EP0234077A1

EP0234077A1 - Improved burner for furnaces employing acoustic energy

Info

Publication number: EP0234077A1
Application number: EP86301340A
Authority: EP
Inventors: Kodavanti Mallikharjuna Swamy; Kallepally Lakshmi Narayana; Josyula Samba Murty
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 1986-02-25
Filing date: 1986-02-25
Publication date: 1987-09-02
Also published as: AU5414086A

Abstract

An improved burner for a chemical or metallurgical furnace comprises first delivery means 1 for :he delivery of a stream of compressed air or other gas to a convergent-divergent nozzle 3 connected to ts outlet. Located concentrically about the first delivery means 1 is a second delivery means 2 which provides an annular passage 10 for fluid fuel which s to be atomised. The annular passage 10 is con- nected to the nozzle 3 by a plurality of radial ducts 7 :hrough which the fluid fuel is drawn into the nozzle, and atomised therein, by the stream of compressed air flowing through the nozzle. The mixture of air and atomised fuel then strikes a resonator 12 connected to but spaced from the divergent mouth of the noz- de 3. The sonic waves created in this way are eflected back onto the air and atomised fuel issuing from the nozzle to further atomise the fuel.

Description

The present invention relates to an improved burner for chemical or metallurgical furnaces which is adapted by means of acoustic energy to bring about finer atomisation of fluid fuel and thereby more efficient burning of the micro-sized fuel particles so produced.
Furnaces such as tunnel furnaces and rotary kilns are known to employ a number of different types of burners. Thus there are conventional fuel oil burners, rotary burners and burners which employ ultrasonic energy produced by piezoelectric means. This last-mentioned type of burner, however, evinces limited applications for low flow rates. The principle underlying any burning or combustion operation envisages the atomisation of fluid fuel within a jet of gas or air. The high speed-high pressure jet breaks up the fuel into tiny droplets which intermingle with the air stream and are maintained in suspension therein. The atomised fuel is then ignited at the mouth of the burner which is located at or within the furnace in question. The finer the atomisation of fluid fuel, the more efficient is the combustion achieved.
Burners which employ acoustic energy, hereinafter referred to as 'acoustic burners', invariably take advantage of the inherent characteristics of functions of compressed gas, chiefly compressed air. These characteristics are essentially twofold. First of all, compressed air generates acoustic waves which can be employed for atomising fluid fuel into fine droplets. Secondly, the stream of compressed air can be employed to perform the function of primary air during a combustion operation. In general, the size of atomised fuel droplets can by varied by adjusting the pressure and volume of compressed gas or air employed and this in turn has a bearing on the flame length, flame velocity and turn down ratio obtained. Burners which employ acoustic energy for atomisation of fluid fuel are reasonably well known and, in general, suffer from the drawback of having first of all to establish varying zones of high and low pressures in order to achieve workable atomisation of fluid fuel. Perhaps the best example of a prior art acoustic burner is that disclosed in Indian Patent No.92387 granted to Sonic Development Corporation of America. According to this Indian patent, a burner includes apparatus for atomising a fluent material in a gaseous medium which apparatus comprises a convergent-divergent nozzle through which a high speed gas stream is adapted to be fed, said gas stream accelerating through said convergent section of the nozzle and expanding in the divergent section. Spaced from the divergent mouth of said nozzle is a pulsator means which is adapted to intercept the high speed stream of gas and develop sonic pressure waves, the space between the nozzle opening and the pulsator constituting the single region of atomisation. Connected to the divergent mouth of said nozzle or opening into the region of atomisation is means for delivering the fluent material to be atomised. As this fluent material, essentially a gaseous fuel, issues it is entrained in the issuing compressed air and atomised by impingement of the sonic waves created by the pulsator.
There is one significant feature common to all prior art acoustic burners, This is their inability to provide for more than a single stage for the atomisation of fluid fuel. As a result, the degree of atomisation that can be achieved is limited and this is reflected in a fall in efficiency during combustion.
It is therefore the essential object of the present invention to devise an improved acoustic burner which makes provision for an additional stage of atomisation of fluid fuel in order to provide an increased degree of efficiency during combustion.
Accordingly, the present invention provides an improved burner for furnaces employing acoustic energy for the atomisation of fluid fuel to be combusted which comprises first delivery means for the delivery of a stream of compressed gas or air, a convergent-divergent nozzle connected to the outlet end of said first delivery means, resonator means connected to but spaced from the divergent mouth of said nozzle, second delivery means located concentrically about said first delivery means to provide an annular passage for the delivery of fluid fuel to be atomised, and a plurality of radial ducts connecting said annular passage with said nozzle upstream of the convergent section thereof whereby with the high pressure-high speed delivery of compressed air or gas through said first delivery means, fluid fuel is drawn through said radial ducts into and atomised within said nozzle which thereby constitutes a first atomisation zone, said stream of atomised fuel and air which issues from the mouth of said nozzle striking said resonator means, the sonic waves thus created reflecting back on to the issuing stream to atomise further the atomised fuel entrained therein, said space between the mouth of said nozzle and said resonator means constituting a second atomisation zone.
