Methods and apparatus for burning fuel with low NO.sub.x formation

Improved methods and burner apparatus are provided for discharging mixtures of fuel and air into furnace spaces wherein said mixtures are burned and flue gases having low NO.sub.x content are formed therefrom. The methods basically comprise discharging a first fuel mixture containing a portion of the fuel and flue gases from the furnace space into the furnace space whereby the mixture is burned in a primary reaction zone therein and flue gases having low NO.sub.x content are formed therefrom, and then discharging the remaining portion of the fuel into a secondary reaction zone wherein the remaining portion of fuel mixes with air and flue gases to form a second fuel mixture which is burned in the secondary reaction zone and additional flue gases having low NO.sub.x content are formed therefrom.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to methods and apparatus for burning fuel-air 
mixtures whereby flue gases having low NO.sub.x content are produced. 
2. Description of the Prior Art 
As a result of the adoption of stringent environmental emission standards 
by government authorities and agencies, methods and apparatus to suppress 
the formation of oxides of nitrogen (NO.sub.x) in flue gases produced by 
the combustion of fuel-air mixtures have been developed and used 
heretofore. For example, methods and apparatus wherein fuel is burned in 
less than a stoichiometric concentration of oxygen to intentionally 
produce a reducing environment to CO and H.sub.2 have been proposed. This 
concept has been utilized in staged air burner apparatus wherein the fuel 
is burned in a deficiency of air in a first zone producing a reducing 
environment that suppresses NO.sub.x formation, and then the remaining 
portion of air is introduced into a second zone. Methods and apparatus 
have also developed wherein all of the air and some of the fuel is burned 
in a first zone with the remaining fuel being introduced into a second 
zone. In this staged fuel approach, an excess of air in the first zone 
acts as a dilutent which lowers the temperature of the burning gases and 
thereby reduces the formation of NO.sub.x. Other methods and apparatus 
have been developed wherein flue gases are combined with fuel-air mixtures 
to dilute the mixtures and thereby lower their combustion temperatures and 
formation of NO.sub.x. 
While the prior art methods and burner apparatus for producing flue gases 
having low NO.sub.x content have achieved varying degrees of success, 
there still remains a need for improvement in such methods and burner 
apparatus whereby low NO.sub.x content flue gases are produced and simple 
economical burner apparatus is utilized. 
SUMMARY OF THE INVENTION 
By the present invention, the above mentioned needs for improved methods of 
burning fuel-air mixtures and improved burner apparatus for carrying out 
the methods are met. That is, the present invention provides improved 
methods and burner apparatus for discharging mixtures of fuel and air into 
furnace spaces wherein the mixtures are burned and flue gases having low 
NO.sub.x content are formed therefrom. The methods each basically comprise 
the steps of mixing a portion of the total fuel needed for the required 
heat release in the furnace space and flue gases from the furnace space to 
form a first fuel mixture. The first fuel mixture is discharged into the 
furnace space whereby it combines with a portion of the total air required 
for forming an at least substantially stoichiometric total fuel-total air 
mixture, and the resultant fuel-flue gases-air mixture is burned in a 
primary reaction zone therein. Because the fuel and air in the mixture are 
diluted with flue gases and, as a result, burn at a relatively low 
temperature, low NO.sub.x content flue gases are formed therefrom. The 
remaining portion of fuel is discharged into a secondary reaction zone in 
the furnace space wherein it mixes with cooled flue gases contained in the 
furnace space and air remaining therein to form a second fuel mixture. The 
second fuel mixture also burns at a relatively low temperature and flue 
gases having low NO.sub.x content contain a portion of the air mixed 
simultaneously with the fuel and flue gases, and a portion of the air can 
optionally be separately conducted to and discharged into the secondary 
reaction zone with the remaining portion of the fuel. 
