Patent Application
20060040223
Mixing two gaseous streams is a very common process in a variety of industrial applications. Examples include oxygen enrichment of a premixed fuel and air stream or oxygen enrichment of secondary, tertiary and over-fire air streams in industrial furnaces and boilers. Oxygen enrichment of gaseous fuel streams has been successfully practiced in a number of industrial processes, such as glass melting, lime and cement kilns, and steel manufacturing, among many others. Coal, on the other hand, is the most abundant fossil fuel currently available. Most of the power generated in the world uses coal as the fuel.
The enrichment of a particle-laden stream, such as a coal/air stream, with oxygen poses additional challenges. These challenges arise due to a large number of factors, as described below.
Firstly, due to safety concerns, high oxygen concentrations close to the walls of the burner primary duct or oxygen lance should be avoided.
Secondly, the coal particles usually have a non-uniform particle size distribution. Most utilities, with coal-fired power plants, use a size fraction that ranges from about 75-120 μm. The coal particle trajectories in the particle-laden stream deviate from the gas streamlines. The larger particles deviate more, whereas the smaller particles follow gas streamlines more closely. In addition, the bends in the line lead to a phenomenon known as “rope phenomena.” As a result, the particle loading is not uniform across the cross-section of the pipe.
Another challenge is the effect of oxygen injection on the particle trajectories. The injection may disturb these trajectories and could lead to a non-uniform loading of the particles in the stream. This could lead to pockets that are fuel lean. This, in turn, has a detrimental effect on the NO, emissions, which tend to increase under such conditions.
Non-uniform distribution of particles downstream the oxygen injection nozzle could lead to fuel lean pockets/zones at the burner primary air duct outlet. This, in turn, has a detrimental effect on the NO, emissions, which tend to increase under such conditions. This negative effect of non-uniform particles loading could be avoided if good oxidant/fuel mixing are achieved. In other words, the oxygen distribution should match the fuel distribution at the burner primary air duct outlet. That way, and in the first stages of coal combustion, oxygen and particles will be able to closely react during coal devolatilization (this reaction is required to decrease the NOx emissions).
Avoiding local increase of axial velocity at the burner outlet is a new challenge raised by oxygen injection. As increased residence times of both oxygen and fuel particles in the hot fuel rich zone are key parameters governing NOx emissions reduction, accelerating the flow at the burner outlet will lead to NOx emissions increase.
Currently, there are a number of injector designs that are used for mixing of two gaseous streams, including that disclosed in U.S. Pat. No. 5,356,213, the contents of which are incorporated herein by reference. This injector design, promoted by Air Liquide under the name Oxynator®, is designed to minimize mixing distance and to prevent high oxygen concentrations near the pipe walls. Oxygen flows radially from the center of the pipe and tangentially swirls in the air stream, improving mixing efficiency.
Other designs include those having a number of nozzle openings in order to efficiently mix two gaseous streams. Typical examples are disclosed in U.S. Pat. Nos. 5,664,733, 5,775,446, and 5,762,007.
For the coal-air flow, it is important to understand that the primary air is surrounded by a secondary air stream, which is injected in the furnace with a swirl to achieve mixing of the secondary air with the primary coal-air stream. The design of these burners is often relatively complex. New designs by Babcock and Wilcox now propose a transition zone between the primary and secondary air streams. These burners, however, must be retrofitted with some injection devices to enrich the primary coal-air stream with oxygen.
Particular pulverized coal burners are disclosed in U.S. Pat. No. 4,556,384. This patent shows various methods for injecting oxygen to enrich the primary coal-air stream. This burner is designed to operate in a stable manner with no risk of explosion with pure oxygen or air highly enriched with oxygen as the reactant. Streams of oxygen and fuel (pulverized coal) are delivered through concentric pipes (concentric tubes and annular rings).
Some other designs are those that are disclosed by U.S. Published Patent Application No. 2004-0074427 A1.
Additionally, four particular lance designs were proposed in U.S. Published Patent Application No. 2004-0185404 A1, which was published on Sep. 23, 2004, and contents of which are incorporated herein by reference. These recent designs, however, suffer from the drawbacks that the oxygen injection lead to a particle-depleted zone and does not allow for complete mixing of oxygen with the coal-air streams.
