Burner for reducing NOx emissions

Abstract: A burner for mixing and burning a gaseous fuel includes an inner pipe (1), carrying the gaseous fuel, and an outer pipe (4), carrying combustion air in the annular space between the inner and outer pipes. A cone-shaped horn (2) is affixed to the end of the inner pipe. The horn extends near the inner surface of the outer pipe, leaving a narrow annular space (5) therebetween. An alternating series of air jetting portions (6) and blind portions (7) around the periphery of the large end of the horn encourage the formation of mixing vortices as air passes through the air jetting portions. A plurality of radially directed fuel jetting openings (9) in the inner pipe, upstream of the horn, inject gaseous fuel into the combustion air to form a lean air-fuel mixture. A plurality of jet openings (8) at the end of the inner pipe, within the horn, direct gaseous fuel generally parallel to the diverging wall of the horn. Openings (3) in the wall of the horn permit the entry of the lean mixture into the interior of the horn, where it mixes with the fuel flowing generally parallel to the wall of the horn. A spacial distribution, and a size distribution of the openings in the wall of the horn encourages the formation of a large number of generally independent flames, thereby encouraging stable combustion.

Description: The invention relates to a burner comprising: an inner pipe for jetting a gaseous fuel; an outer pipe for jetting air concentrically disposed about said inner pipe and thereby forming an annular space with said inner pipe; a diverging horn affixed to an end of said inner pipe thereby forming an annular gap with said outer pipe; a plurality of openings in said horn; and a plurality of openings being in an end of said inner pipe within said horn and with their axes directed generally parallel to a wall of said horn. Such a burner is known from DE-A-2035461. In said known structure the part of the horn joining the inner pipe extends perpendicular to the axis of the inner pipe and only in said part of the horn there has been provided openings with their axes parallel to the axis of the inner pipe. It is an object of the invention to provide a burner capable of reducing NO x emissions with a low manufacturing cost, and which does not require additional devices to reduce combustion temperature. It is a further object of the invention to provide improved mixing of gaseous fuel and air in a burner in order to reduce NO x emissions. It is a still further object of the invention to provide a burner wherein a large number of generally independent flames are maintained in order to achieve stable combustion. It is a still further object of the invention to provide a burner wherein gaseous fuel is jetted into a stream of combustion air upstream of a perforated horn. The lean mixture thus produced is introduced into the interior of the horn where it mixes with additional gaseous fuel jetted generally parallel to the wall of the horn. Additional mixing takes place downstream of a gap between the perimeter of the horn and the inner wall of an outer pipe. The burner according the invention is characterised in that a plurality of openings is provided in said inner pipe upstream of said horn and being directed radially outward into said annular space for jetting a first flow of said gaseous fuel into said annular space whereas the second flow is jetted through the openings into said horn and that an alternating plurality of jetting openings and blind portions about the outer perimeter of said horn within said annular gap is provided. Part of the gas flowing in the interior of the inner pipe is jetted radially from a plurality of gas jetting openings formed upon the inner pipe and then well mixed with a combustion air flowing in the interior of the outer pipe. Part of this lean mixture is introduced into the interior of the horn through the plurality of openings where it is well mixed with the gas jetted along the internal wall from the plurality of jet openings at the end of the inner pipe, within the horn, directing gaseous fuel generally parallel to the diverging wall of the horn. The openings in the horn are concentrated in the vicinity of the narrow end, with few, if any, openings near the wide end of the horn. This spacial distribution, and a size distribution of the openings in the wall of the horn encourages the formation of a large number of generally independent flames, thereby encouraging stable rich combustion concentrated near the narrow end of the horn. The lean mixture, jetting past the large end of the horn, supports lean combustion in that area, generally independently of the rich combustion taking place near the narrow end of the horn. This prevents localized high-temperature combustion and thus permits efficient reduction of NO x emissions. Advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is schematic section view of a conventional burner. Fig. 2 is a longitudinal section view of an embodiment of a burner according to an embodiment of the present invention. Fig. 3 is a front view of the embodiment in Fig. 2. Fig. 4 is a graph of NO x concentration in exhaust gases of a conventional burner as well as of a burner according to this invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 1, a typical conventional burner for use in a boiler or the like, includes an inner pipe 1 within an outer pipe 4. A perforated horn 2, having a plurality of openings, is affixed to an end of inner pipe 1. This conventional burner provides stable combustion, but it also produces a high level nitrogen oxide(NO x ) emissions of, for example, 75 to 100 ppm (O₂ = 0%). Compliance with emission standards requires the addition of devices for recirculating exhaust gas or injecting water in the burner to reduce temperature, thereby reducing NO x emission. Such additional devices increase the cost of the boiler. For example, the manufacturing cost of a small boiler for industrial use may be increased by 20 to 30%. Referring now to Fig. 2, a burner according to an embodiment of the present invention includes an outer pipe 4 and an inner pipe 1. A horn 2 is affixed to an end of inner pipe 1. Near its small end, horn 2 includes a plurality of openings 3 permitting the passage of a lean mixture of gas and air therethrough. The remainder of horn 2 is solid, without perforations. An outer diameter of the larger end of horn 2 is slightly smaller than the inner diameter of outer pipe 4, thereby forming an annular gap 5 between outer pipe 4 and horn 2. Referring now to Fig. 3, a plurality of mixed air jetting portions 6 and a plurality of blind portions 7 alternate around annular gap 5. A plurality of openings 8 are formed on a front surface of inner pipe 1. A first flow of gaseous fuel is jetted through openings 8 into the interior of horn 2. Openings 8 are directed generally parallel to the wall of horn 2, whereby the first flow of gaseous fuel tends to flow parallel to the wall of horn 2. Referring again to Fig. 2, a plurality of openings 9 jet a second flow of gaseous fuel into an annular space between inner pipe 1 and outer pipe 4, upstream of horn 2. It will be recognized that openings 9 jet gaseous fuel in a generally radial direction. The amount of gaseous fuel jetted through openings 9 produce a lean mixture of fuel and air in the annular space. If the openings 3 were distributed uniformly over the surface of horn 2, the flame would spread throughout horn 2, including the larger end. This could enable localized high temperature combustion. This prevents achievement of a dense, rich combustion in one location, and lean combustion in another location. Consequently, it is impossible to obtain efficient reduction in NO x emissions. The openings 3 may be distributed on horn 2 in an irregular array, and their sizes may differ over a substantial range. In particular, the openings 3 are distributed in the vicinity of the narrow end of horn 2, with few, or none, in the vicinity of the larger end. This enables the desired dense, rich, combustion in the vicinity of the narrow end of horn 2, while permitting separate lean combustion in, and just downstream of annular gap 5. When a gaseous fuel flows in inner pipe 1 and air flows in outer pipe 4, part of the gaseous fuel flowing within inner pipe 1 is jetted radially from second gas jetting openings 9 and is well mixed with the air flowing within outer pipe 4 to form a lean mixture. Part of the lean mixture is introduced into the interior of horn 2 through opening 3, and is then well mixed with the gas jetted along the internal wall of horn 2 from first gas jetting openings 8. As a result, the gas mixes well and produces a rich and dense combustion. If the plurality of openings 3 are disposed in an irregular manner or their sizes are different, a large number of different independent flames are produced simultaneously. This tends to establish a constant, stable combustion. The remainder of the lean mixture is jetted through the plurality of mixed air jetting portions 6 at the large perimeter of horn 2. The air passing through jetting portions 6 form vortices in the proximity of blind portions 7 where enrichment of the lean mixture with additional gaseous fuel within horn 2 permits further combustion to occur. As discussed above, the present invention enables combustion within horn 2 relatively independently of combustion within the outer periphery of horn 2. Since a large number of generally independent flames are generated, localized high temperature combustion is avoided. The limiting of temperatures reduces the production of NO x emissions. Fig. 4 shows a comparative graph showing NO x emissions from a burner according to the present invention and a conventional burner. As clearly shown in Fig. 4, whereas NO x emission of the burner of the present invention is limited to no more than 50 ppm (O₂ = 0%), the NO x emission of a conventional burner is much higher i.e. 90 to 60 ppm. It is clear that the burner of the present invention greatly reduces NO x emission compared to the conventional burner. The NO x reduction by the present invention is achieved without the addition of exhaust gas recirculation or water jet devices. In other words, the reduction of NO x emissions is achieved by the unique construction of the burner itself. The present burner lends itself to compact construction, and low burner cost.

