GB2226121A

GB2226121A - Burner with water supply

Info

Publication number: GB2226121A
Application number: GB8829519A
Authority: GB
Inventors: Brian J Wills; Uwe Voigt
Original assignee: Fives North America Combustion UK Ltd
Current assignee: Fives North America Combustion UK Ltd
Priority date: 1988-12-17
Filing date: 1988-12-17
Publication date: 1990-06-20
Anticipated expiration: 2008-12-17
Also published as: GB8829519D0; GB2226121B

Abstract

A burner (11) has an atomiser (33) for atomising water and delivering a mist of water droplets with fuel to a position where the fuel mixes with preheated air and burns in the air. The burners may be associated with a similar burner in a regenerator, and the fuel may be gas. <IMAGE>

Description

Title:"Burner and method of operating same" Description of Invention The present invention relates to a burner for mixing fuel with air to permit combustion of the fuel and is primarily concerned wth a burner suitable for mixinq a fluent fuel with air.

In order to achieve efficient use of the heat released by combustion, it is common to extract heat from products of combustion which are to be exhausted to the atmosphere and to transfer that heat to combustion air.

One consequence of this is that relatively high flame temperatures are attained and the formation of oxides of nitrogen is promoted.

In U.S. 4,768,949. it is proposed to reduce the formation of oxides of nitrogen in a flame by mixing steam with the combustion air and then imparting to the mixture of steam and combustion air heat which has been recovered from products of combustion. The hot mixture is then mixed with the fuel to support combustion of the fuel. The steam introduced with the combustion air leaves the combustor with the products of combustion and heat is extracted from both this steam and the products of combustion. The published specification indicates that, because heat is recovered from the steam and because the steam supplied with the combustion air is pre-heated, little additional heat input is necessary to raise the temperature of the steam to process temperature and the overall efficiency of the process is not unduly reduced.

The process described in U.S. 4,768,949 does, however, suffer from disadvantages. It is necessary to deliver water to the top of the regenerator bed ond it is also necessary to arrange the flow of gases through the bed so that the top of the bed is the coolest part of the bed. Even with this precaution, there is a risk of the bed being damaged by impingement of relatively cool water and the choice of materials for forming at least the upDer part of the bed is restricted to materials which can withstand thermal shock well. The published specification describes arrangements for draining water away from the bottom of the regenerator bed. The apparatus disclosed is inconvenient and the provision for the addition of steam to the combustion air adds substantially to the overall cost of a burner.

According to a first aspect of the present invention, there is provided a method of operating a burner wherein air is directed along an airflow poth to the burner, a stream of fuel is directed along a fuel flowpath to the burner, a stream of water is directed along a water flowpath towards the burner, the stream of water is disrupted to form droplets, the air, fuel and water droplets are mixed together in the burner and the fuel burns in the air.

By disrupting the stream of water to form droplets, the water can be mixed with the air and the fuel whilst the temperature of the water is sufficiently low for water to be present in the liquid phase. Pre-heating of the water to convert the water to steam and to heat the steam to a relatively high temperature is unnecessary.

In a method in accordance with the first aspect of the invention, the stream of water can be disrupted to form the droplets at a position which is close to a mixing position where the mixture of fuel, air and water is produced. Accordingly, the configuration of apparatus associated with the burner, for example of a regenerator, is not determined by the need to introduce and heat water.

Once formed, the droplets are preferably entrained either in the stream of fuel which flows along the fuel flowpath or in the combustion air which flows along the airflow path. It is usual for the fuel to enter at ambient temperature and for there to be no substantial heating of the fuel before the fuel mixes with pre-heated air. Conveniently, the droplets of water can be entrained in the fuel so that the droplets are carried along the fuel flowpath to the mixing position. Preferably, heating of the water upstream of the mixing position is avoided. It will be understood that, in a case where the air is heated upstream of the mixing position, mixing of the fuel and water with the air will result in the transfer of heat to the fuel and to the water so that the water droplets may be converted to steam.Generally, combustion will occur immediately the mixture of fuel, air and water is formed. The combustion will further heat the water or the steam produced therefrom so that the droplets of water will not survive in the flame.

