Fuel jet burner and combustion method

A burner and combustion method employing oxygen or oxygen-enriched air as the oxidant comprising a low velocity fuel stream in proximity to high velocity main fuel within a combustion zone enabling efficient combustion with a stable flame at very high fuel velocities.

TECHNICAL FIELD 
This invention relates to post-mixed burners employing oxygen or 
oxygen-enriched air as the oxidant. 
BACKGROUND ART 
A post-mixed burner is a burner in which the fuel and oxidant are injected 
separately from the burner. The fuel and oxidant mix and react outside the 
burner. Most industrial furnaces use post-mixed burners. 
A number of advantages can be identified using post-mixed burners in which 
oxidant, comprising pure oxygen or oxygen-enriched air, is supplied to the 
combustion zone as high velocity jets and the fuel gas is entrained into 
the oxidant jets. One such advantage is that the burner can be designed to 
be very flexible because a wide variety of flame patterns are possible. 
The heat transfer pattern in a furnace can be altered substantially just 
by changing the oxidant nozzle. Another advantage is that the circulation 
patterns brought about the high velocity oxidant jets result in uniform 
heating of the furnace. A third advantage is that the flame can be 
directed so as to increase the heat transfer rate to the workload. A 
further advantage is that mixing of the fuel and oxidant is enhanced so as 
to ensure complete combustion. Yet another advantage is that the formation 
of nitrogen oxides is reduced with high velocity jets due to the short 
contact at high flame temperatures within the jet. 
A recent significant advance in the field of post-mixed burners is the 
aspirating burner and method developed by Dr. John E. Anderson which is 
disclosed and claimed in U.S. Pat. Nos. 4,378,205 and 4,541,796. 
There are situations, however, when oxygen or oxygen-enriched air is not 
readily available at high pressure but is readily available at a lower 
pressure. One example of such a situation is the waste oxygen from a 
cryogenic air separation plant for producing nitrogen. In these 
situations, this oxidant cannot be employed at very high velocity in a 
combustion process. Therefore, in order to attain the advantages that 
would have been possible with use of high velocity oxidant, one could 
attempt to carry out combustion by injecting fuel into the combustion zone 
as high velocity jets and entraining oxidant into the high velocity fuel 
jets. The major problem with using high velocity fuel jets is that the 
combustion flame becomes unstable before a very high velocity can be 
employed. 
It is desirable to have a post-mixed burner and method employing high 
velocity fuel jets wherein good flame stability is attained. 
Accordingly, it is an object of this invention to provide a post-mixed 
burner and method employing oxygen or oxygen-enriched air as the oxidant 
wherein the fuel may be injected directly into the furnace zone at high 
velocity and wherein good flame stability is attained. 
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 combusting gaseous fuel and oxidant comprising: 
(A) injecting into a combustion zone the major portion of the gaseous fuel 
required for the combustion as at least one stream at a high velocity V 
which is greater than 5P where P is the volume percent oxygen in the 
oxidant and V is in feet per second; 
(B) injecting into said combustion zone in proximity to the major fuel, a 
minor portion of the gaseous fuel required for the combustion, said minor 
portion comprising at least 1 percent of the total gaseous fuel injected 
into the combustion zone, at a low velocity less than 0.5 V; 
(C) injecting oxidant, comprising at least 30 volume percent oxygen, into 
the combustion zone in proximity to the minor fuel to form an interface 
between said oxidant and said minor fuel, at a velocity such that the low 
velocity of the minor fuel is within 200 feet per second of the velocity 
of the oxidant at the interface; 
(D) combusting minor fuel with oxidant at the interface; 
(E) entraining minor fuel into the high velocity major fuel immediately 
after injection of the major fuel into the combustion zone, and thereafter 
entraining oxidant into the high velocity major fuel; and 
(F) drawing hot combustion products from the interface into the high 
velocity major fuel, said hot combustion products serving as a continuous 
source of ignition for the oxidant and major fuel, and combustion oxidant 
and high velocity major fuel in a stable flame. 
Another aspect of the present invention is: 
Burner apparatus for use with pure oxygen or oxygen-enriched air as the 
oxidant comprising: 
(A) means for providing major fuel for injection into a combustion zone, 
said major fuel provision means comprising a central fuel supply tube and 
a nozzle at the injection end of the supply tube, said nozzle having at 
least one orifice therethrough for passage of gaseous fuel from the supply 
tube into the combustion zone; 
(B) an annular opening around the nozzle for providing minor fuel to the 
combustion zone in proximity to the major fuel injection so that minor 
fuel is entrained into major fuel immediately after injection of major 
fuel into the combustion zone; and 
(C) means for providing oxidant to said combustion zone in proximity to the 
minor fuel provision means, said oxidant provision means connected by 
conduit means to a source of oxidant comprising at least 30 volume percent 
oxygen, so that said oxidant and minor fuel form an interface within the 
combustion zone prior to contact between oxidant and major fuel. 
