US3880571A

US3880571A - Burner assembly for providing reduced emission of air pollutant

Info

Publication number: US3880571A
Application number: US383008A
Authority: US
Inventors: Richard R Koppang; Jr Harland L Burge; Wallace A Carter; Ellis W Sheffield
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1973-07-26
Filing date: 1973-07-26
Publication date: 1975-04-29
Anticipated expiration: 1992-04-29
Also published as: DE2431573A1; NL7410032A; CA991527A; SE7409632L; FR2328372A5; AU7166574A; BE805796R; IT1045989B; JPS5043532A; SE412637B; GB1468364A

Abstract

A pair of conduits, coaxially arranged, provide flow paths for carrying combustible reactants to a combustion zone. The central conduit, otherwise known as a distribution tube, extends to within the combustion zone and includes a disc element at the combustion zone end thereof for deflecting an oxidizing reactant moving axially along the exterior of the distribution tube. A fuel reactant introduced into the distribution tube is carried to the combustion zone end thereof where it issues through radially aligned orifices and impinges the oxidizing reactant to produce a hollow cone flame. Additionally, a mixing element may be included within the distribution tube to impart centrifugal motion to liquid fuels which are then subject to being atomized in the region of the radial ports. Proper control and selection of both the dynamic and physical (dimensional) parameters provides for a combustion process to minimize oxides of nitrogen, and increases heat transfer rates in the combustion zone.

Description

' 1 1 Apr. 29, 1975 Primary ExaminerCarroll B. Dority, Jr. Attorney, Agent. or FirmDaniel T. Anderson, Esq.; Edwin A. Oser. Esq.; Jerry A. Dinardo [57] ABSTRACT A pair of conduits, coaxially arranged, provide flow paths for carrying combustible reactants to a combustion zone. The central conduit, otherwise known as a distribution tube, extends to within the combustion zone and includes a disc element at the combustion zone end thereof for deflecting an oxidizing reactant moving axially along the exterior of the distribution tube. A fuel reactant introduced into the distribution tube is carried to the combustion zone end thereof where it issues through radially aligned orifices and impinges the oxidizing reactant to produce a hollow cone flame. Additionally, a mixing element may be included within the distribution tube to impart centrifugal motion to liquid fuels which are then subject to being atomized in the region of the radial ports. Proper control and selection of both the dynamic and BURNER ASSEMBLY FOR PROVIDING REDUCED EMISSION OF AIR POLLUTANT Inventors: Richard R. Koppang, Cypress;

Harland L. Burge, Jr., Tarzana; Wallace A. Carter, Fullerton; Ellis W. Sheffield, Upland, all of Calif.

Assignee: TRW Inc., Redondo Beach. Calif.

Filed: July 26, 1973 Appl. No.: 383,008

US. Cl. 431/8; 239/402.5; 431/116; 431/177; 239/400; 431/115 Int. F23m 3/00 Field of Search................ 431/8. 177. 182-184, 431/187, 188. 348, 350-354; 239/417, 422, 425, 399, 400

References Cited UNITED STATES PATENTS United States Patent Koppang et al.

8 Claims, 13 Drawing Figures physical (dimensional) parameters provides for a combustion process to minimize oxides of nitrogen, and increases heat transfer rates in the combustion zone.

nh iuum 1 w IIIIIIIII A link et al. Schreter et a1 Koppang et PHENTEU APR 2 91975 SHEET 2 BF 5 Fig. 3

RESIDUAL OIL (N06) I3 GPH AIR ATOM lZED NUMBER OF ATOMIZING ORIFICES Fig. 5

NATURAL GAS L5: I07 BTU/HR INPUT UNCOOLED FURNACE 20% EXCESS AIR AI R BLOCKAGE RATIO :ii o 4.. 8 56518 62 PATENIEBAPMQIQYE, 3.880.571

sum 38? 5 E & RESIDUAL OIL (No.6) v IOO GPH 290 STEAM ATOMIZED o o r0 9 280 O LIJ U LLI E 270 8 i o 9 E 260 LU 0.20 0.30 0.40 0.50 0.60 0.70 AIR BLOCKAGE RATIO Fig. 6

I00 GPH RESIDUAL OIL STEAM ATOMIZED 240 3O EXCESS AIR 3 & v 220 x O z I80 I 1 l FUEL DROPLET PENETRATION Xm/A Fig. 8

PQTENIEDAFRZQIBYS R.880,571

SHEET u BF 5 NATURAL GAS I 6 E 2 x l0 BTU/HR INPUT 0.

