WO1991006804A1

WO1991006804A1 - BURNER AND METHOD FOR REDUCING NOx FORMATION

Info

Publication number: WO1991006804A1
Application number: PCT/SE1990/000708
Authority: WO
Inventors: Lars Saltin
Original assignee: AGA AB
Current assignee: Linde Sverige AB
Priority date: 1989-11-01
Filing date: 1990-10-31
Publication date: 1991-05-16
Anticipated expiration: 1992-05-01
Also published as: SE464542B; SE8903654D0

Abstract

A method and a burner for combusting fuel with oxygen, the fuel being delivered to a combustion zone from a center exit nozzle (5). Oxygen travelling at low speed is delivered from a first oxygen nozzle (3) at a first radial distance from the center fuel jet, and oxygen travelling at high speed is delivered from a second oxygen nozzle (6) at a second, greater radial distance from the center fuel jet. This results in incomplete combustion of the fuel with the oxygen moving at low speed in a first region, and incomplete combustion with the oxygen moving at high speed in a second region downstream of the first region, these regions together forming the combustion zone.

Description

BURNER AND METHOD FOR REDUCING NO^ FORMATION

The present invention relates to a method of burning fossil fuel, preferably gas or liquid fuel, and particular, but not exclusively, fuel oil, with oxygen with the intention of reducing the amount of nitrogen oxides formed with this combustion, and also to a burner for carrying out the method.

When burning liquid fuel, e.g. fuel oil, it is necessary to atomize or finely divide the fuel in order to be able to initiate combustion, and also in order to sustain combustion of the fuel.

The liquid fuel can be finely divided with the aid of a gaseous medium. The medium used is normally air or steam. Other gases can also be used, however, such as natural gas, propane and carbon dioxide, for instance.

Normally, compressed air is the gas that is most often used for economic reasons, this air being delivered at a pressure of, e.g. 2-5 bars, so as to finely divide the liquid fuel at the moment of its exit from the burner nozzle. The supply of air, and also to a lesser extent the supply of natural gas, results in the introduction of nitrogen to the combustion process, since nitrogen is present in natural gas. The actual fuel oil itself may contain up to 0.5% by weight nitrogen. This nitrogen reacts with the oxygen supplied to sustain combustion, therewith resulting in the production of nitrogen oxides, normally nitrogen monoxide and nitrogen dioxide.

In recent times, the negative effect of nitrogen oxides on the environment has been recognized to progressively greater extends. The restrictions of permitted emissions are practiced in the majority of industrial countries, and in some countries, the emission of environmentally contamina¬ ting substances will, in the future, result in prosecution and the levy of fines. One object of the present invention is to provide a method and a device for combusting fossil fuels in a manner which will avoid the formation of nitrogen oxides to the greatest possible extent, irrespective of whether nitrogen is present naturally in the fuel or is supplied thereto sepa¬ rately, for instance in the form of air.

Another object is to provide a method of combusting liquid, fossil fuels without supplying atomized substances to the fuel, and to provide a device for carrying out the method.

This primary object is achieved with a method in which the fuel is delivered to a combustion zone from a central exit nozzle, in which oxygen is supplied at a low speed in the form of at least one first jet located at a first radial distance from the central fuel jet, and in which oxygen is delivered at high velocity in the form of at least one second jet at a second radial distance from the central fuel jet, wherein said second distance is greater than said first distance, for the purpose of generating incomplete combustion of the fuel with the oxygen of low speed in a first region and complete combustion with the oxygen of high speed in a second region downstream of the first region, said regions together forming the combustion zone. The burner used for carrying out the method includes a central exit nozzle for delivering fuel to a combustion zone, and is characterized by at least one first oxygen nozzle which is arranged at a first radial distance from the central exit nozzle and which delivers oxygen at a low speed, and at least one second oxygen nozzle which is arranged at a second radial distance from the central exit nozzle and which delivers oxygen at a high speed, said second distance being greater than said first distance, such as to generate incomplete combustion of the fuel with the oxygen of low speed in a first region and complete combustion of the fuel with the oxygen of high speed in a second region downstream of the first region, said regions together forming the combustion zone.

