US20030157451A1

US20030157451A1 - Low NOx particulate fuel burner

Info

Publication number: US20030157451A1
Application number: US10/319,796
Authority: US
Inventors: Michael McCabe
Original assignee: Mccabe Michael I.
Current assignee: Cemex Inc
Priority date: 2001-12-13
Filing date: 2002-12-13
Publication date: 2003-08-21
Also published as: AU2002346728A1; WO2003050449A1

Abstract

A pulverized fuel burner having reduced NO_xemissions comprises a first precessing jet nozzle surrounded by a second nozzle. The second nozzle feeds pulverized fuel into a precessing jet of air formed by the first nozzle.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the U.S. Provisional Application Serial No. 60/341,396 filed Dec. 13, 2001.[0001]

FIELD OF THE INVENTION

The present invention relates generally to particulate fuel burners and, more particularly to a low NO _xparticulate fuel burner having a precessing jet of air and one or more turbulent diffusion jets.

BACKGROUND OF THE INVENTION

Nitrogen oxides (NO _x) are major atmospheric pollutants and are a precursor to acid rain, photochemical smog, and ozone accumulation. The oxides are mainly nitric oxide (NO) and nitrogen dioxide (NO₂), both of which are corrosive and hazardous to human health. Stationary sources of NO_xemissions such as industrial manufacturers (metallurgical processors, glass manufacturers, cement kilns, power generators, etc.) are now being subjected to more stringent standards in many areas of the U.S. Because of the environmental concerns posed by air pollution, a great deal of research time and money is being expended to develop methods for controlling and/or reducing NO_xemissions.

In 1998, the United Stated EPA published the “Federal Implementation Plans to reduce the Regional Transport of Ozone.” In this document, low-NO _xburners are identified as an approved control technique for reducing NO_xemissions from stationary sources such as cement kilns. The science and technology behind designing a low NO_xburner has been based on turbulent diffusion jet (TDJ) theory, which was developed as far back as the 1930's. However, a fundamental challenge for lowering NO_xemissions TDJ technology is that the TDJ theory was developed without regard to NO_xformation. The TDJ theory was developed with the intent to maximize flame intensity and combustion efficiency, which in turn tends to maximize the formation of NO_xemissions. Thus, attempts to lower NO_xemissions with this type of burner (TDJ) usually results in poorer combustion and a lower temperature flame. Installing a low-NO_xturbulent diffusion jet burner may result in a less efficient and more costly manufacturing process.

In the mid-1980's, the University of Adelaide published information about a new type of burner that reportedly reduced NO _xemissions from natural gas flames while improving combustion efficiency. The burner used a nozzle that was based on a new theory known as a “precessing jet” (PJ), instead of the TDJ theory. The precessing-jet (PJ) nozzle is mechanically very simple, but the resulting behavior of the fluid flow is extremely complex. In its simplest form, the nozzle is a cylinder with a small concentric inlet orifice at one end, and an axisymmetric lip at the other end. When the dimensions of the PJ nozzle fall within criteria defined in the 1988 patent, the jet flow through the inlet orifice is purportedly subjected to an unstable lateral deflection and attaches asymmetrically to the internal wall of the cylinder. The exit lip at the nozzle exit then deflects the emerging eccentric jet so that it wobbles or precesses at a large angle to the nozzle axis. If a large centerbody is placed near the exit end of the cylinder, the precession is less irregular and more rotary in nature.

SUMMARY OF THE INVENTION

A pulverized fuel burner having reduced NO _xemissions is disclosed. The pulverized fuel burner includes a first nozzle and a second nozzle. The first nozzle creates a precessing jet of air adjacent the nozzle exit. The second nozzle conveys the pulverized fuel into the precessing jet of air for combustion. In one embodiment, the first and second nozzles are concentric. The first nozzle may be the inner nozzle, and the second nozzle may be the outer nozzle. The pulverized fuel may be coal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, a preferred embodiment, and other aspects of the present invention will be best understood with reference to a detailed description of specific embodiments of the invention, which follows, when read in conjunction with the accompanying drawings, in which [0007]
FIGS. [0008] 1A-I illustrate embodiments of precessing jet nozzles.
FIGS. [0009] 2A-I illustrate applications of the precessing jets, where the mixing of two flows is required.
FIG. 3 is a sectional view of the Low NO[0010] _xFuel Burner according to the present invention.
FIGS. 4 and 5 illustrate other implementations of the present invention that comprise multiple turbulent diffusion jet (TDJ) nozzles and a single precessing jet (PJ) nozzle.[0011]
While the invention is susceptible to various modifications and alternative forms, specific embodiments of the present invention have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims. [0012]

