Swirling-flow burner

Abstract: A swirling-flow burner comprising: a burner tube (1) and an oxidizer supply tube (2) concentric with and spaced from the burner tube (1), defining an annular fuel gas conduit (3) between the tubes (142), the oxidizer supply tube (2) and the fuel gas conduit (3) having separate inlet ends and separate outlets ends; a fuel gas injector (18) connected to the outlet end (20) of the fuel gas conduit (3), which fuel gas injector having a substantially U-shaped cross section around a common axis (21) of symmetry of the burner tube (1) and the injector and having an annular outlet (20) directed inwardly towards the axis (21) and a gas mixing zone; an oxidizer injector (5) connected to the outlet end (11) of the oxidizer supply tube (2) and having an axis (21) of symmetry common with the fuel gas injector (18), wherein the outlet (11) of the oxidizer injector (5) is directed towards the gas mixing zone; static swirler means (7,10) provided in the oxidizer injector (5) for producing a swirling movement of the oxidizer around the axis (21) before the oxidizer can be discharged from the oxidizer injector (5); the static swirler means (7,10) consists of a partition dividing the flow of oxidizer in an upstream (4) and a downstream (9) section and provided with at least one passageway (10) for the passage of the oxidizer from the upstream (4) to the downstream (9) section, where the passageway (10) is designed as to change the flow direction of oxidizer from a direction parallel with the axis (21) of symmetry upstream of the partition to a direction transversely thereto and tangentially in relation to the axis (21) downstream of the partition, thereby forming a rotating flow of oxidizer around the axis (21).

Description: Background of the Invention The invention concerns a swirling-flow burner comprising a burner tube and an oxidizer supply tube concentric with and spaced from the burner tube, defining an annular fuel gas conduit between the tubes, the oxidizer supply tube and the fuel gas conduit having separate inlet ends and separate outlet ends, a fuel gas injector connected to the outlet end of the fuel gas conduit, which fuel gas injector having a substantially U-shaped cross section around a common axis of symmetry of the burner tube and the injector and having an annular outlet directed inwardly towards the axis and a gas mixing zone, an oxidizer injector connected to the outlet end of the oxidizer supply tube and having an axis of symmetry common with the fuel gas injector, wherein the outlet of the oxidizer injector is directed towards the gas mixing zone, further comprising static swirler means provided in the oxidizer injector for producing a swirling movement of the oxidizer around the axis before the oxidizer can be discharged from the oxidizer injector. Such a burner for use in gas-fired catalytic reactors is disclosed in EP B1 545,440. The main field of application of the known burner is the production of hydrogen and carbon monoxide process gas by primary and secondary steam reforming or by autothermal catalytic reforming of a hydrocarbon fuel, where the problem is to produce a process gas with a low carbon soot content as the process has to take place with a substoichiometric oxidizer-supply. According to the previous art, this is counteracted by providing multiple mixing points for the fuel gas and the oxidizer by generating a central swirling flow of oxidizer that is guided to the mixing zone, where the fuel is supplied peripherally to the oxider flow. The swirling or rotating movement of the oxidizer is generated by pitched blades in the swirler. Furthermore, as the combustion temperature is high, often above 1000°C, there is a risk for overheating of the burner surface close to the combustion zone, when designing the burner for recirculation of hot gases for mixing purposes towards the burner surfaces. According to EP B1 545,440 the design of the burner directs the flow of combustion products away from the burner along the central axis causing the recirculation of the combustion products to take place at the cooler periphery of the combustion zone, whereby the gases are cooled before they reach the burner face and are mixed with and reheated by the central flow. away from the burner. As steam is usually supplied to the hydrocarbon fuel for moderating the flame temperature and enhancing hydrocarbon conversion, this also has the effect of suppressing soot formation. The swirler incorporated in the burner according to the known art has a mixing effect depending on the pitch angle of the blades of the swirler, and experiments have shown that an increasing pitch angle makes it possible to feed with a lower steam to hydrocarbon ratio, hereafter referred to as S/C ratio. While a blade angle of e.g. 30° results in a soot limit at a S/C ratio of 0.9, a blade angle of 60° lowers the soot limit at a S/C ratio of 0.6. It is desirable to lower the soot limit further, but physical conditions limit the blade pitch angles to about 60 to 75°. Summary of the Invention It is a general object of the invention to increase the mixing effect of the oxidizer swirler means in a swirling-flow burner. It is a further object of the invention to lower the tendency to carbon soot generation of effluent gas produced by a swirling-flow burner with sub-stoichiometric oxidizer supply. These and other objects are achieved in a swirling-flow burner as indicated in the preamble of claim 1 and which comprises static swirler means consisting of a partition dividing the flow of oxidizer in an upstream and a downstream section and provided with at least one passageway for the passage of the oxidizer from the upstream to the downstream section, wherein the passageway is designed to provide change of the flow direction of oxidizer from a direction