At least the major part of the water, by weight, is preferably converted to droplets having a size generally described as a mist. By a mist, we mean a suspension of water droplets in air or another gas in which the water droplets are sufficiently small not to separate readily from still air, under the action of gravity.

For disrupting the stream of water, there may be provided a device generally termed an atomiser.

According to a second aspect of the invention, there is provided a burner comprising means defining a mixing position, an air flowpath leading to the mixing position, a fuel flowpath leading to the mixing position and a water flowpath leading towards the mixing position, wherein there is further provided means for disrupting a stream of water flowing along the water flowpath to provide water droplets which flow to the mixing Position.

An example of a burner according to the second aspect of the invention and which is used in a method embodying the first aspect of the invention will now be described, with reference to the accompanying drawing, wherein: FIGURE I shows a block diagram of two burners and further parts associated with a furnace and; FIGURE 2 shows a cross-section through one of the burners of Figure The apparatus illustrated in Figure 1 comprises a furnace chamber 10 in which heat is required to be imparted to a heating load, which may be materials or workpieces introduced into the chamber 10 or which may be tubes or other heat exchange elements containing a medium which is to be heated. Burners II and 12 are mounted in or on walls of the chamber 10 to be operated alternately to deliver a burning mixture of fuel and air into the chamber, where combustion takes place. The products of combustion are exhausted from the chamber through that burner which is not firing.

Respective reaenerators 13 and 14 are associated with the burners II and 12 in a known manner. The regenerators may be of known construction and may be separate from the burners or incorporated therein. The regenerators communicate alternately via a changeover valve 15 with a source of combustion air, which is generally in the form of a fan 16. The changeover valve also provides for communication between one of the regeneretors not connected with the fan and a stack through which products of combustion are exhausted to the atmosphere.

Meons is provided for defining a fuel flowpath from a bulk supply of ftuent fuel, for example a gas main 17, to the burners II and 12. By way of example, there is represented in Figure 1, a fuel supply pipe 18 leading from the gas main to the burner II and a fuel supply pipe 19 leading from the gas main to the burner 12. In the fuel supply pipes, there are provided respective control valves 20 and 21.

There is further provided means defining a water flowpath along which water can flow to each of the burners 11 and 12. This means comprises a bulk supply of water, for example a water main 22, a water supply pipe 23 leading from the water main to the burner 11 and containing a control valve 24 and a further water supply pipe 25 leading from the water main to the burner 12 and containing a control valve 26.

The burner 11 may be of generally known construction and is disposed at least partly in an opening in a wall 27 of the furnace chamber 10. The burner defines a mixing position 28 where, during operation of the burner in a firing mode, there is produced a mixture comprising combustion air, fuel and water. Typically, there is provided at the mixing position a flame stabiliser 29 which promotes turbulence at the mixing position. Known means (not shown) is provided for igniting the fuel in the burner.

The burner includes a hollow body 30 which, in the example illustrated, is elongated and within which the mixing position 28 is disposed. A tube 31 extends along the interior of the body 30 from one end thereof to the mixing position. The fuel supply pipe 18 communicates with the tube 31 at an end of the burner which is remote from the chamber 10 so that the interior of the tube constitutes a downstream portion of the fuel flowpath to the mixing position. The space inside the body 30 surrounding the tube 31 constitutes a downstream portion of the air flowpath to the mixinq position and communicates with the regenerator 13 through a duct 32.

A downstream end portion of the water supply pipe 23 extends along the interior of the tube 31 towards the mixing position but terminates at a position inside the tube and spaced somewhat from the end thereof which is immediately adjacent to the mixing position. At this end of the water supply pipe, there is provided means for disrupting a stream of water flowing along the pipe towards the mixing position. This means is an atomiser 33 which, in the exarrple i1ustroted, is a twin fluid atomiser. A compressed air line 34 alsx expends along the interior of the tube 31 to the atomiser. The airline comminicates via a control valve 35, with a source 36 of compressed air.

