CA1173344A - Two stage low no.sub.x combustor - Google Patents

Abstract

ABSTRACT
The method of producing low NOx emissions in a two stage combustion system includes combusting in a first stage within a high velocity combustor with an air-fuel mixture at less than stoichio-metric while avoiding the introduction of any flue gas into the first stage. Thereafter, substantial volumes of recirculated flue gas from the exhaust stack of a process furnace are introduced dowmstream of the first stage. This gas has a high exit velocity into the process furnace where it reacts with the partially burnt fuels to produce ad-ditional products of combustion in a second stage. The apparatus includes a refractory lined process furnace defining a chamber or fire box therein, a flue gas stack exiting the furnace, a recirculated flue gas duct extending from the furnace and a combustor defining a combustion chamber mounted to the furnace. The preferred com-bustor includes a plurality of air nozzles positioned at the opposite end of the combustor from a fuel gun for directing combustion air in counterflow relationship to the fuel so as to create a toroidal vortex in the combustor chamber. Downstream of the air nozzles are a plurality of flue gas nozzles which extend adjacent the combustion chamber and throat thereof for directing flue gas or a flue gas-combustion air mixture through the throat into the process furnace at a high exit velocity.

Description

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TWO STAGE LOW NO COMBUSTOR
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FIELD OF THE INVENTION
My invention relates to combustion systems and, more particularly, to a two stage combustion system for ~rJinimizing NO
5 formation in a process furnace.
DESCRIPTION OF THE PRIOR ~RT
The need to minimize the formation of oxides of nitro-gen during combustion has become a requirement over the last few years. Many existing fired heaters, boilers and furnaces are not 10 discharging satisfactory emissions to meet present and projected strin-- - gent regulations in respect of MO . The probable- use of newly availa-bel synfuels containing large amounts of combined nitrogen, as ~vell as the increasing need to conserve energy, will further increase the demands for low NO emissions in the future. Under typical indus-15 trial combustion conditions, nitric oxide (NO) rather than nitrogen dio~ide (NO2) is predominant in the Nx content of stack gases from process furnaces.
There are two basic sources of NO formation, namely, from the nitrogen in the air, often called nitrogen fixation, and that 20 resulting from fuel bound nitrogen. In process furnaces NOX forma-tion is generally developed more from the fuel bound nitrogen than the nitrogen in the air.
The percentage of conversion from fuel bl~und nitrogen is more dcpendent upon oxygen availability than high ternperature and 25 NO formed from fuel nitrogen is reduced under oxygen deficient con-ditions. On the other hand, nitrogen fixation is strongly dependent upon temperature and is only significant above 2800~.
In addition to these basic factors, there are a n~lmber of practical Eactors in industrial applications which influence the flam~
30 envelope conditions and, therefore, the NO formation. These factors inch1de excess air, type of process furnace, dimensions and conditions inside the Eire box which will determine rates of heat transfer and, therefore, mean Elame temperatures, the use of combustion air preheat, thc amount of internal and external burner recirc~llation of reacted 35 and part;ally reacted products oE combustion. the use of multi-stage comb~lstion air, the Sl~e of the f amF envelope, the type and shape ~' : ' ~

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of the flame and use of combustion air vit;ated with flue gases to reduce flame temperatures, the quali-ty of atomi~ation with liquid ` fuels, the use of multi-stage fuel injectin to split up the flame and reduce the mean temperature and the use of ~vater or steam injection to r educe flame temperature.
Even -though all these factors are known, burner de-signs and the method of operating these burners must be such that there is no sacrifice of the primary objectives of the burner, namely, satisfactory combustion performance and process efficiency. Many existing approaches to low NOX have sacrificed one or both of these obj ecti~es .
Heretofore, recirculatecl flue gas has been employed - in multiple stage combustion apparatus. Generally speaking, in a two stage burner the flue gas has been introduced into both the first stage and second stage of combustion. Flue gas andlor combustion air have even been blown into the process furnace chamber directly.
This latter approach is not effective because the flame ;s normally out of control by this time and impingement on the tubes or lining causes overheating and deterioration. Representative patents in the - 20 area of multiple stage combustion include United States Patent No.4,135,374, United States Patent No. 4,0~3,921, United States Patent No. 3,868,211, United States Patent No. 3J~380, 570 and United States Patent No . 4 ,128, 065 .
SUMMARY OF THE INVENTION
My invention reduces the NO formation and, there-fore, the NO emissions without sacrificing overall combustion per-formance or process efficiency. It further eliminates flue gas or combustion air introcluction directly into the process furnace chamber and thereby avoids the pitfalls associated with such a practice. It 3() accomplishes this through the recognition that no flue gas shou]d be introducecl into the first stage combustion and that the first stage combustion should tal~e place at less than stoichiometric, that is, in a reducing mode. All of the recirculated flue gas must be added to the second stage of combustion. The use of combustion air only in the firs, stage permits combusting under lower stoiclliometric conditions.
Introducing flue gas into the second stage of combustion but not .. ..