The improved burner apparatus of the present invention which is relatively 
simple and economical utilizes a primary fuel jet mixer-nozzle assembly 
for mixing a portion of the fuel and inspirated flue gases drawn from the 
furnace space and discharging the resultant first fuel mixture into a 
primary reaction zone in the furnace space. A portion of the air can 
optionally be inspirated into the primary mixer-nozzle assembly and 
simultaneously mixed with the first fuel mixture. 
The remaining portion of the fuel is discharged into the furnace space by 
way of one or more secondary fuel nozzles positioned adjacent to the 
primary nozzle whereby the fuel enters a secondary reaction zone 
sequentially following the primary reaction zone. A portion of the air 
flows into the primary reaction zone wherein it combines with the first 
fuel mixture discharged from the primary mixer-nozzle assembly, and 
optionally, a portion of the air can be separately conducted to the 
location of each secondary fuel nozzle utilized whereby air is discharged 
along with the fuel into the secondary reaction zone. 
It is, therefore, a general object of the present invention to provide an 
improved method and burner apparatus for discharging a mixture of fuel and 
air into a furnace space wherein the mixture of fuel and air into a 
furnace space wherein the mixture is burned and flue gases having a low 
NO.sub.x content are formed therefrom. 
A further object of the present invention is the provision of an improved 
low NO.sub.x burner apparatus which is of simple and economical 
construction. 
Other and further objects, features and advantages of the present invention 
will be readily apparent to those skilled in the art upon a reading of the 
description of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, a presently preferred embodiment of burner 
apparatus of the present invention is illustrated and generally designated 
by the numeral 10. The burner 10 includes a cylindrical housing 12 which 
is connected at an open end 14 thereof over a complimentary opening 16 in 
a furnace wall 18. As will be understood by those skilled in the art, the 
furnace wall 18 generally includes an internal layer of insulation 
material 20, and the wall 18 and insulation material 20 together with a 
portion of the interior of a burner tile 48 which will be described 
further hereinbelow define a furnace space 21 within which fuel and air 
are burned to form hot flue gases. 
As illustrated in FIG. 1, the burner housing 12 includes an annular flange 
22 at the open end 14 thereof. The flange 22 is bolted to the furnace wall 
18 by a plurality of bolts 24. The opposite end of the housing 12 is 
closed by an end wall 26, and a plurality of air inlet openings 28 are 
disposed in spaced relationship around the cylindrical side of the housing 
12. A cylindrical damper 30 is rotatably positioned over the cylindrical 
side of the housing 12 having a handle 32 attached thereto. The damper 30 
includes air openings 32 which are complimentary to the air openings 28 
whereby the damper 30 can be rotated, using the handle 32, between a 
closed position whereby the openings 28 are covered by solid portions of 
the damper 30, a partially open position and a fully open position whereby 
the openings 28 are in registration with the openings 32 as shown in FIG. 
1. 
Positioned co-axially within the housing 12 is a primary fuel jet 
mixer-discharge nozzle assembly designated by the numeral 34. The assembly 
34 is comprised of an elongated fuel jet mixer 36 connected to a discharge 
nozzle 38. The mixer 36 attached to the end plate 26 of the housing 12 
includes a pressurized fuel inlet connection (not shown) to which a 
conduit 40 (via an opening in the end plate 26) is connected. The conduit 
40 is in turn connected to a header or conduit 42 which conducts 
pressurized fuel from a source thereof to the burner 10. The mixer 36 also 
includes four flue gases inlet connections 46 which are positioned in 
equally spaced relationship around the base thereof. 
