High efficiency, low NOX emitting, staged combustion burner

A staged, high efficiency burner for gaseous, liquid or pulverized solid fuels (including fuels having a high nitrogen content) in which NOX emissions are minimized. The burner includes a burner basket having a base including a throat and a concentric, tubular, frustroconically shaped wall that defines a primary combustion space. The fuel and primary air of about 75% of stoichiometric air are introduced into the basket which has a length so that the fuel has a residence time therein of between about 0.1 to 0.5 second. Secondary air is introduced into the flame downstream of the basket in the form of a multiplicity of individual air streams which are oriented to penetrate the flame and spin it about its axis so as to maintain a substantially cylindrical flame periphery. Part of the secondary air can be introduced through the tubular wall of the basket to shorten the flame length while maintaining the low level NOX emissions and the high efficiency of the burner by directing that part of the secondary air into the primary space so that the formation of pockets which have a high or excess air content cannot form. The flame basket can be disposed fully within the combustion chamber of the furnace or, to prevent it from penetrating excessively into the chamber, part of the basket can be disposed outside the chamber.

BACKGROUND OF THE INVENTION 
The increasing scarcity and the resulting increasing cost of fuel make it 
mandatory that today's burners operate at top efficiency. Further, 
environmental concerns require that the discharge of pollutants, and 
particularly of nitrogen oxides (hereinafter NOX), be minimized. 
Amongst the measures to attain high efficiency, operating of the burner 
with the least amount of air is of utmost importance. This means that a 
burner should be operated with the least amount of excess air, that is the 
theoretically required amount of air to fully combust the fuel 
(hereinafter in the specification and the claims sometimes referred to as 
"stoichiometric air") plus the amount of excess or additional air that 
must be supplied to assure that all fuel will be combusted or oxidized. 
With today's best burner designs that typically means an amount of air 
only about 5% above the stoichiometric air. 
To minimize the NOX emissions, more intricate measures must be taken. NOX 
is generated at high temperatures, that is temperatures in excess of about 
3000.degree. F. Because of the greater affinity of oxygen (in the 
combustion air) to combustible materials in the fuel as compared to 
nitrogen, practically no NOX is generated while there is an oxygen 
deficiency, that is when the fuel is combusted in the presence of less 
than the theoretically required amount of air to fully combust the fuel 
(hereinafter in the specification and the claims also sometimes referred 
to as "off-stoichiometric air"). Further, practically no NOX forms at 
temperatures below about 3000.degree. F. Thus, as long as the fuel is 
combusted with off-stoichiometric air or the temperature is kept below 
3000.degree. F. there are substantially no NOX emissions. 
In large industrial furnaces such as boilers for electric generating 
plants, which employ multiple burners, these objectives are attained by 
controlling the air supply so that there is a "staged combustion", that is 
a combustion which, during a first stage, takes place in an oxygen 
deficient environment and, in a second stage, takes place at a relatively 
low temperature. Systems to accomplish this typically employ auxiliary air 
inlets spaced from and in relation to the burners so that the secondary 
combustion air, that is the air to bring about a complete combustion of 
the fuel, is introduced downstream of the burners. This approach has 
proved most successful. 
However, small furnaces such as package boilers have only a single burner 
and a pre-assembled combustion chamber which does not permit an 
arrangement as disclosed in the preceding paragraph because of the size, 
shape, orientation and construction of the combustion chamber which 
typically is horizontal and has a tubular shape through which the flame 
extends. There is no space to arrange separate air inlets above the burner 
or in the side walls of the combustion chamber. 
Consequently, prior art package boilers and furnaces employing only one 
burner either had to be operated at low temperatures to avoid the 
generation of NOX, which meant they had to be operated with large amounts 
of excess air which reduced their efficiency, or if high efficiency was 
most important, high levels of NOX emissions had to be accepted. Neither 
alternative is acceptable under today's economic conditions and concerns 
for the protection of the environment. 
Further, when operating such a burner at the required relatively low 
temperature to prevent NOX emissions the boiler had to be de-rated, that 
is its steam producing capacity had to be lowered to prevent the formation 
of excessively long flames which would extend into the boiler convection 
section. 