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The disclosures of U.S. Pat. No. 5,060,867, entitled “Controlling the Motion of a Fluid Jet” and U.S. Pat. No. 5,769,624, entitled “Variable Flame Burner Configuration” are incorporated herein by reference for all purposes. Particular attention is directed to the figures and preferred embodiments of the precessing jet nozzles disclosed therein. [0013]
Referring to FIGS. [0014] 1A-E, embodiments of precessing jet or mixing nozzles disclosed in U.S. Pat. No. 5,060,867 for use with a low NO_xparticulate burner of the present invention are illustrated. The nozzle comprises a conduit 15 containing a chamber 16. The chamber 16 is defined by the inner cylindrical face of the conduit 15, by orthogonal end walls defining an inlet plane 12, and an exit plane 13. The inlet plane 12 contains an inlet orifice 11 of diameter d₁, the periphery of which thereby serves as means to separate a flow through the inlet orifice 11 from the walls of the chamber 16. The exit plane 13 essentially comprises a narrow rim or lip 13 a defining an outlet orifice 14 of diameter d₂somewhat greater than d₁. The rim or lip 13 a may be tapered as shown at its inner margin, as may be the periphery of the inlet orifice 11. Fluid is delivered to the orifice 11 via a supply pipe 10 of diameter d₀.
All five embodiments illustrated in FIGS. [0015] 1A-E consist of a substantially tubular chamber 16 of length L and diameter D, wherein diameter D is greater than the inlet flow section diameter d₁. The chamber 16 need not be of constant diameter D along its length in the direction of the flow. Preferably, a discontinuity or other relatively rapid change of cross-section occurs at the inlet plane 12 such that the inlet throat diameter is d₁. The relationship between the diameter d₀of the upstream conduit 10 and the inlet diameter d₁is arbitrary, but d₀is greater than or equal to d₁.
Typical ratios of dimensions L to D lie in the range 2.0≦L/D≦5.0. A ratio of L/D≅2.7 has been found to give particularly good enhancement of the mixing. Typical ratios of dimensions d[0016] ₁to D lie in the range 0.15≦d₁/D≦0.3. Typical ratios of dimensions d₂to D lie in the range 0.75≦d₂/D≦0.95.
These ratios are typical for the embodiments illustrated in FIGS. [0017] 1A-E but are not exclusive and are not necessarily those applicable for all embodiments. It should be noted that the range of geometric ratios for which mixing enhancement is consistently stable is increased substantially by means of the embodiment illustrated in FIG. 1E.
In FIG. 1E, a [0018] body 17 is suitably suspended in the flow for the aforementioned purpose of preventing intermittency, i.e. reversals of the direction of precession. The body 17 may be solid or it may be hollow. It may also be vented from its inside surface to its outside surface. The body 17 may have any upstream and downstream shape found to be convenient and effective for a given application. For instance, it may be bullet shaped or spherical. It may further provide the injection point for liquid or particulate fuels. The length of the body (x₂−x₁) is arbitrary but is usually less than half the length L of the cavity 16 when the body 17 is hollow; and is typically less than D/4 when the body 17 is solid. It is typically placed within the cavity 16 as illustrated in FIG. 1E, in which case both x₂<L and x₁<L; it may also be placed spanning the exit plane 13, in which case x₂>L and x₁<L; or it may be wholly outside the exit plane 13 of the nozzle, in which case x₂>L and x₁>L. The outside diameter d₃of the body is less than the cavity diameter D, and the inside diameter d₄may take any value from zero (solid body) up to a limit which approaches d₃. The body 17 is typically placed symmetrically relative to the conduit 15, but it may be placed asymmetrically.
The embodiments of FIGS. 1F, 1G, and [0019] 1H differ in that the chamber 16 diverges gradually from the inlet orifice 11. In this case, the angle of divergence and/or the rate of increase of the angle of divergence must be sufficient to cause full or partial separation of flow admitted through and fully occupying the inlet orifice 11 for precession of the jet to occur.