Q I l 1 l l l PENETRATION INDEX, l/A

Fig. 7

2 x :0 BTU/HR GAS FIRED BURNER 0 5 IO l5 MOMENTUM RATIO, AIR/FUEL A Fi 9 z 9 n. 5, 300

0 i? 250 Fig. 10 Q E 200 g RESIDUAL on. (NO. 6) 0: I00 GAL lHR g STEAM ATOMIZED 0 I50 1 1 0 5 l0 I5 g MOMENTUM RATIO, AIR/FUEL mEI-mnmzsms 3,880,571

SHEET 5 OF S FUEL m A smum. on. NNXTURE (0.6 I. E 200 RE 2;

N O I50 8 F '0 12 a LU I00 6: g D\ST\LLATE on. g 50 NATURAL GAS Z 0 I I I l HEAT INPUT BTU/HR TRANSLATOR 47 BURNER ASSEMBLY FOR PROVIDING REDUCED EMISSION OF AIR POLLUTANT BACKGROUND OF THE INVENTION The invention relates generally to burner assemblies used for the combustion of fossil and manufactured fuels. More particularly the invention relates to power burner assemblies which herein means burners of such a size as are normally used for industrial and commercial applications. e.g.. boilers. dryers. and process heat ers.

Typical burner assemblies provide two or more orifices through which a fuel or an oxidizer is ejected into an external mixing zone. Mixing takes place in the external mixing zone by impingement of one reactant stream which. in the case of liquid fuels provides additional atomization of the liquid fuel. Once the reactants have been ejected from the orifices. further treatment of the reactant stream generally is not attempted. However. simple impingement of one reactant stream with another does not result in optimum mixing.

Atomization of liquid fuel takes place in the external mixing zone by the impingement of the fuel stream with an oxidizer stream. Preconditioning of the fuel prior to its ejection from the orifice is seldom attempted. Typical burner assemblies may be found in the 1933 patent to Zulver. US. Pat. No. l.934.837; the I966 patent to Schreter et al.. U.S. Pat. No. 3.254.846 and the 1965 patent to elverum. .lr. US. Pat. No. 3.205.656.

One method of reducing emissions of oxides of nitrogen is to minimize the residence time that hot nitrogen molecules are in contact with unreacted oxygen. and to conduct the combustion and post combustion process at minimum temperatures. The residence time may be controlled by completing the combustion process rapidly and at near homogenous conditions. Flame temperature reduction may be accomplished by radiating and conducting heat away from the flame and by diluting the reactants with an inert gas. These conditions will also result in reduction of carbon monoxide. hydro carbon. and particulate emission also.

Accordingly. it is an object of the present invention to provide a burner assembly in which there is improved mixing of the reactant in the external mixing zone.

It is also an object of the present invention to provide a burner assembly in which pretreatment of the fuel may be provided prior to injection into the external mixing zone.

It is a further object of the present invention to provide a burner assembly in which the dynamic and dimensional parameters are selected to control the reaction kinetics of the combustion process and to shape the flame to maximize heat transfer.

It is yet another object of the present invention to provide a burner assembly in which there is a reduced emission of gaseous and particulate air pollution.

SUMMARY OF THE INVENTION In accordance with the teachings of the invention a burner assembly of the forced draft type includes a set of coaxial conduits defining an annular flow path therebetween which exits into a combustion zone. A distribution tube. or inner conduit. includes an array of radially aligned orifices near the combustion zone end of the distribution tube. Thus. air or any gaseous oxidizer exiting from the annular flow path is intercepted by a fuel exiting from the radial orifices. Mixing occurs as a result of an exchange of momentum between the reactant streams and an aerodynamically formed screen effect. Combustion is further enhanced by several parameters which relate both to the dynamics of combustion and to the physical dimensions of the burner.

The burner may further include means within the distribution tube for causing liquid fuels to be entrained upon the interior walls of the distribution tube. This fuel will then be subjected to shear forces at the radial ports that will assist in atomizing liquid fuels. By controlling the momentum of the reactant streams. properly sizing the deflector disc. and selectively sizing and positioning the array of orifices. the flame will take the shape of a hollow cone having a relatively thin wall.

Other objects, advantages. and inventive features of the invention will be apparent from the more detailed description which follows and forms the drawings wherein like characters indicate like parts and which form a part of this application.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in partial cross section and partly in block diagram form showing a burner assembly in com bination with an industrial type boiler unit;

FIG. 2 is a view in partial cross section showing the burner assembly of FIG. I and portion of the boiler combustion chamber;

FIG. 3 is a graph representing conditions of combustion.