SUMMARY OF THE INVENTION 
The present invention provides a self-contained burner for the staged 
combustion of gaseous, liquid or pulverized solid fuels, including fuels 
having a relatively high, chemically bound nitrogen content such as, for 
example, California (Kern County) heavy grade crude oil, in which the 
emission of objectionable NOX is substantially eliminated. Yet, this 
burner can be operated with as little as 5% excess air, the amount of 
excess presently considered necessary to effect a complete combustion of 
all fuel. Thus, the burner can be used for packaged boilers without 
sacrificing efficiency, without generating objectionable levels of NOX and 
without the need for de-rating the boiler. 
Generally speaking, the present invention accomplishes this by providing a 
flame basket which forms part of the burner and which defines a primary 
combustion space in which the fuel is combusted with off-stoichiometric 
air, usually not less than about 65% and preferably about 75% of 
stoichiometric air. The primary combustion air rotates about the flame 
axis at a sufficient rate to effect a rapid and uniform mixing of the air 
and the fuel. Typically, this requires a (primary air) swirl No. 
.gtoreq.1. The flame basket has a sufficient length so that the retention 
time of the flame in the basket is in the range of between about 0.1 to 
about 0.5 seconds, the shorter time periods of the range being applicable 
to nitrogen-lean fuels and the longer time periods to nitrogen-rich fuels 
because extended flame retention times in the first stage facilitate the 
reduction of NOX emissions. 
Further, the flame basket has cut-outs, preferably in the form of 
undulations in its free-end (which communicates with the main combustion 
chamber of the furnace) to achieve a direct (substantially perpendicular 
to the flame axis) radiation heat transfer to heat exchange surfaces, e.g. 
the boiler tubes surrounding the flame basket. The resulting drop in the 
temperature of the flame as it exits the primary combustion space of the 
basket (first stage combustion) and enters the main combustion chamber of 
the furnace (second or burnout stage) helps to maintain a second stage 
flame temperature below about 3000.degree. F., the temperature level below 
which NOX is not generated in objectionable amounts. 
The balance of the combustion air, that is the secondary combustion air, is 
then injected into the flame in the combustion chamber of the furnace. In 
accordance with the invention, this is done by forming a multiplicity of 
secondary combustion streams which are equally distributed about the flame 
and are directed into the flame at axially spaced points. This assures 
good and uniform mixing of the secondary air with the flame and prevents 
the formation of a secondary air sheath in the flame which can adversely 
affect the combustion process. 
To assure a uniform dispersal of the secondary air into the flame and a 
complete combustion of all fuel, the secondary air streams converge in a 
downstream direction towards the flame axis. Since this arrangement has a 
tendency of compressing the flame periphery, the secondary airstreams 
further lie in planes which are non-parallel and angularly inclined with 
respect to the flame axis to impart a rotary motion to or to spin the 
flame at a rate selected to offset the peripheral flame compression caused 
by the converging secondary airstreams. As a result, the flame periphery 
throughout the combustion chamber remains substantially cylindrical. 
By operating the burner in the manner described in the preceding 
paragraphs, the off-stoichiometric first stage combustion combined with 
the intimate and uniform mixing of primary air with fuel prevents the 
formation of high oxygen pockets in the flame so that substantially no NOX 
is generated there. The cooling of the flame before it enters the second 
stage combustion prevents the flame from exceeding the 3000.degree. F. 
temperature level which is critical to prevent NOX generation where excess 
oxygen is present. The uniform mixing of the secondary combustion air with 
the flame in the second stage so that the flame shape is substantially 
cylindrical makes it possible to complete the combustion process with 
little, e.g. 5% excess air, while maintaining a relatively short flame 
length even though it is of a relatively low temperature. Previously 
encountered problems caused by excessive flame lengths, including 
especially the need for de-rating the boiler, are thereby prevented. 
Nevertheless, large burners size, say burners having a throat diameter of 
between 18 or 20 inches up to as much as 30 inches or more, however, can 
still have excessively long flames and/or of incomplete combustion because 
it becomes increasingly difficult to uniformly distribute the secondary 
air throughout the relatively large flame. To alleviate this problem the 
present invention contemplates to introduce a portion of the secondary air 
into the primary combustion space defined by the flame basket downstream 
of the throat but upstream of the basket outlet. 