Referring to FIGS. [0020] 2A-E, typical geometries for the mixing of two fluid streams are illustrated as disclosed in U.S. Pat. No. 5,060,867 to Luxton et al. for use with a low NOx particulate fuel burner of the present invention. In FIGS. 2A-E, in which the same reference numbers used in FIGS. 1A-H indicate substantially similar elements, one inner flow and another, outer flow are designated by FLOW 1 or FLOW 2 respectively. In contrast to the present invention, the Luxton et al. patent contemplates that either FLOW 1 or FLOW 2 may represent e.g. a fuel and that either or both FLOW 1 and/or FLOW 2 may contain particulate material or droplets. In the case of FIG. 2A, FLOW 1 may be introduced in such a manner as to induce a swirl, the direction of which is preferably, but not necessarily, opposed to that of the jet precession. Alternatively, FLOW 1 may be unswirled. The relationship between diameters D and d may take any physically possible value consistent with the achievement of the required mixture ratio between the streams. The expansion 18 is a quarl the shape and angle of which may be chosen appropriately for each application.
FIG. 2B depicts a variation of FIG. 2A in which a [0021] chamber 20 has been formed by the addition of a combustion tile 19 through which the burning mixture of fuel and oxidant is contracted from the quarl diameter d_Qto form a burning jet from an exit 21 of diameter d_Eor from an exit slot 21 of height d_Eand whatever width may be convenient. In this configuration, by suitable choice of the shape and expansion angle of the quarl 18 relative to the swirl of FLOW 1 and the precession rate of FLOW 2, a vortex burst may be caused to produce fine-scale mixing between the fluids forming FLOW 1 and FLOW 2, in addition to the large-scale mixing which is generated by the precession of the jet.
A nozzle according to the present invention is preferably constructed of metal. Other materials can be used, either being molded, cast, or fabricated, and the nozzle could be made, for example, of a suitable ceramic material. Where a combustion tile is employed, both the tile and the quarl should ideally be made of a ceramic or other heat resisting material. For non-combustion applications in which temperature is relatively low, plastic, glass, or organic materials such as timber may be used to construct the nozzle. [0022]
The nozzles of the present invention are preferably circular in cross-section, but may be of other shapes such as square, hexagonal, octagonal, elliptical, or the like. If the cross-section of the cavity has sharp corners or edges some advantage may be gained by rounding them. As described hereinbefore, there may be one or more fluid streams, and any fluid stream may carry particulate matter. The flow speed through the [0023] inlet orifice 10 of diameter d₁may be subsonic or, if a sufficient pressure ratio exists across the nozzle, may be sonic. That is, it may achieve a speed equal to the speed of sound in the particular fluid forming the flow through orifice 11. Other than in exceptional circumstances in which the supply pipe 10 is heated sufficiently to cause the flow to become supersonic, the maximum speed through orifice 11 will be the speed of sound in the fluid. In most combustion applications, the speed is likely to be sub-sonic. In some applications, it may be appropriate to follow the throat section d₁with a profiled section designed to produce supersonic flow into the chamber 16.
Referring to FIG. 2F, an embodiment of a burner configuration as disclosed in U.S. Pat. No. 5,769,624 for use with a low NOx particulate burner of the present invention is illustrated. The burner configuration includes a pair of generally [0024] tubular nozzles 30, 60 arranged side-by-side with their longitudinal axes parallel. In contrast to the present invention, it should be noted that the nozzles 30, 60 are both supplied with fuel, typically natural gas, by respective feed pipes 32, 62, from a common delivery pipe 38 via respective control valves 36, 66.
[0025] Nozzle 30 is a precessing jet nozzle, and nozzle 60 a simple turbulent jet nozzle. An example of a suitable precessing jet nozzle 30 is depicted in FIG. 1I, and includes an axisymmetric chamber 40 with a simple 42 or profiled 42′ inlet aperture defining a large sudden expansion at the chamber's inlet end, and a small peripheral lip 44 defining an exit port 46.
The complex behavior of the fluid in the [0026] precessing jet nozzle 30 of FIG. 1I will be briefly discussed. Unlike the present invention where the precessing jet is of air, the nozzle 30 uses a fuel jet 48, which enters the chamber 40 at the aperture 42 or 42′ and is there separated from the chamber wall. The jet then reattaches asymmetrically at 50 to the inside of the wall and at the nozzle exit is deflected (52) at a large angle (e.g. 45-degrees) from the nozzle axis by strong local pressure gradients. There are also strong azimuthal pressure gradients, which cause the jet, and the entire flow field within the chamber, to precess about the nozzle axis. These pressure gradients and fields induce air 54 through the outlet 46, and this air swirls in the chamber at 55 between the flow separation and the reattachment and in part induces the precession of the separated/reattached flow. This precession enhances mixing of the fuel flow with the air from the exterior of the chamber 40.
