METHOD OF OPERATING A STAGED-AIR VORTEX BURNER

Abstract: A burner for low heating value fuel includes a swirl chamber (29) for creating a swirling flow path for primary combustion air. The burner also includes means (37) communicating with the swirl chamber (29) for introducing low heating value gas to the primary combustion air for discharge with the air into a flame zone. Also provided are means (46, 48) in a gap (44) adjacent to each other for introducing secondary air and for introducing secondary low heating value gas, positioned so that the secondary air and secondary low heating value gas intersect the flame zone, preferably above the region of the recirculating core of the flame, and at such a point that the secondary air and low heating value gas have a minimal effect on the swirl aerodynamics of the flame.

Description: The present invention relates to a method of operating a high-intensity staged-air vortex burner. More particularly, the present invention relates to a novel and improved method of operating a staged-air vortex burner which is suitable for utilizing low heating value fuel under forced-draft service. In U.S. Patent 3,476,494, there is disclosed a vortex burner which includes a cylindrical combustion chamber and a constant diameter air swirl chamber, the functions of which are to contain and confine the burning process and to achieve a high degree of mixing to ensure completeness of combustion. In U.S. Patent 3,671,173 there is disclosed an improved high-intensity burner, which eliminates the necessity for a combustion chamber but which nonetheless achieves a high temperature in a relatively small but structurally unconfined volume. The device disclosed in that patent relies upon the principle of vortex air flow to achieve the high temperature in the structurally unconfined volume. In U.S. Patent 3,746,499, there is disclosed a chamberless vortex burner which employs a swirling auxiliary air flow to achieve a high concentrated degree of fuel-air mixing and high temperature in a relatively small but structurally unconfined volume. In U.S. Patent 4,175,920, there is disclosed a staged-air vortex burner which is provided with at least two fuel supply means, making it possible to fire fuel simultaneously at least at two different flow paths. Each of the foregoing burners, and particularly the last three, have been proven to be entirely satisfactory for their intended purposes. They are not satisfactory, however, for combusting low heating value, i.e., low Btu, gas fuels under forced-draft service. Lastly, U.S. patent 4,095,929, discloses a low BTU (British Thermal Unit) gas burner in which, the gas flow is separated into two independent (primary and secondary) flow streams which are respectively exhausted into primary and secondary air streams prior to combustion. The secondary gas is supplied to the flame via a gas annulus. Broadly stated, the present invention is predicated on the discovery that a high-intensity, swirl-stabilized, vortex burner is capable of operating with low heating value gas under forced-draft service provided that the low heating value gas is injected at spaced locations in the burner where the effect on swirl aerodynamics is minimized. According to the invention there is provided a method of operating a staged-air vortex burner for use in burning fuel under forced-draft service, said burner comprising a flame zone in which a flame is produced in operation of the burner; a swirl chamber for creating a swirling flow path for primary combustion air and primary fuel; means for introducing primary combustion air and primary fuel into the swirl chamber and from there to said flame zone; an annular gap surrounding the flame zone for introducing a mixture of secondary air and secondary fuel so as to intersect the flame zone; and adjacent annular fuel and air inlet openings into said annular gap, in which method, for burning a fuel having a calorific value in the range of about 90 BTu/SCF (∼3.35 kJ/dm³) to about 400 BTu/SCF (∼14.9 kJ/dm³) in the burner, part of the fuel and primary air are introduced into the swirl chamber through said primary fuel and air introducing means while the rest of the fuel and secondary air are introduced through the fuel and air inlet openings and into the flame zone, under conditions such that the mixture of secondary air and fuel intersects the burner flame at a point located beyond the recirculating core of the flame, whereby the secondary air and secondary low heating value fuel have minimal effect on the aerodynamics of the flame. In a preferred embodiment of the present invention, the burner includes means for introducing an additional fuel into the swirl chamber for combustion in the flame zone, thereby permitting operation of the burner with either one or both fuels. The present invention will be better understood from the following description, given by way of example and taken in conjunction with the appended drawings, in which: Figure 1 is a sectional elevation through one form of staged-air burner according to the present invention; Figure 2 is a perspective view of swirl chamber 29 of the burner of Figure 1; Figure 3 is a fragmentary sectional elevation of the structure of Figure 1 showing the secondary air and fuel supply means; and Figure 4 is a detailed view of a preferred wall member 40a of Figure 3. The embodiment of this invention to be described is designed to combust a low heating value gas, a gas having