As used herein the term "combustion zone" means a volume in which fuel and 
oxidant mix and react to release heat. 
As used herein, the term "pure oxygen" means a gas having an oxygen 
concentration of at least 99.5 volume percent. 
As used herein, the term "interface" refers to the plane or space where the 
oxidant and the minor fuel interact. The interface has a finite thickness 
as the gaseous fuel diffuses into the oxidant and the oxidant diffuses 
into the gaseous fuel so as to form a combustible mixture. 
As used herein, the term "gaseous fuel" means a fuel composed of one or 
more of the following: one or more gaseous components some or all of which 
are combustible; liquid fuel droplets dispersed in a gaseous medium; solid 
fuel particles dispersed in a gaseous medium. Specific examples of gaseous 
fuel include natural gas, hydrogen, coke oven gas, and propane. 
As used herein, the term "apparent jet velocity" means the volumetric flow 
rate, at ambient pressure, leaving an orifice divided by the cross 
sectional area of the orifice.

DETAILED DESCRIPTION 
The invention will be described in detail with reference to the Drawings. 
Referring now to FIG. 1, passage 1 is connected by conduit means 2 to a 
source (not shown) of gaseous fuel. The fuel passes out of passage 1 
through a nozzle 9 having one or more orifice openings 12 and into 
combustion zone 3. 
FIG. 1 illustrates a preferred embodiment of the invention wherein the low 
velocity fuel stream proximate to the high velocity fuel stream is an 
annular stream which forms an annular envelope around the high velocity 
stream. However, the low velocity fuel stream need not completely surround 
the high velocity fuel stream. The annular stream could also comprise a 
number of low velocity jets from a number of holes rather than a 
continuous annular opening. Furthermore, if, for example, the high 
velocity fuel stream were not round but were a plane jet, the low velocity 
fuel stream could be an adjacent plane stream. 
Referring back now to FIG. 1, coaxial to passage 1 is annular passage or 
opening 4 which is connected by conduit means 5 to a source (not shown) of 
gaseous fuel. Gaseous fuel passes out of annular passage 4 directly into 
combustion zone 3 and forms an envelope surrounding the fuel passing out 
of central passage 1 at, and for a short distance beyond, their respective 
injection points. 
Oxidant comprising at least 30 volume percent oxygen is provided to 
combustion zone 3 by means separate from the fuel passages so that at the 
start of the combustion zone, the oxidant is proximate to the low velocity 
fuel stream. In the embodiment illustrated in FIG. 1, oxidant is provided 
directly to the combustion zone 3 through passage 6 which is coaxial and 
next to the outer surface of the low velocity fuel annular stream at their 
respective injection points. Passage 6 is connected by conduit means to a 
source (not shown) of pure oxygen or oxygen-enriched air comprising at 
least 30 volume percent oxygen. Sources of oxygen include, for example, a 
gas storage cylinder, a liquid oxygen tank form which oxygen is vaporized 
prior to use and, for larger requirements, an air separation plant such as 
a cryogenic rectification plant or a pressure swing adsorption plant. 
Oxygen-enriched air may also be produced by combining high purity oxygen 
with air and passing the combined stream to oxidant passage 6. 
Passage 1 carries the major portion of the gaseous fuel necessary for 
combustion with the oxidant, and coaxial passage 4 carries the minor 
portion or the remainder of the total fuel. The minor fuel comprises at 
least 1 percent of the total fuel supplied to the combustion zone. 
Preferably the minor fuel comprises less than 10 percent of the total fuel 
supplied to the combustion zone. 
The major fuel is injected into combustion zone 3 from passage 1 as a high 
velocity jet 8 with a velocity V at the exit orifice 12 greater than 5P, 
where V is velocity in ft/sec and P is the volume percent oxygen in the 
the oxidant. 
The minor fuel is injected into the combustion zone 3 from annular passage 
or opening 4 at a low velocity less than 0.5 V such that the low velocity 
of the minor fuel is within 200 feet per second of the velocity of the 
oxidant at their interface. Preferably the velocity of the minor fuel is 
less than 100 ft/sec. 