To this end, secondary air orifices are placed in the basket wall and 
oriented so that resulting secondary airstreams are tangent to an 
imaginary cylinder (which is concentric with the flame axis) at axially 
spaced points thereon. The secondary air streams in the primary combustion 
space are low volumetric flows best attained by providing a relatively 
large number of correspondingly small diameter orifices to assure a quick 
and uniform dispersal of the secondary air into the flame without the 
formation of excess air pockets which could lead to the generation of NOX. 
Since the small volumetric secondary air flows cannot penetrate deeply into 
the flame, the larger burners can further be provided with additional 
secondary air orifices which are located upstream of the first mentioned 
orifices and which direct secondary air towards the center of the flame. 
Typically, the number of these additional orifices will be smaller than 
the number of the first mentioned secondary air orifices to facilitate the 
desired deeper penetration of the air towards the vicinity of the center 
of the flame. 
The arrangement in which secondary combustion air is introduced into the 
flame basket has the advantage that it reduces the overall length of the 
flame without causing a heat up of the flame in the combustion chamber 
above the critical 3000.degree. F. level. Yet, the burner can be 
efficiently operated with as little as 5% excess air. 
Regardless of the specific arrangement of the secondary air orifices, it is 
preferred that the ratio of the volume of secondary air introduced into 
the primary combustion space and into the (second stage) combustion 
chamber does not exceed 1:2. The distribution of the secondary air between 
the first and second set of orifices is preferably roughly equal. 
In a presently preferred embodiment in which the flame is operated with 5% 
excess air, the primary combustion air comprises about 75% of 
stoichiometric air and the secondary combustion air comprises the 
remainder of the required combustion air, e.g. 30% of stoichiometric air. 
If secondary air is also introduced into the flame basket 10% of 
stoichiometric air is introduced there while 20% of stoichiometric air is 
introduced into the second stage or burnout zone. 
An actual burner capable of operating as above-described and constructed in 
accordance with the present invention comprises a burner basket 
constructed of refractory material. The basket has a base that defines a 
burner throat through which the fuel and the primary combustion air enter. 
A burner wall extends from the base towards the outlet end and includes 
the undulations which form the heat radiation cut-outs. Secondary 
combustion air conduits are usually embedded in the wall and terminate at 
the outlet end of the wall at axially spaced locations determined by the 
wall undulations. The basket is adapted to be demountably attached to a 
furnace wall and is connected with a primary air wind box and a register 
which rotates the primary air entering through the throat sufficiently so 
that centrifugal forces expand the flame within the basket and assure that 
the flame at all times contacts the basket wall. To facilitate this, the 
throat is flared outwardly in a downstream direction and is given a convex 
shoulder of a radius of up to 18 inches. In addition, a secondary air 
plenum for the secondary air conduits and, optionally, means for 
regulating the relative air flow through the conduits are provided. 
The fuel is introduced into the basket centrally with respect to the throat 
through suitable gaseous, liquid or pulverized solid fuel nozzles. The 
construction and operation of such nozzles is well-known and is not 
further described herein. 
To assure the required flame retention time in the basket, the basket has a 
preferred length of between about 24 inches for low nitrogen fuels to as 
much as 96 inches for high nitrogen fuels although the basket can be 
lengthened or shortened as required for a particular installation. For 
relatively long baskets, a portion of the basket can be disposed outside 
the furnace wall to prevent the basket from protruding too far into the 
main combustion chamber of the furnace. 
A burner constructed and operated as described above has an efficiency 
which compares favorably with the efficiency attained in furnaces 
employing multiple burners, auxiliary air inlets and the like. Yet, it is 
self-contained and can be installed as a single burner in relatively small 
boilers, industrial furnaces and the like. 
In addition to its efficiency, the burner of the present invention reduces 
the NOX generation when natural gas is burned from about 175-225 ppm to 
about 50-60 ppm. For low nitrogen containing fuel oil NOX production is 
reduced from about 300 ppm to around 90 ppm. Similar NOX reductions are 
attained when burning high nitrogen content fuel oil or coal (pulverized). 
Thus, the burner of the present invention is an environmentally sound, 
energy-efficient and, therefore, economic burner which is expected to find 
widespread acceptance wherever single burner furnaces are operated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a typical "package boiler" 2 has a main, generally 
circular body 4, the downstream end of which terminates in a flue 6. 
Disposed within the boiler are a multiplicity of heat exchange tubes 8. 