FIGS. 3 I0 are graphical representations showing the effects of various dimensional and dynamic parameters on NO, emissions;

FIG. II is a view in partial cross section showing a specific fuel distribution design;

FIG. 12 is a graph representing the trend in NO, emissions as a function of burner rating; and

FIG. 13 is an alternative embodiment of the burner assembly of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, the present invention is shown embodied in an industrial boiler 10 having a cylindrical inner wall 11, and an outer wall I2 defining a coolant jacket. The boiler further has an inlet 14 for introducing a coolant such as water. The inner wall I I serves as a combustion chamber wall and defines a combustion zone 16 wherein combustion products are introduced by a burner assembly I7. An oxidizer from an oxidizer reservoir 18 is introduced into the assembly 17 by an oxidizer pump 19 and an oxidizer conduit 20. In many cases. the oxidizer is air taken directly from the atmosphere. Similarly. a fuel from a fuel reservoir 21 is introduced into the assembly 17 by a fuel pump 22 and fuel conduit 23. A fuel atomizing gas. which may be an oxidizer. such as air. or an inert gas, may be supplied to the assembly I7 by a mixing gas pump 24 and a mix ing gas conduit 25.

The burner assembly 17 is further detailed in FIG. 2 wherein the combustion zone 16. in addition to the chamber walls 11 is further defined by an end wall 26.

The oxidizer tube 27 enters the combustion chamber through the end wall 12. A second conduit 28 hereinafter referred to as a distribution tube. is coaxially mounted within the oxidizer tube 27 and extends to within the combustion chamber. The one end 30 of the distribution tube 28 carries a circular disc element 3!. The distribution tube 28 is further characterized by an array of radially aligned orifices 32 which are circumferentially disposed around the distribution tube near the one end 30 thereof. The oxidizer tube 27 together with the distribution tube 28 serve to define an annular flow path 33.

Oxidizer from oxidizer conduit (FIG. I) is introduced into the annular oxidizer flow path 33. Atomizing gas from conduit may be introduced into distribution tube 28. Fuel from fuel conduit 23 is introduced into the distribution tube 28.

In operation. an oxidizing gas. such as air. is intro duced into the oxidizer supply tube 27 as indicated by the arrow at 54. The oxidizer gas flows down the annular parh 33 to create a shroud of oxidizer gas around the distribution tube 28.

A fuel is introduced into the fuel supply tube 22 as indicated at arrow 35. The fuel enters the distribution tube 28 through an inlet orifice 36. The fuel may be joined by an atomizing gas introduced as indicated by the arrow 37. The atomizing gas may simply be an oxidizer gas as indicated in FIG. 1, an inert gas such as steam. nitrogen. or combustion gas products recirculated from the combustion chamber.

The fuel and. if used. the atomizing gas pass down the distribution tube 28 toward the radial ports 32 and in the case of gaseous fuels such as methane become an admixture through turbulent mixing. The fuel passes through the radial ports creating a plurality of jets around the distribution tube each of which impinges the shroud of oxidizing gas.

In the case of liquid fuels. the inclusion of a mixing element 38 having helical surfaces 39 mounted or formed within the distribution tube 28 serves to impart centrifugal forces to the fuel and mixing gas. When liquid fuels. such as oil. are used. the liquid is centrifuged outwardly and entrained on the internal walls of the distribution tube 28. When the liquid fuel reaches the radial ports 32 atomizing gas exiting through the orifices subject the liquid to high shear forces which break up the liquid into a fine fog like mist. There is thereby provided means for atomizing liquid fuels. The centrifuging of liquid fuels is enhanced by introducing the fuel tangentially into the distribution tube 28 thereby imparting to the fuel centrifugal motion prior to reaching the mixing element 38.

The impingement of the fuel exiting through the orifices 32 with the oxidizer stream results in a mixing of the fuel and the oxidizing gas in an external mixing zone generally designated by the numeral 40. The gas flow is generally shown by the arrows at 41. The external mixing is enhanced by any premixing which occurred within the distribution tube 28. The external mixing is further enhanced by any oxidizing gas which passes between the fuel jets and is reflected into the external mixing zone 40 by the deflector disc 3|. The deflected oxidizing gas. indicated at 42. impinges the main stream. where further mixing occurs. Thus. there is provided a very thoroughly mixed set of reactants in both the radial and circumferential dimensions which aids in assuring a complete combustion process. local control of oxidizer and fuel mixture ratio. and therefore. as will be seen from later discussions. reduced emissions of air pollutants.

