Swirler burner

Abstract: Swirling-flow burner with a burner tube comprising a central oxidiser supply tube and an outer concentric fuel supply tube, the oxidiser supply tube being provided with a concentric cylindrical guide body having static swirler blades and a central concentric cylindrical bore, the swirler blades extending from outer surface of the guide body to inner surface of oxidiser supply tube being concentrically arranged within space between the guide body and inner wall at lower portion of the oxidiser supply tube.

Description: The present invention is directed to combustion of hydrocarbon fuel and in particular to a burner with a swirler body for use in hydrocarbon fuelled combustion reactors. Burners with a swirling flow of a combustion reactant are mainly used for firing gas-fuelled industrial furnaces and process heaters, which require a stable flame with high combustion intensities. Conventionally designed swirling-flow burners include a burner tube with a central tube for fuel supply surrounded by an oxidiser supply port. Intensive mixing of fuel and oxidiser in a combustion zone is achieved by passing the oxidiser through a swirler installed at the burner face on the central tube. The stream of oxidiser is, thereby, given a swirling-flow, which provides a high degree of internal and external recirculation of combustion products and thus a high combustion intensity. As a general drawback of conventional swirling-flow burners of the above design, the burner face is at high gas flow velocities, as required for industrial burners of this design, exposed to overheating caused by the high degree of internal recirculation along the central axis of the combustion zone. Hot combustion products flow, thereby, back towards the burner face, which results in rapid heating up to high temperatures and, consequently, destruction of the face. A swirling burner for use in small and medium scale applications with substantially reduced internal recirculation of combustion products toward the burner face is disclosed in US patent No. 5,496,170. The burner design disclosed in this patent results in a stable flame with high combustion intensity and without detrimental internal recirculation of hot combustion products by providing the burner with a swirling-flow of oxidiser having an overall flow direction concentrated along the axis of the combustion zone and at the same time directing the fuel gas flow towards the same axis. The disclosed swirling-flow burner comprises a burner tube and a central oxidiser supply tube concentric with and spaced from the burner tube, thereby defining an annular fuel gas channel between the tubes, the oxidiser supply tube and the fuel gas channel having separate inlet ends and separate outlet ends. U-shaped oxidiser and fuel gas injectors are arranged coaxial at the burner face. The burner is further equipped with a bluff body with static swirler blades extending inside the oxidiser injector. The swirler blades are mounted on the bluff body between their upstream end and their downstream end and extend to the surface of the oxidiser injection chamber. In burners for large-scale reactors, the swirler blades will have an extended length and area, which decreases the mechanical stability of the blades. Alternatively, the bluff body has to be constructed with a larger size to reduce length of the swirler blades. Disadvantageously, the swirler blades in a large-scale swirler burner have a size, which causes mechanical stability problems and unintended vibrations. Alternatively, when upscaling the swirler bluff body, pressure drop of oxidiser flowing around the body will disadvantageously increase. Widening the outlet end of the fuel and/or oxidiser supply tube may compensate the increasing pressure drop. However, the desired flow pattern around axis of the combustion zone will then be disadvantageously scattered around the axis. Thus, the main object of the invention is to obtain a swirler body, preferably for use in large-scale swirler burners with guide body and swirler blades with a size and shape without the above problems in large-scale swirler burners. Accordingly, this invention is a swirling-flow burner with a burner tube comprising a central oxidiser supply tube and an outer concentric fuel supply tube, the oxidiser supply tube being provided with a concentric cylindrical guide body having static swirler blades and a central concentric cylindrical bore, the swirler blades extending from outer surface of the guide body to inner surface of oxidiser supply tube being concentrically arranged within space between the guide body and inner wall at lower portion of the oxidiser supply tube. In further an embodiment, the burner further comprises in the central bore static swirler blades and a central bluff body, these static swirler blades extending from surface of the bluff body to surface of the guide body. Additional stabilisation of the swirler blades during operation is obtained by fixing the outer swirler blades in the above burner on the inner surface of the oxidiser supply tube. The guide body is then mounted on outer edge of the inner swirler blades. The inner swirler blades are preferably formed by machining the outer surface of the bluff body. The swirling-flow induced in the swirler promotes mixing of fuel gas and oxidiser by increasing the area of their contact. Effective mixing is obtained, when adjusting the pitch angle of the swirler blades to an angle of between 15° and 75°, preferably between 20° and 45°. An increased mixing of fuel gas and oxidiser is additionally provided, when arranging the inner swirler blades around central part of the bluff body and inside upper portion of the guide body, and arranging the outer swirler blades around lower portion of the guide body and inside lower portion of the oxidiser supply tube. Inwardly directed flow pattern of combustion reactants along axis of a combustion zone adjacent to the burner face is obtained by U-shaped contours of outlet ends of the fuel and oxidiser supply tubes and prevents recirculation of hot combustion products in high temperature region around axis of the combustion zone, which otherwise leads to overheating and destruction of the burner face. The inwardly directed flow pattern leads to a high degree of external recirculation in low temperature outer region of the combustion zone. From this region only cooled combustion products flow back to the burner face, where the products are being sucked into the hot combustion zone area and reheated there. To maintain substantially the above flow pattern it is additionally preferred that contour of the guide body follows the contour of the inner wall of the oxidiser supply tube. When operating a burner according to the invention in gas fired reactors, the recycle stream of cooled combustion products protects advantageously the reactor walls surrounding the combustion zone against impingement of hot combustion products and prolongs the lifetime of the reactor. The temperature at the burner face close to the outlet end of the injection chambers may further be lowered by forming the oxidiser tube at the outlet end sharp-edged with a minimum tip angle. Reduced heating and suitable mechanical strength of the injector are obtained at tip angles of between 15° and 60°, preferably between 15° and 40°. As a further advantage of the burner according to the invention, the high degree of external recirculation of cooled combustion products provides a homogeneous temperature distribution in the combustion outlet zone. This is of great importance during operation of fired catalytic reactors, where the product yield highly depends on the temperature distribution in the catalyst bed, which typically is arranged in the combustion outlet zone. In another embodiment of the invention the guide body is solid and provided with a bore in the middle concentric with the axis of the burner. This ensures the swirling effect as above with the same radius of outer swirler blades and simultaneously low pressure drop when operating with high oxidiser gas flows. The hot reaction zone is forced away from the burner tip, still maintaining the rotation of the reacting gas around the axis of the burner. Accordingly, the burner of this invention is particularly useful in large-scale gas-fuelled reactors with heating and catalytic processes without creating additional pressure drop or mechanical instability. The above objects and advantages of the invention are explained in more detail in the following description by reference to the drawings, in which the figures show a cross sectional view of the lower portion of a fuel and oxidiser supply tube in a swirling-flow burner according to two specific embodiments of the invention. Fig. 1 shows one embodiment of the invention. The fuel supply tube 1 of the burner concentrically surrounds the oxidiser supply tube 2, which comprises the guide body 3 provided with swirler blades 4. Swirler blades 4 are arranged in a plane B around the lower part of the guide body. Guide body 3 is provided with a bore 5 for oxidiser supply concentrically arranged in the burner. Referring to Fig. 2, lower portion of oxidiser supply tube 2 is provided with a guide body 3 and a central bluff body 7. The oxidiser supply tube 2 is surrounded by a fuel supply tube 1, and further comprising inner swirler blades 8 and outer swirler blades 9. Bluff body 7 is provided with a domeshaped upstream end and a tapered downstream end. Swirler blades 8 are an integrated part of bluff body 7 obtained by machining the surface of body 7. Blades 8 extend thereby from outer surface of body 7 to the guide body 3, which is arranged coaxial with and between bluff body 7 and oxidiser supply tube 2. Blades 8 are arranged between upper portion of bluff body 7 and upper portion of guide body 3 around an axis A. Blades 8 are fixed with suitable tolerance for thermal expansion into guide body 3 by means of slots 10 and tongues 11 provided in the guide body 3 and on the blades 8, respectively. Outer swirler blades 9 are arranged in the oxidiser supply tube in space between guide body 3 and tube 2 around axis A and with the centres of gravity in a plane B, perpendicular on axis A and going through lower portion of bluff body 7 and oxidiser supply tube 2. Similar to the inner blades, outer swirler blades 9 are fixed with tolerance to oxidiser tube wall by tongues 12 on the blades resting in slots 13 formed in inner wall of tube 2. Blades 9 are further mounted on surface of guide body 3. Alternatively, blades 9 may be formed as an integrated part of the guide body 3. At outlet end 16 of tube 2, this tube and guide body 3 have a U-shaped cross sectional inner surface around axis A. The U-shaped form may conveniently be obtained by machining a suitable metallic body having a cylindrical part and a conical part. The transition angle between the cylindrical and conical part is thereby preferably in the range of 115° and 170°. The edge of oxidiser supply tube 2 surrounding outlet end 16 is tapered with a minimum tip angle γ in order to protect the edge against overheating as described more detailed below. The tip angel is typically 15°-60°, preferably 15°-40°. When operating the burner according to the invention, an oxidiser stream is brought into swirling-flow by passage through swirler blades 8 and 9. Furthermore, by means of bluff body 7 and the U-shaped contour of outlet ends of guide body 3, oxidiser tube 2 and fuel tube 1, the swirling oxidiser stream is discharged into a combustion zone in an overall flow directed around the axis of the combustion zone. As a result, mixing of the oxidiser and fuel gas stream is mainly accomplished in the high temperature region around the axis of combustion zone. Thereby, deleterious internal recirculation of hot combustion products within this region is prevented. Recirculation is only established in the low temperature outer region of the combustion zone, resulting in reduced material temperatures close to the outlet ends of the injection chambers. As mentioned hereinbefore, the temperature in this region may further be controlled by angle γ of the oxidiser injector edge around the outlet end of the oxidiser injection chamber, whereby the mixing zone of oxidiser and fuel gas is kept at an increasing distance from the edge at decreasing tip angles. In applications requiring very high combustion intensities the burner face may further be protected against high temperatures by addition of an inert gas or steam in the region of the outlet end.