The upstream end of the boiler is defined by an end wall 10. A burner 12 
constructed in accordance with the present invention is demountably 
secured to the end wall with brackets 14 or the like. A schematically 
illustrated wind box 16 is fluidly connected to a source of combustion air 
18. For purposes more fully described hereinafter, a mixer 20 may be 
provided for adding to the combustion air flue gas which has been cooled 
in a flue gas cooler 22. A valve 24 controls the flow of cooled flue gas 
to the mixer. 
A fuel nozzle 26 is connected to a suitable fuel source 28 and disperses 
fuel via a burner throat 30 into the burner. There the dispersed fuel is 
mixed with the combustion air and ignited to form an elongate flame 32 
within a combustion chamber 34 of the boiler. Any suitable fuel such as 
natural gas, fuel oil or pulverized coal can be used. 
Referring now to FIG. 2, burner 12 of the present invention comprises a 
flame basket 36 constructed of refractory material. It has a relatively 
thick base 38 which defines the burner throat 30 and a tubular wall 40 
which projects away from the base in a downstream direction into 
combustion chamber 34 of boiler 2. The base and wall define a primary 
combustion space or zone 39. The tubular wall terminates in a scalloped 
outlet end 42 formed by undulations 44 which define a multiplicity, say 12 
generally triangularly shaped cut-outs 46 which extend from the outlet end 
42 in an upstream direction. 
On the outside of the flame basket, that is the side of base 38 facing the 
exterior of the boiler is mounted a register wind box 48 which fluidly 
communicates with a source of primary combustion air (not separately shown 
in FIG. 2). A register 50 of a conventional construction such as, for 
example, the one sold by Coen Company of Burlingame, Calif., under the 
designation SAZ-15 is disposed within the wind box. The register is 
constructed (or the Coen Co. register is modified to) spin or swirl 
primary air entering the burner throat 30 at a relatively high rate about 
burner axis 58 to achieve a swirl No. .gtoreq.1 so that the air, upon 
entering primary combustion space 29, hugs or contacts flame basket wall 
40. To facilitate this, the section 68 of the burner throat contiguous 
with the primary combustion space is flared outwardly (in a downstream 
direction) and is given a relatively large radius "R" of between about 12" 
to 18" and typically about 15". 
Also disposed on the exterior side of flame basket base 38 is a secondary 
combustion air plenum 52 which is ring-shaped and generally surrounds the 
wind box 48. It is connected to a source of secondary combustion air (not 
separately show). A multiplicity of equally spaced, secondary combustion 
air conduits 54 are disposed within basket wall 40 and extend from the 
secondary air plenum 52 in a downstream direction to outlet end 42 where 
each conduit forms a discharge opening 56. Preferably the conduits are 
formed by metal, e.g. steel tubes embedded in the refractory material of 
the basket wall, although, if desired, the conduits may be directly formed 
in the refractory material. 
The conduits (as well as the tubular wall 40 in which they are embedded) 
are angularly inclined with respect to the burner axis 58 so that their 
projections converge in a downstream direction on the burner axis at an 
angle 60 which is in the range of between about 7.degree. to 15.degree.. 
Additionally, the conduits lie in planes 62 which are non-parallel to the 
burner axis, that is which form an angle 64 with the axis of between about 
10.degree. to 20.degree.. When secondary air is discharged into combustion 
chamber 34 from secondary air conduits 56, the resulting secondary air 
streams are tangent to an imaginary cylinder 66 (shown in FIG. 3) which is 
concentric with the burner axis. Typically, it has a diameter no larger 
than the diameter of burner throat 30. 
In use, low pressure air, typically having no more than a 21/2" to 4" water 
column pressure, is introduced into the wind box 48 and the register 50 
swirls the air at a sufficient rate so that the air in the primary 
combustion space 39 at all times contacts the flame basket wall 40. The 
primary combustion air flow is limited to an off-stoichiometric amount, 
typically at least about 65% and preferably about 75% of stoichiometric 
air. Fuel dispersed in the primary combustion space 39 by nozzle 26 is 
intimately mixed with the swirling primary air and ignited to form flame 
32 which propagates in a downstream direction toward and past basket 
outlet end 42 into the combustion chamber 34. The intimate and rapid 
mixing of the fuel with the swirling air assure a uniform oxygen 
deficiency throughout the entire primary combustion space so that the 
formation of "excess air pockets" within that space, and the generation of 
NOX resulting therefrom, is prevented. 
