Method of low-NOx combustion and burner device for effecting same

A method of low-NOx combustion and a burner device for effecting the same, in which a primary fuel is injected in a direction from tile periphery of stream of a combustion air towards that same combustion air, effecting a first combustion, so as to create a generally cylindrical primary flame covering the combustion air, whereby a secondary fuel injected towards the combustion air is shielded or intercepted by such primary flame from the combustion air, while causing NOx in tile primary flame to be reduced by the secondary fuel, after which, a second combustion is effected by bringing the secondary fuel to contact with a portion of the combustion air penetrating through the primary flame, at a downstream side. This arrangement permits more positive decrease of NOx density in an exhaust gas.

BACKGROUND OF INVENTION 
1. Field of the Invention 
The present invention relates to a method of low-NOx combustion and a 
burner device for effecting the same. More particularly, the invention is 
directed to an improvement of a two-staged low-NOx combustion method and a 
two stage firing burner device for carrying out tile method. 
2. Description of Prior Art 
Among various conventional low-NOx combustion methods, there has been known 
a two-staged method comprising two fuel supply stages for doing the 
combustion at two stages, as disclosed, for instance, from tile Japanese 
Patent No. 1104160. (Hereinafter, tills method will be referred to as 
"two-stage fuel combustion method".) Such two-stage fuel combustion method 
is normally executed by a burner device as shown in FIG. 1. According 
thereto, there is provided a burner device BD' which has a burner throat 
103 formed therein and one piece of primary fuel nozzle 101 disposed 
within the burner throat 103. Further, a plurality of secondary fuel 
nozzles 102 are provided around the outside opening of tile burner throat 
103. Each of those secondary fuel nozzles 102 is oriented toward a primary 
flame which is to be flowed out from the burner throat 103. With this 
device, a whole amount of combustion air (as designated by "Air" in FIG. 
1) is supplied in the throat 103, and a primary fuel is injected from the 
primary fuel nozzle 101 toward the combustion air, such that the primary 
fuel is embraced or circumscribed by the air, to thereby effect a 
combustion and create the primary flame. Then, in the vicinity of the 
opening of burner throat 103, a secondary fuel is injected from the 
secondary fuel nozzles 102 toward the thus-created primary flame, creating 
thus a secondary flame. Namely, in tills sort of combustion method, the 
first combustion stage uses the whole amount of combustion air to burn the 
primary fuel under a proper excess air condition set by an suitable excess 
air ratio (i.e. the so-called "air rich" condition), and then, the 
secondary fuel is injected to such first combustion, reducing a part of 
NOx existing in the primary flame and thereafter bringing the primary fuel 
in contact with the downstream portion of combustion air which remains not 
burned through the primary flame, so as to effect a second combustion, 
creating a secondary flame. 
However, the above-described conventional method and burner device inject 
out the combustion air from the burner throat 103, in such a way that the 
primary flame is surrounded by the air, which has been found defective in 
that the combustion air, which flows in tile thin-arrow direction in FIG. 
1, results in expanding its stream at tile exit of burner throat 103 as 
indicated by the arrow As, and the expanded portion of air directly 
contacts the secondary fuel injected from the secondary fuel nozzles 102, 
causing a combustion in this particular area. Hence, a part of tile 
secondary fuel is directly contacted with such leaked air (As) before 
contact with the primary flame, starting thus a secondary combustion in 
advance. Consequently, tile combustion air is not fully used to reduce the 
NOx in the primary flame and there is a problem of insufficient NOx 
reduction. Although this prior-art technique serves the low NOx purpose 
based on the thick and thin fuel combustion principle, more effectively 
than most of ordinary combustion techniques, yet there is a room of 
improvement for the reason above. 
SUMMARY OF THE INVENTION 
In view of the above-stated drawbacks, it is therefore a primary purpose of 
the present invention to provide a method of low-NOx combustion which 
enables more positive decrease of NOx density. 