An extensive experimental test program has been conducted for the purpose of studying the influence exertcd by various dynamic and dimensional parameters on the generation of oxides of nitrogen and for devising a method of design optimization for minimum emission of oxides of nitrogen (NO It will be appreciated that the generation of such oxides may be the result of either thermal formation or chemical release. In the latter case. the fuel itself. such as fuel oil. is characterized by having large quantities of nitrogen chemically bonded into its molecular structure. The degree to which such nitrogen is present is a function of the particular grade of oil and its geographic source.

In the case of thermally generated oxides. atomic oxygen combines with free nitrogen to form nitrous oxide and atomic nitrogen. i.e.. O N N0 N. This reaction may be followed by the atomic nitrogen combining with molecular oxygen. if available. to form more nitro gen oxide plus atomic oxygen, i.e.. N -l- 0 NO 0. Thermal generation of nitrogen oxide is dependent upon time. temperature. and the availability of molecular oxygen.

Thus. nitrogen oxide may be reduced by sufficiently reducing the time during which 0 is exposed to N or N. i.e.. reducing the resident time. Conceptually. FIG. 3 shows that as a result of combustion. the temperature rises to some peak which is limited in part by the heat transfer characteristics of the system. After combustion. the temperature drops off gradually. Molecular oxygen of course decreases rapidly during combustion. but a certain residual quantity remains post combustion. It is known that thermal generation of NO requires temperatures in excess of some nominal temperature which is about 2.200 F.

Since in most industrial applications the oxidizer will be air. the O to the extent that it exceeds the needs for combustion. is available to combine with the N from the air during the combustion step. Further. since this is a rate limiting step. rapid combustion contributes to reducing NO generation. The presence of N is further compounded by chemically bound N, in the fuel.

The quantity of excess 0 at any particular time may vary greatly throughout the fuel-oxygen mixture. Thus. stoichiomatric mixture ratios do not assure that local conditions throughout the combustion gases will also be at stoichometric conditions. Thus. it is important to assure that the reactants are thoroughly mixed in a manner which will reduce the liklihood that there will be local pockets of excess 0 As already stated. thermal generation of NO occurs only above about 2.200 F. Therefore. if the post combustion temperatures of the unburned fuel/air components are rapidly reduced to less than this critical temperature. there will be no further NO formation.

NO formation will occur downstream of the flame front due to the presence of oxygen and nitrogen at elevated temperatures. It is desirable to reduce the post combustion gas temperatures to less than such a critical temperature as rapidly as possible. This will of course depend in part upon the heat transfer characteristics of of the surrounding media. It is also known to induce cooler gases from an external source as by some recirculation scheme.

it is believed that the substantially short conical flame 45 as produced by the invention herein results in higher gas velocities near the combustion chamber walls when the momentum ratio and the air blockage ratio are properly selected thereby providing heat transfer by convection in addition to radiation. Heat transfer measurements have confirmed that a burner constructed and operated as disclosed herein displays better heat transfer characteristics than conventional burners of comparable rating.

in addition to controlling the above variables for the purpose of assuring a homogeneous reactant mixture ratio, it is beneficial to provide rapid step combustion to minimize the O and N resident time at elevated temperatures.

To this end, the shape of the flame produced by the subject burner and the homogeneous mixing yield a flame shape substantially in the form of a hollow cone. This shape is primarily dictated by the deflector 31 which serves as a flame holder and by the degree of combustion and swirl or momentum. The degree to which the flame will be thrown radially outwardly toward the combustion chamber walls will be influenced by the air blockage ratio, discussed above.

A large blockage ratio indicates a relatively large disc which would tend to throw the flame more radially outward. The primary combustion air velocity (momentum) will also substantially affect the flame shape by throwing the flame more forward at higher velocities. The resultant flame of the burner herein described has a relatively short combustion distance, i.e., the conical flame has a thin reaction zone. thereby indicating rapid combustion as desired.

It has been discovered that a central fuel injection pintle type of burner as disclosed herein when designed and operated in accordance with the guidelines as herein enumerated will provide reduced NO emission compared to conventional burners of comparable heat output. To this end, guidelines are given so as to influence the local fuel to air (oxidizer) mixture ratio to be homogenous, to provide rapid combustion, and to induce rapid dissipation of heat after combustion.

The physical size and various dimensions are of course dependent upon the input rating of the burner and the heat content of the fuel. As in any burner. the total flow rates of the reactants are determined by the net heating value of the fuel selected. thus the oxidizer annual flow path 33, otherwise designated a venturi section. may be calculated for selected inlet pressures.