At the open end 14 of the housing 12 is an annular burner tile 48 formed of 
flame and heat resistant material. As shown in FIGS. 1 and 2, the burner 
tile 48 includes four passageways 50 which extend from the end 49 thereof 
adjacent the open end 14 of the housing 12 to the exterior side 51 thereof 
within the furnace 21. Connected to each of the flue gases inlet 
connections 46 of the mixer 36 are the ends of four conduits which are 
disposed within the housing 12, the other ends of which extend into the 
passageways 50 formed in the burner tile 48. Thus, the four conduits 52 
connect the four flue gases inlet connections 46 of the primary 
mixer-nozzle assembly 34 to the passageways 50 in the burner tile 48. As 
best shown in FIG. 2, the passageways 50 with the conduits 52 extending 
therein are positioned in equally spaced relationship around the primary 
mixer-nozzle assembly 34. As will be understood, more or less than four 
conduits 52 and inlet connections 46 may be utilized in the burner 
apparatus 10 depending upon various design considerations known to those 
skilled in the art. 
The nozzle 38 of the primary mixer-nozzle assembly 34 includes one or more 
orifices 54 formed therein through which, as will be described further 
hereinbelow, a mixture of fuel and flue gases is discharged into a primary 
reaction zone in the furnace space 21. 
Four additional passageways 56 are disposed in the burner tile 48 extending 
from the end 49 thereof to the other end 53 thereof. As best shown in FIG. 
2, the opening 56 are positioned in spaced relationship around the burner 
tile 48 between the passageways 50 therein, Disposed within the 
passageways 56 are four secondary fuel discharge nozzles 60. The discharge 
nozzles 60 each include one or more discharge orifices 62 in the external 
ends thereof, and are each snugly fitted within a passageway 56. The 
internal ends of the nozzles 60 are connected to conduits 64 which extend 
through the passageways 56 of the burner tile 48, through the interior of 
the housing 12 and through complimentary openings 58 in the end wall 26 of 
the housing 12. The conduits 64 are connected to a pressurized fuel source 
by way of the conduit 42. As will be described further hereinbelow, the 
fuel nozzles 60 discharge fuel into the furnace space 21 wherein the fuel 
mixes with cool flue gases contained in the furnace space 21 and air 
remaining therein. The resulting mixture is burned is a secondary reaction 
zone in the furnace space 21 adjacent to and downstream from the primary 
reaction zone. More or less than four fuel nozzles 60 can also be utilized 
in the apparatus 10 based on known design considerations. 
In the operation of the furnace of which the burner apparatus 10 is a part, 
fuel and air are discharged into the furnace space 21 and burned therein 
to form hot flue gases. The hot flue gases are cooled as they circulate 
through the furnace space 21 and lose heat prior to being vented to the 
atmosphere. In order to meet environmental emission standards, the flue 
gases must have low NO.sub.x content. 
The required flue gases low NO.sub.x content is accomplished in accordance 
with the present invention by: (a) discharging into the furnace space 21 
the air required for producing at least a substantially stoichiometric 
mixture of fuel and air therein by way of the opening 14 in the housing 
12; (b) mixing, within the primary mixer-nozzle assembly 34, a portion of 
the total fuel needed for the required heat release within the furnace 
space 21 and flue gases from the furnace space 21 to thereby form a first 
fuel mixture, i.e., fuel diluted with flue gases; (c) discharging the 
first fuel mixture into the furnace space 21 by way of the orifices 54 of 
the nozzle 38 whereby the mixture combines with air discharged into the 
furnace space 21, the resulting fuel-flue gases-air mixture is burned in a 
primary reaction zone therein and flue gases having low NO.sub.x content 
are formed therefrom; and (d) discharging the remaining portion of the 
fuel by way of the nozzles 60 into a secondary reaction zone which 
sequentially follows the primary reaction zone in the furnace space 21 
whereby the fuel combines with cooled flue gases from the furnace space 
21, with the products of combustion from the primary reaction zone and 
with air in the furnace space 21 to form a second fuel mixture which is 
burned in the secondary reaction zone and additional flue gases having low 
NO.sub.x content are formed therefrom. 
Referring to FIGS. 1 and 2, atmospheric air is introduced into the housing 
12 of the burner apparatus 10 by way of the openings 28 therein and is 
discharged, in accordance with step (a) described above, through the open 
end 14 of the housing 12, through the open interior of the burner tile 48 
and into the furnace space 21. As is well understood, the damper 30 is 
utilized to control the rate of total air introduced into the housing 12 
at a level whereby at least a substantially stoichiometric mixture of 
total air and total fuel results in the furnace space 21. 