In order to achieve such purpose, in accordance with the present invention, 
there is basically provided the steps of: 
injecting a substantially whole amount of combustion air through a burner 
throat; 
then, at a first fuel supply stage, injecting a primary fuel from the 
periphery of stream of tile combustion air towards tile same combustion 
air, thereby subjecting tile primary fuel to a first combustion so as to 
create a generally cylindrical shape of primary flame covering the stream 
of combustion air; and 
at a second fuel supply stage, injecting a secondary fuel towards tile 
thus-created primary flame from outside thereof, and further subjecting 
tills secondary fuel to a second combustion with a portion of tile 
combustion air which penetrates through the primary air at a downstream 
side so as to create a secondary flame, 
whereby the combustion air is initially covered with the primary flame 
before a point where the secondary fuel is injected, so that tile 
secondary fuel, immediately after being injected towards said combustion 
air, is shielded or intercepted by said primary flame from said combustion 
air, thus causing contact of said secondary fuel with said primary flame 
to reduce NOx, and thereafter, the second combustion is carried out. 
It is a second purpose of the present invention to provide an improved 
burner device for effecting the above-mentioned low-NOx combustion method. 
To attain this purpose, in accordance with the present invention, there is 
basically provided a burner device for the low-NOx combustion which 
comprises: 
a burner throat means through which a substantially whole amount of 
combustion air is injected; 
a first injection means for injecting a primary fuel towards said 
combustion air, which first injection means is provided in the burner 
throat means and having an injection axis oriented towards a central axis 
of the burner throat; 
a secondary injection means for injecting a secondary fuel in a direction 
towards the primary fuel from outside thereof. 
In both of the foregoing method and burner device, it is preferable that 
the injection axis of the first injection means is oriented at an angle in 
a direction towards a downstream with respect to tile combustion air in 
order to inject the primary fuel to the combustion air in such direction. 
In one aspect of the invention, the injection axis of the first injection 
means may be oriented in a direction tangential to the inner surface of 
the burner throat means, to thereby inject the primary fuel to the 
combustion air it that tangential direction so as to create a generally 
cylindrical primary flame in a vortex manner. 
In another aspect of the invention, the burner throat manes may be of a 
generally cylindrical shape and the first injection means may comprise a 
plurality of nozzles disposed along such cylindrical shape of burner 
throat means, so that the primary fuel is injected in a direction from the 
circumference of circle towards the combustion air, thereby creating a 
generally circular cylindrical shape of primary flame. 
In still another aspect of the invention, the burner throat means may be of 
a rectangular cylindrical shape and the first injection means may comprise 
a plurality of nozzles along such rectangular cylindrical shape of burner 
throat means, so that the primary fuel is injected in a direction from the 
rectangular line towards the combustion air, thereby creating a flat 
configuration of primary flame having a generally rectangular 
cross-section. 
Preferably, two or more first injection means may be provided equidistantly 
in the inner surface of the burner throat means, and also two or more 
second injection means be provided adjacent to the exit of the burner 
throat means. 
Preferably, the burner throat means may a burner tile throat disposed at at 
downstream side with respect to the combustion air, and inner throat 
member disposed at an upstream side with respect to the same air, the 
inner throat member extending towards the inside of burner the throat in 
registry with an inner surface of the same burner tile throat, and further 
tile first injection means may be provided between those burner tile 
throat and inner throat member. In this case, more preferably, the second 
injection means may be provided adjacent the exit of burner tile throat 
such as to be oriented towards tile central axis of those burner throat 
elements. 
In addition, a baffle plate may preferably be provide adjacent to the 
injection holes of the first injection means and further disposed at an 
upstream side relative to the combustion air. 
It is also preferable that an air velocity adjustment means be provided 
within the burner throat means such as to be disposed coaxially relative 
to tile central axis of burner throat means, whereby a velocity 
distribution of the combustion air injected through the burner throat 
means may be adjusted properly in order to insure a better formation of 
the cylindrical primary flame. 
Accordingly, tile formation of generally cylindrical primary flame serves 
to cover or encircle the combustion air, earlier than the injection of the 
secondary fuel to the air, to thereby shield the air form tile secondary 
fuel while at the same time, the NOx in the primary flame is reduced by 
the secondary fuel. Thereafter, a second combustion is effected by 
bringing the secondary fuel to contact with the portion of combustion air 
at the downstream side. With this arrangement, it is possible to decrease 
the NOx density in the exhaust gas emitted, in a more positive way.