about 125 Btu/SCF (∼ 4.7 kJ/dm³) and typically in the range of about 90 Btu/SCF (∼3.35 kJ/dm³) to about 400 Btu/SCF (∼14.9 kJ/dm³), under forced-draft service. With the foregoing in mind, reference is now made to Figure 1, in which is shown a chamberless, high intensity vortex burner, generally designated as 10, maintained in a furnace floor or wall by suitable welds or brackets (not shown). The burner 10 has a plenum chamber 20 defined by an outer, substantially cylindrical wall 22 covered on its inner surface with a suitable layer of thermal insulation 23. The plenum chamber 20 also includes an inner end-wall 24 also lined with a suitable thermal insulation 25. The plenum chamber 20 has an inlet conduit 26 through which combustion air, which may be preheated, for example, is forced by means of an external fan (not shown) or the like into the plenum chamber 20. A flange 27 is provided on the inlet conduit 26 of the plenum chamber for connecting the air inlet conduit 26 to an appropriate source of forced air, e.g., an air supply means (not shown). Also, as can be seen from Figures 1 and 2, the swirl chamber 29 has a plurality of tangential duct means 33 communicating with the interior of the swirl chamber 29 such that when combustion air is fed under pressure to the plenum chamber 20 it will flow through the duct means 33 along a tangential flow path into the swirl chamber 29, creating a swirling flow of primary combustion air which mixes with the fuel provided to the swirl chamber 29. As can be seen in Figure 1, the upper portion of the swirl chamber 29 is surrounded by a concentric gas plenum 34. A conduit 35 extends through the floor 24 of the air plenum 20 and through the floor 36 of the gas plenum 34 for providing a low Btu gas from a gas source (not shown) to the plenum 34 for ultimate discharge into the burner for combustion therein. A plurality of tangential ducts 37 are provided which communicate with the gas plenum 34 and the upper section of the swirl chamber 29 for the tangential and swirling introduction of the primary low Btu gas into the swirl chamber. Ducts 37 are oriented in the same direction of air ducts 33 to impart the same tangentially directed swirl to the low Btu gas as is applied to the primary air stream. The relative size of the ducts 33 and 37 is predetermined based on the desired flow of low Btu gas and air into the swirl chamber 29. In general, the relative size is sufficient to provide up to about 20 percent excess air for complete combustion of the fuel. As can be seen from Figures 1 and 3, annular primary burner tile 41 made of a suitable, high-temperature castable refractory material meets with the top of the swirl chamber 29. It may be secured thereto by suitable refractory cement and a suitable support plate, such as plate 40. The upper surface of the primary burner tile 41 is outwardly sloping. A second annular burner tile 42 is provided, which has an upper annular surface 43 having a slope substantially similar to and coplanar with the outwardly sloping surface of the primary annular burner tile 41. As shown in Figures 1 and 3, the space 44 between tiles 38 and 42 forms a continuous, uniform annular gap for secondary air and secondary low Btu gas. As can be seen more readily in Figure 3, the outer wall 45 of the low Btu gas plenum 34 extends upwardly beyond the base of the primary annular ring tile 41 and then extends inwardly toward and terminating just within the entrance to gap 44, thereby defining with vertical wall member 40a, annular opening 46 through which low Btu gas enters into gap 44 and thence into the burner. Additionally, the inwardly-directed wall 45a of the low Btu gas plenum also serves to define with the sloping bottom surface 47 of the secondary tile 42 a gap 48 through which air flows into gap 44 adjacent to the low Btu gas which enters gap 44 via annular gap 46. Thus, the burner includes substantially a uniform, annular gap extending through the refractory through which secondary low Btu gas and secondary air can be injected adjacent to each other in a convergent direction toward the primary air flow so as to contain the flame envelope and obtain precise flame control and intimate mixing of fuel and air at the approximate point of fuel injection. It is particularly important in the practice of the present invention that gap 44 is positioned so as to inject the secondary low Btu gas and secondary air into the flame zone at a point of intersection with the flame, preferably above the recirculating core of the main flame. In this way, the secondary gas streams do not penetrate the recirculating core of the flame and have a minimal effect on the aerodynamics of the swirl-stabilized flame. It is also particularly preferred that the secondary gap be adjustable. Thus, a number of air gap spacers 52, for example three or more, are provided, the size of which are predetermined based on the desired air flow through gap 48. Indeed, gaps 48 and 46 are sized so as to provide that about 40 to 50 percent of the low Btu gas and air flows through the primary swirl chamber 29 and the balance flows through the secondary gap 44. This preferably sized ratio will generally require that the width of gap 46, if it is continuous, be relatively small. Therefore, it is particularly preferred to control the flow of low Btu gas into gap 44 by designing gap 46 to consist of a plurality of uniformly distributed slots. Thus, as is shown in Figure 4, wall member 