A burner for secondary reformers

Abstract

A burner for a secondary reformer, comprising an oxidant channel (22) for a flow of oxidant (4) and a swirler (7) located in the oxidant channel; wherein the swirler includes: an inner body (16) with a central passage (30) axially arranged with respect to the oxidant channel; an outer body (15) coaxial and external with respect to the inner body; a first set of stationary vanes (14) extending from the inner body; a second set of stationary vanes (13) extending from the outer body; each set of vanes arranged to induce swirling of the flow of oxidant.

Description

[0001] A burner for secondary reformers

[0002] DESCRIPTION

[0003] Field of application

[0004] The present invention relates to the field of burners for secondary reformers.

[0005] Prior art

[0006] A burner of a secondary reformer is designed to feed a process gas and an oxidant to a combustion chamber. The process gas is typically a partially reformed gas from a primary reformer, for example comprising methane, hydrogen, carbon monoxide and carbon dioxide. The oxidant is typically air, oxygen-enriched air or oxygen of desired purity. The process gas is at a high temperature around 700 - 800 °C, while the air is at a lower temperature, generally around 450 °C to 600 °C.

[0007] The burner is mounted vertically on top of the pressure vessel of the secondary reformer, above a combustion chamber. The burner comprises essentially an oxidant channel coaxially arranged with a process gas channel. Commonly, the process gas channel is an annular channel arranged coaxially around the oxidant channel. The process gas and the oxidant remain separate while flowing through the coaxial channels of the burner and mix at the outlet of the same.

[0008] It is a known practice to provide a swirler located in the oxidant channel. The swirler is designed to induce a swirling motion of the oxidant around a vertical axis, to improve mixing with the process gas at the inlet of the combustion chamber with formation of a diffusion flame.

[0009] In a common design of the prior art, the swirler includes a straight central passage surrounded by an annular channel including a set of swirling vanes. The central passage does not include swirling vanes, so that the incoming flow of oxidant is split into two parts: an inner flow through the central passage which is not swirled, and an outer flow through the annular channel which is swirled by the vanes. The inner flow and outer flow rejoin downstream the swirler.

[0010] The reason of the above design is that the angle of the swirling vanes relative to the axis of flow (vertical axis) is dictated by the manufacturing technique of shaping the vanes from a metal sheet and subsequent welding onto a machined body. Consequently, the angle of the vanes is typically greater than the optimum for the fluid-dynamics and causes a swirl motion greater than desired. An excessive swirl motion of the flow may cause a highly undesired backflow of the mixture from the combustion chamber into the burner body, with the related risk of the flame rising within the burner which would damage or destroy the same.

[0011] To avoid this risk, the prior art burners are designed with a relatively large central passage, having a cross sectional area around 10% of the area of the annular channel. Taking into account the different pressure drop, the non-swirled inner flow is typically around 20% to 30% of the total flow. The inner flow reduces the overall swirl, given that only a fraction passes through the vanes; secondly, the non-swirled inner flow acts as a deterrent against the formation of recirculation on the burner axis, thereby preventing the backflow of syngas from the combustion chamber.

[0012] The above-described design of the swirler, however, is far from optimal. In most cases, the swirl given to the outer flow is higher than required, which is then balanced by mixing with the inner flow. This process is inherently inefficient and causes a large pressure drop, increasing the energy consumption for the compression of the oxidizer.

[0013] As a further drawback, the current design involves crafting vanes manually from metal sheets, which are then welded to the middle body. This construction process is time-consuming and prone to errors. There is a significant risk that the manual construction of the vanes and the welding process may result in an asymmetric swirler structure. Such asymmetry could induce vibrations in the burner, leading to uneven flames and temperature fields in the combustion chamber. Additionally, it may cause nonuniform cooling of the burner surfaces, which could have detrimental effects on the burner's lifespan, potentially causing damage to the refractory lining of the reformer and pressure vessel, and increasing methane slip.

[0014] US 5,252,447 discloses an air fuel mixer specifically for gas turbine combustors, wherein the mixer includes inner and outer counter-rotating swirlers which are separately rotatable and separated by a hub. WO 2023 / 227547 discloses a burner for a reforming reactor.

[0015] Summary of the invention

[0016] The aim of this invention is to overcome the above-mentioned problems and limitations of the prior art. The invention aims to solve the problem of non-optimal design of the swirler and related drawbacks, including the large pressure drop.

[0017] The aim of the invention is reached with a burner according to claim 1 .

