Method and burner for reducing nitrogen oxide emissions during the combustion of a gaseous fuel

Abstract

A method for reducing nitrogen oxide NOx emissions during combustion of a gaseous fuel in a burner intended for a naked-flame or controlled-atmosphere reheating furnace, for reheating steel products or for continuous coating and/or annealing of metal strips, wherein a first dilution is carried out by mixing combustion air with combustion products upstream from or in the body of the burner, and a second dilution is carried out directly at the level at which the gaseous fuel reacts with the combustion air, mixing the fuel with a recirculated portion of the flame or products of partial combustion, the double dilution enabling the physical and chemical properties of the gas to be modified in order for the burner to operate with low oxygen rates and obtain a flame that produces a very low level of NOx production regardless of the temperature of the enclosure in which the combustion takes place.

Claims

1. A process for reducing the emission of nitrogen oxides NOx during the combustion of a gaseous fuel in a burner intended for a direct flame or controlled-atmosphere reheating furnace, for reheating steel products or for continuous coating and/or annealing of metal strips, especially steel strips, according to which process, a first dilution is achieved by mixing combustion air with combustion products upstream of the burner or in the body of the burner, wherein the combustion air mixed in the first dilution is all the combustion air used in the process, a second dilution is achieved by mixing the gaseous fuel with a recirculated portion of the flame or the partial combustion products, wherein the gaseous fuel in the second dilution is un-diluted and un-premixed before the second dilution, this double dilution resulting in the modification of the physical and chemical characteristics of the gas for a stable operation of the burner, in particular with a highly diluted oxidant having an oxygen content close to 10% by volume, for the purpose of reducing the production of NOx, this being for all the operating temperatures of the chamber in which the combustion takes place, wherein the second dilution is achieved at the burner nozzle by recirculation of products resulting from the reactive zone of the flame, in particular with free radicals, used for initiating thermochemical reactions in the fuel.

2. The process as claimed in claim 1, wherein the second dilution is achieved by injecting at least two gaseous fuel jets that are substantially parallel, at a distance from one another and suitable for inducing a vacuum in a zone located between the jets, which leads to a circulation of partial combustion products in this zone, and ensures the mixing of the gaseous fuel with a recirculated portion of the flame or the partial combustion products.

3. The process as claimed in claim 2, wherein the gaseous fuel jets are distributed along a closed contour, in particular in a ring, and the vacuum zone is located on the inside of the closed contour, in particular of the ring, leading to a circulation of the partial combustion products in this zone.

4. The process as claimed in claim 2, wherein the gaseous fuel jets are distributed in a circular ring, the diameter of which is between 80 and 120 mm.

5. The process as claimed in claim 1, wherein the initial velocity of the gaseous fuel jets is at least equal to 120 m/second for natural gas.

6. The process as claimed in claim 1, wherein the mixture of combustion air and of combustion products, in particular flue gases, is distributed in an annular zone surrounding the gaseous fuel jets.

7. The process as claimed in claim 1, wherein the oxygen content of the mixture of combustion air with combustion products, resulting from the first dilution, is less than 15% by volume, in particular close to 10% by volume.

8. A gaseous fuel burner intended for a direct flame or controlled-atmosphere reheating furnace, for reheating steel products or for continuous coating and/or annealing of metal strips, especially steel strips, wherein the burner is designed to achieve a double dilution, a first dilution is achieved by mixing combustion air with combustion products upstream of the burner or in the body of the burner, wherein the combustion air mixed in the first dilution is all the combustion air used in the process, the second dilution is achieved by mixing the gaseous fuel with a recirculated portion of the flame or the partial combustion products, wherein the gaseous fuel in the second dilution is un-diluted and un-premixed before the second dilution, this double dilution resulting in the modification of the physical and chemical characteristics of the gas to enable a stable operation of the burner, in particular with a highly diluted oxidant having an oxygen content close to 10% by volume, for the purpose of reducing the production of NOx, this being for all the operating temperatures of the chamber in which the combustion takes place, the burner comprising a burner nozzle composed of a cylindrical portion attached to which, perpendicular to the geometric axis of the cylindrical portion and set back from the opening plane of the cylindrical portion, is a disk pierced with a plurality of orifices, the axes of which are substantially parallel to the axis of the cylindrical portion, that are located over a diameter close to the external diameter of the disk, and a tube having a diameter smaller than that of the cylindrical portion is attached coaxial to this portion, one of its ends being located inside said portion while leaving a distance between this end and the front face of the disk, the other end of the tube being located outside of the cylindrical portion.