According to a preferred feature of the invention, the convergent-divergent nozzle is provided in the form of a venturi with a convergent inlet section, a narrow cylindrical central or 'stabilisng' section and a divergent outlet section. Compressed air delivered from the first delivery means to the nozzle accelerates in the convergent section and expands in the divergent section. This leads to the creation of vacuum conditions at the narrow wasp- waisted central section of the nozzle. Accordingly, while the radial ducts for delivery of fluid fuel from the annular passage can connect with the nozzle anywhere upstream of the convergent section thereof, it is most advantageous for such ducts to connect with the narrow central section of the convergent-divergent nozzle where, because of the vacuum created, suction of fluid fuel from the annular passage is easiest.
Conveniently, the first and second delivery means comprise a pair of concentrically disposed pipes for delivering the compressed gas or air and fluid fuel, respectively.
In accordance with a preferred feature, there are from four to twelve radial ducts connecting said annular passage with the narrow central section of the convergent-divergent nozzle, said ducts being disposed at regular intervals from one another, e.g. at intervals of 90° where there are four ducts, 45° when there are eight ducts and 30° when there are twelve ducts.
The resonator means can be spaced from the divergent mouth of the nozzle by any convenient spacing such as a plurality of rod or leg-like members. Preferably, the leg-like members extend from the mouth of the nozzle in a converging manner with the resonator means supported between the opposite ends of said members.
While the burner of the present invention can be employed with any fluid fuel, it is particularly adapted for operation with a liquid fuel such as an oil. The most convenient compressed gas employed as the atomising agent is compressed air.
The invention will now be described in greater detail with reference to the accompanying drawing which illustrates an axial section of the acoustic burner of this invention.
The burner illustrated in the drawing comprises inner tubular member 1 though which there is adapted to be delivered a stream of compressed gas such as air. Provided externally about tubular member 1 in a concentric manner is outer tubular member 2 which is adapted to be fed through opening 11 with a source of fluid fuel, in this instance fuel oil, which travels up the annular passage 10 between members 1 and 2. Inner and outer members 1 and 2 are provided with the requisite connections in the form of packing 8 and gland nut 9 to ensure that the compressed air and fuel oil do not mix at their points of entry in the burner.
At its outlet end, tubular member 1 is connected to a convergent-divergent nozzle 3 composed of a convergent inlet section 4, a narrow- waisted cylindrical central section 5 and a divergent outlet section 6. Four ducts 7 disposed at regular 90° intervals from one another extend radially inwardly from annular passage. 10 to connect with cylindrical section 5 of the convergent-divergent nozzle 3. Connected to nozzle 3 but spaced externally of the opening thereof is resonator 12 incorporating resonating chamber 13. This connection of resonator 12 is effected through the medium of convergingly disposed leg members 14 which extend from nozzle 3 with resonator 12 mounted between the extremities thereof in the direct path of issue of the air stream from nozzle 3.
The operation of the burner of the invention is described hereafter. Compressed air delivered through inner tubular member 1 enters convergent-divergent nozzle 3 where its rate of delivery accelerates in convergent inlet section 4 before expanding in divergent outlet section 6. This action creates a vacuum at the narrow cylindrical central section 5. As a result of this, fuel oil which has been fed from oil inlet 11 in outer tubular member 2 to annular passage 10 is drawn into central section 5 through radial ducts 7 connecting such section with annular passage 10. The in-drawn oil is atomised by the onrushing air within nozzle 3 itself, specifically within central section 5 which thus constitutes a first atomisation zone, and the stream of compressed air with atomised oil particles entrained therein exits from the divergent mouth 15 of nozzle 3 to impinge against resonating chamber 13 of resonator 12. The impact of the high speed air on resonator 12 sets off a sonic wave which reflects back on the issuing stream of air and shatters the already atomised oil particles in such stream into even finer substantially uniform microsized particles thus bringing about a second stage atomisation. The extremely finely atomised oil particles are delivered to a furnace where their micro-fineness enables combustion thereof to take place very efficiently. It has been found that the burner of the present invention is capable of achieving through its two-stage atomisation atomised particles of from 10 to 30 microns which compares very favourably with the particles of 50 to 100 microns achieved by conventional burners.
The entire burner of the present invention is preferably made of stainless steel or any alternative metallic alloy capable of withstanding furnace operating temperatures.

