Burner assembly and method for combustion of gaseous or liquid fuel

Abstract

Burner assembly and method for combustion of gaseous of liquid fuel The invention relates to a burner assembly (1) and a method for combustion of gaseous or liquid fuel to heat an industrial furnace (9) having a combustion chamber (2), at least one main combustion air inlet (3) for the supply of preheat-ed combustion air (4) into the combustion chamber (2), a burner (5) with at least one fuel feed (7) and at least one air feed (8) for supply of fuel and primary air into a the combustion chamber (2), wherein the burner (5) is positioned adjacent to a combustion zone of the combustion chamber (2) such that the combustion air (4) flowing into the combustion chamber (2) through the main combustion air inlet (3) is passing the burner (5) in the combustion zone and is then deflected such that the flow of preheated combustion air and the smaller flows of fuel and primary air are flowing mainly in parallel from the burner (5) to the furnace (9), and a control unit for controlling the supply of fuel and maybe primary air into the combustion chamber (2). The control unit is adapted to supply the fuel and/or the primary air from the fuel and/or air feed (7, 8) into the combustion chamber (2) with an exit velocity higher than 150 m/s.

Claims

1. Method for combustion of gaseous or liquid fuel to heat an industrial furnace (9) having process air with an amount of oxygen of 16 to 18 Vol-% comprising: providing a burner assembly (1) having a combustion chamber (2), at least one main combustion air inlet (3) for supplying preheated combustion air (4) with an amount of oxygen of ambient air into the combustion chamber (2), a burner (5) with at least one fuel feed (7) and at least one air feed (8) for the supply of fuel and primary air into a the combustion chamber (2), positioning the burner (5) adjacent to a combustion zone of the combustion chamber (2) such that the combustion air (4) flowing into the combustion chamber (2) through the main combustion air inlet (3) is passing the burner (5) in the combustion zone and is then deflected such that the flow of preheated combustion air and the smaller flows of fuel and primary air are flowing mainly in parallel from the burner (5) to the furnace (9); and controlling the supply of fuel from the fuel feed (7) into the combustion chamber to produce an exit velocity that is higher than 150 m/s and less than the sound velocity; and controlling the supply of primary air from the air feed (8) into the combustion chamber (2) such that the excess air ratio of primary air to fuel during the supply of fuel and primary air into the combustion chamber (2) is between 0.1 and 0.6, wherein controlling the supply of primary air from the air feed (8) into the combustion chamber (2) includes controlling the primary air from the air feed (8) such that the velocity of the primary air is higher than the velocity of the fuel feed by a factor of about 2.0.

2. Method according to claim 1, wherein positioning the burner (5) adjacent to a combustion zone includes feeding combustion air (4) into the combustion chamber (2) through the main combustion air inlet (3) with a temperature of more than 750 C.

3. Method according to claim 1, wherein controlling the supply of primary air from the air feed (8) includes, after ignition of the burner, initially controlling the supply of the primary air to produce an exit velocity lower than 150 m/s through the fuel feed (8) into the combustion chamber (2) until the combustion air (4) in the main combustion air inlet (3) and/or in the combustion chamber (3) has a temperature higher than a predefined temperature value, and, upon exceeding the predefined temperature value, subsequently controlling the primary air through the air feed (8) into the combustion chamber (2) to produce an exit velocity higher than 150 m/s and less than the second velocity.

4. Method according to claim 1, wherein controlling the supply of primary air from the air feed (8) includes controlling the primary air from the air feed (8) into the combustion chamber (2) via a compressor.

5. Method according to claim 1, wherein controlling the supply of fuel from the fuel feed (7) includes pressurizing the fuel in the fuel feed such that the exit velocity is attained.

6. Method according to claim 1, wherein controlling the supply of fuel from the fuel feed (7) includes controlling the supply of fuel from the fuel feed (7) such that excess air ratio of primary air to fuel during supply of fuel and primary air into the combustion chamber (2) is between 0.2 to 0.5.