Referring to FIGS. [0027] 2G-I, embodiments of burner configurations are illustrated as disclosed in U.S. Pat. No. 5,769,624 to Luxton et al. for use with a low NOx particulate fuel burner of the present invention. The arrangement shown in FIG. 2G comprises a concentric pipe burner configuration 70, consisting of a precessing jet nozzle 72 mounted substantially concentrically within an outer pipe 75 defining a co-annular burner pipe. The co-annular pipe 75 may or may not have a flow-directing nozzle in the end and may or may not be used to cool the inner nozzle/burner 72. In the case where a flow-directing nozzle 85 is used to swirl the co-annular flow, a co-annular swirl burner 80 is produced: this is depicted in FIG. 2H, in which the swirl flow is indicated by arrow lines 86. FIG. 2I is an end view of a multi-pipe burner configuration 90, consisting of a ring of four equiangularly spaced precessing jet nozzles/burners 92 arranged around one or more turbulent jet nozzles/burners 96. Jet nozzles/burners 92 are supported by radial spacer elements 94. The converse—a ring of turbulent jet nozzles/burners around one or more precessing jet nozzles/burners—is of course also an option. It should be noted that, in contrast to the present invention, the burner configurations in U.S. Pat. No. 5,769,624, like those discussed above in FIGS. 2A-B, are also disclosed as having fuel in all of the multiple nozzles of the burner.
To date, the precessing jet nozzles as described in the above incorporated patents have been successfully applied to several natural gas fired processes, including cement kilns. However, the vast majority of all cement kilns in the United States are heated with a particulate fuel, such as pulverized coal, not a gaseous fuel, such as natural gas. The present invention combines turbulent diffusion jet theory and precessing jet theory to achieve a particulate fuel burner having reduced NO[0028] _xemissions.
One implementation of the present invention comprises a [0029] burner 100 having two concentric nozzles 120 and 130. As shown in FIG. 3, the inner nozzle 120 is a precessing jet-type (PJ) nozzle and is used to create a precessing jet of air 122 at or adjacent the exit 102 of the burner 100. The outer nozzle 130 is a turbulent diffusion jet (TDJ) nozzle. A pulverized fuel 140, such as coal, is carried down the annulus of the burner 100 by a mass of conveying air 150. Upon exiting the burner 100, the pulverized fuel 140 and the precessing jet of air 122 interact to create a flame (not shown) of lower NO_xemissions than conventional pulverized fuel burners. Additionally, the amount of air 150 used for conveying the fuel 140 (in contrast to draft air) is minimized to a percentage typically less than 3% of the total air required for combustion.
The [0030] TDJ nozzle 130 of FIG. 3 is designed to produce an exit velocity V_TDJbetween 10-80 meter/sec. At an exit velocity below 10 meter/sec., the TDJ nozzle 130 may not have sufficient velocity to adequately entrain the pulverized fuel particles 140 in the conveying air 150. Thus, the fuel particles 140 may undesirably drop out of the air stream 150. At an exit air velocity greater than 80 meter/sec., the particulate fuel jet 132 may have too much velocity for adequate entrainment of the fuel particles 140 by the precessing jet 122.
The quantity of conveying [0031] air 150 is dependent on the type of solid fuel grinding and handling system. The two basic types of solid fuel grinding and handling systems are applicable to this invention: indirect and direct. Indirect systems consist of all systems in which the solid fuel is stored in a holding bin after being pulverized in a grinding mill and then metered to the burner. The fuel is metered and conveyed by an air blower (not shown) to the burner. The amount of air used by the indirect system for conveying the pulverized feed is typically between 1-10% of the stoichiometric air requirement, and the TDJ nozzle 130 is sized to produce an exit velocity V_ABbetween 10-40 meter/sec. For example, if 10 lbs. per minute of pulverized coal is being conveyed to a burner, and the stoichiometric air requirement to combust 1 lb. of coal is 10 lbs. of air, then the total stoichiometric air requirement is 100 lbs. per minute. Thus, in this example 10% of the stoichiometric air requirement is equal to 40 lbs./min.