The construction, including the size of the flame basket, is of importance 
to minimize NOX. As already mentioned above, maintaining an oxygen 
deficiency in the primary combustion space is of utmost importance. 
However, the generation of NOX is also influenced by the residence time of 
the flame in the primary combustion space. If the residence time is 
insufficient, then the original fuel nitrogen (nitrogen present in the 
fuel as constrasted with the nitrogen present in the combustion air) will 
exist in the gaseous state as some nitrogen compound (hereinafter "XN") 
which can be converted into NO in the second stage or zone. Thus, to 
minimize or eliminate NX production in the primary combustion zone the 
flame basket (which can become NOX in the secured stage) a residence time 
of at least about 0.1 second is required for burning fuels having a very 
low nitrogen content, such as natural gas. For relatively high nitrogen 
content fuel oil, such as Kern County crude oil which has a nitrogen 
content of about 0.8%, a residence time of about 0.3 to 0.4 seconds is 
desirable while for even higher nitrogen content fuels, such as certain 
coals, residence times of up to 0.45-0.5 seconds are indicated. This 
translates into a flame basket length (from the basket base 38 to the wall 
outlet end 42) which has an outside range of about 12 inches to 96 inches. 
For most practical applications, and the presently preferred basket length 
range is between 24 to 96 inches. 
The relatively long flame retention times in the basket has the additional 
advantage that the burn rate is relatively low and, consequently, the 
maximum temperature is maintained lower. This is further aided by the 
off-stoichiometric firing of the fuel, that is the more fuel-rich the 
flame is the lower will be its maximum temperature. 
As the flame propagates through the primary combustion space it first 
reaches cut-outs 46 where it radiates heat in the most efficient manner, 
that is perpendicular to its axis, to the heat exchange tubes 8 
surrounding the flame basket. Consequently, the flame temperature drops 
before the flame reaches its burn-out zone in the combustion chamber 34 of 
boiler 2. This aids in maintaining the maximum temperature in the burn-out 
zone below the critical 3000.degree. F. level. 
As the flame propagates further into the combustion chamber, it is mixed 
with secondary combustion air that issues from conduit ends 56 to effect 
the burn-out or complete combustion of all fuel. The converging secondary 
combustion air fuel penetrates into the flame towards the center thereof 
to assure an equal distribution of the secondary air throughout the flame 
which faciliates the complete combustion of all fuel. Since the converging 
airstreams have a tendency to compress the flame towards the burner axis 
58, the secondary air conduits 54 are arranged so that the air streams 
therefrom are tangent to an imaginary circle and thus swirl or rotate the 
flame. The rate of rotation is selected to generate a centrifugal force in 
the flame which offsets the tendency of the convering airstreams to 
compress the flame so that the flame periphery remains relatively constant 
and cylindrical over its length. To achieve the necessary penetration of 
the flame, and an intimate and turbulent mixing of the secondary air with 
the flame and the unburned fuel, the secondary air is normally at a 
relatively higher pressure than the primary air of between about 6 to 8 
inches water column. 
An efficient combustion process also requires that the secondary air is 
quickly and evenly introduced into the flame. Aside from the above 
discussed orientation of the secondary air conduits 54 and the relatively 
higher secondary air pressure, this is achieved by providing a relatively 
large number of relatively small diameter secondary air conduits. In a 
presently preferred embodiment, the conduits are arranged so that their 
discharge ends 56 are equally spaced apart no more than between about 1 to 
2 inches. 
To prevent the secondary air from developing an air curtain or sheath in 
the flame having areas or pockets of high excess oxygen, the secondary air 
discharge ends 56 terminate at axially spaced points as determined by the 
basket undulations 44. In this manner, parts of the flame periphery come 
in contact with secondary air earlier than other parts. Furthermore, due 
to a relatively higher pressure in the primary combustion space 39 the 
flame, as it passes wall cut-outs 46, begins to expand outwardly into the 
cut-outs. Thus, parts of the flame become mixed with the secondary air at 
the upstream ends of cutout 46 while other parts of the flame become mixed 
with secondary air further downstream, e.g. at the downstream tip of the 
undulations. This also increases the flame turbulence and therefore the 
intimacy with which the secondary air is mixed with the flame, all of 
which facilitates an even and complete combustion of all fuel with a 
minimum, e.g. 5% excess air. 
