Coherent jet combustion

Abstract: Combustion method and apparatus establishing coherent jet oxidant flow (27) within a cavity (22) recessed from the main combustion area (24) enabling operation without need for water cooling.

Description: Technical Field The invention relates generally to the field of combustion processes and is particularly advantageous for use with burners and combustion employing oxygen or oxygen-enriched air as the oxidant. Background Art In the operation of a burner to carry out combustion, especially in an industrial setting, a major problem is heat damage to the burner. One conventional method for reducing heat damage to a burner is to circulate a coolant such as water through the burner. While this method provides adequate cooling to the burner, it has several disadvantages. A supply of clean water is required at the requisite pressure and such a supply may not always be readily available without considerable effort and additional equipment. The burner design itself is considerably more complicated to accommodate the coolant passages. The cooled burner surface temperature may be low enough to cause condensation of furnace gases which may corrode the burner. Coolant leaks may develop within the burner which can cause damage to the burner or a shutdown of the combustion operation. For all of these reasons it is desirable to have a combustion method which can operate without the need for employing circulating coolant. One method for addressing this problem which has found use in industrial combustion operations comprises recessing the burner from the furnace or combustion zone. Generally such a burner is recessed within a cavity in the furnace wall. In this way less heat or energy from the combustion zone is radiated to the burner surface and thus a separate coolant is not needed. Heat transfer by radiation from the furnace decreases as the burner is withdrawn into the cavity. However, with a burner recessed within a cavity, combustion may, and usually does, occur within the cavity thus generating heat close to the burner surface and thereby increasing the heat to the burner. With air as the oxidant there is a large volume of oxidant flow which can be used to cool the burner and refractory walls. Moreover, the flame temperature for combustion with air is lower than that for combustipn with oxygen or oxygen-enriched air so that combustion within the cavity usually does not have serious consequences. However the problem of heat damage to a recessed burner by combustion within a cavity becomes more acute as the oxygen concentration of the oxidant is increased to concentrations significantly greater than that of air. Accordingly, in such situations a burner is recessed only a small distance from the combustion zone thus reducing the protective effect of the recession, and/or the flowrates of fuel and oxidant are very carefully controlled to diminish combustion proximate the burner which serves to complicate the combustion operation and to reduce its efficiency. In a method for carrying out combustion in a combustion zone communicating with a walled cavity, known from US-A-3 044 539, a relatively high velocity central main air stream is supplied to the cavity through a central large air opening. Furthermore, a small amount of secondary air is directed to depressions in the front face of a burner ring through small passages, which are circumferentially spaced around the central large air opening and are inclined forwardly and radially outwardly at a small angle, to burn with fuel gas which is supplied to these depressions through small passages which likewise are circumferentially spaced around the central large air opening and are inclined forwardly and radially inwardly. Excess fuel gas which is not burned with the secondary air mixes with the main air stream for being combusted. This combustion creates flame turbulence within the walled cavity. It is an object of this invention to provide a combustion method which can operate efficiently without the need for water cooling. It is a further object of this invention to provide a burner which can operate without damage caused by heat and without the need for water cooling. It is yet another object of this invention to provide a combustion method employing oxygen or oxygen-enriched air as the oxidant which can be carried out without the need for water cooling. Summary of the Invention The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention which is: A method for carrying out combustion in a combustion zone communicating with a walled cavity, (A) providing fuel into the cavity and flowing the fuel through the cavity near the cavity wall; (B) providing main oxidant into the cavity and flowing the main oxidant at a high velocity through the cavity; (C) providing secondary oxidant into the cavity and flowing the secondary oxidant, at a velocity less than that of the main oxidant, through the cavity between the flowing fuel and the flowing main oxidant to maintain the flow of main oxidant substantially coherent as it flows through the cavity; and (D) passing said fuel and said main and secondary oxidant from the cavity into the combustion zone wherein the flow of main oxidant is not coherent and mixes with and combusts with the fuel. As used herein the term "water cooling" means the use of water or a water-containing liquid to cool a burner. As used herein the term "coherent" means the flow of gas with little or no entrainment of ambient gas into the flowing gas. As used herein the term "jet" means a stream of gas leaving a hole in a nozzle at a substantially high velocity. In a conventional jet there is entrained into the jet a substantial amount of ambient gas due to turbulence at the jet-ambient gas interface causing the jet to expand as it flows from the nozzle. In a coherent jet only a minimal amount of ambient gas is entrained into the jet and the jet expands very little as it flows from the nozzle face. Brief Description of The Drawings Figure 1 is a cross-sectional representation of one preferred embodiment of the invention. Figure 2 is a simplified cross-sectional