With the use of liquid fuel, e.g. fuel oil, the fuel is caused to leave the central nozzle in the form of a conical mantle, wherein the fuel is finely divided or atomized, and the radial extension of the conical fuel jet is restricted with the aid of an annular third oxygen jet located at shorter radial distance from the central exit nozzle than th first oxygen jet. For the use of liquid fuel, the burne includes a central nozzle which delivers the fuel in the for of a conical mantle, wherein the fuel is finely divided o atomized, and is characterized by at least one third oxyge nozzle which is arranged radially between the central exi nozzle and the first oxygen nozzle and which delimits th radial extension of the conical fuel nozzle. Advantageous embodiments of the inventive method and th inventive burner are set forth in the following method an apparatus Claims.

The inventive method and device will now be described i more detail with reference to the accompany drawings, i which

Figure 1 is a schematic sectional view along the lon¬ gitudinal axis of the inventive burner;

Figure 2 is a schematic sectional view, in larger scale, of the forward part of the burner in Figure 1; and Figure 3 is a schematic view from the front of the inventive burner.

The burner 1 in Figures 1-3 comprises a casing 12 which delimits a cylindrical oxygen chamber 16 and which is surrounded, at least at its forward part, by a cylindrical cooling jacket 13 having a cooling agent inlet 14 and a cooling agent outlet 15, this cooling agent normally being water. The rear part of the casing 12 is provided with a flange 17 to which a wall 18 defining the oxygen chamber 16 is sealingly attached with the aid of a number of bolts, for instance, not shown. Connected to the oxygen chamber 16 is an inlet 19 for oxygen-containing gas, preferably pure oxygen. The casing 12 functions as the inner wall of the cooling jacket 13 surrounding the burner, at the forward end of said burner. The burner 1 also includes a center body 9 which is arranged in the forward part of the burner. This body forms the forward limitation of the oxygen chamber 16. The center body 9 has a forward part 8 having a diameter such that said body will sealingly abut the inner surface of the casing 12. Optionally, the cylindrical mantle surface of the forward part 8 may be configured with a groove for accommodating an O-ring 20. The center body 9 also has a rear part 7 whose axial length is longer but whose radius is smaller than the forward part 8. There is formed between the rear part 8 of the center body 9 and the casing 12 an annular gap which extends up to the forward part 8, which forms an annular end wall 9 and thus a forward defining wall 8 of the oxygen chamber 16. This annular end wall 8 is provided with a plurality of openings 6 which widen conically in the direc¬ tion of the burner and which are preferably laval nozzles. These openings 6 are disposed concentrically in a ring and are spaced at regular distances apart.

A fuel line 21 extends through the center of the end wall 18. The fuel line 21 extends through the whole of the oxygen chamber 16 and opens into the center body 9, where the forward end of said fuel line terminates in a fuel nozzle 5. The fuel nozzle 5 is the center nozzle of the burner 1.

The center body 9 is also provided with a number of channels which extend axially through the whole body. These channels have a circular cross-section and comprise a first rearward part 4 and a second forward part 3, this latter channel part having a larger diameter than the rear channel part. The rear channel part 4 functions as a constriction for the forward channel part 3, which forms an oxygen nozzle.

The forward circular area of the center body 9 is spaced from the forward end of the casing 12. The distance from the forward end of the casing 12 or the cooling jacket 13 to the forward side of the center body 9 is normally 0.5-1.5 times the internal diameter of the casing.

When the burner 1 is in use, cooling water is delivered through the inlet 14 and flows through the cooling jacket 13 and is discharged through the outlet 15. A gaseous fuel, for instance natural gas, is passed through the line 21 to the nozzle 5, from where it is spread in front of the center body 9. Oxygen is delivered through the inlet 19 to the oxyge chamber 16, where a pressure of up to 5 bars is built up. Th oxygen leaves the chamber 16 partly through the opening 6 i the forward, wall-like part 8 of the center body 9 and partl through the channels 4, and thereafter through the channel 3 forming the oxygen nozzles 3. The respective diameters o the channel 4 and channel 3 are selected so that the oxyge leaves the oxygen nozzle 3 at a speed of 50-150 meters pe second, preferably about 100 meters per second or lower. The peripheral oxygen nozzles 6 are configured so tha oxygen leaves these nozzles at supersonic speed, i.e. at speed of from 350-500 meters per second. These nozzles will preferably form laval nozzles.