Claims: Burner comprising: an inner pipe (1) for jetting a gaseous fuel; an outer pipe (4) for jetting air concentrically disposed about said inner pipe (1) and thereby forming an annular space with said inner pipe (1); a diverging horn (2) affixed to an end of said inner pipe (1) thereby forming an annular gap with said outer pipe (4); a plurality of openings (3) in said horn (2); and a plurality of openings (8) being in an end of said inner pipe (1) within said horn (2) and with their axes directed generally parallel to a wall of said horn, characterised in that a plurality of openings (9) is provided in said inner pipe (1) upstream of said horn (2) and being directed radially outward into said annular space for jetting a first flow of said gaseous fuel into said annular space whereas the second flow is jetted through the openings (8) into said horn (2) and that an alternating plurality of jetting openings (3) and blind portions about the outer perimeter of said horn (2) within said annular gap is provided. Burner according to claim 1, wherein said openings (8) in said horn (2) are unevenly distributed on said horn (2). Burner according to claim 1 or 2, wherein said openings (8) in said horn (2) have different sizes.

Inventors: IDO TOSHIMITSU, KAIYA KAZUO, IDO, TOSHIMITSU, KAIYA, KAZUO
Applicants: TOKYO GAS CO LTD, TOKYO GAS CO., LTD.
Application year: 1990
Main classification: F23D14
IPC codes: F23C99, F23D14