Claims: Swirling-flow burner with a burner tube comprising a central oxidiser supply tube (2) and an outer concentric fuel supply tube (1), the oxidiser supply tube being provided with a concentric cylindrical guide body (3) having static swirler blades (4) and a central concentric cylindrical bore (5), the swirler blades extending from outer surface of the guide body to inner surface of oxidiser supply tube being concentrically arranged within space between the guide body and inner wall at lower portion of the oxidiser supply tube. The swirling-flow burner of claim 1, wherein the swirler blades are arranged in the swirler with a pitch angle of 15°-75°, preferably of 20°-45°. The swirling-flow burner of claim 1, wherein the central oxidiser supply tube and the outer fuel supply tube have a tip angle of 15°-60°, preferably 15°-40° at outlet end. The swirling-flow burner of claim 1 further comprising in the central bore a central bluff body and static swirler blades extending from surface of the bluff body to surface of the guide body. Use of a burner according to anyone of the preceding claims for carrying out catalytic processes in a gas fuelled reactor.

Inventors: PRIMDAHL IVER IVARSEN, PRIMDAHL, IVER IVARSEN
Applicants: TOPSOE HALDOR AS, HALDOR TOPSOE A/S
Application year: 2001
Main classification: F23D14
IPC codes: F23C7, F23D14