the flare pilot head to reduce the flame speed of the fuel used in the air-fuel combustion mixture.

[0015] In some embodiments of the third aspect, the method further includes a step of mixing combustion air and flare pilot fuel in a pilot inlet pipe to form the air-fuel combustion mixture.

[0016] In some embodiments of the third aspect, the method further includes a step of routing the portion of the products of combustion at the flare pilot head to a pilot mixer.

[0017] In some embodiments of the third aspect, the method further includes a step of recirculating the portion of the products of the combustion through a recirculation circuit.

[0018] In some embodiments of the third aspect, the method further includes a step of cooling the portion of the products of combustion in the recirculation circuit.

[0019] In some embodiments of the third aspect, the step of cooling the portion of the products of the combustion in the recirculation circuit includes convective cooling.

[0020] In some embodiments of the third aspect, the step of cooling the portion of the products of the combustion in the recirculation circuit includes radiation cooling.

[0021] In some embodiments of the third aspect, the method further comprises a step of forming a vacuum in the recirculation circuit to draw the products of the combustion into the air-fuel combustion mixture.

[0022] In some embodiments of the third aspect, the step of forming the vacuum comprises directing the flow of combustion gas through a pressure differential to generate the vacuum in the recirculation circuit.

[0023] In some embodiments of the third aspect, the pressure differential forms a venturi.

[0024] Additional features, which alone or in combination with any other feature(s), such as those listed above and / or those listed in the claims, can comprise patentable subject matter and will become apparent to those skilled in the art upon consideration of the following detailed description of various embodiments exemplifying the best mode of carrying out the embodiments as presently perceived.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The detailed description particularly refers to the accompanying figures in which:

[0026] Fig. 1 is a diagrammatic representation of an industrial flare of the present disclosure, the industrial flare including a pilot that has a recirculatory approach to directing a portion of the flue gas to a pilot fuel mixer to mix the flue gas with pilot fuel to dilute the pilot fuel and reduce the potential for flashback in the pilot; and

[0027] Fig. 2 is a diagrammatic representation of a pilot fuel mixer that employs a venturi to draw flue gas from the flare pilot head.DETAILED DESCRIPTION

[0028] The present disclosure is directed to the use of the products from combustion in a pilot for a flare tip as additives to the pilot fuel to reduce the flame speed of the pilot fuel, thereby mitigating the potential for flashback in the flare pilot. This technology targets a wider range of fuel compositions compared to existing technology, specifically, very high (up to 100%) levels of hydrogen as well as natural gas and refinery gas blends. Referring now to Fig. 1, a system 100 for combusting waste gas includes a flare stack 10 that has a flare 12. A flare pilot 16 is positioned at a flare tip 14 of the flare 12, asis known in the art. The flare stack 10 receives waste gas from a waste gas inlet 56, the waste gas being ignited by the flare pilot 16 to cause the waste gas to be combusted.

[0029] The present disclosure provides a structure and method for operating the system 100 with a focus on reducing the potential for flashback in the flare pilot 16. The disclosed system 100 includes typical structures, but is modified to include a capability for recirculating a portion of the flue gas generated by the flare pilot 16, the recirculated flue gas being used to mix with incoming pilot fuel so as to reduce the flame speed of the pilot fuel. In particular, the system 100 is configured to operate with pilot fuel that comprises 100% hydrogen, with the recirculated flue gas being introduced into the fuel stream so that the inert byproducts of combustion that are introduced mix with the hydrogen fuel. It has been found, for example, that providing the recirculated flue gas at a mixture rate of approximately 15% relative to the hydrogen fuel, significantly reduces the flame speed of the hydrogen fuel, mitigating the risk for flashback into a combustion air-fuel feed stand 38.