Yet, inspite of the low excess air operation of the burner of the present 
invention, the two-stage combustion achieved with the flame basket, 
coupled with the extended flame retention times in the basket, the 
radiation cooling of the flame via the cut-outs, and the uniform mixing of 
the flame with secondary air, make it possible to maintain flame 
temperatures below the 3000.degree. F. level, particularly in the burn-out 
zone where, in contrast to the primary combustion space, there is an 
oxygen surplus rather than deficiency. 
For larger burners, say burners having throat diameters in excess of 20 
inches, the above-described staged fuel combustion leads to relatively 
long flames which can only be reduced by increasing the excess air above 
what is presently considered to be about the absolute minimum of 5%. This, 
in turn, reduces the efficiency of the burner and may even increase NOX 
levels. To avoid this, the present invention contemplates to add up to 
one-third of the secondary air, or about 10% of stoichiometric air, to the 
flame in the primary combustion space 39 after the fuel has been ignited. 
Referring now to FIGS. 2, 4 and 5, for intermediate size burners, such 
secondary air is introduced into the primary combustion space 39 through a 
plurality of orifices 70 in wall 40 of the flame basket. The orifices and 
the associated passageways 72 orient the resulting secondary air flows 
(schematically illustrated in FIGS. 2 and 4 by phantom lines 74) so that 
each stream is tangent to an imaginary cylinder 76 which is concentric 
with burner axis 58 and may have, for example, about the same diameter as 
throat 30. 
To forestall the formation of NOX, it is important to quickly and uniformly 
disperse the secondary combustion air introduced via orifices 70 
throughout the flame to prevent the formation of pockets in the primary 
combustion space where the oxygen content approaches or exceeds 
stoichiometric air. To this end the orifices (and the associated 
passageways 72) direct the secondary air flows 74 into the primary 
combustion zone so that the points of tangency 78 between the air flows 
and the imaginary cylinder 76 are spaced apart in an axial direction. 
Further, the orifices have relatively small diameters so that the inertia 
of the secondary air flows is relatively low which results in a rapid 
diffusion of the secondary air throughout the flame. This also facilitates 
the construction of the passageways since they must extend through the 
relatively narrow basket wall spaces 80 between adjoining secondary air 
conduits 54. Thus, in a preferred embodiment of the invention where the 
10% secondary air introduced into the flame basket is introduced at six 
equally spaced locations, for example, each location is provided with a 
pair of orifices the associated passageways of which straddle a secondary 
air conduit 54 located between them as is best shown in FIG. 5. 
To effect the desired axial spacing of the tangent points 78 on the 
imaginary cylinder 76, each orifice (of the six orifice pairs, for 
example) lies on an inclined plane 82 as is shown in both FIGS. 2 and 5 to 
effect the desired axial spacing of the tangent points shown in FIG. 2. 
Alternatively, all orifices can be located in a common plane that is 
perpendicular to the burner axis. In such an event, the passageways have 
varying angular inclinations to obtain the desired axial spacing of all 
tangent points 78. 
In use the burner including secondary air orifices in the flame basket wall 
is operated as described above except that the secondary air introduced 
into the flame in the burn-out zone is reduced, say from 30% to 20% of 
stoichiometric air. The balance, e.g. 10% of stoichiometric air is 
introduced into the primary combustion space via orifices 70. This 
arrangement has the advantage that more fuel is combusted in the primary 
combustion space so that the burn-out zone can be shortened which leads to 
a corresponding shortening of the flame length. By introducing the 
secondary air downstream of the point where the primary air enters 
(through burner throat 30) the potential of excess air pockets in the 
primary combustion zone, and particularly in the vicinity of the burner 
base is reduced or eliminated. Further, by introducing the secondary 
combustion air into the primary combustion zone by way of a multitude of 
relatively small secondary air flows spaced in an axial direction and 
maintained tangent to the imaginary cylinder 76, the secondary air is 
rapidly dispersed. Excess air containing pockets, which could form in the 
flame if high volume secondary airstreams were introduced, are avoided. 
Thus, the danger of NOX formation due to the localized presence of excess 
oxygen is effectively prevented. 