The invention provides a system for improving combustion of oxygen and a mixture of a non-gaseous fuel and conveying gas for improved combustion in a combustion chamber. The system includes: 1) a source of a mixture of non-gaseous fuel and conveying gas; 2) a source of oxygen; 3) a burner operatively associated with a combustion chamber; 4) a fuel duct in fluid communication with the source of mixed non-gaseous fuel and conveying gas; 5) a tubular oxygen lance fluidly communicating with the source of oxygen; and 6) at least first and second injection elements in fluid communication with the source of oxygen. The fuel duct includes a portion that extends along an axis towards the burner. The lance is disposed along the axis and has a diameter D. The at least first and second injection elements are configured to inject oxygen into, and mix therewith, a flow of the mixture upstream of, or at, the burner. At least one of the first and second injection elements receives oxygen from the lance. The first and second injection elements are spaced apart by a distance X, which is greater than the length of diameter D.
The invention also provides a method of improved combustion of oxygen and a mixture of a non-gaseous fuel and conveying gas for improved combustion in a combustion chamber. Utilizing the system as described above, a mixture of the non-gaseous fuel and the conveying gas is first allowed to flow into the fuel duct. Oxygen is then allowed to flow from the first and second injection devices, such that the oxygen and the mixture of non-gaseous fuel and conveying gas are mixed. The thus-mixed oxygen, non-gaseous fuel, and conveying gas is then combusted within the combustion chamber.
For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
The method and system according to the invention achieve improved mixing of oxygen injected into a stream of mixed non-gaseous fuel and a conveying gas. Improved mixing leads to improved combustion. The injection is accomplished using at least two injection elements that are spaced apart to allow greater mixing over a shorter distance.
The term, “oxygen”, is defined as a gas or gas mixture containing at least 90% by volume of oxygen.
While an oxygen lance extending down an axially extending fuel duct feeds oxygen to at least one of the injection elements, it may feed one or more injection elements. At least two of the injection elements may lie roughly in a same plane and axial position with respect to the axis of the fuel duct. Alternatively, at least two of the injection elements lie at different axial positions with respect to the fuel duct axis. In either case, this leads to enhanced mixing of the oxygen into the mixture of non-gaseous fuel and conveying gas.
The oxygen lance has a diameter D. In contrast to other oxygen injection configurations, at least two of the injection elements are spaced apart from one another by a distance greater than D. This greater distance avoids local high concentrations of oxygen because the oxygen fed into the stream of mixed non-gaseous fuel and conveying gas is spread out.
One benefit of the invention is realized when oxygen is injected at two different axial positions because it does not require the use of “oxygen clean” piping and components. Ordinarily, an oxygen concentration above 25% in a fuel stream requires the use of piping and components that are “oxygen clean”. This requirement may be avoided by injecting oxygen into the fuel stream with a first injection element in an amount such that the overall oxygen concentration remains below 25%. A remaining quantity of oxygen can then be injected with a second injection element axially spaced from the first injection element (downstream of the first injection element) to bring the overall concentration of oxygen to a concentration greater than 25%.
The invention also includes several new injection element designs. These allow effective mixing of oxygen and the two-phase flow of combined non-gaseous fuel and conveying gas over a relatively short distance while causing minimal disturbance to the two-phase flow. For solid fuels, the particle loading and the species concentrations at the exit of the injector are uniform.
The fuel used in the invention is present in either solid or liquid form. Suitable solid fuels include coal, pet coke, biomass, and the like. Suitable liquid fuels include hydrocarbons, such as resid oils. The preferred fuel is coal. The preferred application of this invention is in a coal-fired utility boiler in the power generation industry. Preferably, the conveying gas is air.
Due to safety concerns, the injection of oxygen in a premixed fuel-oxidant stream poses some challenges. In the case of coal-fired boilers, oxygen injection for enrichment of the primary air-coal stream is delayed until very close to the boiler inlet in order to lessen the chance that the fuel will ignite earlier than desired. However, this provides relatively less distance and time for the oxygen and the primary air-coal stream to adequately mix. This could potentially result in local pockets of fuel-lean mixture, and as a result, could increase NOx emissions.
The characteristics and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the preferred embodiments, which are discussed below with reference to the attached drawings.
As illustrated in
As an example, a utility boiler using coal as fuel is considered. A final oxygen concentration of more than 25% in the primary air-coal stream is intended. Ordinarily, an oxygen concentration above 25% in the primary air-coal stream requires the use of piping and components that are “oxygen clean”. This requirement may be avoided by injecting oxygen into the primary air-coal stream with injection element 3 in an amount at a distance L1 from the exit, such that the overall oxygen concentration remains below the 25%. The remaining quantity is injected with injection element 5 relatively close to the exit at a distance L2 from the exit. This brings the overall concentration of oxygen to the desired level greater than 25%. In this aspect of the invention, L1 is greater than L2. This double injection provides sufficient distance and time for the oxygen injected far upstream to mix well with the primary air-coal stream. The second injection allows additional mixing over the remaining distance and yields the desired oxygen level.