Referring again to FIG. 1, a low velocity stream of fuel flows from exit 7 
at the end of passage 4. This low velocity stream forms an envelope around 
the high velocity jet 8 of the major fuel exiting orifice 12 at the end of 
passage 1. The high velocity jet from orifice 12 entrains surrounding 
gases as it passes through combustion zone 3. The first gas entrained 
after leaving the nozzle is the low velocity annulus stream of fuel which 
is entrained into the major fuel immediately after injection of the major 
fuel into the combustion zone. This is followed by the entrainment of 
oxidant. The flow lines for the oxidant being entrained into the fuel jet 
8 are represented by the dotted lines 10 in FIG. 1. A combustion interface 
11 is formed between the low velocity fuel stream and the oxidant as both 
streams are drawn into the high velocity fuel jet. Ignition of the oxidant 
and low velocity fuel is initiated by any suitable ignition means. Since 
the annulus stream of the minor fuel is moving at a low velocity relative 
to the oxidant, a stable flame can be maintained at the interface 11. The 
hot combustion products from this flame are drawn into the jet at point 
13, generally less than a distance equal to 6 times the diameter of 
orifice 12 in nozzle 9, and before any substantial dilution of the major 
fuel by furnace gases can occur. This is the exact point where oxidant is 
starting to be drawn into the high velocity fuel jet and starts mixing 
with the major fuel. For the embodiment of the invention as shown in FIG. 
1, the intersection 13 is a circle around the periphery of the fuel jet. 
The hot combustion products from the annulus fuel-oxidant flame serve as a 
continuous source of ignition at the intersection point 13 wherein the 
oxidant and major fuel first meet. This is the ideal location for 
ignition. This continuous source of ignition prior to any substantial 
dilution of the major fuel by furnace gases, makes it possible to maintain 
stable combustion with the oxidant and fuel at the interface of the high 
velocity fuel jet 8. This is accomplished without changing the integrity 
of the jet of major fuel. 
FIGS. 2-4 illustrate the embodiment of the invention in a burner using a 
single orifice for the high velocity fuel. The fuel nozzle is shown 
separately in FIG. 3 and is shown incorporated into the burner in FIGS. 2 
and 4. The flame is illustrated in FIG. 4. Referring to FIG. 4, the high 
velocity fuel is injected through the single orifice 20 while the low 
velocity fuel is injected through the annulus passage 21. Oxidant is 
injected into the combustion zone through passage 26. As the low velocity 
fuel and oxidant are entrained into the high velocity fuel jet 22, a 
stable flame is formed at the interface 27 with the low velocity fuel on 
one side and oxidant on the other side. This flame at the interface forms 
an envelope around the nozzle meeting the high velocity fuel jet 22 at 
point 24. This point is where oxidant is first drawn into the high 
velocity fuel jet. Continuous ignition between the oxidant and major fuel 
is established and maintained at point 24 by the supply of hot combustion 
products from the flame at interface 27. Nozzle 23 is removeable and is 
screwed into the fuel supply tube 29 at threads 28. In this way the major 
fuel nozzle can be easily replaced and the burner altered to operate in a 
different mode as desired. 
In a preferred embodiment of the invention, the high velocity fuel is 
injected into the combustion zone through a plurality of small diameter 
orifices. With a single, large diameter orifice, the high velocity jet 
entrains the oxidant over an extended distance and the entrainment near 
the high velocity fuel orifice may be insufficient to overcome buoyancy 
and natural convection forces. As a result, oxidant can escape the high 
velocity fuel jet and remain unreacted. FIGS. 5 and 6 illustrate the 
embodiment of the invention in a burner using four orifices for the high 
velocity fuel. The fuel nozzle is shown separately in FIG. 6 and is shown 
incorporated into the burner in FIG. 5. As illustrated in FIG. 5, the four 
orifices 30 are angled out from the burner axis so that the high velocity 
fuel jets do not interfere with each other. By changing the single orifice 
nozzle 23 in FIG. 4 with the nozzle 31 in FIG. 5 containing four orifices, 
the entrainment of the oxidant occurs closer to the high velocity fuel 
orifice and the tendency to forego oxidant reaction due to buoyancy and 
natural convection forces is greatly reduced. In this preferred embodiment 
of the invention, it is particularly preferred that there be more than 
three orifices for the high velocity fuel and that each orifice be less 
than 1/4 inch in diameter. 
Another embodiment of the invention is shown in FIGS. 7, 8, and 9. The fuel 
nozzle is shown separately in FIG. 8 and is shown incorporated into the 
burner in FIGS. 7 and 9. The flame is illustrated in FIG. 9. The single 
orifice nozzle 23 in FIG. 4 is replaced by a multi-orifice nozzle 50 as 
shown in FIG. 9. Twelve orifices 51 are evenly spaced around a circle on 
the face of the nozzle 52. The orifices are angled out from the axis of 
the burner. In order to accommodate the orifices with adequate spacing 
between adjacent orifices, the nozzle is extended beyond exit 53 of the 
annulus fuel passage 54 and enlarged to provide greater area for the 
nozzle face 52. In this embodiment of the invention, the low velocity 
annulus stream of fuel leaving exit 53 flows along the periphery of the 
nozzle 50 before being entrained into the high velocity jets 55 of the 
major fuel. The flame at the interface 56 of the low velocity fuel and the 
oxidant forms an envelope around the portion of the nozzle extending into 
the combustion zone. Continuous ignition of the oxidant from passage 57 
and the high velocity fuel jets 55 is provided at the point 58 where the 
oxidant-annulus fuel interface 56 intersects with the high velocity jets 
55. 