[0030] The system 100 includes a pilot ignitor 60 that includes an ignition fuel inlet 62 that is connector to an ignitor controller 64. Ignition fuel is fed to the ignitor controller 64 and ignited fuel is transferred through an ignition line 66 to provide the initial ignition for the flare pilot 16. The flare pilot 16 is operated by receiving pilot fuel at a pilot fuel inlet 18 which is connected to a pilot fuel line 20. As in typical applications, the pilot fuel line 20 is connected to a fuel-air mixer 26 which is operable to entrain ambient air into the pilot fuel with the mixture being conducted to the flare pilot 16 through the air-fuel feed stand 38. However, the present disclosure includes a pilot fuel mixer 32 that receives the pilot fuel from the pilot fuel inlet 18. The mixer 32 is connected to a flue gas recirculation line 28 at a tap 40. The mixer 32 is operable to draw from the flue gas recirculation line 28 and mix flue gas with the pilot fuel. The mixer 32 is configured to create a vacuum in the flue gas recirculation line 28. The flue gas recirculation line 28 is coupled to the flare pilot head 30 by a flue gas recirculation tap 36. The vacuum created in the flue gas recirculation line 28 is applied to the flare pilot head 30 so that a portion of the products of combustion at the flare pilot 16 are drawn into the flue gas recirculation tap 36 by the mixer 32.

[0031] The flue gas recirculation line 28 engages with a flue gas recirculation line heat exchanger 34 which is operable to remove heat from the flue gas recirculation line 28, thereby reducing the temperature of the recirculated flue gas before it is introduced into the pilot fuel stream at the mixer 32. The heat exchanger 34 is shown diagrammatically. In some embodiments, the heat exchanger 34 could be configured to provide radiant heat exchange from the flue gas recirculation line 28. In other embodiments, the heat exchanger 34 may be a convective heat exchanger utilizing a water jacket, for example.

[0032] Referring now to Figure 2, a diagrammatic representation of the pilot fuel mixer 32 connected to the tap 40, the pilot fuel inlet 18, and the pilot fuel line 20 shows that the mixer 32 may be configured as venturi 42. As shown in Figure 2, the pilot fuel inlet 18 enters an enclosure 44 and feeds an inlet 46 of the venturi 42 within the enclosure 44. The pilot fuel line 20 is coupled to the outlet 48 of the venturi 42 within the enclosure 44. The tap 40 is coupled to the venturi tube 50 so that the low pressure generated by the venturi 42 tends to cause a vacuum to be created in the tap 40 which is transferred to the flue gas recirculation line 28, causing a vacuum in the flue gas recirculation line 28 that draws a portion of the products of combustion from the flue gas recirculation tap 36 at the flare pilot head 30. The venturi 42 is configured such that the normal operating pressure of the pilot fuel introduced into the pilot fuel inlet 18 creates an appropriate vacuum to introduce inert components of combustion to affect the appropriate reduction in the flame speed of the pilot fuel. As discussed above, in one example, introducing inert products of combustion at a ratio of about 15% of the pilot fuel provides an appropriate reduction of flame speed when 100% hydrogen fuel is used as the pilot fuel.

[0033] It should be understood that the use of the venturi 42 is one method of utilizing the flow of pilot fuel to create a vacuum in the flue gas recirculation line 28. Other arrangements may also to be used to create the vacuum in the flue gas recirculation line 28. A benefit of the use of a mechanism for creating a pressure drop in the flue gas recirculation line 28 based on the flow of pilot fuel is that such an arrangement reduces the likelihood of errant pilot fuel from being introduced into the flue gas recirculation line 28 and delivered to the flare pilot 16.

[0034] Although this disclosure refers to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the subject matter set forth in the accompanying claims. For example, while the disclosure has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The disclosure is not limited to the disclosed embodiments. From reading the present disclosure, other modifications will be apparent to a person skilled in the art. Such modifications may involve other features, which are already known in the art and may be used instead of or in addition to features already described herein. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.

[0035] While only certain features of the described apparatus have been illustrated and described in this application, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes that fall within the true spirit of the invention.

[0036] The following numbered clauses include embodiments that are contemplated and non-limiting:

[0037] Clause 1. A flare pilot for a flare stack comprising a fuel line.

[0038] Clause 2. The flare pilot of clause 1, any other clause, or any combination of clauses, wherein the flare pilot comprises an air inlet coupled to the fuel line and operable to entrain air in with the fuel line to form an air-fuel mixture.

[0039] Clause 3. The flare pilot of clause 2, any other clause, or any combination of clauses, wherein the flare pilot comprises a fuel-air mixture inlet pipe coupled to the air inlet.

[0040] Clause 4. The flare pilot of clause 3, any other clause, or any combination of clauses, wherein the flare pilot comprises a pilot head coupled to the fuel-air mixture inlet pipe and operable to combust a fuel-air mixture.

[0041] Clause 5. The flare pilot of clause 4, any other clause, or any combination of clauses, wherein the flare pilot comprises a flue gas recirculation line coupled to the pilot head.

