Coherent jet combustion

Combustion method and apparatus establishing coherent jet oxidant flow within a cavity recessed from the main combustion area enabling operation without need for water cooling.

TECHNICAL FIELD 
The invention relates generally to the field of burners and combustion 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 burner and 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burner is withdrawn into the cavity.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 combustion 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. 
Accordingly 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. 
It is a still further object of this invention to provide a burner which 
can use oxygen or oxygen-enriched air as the oxidant while not requiring 
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 one aspect of which is: 
A method for carrying out combustion comprising: 
(A) providing a walled cavity communicating with a combustion zone; 
(B) providing fuel into the cavity and flowing the fuel through the cavity 
near the cavity wall; 
(C) providing main oxidant into the cavity and flowing the main oxidant at 
a high velocity through the cavity; 
(D) 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 
(E) passing fuel and oxidant into the combustion zone for combustion 
therein. 
Another aspect of the invention comprises: 
Non-water-cooled combustion apparatus comprising: 
(A) a walled cavity having an output end; 
(B) means for providing fuel into the walled cavity recessed from the 
output end; 
(C) means for providing main oxidant into the cavity recessed from the 
output end; and 
(D) means for providing secondary oxidant into the cavity recessed from the 
output end between the fuel and the main oxidant, said cavity having a 
length sufficient to maintain the main oxidant and fuel substantially 
unmixed within the cavity by the interpositioned secondary oxidant. 
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.

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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 0.125 to 0.5 inch. 
Nozzle 14 may have any effective configuration. One such configuration is 
shown in FIGS. 3 and 4. Referring now to FIGS. 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 FIGS. 3 and 4 has eight such jet passages 32. FIG. 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 
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 500 fps. Referring back to FIG. 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 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 FIG. 1 there is shown 20 the use 
of the igniter disclosed in U.S. Pat. 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. 
FIG. 2 illustrates in simplified form the operation of the invention to 
achieve coherent jet flow through the walled cavity. Referring now to FIG. 
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 FIG. 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&gt;0.5 F and L&lt;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 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 1000 fps and the secondary oxidant velocity was about 80 
fps. The main oxidant comprised 8 high velocity jets. The burner was fired 
at a firing rate of 10 MM Btu/hr. A thermocouple was set in a stainless 
steel nozzle 1/8 inch back from the burner face. The burner was recessed 
in a cavity 4.5 inches in diameter. As the recess length was increased 
form 1 to 6 to 8 inches, the measured nozzle temperatures were 
425.degree., 398.degree. F. and 412.degree. 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. 
Although the invention has been described in detail with reference to 
certain embodiments, those skilled in the art will recognize that there 
are other embodiments of the invention within the spirit and the scope of 
the claims.