9. The burner as claimed in claim 8, wherein to achieve the second dilution, the burner further comprises at least two ports for injection of gaseous fuel jets that are substantially parallel, at a distance from one another and suitable for inducing a vacuum in a zone located between the jets.

10. The burner as claimed in claim 9, wherein the ports for injection of gaseous fuel are distributed along a closed contour, in particular in a ring.

11. The burner as claimed in claim 8, wherein the burner is positioned in a pipe for the mixture of combustion air and combustion products, in particular flue gases, which is distributed in an annular zone surrounding the portion of the burner equipped with ports for the gaseous fuel jets.

12. The burner as claimed in claim 8, wherein the mixture of combustion air and flue gases is distributed around the cylindrical portion (26), and the gas jets (18) from the ring of orifices (19) induce a vacuum inside the tube (27), which enables a return of flame to the burner.

13. The burner as claimed in claim 8, wherein the fuel inlet (22) comprises a tubular portion of small diameter (23), followed by a cone (24) coupled to the cylindrical portion (26).

14. The burner as claimed in claim 8, further comprising a stack of tubes (25, 26) and of a ring of holes (19) in a distribution plate (25) in order to produce a suction zone (A) in the location of start up for the oxidation of the fuel by the oxidant.

15. The burner as claimed in claim 9, wherein the burner is positioned in a pipe for the mixture of combustion air and combustion products, in particular flue gases, which is distributed in an annular zone surrounding the portion of the burner equipped with ports for the gaseous fuel jets.

16. The burner as claimed in claim 12, wherein the fuel inlet (22) comprises a tubular portion of small diameter (23), followed by a cone (24) coupled to the cylindrical portion (26).

17. The burner as claimed in claim 12, further comprising a stack of tubes (25, 26) and of a ring of holes (19) in a distribution plate (25) in order to produce a suction zone (A) in the location of start up for the oxidation of the fuel by the oxidant.

Description

(1) The invention consists, apart from the arrangements disclosed above, of a certain number of other arrangements that will be mentioned more explicitly hereinbelow with respect to an exemplary embodiment described with reference to the appended drawings, but which is in no way limiting. In these drawings:

(2) FIG. 1 is a schematic drawing of a radiant tube with burner according to the prior art;

(3) FIG. 2 is a schematic drawing of equipment with burner according to the prior art;

(4) FIG. 3 is a schematic drawing of vertical cross section of a burner according to the invention.

(5) The solution of the invention is illustrated in FIG. 3 which schematically presents the burner 10 and the first leg of the radiant tube 12a, as shown in FIG. 1.

(6) Seen in FIG. 3 is the port 21 corresponding to the inlet of recirculated flue gases such as 9 and of combustion air 7 preheated in a recuperator, not represented in FIG. 3, but similar to the recuperator 6 from FIG. 1. The same result may be obtained with an inlet of a pre-established mixture of recirculated flue gases 9 and of combustion air 7.

(7) The fuel inlet 22 is composed of a tubular portion 23, for example of diameter DN 20 for a natural gas, a cone 24, followed by a cylindrical portion 26. Inside the cylindrical portion 26 a disk 25 is attached orthogonal to the geometric axis of the portion 26, in particular welded to the inside of said tube, so that there is a distance 1, for example of between 30 and 60 mm for the natural gas, between the front face of this disk and the opening plane of the tube 26. The disk 25 is pierced with a plurality of orifices 19, for injection of fuel, the axes of which are substantially parallel to the axis of the tube 26, that are located over a diameter, in particular of 10 mm, smaller than the external diameter of the disk.