[0018] The burner of the invention includes a swirler located in the oxidant channel that has an improved design with respect to the prior art. The swirler includes a double set of vanes that allows for lower pressure drop of the oxidant flow and reduces the problem of backflow. In a highly preferred embodiment, the swirler is made with additive manufacturing (AM) which eliminates the drawbacks of the manual crafting from metal sheets.

[0019] Description of the invention

[0020] The invention discloses a burner for a secondary reformer, comprising an oxidant channel, such as an oxidant pipe, coaxial to a process gas channel. Typically, the process gas channel is an annular channel around the oxidant channel. The process gas channel may be part of the burner or of the upper neck of the vessel of the reformer. The oxidant may be air, enriched air or oxygen, while the process gas may be the effluent of a partial reforming process, such as the effluent of a primary reformer or of a gas-heated reformer upstream the secondary reformer.

[0021] The term “secondary reformer” used in this description and claims may include also an autothermal reformer (ATR).

[0022] A swirler located in the oxidant channel includes an inner body and an outer body. The inner body has a central passage for a portion of the oxidant flow. The outer body is arranged around and coaxial with respect to the inner body. The swirler has, therefore, a first annular passage between the inner body and the outer body and a second annular passage outside the outer body and within the oxidant channel. The inner body and the outer body have preferably a cylindrical shape.

[0023] The swirler includes a first set of vanes (inner vanes) extending from the inner body in the first annular passage, and a second set of vanes (outer vanes) extending from the outer body in the second annular passage.

[0024] Each set of vanes is arranged to induce swirling of the flow of oxidant. In preferred embodiments, each inner vane is helically arranged around the inner body, and / or each outer vane is helically arranged around the outer body. The inner vanes may extend radially through part or all of the first annular passage; similarly, the outer vanes may extend radially through part or all of the second annular passage. In some embodiments, the outer vanes may connect to an outer ring (shroud).

[0025] The inclination of the vanes can be defined relative to a swirler axis. Said swirler axis is a central axis of the swirler and is coaxial with the oxidant channel and the process gas channel. In most applications, the burner is mounted vertically on top of a pressure vessel and therefore said axis is a vertical axis. The vanes of the first set and the vanes of the second set are stationary, i.e. they do not move or rotate, so that they define a fixed swirling path for the oxidant flow. The vanes of the first set and the vanes of the second set are configured to induce swirling with the same rotation direction. The rotation direction is understood as the sense of rotation (clockwise or counterclockwise) imparted by the vanes and is the same for the vanes of the first set and the vanes of the second set.

[0026] In a preferred embodiment, each of the inner vanes has a first inclination angle with respect to said swirler axis which is less than 65°, preferably in the range 40° - 60°, even more preferably 45° - 55°. Each of the outer vanes has preferably a second inclination angle with respect to said swirler axis which is less than 75°, more preferably in the range 50° - 70°, even more preferably 55° - 65°.

[0027] The inner vanes and outer vanes are arranged to swirl the oxidant flow in the same direction, clockwise or counter-clockwise. Said first inclination angle and second inclination angle may be the same or different, according to various embodiments. The above upper limits are not limiting and may be exceeded in specific cases where high swirl is required.

[0028] In a preferred embodiment, the cross-sectional area of the central passage is preferably 0.2% to 5.0% of the cross-sectional area of the first annular passage, preferably 0.5% to 3.0%, and more preferably 0.5% to 1.0%, such as 0.8% or around 0.8%. The cross-sectional area of the first annular passage is preferably 10% to 25% of the cross-sectional area of the second annular passage, preferably 15% to 25%, and more preferably 20% or around 20%. The cross- sectional area of said passages is understood as area in a plane perpendicular to the swirler axis.

[0029] The central passage within the inner body is a free passage without swirling vanes. Accordingly, the portion of the oxidant flowing into the central passage bypasses the swirling process. More in detail, a flow of oxidant passing through the swirler of the invention is divided into three streams: an inner flow passing through the central passage, which is not swirled; a middle flow passing through the first annular passage, which is swirled by the inner vanes; an outer flow passing through the second annular passage and swirled by the outer vanes.

[0030] The invention allows to make a swirler with a central passage smaller than the prior art, still avoiding the risk of a backflow. In embodiments of the invention, said inner flow is less than 10% or less than 5% of the total flow of oxidant passing through the oxidant channel.

[0031] A further aspect of the present invention is a method of feeding process gas and oxidant to a secondary reformer, using a burner as described above, wherein the flow of oxidant passing through the above-mentioned central passage is less than 10% of the total flow of oxidant.

[0032] The invention has the following advantages.

[0033] The invention allows swirlers with an inclination angle of the vanes smaller than the prior art, for a given swirl effect, thanks to the provision of a double set of vanes and even more particularly in the embodiments with additive manufacturing.