7. Method according to claim 1, wherein controlling the supply of fuel from the fuel feed (7) includes, after ignition of the burner, initially controlling the supply of the fuel to produce an exit velocity lower than 150 m/s through the fuel feed (7) into the combustion chamber (2) until the combustion air (4) in the main combustion air inlet (3) and/or in the combustion chamber (3) has a temperature higher than a predefined temperature value, and, upon exceeding the predefined temperature value, subsequentialy controlling the supply of the fuel through the fuel feed (7) into the combustion chamber (2) to produce an exit velocity higher than 150 m/s and less than the sound velocity.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 schematically shows a section through a burner assembly according to a preferred embodiment of the invention;

(2) FIG. 2 schematically shows different flame types of the burner attainable with the burner assembly according to FIG. 1;

(3) FIG. 3 schematically shows a section through an induration furnace with two burner assemblies;

(4) FIG. 4 shows a perspective view from the rear of a burner with the mixing tubes according to a preferred embodiment;

(5) FIG. 5 shows a perspective view from the front of the burner according to FIG. 4 with the openings of the supply tubes directed into the combustion chamber;

(6) FIG. 6 shows a section through a preferred supply tube according to a first embodiment; and

(7) FIG. 7 shows a section though a preferred supply tube according to a second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(8) In FIG. 1 a burner assembly 1 according to a preferred embodiment of the invention is shown in a sectional view. The burner assembly 1 has a combustion chamber 2, a main combustion air inlet 3 for the supply of preheated combustion air 4 into the combustion chamber 2 and a burner 5 positioned in a wall 6 of the combustion chamber 2 such that combustion air 4 flowing into the combustion chamber 2 through the main combustion air inlet 3 is passing the burner 5. The burner 5 has at least one fuel feed 7 and at least one air feed 8 for the supply of fuel and primary air, respectively, into the combustion chamber 2. Further, with the burner assembly 1 a control unit or equipment (not shown) is provided for controlling the supply of fuel and primary air into the combustion chamber.

(9) The combustion chamber 2 is open towards the industrial furnace 9, in which the desired treatment of material is performed.

(10) According to a preferred embodiment of the invention, the industrial furnace 9 is an indurating furnace with a travelling grate 10 of a pelletizing plant. In such a pelletizing plant, the main combustion air inlet is typically a down corner 3 through which secondary air, i.e. the preheated combustion air 4, is flowing downwards into the combustion chamber 2 thereby passing the burner block 5 and then diverted vertically into the furnace 9 through the combustion chamber 2. In the combustion chamber 2, the fuel supplied into the combustion chamber 2 through the fuel feed 7 is mixed with the preheated combustion air 4 building a flame to heat the furnace 9 to the desired temperature.

(11) In a conventional burner assembly 1, the flame in the combustion chamber 2 is a standard flame 11 (as shown in FIG. 2a). The standard flame 11 is ignited just at the end of the fuel feed 7 and open towards the combustion chamber 2. Accordingly, the standard flame 11 is contained almost completely inside the combustion chamber and has a stable, conventional flame-like form, whereas the standard flame 11 is stabilized very close to the burner tip. The peak temperature in such a standard flame 11 is typically higher than 1500 C. whereas the temperature in the furnace 9 will be about 1300 C. (at least for an indurating furnace 9 of a pelletizing plant).

(12) Due to the high flame temperature, there is a high amount of thermal NOx produced by the flame. In order to reduce this thermal NOx-emission, it would be necessary to reduce the flame mid temperature and to avoid high peak temperatures.

(13) In order to achieve this goal, the invention proposes to destabilize the standard flame 11 as shown in FIG. 2a.

(14) In contrast to the prior art proposing a burner with a staged combustion and a multi-point fuel supply into the combustion chamber of the burner (see U.S. Pat. No. 8,202,470 B2), the invention proposes to enhance the exit velocity of at least the fuel supplied into the combustion chamber through the fuel feed 7 of the burner block 5. Due to the high velocity of the fuel introduced into the combustion chamber, the stabilization of a standard flame 11 right in front of the fuel outlet openings is not possible and the fuel mixes much better with the secondary air (preheated combustion air 4) before ignition.