(8) A tube 27 is welded in the axis of the tube 26, one of its ends being located inside the tube 26 while leaving a distance 2, in particular of between 5 and 30 mm for a natural gas, between this end and the front face of the disk 25. The tube 27 extends over a distance 3, in particular of between 100 and 250 mm, beyond the end of the tube 26.

(9) The mixture of combustion air and flue gases is distributed, in the pipe 12a, along an annular zone 20, around the cylinder 26 and gas jets 18 from the ring of orifices 19. The injections 18 of gas at high velocity, greater than 120 m/sec of natural gas, induce a vacuum in the tube 27, which leads to a suction of the combustion products along the path 28 illustrated in the tube 27 from the zone B, located in the vicinity of the end of the tube 27 far from the disk 25, to a zone C located between the end of the tube 27 close to the disk 25 and the disk.

(10) The zone B is in the reaction zone of the fuel and of the oxidant, that is to say in a very high temperature flame zone, in particular above 1500 K and in a zone where the development of the combustion produces a large amount of partially oxidized and reactive chemical species including free radicals present in a plasma-type state of these combustion products. It may also be noted that, contrary to what occurs when a recirculation of flue gases is implemented conventionally, for which the increase in the recirculation degrades the stability of the flame, the implementation of the dilution of the fuel at the burner nozzle as presented by the invention in the presence of an oxidant having a low oxygen content, in particular 10% by volume, extends the stability range of the flame. The energy provided by this recirculation 28 of very high temperature gas at the meeting point D with the fuel modifies its physicochemical characteristics, in particular partially achieves the partial thermal cracking of the fuel which ensures the development of the combustion in the zone A, around the tube 27. This is obtained despite the low concentration of oxygen present in the mixture of flue gases and air 20. By this means, it is possible to achieve the ignition and stabilization of the reaction zone even with very low oxygen contents via a local supply of thermal energy and the modification of the thermochemical properties of the fuel, which makes it possible to extend the inflammability limits of the air/fuel gas mixture, in particular at an oxygen content of 10% by volume.

(11) The reactions involved may be, for example, of the type:
CH.sub.4+H.sub.2O=3H.sub.2+CO
CH.sub.4.fwdarw.C+2H.sub.2
CO+H.sub.2O=CO.sub.2+H.sub.2

(12) From these equations, the formation of hydrogen may be noted, which will promote the ignition of the fuel despite a low concentration of oxygen.

(13) This device makes it possible to maintain a stable flame with oxygen contents lower than those used according to the prior art and thus to obtain levels of NOx produced that are lower than those obtained according to the prior art, this whatever the temperature of the chamber in which the combustion develops.

(14) It may also be noted that the implementation of the recirculation of the combustion products at the burner nozzle as presented by the invention in the presence of a mixture of air and flue gases having a low oxygen content, in particular 10% by volume, increases the stability of the flame by facilitating the combustion, or the ignition of the fuel.

(15) It is seen that the operation of this burner is based on a double dilution, the first dilution achieved by the mixing of the combustion air with combustion products upstream of the reaction zone, the second dilution achieved directly in the reaction zone by the dilution of the fuel with the reactants of the high-temperature flame directly at the burner nozzle. This second dilution is different since it does not have the simple effect of diluting the gases, but also, due to the input of thermal energy greater than the self-ignition temperature, it contributes to the modification of the thermochemical properties of the fuel gas via complex phenomena that can be likened to a pyrolysis. The mixture of fuel gas and of incomplete combustion products reacts in order to produce in particular hydrogen, resulting in a modification of the thermochemical properties of the gas.

(16) It is understood that the preceding description of the invention was given for an application to a radiant tube but that the disclosed arrangements can be transposed to direct flame burners for which the first dilution is achieved y the mixing of combustion products inside the furnace, along the paths 14 and 16 from FIG. 2, and that the second dilution may be achieved at the burner nozzle with a device as presented in FIG. 3.