DETAILED DESCRIPTION OF PREFERRED 
EMBODIMENTS OF THE INVENTION 
Now, a specific description will be made of the processes and constructions 
of a low-NOx combustion in accordance with the present invention, with 
reference to FIGS. 2 through 12. 
FIG. 2 schematically shows a principle of low-NOx combustion in the present 
invention. Basically, this is similar to the previously described 
prior-art two-stage fuel combustion method in terms of the first and 
second fuel supply stages involving injection of primary fuel to the 
combustion air and subsequent injection of secondary fuel to the 
downstream portion of the air. According to the invention, as shown in 
FIG. 2, a substantially whole amount of combustion air A is supplied and 
subject to a first combustion by a primary fuel F1 being injected thereto, 
and then, the downstream portion of the same air A .(adjacent to the 
inside of combustion chamber CH) is subject to a second combustion by 
injection of a secondary fuel F2 thereto. 
It should be noted that the definition of "a substantially whole amount of 
combustion air A" as above is intended to entail the case where a part of 
the air A may be utilized as a cooling air for cooling tile secondary 
nozzles 4. But, in tile actual combustion process, it can be regarded as a 
whole amount of combustion air A to which the primary fuel F1 is injected. 
In this context, the ratio of distribution between the primary and 
secondary fuels F1, F2 with respect to the combustion air A may be set at 
any proper degree, which is not limitative, but for example, may be set by 
a proper ratio out of 90-30% by volume of secondary fuel F2 against 10-70% 
by volume of primary fuel F1. 
Designations 1, 4 and 19 denote a primary fuel nozzle for injecting the 
primary fuel F1, a secondary fuel nozzle or injecting the secondary fuel 
F2 and a burner throat, respectively. 
As understandable from FIG. 2, the low-NOx combustion method in the present 
invention essentially includes a first stage where the primary fuel F1 is 
injected in a direction from tile periphery of stream of the combustion 
air A flowing in the burner throat 19, towards tile air A per se, and 
ignited by a pilot burner (not shown) or the like to start a combustion 
and create a generally cylindrical shape of primary flame B, confirming 
generally to the inner surfaces of burner throat 19, so that tile primary 
flame B1 surrounds or circumscribes the combustion air A. For that 
purpose, at least two or more primary fuel nozzles 1 should be provided in 
order to produce such cylindrical primary flame B1 and preferably those 
plural nozzles 1 be disposed equidistantly along the inner surfaces of or 
circumferentially of the burner throat 19. At this point, a part of the 
combustion air A is subject to this particular first combustion, creating 
the cylindrical primary flame B1 immediately from the exit of burner 
throat 19 and a remainder of tile air A passes through within the 
cylindrical primary flame B1 to the downstream side (see the designation 
A' in FIG. 2). Then, the secondary fuel F2 is injected toward that primary 
flame B, from the secondary fuel nozzles 4 which are disposed outside the 
primary flame B1. At this moment, it is seen from FIG. 2 that, since the 
combustion air A is initially covered with the primary flame B1 from the 
exit of burner throat 19, time secondary fuel F2, immediately after its 
injection towards the air, is inevitably contacted with the primary flame 
B1 and thus intercepted or shielded by tile same flame B1 per se from the 
stream of air A passing centrally therewithin. 
Under this state, it is also seen that the primary flame B1 is placed in 
the condition containing an excessively small amount of residual oxygen 
therein, and the secondary fuel F2 applied to such primary flame B1 causes 
a high efficient reduction of NOx in the primary flame B1 at the area 
contacting therewith as shown in FIG. 2, which will also be explained 
later. 
Next, at the downstream side away from the primary flame B1, the secondary 
fuel F2 is contacted with the remaining combustion air A' penetrating 
through that primary flame B1, to thereby perform a second combustion. At 
this second combustion stage, a secondary flame as designated by B2 is 
created at the side of combustion chamber CH. 