For even larger burners, say a burner having a 30-inch diameter throat, it 
is of importance to distribute the secondary air introduced into the 
primary combustion space 39 into the vicinity of the core or center of the 
flame (which surrounds burner axis 58). The relatively small, low inertia 
secondary airstreams discharged from orifices 70 are typically unable to 
reach the flame center so that a non-uniform oxygen distribution 
throughout the primary combustion air space may result. In such instances, 
the present invention provides an additional set of relatively larger 
diameter orifices 84 which are located upstream of orifices 70 and which 
may be at the same circumferential location as the first mentioned 
orifices. However, to provide the resulting secondary air flows 86 with 
the desired inertia to penetrate to the flame core there is only one 
orifice at each location. Since the secondary air conduit 54 in burner 
wall 40 diverge in an upstream direction the wall spaces 80 between 
adjoining conduits provide sufficient room therefor in the vicinity of 
basket base 38. The larger orifices 84 are constructed so that airflows 
therefrom are tangent to a relatively small diameter imaginary cylinder 88 
and they are oriented, in the manner discussed above in connection with 
the description of orifices 70, so that the points of tangency 90 are 
spaced apart in an axial direction. 
Secondary combustion air for orifices 70, 84, is provided from secondary 
air plenum 52 via suitably arranged supply tubes 92. The volumetric flow 
of air through the orifices is controlled by appropriately sizing the 
associated passageways 72 and 85. Alternatively, where it is desirable to 
vary the pressure of the secondary airstreams 74 or 86 an appropriate 
pressure regulator, control valves or an entirely separate air plenum (not 
separately shown in the drawings) may be provided. Typically, however, 
this will neither be necessary nor desirable. 
For optimum efficiency, it is desirable that the periphery of the flame in 
the combustion chamber 34 is equally spaced from the surrounding heat 
exchange tubes 8. Referring now to FIGS. 6 and 7, in instances in which 
the boiler body has an oval cross-section, and to maintain the desired 
constant spacing between the flame periphery and the heat exchange tubes, 
a flame basket 94 otherwise constructed in the same manner as basket 36 
described above has a cylindrical base 96 and a tubular wall 98 which 
converges in a downstream direction. It has an outlet end 100 of an oval 
configuration complementary to that of the boiler body (not shown). To 
accommodate this construction the tubular wall 98 has a cross-section 
which changes from circular proximate the base to the oval configuration 
at the outlet end. 
The flame basket 102 illustrated in FIG. 7 is constructed similarly to 
flame basket 94 shown in FIG. 6. It differs therefrom only in that its 
outlet end 104 has a rectangular (which includes square) configuration 
complementary to a rectangular (or a square) configuration of a boiler 
body (not shown) so that the periphery of the resulting flame is generally 
rectangular. The remainder of the construction of burner basket 102 is the 
same as that of flame basket 94 shown in FIG. 6 and, therefore, has the 
same reference numerals. The operation of flame basket 94 and 102 is as 
described for flame basket 36 shown in FIGS. 2-5. 
Referring again to FIGS. 1 and 2, when the burner 12 of the present 
invention is operated with fuel having a high nitrogen content, such as 
certain pulverized coals, the flame retention times to prevent the 
formation of NOX may require a basket length which can interfere with the 
operation of the boiler. In such instances, brackets 14 which mount the 
burner to boiler end wall 10 can be moved on the burner in a downstream 
direction so that a relatively long segment 106 of the flame basket is 
disposed outside the boilers. 
Alternatively, or in addition thereto, the rate of combustion (and 
therewith the temperature of the flame) can be lowered by circulating 
cooled flue gas from flue gas cooler 22 via valve 24 into the combustion 
air, normally the secondary combustion air introduced into the flame 
downstream of the basket. The presence of flue gas in the combustion air 
lowers the overall oxygen content thereof and thus slows down the 
combustion process. In addition, there will be an overall increase in the 
amount of inert substances in the combustion gas which, in turn, lowers 
the overall flame temperature. This further aids in reducing or 
eliminating the formation of NOX which might be of utmost importance for 
users of high nitrogen containing fuel. A drawback of this alternative, 
however, is that there is some loss of efficiency. Consequently, this 
manner of operating the burner of the present invention should normally 
only be employed where the fuel contains relatively high levels of bound 
nitrogen.