[0042] Clause 6. The flare pilot of clause 5, any other clause, or any combination of clauses, wherein the flare pilot comprises a pilot mixer coupled to the fuel line prior to the air inlet, the pilot mixer further coupled to the flue gas recirculation line and operable to draw from the flue gas recirculation line and mix flue gas with pilot fuel.

[0043] Clause 7. The flare pilot of clause 6, any other clause, or any combination of clauses, wherein the flue gas recirculation line is configured to collect inert gases from combustion products of the pilot head.

[0044] Clause 8. The flare pilot of clause 7, any other clause, or any combination of clauses, wherein the flue gas recirculation line is configured to provide cooling of flue gases passing through the flue gas recirculation line.

[0045] Clause 9. The flare pilot of clause 8, any other clause, or any combination of clauses, wherein the pilot mixer is configured to create a vacuum to draw the inert gases from the combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

[0046] Clause 10. The flare pilot of clause 9, any other clause, or any combination of clauses, wherein the pilot mixer comprises a venturi.

[0047] Clause 11. The flare pilot of clause 7, any other clause, or any combination of clauses, wherein the pilot mixer is configured to create a vacuum to draw the inert gases from the combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

[0048] Clause 12. The flare pilot of clause 11, any other clause, or any combination of clauses, wherein the pilot mixer comprises a venturi.

[0049] Clause 13. The flare pilot of clause 6, wherein the pilot mixer is configured to create a vacuum to draw inert gases from combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

[0050] Clause 14. The flare pilot of clause 13, any other clause, or any combination of clauses, wherein the pilot mixer comprises a venturi.

[0051] Clause 15. A flare pilot for a flare stack comprising a pilot head operable to combust a fuel-air mixture.

[0052] Clause 16. The flare pilot of clause 15, any other clause, or any combination of clauses, wherein the flare pilot comprises a flue gas recirculation line coupled to the pilot head.

[0053] Clause 17. The flare pilot of clause 16, any other clause, or any combination of clauses, wherein the flare pilot comprises a pilot mixer operable to receive high pressure fuel, create a vacuum in the flue gas recirculation line to draw products of the combustion of the fuel-air mixture into the pilot mixer, and mix the high pressure fuel and products of combustion form a diluted fuel gas to be used in the fuel-air mixture at the pilot head.

[0054] Clause 18. The flare pilot of clause 17, any other clause, or any combination of clauses, wherein the pilot mixer comprises a venturi.

[0055] Clause 19. The flare pilot of clause 18, any other clause, or any combination of clauses, further comprising an air mixer coupled to the pilot mixer, wherein the air mixer is operable to entrain air in the diluted fuel gas to form the fuel-air mixture.

[0056] Clause 20. The flare pilot of clause 19, any other clause, or any combination of clauses, further comprising a heat exchanger operable to cool the flue gas recirculation line.

[0057] Clause 21. A method of mitigating flashback in a flare pilot comprising operating a flare pilot head by combusting an air- fuel combustion mixture to form products of combustion.

[0058] Clause 22. The method of clause 21, any other clause, or any combination of clauses, wherein the method comprises recirculating a portion of the products of combustion with additional air-fuel combustion mixture being fed to the flare pilot head to reduce a flame speed of the fuel used in the air-fuel combustion mixture.

[0059] Clause 23. The method of clause 22, any other clause, or any combination of clauses, further comprising a step of mixing combustion air and flare pilot fuel in a pilot inlet pipe to form the air-fuel combustion mixture.

[0060] Clause 24. The method of clause 22, any other clause, or any combination of clauses, further comprising a step of routing the portion of the products of combustion at the flare pilot head to a pilot mixer.

[0061] Clause 25. The method of clause 22, any other clause, or any combination of clauses, further comprising a step of recirculating the portion of the products of combustion through a recirculation circuit.

[0062] Clause 26. The method of clause 25, any other clause, or any combination of clauses, further comprising the step of cooling the portion of the products of combustion in the recirculation circuit.

[0063] Clause 27. The method of clause 26, any other clause, or any combination of clauses, wherein the step of cooling the portion of the products of combustion in the recirculation circuit includes convective cooling.

[0064] Clause 28. The method of clause 26, any other clause, or any combination of clauses, wherein the step of cooling the portion of the products of combustion in the recirculation circuit includes radiation cooling.