It is therefore appreciated that the combustion air A injection from the 
burner throat 19 is shielded on the peripheral region by the primary flame 
B, from the secondary so as to insure that the NOx in the primary flame B1 
is reduced by the secondary fuel F2, and thereafter the air is fully 
burned by the same secondary fuel F2. 
Referring now to FIGS. 3 through 6, there is illustrated a first embodiment 
of burner device for effecting the above-described low-NOx combustion 
method. 
In the present embodiment, there is presented a cylindrical burner device 
BD1 having a cylindrical burner casing 15. Arranged in the burner casing 
15, are a burner tile 17, a burner tile throat 19 and an inner throat 
member 8. Both burner tile throat 19 and inner throat member 8 form a 
burner throat in this particular device BD1, which also refers to the 
throat 19 schematically in the aforementioned method. The burner tile 
throat 19 is formed cylindrically in the center of the burner tile 17, 
facing towards the combustion chamber CH. The inner throat member 8 has 
cylindrical wall extending in registry with the inner surface of the 
burner tile throat 19 in a direction inwardly of the casing 15. 
As shown in FIG. 4, an annular header 2 is arranged between the 
above-stated burner tile throat 19 and inner throat member 8 in a manner 
surrounding the circumference of those two elements. The primary fuel 
nozzles 1 are connected to this annular header 2, as will be explained 
later. 
More than one or preferably more plural secondary nozzles 4 are disposed 
via lance pipe holes 18 outwardly of the burner tile throat 19. In the 
embodiment shown, four secondary nozzles 4 are arranged in the burner tile 
17 such that they are disposed equidistantly along the circumference of a 
circle in a coaxial manner relative to the central axis of burner tile 
throat 19. The number of such secondary fuel nozzles 4 is not limited 
thereto, but the experiments show that such equidistant disposition of 4 
to 6 secondary fuel nozzles is most effective in reducing NOx in tile 
primary flame B1. The secondary fuel nozzles 4 may be disposed at the 
burner tile front 20 or in tile neighborhood thereof, for instance, and 
adopted to inject a predetermined amount of the secondary fuel F2 toward 
the inside of combustion chamber CH. As shown in FIG. 6, each of the 
secondary fuel nozzles 4 has an injection hole 4a which is oriented at a 
given angle toward a central axis of the burner throat (19, 8) so that the 
secondary fuel F2 is injected at an angle .alpha.2 toward the primary 
flame B1. Preferably, such injection angle .alpha.2 may be set from the 
range between 0 to 60 degrees, but this is not necessarily limitative. 
Although not clearly shown, those secondary fuel nozzles 4 are normally 
connected to a fuel supply header 6 located outside the casing 15, via 
their respective fuel supply pipes or the so-called lance pipes 5. The 
fuel supply header 6, as shown in FIG. 4, is formed in an annular shape, 
having a connecting pipe portion 6a provided therein. This annular header 
6 is communicated with the four lance pipes 5 as understandable from FIGS. 
3 and 4 and further communicated with the upper annular header 2 via a 
pipe 3. The connecting pipe portion 6a, though not shown, is connected to 
an external fuel supply system. Thus, a full amount of fuel supplied from 
such supply system is introduced through the connecting pipe portion 6a 
into each of tile upper and lower headers 2, 6 as can be seen in FIG. 4, 
whereby tile fuel is distributed into each of the primary and secondary 
fuel nozzles 1, 4. 
It is noted that the foregoing lance pipe hole 18, through which each lance 
pipe 5 extends, may be so formed to have an inner diameter slightly 
greater than the outer diameter of the lance pipe 5, providing thus a 
slight clearance between the lance pipe 5 and the inner surface of hole 18 
in order to allow a part (a few percent) of the combustion air A to pass 
through that clearance, thereby cooling each secondary fuel nozzle 4. 
As shown in FIG. 3, an air supply connecting pipe 14 is formed on the 
lateral wall of tile burner casing 15. This pipe 14 has, provided therein, 
a rotary air damper member 13 which is rotatable to permit adjusting the 
opening degree of the pipe 14. In other words, the pipe 14 works as an air 
damper device. Though not shown, an external air supply system is 
connected to such connecting pipe 14, allowing supply of the combustion 
air into the burner casing 15. The amount of combustion air to be supplied 
into the casing 15 may be adjusted by operation of the rotary air damper 
member 13. 