parallel with the axis of symmetry upstream of the partition to a direction transversely thereto and tangentially in relation to the axis downstream of the partition, thereby forming a rotating flow of oxidizer around the axis. By the invention it is possible to change the direction of the oxidizer flow up to 90° in relation to the original direction, while the tangentially directed passageway causes a swirling motion of the oxidizer. Thereby, an increased rotation of the oxidizer is achieved before it enters the mixing zone, while passing through the outlet, i.e. along the axis, and, thus, an increased mutual mixing of fuel and oxidizer as a non-rotating radial stream of fuel gas hits the rotating oxidizer gas and causes turbulence between the gases. In a preferred embodiment according to the invention, the fuel gas injector is provided with a substantially plane internal surface surrounding the fuel gas outlet for directing the fuel gas flow mainly perpendicularly against the flow of oxidizer in the gas mixing zone. The fuel gas is forced radially towards the oxidizer flow in the center, while forming a collar-like stream without any velocity component parallel with the oxidizer. As the fuel gas and the oxidizer meet without any common motion components, a maximum of mixing effect is achieved. In another preferred embodiment according to the invention, the partition is substantially shaped as an inverted cup with the top pointing against the flow direction of oxidizer and having an annular wall section extending along the inside of the oxidizer supply tube, where the passageway for the oxidizer is provided in the wall section. The partition may then be designed rather simply as an insert sealing the end of the oxidizer supply tube, except for one or more machined passageways for oxidizer flow. Thereby, the oxidizer flow may pass between the oxidizer tube and the wall section until it is turned transversely to the original flow direction by one or more passageways in the partition. In order to ensure a symmetric flow upstream and downstream of the partition while minimizing pressure loss caused by turbulence, there are preferably 2-4 passageways. The wall section of the partition will typically have a cylindrical outer surface, which is a simple and cost-saving design. In a further embodiment according to the invention, a swirling chamber with an annular cross-section around the axis and with a larger diameter than the oxidizer outlet is provided in the oxidizer injector between the partition and the oxidizer outlet and wherein the surface section constituting the transition between the swirling chamber and the oxidizer outlet is rounded. The swirling chamber enhances the rotational movement of the oxidizer before leaving it through the outlet, and the rounded transition keeps turbulence low, thereby also avoiding a disturbance in the rotation. When the outlet of the oxidizer injector is provided with an annular lip extending into the gas mixing zone, the mixing point of fuel and oxidizer is moved away from the burner face, especially when designing the lip with a very sharp angle. Preferred embodiments according to the invention are described in detail in the following with reference to the drawing, where: Fig. 1 shows an elevated section of a preferred embodiment of the burner according to the invention, Fig. 2 shows a section along the line II-II on Fig. 1, and Fig. 3 shows an elevated section of a second embodiment according to the invention. Description of Embodiments In a first, preferred embodiment according to the invention, a burner for gaseous fuel comprises an outer burner tube 1, in which an oxidizer supply tube 2 of smaller diameter is concentrically provided. Burner tube 1 and oxidizer tube 2 define a duct 3 there between for the supply of fuel gas. The oxidizer tube defines a duct 4 for supply of oxidizer. Ducts 3 and 4 have separate, not shown inlets. The outlet of oxidizer tube 2 is furnished with an oxidizer injector generally designated 5, comprising a partition 6 shaped as an inverted cup and provided with a cylindrical skirt 7. Other embodiments may comprise dome-shaped, tapered, or discshaped partitions. The rim of skirt 7 is embedded in an end piece 8 welded onto the end of tube 2, thereby defining a swirling-chamber 9 between parts 6, 7 and 8. Partition 6 is kept in place with a stud 14 screwed and welded into the wall of tube 2, but other embodiments without this feature are possible, e.g. with the partition welded to the end piece 6. As shown in Figs. 1 and 2, partition 6 has three passageways for oxidizer formed as slits 10 machined into skirt 7. The passageways may have other geometrical shapes, e.g. as round holes, and may be more or less in numbers. Slits 10 are directed tangentially in relation to the cylindrical inwardly facing side of partition 6. Centrally in end piece 8 oxidizer injector outlet 11 is placed, with a rounded transition 12 for reducing turbulence between outlet 11 and the flat upper side 13 of end piece 8, making outlet 11 nearly funnel-shaped. An annular lip 15 surrounding outlet 11 projects from the otherwise flat outer surface of end piece 8. Lip 15 has a tip angle γ, which is preferably in the range 15-40°. A stud 16 mounted in burner tube 1 keeps oxidizer tube 2 fixed against a projection 17 at the bottom end of tube 1, but other means of fastening tube 2 are possible. Fuel gas supply duct 3 is connected to a generally U-shaped fuel injector 18, wherein the bottom of the U is generally plane with a centrally provided circular aperture 19 concentric with axis 21 of symmetry of tubes 1, 2 and end piece 8. Outlet 20 of the fuel gas tube is thereby defined as an annular aperture between the inner edge of aperture 19 and lip 15 on end piece 8. A gas mixing zone is thus created between outlets 