The sizing ofthe central pintle or distribution tube 28 has been developed from analytical and experimental investigations. These parameters are important in controlling the mixing characteristics. The number and size of the fuel orifices 32. FIG. 2, may be determined from the fuel stream to air stream aspect ratio which herein means the ratio of the total fuel orifice area to the circumference of the distribution tube at the fuel orifice location. The fuel orifice total area equation is as follows:

where:

Q flow rate (cu. ft. per minute) A orifice area (sq. inches) C constant including the coefficient of discharge and a unit conversion factor AP pressure differential (psi) p specific weight of fluid (lbs/fr) substituting a nominal inlet pressure drop of 1 psi, an orifice coefficient of 0.65 and a specific weight for natural gas of 0.05 lb/ft" yields Q/A equal to 338.8. The flow rate Q is determined from heat rating.

The total fuel injection area is then divided into a number of radial orifices distributed circumferentially around the central element in order to determine the fuel injection orifice diameter.

Generally, it is desirable to have a large number of fuel orifices (at least 25 or more) as indicated by FIG. 4 which show that with residual oil 6 increasing the number of orifices improves the circumferential distribution of fuel resulting in a reduction of the NO emis sions; however, manufacturing and economic consider ations places a practical limit on the number selected. Also, as will be seen, the radial momentum of the fuel stream should be held within a predetermined range to assure proper radial penetration of the fuel into the oxidizer stream. This constraint also limits the practical useful diameter and therefore the numbers of orifices. A sharp rise in NO, emissions as shown in FIG. 4 would occur when the orifice diameter becomes sufficiently small as to unduly limit fuel stream momentum.

The ratio of total number of fuel orifices times their diameters to the fuel distribution tube circumference is commonly called the fuel blockage, 8,. This ratio can be expressed as:

where:

B fuel blockage ratio N number of radial fuel orifices d,= fuel orifice diameter (inches) D pintle diameter For optimum performance with natural gas, the blockage ratio should be maintained at about 25% to about 40% of the oxydizer injection flow area, as will be evident from FIGS. 5 and 6. Applying the above equations to the specific application will provide the pintle and fuel orifice diameter.

Another design variable which has a considerable effect on both NO emissions and burner heat flux profile is the air blockage ratio. This ratio is defined as the ratio of the combustion air injection area (annular path 33) to the cross-sectional area of the deflector 31.

Experimental investigations have been conducted to evaluate the effect of air blockage ratio on the NO emissions which is shown in FIG. 5 for a gas burner with an input rating of l.7Xl0 BTU/hr. This figure illustrates that as the air blockage ratio (deflector diameter) is increased above about 0.45 the NO, emissions begin to increase significantly. This is primarily due to the increase in combustion intensity which results in higher peak gas temperatures due to the flatter flame. This flame has poorer radiant heat transfer characteristics because of its smaller surface area and its closer proximity to the insulated fire wall typical of most applications.

Decreasing the blockage ratio past on optimum causes increases in post combustion generated NO because of greater residence times above a cited temperature of about 2,2000 F.

A similar result occurs with an oil firing burner of [00 gph (gallons/hour). As shown by FIG. 6 emissions rise sharply for ratios above about 0.4. The data suggests that the effective blockage area of the deflector 31 should be about 20 to 40 or even 55 percent of the oxidizer injection flow area,

The combustion air requirements are based upon the desired fuel input rating and the design excess air. The air required for combustion varies with the type of fuel; however. a representative value for natural gas is SCF air/SCF gas (standard cubic foot). The actual combustion requires some excess air, therefore. the burner should be sized for about it] 30 percent excess air.

The annular combustion air sheet must be suffi ciently thick to preclude penetration by the radial fuel jet prior to the final mixing by the deflector.

The penetration distance of the radial gas (natural gas) et into the air stream can be estimated from the following empirical equation I: vi 1),

l depth of penetration, inches L orifice coefficient. dimensionless D I jet orifice diameter, inches p; 1 fuel jet density. lb/ft p,, air density, lb/ft" V fuel jet velocity, ft/sec l',, 3 air jet velocity, t t/sec 5 axial distance from the plan of the fuel injection to the beginning of the flame zone, inches in empirical exponent, f(orifice geometry) 0.95 for square orifices I 0.65 for circular orifices The combustion air sheet thickness must be greater than the estimated penetration distance in order to ob tain complete combustion within the desired flame envelope. The effect of the natural gas fuel jet penetration on the NO emissions is shown in FIG. 7 for a natural gas burner rated at 2X [0" BTU/hr. The penetration distance in this figure has been normalized by dividing by the air sheet thickness A. it is evident from the data that as the fuel jet penetration approaches the air sheet thickness, the NO, emissions increase.