The primary fuel nozzles 1, in this embodiment, are located between the 
burner tile throat 19 and inner throat member 8, the arrangement thereof 
being such that the nozzles 1 are disposed along the circumference of a 
circle generally equal in diameter to the diameter of those two throat 
elements 19, 8 and that each of the same nozzles 1 is oriented such as to 
inject the primary fuel F1 in the direction from the periphery of the 
stream of combustion air A flowing in the burner throat (19, 8) towards 
that particular combustion air A. In other words, the primary fuel F1 is 
injected in the direction from the circumference of circle towards the 
combustion air A, to thereby create a generally circular cylindrical 
primary flame B, having a generally annular cross-section. The illustrated 
primary fuel nozzles 1 are each formed with an injection hole 1a. The 
injection holes 1a are formed equidistantly in the inward surface of the 
annular header 2 and opened inwardly thereof, as understandable from FIG. 
4 at the designation 1. The formation of each injection hole 1a is 
generally shown in FIG. 5. Namely, tile injection hole 1 of primary fuel 
nozzle 1 is oriented at a given injection angle a, relative to the axis X 
orthogonal with the axis Ax of combustion air flow, directing its 
injection axis fx towards the downstream portion of the combustion air A 
or in a direction to intersect the combustion air flow axis Ax. With this 
arrangement, the primary fuel F, will be injected at that injection angle 
.alpha.1 toward the primary flame B1 at the downstream side. For instance, 
the injection angle .alpha.1 may preferably be set from tile range within 
0 to 60 degrees. Of course, this angle is not limited thereto. 
With regard to the number of the injection holes 1a, the inventors 
conducted experiments and found that more than eight injection holes 1a 
are most effective in setting the primary fuel injection points enough to 
create a complete cylindrical primary flame B, which completely 
circumscribes tile combustion air A as seen in FIG. 2. Needless to 
mention, the injection holes la may be formed in any number insofar as 
they achieve such complete cylindrical primary flame. 
A baffle plate 7 of a ring-like plate configuration is integrally formed on 
and along tile inward peripheral surface of the header 2 such as to be 
located adjacent the foregoing injection holes 1a of primary fuel nozzles 
1. As best seen from FIG. 4, the baffle plate 7 is situated at the 
downstream side within the burner throat, projecting a small distance 
inwardly thereof so as to provide a proper efficiency for protecting the 
primary flame B1 from direct blow of combustion air A at tile injection 
holes 1a. Otherwise stated, the plate 7 serves to prevent a direct flow of 
tile air A into the area in the proximity of the injection holes 1a, 
thereby holding stable the root portion of the primary flame B1. 
Reference is made to FIG. 4. The present invention further contemplates a 
ratio of the diameter D of burner tile throat 19 against the distance L 
between the primary fuel nozzle injection holes 1a and burner tile front 
20 in order to set an optimal position of tile primary fuel nozzles 1 that 
insures expanding the primary flame F1 to a sufficient degree within the 
burner tile throat 19 and forming the intended complete cylindrical shape 
of primary flame F1. In this instance, such L/D ratio should be more than 
0.5, but it may be set properly, depending on the structural dimensions of 
the burner device to be used and the like. 