Another aspect of the invention includes relocating the coal diffuser, impellers, concentrators, and/or other bluff-body devices located in the primary air duct to a position downstream of the oxygen injection location. This configuration uses the turbulence introduced by these devices to further mix the oxygen with the non-gaseous fuel/conveying gas. Alternatively, the oxygen injection may be performed upstream the existing location(s) of one or more of these devices. Still another retrofit application includes addition of a second coal diffuser downstream of the oxygen injection.
The oxygen lance length has a significant effect on oxygen jet injection and behavior. Because the lance is cooled by the cold flow of oxygen to be injected into the fuel duct, it is possible to extend it after the actual position of oxygen injection.
Radially Injecting Injection Elements Designs:
One configuration of the invention includes locating at least one injection element at the end of the oxygen lance in the fuel duct. A second or more injection elements could be located in a portion of the oxygen lance upstream the end or could be located elsewhere.
As illustrated in
The length, D1, and width, D2, of these apertures, as well as the circumferential arc distance, D0, between two adjacent apertures may be varied to control the momentum ratio J (ratio of the oxygen jet momentum to the momentum of the stream of non-gaseous fuel/conveying gas). D1, D2, and D0 also control the penetration of the injection gas into the primary stream. A small D2/D1 ratio (streamlined rectangular apertures) will minimize the perturbation to solid fuel particles, such as coal. A big D2/D1 ratio (bluff-body slots) will have a greater influence on the solid phase and will push solid fuel particles, such as pulverized coal, away from the centerline of the burner primary air duct. Those two different aspect ratios will lead to different distribution of particles and oxygen at the duct outlet.
Those three parameters, S1, D1, and D2, in turn, control the penetration of the injection gas into the primary stream. A small D2/D1 ratio (streamlined slots) will minimize the perturbation to the solid phase. A big D2/D1 ratio (bluff-body slots) will have a greater influence on the solid phase and will push the coal particles away from the centerline of the burner primary air duct. Those two different aspect ratios will lead to different distribution of particles and oxygen at the duct outlet. As shown in
As depicted in
As best illustrated in
As shown in
As depicted in
Similar to the injection element designs 10, 20, 30, 40, the apertures 53 may be staggered and vary in size in the axial and azimuthal directions. The distance between apertures 53, the number of rows of apertures 53, or the surface area of apertures 53 could also be varied.
This injection element 50 has a particularly beneficial application to coal-fired boilers whose burner geometry include coal concentrators or splitters (identified technique in the prior art for reducing NOx emissions from pulverized coal burners). Varying levels of oxygen injection may be located to achieve higher concentration of oxygen in coal richer zones. As a result, the equivalence ratio between coal and oxygen can be controlled in the coal richer zone (concentrated zone) as well as in the coal leaner zones.
As shown in FIGS. 3A-D, the apertures 13A-13B can have various shapes such as circular, rectangular, triangular, elliptical, and still others. In addition, all the aperture geometries and arrangements described in
As depicted in FIGS. 3A-D, the apertures 13A-13D for each of the foregoing tubular designs can have various shapes such as circular, rectangular, triangular, elliptical, and still others.
Aerodynamic Injection Element Designs:
As depicted in
Referring to the injection element 100 of
Referring to the injection element 100 of
Referring to the injection element 110 of
As illustrated in
As shown in
Swirl-Type Injection Element Designs:
The designs presented in this section are based upon the patented Oxynator® (U.S. Pat. No. 5,356,213) concept. It is designed to minimize mixing distance and to prevent high oxygen concentrations near the pipe walls. Although there are/may be other applications of this device, it is generally used for oxygen enrichment of an air stream in a typical combustion application.
In operation, oxygen exits at a high velocity through a very narrow slit with a swirl in a radial direction, thereby improving mixing efficiency. In a particle or liquid droplet laden stream, the high flow velocity would ordinarily tend to undesirably push the particles outwards. In order to help diminish this problem, the designs below are proposed.
If two Oxynators® are to be used in succession with oxygen feeds from the oxygen lance, the design has to be slightly modified in order to allow an appropriate distribution of the oxygen flow through the different successive oxynators (i.e., an orifice allows part of the oxygen flow to pass through the upstream Oxynator®.