The size of the channels 4 and the peripheral oxyge nozzles, and also the number of such channels and nozzles is selected so that about 25-75 percent of the oxygen will leave the oxygen chamber 16 through the openings 3. The remaining part of said oxygen leaves the oxygen chamber through the peripheral nozzle 6. As a result of the low speed at which the oxygen leaves the burner through the opening 3, and the positioning of these openings between the fuel nozzle and the peripheral nozzle 6, the fuel will first come into contact with oxygen exiting from the low-speed nozzles 3. The subsequent reac- tion, i.e. combustion, is sub-stoichiometric and takes place in a first combustion zone nearest the burner.

The oxygen flowing through the laval nozzles 6 moves at high speed and is located radially further away from the nozzle than the oxygen which exits from the openings 3. Consequently, this oxygen will come into peripheral contact with residual fuel, and also the reaction products, in an axial direction more distal from the burner than the first combustion zone and form a second combustion zone. These combustion zones normally overlap one another, i.e. they merge with one another and a total combustion zone. Due to the high speed of the peripheral oxygen, ambient atmosphere will be drawn into the oxygen which is therewith diluted and the flame or reaction temperature consequently lowered. Consequently, the reaction rate at which nitrous gases, NO_x, are formed will be lower. The nitrogen consumed in forming these nitrous gases may come from the ambient atmosphere or from the fuel. Oxygen is supplied to the burner in an amount corresponding to substantially stoichiometric combustion, preferably with a small oxygen surplus of at most 5-10 percent.

According to one preferred embodiment, the inventive burner is intended for burning fuel oil or some other liquid fuel.

To this end, particular advantage is afforded when the burner 1 is provided with a third nozzle 24. This third nozzle 24 is configured as an annular channel in the center body 9, this channel surrounding the fuel nozzle 5 con¬ centrically and being located at a shorter distance from the center axis of the burner 1 than the oxygen nozzle 3. The rear part of this third oxygen nozzle is connected with the oxygen chamber 16 and consists in an annular channel 25 and has a relatively large width exten¬ sion. The channel 25 merges with a channel 26 of smaller width in the center body 9. The width and diameter of the channel 25 are so adapted that 5-10 percent of the oxygen delivered to the oxygen chamber 16 exits through the nozzle 24. According to this preferred embodiment, the fuel nozzle delivers the liquid fuel in the form of a conical mantle or spray.

When practicing the preferred method and using liquid fuel, the fuel is supplied to the line 19 at a pressure of 20-30 bars. The fuel leaves the nozzle 5 in the form of a conical spray or mantle, the cone angle being dependent on the nozzle configuration. The fuel is therewith finely divided into discrete liquid droplets or is atomized. It is particularly preferred to configure the nozzle 5 so that the fuel is given a swirling motion as it exits from the nozzle. The oxygen exiting from the third, annular nozzle 24 will preferably flow in the form of a hollow cylinder externally of the diverging fuel jet and impinges on said jet at a distance from the forward side 8 of the center body 9. The oxygen jet exiting from the nozzle 24 may also diverge slightly. The oxygen jet exiting from the third nozzle 24 is given an annular cross-sectional shape in order to limit the radial propagation of the fuel.

When using an inventive burner which includes the third oxygen nozzle, 5-10 percent by volume of the oxygen supplied to the burner will flow through this third nozzle 24, whereas 70-20 percent will pass through the peripheral nozzle 6 and 20-70 percent through the nozzle 3. The oxygen pressure in the chamber 16 is 4-5 bars. Liquid fuel is delivered to the burner in an amount of 50-1000 liters per hour. Combustion with liquid fuel takes place in a similar manner in two zones, wherein combustion in the first zone consumes oxygen from the third nozzle 24 and the oxygen nozzle 3. Combustion is effected with an oxygen surplus of at most 5-10 percent also in this case.

The embodiment which includes the third nozzle may also be used for gaseous fuel.

The peripheral oxygen nozzle 6 may be replaced with an annular oxygen nozzle having a width which widens in the forward direction of said nozzle. The annular nozzle is preferably configured so that at least substantially the same flow profile is obtained as that obtained through a laval nozzle. The nozzles 3 through which oxygen exits at a low speed may also be replaced with an annular nozzle. These nozzles are configured so that the oxygen will leave the burner in at least substantially mutually parallel jets parallel with the burner center line.