As shown in FIG. 4, an air velocity adjustment device 16 is provided 
inwardly of the inner throat member 8 and at the upstream side from the 
above-described primary fuel nozzles 1. The air velocity adjustment device 
16 extends along the central axis of burner casing 15 or the axis of 
burner throat in the present burner device BD1, comprising a cylindrical 
shutter member 10 fixed on the inner surface of bottom wall of burner 
casing 15, and a tubular movable member 9 slidably fitted in the shutter 
member 10, the tubular movable member 9 penetrating through the bottom 
wall of burner casing 15 and being movable vertically along tile burner 
throat axis. Such movable member 9 has, perforated in its peripheral 
surface, a pair of spaced-apart air inlet holes 11. As shown by the solid 
line in FIG. 4, the air inlet holes 11 are completely closed by the 
shutter member 10, but to push and move the movable member 9 upwardly as 
indicated by the two-dot chain line will open the air inlet holes 11 to 
allow a part of the combustion air A to flow through the holes 11 into the 
movable member 9, thereby flowing the air upwardly in the arrow direction 
towards the exit of burner tile throat 19. Namely, the air, after passing 
through the inlet holes 11, is directed towards the center of burner 
throat, then injected in that direction along the axis of burner throat 
(8, 19), and jetted out towards the combustion chamber CH. In practice, an 
operator depresses and draws the movable member 9 in the longitudinal 
direction along the burner throat axis so as to adjust the opening degree 
of the air inlet holes 11 relative to the shutter member 10. In this way, 
it is readily possible to adjust the amount of air (designated at 22) into 
the movable member 9 and jet out the air at a proper velocity. A flange 12 
is formed at the free end of the movable member 9 which projects from the 
bottom of burner casing 15, the flange 12 facilitating the ease with which 
an operator grasps the movable member 9 more positively to assure its 
movement. 
As seen in FIG. 4, the cylindrical wall of the inner throat member 8 
extends in the direction toward the upstream side away from the level at 
which the primary fuel nozzles 1 lie at the downstream side, with respect 
to the stream of combustion air or the burner throat axis, and terminates 
at a point spacing apart from the bottom wall of burner casing 15. This 
construction defines a main air inlet passage for allowing a substantially 
whole amount of the combustion air supplied from the connecting pipe 14 to 
smoothly flow into the upstream-side opening of inner throat member 8. The 
thus-introduced air is partly flowed into the above-stated movable member 
9 of air adjustment device 16 through the two air inlet holes 11 thereof 
as indicated at 22, whereas other part of the air is flowed outside the 
movable member 9 as indicated by a designation 21. 
It is thus understood that in the air velocity adjustment device 16, the 
combustion air is bifurcated into the above-mentioned two air streams 
designated by 21 and 22. Namely, the former 21 flows through the annular 
spacing between the inner throat member 8 and movable member 9, and the 
latter 22 flows within the movable member 9 along the central axis of 
burner throat. Accordingly, as with usual velocity distribution found in a 
pipe, the central air stream 22 flows at a far greater velocity than the 
surrounding or peripheral air stream 21, whereupon it is possible by 
operation of the foregoing device 16 to adjust such velocity distribution 
so as to cause the central air stream 22 to penetrate through the primary 
flame B1 which is created mainly from the peripheral air stream 21. 
FIG. 7 shown another mode of injection hole of the primary fuel nozzle 1. 
In this embodiment, there are formed another primary fuel nozzles 
designated by 1' in the inward circular surface of annual header 2, 
although they are shown to be in a singular form. Each of these nozzles 
1', in addition to being formed in the same manner with the one 1, is 
provided with a differently formed injection hole 1'a which is oriented in 
the direction tangential to a circle along which there extend the inner 
circular surfaces of burner throat (8, 19). More specifically, referring 
to FIG. 7, the injection hole 1'a is formed such that it is not only 
oriented at an angle equal to the above-noted angle .alpha.1 in respect to 
the axis "z" orthogonal with the combustion air flow axis Ax, but also 
oriented at a certain angle in respect to the axis "x" which forms a 
tangent line touching the circle along which the inner circular surfaces 
of burner throat extend, so as to define a new primary fuel injection axis 
"fx'". 
In the present embodiment, experiments reveals that the primary flames B1 
created from tile foregoing new injection holes 1'a are curled or assume a 
vortex-like flow in the above-said tangential direction and Jetted around 
the combustion air A with respect to the axis Ax thereof, as shown in FIG. 