In contrast, direct systems consist of all systems in which the solid fuel is pulverized in a grinding mill, and conveyed directly to the burner. A derivative of the direct system, called semi-direct, includes a cyclone separator between the grinding mill and the burner. The fuel and air are separated by the cyclone. The air is then passed through a fan and sent to the burner and the pulverized fuel is passed through a pump and sent to the burner. The amount of air for a direct or semi-direct system is typically between 10 to 40% of the stoichiometric air requirement. If the solid fuel firing system is either direct or semi-direct, then the [0032] TDJ nozzle 130 is sized to produce an exit velocity V_TDJbetween 40-80 m/s. Typical examples of solid fuel are coal and petroleum coke, although the present invention is not limited to these fuels.
Proper operation of the [0033] precessing jet nozzle 120 can be achieved with an air pressure between 30-75 psig. The amount of air required to properly operate the precessing jet nozzle 120 falls within a range between 1-10% of the stoichiometric air required to combust the pulverized fuel 140 that is being conveyed by the burner 100. The burner 100 features a control valve (not shown) used to regulate the amount of air sent to the precessing jet nozzle 120.
By combining the [0034] precessing jet 122 of air and turbulent diffusion jet 132 of pulverized fuel 140 & air 150, the initial entrainment of the solid fuel particles 140 and surrounding air increases, and generates very large turbulent structures. Regions of fuel-rich fluid are exposed to high temperatures for relatively long periods of time. This provides pyrolitic reaction pathways that promote the formation of soot within the flame, thus increasing flame emissivity and radiant heat transfer. Heat loss by radiation reduces the characteristic flame temperature so that open precessing jet flames have lower NO_xemissions than simple jet flames. The structures of precessing-jet turbulence promote a clustering of the pulverized-fuel particles, which is believed to be the primary cause for reduction in NO_xemission. Thus, the resulting combination of the precessing jet 122 of air and turbulent diffusion jet 132 of pulverized fuel 140 & air 150 is a low-NO_xflame suitable for efficient use in a cement kiln. The application of this burner design is intended but not limited for use in cement plants.
The present invention may be implemented in other manners as well. FIGS. 4 and 5 show other [0035] concentric nozzles 160, which can be located either outside the primary fuel TDJ nozzle 130 as in FIG. 4 or can be located inside the primary fuel TDJ nozzle 130 as in FIG. 5. This other, concentric nozzle 160 can be used for transporting alternative fuels 170 and/or air 180 to the furnace. Alternative fuels 170 may consist of oil, wood chips, sludge, tire chips, and/or bio-waste. Air alone may be used in an additional TDJ nozzle for flame shaping, i.e., increasing or decreasing the length of the flame.
Thus, the present invention contemplates combining a [0036] precessing jet nozzle 120 with a turbulent jet nozzle 130 such that the pulverized fuel 140 delivered by the TDJ nozzle 130 will become adequately entrained in the precessing jet 122 of air established by the PJ nozzle 120. It will be appreciated from this disclosure that the energy of the particulate fuel 140 as it exits the burner (e.g., as represented by exit velocity or momentum) must be sufficient to propel the fuel 140 into the precessing jet 122 of air while not “blasting” the fuel through the precessing jet 122. Similarly, the energy of the precessing jet 122 of air must be sufficient to entrain or catch the pulverized fuel 140 as it exits the burner.
The present invention is susceptible to implementation in a variety of ways without departing from the spirit disclosed herein. Depicting the invention in terms of the selected implementations discussed herein is not intended to limit the breadth of the invention, but merely to illustrate the invention. [0037]

Claims

What is claimed is:

1. A pulverized fuel burner, comprising:

a first nozzle that creates a precessing jet of air adjacent the nozzle exit; and

a second nozzle that conveys the pulverized fuel into the precessing jet of air for combustion.

2. The burner of claim 1, wherein the first and second nozzles are concentric.

3. The burner of claim 2, wherein the first nozzle is the inner nozzle and the second nozzle is the outer nozzle.

4. The burner of claim 1, wherein the pulverized fuel is coal.