The inventive burner can be used with liquid fuel without using, for instance, air for atomizing the fuel. Because lower flame temperatures are obtained when using the inventive burner, the formation of nitrous gases due to the presence of nitrogen in the fuel and the supply of air is suppressed.

According to the present invention, oxygen is to be interpreted as an oxygen containing gas containing at least 90 per cent by volume of oxygen, preferably at least 95 per cent by volume of oxygen. It is especially preferred that the gas contains at least 99 per cent by volume of oxygen. By oxygen nozzle is meant to define a nozzle for the oxygen containing gas comprising at least 90 per cent by volume of oxygen.

Claims

1. A method of combusting fossil fuel, preferably gaseous or liquid fuel with oxygen, in which the fuel is delivered to a combustion zone from a central exit nozzle, oxygen at a low speed is delivered in the form of at least one first jet at a first radial distance from the central fuel jet; and oxygen at a high speed is delivered in the form of at least one second jet at a second radial distance from the central fuel jet, wherein said second distance is greater than said first distance, such as to produce incomplete combustion of the fuel with the oxygen of low speed in a first region and complete combustion with the oxygen of high speed in a second region downstream of the first region, said regions together forming the combustion zone, c h a r a c t e r i z e d in that the exit velocity of the second jet is exceeds the sonic velocity.

2. A method according to Claim 1, c h a r a c t e r ¬ i z e d in that the first oxygen jet consists of several oxygen jets which are disposed in a ring concentrical with the center exit nozzle.

3. A method according to Claim 1, c h a r a c t e r i ¬ z e d in that the first oxygen jet consists of a jet of annular cross-section which surrounds the central exit nozzle concentrically therewith.

4. A method according to Claim 1, 2 or 3, c h a r ¬ a c t e r i z e d in that the second oxygen jet consists of several oxygen jets which are disposed in a ring concentrical with the center exit nozzle.

5. A method according to one or more of Claims 1-4, in which the fuel is a liquid fuel, c h a r a c t e r i z e d by causing the fuel to exit from the center nozzle in the form of a conical mantle, said fuel being finely divided, and by restricting the radial propagation of the conical fuel jet with the aid of an annular third oxygen jet located at a shorter radial distance from the center exit nozzle than the first oxygen jet.

6. A method according to Claim 6, c h a r a c t e r - i z e d in that the third oxygen jet contains 4-10 percent by volume of the total amount of oxygen delivered.

7. A burner for burning fossil fuel with oxygen, preferably gaseous or liquid fuel, said burner (1) comprising a center exit nozzle (5) for delivering the fuel to a combustion zone, at least one first oxygen nozzle (3) which is located at a first radial distance from the center exit nozzle (5) and which is intended to deliver oxygen at low speed, at least one second oxygen nozzle (6) which is located at a second radial distance from said center exit nozzle (5) and which is intended to deliver oxygen at high speed, said second distance being greater than said first distance, such as to generate incomplete combustion of the fuel with the oxygen of low speed in a first region and complete combustion of the fuel with the oxygen of high speed in a second region downstream of said first region, said regions together forming the combustion zone, c h a r a c t e r i z e d in that the second oxygen nozzle (6) is configured as a laval nozzle.

8. A burner according to Claim 7, c h a r a c t e r ¬ i z e d in that the first oxygen nozzle (3) consists of several oxygen nozzles which are disposed in a ring con¬ centrical with the center exit nozzle (5) .

9. A burner according to Claim 8, c h a r a c t e r - i z e d in that the first oxygen nozzle (3) is an annular nozzle which concentrically surrounds the center exit nozzle.

10. A burner according to Claim 7, 8 or 9, c h a r ¬ a c t e r i z e d in that the second oxygen nozzle (6) consists of several oxygen nozzles which are disposed in a ring concentrical with the center exit nozzle (5) .

11. A burner according to Claim 8, in which the fuel is a liquid fuel which exits from the center nozzle (5) in the form of a conical mantle, preferably with a swirling motion, wherein the fuel is finely divided, c h a r a c t e r - i z e d by at least one third oxygen nozzle (24) which is located radially between the center exit nozzle (5) and the first oxygen nozzle (3) and which limits the radial propa- gation of the conical fuel jet.

12. A burner according to Claim 14, c h a r a c ¬ t e r i z e d in that the third oxygen nozzle (24) is nozzle of annual cross-section.