[0034] The inventive design allows achieving the same swirl level as the prior art design but with smaller vane angles, resulting in reduced pressure drops. The inner body is retained for ease of construction and to provide a small non-swirled inner flow, preventing the formation of even small recirculation regions in front of the inner vanes. The outer body serves the purpose of finely adjusting the angles of the outer vanes, enabling control over the flow distribution between the first annular passage and the second annular passage and determining the desired level of swirl. By optimizing the angles of the inner and outer vanes, the pressure drop can be minimized. As a result, the new design offers a significant reduction in pressure drop, approximately 10 to 30% compared to the conventional design.

[0035] In an embodiment the proposed design leverages on additive manufacturing (AM) construction. AM eliminates constraints on vane angles imposed by conventional manufacturing, allowing for lower angles and mitigating the risk of recirculation on the swirler axis. Additionally, relying on additive manufacturing, the risk of asymmetric construction is eliminated, as well as all other construction- related issues outlined in the prior art description. In a preferred embodiment, a portion of a duct around the swirler delimiting the oxidant channel is integrally made with the swirler by additive manufacturing. The piece including the portion of the duct and the swirler can be joined (for example welded) to one or more sections of conventional pipe for making the oxidant channel. A related advantage is that the construction is made simpler and the number of welds is reduced, compared to the prior art, as the item integrally made by AM does not require welds.

[0036] Description of the Figures

[0037] Fig. 1 is a sketch of a secondary reformer.

[0038] Fig. 2 and 3 illustrates a swirler according to an embodiment of the present invention.

[0039] Fig. 4 represents a longitudinal section of the swirler of Figs. 2-3.

[0040] Figs. 5 and 6 illustrate different projections on the frontal plane of the new swirler design.

[0041] Fig. 7 is a longitudinal section of a conventional swirler of the prior art.

[0042] Fig. 8 illustrates another embodiment of the invention.

[0043] The accompanying drawings are given by way of illustration only, and thus are not limitative of the present invention.

[0044] Fig. 1 illustrates a secondary reformer with a burner 1 above a combustion chamber 2. The burner 1 includes an oxidant pipe 22 arranged coaxially to a process gas channel 21 . The oxidant pipe 22 is connected to an oxidant inlet 41 and the process gas channel is connected to a process gas inlet 51 .

[0045] The oxidant inlet 41 is connected to a suitable source of an oxidant flow 4, such as an air compressor. The oxidant 4 may be air, enriched air or oxygen. The process gas inlet 51 is connected to an upstream equipment such as a primary reformer or a gas-heated reformer, providing a partially reformed process gas 5.

[0046] The process gas channel 21 includes a process gas distributor 6 and the oxidant pipe 22 includes a swirler 7. The gas channel 21 may be part of the burner or, in some embodiments, the gas channel 21 is made in the upper neck of the refractory vessel of the secondary reformer.

[0047] The oxidizer 4 entering from the inlet 41 is directed to the combustion chamber 2 through the oxidant pipe 22 and is swirled around a vertical axis A-A by the swirler 7. At the outlet of the pipe 22, the oxidizer 4 meets the process gas 5 with formation of a diffusion flame 8 in the combustion chamber 2. The arrows in the combustion chamber denote the recirculation of gas. The reformer may include a catalytic bed 3 for completion of the reforming reaction.

[0048] The swirler axis A-A passes through the centre of the swirler and is also the axis of the oxidant pipe 22.

[0049] Figs. 2 and 3 illustrate the swirler 7 in a preferred embodiment of the invention. The swirler includes coaxial inner body 16 and outer body 15. The inner body 16 is a hollow pipe defining a central passage 30. The outer body 15 defines a first annular passage 40 between the inner body 16 and the outer body 15, and a second annular passage 50 outside the outer body 15. Said second annular passage 50 may be delimited externally by the inner surface 101 of the oxidant pipe 22 (Fig. 4).

[0050] The swirler 7 has a set of inner vanes 14 and a set of outer vanes 13. The inner vanes 14 are located in the first annular passage 40 and extend from the inner body 16. The outer vanes 13 are located in the second annular passage 50 and extend from the outer body 15.

[0051] Preferably, the inner vanes 14 and outer vanes 13 are arranged helically around the inner body 16 and outer body 15, as shown in Figs. 2, 3. The inner vanes 14 and the outer vanes 13, as shown, follows the same (concurrent) sense of rotation The vanes of the first set and the vanes of the second set are configured to induce swirling with the same rotation direction. The rotation direction is understood as the sense of rotation (clockwise or counterclockwise) imparted by the vanes and is the same for the vanes of the first set and the vanes of the second set.