40a is provided with a plurality of slots at the top edge thereof. In this way, wall member 40a can be and preferably is extended up to meet with and support inwardly directed wall 45a, with the slots providing proper passage of low Btu gas into gap 44. It should be noted that the burner is provided with the usual pilot guide tube such as tube 50. In a particularly preferred embodiment of the present invention, means for combusting an auxiliary or alternate fuel is provided. Thus, as is shown in Figure 1, extending through the end wall 24 of the plenum chamber 20 in a central aperture is a conduit 28 which extends upwardly and is in operable communication with centrally- and coaxially-disposed swirl chamber 29. Bushing means, not shown, can be used to assure that conduit 28 maintains a fluid-tight seal around its periphery between the conduit and the base of the plenum chamber 24 and the base 30 of air swirl chamber 29. An oil or high Btu gas fuel supply means 31 having a nozzle 32 is adapted to be placed within conduit 28 with the nozzle extending into the swirl chamber for delivery of fuel therein if so desired. This arrangement, of course, permits simultaneous firing of the burner with the low Btu and auxiliary fuel as well as firing with either fuel. In operation, air is forced through air duct 26 into plenum 20 through ducts 33 into the swirl chamber 29. The air entering the swirl chamber constitutes the primary air flow. Since it is introduced tangentially into the swirl chamber, a strong vortex flow is established. The remaining portion of the inlet air forced into plenum 20 enters the flame zone through the continuous, annular gap 44. This is the secondary air flow. The low Btu gas enters plenum 34 and through tangential ducts 37 into the air swirl chamber 29 so as to establish a vortex flow and mix with the air and hot gases that are recirculating therein. The portion of the low Btu gas not entering swirl chamber 29 flows through plenum 34 and gap 46 and then through the uniform, annular gap 44 into the burner. Thus, both the secondary air and low Btu gas flow through the gap 44 adjacent to each other into the burner in a direction which shapes the flame inwardly and upwardly to provide a high turbulence zone for mixing the fuel and air to achieve complete combustion. An auxiliary fuel, oil or gas, optionally is fed into the burner via nozzle 32 of the fuel gun. Indeed, it frequently is preferred to operate the burner by simultaneously supplying both low Btu gas and supplementary fuel.

Claims: A method of operating a staged-air vortex burner for use in burning fuel under forced-draft service, said burner comprising a flame zone in which a flame is produced in operation of the burner; a swirl chamber (29) for creating a swirling flow path for primary combustion air and primary fuel; means (33, 37) for introducing primary combustion air and primary fuel into the swirl chamber (29) and from there to said flame zone; an annular gap (44) surrounding the flame zone for introducing a mixture of secondary air and secondary fuel so as to intersect the flame zone; and adjacent annular fuel and air inlet openings (46, 48) into said annular gap (44), in which method, for burning a low heating value fuel having a calorific value in the range of about 90 BTu/SCF (∼3.35 kJ/dm³) to about 400 BTu/SCF (∼14.9 kJ/dm³) in the burner, part of the fuel and primary air are introduced into the swirl chamber (29) through said primary fuel and air introducing means (33, 37) while the rest of the fuel and secondary air are introduced through the fuel and air inlet openings (46, 48) and into the flame zone, under conditions such that the mixture of secondary air and fuel intersects the burner flame at a point located beyond the recirculating core of the flame, whereby the secondary air and secondary low heating value fuel have minimal effect on the aerodynamics of the flame. A method as claimed in claim 1, wherein about 40 to 50 percent of the low heating value fuel and air forms the primary fuel and air and the remainder forms the secondary fuel and air. A method as claimed in claim 1 or 2, wherein the burner includes a common plenum chamber (20) for said primary and said secondary air and a common plenum chamber (34) for said primary and secondary low heating value fuel, said air and said low heating value fuel plenum chambers (20, 34) communicating with said swirl chamber (29) and with said annular gap (44) such that a predetermined ratio of air and low heating value fuel is fed into said swirl chamber (29) and into said annular gap (44). A method as claimed in any preceding claim, wherein the burner includes an auxiliary fuel supply means (31, 32) positioned to discharge auxiliary fuel in said swirl chamber (29). A method as claimed in any preceding claim, wherein the burner includes an annular refractory tile (41, 42) surrounding said flame zone defines said annular gap (44) therein. A method as claimed in any preceding claim, wherein said burner includes a wall member (40a) having a top edge with a plurality of slots along said top edge and extending across said annular fuel inlet opening (46), so that said low heating value fuel passes through said slots into said annular gap (44).

Inventors: DAVIS, LESTER WARREN JR., GEOSITS, JOHN PETER, JUEDES, DENNIS LESTER, KICZEK, EDWARD FRANCIS
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1988
Main classification: F23C7
IPC codes: F23C7, F23D17, F23D14