In accordance with step (b), pressurized fuel flows by way of the conduit 
40 into the primary mixer-nozzle assembly 34. The pressurized fuel, which 
can be fuel gas or vaporized liquid fuel, is formed into a high velocity 
jet as it enters the mixer 36 which causes a suction to be created at the 
flue gases inlet connections 46, the conduits 52 and the passageways 50. 
This in turn causes flue gases contained within the furnace space 21 to be 
drawn into the passageways 50 from the furnace space 21 and to flow by way 
of the conduits 52 to the mixer 36 wherein the flue gases are inspirated 
into and mixed with the fuel to form a first fuel mixture. 
In accordance with step (c) described above, the first fuel mixture is 
discharged through the orifices 54 of the discharge nozzle 38 of the 
primary mixer-nozzle assembly 34 into a primary reaction zone adjacent 
thereto. Upon being discharged from the nozzle 38, the first fuel mixture 
combines with air flowing into the furnace space 21 by way of the open end 
14 of the housing 12 and the interior of the burner tile 48 (as shown by 
the arrows 44), and the resultant flue gases-fuel-air mixture is burned in 
the primary reaction zone. Because the burning of the mixture takes place 
at a relatively low temperature due, at least in part, to the presence of 
the flue gases therein, the flue gases formed have a low NO.sub.x content. 
The term "relatively low temperature" is used herein to mean a temperature 
that is lower than the temperature at which the same fuel-air mixture, but 
undiluted with fuel gases, would burn. 
Generally, the portion of fuel introduced into the primary mixer-nozzle 
assembly 34 and contained in the first fuel mixture discharged into the 
primary reaction zone is an amount in the range of from about 10% to about 
50% by volume of the total fuel required. The flue gases which are drawn 
into and mixed with the fuel in the primary mixer-nozzle assembly 34 are 
preferably present in an amount in the range of from about 30% to about 
400% by volume of the fuel depending on the composition of the fuel and 
other factors. As will be understood, the fuel utilized in a burner or 
furnace apparatus is normally expressed as a rate, i.e., a volume of fuel 
per unit time. The term "% by volume" as used herein means the stated % of 
the rate of fuel referred to. While the rate of the air discharged into 
the furnace space 21 can be varied, the rate of air utilized preferably 
results in at least substantially stoichiometric fuel-air mixture. The 
term "stoichiometric fuel-air mixture" is used herein to mean a mixture in 
which the relative portions of fuel and air are such that when the mixture 
is burned to completion, no excess oxygen or fuel remains. 
In accordance with step (d), the remaining portion of the fuel flows to the 
secondary nozzles 60 by way of the conduits 64 connected thereto and to 
the conduit 42. The fuel is discharged by way of the orifices 62 in the 
secondary nozzles 60 into the furnace space 21. That is, the portion of 
the fuel discharged by the secondary fuel nozzles 60 into the furnace 
space 21 mixes with air therein, with cooled flue gases contained within 
the furnace space 21 and with products of combustion, i.e., flue gases, 
from the primary reaction zone to form a second fuel mixture. Like the 
first fuel mixture, the second fuel mixture, at least in part as a result 
of the dilution thereof with flue gases, is burned in the secondary 
reaction zone at a relatively low temperature whereby the flue gases 
formed have a low NO.sub.x content. 
Because the secondary fuel nozzles 60 are located adjacent to and 
downstream from the nozzle 38 of the primary mixer-nozzle assembly 34, the 
secondary reaction zone in which the second fuel mixture is burned 
sequentially follows the primary reaction zone in which the first fuel 
mixture is burned. Stated another way, the primary reaction zone extends 
from the primary nozzle 38 into the furnace space 21 and the secondary 
reaction zone substantially surrounds and extends outwardly from the 
primary reaction zone. 