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directly into the process furnace provides a greater volume of gas and e~cit velocity. This increased volume in the seconcl stage gives aclded turbulence and the velocity for more efEective combustion, in-duced recirculcltiQn and less residexlce time.
:, My invention is a method of producing low NOX emis-sions in a two stage combustion system by combusting in a first stage with an air to Euel mixture at less than stoichiometric while avoidinv the introduction of any flue gas into the first stage. Substantial volumes of recirculated flue gas from the furnace chamber are intro-duced into the second stage downstream of the first stage to react with the partially burnt fuel and produce additional products of com-bustion having a high exit velocity into the process furnace chamber.
In a preferred embodiment, the method includes pr~)viding first stage combustion air downstream of the fuel and in counterflow relationship la thereto so as to create a toroidal vortex in the first stage. In acldi~
tion, the recirculated flue gas may be mixed with air prior to intro-ducing the mixture into the second stage.
The combustor apparatus inc}udes a refractory lined combustion chaml~er terminating in a throat in registry with -the pro-cess furnace chamber. Fuel means are positioned at one end of the combustor and preferably a combustion air plenum surrounds the combustion chamber to provide combustion air through a plurali ty of nozgles at an opposing end of the combustor to the Iuel means so as to provide combustion air in counterflow relationship to the fuel so as

2~ to create a toroidal vortex in the chamber. A plurality of nozæles direct flue gas from a flue gas duct into the area adjacent the throat so as to direct the flue gas through the throat into the process fur~
nace chamber at a high exit velocity. Air supply means may be associated with the flue gas duct for supplying controlled amounts of air for mi~iing with the flue gas prior to the entry into the com-bustor chamber.
BRIEF DESCRIPTION OF TE~E DRA~\1INGS
Fig. 1 is a schematic of an o;l field steamer including my combustor:
Fig. 2 is a section tal;en along a longitudinal center line throueh ehe co~l-b~ tor part of the proceas furn.lcl3 systen~ of . ~ , ~'' , 1~3~
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Fig. 3 is a graph of NO in parts per million as a function of the percent of stoichiometric air.
DESCRIP~ION OE~ TI-IE PREFERRED EMBODIMENTS
~vly two stage comb~lstion system and the method of operation thereof can be ~Ised in a wide variety of process furnaces including, but not limited to, rotary driers and kilns, refinery heat-ers, malt kilns, steam boilers, direct fired air heaters, spray driers, submerged combustion plants, heat exchangers, drying hoods and ef~luent incineration. I have illustrated my invention in connection with an oil field steamer in Fig. 1. Such a steamer is used to gen-erate typically 1000 psig steam at . 85 dryness for injecting into the ground to aid in the extraction of petroleum crudes.
The oil field steamer, generally designated 10, com- ^ ~-prises a process furnace 12 having a combustor 14 connected at one encl thereof. The furnace 12 includes a large furnace chambcr 16 housing a series o~ interconnected hlbes ~vhich form a heat transfer radiant section 18. Do~vnstream from the heat transfer radiant sec-tion 18 are a series of shock tllbes 20 located subjacent a heater convection section 22. Exiting from the furnace 12 is a stack 24 ~vherc the spent products oE cornbustion containing the NO~ pass into the environs. A recirculated flue gas duct 26 extends from the furnace 12 at the foot of the stack 24 and extends the length of the furnace 12 to the combustor 14 as will be described in more 2 S detail hereinafter .
The recirculated flue gas duct 26 terminates in an el-bow 27 which feeds into a flue gas control damper 28. The flue gas is drawn through the flue gas duct 26 by means of a force draft fan 30 adjacent the fl~le gas control damper 28. The forced draft fan 30 is driven by motor 32 mounted adjacent the fan 30. A combustor flue gas inlet 42 directs thc tlue gas into the combustor 14. Fresh air (preheated or ambient) can be drawn into the flue gas duct 26 throug~
a control damper 40 ~.vhich connects into the flue gas duct 26.
Combustion air is fed into the combustor 14 through a combustion air inlet 3~ in registry with a frcsh air control damper 34.
The combustion air can be ambient or preheated through a variety af ~ ~'733~