[0065] Clause 29. The method of clause 25, any other clause, or any combination of clauses, further comprising the step of forming a vacuum in the recirculation circuit to draw the products of combustion into the air-fuel combustion mixture.

[0066] Clause 30. The method of clause 29, any other clause, or any combination of clauses, wherein the step of forming the vacuum comprises directing a flow of combustion gas through a pressure differential to generate the vacuum in the recirculation circuit.

[0067] Clause 31. The method of clause 30, any other clause, or any combination of clauses, wherein the pressure differential forms a venturi.

Claims

CLAIMS1. A flare pilot for a flare stack comprising: a fuel line; an air inlet coupled to the fuel line and operable to entrain air in with the fuel line to form an air-fuel mixture; a fuel-air mixture inlet pipe coupled to the air inlet; a pilot head coupled to the fuel-air mixture inlet pipe and operable to combust a fuel-air mixture; a flue gas recirculation line coupled to the pilot head; and a pilot mixer coupled to the fuel line prior to the air inlet, the pilot mixer further coupled to the flue gas recirculation line and operable to draw from the flue gas recirculation line and mix flue gas with pilot fuel.

2. The flare pilot of claim 1, wherein the flue gas recirculation line is configured to collect inert gases from combustion products of the pilot head.

3. The flare pilot of claim 2, wherein the flue gas recirculation line is configured to provide cooling of flue gases passing through the flue gas recirculation line.

4. The flare pilot of claim 3, wherein the pilot mixer is configured to create a vacuum to draw the inert gases from the combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

5. The flare pilot of claim 4, wherein the pilot mixer comprises a venturi.

6. The flare pilot of claim 2, wherein the pilot mixer is configured to create a vacuum to draw the inert gases from the combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

7. The flare pilot of claim 6, wherein the pilot mixer comprises a venturi.

8. The flare pilot of claim 1, wherein the pilot mixer is configured to create a vacuum to draw inert gases from combustion products of the pilot head through the flue gas recirculation line and entrain the inert gases with the fuel-air mixture.

9. The flare pilot of claim 8, wherein the pilot mixer comprises a venturi.

10. A flare pilot for a flare stack comprising: a pilot head operable to combust a fuel-air mixture; a flue gas recirculation line coupled to the pilot head; and a pilot mixer operable to receive high pressure fuel, create a vacuum in the flue gas recirculation line to draw products of the combustion of the fuel-air mixture into the pilot mixer, and mix the high pressure fuel and products of combustion form a diluted fuel gas to be used in the fuel-air mixture at the pilot head.

11. The flare pilot of claim 10, wherein the pilot mixer comprises a venturi.

12. The flare pilot of claim 11, further comprising an air mixer coupled to the pilot mixer, wherein the air mixer is operable to entrain air in the diluted fuel gas to form the fuel-air mixture.

13. The flare pilot of claim 12, further comprising a heat exchanger operable to cool the flue gas recirculation line.

14. A method of mitigating flashback in a flare pilot comprising the steps of: operating a flare pilot head by combusting an air-fuel combustion mixture to form products of combustion; and recirculating a portion of the products of combustion with additional airfuel combustion mixture being fed to the flare pilot head to reduce a flame speed of the fuel used in the air-fuel combustion mixture.

15. The method of claim 14, further comprising a step of mixing combustion air and flare pilot fuel in a pilot inlet pipe to form the air-fuel combustion mixture.

16. The method of claim 14, further comprising a step of routing the portion of the products of combustion at the flare pilot head to a pilot mixer.

17. The method of claim 14, further comprising a step of recirculating the portion of the products of combustion through a recirculation circuit.

18. The method of claim 17, further comprising the step of cooling the portion of the products of combustion in the recirculation circuit.

19. The method of claim 18, wherein the step of cooling the portion of the products of combustion in the recirculation circuit includes convective cooling.

20. The method of claim 18, wherein the step of cooling the portion of the products of combustion in the recirculation circuit includes radiation cooling.

21. The method of claim 17, further comprising the step of forming a vacuum in the recirculation circuit to draw the products of combustion into the air-fuel combustion mixture.

22. The method of claim 21, wherein the step of forming the vacuum comprises directing a flow of combustion gas through a pressure differential to generate the vacuum in the recirculation circuit.

23. The method of claim 22, wherein the pressure differential forms a venturi.

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