Referring now to FIGS. 3 and 4, an alternate embodiment of the burner 
apparatus of the present invention is shown and generally designated by 
the numeral 100. The burner 100 includes a cylindrical housing 112 which 
is connected at an open end 114 over a complimentary opening 116 in a 
furnace wall 118. An internal layer of insulation material 120 is provided 
adjacent the wall 118; and the wall 118, the insulation material 120 and a 
portion of the interior of a burner tile 148 define a furnace space 121 
within which fuel and air are burned to form hot flue gases. The burner 
housing 112 includes an annular flange 122 at the open end 114 thereof 
which is bolted to the furnace wall 118 by a plurality of bolts 124. The 
opposite end of the housing 112 is closed by an end wall 126, nd a 
plurality of air inlet openings 128 are disposed in spaced relationship 
around a cylindrical side of the housing 112. Like the burner apparatus 
10, the apparatus 100 includes a cylindrical damper 130 rotatably 
positioned over the cylindrical side of the housing 112 having a handle 
132 attached thereto. 
A primary fuel jet mixer-discharge nozzle assembly generally designated by 
the numeral 134 is positioned co-axially within the housing 112. The 
assembly 134 is comprised of an elongated fuel jet mixture 136 connected 
to a discharge nozzle 138. The mixer 136 includes a pressurized fuel inlet 
connection to which a conduit 140 is connected. The conduit 140 is in turn 
connected to a source of pressurized fuel by a conduit 142. The primary 
mixer-nozzle assembly 134 also includes an air inlet 144, and four flue 
gases inlet connections 146 which are positioned in equally spaced 
relationship around the mixer 136. 
In the embodiment illustrated in FIGS. 3 and 4, a conical shield 141 is 
attached to the nozzle 138 to enhance flame stability thereto. The 
shielding cone 141 is dish-shaped and includes a plurality of openings 143 
formed therein for allowing the passage of a limited amount of air 
therethrough. The shielding cone 141 functions to create a protected area 
adjacent the nozzle 138 whereby air flowing in the direction indicated by 
the arrows 145 is deflected and instability of flame adjacent the nozzle 
138 is reduced. The shielding cone 141 further includes tabs 147 extending 
therefrom towards and adjacent the secondary fuel nozzles 160 to be 
described further hereinbelow. The shielding tabs 147 function to enhance 
flame stability to the secondary fuel nozzles 160 by deflecting the flow 
of air in areas adjacent thereto. 
An annular burner tile 148 is connected at the open end 114 of the housing 
112. Like the burner tile 148 of the apparatus 10, the burner tile 148 
includes four passageways 150 which extend from the inner end 149 thereof 
to the exterior side 151 within the furnace space 121. Connected to each 
of the flue gases inlet connections 146 of the mixer 136 are the ends of 
four conduits 152, the other ends of which extend into the passageways 150 
formed in the burner tile 148. The four conduits 52 connect the four flue 
gases inlet connections 146 of the mixer 136 to the passageways 150 in the 
burner tile 148. The passageways 150 with the conduits 152 extending 
therein are positioned in equally spaced relationship around the primary 
mixer-nozzle assembly 134. The nozzle 138 of the primary mixer-nozzle 
assembly 134 includes one or more orifices 154 formed therein through 
which a fuel-air mixture diluted with flue gases is discharged into a 
primary reaction zone in the furnace space 121. 
Four enlarged passageways 156 are disposed in the burner tile 148 extending 
from the inner end 149 thereof to the exterior end 153 thereof. The 
passageways 156 are positioned in spaced relationship around the burner 
tile 148 between the passageways 150 therein. Disposed within the 
passageways 156 are four secondary fuel discharge nozzles 160, each 
including one or more discharge orifices 162 in the external ends thereof. 