11 and 20, and from here the mixing zone extends further away from the burner along axis 21. The bottom of fuel gas injector 18 is provided at the outer side with a obtuse-angled conical surface 22 surrounding aperture 19 for avoiding contact with the hot combustion products, which otherwise may overheat injector 18. Another embodiment according to the invention as shown on Fig. 3 features common details with the first embodiment described above, except that the oxidizer injector is provided with an elongate bluff body 23 shaped like an icicle. Body 23 is fastened to or is integrated with the top of partition 6 and stretches through the oxidizer outlet 11 and aperture 19. The presence of the bluff body 23 provides an annular space 24 within the oxidizer injector 5 and at the top of the gas mixing zone and eliminates immobile or slowmoving central parts of oxidizer gas. Fuel gas, containing hydrocarbon compounds, and steam, and possibly carbon monoxide from a previous process step, is supplied with high pressure through duct 3 and flows through fuel injector 18 towards outlet 20, the last part of the flow being directed radially and collar-like towards axis 21. The straight flow of oxidizer, which may be air, oxygen, or oxygen-enriched air, supplied to duct 4, is turned 90° as it reaches end piece 8 between partition 6 and tube 2 and is forced through slits 10. Because of the tangential direction of slits 10, the substreams of oxidizer gas entering swirling chamber 9 induce a rotational movement of the gas inside chamber 9 with a rather small component of motion, or none at all, in the axial direction. As the oxidizer is discharged through outlet 11, it has still a rotating or swirling movement when it reaches the gas mixing zone and is hit by the transversely directed flow of fuel gas through outlet 20. As the oxidizer and the fuel flows by lip 15 the geometry and tip angle of lip 15 cause the two gas streams to flow together at a distance from the outlets thereby moving the hot combustion zone away from the burner. Because of the intense rotational movement of the oxidizer a better mixing of fuel and oxidizer in the mixing zone is obtained, thereby lowering the tendency of this sub-stoichiometric process to produce carbon soot. The mixing effect is enhanced by the substantially perpendicular injection of the fuel gas towards the axially moving oxidizer. The burner according to the invention may be used for in other combustion purposes than indicated in the above.

Claims: A swirling-flow burner comprising: a burner tube (1) and an oxidizer supply tube (2) concentric with and spaced from the burner tube, defining an annular fuel gas conduit (3) between the tubes, the oxidizer supply tube and the fuel gas conduit having separate inlet ends and separate outlets ends; a fuel gas injector (18) connected to the outlet end of the fuel gas conduit (3), which fuel gas injector having a substantially U-shaped cross section around a common axis of symmetry (21) of the burner tube (1) and the injector and having an annular outlet (20) directed inwardly towards the axis (21) and a gas mixing zone; an oxidizer injector (5) connected to the outlet end of the oxidizer supply tube (2) and having an axis of symmetry (21) common with the fuel gas injector (18) wherein the outlet (11) of the oxidizer injector (5) is directed towards the gas mixing zone; static swirler means provided in the oxidizer injector (5) for producing a swirling movement of the oxidizer around the axis (21) before the oxidizer can be discharged from the oxidizer injector (5); characterized in that the static swirler means consists of a partition (6) dividing the flow of oxidizer in an upstream and a downstream section, the partition being provided with at least one passageway (10) for the passage of the oxidizer from the upstream to the downstream section, where the passageway forces the flow direction of oxidizer from a direction parallel with the axis of symmetry (21) upstream of the partition to a direction transversely thereto and tangentially in relation to the axis downstream of the partition, thereby forming a rotating flow of oxidizer around the axis. A swirling-flow burner according to claim 1, wherein the fuel gas injector (18) is provided with a substantially plane internal surface surrounding the fuel gas outlet (20) for directing the fuel gas flow mainly perpendicularly against the flow of oxidizer in the gas mixing zone. A swirling-flow burner according to claim 1 or 2, wherein the partition (6) is substantially shaped as an inverted cup with the top pointed against the direction of flow of oxidizer and having an annular wall section (7) extending along the inside of the oxidizer supply tube (2) where the passageway (10) for the oxidizer is provided in the wall section (7). A swirling-flow burner according to claim 3, wherein the partition (6) is provided with passageways (10) between 3 and 10. A swirling-flow burner according to claim 3, wherein the wall section (7) has a cylindrical outer surface. A swirling-flow burner according to any of the preceding claims, wherein a swirling chamber (9) of annular cross-section around the axis and with greater diameter than the oxidizer outlet (11) is provided in the oxidizer injector (5) between the partition (6) and the oxidizer outlet (11) and where the surface section (12) constituting the transition between the swirling chamber (9) and the oxidizer outlet (11) is rounded. A swirling-flow burner according to any of the preceding claims, wherein the outlet (11) of the oxidizer injector (5) is provided with an annular lip (15) extending into the gas mixing zone.

Inventors: PRIMDAHL IVAR IVARSEN, PRIMDAHL, IVAR IVARSEN
Applicants: TOPSOE HALDOR AS, HALDOR TOPSOE A/S
Application year: 1998
Main classification: F23D14
IPC codes: F23D14