representation of the invention in operation. Figure 3 is a cross-sectional representation of one embodiment of a main oxidant nozzle useful with the invention. Figure 4 is a head-on view of the nozzle illustrated in Figure 3. Detailed Description The invention comprises, in general, the establishment and use of one or more high velocity coherent oxidant jets within a cavity recessed from a combustion zone which serve to retard combustion within the cavity but which serve to promote stable combustion upon passage from the cavity output end into the combustion zone. The invention will be described in detail with reference to the Drawings. Referring now to Figure 1 which illustrates one preferred embodiment of the invention, burner 1 comprises fuel tube 2 and oxidant provision means 3 which provide fuel and oxidant into walled cavity 4 which communicates at output end 5 with combustion zone 6. The fuel and oxidant may be provided into cavity 4 at its input end 7 as illustrated in Figure 1 or at any point within the walled cavity recessed from its output end. The walled cavity may have any desired configuration. For example the walled cavity may have outwardly tapered walls 8 so that it is conical as illustrated in Figure 1 or may have straight walls so that it is cylindrical. The walled cavity is situated within refractory 9 which is generally part of a furnace wall. The burner 1 is secured to the refractory wall cavity by means of block plate 10 and flange 11. Fuel is provided into cavity 4 through outer annular passageway 12 and flows through cavity 4 near cavity wall 8 and then through output end 5 into combustion zone 6. The fuel is a gaseous fuel such as natural gas, methane, propane, or coke oven gas. Oxidant is provided into cavity 4 through oxidant provision means 3. In the embodiment illustrated in Figure 1 the oxidant provision means comprises outer tube 13 containing screwable removable nozzle 14 which is slightly smaller than outer tube 13 so as to define an annular passageway 15. Preferably, as illustrated in Figure 1, the end 16 of oxidant provision means 3 is recessed from the inlet end 17 of the cavity such as by the distance x illustrated in Figure 1. The distance x will vary depending upon the absolute size of the burner apparatus. The injection end of the burner may be upstream of or within the walled cavity. In addition the end 18 of nozzle 14 is preferably recessed from end 16 by a distance y which may be within the range of from 3.18 to 12.7 mm (0.125 to 0.5 inch). Nozzle 14 may have any effective configuration. One such configuration is shown in Figures 3 and 4. Referring now to Figures 3 and 4, nozzle 30 comprises threads 31 with which it may be inserted into the outer tube. Nozzle 30 also comprises a plurality of individual jet passages 32 through which oxidant is provided at a high velocity into the walled cavity. The nozzle illustrated in Figures 3 and 4 has eight such jet passages 32. Figure 4 illustrates a preferred arrangement wherein the jet passages are in a circular arrangement. Nozzle 30 also has annulus oxidant feed means 33 through which oxidant flows into the annular passageway. The oxidant may be any effective oxidant. The advantages attainable with this invention are most noticeable when the oxidant has an oxygen concentration of 30 percent or more. A particularly preferred oxidant is technically pure oxygen having an oxygen concentration of 99.5 percent or more. Main oxidant is provided into the walled cavity and flows therethrough at a high velocity. Generally the velocity of the main oxidant is greater than 1.52 P where P is the volume percent of oxygen in the main oxidant and the velocity is in m/s (5P where P is the volume percent of oxygen in the main oxidant and the velocity is in feet per second (fps)). Preferably the main oxidant velocity is at least 152 m/s (500 fps). Referring back to Figure 1, the main oxidant is provided into walled cavity 4 as one or more high velocity jets through jet passages 19. Preferably the main oxidant is provided into and flows through the walled cavity as a plurality of jets. The jets will number generally within the range of from 1 to 16, preferably within the range of from 1 to 8. Secondary oxidant is provided into the walled cavity and flows therethrough at a velocity which is less than that of the main oxidant. Generally the velocity of the secondary oxidant is less than one half of the velocity of the main oxidant and preferably is less than 30.5 m/s (100 fps). The secondary oxidant will generally comprise from about 1 to 10 percent of the total oxidant provided into the walled cavity. Ignition of the combustible mixture may be attained by any convenient means. In the embodiment illustrated in Figure 1 there is shown 20 the use of the igniter disclosed in U.S. Patent No. 4,892,475-Farrenkopf et al.as the means for igniting the combustion reaction. In operation, the high velocity main oxidant passes through the walled cavity as one or more high velocity jets while the lower velocity secondary oxidant flows through the walled cavity between the high velocity main oxidant and the fuel thus serving to maintain the flow of main oxidant substantially coherent as it flows through the walled cavity. In a particularly preferred embodiment the secondary oxidant due to its lower velocity mixes with and combusts with fuel within the cavity and this combustion forms a thin envelope around the high velocity oxidant. It is believed that if a combustion reaction is occurring at the main oxidant jet boundary, then the temperature will increase with a corresponding increase in gas volume. The gas at the interface will expand resulting in a velocity component in the radial direction. As the gas moves out radially, the turbulent boundary layer that is initiated at the high velocity - low velocity interface is continuously removed along the length of the jet in the same way as may happen with the use of suction for gas flow over a flat plate wherein the suction continuously removes the turbulent boundary layer and prevents it from growing. If the turbulent boundary layer is continuously