known means. The air is drawn into the system by means of a force draft fan 36 driven by motor 31. The combustion air enters the combustor at air inlet 44.
The combustor 14 comprises an outer casing or housing 50 and an inner casing 56 which includes a refractory lining 52, Fig. 2. The outer housing 50 and the inner casing 56 define an air plenum 46 therebetween which extends about ` the combustor 14, which is generally cylindrical in shape. Combustion air inlet 44 is in communication with air plenum 46 at the distal end of combustor 14.
Refractory lining 52 defines chamber 54 which functions as the first stage pre-combustion chamber.
An appropriate fuel source such as multiple fuel gun 58 ex~ends through the distal end of the combustor 14 so as to provide a -fuel tube 66 which feeds axially into combustion chamber 54. The fuel gun 58 may be of a variety of types and I have illustrated the fuel gun 58 as including a steam atomizer in-let 62~ and an oil inlet 64 and a gas fuel tube 60. While the multiple fuel gun illustrated in Fig. 2 is designed basically to burn liquid and gaseous fuels in combination or separately, the apparatus in Fig. 2 can be arranged to burn solid fuels singly, or in combination with the other fuels in almost any ratio.
Adjacent ~he air plenum 46 and downstream thereof is a flue gas plenum 48 which is fitted to the combustor 14 and connects to flue gas duct 26 ~Fig. 1~
by means of flue gas inlet 42. The downstream end of the combustor 14 defines a frustoconical shaped throat 70 which feeds into the process furnace chamber 16 of the oil field steamer or other appropriate furnace. The combustor 14 is attached to the furnace by means of fixing flange 58 at the downstream end of combustor 14.
Extending radially inward at the juncture of the air plenum 46 and the flue gas plenum 48 is refractory shoulder 72. Refractory shoulder 72 is upstream of and adjacent to throat 70. A plurality of stainless steel tubes or iets 74 . :
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extend through the shoulder 72 from the air plenum 46 into the combustion chamber 54. Jets 74 are directed into combustion chamber 54 in an axial direc-tion substantially adjacent the refractory lining 52 and in counterflow relation-ship to the fuels exiting the fuel tube 66.
A second series of tubes, namely, flue gas jets 76, also extend through the shoulder 72 and commlmicate between flue gas plenum 48 and throat 70.
Flue gas jets 76 extend completely around the combustor 14. A typical applica-tion may include as many as 48 jets or more feeding into a throat having a minordiameter on the order of 30 inches. Flue gases enter the throat area 7~ from the jets 76 at an angle to the axial center line of the combustor 14.
The operation of the oil field steamer lO is as follows. Water enters the piping of the heater convection section 22 and circulates therethrough into , the shock tubes 20 and then into the heat transfer radiant section 18, Fig. l.
Heat transfer radiant section 18 converts the water into steam which exits the section 18 where it is piped into the ground (not shown~. I`he heat input is d~veloped by the combustor 14 through two stage combustion with the first or pre-combustion stage being in combustion chamber 54 (Fig. 2) and the second stage occurring within the process furnace chamber 16 in the area of the heat transferradiant section 18 (Fig. 1).
~he appropriate fuel is fed through the fuel tube 66 and/or 60 essenti-ally axially into the combustion chamber 54. Combustion air is fed from plenum 46 through jets 74 in counterflow relationship to the fuel so as to create a toroidal vortex in the combustion chamber 54 as illustrated by the arrows. An appropriate igniter (not shown) initiates the combustion in the precombustion chamber 54. The combustion air is fed in less than stoichiometric amounts (e.g.
less than 75%) so that burning takes place substoichiometrically under highly reducing conditions. ~s little as 50% of the fuel or less may be burned in the L'~33~'1 first stage of combustion. It will be noted that the combustion air in plenum 46 also extends about the fuel gun 58 so circulating combustion air provides cooling thereto and keeps the fuel tube 66 clean.
The flue gas which can contain as little as 1% ~o 2% oxygen is fed through the jets 76 in large volumes. The flue gas is taken from stack 24 through flue gas duct 26. The large volumes oE the lean oxygen flue gas give added turbulence and velocity for more effective combustion and induced recircu-lation in less residence time.