8. Further, the experiments teach that such vortex-like flow of air serves 
to expand tile primary flames B1 circumferentially of the combustion air 
flow, more widely than the aforementioned first mode of injection holes 
1a, and this is found to cover a sufficient cylindrical range of primary 
flames even if tile associated primary fuel nozzles 1' are provided in a 
smaller number than eight. In other words, such curling effect of flames 
compensates for a less number of primary fuel nozzles 1' used than the 
ideal number of eight, and results in attaining the sufficient shielding 
effect that shields the central stream of combustion air by the primary 
flames B1 as explained above. For instance, from the results of 
experiments, at least more than two primary fuel nozzles 1' were found to 
suffice in achieving such flame vortex effect and air shielding effect. 
Hence, in terms of reduction of injection holes and the air shielding 
effect, this tangential orientation of second injection holes 1'a is more 
advantageous than the first injection holes la which are merely oriented 
in the direction along a normal relative to the tangential direction of 
the second ones 1'a. 
Of course, the injection nozzles (1a or 1'a) may be increased on the 
contrary in an attempt to make smaller each of the primary flames B1 per 
nozzle while increasing the surface area of total flames, to thereby avoid 
the heat residing phenomenon within the flames B1. This is also naturally 
effective in lowering the generation of NOx. The same goes for tile 
secondary fuel nozzles 4. 
With the burner device B1 constructed above, a description will be made of 
its combustion processes in more details as follows. 
Firstly, a substantially whole amount of combustion air A is encircled or 
circumscribed by the primary fuel F1 injected from the primary fuel 
nozzles (1 or 1') and then Jetted out from the burner tile throat 19, 
creating the cylindrical shape of primary flame B1 which conforms to the 
inward circular surfaces of the burner tile throat 19. Theoretically 
stated in this regard, the primary fuel F1 being injected from the nozzles 
(1 or 1') is forcibly changed its flowing direction by the momentum of 
combustion air A intersecting it, within tile burner throat, and flowed in 
the downstream direction to the exit of burner tile throat 19. Then, tile 
primary fuel F1, upon coming out of the burner tile throat 19, is quickly 
burned with the peripheral portion of air A by a pilot burner (not shown) 
at the same time, creating thus a generally cylindrical shape of primary 
flame F, which conforms generally to the inner circular surface of burner 
throat 19. 
It is noted here that if for example the fuel is distributed into the 
primary and secondary fuel nozzles 1, 4 at the ratio of 50/50, then the 
primary fuel F, injected from the primary nozzles 1 is burned under an 
excess air ratio twice as much as the theoretical amount of air normally 
required, because the substantially whole amount of combustion air A is 
flowed into the burner throat (8, 19) as stated above. Consequently, it is 
possible to suppress the generation of NOx down to a lowest possible level 
at a lower flame temperature in comparison with the hitherto ordinary 
diffusion flame method which shows such NOx characteristics in FIG. 11, 
which will be explained more specifically later with reference to FIG. 12. 
Thus, taking the advantage of the foregoing remarkable excess air ratio 
and, if desired, increasing the primary and secondary fuel nozzles (1 or 
1' and 4), may amplify the lowering of the flame temperature and 
contribute to minimize the amount of NOx to be generated in the flames. 
Now, at this first combustion stage, the cylindrical primary flame B1 
completely circumscribes the combustion air A, as in FIG. 2. Then, the 
secondary fuel F2 is injected from the secondary nozzles 4 towards the 
primary flame B1, but the cylindrical flame wall formed by that primary 
flame B1 has already been emitted outwardly from the point before the 
position of secondary fuel nozzles 4, thereby initially encircling the 
combustion air prior to tile next injection of secondary fuel F2 thereto 
and thus keeping the secondary fuel F2 away from contact with the central 
stream of combustion air penetrating through tile primary flame B1. For 
this reason, the secondary fuel F2, even though it may be injected towards 
the air immediately after the creation of primary flame B1, is inevitably 
contacted with the primary flame B1 and intercepted thereby from the 
stream of combustion air. 
At that moment, such contact of tile secondary fuel F2 with the primary 
flame B1 brings about a combustion reaction on the outer peripheral 
surfaces of tile primary flame B1 to reduce NOx present therein. It is 
important to note that, as a result of the earlier first combustion stage 
mentioned above, the density of residual oxygen in the outer peripheral 
surfaces of primary flame B1, is extremely lowered, which generates an 
extremely-low-oxygen thin layer of combustion gas surrounding the primary 
flame B1, and immediately thereafter, the secondary fuel F2 is injected 
for direct contact with such extremely-low-oxygen thin layer of combustion 
gas, with the result that a rapid oxidation reaction is avoided and 
simultaneously the partial reduction of NOx is expedited. 