Claims

1. An improved burner for furnaces employing acoustic energy for the atomisation of fluid fuel to be combusted which comprises first delivery means for the delivery of a stream of compressed gas or air, a convergent-divergent nozzle connected to the outlet end of said first delivery means, resonator means connected to but spaced from the divergent mouth of said nozzle, second delivery means located concentrically about said first delivery means to provide an annular passage for the delivery of fluid fuel to be atomised, and a plurality of radial ducts connecting said annular passage with said nozzle upstream of the convergent section thereof whereby with the high pressure-high speed delivery of compressed air or gas through said first delivery means, fluid fuel is drawn through said radial ducts into and atomised within said nozzle which thereby constitutes a first atomisation zone, said stream of atomised fuel and air which issues from the mouth of said nozzle striking said resonator means, the sonic waves thus created reflecting back on to the issuing stream to atomise further the atomised fuel entrained therein, said space between the mouth of said nozzle and said resonator means constituting a second atomisation zone.

2. A burner as claimed in claim 1 wherein said convergent-divergent nozzle is in the form of a venturi with a convergent inlet section, a narrow cylindrical central section and a divergent outlet section.

3. A burner as claimed in claim 2 wherein said plurality of radial ducts from said annular passage connect with said narrow cylindrical central section of said nozzle.

4. A burner as claimed in claim 3 wherein from four to twelve radial ducts are provided for connecting said annular passage with said cylindrical central section of said nozzle, said ducts being disposed at regular intervals of from 30° to 90° from one another.

5. A burner as claimed in any of claims 1 to 4 wherein said first and second delivery means comprise a pair of concentrically disposed pipes.

6. A burner as claimed in any of claims 1 to 5 wherein said resonator means is connected to and spaced from the mouth of said nozzle by a plurality of leg-like members extending from the mouth of said nozzle in a converging manner with said resonator means supporting between the opposite ends of said leg-like members.

7. A burner as claimed in any of claims 1 to 6 wherein the fluid fuel atomised is a liquid fuel such as oil and the burner body is made entirely of stainless steel or similar material compatible with high furnace temperatures.

8. An improved burner for furnaces employing acoustic energy for the atomisation of fluid fuel to be combusted substantially as herein described with reference to and as illustrated in the accompanying drawing.