The nozzles 160 are connected by conduits 164 to the pressurized fuel 
conducting conduit 142. The diameters of the passageways 156 are sized 
with respect to the external sizes of the secondary fuel nozzles 160 such 
that annular air conducting conduits 161 are provided between the external 
surfaces of the nozzles 160 and the interiors of the passageways 156. 
Thus, as indicated by the arrows 157 in FIG. 3, air from within the 
housing 12 flows by way of the annular conduits 161 provided between the 
passageways 156 and the nozzles 160 into the secondary reaction zone above 
and adjacent to the secondary fuel nozzles 160. The particular rate of air 
which flows through the annular conduits 161 is controlled by the sizes of 
the annular conduits 161. 
The fuel nozzles 160 discharge fuel into the furnace space 121 wherein the 
fuel mixes with the air entering the furnace space 121 by way of the 
annular conduits 161. As described above in connection with the burner 
apparatus 10, the fuel-air mixture combines with cool flue gases contained 
in the furnace space 121, products of combustion from the primary reaction 
zone and with any air remaining in the furnace space 121, and the 
resulting mixture is burned in a secondary reaction zone within the 
furnace space 121. 
In order to further lower the production of NO.sub.x within the furnace 
space 121, a steam injection nozzle 170 connected to a steam conduit 172 
is disposed within the housing 112. Alternatively the steam can be 
introduced into the primary mixer nozzle assembly 134 by way of a conduit 
174 connected thereto. The steam injection contributes to low NO.sub.x 
production as is well known by those skilled in the art. 
The operation of the apparatus 100 is similar to the operation of the 
apparatus 10 described above, except that a portion of the air which flows 
into the housing 112 by way of the openings 128 is drawn into the primary 
mixer-nozzle assembly 134, mixed with the fuel and flue gases therein and 
the resulting flue gases-fuel-air mixture is discharged into the furnace 
space 121 by way of the nozzle 138. In addition, a portion of the air 
within the housing 112 flows by way of the annular conduits 161 directly 
into the secondary reaction zone in the furnace space 121. More 
specifically, a portion of the total fuel needed for the required heat 
release is mixed within the primary mixer-nozzle assembly 134 with a 
portion of the total air required for at least the substantial 
stoichiometric combustion of the total fuel and with flue gases from the 
furnace space 21 to thereby form a first fuel-air mixture diluted with 
flue gases. 
Generally, the portion of the total fuel which is introduced into the 
primary mixer-nozzle 134 and contained in the first fuel-air mixture 
diluted with flue gases discharged into the primary reaction zone is as 
amount in the range of from about 10% to about 50% by volume of the total 
fuel. The flue gases which dilute the first fuel-air mixture are 
preferably present therein in an amount in the range of from about 30% to 
about 400% by volume of the fuel in the fuel-air mixture depending on the 
composition of the fuel and other factors. The portion of the total air 
which is drawn into the mixer 136 by way of the air inlet 144 and which is 
contained in the first fuel-air mixture diluted with flue gases discharged 
into the furnace space 121 is an amount in the range of from about 50% to 
about 500% by volume of the fuel in the first fuel-air mixture depending 
on the composition of the fuel and other factors. As will be understood, 
the amounts of flue gases and air drawn into the mixer 136 are 
substantially set when the design of the burner apparatus 100 is finalized 
and the number and sizes of the various inlets, passageways, conduits, 
etc. are selected. However, some adjustments are normally possible. 
The first fuel-air mixture diluted with flue gases is discharged into the 
furnace space 121 by way of the orifices 154 of the nozzle 138 whereby the 
mixture combines with a further portion of the total air which is 
discharged from the housing 112 into the furnace space 121 by way of the 
open end 114 of the housing 112 as illustrated by the arrows 145. The flow 
of air is deflected and slowed down adjacent the nozzle 138 by the 
shielding cone 141 to insure stability of the flame adjacent the burner 
138 in the primary reaction zone. The resulting fuel-air mixture diluted 
with flue gases is burned in the primary reaction zone and flue are formed 
therein having low NO.sub.x content as a result at least in part of the 
presence of the diluting flue gases causing the burning to take place at a 
relatively low temperature. 