During operation of the burner II in the firing mode, air flows from the fan !6 through the valve 15 and the regenerator 13 to the mixing position in the burner. After start-up, the regenerator 13 supplies heat to combustion air flowing through the regenerator to the burner 11 so that the air which enters the burner is at an elevated temperature, for example in the region of 700 0C. A stream of gaseous fuel flows along the interior of the tube 31 to the mixing position. A stream of water flows along the water supply pipe 23 to the atomiser 33 and is disrupted in the atomiser to form a mist of water droplets which is discharged into the stream of fuel and is carried with the fuel along the tube 31 to the mixing position. Neither the fuel nor the air is heated substantially, prior to reaching the mixing position.To reduce transfer of heat from the combustion air to the fuel and to the mist of water, the tube 31 may incorporate thermal insulation and may be of double-well construction.

At the mixing position 28, the air, fuel and water mix together and the fuel burns in the air, producing a flame which emerges from the burner 11 into the chamber 10. The hot products of combustion are exhausted from the chamber 10 through the burner 12 and the regenerator 14 to the changeover valve 15. As the products of combustion, including steam produced by evaporation of the water atomised by the atomiser 33, passes through the regenerator 14, heat is transferred to the regenerator.

The stream of water which flows along the water supply pipe 23 is atomised by the combined effect of the atomiser 33 and compressed air from the line 34. Effective atomisation can be sustained even when the rate of flow of water is varied throughout a wide range, for example by a factor of 20.

The rate at which fuel and air are delivered to the mixing position is varied according to the heating load in the chamber 10. It will beunderstood that, when the air and fuel are delivered at a relatively low rate, the rate of supply of water to the atomiser also is required to be relatively low.

The burner 12 is identical with the burner 11 and is provided with an airline 37 for the supply of compressed air from the source 36 to an atomiser corresponding to the atomiser 33. When the burner 12 is operating in a firing rrS combustion air flows to that burner through the regenerator 14 and is hefted therein. The air is mixed with fuel and with water droplets at the mixing position of the burner 12 and a burning mixture is delivered from this burner into the combustion chamber 10. The hot products of combustion are exhausted from the chamber through the burner II and pass from that burner along the duct 32 to the regenerator 13. Whilst the burner 12 is operating in the firing mode, the valves 20, 23 and 35 are closed. There is no flow of products of combustion into the tube 31.

We have found that the rate at which water must be introduced to the mixing position as a mist in a method in accordance with the present invention to achieve a certain concentration of oxides of nitrogen in the flue gases is less than the rate at which steam must be supplied in the manner described in U.S. 4,768,949 to attain the same concentration of oxides of nitrogen in the flue gases, when fuel is being burned at the same rate and under corresponding conditions.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

I. A method of operating a burner wherein air is directed along an air flowpath to the burner, a stream of fuel is directed along a fuel flowpath to the burner, a stream of water is directed along a water flowpath towards the burner, the stream of water is disrupted to form droplets, the air, fuel and water droplets are mixed together in the burner and the fuel burns in the air.
2. A method of operatina a burner wherein air is directed along an air flowpath to the burner, a stream of fuel is directed along a fuel flowpath to the burner, water is atomised to form a mist, the air, fuel and the mist are mixed together in the burner and the fuel burns in the air.
3. A method according to Claim I or Claim 2 wherein the water is mixed with the fuel to form a mixture of fuel and water and this mixture is mixed with the air.
4. A method according to Claim I wherein there is no substantial transfer of heat to the water in the water flowpath prior to mixing of the water with the air and the fuel.
5. A burner comprising means defining a mixinq position, an air flowpath leading to the mixing position, a fuel flowpath leading to the mixing position and a water flowpath leading towards the mixinq position, wherein there is further provided means for disrupting a stream of water flowing along the water flowpath to provide water droplets which flow to the mixing position.
6. A burner according to Claim 5 wherein said means for disrupting a stream of water is disposed in the fuel flowpath so that the droplets flow to the mixing position with the fuel.
7 A burner according to Claim 5 wherein said means defining the fuel frlowpoth comprises a tube having an open end adjacent to the mixing position and wherein said means for disrupting a stream of water is disposed in the tube to deliver the water droplets along the interior of the tube to the mixing position.
8. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.