[0052] The preferred material for the swirler 7 include: Cr-Ni alloys; Fe-Ni-Cr alloys; stainless steel.

[0053] As illustrated in Fig. 4, the oxidizer flow 4 is divided into three separate streams: an outer flow 17 in the channel 50, a middle flow 18 in the channel 40 and an innermost flow 19 in the central passage 30. The flows 17, 18 are swirled by the outer vanes 13 and inner vanes 14 respectively; the inner flow 19 (“bypass flow”) is not swirled since there are no vanes inside the small passage 30.

[0054] Fig. 5 represents a projection on a plane of the inner vanes 14 and illustrates the inclination angle V1 of the vanes 14 relative to the axis A-A. Similarly, Fig. 6 represents a projection on a plane of the outer vanes 13 and illustrates the inclination angle V2 of said vanes 13 relative to the axis A-A.

[0055] Fig. 7 represents a sectional view of a conventional swirler design. The conventional swirler includes a single set of vanes 9 and a comparatively large bypass channel 60. By comparing Fig. 7 and Fig. 4, it can be seen that the central passage 30 of the inventive swirler 7 is significantly smaller than the central passage of known swirlers. Accordingly, the bypass flow (traversing the swirler without being swirled) is smaller.

[0056] Fig. 8 is an example of an embodiment wherein a portion of the oxidant pipe 101 is integrally made with the swirler 7 by additive manufacturing. The item of Fig. 8 (including the swirler 7) is a single piece made by AM that can be welded to other portions of the oxidant pipe.

Claims

CLAIMS1 ) A burner (1 ) for a secondary reformer, comprising an oxidant channel (22) arranged to feed a flow of oxidant (4); a swirler (7) located in the oxidant channel (22); wherein the swirler includes: an inner body (16) with a central passage (30); an outer body (15), coaxial and external with respect to the inner body (16), defining a first annular passage (40) between the inner body and the outer body and a second annular passage (50) outside the outer body (15) within the oxidant channel (22); a first set of stationary vanes (14) extending from the inner body in the first annular passage (40); a second set of stationary vanes (13) extending from the outer body in the second annular passage (50); said first set of vanes and said second set of vanes being arranged to induce swirling with the same rotation direction to the flow of oxidant (4).2) A burner according to claim 1 wherein the cross-sectional area of said central passage (30) is between 0.2% and 5.0% of the cross-sectional area of the first annular passage (40), preferably between 0.5% and 3.0%, more preferably around 0.8%.3) A burner according to claim 1 or 2 wherein the cross-sectional area of the first annular passage (40) is between 10% and 25% of the cross-sectional area of the second annular passage (50), preferably between 15% and 25%, more preferably around 20%.4) A burner according to any of the previous claims wherein the swirler is produced with additive manufacturing.5) A burner according to claim 4, wherein a portion of a duct (101 ) around the swirler (7) delimiting the oxidant channel is integrally made with the swirler (7) by additive manufacturing.6) A burner according to any of the previous claims wherein said inner body (16) and said outer body (15) have a cylindrical shape.7) A burner according to any of the previous claims wherein each vane of the first set of vanes (14) is helically arranged around the inner body (16).8) A burner according to any of the previous claims wherein each vane of the first set of vanes is arranged around the inner body (16) with a first inclination angle (V1 ) with respect to a swirler axis (A-A), wherein the first inclination angle (V1 ) is less than 65°, preferably in the range 40° - 60°, even more preferably 45° - 55°.9) A burner according to any of the previous claims wherein each vane of the first set of vanes (14) extends from the inner body (16) to the outer body (15).10)A burner according to any of the previous claims wherein each vane of the second set of vanes (13) is helically arranged around the outer body (15).11 )A burner according any of the previous claims wherein each vane of the second set of vanes is arranged around the outer body (15) with a second inclination angle (V2) with respect to a swirler axis (A-A), wherein the second inclination angle (V2) is less than 75°, preferably in the range 50° - 70°, even more preferably 55° - 65°.12)A burner according to any of the previous claims wherein each vane of the second set of vanes (13) extends from the outer body (15) to the oxidant channel (22).13)A burner according to any of the previous claims wherein the first set of vanes and the second set of vanes are arranged in the same direction, clockwise or counter-clockwise.)A method of feeding process gas (5) and oxidant (4) to a secondary reformer, using a burner according to any of the previous claims, wherein the flow of oxidant (4) passing through the central passage (30) is less than 10% of the total flow of oxidant passing through the oxidant channel (22), preferably less than 5%.