The remaining portion of the fuel is discharged by way of the fuel nozzles 
160 into a secondary reaction zone which sequentially follows the primary 
reaction zone. The discharged fuel combines with the air which is 
separately conducted to the secondary reaction zone by way of the annular 
conduits 161 formed within the passageways 156 around the nozzles 160. The 
air mixes with the fuel, with the products of combustion from the primary 
reaction zone and with cooled flue gases and any air contained in the 
furnace space to form a second fuel-air mixture diluted with flue gases. 
The second diluted fuel-air mixture is burned in the secondary reaction 
zone at a relatively low temperature thereby forming additional flue gases 
having a low NO.sub.x content. 
Generally, the portion of the air which flows by way of the annular 
conduits 161 directly to the secondary reaction zone is an amount of air 
in the range of from about 10% to about 100% by volume of the fuel which 
is discharged into the secondary reaction zone by way of the nozzles 160. 
Referring now to FIGS. 5 and 6, alternate forms of burner apparatus of the 
present invention are illustrated. Referring to FIG. 5, a rectangular 
shaped burner apparatus 200, often referred to as a flat flame burner, is 
illustrated. The burner apparatus 200 is generally the same design as the 
burner apparatus 100 described above except that it includes an elongated 
rectangular primary nozzle 210 with a rectangular shield 212 for providing 
flame stability attached thereto. Flue gases passageways 214 and conduits 
216 are provided for drawing flue gases into the primary mixer-nozzle 
assembly, and a plurality of secondary fuel nozzles 218 are disposed in 
passageways 220. Air is discharged around the nozzles 218 by way of 
annular conduits 219 formed between the passageways 220 and nozzle 218. 
The passageways 214 and 220 are disposed in a rectangular burner tile 222 
attached to the burner housing (not shown). 
FIG. 6 illustrates another alternate form of burner apparatus of the 
present invention generally designated by the numeral 300. The apparatus 
300 is similar to the apparatus 10 and includes a cylindrical burner tile 
310 attached to a cylindrical burner housing (not shown). Instead of a 
circular burner nozzle with or without a flame stability shield the 
apparatus 300 includes a primary mixer-nozzle assembly wherein the nozzle 
312 thereof includes a plurality of radially extending fingers 314. The 
configuration of the nozzle 312 is commonly referred to as a "spider" 
configuration. The apparatus 300 includes a plurality of flue gas intake 
passageways 316 and conduits 318 as well as a plurality of secondary fuel 
nozzles 320 disposed in passageways 322. 
The burner apparatus 200 and 300 can include the structure and can be 
operated as described above in connection with the burner apparatus 10, or 
the burners 200 and 300 can include the structure and be operated as 
described above in connection with the burner 100, or various combinations 
of the structure and operation steps can be utilized depending upon the 
particular applications in which the burners are used. That is, for a 
particular application, a burner apparatus of the present invention may be 
rectangular, cylindrical or other shape, may or may not include a nozzle 
flame stabilizing shield, may or may not inspirate air into the primary 
mixer-nozzle assembly, may or may not separately conduct air directly to 
the secondary reaction zone or may or may not inject steam. Also, the 
apparatus may utilize natural air draft or forced air draft. The term 
"air" is used herein to mean atmospheric air, oxygen enriched atmospheric 
air or air which otherwise includes more or less oxygen therein than 
atmospheric air. The selection of a particular embodiment of the burner 
apparatus of this invention and its operation depends on the particular 
application in which the burner apparatus is used and various design 
considerations relating to that application which are well known to those 
skilled in the art. 
In order to facilitate a clear understanding of the methods and apparatus 
of the present invention, the following examples are given. 
EXAMPLE I 
A burner apparatus 10 designed for a heat release of 10,000,000 BTU/hour by 
burning natural gas having a caloric value of 1,000 BTU/SCF is fired into 
the furnace space 21. 