When designing an oil fired burner, it is obvious that the above equation will not account for the injection characteristics of a liquid. The penetration distance of the radial oil jet into the air stream can be estimated from the following empirical equation:

where lustrated in HG. 9 for a 2X10" BTU/hr burner firing natural gas. These data show that for lower values of momentum ratio (ie, l0) the fuel momentum is too great and the NO, emissions increase significantly. The CO emission level increases greatly in this region also. The data indicate the burner should be designed to operate at momentum ratios ofapproximatcly 15:1. Operation at high momentum ratio results in low NO, and CO emissions and the flame appearance is better with no soot buildup.

The effect of air to fuel oil jet momentum ratio on NO emission performance is shown in FIG. [0 for a gph burner firing residual oil (6). The data indicate that lower values of NO; emissions can be realized at both low and high conditions of momentum ratio. On the low side, while the NO is low, the CO is generally high and coking occurs and deposits build up on the furnace wall. This is due to the radial fueljets penetrating through the annular air stream. Operation at high values of momentum ratio (25:1) results in low NO, and CO emissions, the flame appearance is better and no deposit buildup is evidenced. Generally momentum ratios of about 10 to about 20 are suitable for most con ditions.

Yet another parameter affecting the local mixture ratio conditions and the NO, emissions in the case of liquid fuels is the mixing gas flowrate. The mixing gas serves as a means for atomizing the fuel by apparatus which provides for centrifugal action heretofore described and shown in FIG. I, or by the atomizing appa ratus of FIG. 11. or some other such device.

it is generally agreed that greater atomization is desirable because smaller liquid fuel droplets are subjected to greater exposure to oxygen and therefore to greater oxidation and that large droplets do not gasify readily. Yet, there is a limit to the degree of atomization beyond which there is an increase in undesirable emissions. lt is believed that the resultant smaller drop lets lack sufficient momentum to provide adequate penetration of the oxydizer shroud thereby causing a poor distribution of the fuel i.e., there will be local pockets of either excess fuel or oxygen. in the test program, it was found that an atomizing differential pres sure of about 2 to 4 psi for light and heavy oils provided optimum penetration and fuel droplet size.

The atomizing gas may be steam in which case the hardware is simplified by eliminating the need for an air compressor in the atomizing system.

A alternate embodiment of the fuel distribution tube 28 utilized for testing in a YorlcShipply Scotch Marine boiler rated at 50 horsepower (2,100,000 BTU/hr) is shown in FIG. ll. The distribution tube 28, in a configuration suitable for using fueLoil. includes a mating flange 52 for use with the York shippley boiler. A pintle portion 53 projects into the boiler combustion chamber. The pintle base portion 54 extends through the boiler air plennum which distributes air along the pintle toward the combustion chamber substantially as in FIG. 2 except that the deflector disc 31 is not shown. Fuel oil is introduced into a fuel tube 55 while a mixing or atomizing gas is introduced through a gas inlet 56 and directed along the annular mixing gas tube 57 formed between the fuel tube 55 and the wall of the pintle portion 53. Sets of fuel orifices 32 and 320 are matched-drilled through the walls of the pintle S3 and fuel tube 55.

This concentric type of fuel distribution tube pro vides an alternative liquid fuel atomizing means to that of the centrifugal type means of FIG. 2. Fuel and atomizing gas. each under pressure. project fuel droplets mately stoichiometric proportion to achieve a pre determined heat input rating;

providing a preselected number of radially aligned fuel orifices of predetermined diameter and total into the oxydizer shroud flowing past the pintle portion. area ll di ib d about a circumference f Similar testing with the York-Shippley boiler was carthe fuel distribution tube, the total area of said oriried out with a natural gas version distribution tube fices being selected in accordance with the expreswherein the fuel tube was omitted. sion;

The following table sets forth representative data Q A: pertinent to a natural gas fired burner and an oil fired GAMP burner each having substantially the same rating: where:

Oll. GAS \Sxm BTU/hr l7xlt)" BTU/hr Primary air pressure. inches we. l |.(l it) Primary air temperature. F 7t) 70 Primary air flow. standard cubic foot per 209M 3500 minute Oil pressure. psi 30 Oil temperature. F 2110 Oil flow rate. gal/hr lllt) Excess air. percent 4t] Gas pressure. inches we. Gas temperature. F 70 Gas flu. SCFM 285 Design Distribution tube diameter. inches 3.ll (Lil (ombustion air annulus. inches 31] 2.65 Deflector diameter. inches 4-l/2 8.0