(15) This leads at least to a lifted flame 12 as schematically shown in FIG. 2b being one of the preferred flames according to the invention. This lifted flame 12 covers a larger reaction area than the standard flame 11 and provides a more equally distributed temperature inside the flame as the fuel is not concentrated around the nozzle of the fuel feed 7 in the burner 5 but well distributed inside the combustion chamber 2 due to the extensive mixture of the fuel with the secondary combustion air 4. Accordingly, the flame mid temperature is reduced and the flame is lifted away from the burner 5 in the direction of the combustion air flow towards the furnace 9. The lifted flame 12 is generated in the combustion chamber 2 and may partially reach into the furnace 9 itself.

(16) However, according to the present invention, the most preferred flame is a strongly lifted or mild flame 13 as shown in FIG. 2c and FIG. 1.

(17) With the mild (or strongly lifted) flame 13, there is no or nearly no visible flame in the combustion chamber 2 or the furnace 9 itself. Instead, the fuel reacts with the preheated combustion air 4 (secondary air) in a more or less flameless reaction due to the elevated temperature of the secondary air. This is achieved by an even larger distribution of the fuel in the secondary combustion air 4, avoiding thus fuel enriched zones in the combustion air leading to a visible flame 11. The reaction between the fuel and the combustion air 4 regularly takes place mostly at the end of the combustion chamber 2 and in the furnace 9 itself. Accordingly, the mid temperature of this reaction over the complete reaction zone is much lower than the mid temperature in the standard flame 11 or even in the lifted flame 12. According to the present invention, this is attained by the high dilution with combustion air due to the very high exit velocity of the fuel out of the fuel feed 7 in the burner block 5 and/or the excess air ratio.

(18) For clarity reasons, not all reference sign are reproduced in the FIGS. 2b and 2c. They are considered the same as in FIG. 2a.

(19) Additionally, with the fuel supplied into the combustion chamber 2, primary air is fed through an air feed 8 in the burner block 5 into the combustion chamber 2. Advantageously, also the primary air is supplied into the combustion chamber 2 with an exit velocity higher than 150 m/s. The fuel and the primary air might be fed into the combustion chamber with the same exit velocity which preferably is higher than 250 m/s. Of course, it is also possible to have the different exit velocities of the fuel and the primary air.

(20) Contrary to the secondary air being a combustion air preheated to a temperature of about 750 to 1000 C. for example, the primary air has a low ambient-like temperature in the range of preferably 20 C. to 100 C. when being supplied into the combustion chamber. The primary air has the effect, that it cools down the possible reaction zone in front of the fuel and air feeds in the combustion chamber 2 thus avoiding a fast ignition of the fuel in the preheated combustion air. Accordingly, the fuel is transported along with the preheated combustion air 4 more deeply into the combustion chamber 2 and the furnace 9 leading to a lifted flame 12, or optimally to a mild flame 13 with no or nearly no visible flame.

(21) FIG. 3 shows a cross section through a indurating furnace 9 with a traveling grate 10 and two burner assemblies 1 as shown in FIG. 1. The preheated combustion air (secondary air) is fed into the combustion chamber 2 and the furnace 10 through a down corner 3 being the main combustion air inlet. In FIG. 3, a mild flame 13 is shown.

(22) In order to achieve the desired exit velocities of the fuel and the primary air, the fuel and the primary air might be pressurized, controlled by the control unit of the burner assembly 1. Typically, the (gaseous) fuel and the primary air are pressurized with a preferred increase of pressure of about 3 bar or the range of 0.8 to 4 bar.

(23) The primary air might even be pressurized to a higher value such as 6 or 7 bar.

(24) Basically, the higher the pressure of the fuel and the primary air is, the higher is the exit velocity of the fuel and the primary air and the better is the mixture of the fuel with the preheated combustion air 4. In order to achieve even supersonic speed special nozzles, such as Laval type nozzles, may be used. Accordingly, for attaining a mild flame 13, the exit velocity and the pressure, respectively, will be higher than for a lifted flame 12 in the same burner and furnace array. Also an increase of the excess air ratio may change the flame type from lifted flame to mild flame.