removed for the length of the jet, entrainment will be minimized and the jet will be coherent. Figure 2 illustrates in simplified form the operation of the invention to achieve coherent jet flow through the walled cavity. Referring now to Figure 2, burner 21 is placed within walled cavity 22 and is recessed from the output end 23 of cavity 22 which communicates with combustion zone 24. Fuel is provided into cavity 22 through passage 25 and lower velocity secondary oxidant is provided into cavity 22 through annular passage 26. There is established within cavity 22 one or more coherent jets 27 of high velocity oxidant which pass through cavity 22 with minimal combustion of the high velocity main oxidant with the fuel. The combustion of the lower velocity secondary oxidant with the fuel results in a buffer layer of combustion products around the main oxidant. Upon passage into the combustion zone through the output end 23 of cavity 22 the consequent turbulence 28 causes the coherent jet or jets to break up resulting in rapid entrainment or mixing of the fuel with the oxidant resulting in effective stable combustion. As a consequence of the fact that most of the combustion occurs in the combustion zone well away from the burner which is recessed within the walled cavity, very little of the heat released by the combustion is radiated to the burner. Thus the burner need not be water cooled to avoid damage from heat. It is preferred that the burner be as deeply recessed from the output end of the walled cavity as possible because the deeper the recession the greater is the protection of the burner from heat damage. However if the burner is recessed too deeply, the coherency of the high velocity main oxidant jet(s) may break down prior to their flow through the output end into the combustion zone resulting in significant combustion within the walled cavity with corresponding overheating. This defeats the purpose of the invention. The maximum length that the burner may be recessed will vary with each case depending on the absolute size of the burner apparatus. As mentioned, the embodiment of the invention illustrated in Figure 1 wherein the cavity has a conical shape and the burner is upstream of the cavity is a preferred embodiment. Applicants have found that with such an embodiment the most advantageous results are attained where A > 0.0110 F and L < 0.5935 √F where : A = area of the smaller cone opening at the burner end in cm 2 , F = firing rate of the burner in GW and L = refractory cone length in cm (A > 0.5 F and L < 4 (square root of F) where: A = area of the smaller cone opening at the burner end in square inches, F = firing rate of the burner in MMBtu/hr and L = refractory cone length in inches). In addition it is preferred that the cone half angle be less than 15 degrees. The following Example is provided for illustrative purposes and is not intended to be limiting. A burner of this invention was operated within a cylindrical cavity recessed from the cavity output end. The recess was varied from 2.5 to 20.3 cm (1 to 8 inches). The oxidant employed was technically pure oxygen and the fuel was natural gas. The main oxidant velocity was greater than sonic velocity which is about 305 m/s (1000 fps) and the secondary oxidant velocity was about 24 m/s (80 fps). The main oxidant comprised 8 high velocity jets. The burner was fired at a firing rate of 2930 GW (10 MM Btu/hr). A thermocouple was set in a stainless steel nozzle 3.2 mm (1/8 inch) back from the burner face. The burner was recessed in a cavity 11.4 cm (4.5 inches) in diameter. As the recess length was increased form 2.5 to 15.2 to 20.3 cm (1 to 6 to 8 inches), the measured nozzle temperatures were 218°C, 203°C and 211°C (425°F, 398°F and 412°F) respectively. The oxygen jets were coherent within the cavity. These tests demonstrated that the burner apparatus and combustion method can be employed without the need for water cooling while avoiding overheating.

Claims: A method for carrying out combustion in a combustion zone communicating with a walled cavity comprising: (A) providing fuel into the cavity (4, 22) and flowing the fuel through the cavity near the cavity wall; (B) providing main oxidant into the cavity and flowing the main oxidant at a high velocity through the cavity; (C) providing secondary oxidant into the cavity and flowing the secondary oxidant, at a velocity less than that of the main oxidant, through the cavity between the flowing fuel and the flowing main oxidant to maintain the flow of main oxidant coherent as it flows through the cavity; and (D) passing said fuel and said main and secondary oxidants from the cavity into the combustion zone (6, 24) wherein the flow of main oxidant is not coherent and mixes with and combusts with the fuel. The method of claim 1 wherein at least one of the main oxidant and the secondary oxidant has an oxygen concentration of 30 percent or more. The method of claim 1 wherein at least one of the main oxidant and the secondary oxidant comprises technically pure oxygen. The method of claim 1 wherein the velocity of the main oxidant is greater than 152 m (500 feet) per second. The method of claim 1 wherein the velocity of the secondary oxidant is less than one half of that of the main oxidant. The method of claim 1 wherein the secondary oxidant comprises from 1 to 10 percent of the total oxidant provided into the walled cavity. The method of claim 1 wherein the secondary oxidant is provided into the walled cavity as an annular stream around the main oxidant. The method of claim 1 wherein the main oxidant is provided into the walled cavity in a plurality of jets. The method of claim 8 wherein the number of main oxidant jets is within the range of from 1 to 16. The method of claim 1 wherein the secondary oxidant combusts with fuel within the cavity and forms an envelope comprising combustion products around the main oxidant within the cavity.

Inventors: ANDERSON JOHN ERLING, SNYDER WILLIAM JOSEPH, ANDERSON, JOHN ERLING, SNYDER, WILLIAM JOSEPH
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1992
Main classification: F23C7
IPC codes: F23D14