When the various parameters are controlled, the Q flow rate (60- t per t burner emission characteristics will remain favorable A ri a (sq. ihChEiS) over a rather large range of heat output ratings as indi- C a t nclu ing the discharge Coefficient and cated by the trends depicted by FIG. 12. a unit conversion factor Additional temperature dissipation may be provided P pre differential HCI'OSS Said l' (P by introducing cool gas to the interior of the conical p specific flighl 0f fluid (lbs/ft) and the number flame. For this purpose, a cooling gas tube 44, FIG. 2, oforifices being determined in accordance with the is mounted coaxially within distribution tube 28. Any 35 CXPI'eSSiOHI convenient means may be used to provide cooling gas thereto. Cooling gas tube 44 extends through deflector r "Du disc 31 so as to inject cooling gas into the interior of the whet flame 45. Another deflector 46 may be suspended from 0 2 B 0 25 the deflector disc 3! so as to direct the cooling gas radi B,= ratio of total number of fuel orifice times their allv outwardlv. and along the flame front to carry away diameter to fuel distribuuon tube circumference heat" N number of radial fuel orifices Other features of the burner assembly are shown In d; fuel orifice diameter (inches) FIG. 13. The distnbution tube 28 IS coupled through D fuel distribution tube diameter appropriate linkage to a translator device 47 whlch in "hides means [on l v men h providing a deflector disc affixed to the combustion mpa mg m0 6 o e zone end of the fuel distribution tube and having a tribution tube 28. The combustion end. or one end 30,

f th t f t b l d t d 48 diameter selected to provide an effectlve crossz n u l u g :ji sectional area of the disc which is 20 to 55 percent 0 t e a 3 5. g li g i 1 50 of the oxidizer annulus cross-sectional area;

2 g f 2: en 5 u u e controlling the momentum of the fuel and oxidizer is rawn owar e on lzer on e reactant Streams and FIG. 13 is split along centerline 51 to Sh e districontrolling the fuel exiting the fuel orifices to provide bution tube 28 in two of its infinitely selectable posia penetration depth, whereby to minimize the protions. Axial translation results in a change in the oxi- 55 duction of nitrogen oxides. dizer annulus area. this causing the flow rate to change 2. The method of controlling combustion of claim when the upstream supply pressure is h n a In wherein the step of controlling the momentum of the hi m n rn n p r i n m y be pli h fuel and oxidizer reactants further includes the step of: y area throttling Whilfl retaining the gn Point regulating the mass flow rates of the fuel and oxidizer ing characteristics. streams to provide an oxidizer to fuel momentum What is claimed is:

l. A method of controlling the combustion of reactants in a forced draft burner of the type having a combustion zone. an oxidizer tube in communication with the combustion zone, a fuel distribution tube coaxially mounted with the oxidizer tube and extending to within the combustion zone. comprising the steps of:

selecting an oxidizer and fuel flow rate in approxi ratio between about l0 to one and about 20 to one.

3. The method of controlling combustion of reactants claim I wherein the step of controlling the fuel penetration depth in the case of gaseous fuels is detennined in accordance with the expression:

where:

l depth of penetration. inches c orifice coefficient, dimensionless D jet orifice diameter. inches pf 2 fuel jet density, lb/ft pa air density, lb/ft I} fuel jet velocity. ftfsec l',, air jet velocity. ft/sec S axial distance from the point of the fuel injection to the beginning of the flame zone inches m I empirical exponent. ftorifice geometry) (195 for square orifices I tl.65 for circular orifices 4. The method of controlling combustion of claim I wherein the step of controlling the fuel penetration depth in the case of liquid fuels is determined in accordance with the expression:

where fuel droplet penetration distance, inches D fuel oil orifice diameter, inches Re Reynolds number D p l p We Weber number D p V la where.

p fuel jet density, lb/ft" l", I fuel jet velocity. ft/sec viscosity of the fuel p, air density, lb/ft l,, I air jet velocity, ft/sec 0, surface tension of the fuel 5. A forceddraft burner of the type used in packaged-hoiler systems having a combustion Zone, means for supplying an oxidizer under pressure, and means for supplying a fuel under pressure, comprising:

an oxidizer tube in communication with the combustion zone;

oxidizer regulating means operably coupled to said oxidizer tube for reguiating the oxidizer flow,

a fuel distribution tube coaxially mounted within said oxidizer tube. said tubes forming an annular oxidizer flow path, said fuel distribution tube having a plurality of radially aligned fuel orifices formed therein and distributed equidistant on a circumference of said fuel distribution tube proximate the combustion zone end thereof, the total orifice area being determined in accordance with the expression:

where:

Q fuel flow rate (cuv ft. per minute) A orifice area (sqv inches} C 2 constant including the discharge coefficient and a unit conversion factor AP I pressure differential across said fuel orifices (psi) p specific weight of fluid (lbs/ft) and the number of orifices being determined in accordance with the Ill expression:

M1, B rrD where:

t).5 B, 0.25 B, ratio of total number of fuel orifice times their diameters to fuel distribution tube circumference 30 N number of radial fuel orifices 41, fuel orifice diameter (inches) D I fuel distribution tube diameter; and a deflector disc affixed to the combustion end of said fuel distribution tube the diameter of which is selected 15 to provide an effective cross-sectional area which is to 55 percent of the oxidizer annulus cross-sectional area.

6. The method of controlling combustion chemistry and flame shape of claim 1 wherein the penetration 3U depth is controlled within about 40% to about 60% of the oxidizer annulus height.

7. The method of controlling combustion chemistry of reactants of claim 13 further comprising the steps of:

supplying a liquid fuel to the distribution tube; providing means for atomizing the liquid fuel which utilizes an atomizing gas; supplying an atomizing gas to the atomizing means and in operable association with the liquid fuel; and regulating the mass flow rate of the atomizing gas to provide a rate of flow which tends to minimize the production of nitrogen oxides. 8. The method of controlling flame shape of claim 1 further comprising the steps:

providing a preselected deflector disc diameter resulting in a flame shape filling the cross-section of the burner without substantially generating wall deposits; and controlling the momentum of the oxidizer gas stream to selectively shape the resultant flame and the heat transfer characteristics of the system to minimize production of nitrogen oxides.

Claims

1. A method of controlling the combustion of reactants in a forced-draft burner of the type having a combustion zone, an oxidizer tube in communication with the combustion zone, a fuel distribution tube coaxially mounted with the oxidizer tube and extending to within the combustion zone, comprising the steps of: selecting an oxidizer and fuel flow rate in approximately stoichiometric proportion to achieve a predetermined heat input rating; providing a preselected number of radially aligned fuel orifices of predetermined diameter and total area equally distributed about a circumference of the fuel distribution tube, the total area of said orifices being selected in accordance with the expression;

2. The method of controlling combustion of claim wherein the step of controlling the momentum of the fuel and oxidizer reactants further includes the step of: regulating the mass flow rates of the fuel and oxidizer streams to provide an oxidizer to fuel momentum ratio between about 10 to one and about 20 to one.

3. The method of controlling combustion of reactants claim 1 wherein the step of controlling the fuel penetration depth in the case of gaseous fuels is determined in accordance with the expression:

4. The method of controlling combustion of claim 1 wherein the step of controlling the fuel penetration depth in the case of liquid fuels is determined in accordance with the expression:

5. A forced-draft burner of the type used in packaged-boiler systems having a combustion zone, means for supplying an oxidizer under pressure, and means for supplying a fuel under pressure, comprising: an oxidizer tube in communication with the combustion zone; oxidizer regulating means operably coupled to said oxidizer tube for regulating the oxidizer flow; a fuel distribution tube coaxially mounted within said oxidizer tube, said tubes forming an annular oxidizer flow path, said fuel distribution tube having a plurality of radially aligned fuel orifices formed therein and distributed equidistant on a circumference of said fuel distribution tube proximate the combustion zone end thereof, the total orifice area being determined in accordance with the expression:

6. The method of controlling combustion chemistry and flame shape of claim 1 wherein the penetration depth is controlled within about 40% to about 60% of the oxidizer annulus height.

7. The method of controlling combustion chemistry of reactants of claim 13 further comprising the steps of: supplying a liquid fuel to the distribution tube; providing means for atomizing the liquid fuel which utilizes an atomizing gas; supplying an atomizing gas to the atomizing means and in operable association with the liquid fuel; and regulating the mass flow rate of the atomizing gas to provide a rate of flow which tends to minimize the production of nitrogen oxides.

8. The method of controlling flame shape of claim 1 further comprising the steps: providing a preselected deflector disc diameter resulting in a flame shape filling the cross-section of the burner without substantially generating wall deposits; and controlling the momentum of the oxidizer gas stream to selectively shape the resultant flame and the heat transfer characteristics of the system to minimize production of nitrogen oxides.