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The flue gas ~vhich m;ly be mi~;ed with con-trolled amounts of combus-tion air enter the furnace chamber 16 with a high e~it velocity thereby providing a second stage of combustion of the partially burnt fuels in the furnace chamber 16. It should be noted that no flue gas is - 5 utilizecl in the first stage of combustion. The preferred minimum exit llue gas velocity is 250 feet per second hot.
Fig. 3 illustrates the relationship between NOX and parts per million as a function of percent stoichiometric air. The curve was presented in a paper entitled Basic Kinetic Study and ~odel-10 ing of Nitrogen Formation ln Combustion Processes, prepared by Esso Research and Engineering Company and Esso Mathematics and Systems, Inc. The curve shows that NO formation increases rapidly as the percent stoich;ometric air approaches 100. It can be seen that main-taining the stoichiometric at lF ss than 50~6 and down to 30% drastically L5 reduces the NO forrnation. By not using any flue gas in the first stage of combust;on, the e~ctremely lean o~sygen mixture can be employed to assure a minimum amount of NOX formation.
The injection of the flue gas into the throat area of the combustor for second stage combustion only further minimizes NO
20 formation. This occurs because the flue gas may only have 1 to 2~6 oxygen and, therefore, large volumes are necessary to provide the combustion air. These large volumes give the added turbulence and velocity for more effective combustion, induced recirculation and less residence time. The flue gas~ which typically is at a temperature of 2:, 350 to 400F, cools the flame which is normally above 3000F and there-by lowers the adiabatic flame temperature to a temperature below the level where nitric oxides could reform. This is accomplished without placing the flame envelope out of control.

Claims (10)

THE CLAIMS:

1. A method of minimizing NOx emissions in a two-stage combustion system including a burner firing into a process fur-nace chamber having a flue gas recirculation system comprising:
A. combusting in a first stage within the burner with an air-fuel mixture at less than stoichio-metric while avoiding the introduction of any flue gas into the first stage to produce products of combustion and unburnt fuel;
B. introducing substantial volumes of recirculated flue gas from the furnace chamber into the com-bustor downstream of the first stage; and C. exiting the flue gas into the process furnace at a high velocity to react with the unburnt fuel and produce additional products of combustion.

2. The method of Claim 1 wherein the burner com-prises a high velocity combustor.

3. The method of Claim 2 including providing the first stage combustion air downstream of the fuel and in counterflow relationship thereto so as to create a toroidal vortex in the first stage.

4. The method of Claim 2 including mixing the re-circulated flue gas with combustion air prior to introducing the flue gas-combustion air mixture into the second stage.

5. The method of Claim 2 including exiting the flue gas in to the furnace at a velocity of at least 250 feet per second hot.

6. In a method for operating a furnace system utilizing recirculated flue gas including a combustor having a first stage combustion chamber therein for mixing fuel and combustion air in less than stoichiometric ratio, a furnace chamber downstream of the first stage combustion chamber forming a second stage combustion chamber, the improvement comprising:
A. supplying substantially 100% of the recirculated flue gas through a downstream end of the com-bustor; and B. directing the flue gas into the second stage com-bustion chamber at a high exit velocity.

7. A furnace system comprising:
A. a refractory lined furnace defining a furnace chamber therein;
B. a flue gas stack exiting the furnace;
C. a recirculated flue gas duct extending along the furnace:
D. a combustor mounted at one end of the furnace comprising:
i. a refractory lined combustor chamber termi-nating in a throat in registry with the furnace chamber;
ii. fuel means positioned at an end of the com-bustor opposite the throat for directing fuel generally axially into the combustor chamber;
E. a combustion air plenum for supplying combustion air in less than stoichiometric amounts to the combustor chamber;
F. a flue gas plenum in communication with the flue gas duct; and G. a plurality of nozzles in communication with the flue gas plenum and extending into the combustor chamber adjacent the throat so as to direct flue gas through the throat into the furnace chamber at a high exit velocity.

8. The furnace system of Claim 7 including a plurality of air nozzles in communication with the combustion air plenum, said nozzles positioned substantially adjacent the flue gas nozzles for directing combustion air in counterflow relationship to the fuel so as to create a toroidal vortex in the combustor chamber.

9. The furnace system of Claim 8 including a combustion air inlet in registry with said air plenum positioned at the downstream end of said combustor, said air plenum formed by an outer casing surrounding said combustor, said air passing along said combustor in route to said air nozzles.

10. The furnace system of Claim 7 including an air supply means associated with the recirculated flue gas duct for supplying controlled amounts of air for mixing with the flue gas prior to entry into the combustor.