Finally, the unburnt portion of the secondary fuel F2, not subject to 
combustion with the primary flame B1, is brought to contact with tile 
central stream of combustion air penetrating through the primary flame B1, 
at the downstream side away from that primary flame B1, and performing a 
second combustion for creating the secondary flame B2. 
In this way, in accordance with the present invention, it is possible to 
minimize the NOx density in the exhaust gas discharged therefrom. 
FIG. 12 shows an example of data obtained from an actual experiment, using 
the above-constructed burner device BD1. The fuel used was a city gas 
(Class 13A under the Japanese gas classification). The two-stage firing 
burner device BD1 was mounted in a water-cooled type furnace, and the 
experiments were done under the excess air ratio of 1.1. The result is 
shown from the graph of FIG. 12. It is observed that the burner device BD1 
lowers the NOx reduction at 50% in the exhaust gas as compared with the 
conventional two-stage firing burner device. 
Referring to FIG. 9, there is shown a second embodiment of burner device in 
accordance with the present invention, which presents a rectangular shaped 
burner device BD2. This device BD2 forms a flat flame having a generally 
rectangular cross-section, which surrounds the combustion air A in that 
flame configuration and realizes the same low-NOx combustion as the 
foregoing burner device BD1. In the present second embodiment, the burner 
housing 15 is formed in a rectangular shape, so that the burner tile 17, 
burner tile throat 19, inner throat member (not shown), and movable member 
9 of air velocity adjustment device are all shaped in tile likewise 
rectangular form. 
In addition, as shown in FIG. 10, there may be provided another burner 
device BD3 which differs only in the disposition of secondary fuel nozzles 
4 from the above-described two burner devices BD1 and BD2. This embodiment 
suggests that the secondary fuel nozzles 4 be disposed on the inner 
surface of burner of burner tile throat 19. Of course, the secondary fuel 
nozzles 4 must be located adjacent to the exit of burner tile throat 19 or 
at a more downstream side than the primary fuel nozzles 1 in order to 
carry out the same combustion manner as in the foregoing burner device BD1 
or BD2. 
Furthermore, the burner device may be constructed as a multi-fuel 
combustion type by providing a pilot burner and/or oil burner gun in tile 
movable member 9 of air velocity adjustment device 16. 
From the descriptions above, tile low-NOx combustion method and burner 
device therefore in accordance with the present invention produces the 
undermentioned advantageous features. 
(i) At the first combustion stage, tile combustion air is embraced or 
encircled by the generally cylindrical primary flame, whereby the 
secondary fuel injected thereto is shielded or intercepted by that primary 
flame from tile combustion air. Hence, the secondary fuel is contacted 
with the primary flame to reduce NOx present therein, and then subject to 
a second combustion with tile portion of combustion air penetrating 
through the primary flame. In that manner, it is practically possible to 
insure the decrease of NOx density by virtue of the complete air shielding 
effect of tile primary fuel and the NOx reduction effect of the secondary 
fuel. 
(ii) The primary fuel nozzle may be oriented in the direction tangential to 
the circle along which the inner surface of burner throat extends, to 
thereby permit the formation of cylindrical primary flame, more 
positively, even by use of a small number of primary fuel nozzles. 
(iii) The provision of the baffle plate adjacent the primary fuel nozzle 
injection holes at the upstream side is effective in holding stable the 
primary flames emitting from those injection holes. 
(iv) The coaxial disposition of plural secondary fuel nozzles relative to 
the central axis of burner throat will cause a uniform injection of the 
secondary fuel toward the primary flame and therefore will make the NOx 
reduction more efficient. 
While having described the present invention thus far, it should be 
understood that tile invention is not limited to the illustrated 
embodiments and any other modifications, replacements and additions may be 
applied thereto without departing from tile scope and spirit of tile 
appended claims therefor.