(25) One big advantage of the described burner assembly 1 is, that the burner 5 is able to produce also a standard flame 11, for example when heating the furnace 9 and the combustion chamber 2 to the process temperature, by supplying the fuel and the primary air with a much lower velocity below 150 m/s thus obtaining a stable flame, if required.

(26) FIGS. 4 and 5 show a burner block 5 from the rear (FIG. 4) and from the front (FIG. 5), the front being defined as the side of the burner block 5 directed towards the combustion chamber 2.

(27) Inside the burner block 5, there are provided several supply tubes 14, each in form of a duplex lance having a central tube 15 and a surrounding tube 16 as evident from the detail in FIG. 5.

(28) Preferably, the central tube 15 is used as fuel feed tube and the surrounding tube 16 is used as air supply feed tube for the primary air. It has been found, that this configuration of the fuel feed 7 and the air feed 8 in a duplex lance 14 is easy to handle and has good results with regard to the mixture of fuel and primary air into the secondary or preheated combustion air 4 in the combustion chamber 2.

(29) In FIG. 6, the openings of the central tube 15 and the secondary tube 16 include a nozzle with reduced opening diameter, but they may be simply open by a cut through the tubes. However, it is possible to provide at the open end of the central tube 15 and/or the surrounding tube 16 separated nozzles for fuel and primary air, or even one common nozzle for fuel and primary air, as shown in FIG. 6 or 7, in order to influence the entry, mixing and flow of the fuel and the primary air into the combustion chamber 2 according to the desired form of the flame and/or the geometry of the burner 1 and/or furnace 9.

(30) In order to supply the fuel and the secondary air under pressure into the supply tube 14, there are providedas evident from FIG. 5a first port 17 to the central tube 15 and a second port 18 to the surrounding tube 16 for each supply tube or duplex lance 14.

(31) FIG. 6 shows a cross-sectional view of one supply tube or duplex lance 14 with a central tube 15 and the surrounding tube 16 and the respective first port 17 to the central tube 15 and the second port 18 to the surrounding tube 16. As evident from FIG. 6, the cross section of the openings of the central tube 15 and the surrounding tube 16 are brought to a certain form and volume in order to be able to adjust the exit velocity and direction of the fuel and the primary air. Preferably they also adjust the excess ratio of primary air and fuel.

(32) FIG. 7 shows a supply tube 14 as described before with a structured nozzle 19 with an input opening 20 corresponding to the inner diameter of the surrounding tube 16 of the duplex lance 14 and an output opening 21 with a smaller diameter than the input opening 20. The diameter of the output opening 21 might be about the half of the diameter of the input opening 10. The inner wall of the structured nozzle 19 has partially a surface 22 reducing in conical form. This form enhances the outlet velocity and has shown positive test results. Fuel gas and primary air flow through the central tube 15 and the surrounding tube 16 and through the nozzle 21. In this example, the nozzle has only one common channel for both, the fuel and the primary air.

(33) Tests have shown, that the excess air ratio of the primary air related to the fuel should be preferably about 0.2 to 0.5 in order to achieve a low NOx-combustion in the burner 1.

(34) The proposed burner assembly 1 and method are advantageous because the burner 5 itself can be used as conventional burner 5 to produce a stable flame 11 as well as a low NOx-burner producing a lifted flame 12 or mild flame 13 by just amending the exit velocity and/or excess ratio of primary air to fuel.

LIST OF REFERENCE SIGNS

(35) 1 burner assembly 2 combustion chamber 3 main combustion air inlet, down corner 4 preheated combustion air (secondary air) 5 burner, burner block 6 wall of combustion chamber 7 fuel feed 8 air feed for supply of primary air 9 industrial furnace, indurating furnace 10 traveling grate 11 standard flame 12 lifted flame 13 mild flame 14 supply tube, duplex lance 15 central tube 16 surrounding tube 17 first port to central tube 18 second port to the surrounding tube 19 structured nozzle 20 input opening 21 output opening 22 conically reducing surface