Pressurized fuel gas is supplied to the burner 10 at a pressure of about 30 
PSIG and at a rate of 10,000 SCF/hour. A 30% by volume portion of the fuel 
(3,000 SCF/hour) flows into and through the primary mixer-nozzle assembly 
34 wherein it is mixed with about 7,500 SCF/hour of flue gases (about 250% 
by volume of the fuel gas present in the mixture). The remaining portion 
of the fuel gas i.e., 7,000 SCF/hour flows from the conduit 42 to the four 
secondary fuel nozzles 60 from where the fuel gas is discharged into the 
furnace space 21. The rate of air introduced into the housing 12 is 
controlled by means of the damper 30 such that the total rate of air 
introduced into the furnace space 21 is an amount which results in at 
least a substantially stoichiometric total fuel-total air mixture therein. 
The air flows through the open end 14 of the housing 12 into the furnace 
space 21 by way of the interior of the burner tile 48. 
The fuel discharged from the secondary fuel nozzles 60 mixes with the 
remaining air, products of combustion (flue gases) from the primary 
reaction zone and relatively cool flue gases in the furnace space 21 to 
form a second combustion products and flue gases diluted fuel-air mixture 
which is burned in a secondary reaction zone adjacent to and surrounding 
the primary reaction zone in the furnace space 21. 
Because of the dilution of the first and second fuel mixtures with flue 
gases, such mixtures burn at a relatively low temperature whereby the 
additional flue gases formed have a low NO.sub.x content. That is, the 
mixture of flue gases withdrawn from the furnace space 21 has a NO.sub.x 
content of less than about 25 ppm. 
EXAMPLE II 
A burner apparatus 100 designed for a heat release of 10,000,000 BTU/hour 
by burning natural gas having a caloric value of 1,000 BTU/SCF is fired 
into the furnace space 121. 
Pressurized fuel-gas is supplied to the burner 100 at a pressure of about 
30 PSIG and at a rate of 10,000 SCF/hour. A 30% by volume portion of the 
fuel (3,000 SCF/hour) flows into and through the primary mixer-nozzle 
assembly 34 wherein it mixes with 3,000 SCF/hour of air and about 7,500 
SCF/hour of flue gases. The portion of the total air mixed with the fuel 
gas in the primary mixer-nozzle assembly and discharged therefrom results 
in a sub-stoichiometric fuel-air mixture. 
The first flue gases diluted fuel-air mixture discharged from the nozzle 
138 mixes with additional air flowing into the furnace space 121 by way of 
the open end 114 of the housing 112. The resulting mixture is burned in 
the primary reaction zone, and because, at least in part of the presence 
of flue gases, the additional flue gases produced have a low NO.sub.x 
content. 
The remaining portion of fuel, i.e., 7,000 SCF/hour, flows to the nozzles 
160 from where the fuel gas is discharged into a secondary reaction zone 
within the furnace space 121. A 1,000 SCF/hour amount of air is conducted 
directly to the secondary reaction zone by way of the annular conduits 
161. The air flows from the annular conduits 161, mixes with the fuel 
discharged from the nozzles 160, mixes with products of combustion (flue 
gases) from the primary reaction zone and mixes with relatively cool flue 
gas and any air contained in the furnace space 121 to form a second 
products of combustion and flue gases diluted fuel-air mixture which is 
burned in the secondary reaction zone at a relatively low temperature. 
The mixture of flue gases formed in the furnace space 121 and withdrawn 
therefrom has a NO.sub.x content of less than about 25 ppm. 
Thus, the present invention is well adapted to carry out the objects and 
attain the advantages mentioned as well as those inherent therein. While 
presently preferred embodiments of the invention have been described for 
purposes of this disclosure, numerous changes in construction and in the 
arrangement of parts and steps will suggest themselves to those skilled in 
the art which are encompassed within the spirit of this invention as 
defined by the appended claims.