In a preferred embodiment of the invention, the orifices 51 are angled out 
from the burner axis by more than 10 degrees. 
Another embodiment of the invention is shown in FIGS. 10 and 11. The fuel 
nozzle is shown separately in FIG. 11 and is shown incorporated into the 
burner in FIG. 10. The fuel nozzle 61 has a spherical surface 62. The 
orifices 63 for the high velocity fuel jets are drilled perpendicular to 
this surface towards the center of the sphere. The orifices are located on 
three concentric circles 64, 65, and 66 with a common center as shown in 
FIG. 11. Gaseous fuel to the orifices is supplied through passage 67. A 
small portion of the fuel, between 1 and 10 percent of the total fuel, is 
withdrawn from passage 67 through bleed passages 68 to the annular passage 
69. Oxidant is supplied through passage 70 surrounding the nozzle. The 
fuel existing passage 69 provides the low velocity fuel stream required to 
stabilize the flame around the high velocity fuel jets from orifices 63. 
The stream of fuel from annular passage 69 flows along the spherical 
surface prior to being entrained into the high velocity jets. The flame at 
the interface of the annular fuel stream and the oxidant forms an envelope 
around the spherical surface of the nozzle. 
The common center for the concentric circles in FIG. 11 is located along 
the axis of the burner. The invention can also be practiced with the 
center of the concentric circles at a point removed from the axis as shown 
in FIGS. 12 and 13. The fuel nozzle is shown separately in FIG. 13 and is 
shown incorporated into the burner in FIG. 12. A line 83 is shown in FIG. 
12 at an angle 81 with a line 84 drawn through the burner axis. Line 83 
intersects the nozzle surface at point 82. The concentric circles for 
locating the orifices on the nozzle surface have a common center on the 
line 83. The low velocity fuel annulus stabilizes the flames around the 
high velocity fuel jets in the same manner as described for the burner 
configuration in FIGS. 10 and 11. The flame pattern is determined by the 
direction of the high velocity fuel jets. When the invention is practices 
as shown in FIGS. 12 and 13, the jet flames are then angled away from the 
burner axis. In this way, the nozzle can be changed to obtain the same 
effect as angling the whole burner. 
The invention can also be practiced with multi-orifice nozzles with an 
asymmetric hole pattern. 
The following Example serves to further illustrate the present invention or 
to provide comparative results, and is presented for illustrative purposes 
and is not intended to be limiting. 
EXAMPLE 
A burner, similar to that illustrated in FIG. 1, was employed to combust 
fuel and oxidant injected into a combustion zone. The fuel was natural gas 
and the oxidant was pure oxygen. The fuel was injected into the combustion 
zone through a nozzle having a single orifice, 1/16 inch diameter. The 
burner was operated with and without the low velocity annulus fuel. 
Without the fuel flow to the annulus, the flame was stable up to the fuel 
flowrate of 66 cubic feet per hour (CFH) corresponding to a jet velocity 
of 860 ft/sec at the nozzle. When the fuel flowrate was increased further, 
the flame became detached from the nozzle, an unstable condition for 
practical applications. The procedure was repeated with fuel flow through 
the annulus at a flowrate of 4.2 CFH corresponding to a stream velocity of 
1.7 ft/sec. As the fuel flowrate to the nozzle was increased, the flame 
remained stable and attached to the nozzle up to a flowrate of 220 CFH 
corresponding to an apparent jet velocity of 2880 ft/sec at the nozzle. 
This was the highest flow rate that could be obtained through the flow 
lines at the available fuel pressure. The oxygen flowrate was 350 CFH 
corresponding to a stream velocity of 7.4 ft/sec. When the flow of fuel to 
the annulus was shut off, the flame became detached from the nozzle and 
was noisy and unstable. The small flow of fuel to the annulus increased 
the flowrate of fuel that could be passed through the nozzle, while 
maintaining a stable flame attached to the nozzle, by more than three 
fold. 
Now by the use of the burner and method of this invention one can carry out 
efficient and stable combustion at very high fuel velocity using oxygen or 
oxygen-enriched air as the oxidant. 
Although the invention has been discussed in detail with reference to 
certain specific embodiments, those skilled in the art will recognize that 
there are other embodiments of this invention within the spirit and scope 
of the claims.