As illustrated in
As shown in
With respect to industrial boilers, especially coal-fired boilers, a secondary or transition stream with respect to a fuel duct may be arranged in at least two different ways. First, they may be configured as a tube within a tube (concentrically arranged tubes). Second, they may also be configured as a plurality of tubes (secondary or transition stream) radially spaced around a circumference of a tube (fuel duct). In the invention, oxygen may be injected from the secondary or transition stream in either of these manners.
With respect to the first configuration and as illustrated in
Oxygen is injected from the inner wall of fuel duct 231 with a swirl S1 by injection element 234. The directions of swirls S1, S2 may the same or different. The flow passage leading to and from the peripheral injection element 234 could be aerodynamically (like a venturi) designed to cause minimum disturbance to the flow. In other words, shoulders before and after the injection element 234 could be used. It should also be understood that fuel duct 238 need not extend beyond injection element 231A, 231B.
With respect to the second configuration, the conduit 239 may actually be a plurality of conduits surrounding the fuel duct 231, any or all of which feeds injection element 234.
As shown in
As shown in
Similar to the designs of
All of the Oxynator®-based designs of
Bluff Body Injection Element Designs:
Oxygen may be injected at several locations at roughly a single axial position by several different injection elements.
As shown by
As illustrated by
As depicted in
The lance 402 portion terminates in an aerodynamic body 405 having an aerodynamic tip 406. Each of the fins 402 is aerodynamically streamlined in shape. The apertures 403 are configured as circular holes, slots, slits, and other shaped openings such as those depicted in
In all the bluff body designs of
Axially Injecting Injection Element Designs:
Another type of injection element is configured to inject oxygen axially into the flow of non-gaseous fuel/conveying gas from a surface that faces downstream. This surface could have any number of apertures of any shape. Some exemplary shapes 701A-F are best shown in FIGS. 25A-F. The number of apertures, size, shape and angle of injection could be adjusted in order to optimize mixing and solid fuel loading.
Baffles arranged near the outlet end can facilitate a uniform mixing of oxygen in the primary stream (the use of baffles is an improvement over prior art designs as it accomplishes more efficient mixing by increasing the turbulence at the outlet end). Various baffles number, shape and size may be utilized. As the velocity control of the jet outgoing from the pipe is a crucial parameter governing burner aerodynamics, the cross-sectional area of those baffles will be chosen carefully.
Similar types of axially injecting injection elements have a modified cross-section. As gravity has an influence on motion of the particles, a vertical elliptical cross-section, for example, will cause fewer disturbances to the particle trajectories and at the same time could provide improved mixing. Modifications of the cross-section of the pipe allow decreasing or increasing the velocity of the axial oxygen jet. As best illustrated in
As depicted in
Variable Area/Venturi Designs:
As shown in
Other Considerations:
On all the previous designs, control of the three spatial components of the injection velocity could be used to optimize mixing of oxygen and carrier gas as well as particle load distribution. In addition, a forced pulsed oxygen injection can be applied to all the previous designs. Moreover, it is known as a prior art that in some specific geometrical configurations and in high Reynolds number flows (oxygen injection and/or primary stream Reynolds number), self-sustained oscillations of confined jets can occur. Oxygen injections could be designed to create and optimize pulsed jets mixing in the primary stream. The introduction of a pulsed oxygen flow in the primary air duct will force and improve the mixing of the oxygen with the carrier air and the fuel. Finally, all the above-listed concepts are specifically suitable for oxygen injection in the primary air of coal-fired low-NOx burners (LNB). Pulverized coal (PC) fired LNB (Wall or tangential fired) are of particular interest.
It will be understood that many additional changes in the details, materials, steps, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and/or the attached drawings.
This application is a continuation-in-part of co-pending U.S. non-provisional application No. 10/758,607, filed Jan. 15, 2004, which claims priority from U.S. provisional application No. 60/441,508, filed Jan. 21, 2003. This application claims the benefit under 35 U.S.C. §119(e) to provisional application No. 60/602,442, filed Aug. 18, 2004, and provisional application No. 60/605,312, filed Aug. 26, 2004, the entire contents of each of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60441508 | Jan 2003 | US | |
| 60602442 | Aug 2004 | US | |
| 60605312 | Aug 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10758607 | Jan 2004 | US |
| Child | 11206730 | Aug 2005 | US |
