GB2146112A - Split stream burner assembly - Google Patents

Abstract

A burner assembly is disclosed in which an inlet is located at one end of an annular passage (26) defined by inner (22) and outer (20) tubular members for receiving fuel, and an outlet is located at the other end of the passage for discharging the fuel. A plurality of members (60) are disposed within the annular passage for splitting up the fuel discharging from said outlet so that, upon ignition of such discharging fuel, a plurality of flame patterns are formed. <IMAGE>

Description

1 GB 2 146 11 2A 1

SPECIFICATION

Split stream burner assembly This invention relates to burner assemblies and is specifically directed at a burner as sembly which operates in a manner to reduce the formation of nitrogen oxides as a result of fuel combustion. It is particularly concerned with the combustion of coal in such burners, typically in pulverized form.

Considerable attention and efforts have re cently been directed to the reduction of nitro gen oxides resulting from the combustion of fuel, and especially in connection with the use 80 of coal in the furnace sections of relatively large installations such as vapour generators and the like. In a typical arrangement for burning coal in a vapour generator, several burners are disposed in communication with 85 the interior of the furnace and operate to burn a mixture of air and pulverized. coal. The burners used in these arrangements are gener ally of the type in which a fuel air mixture is continuously injected through a nozzle so as to form a single relatively large flame. As a result, the surface area of the flame is rela tively small in comparison to its volume, and therefore the average flame temperature is relatively high. However, in the burning of coal, nitrogen oxides are formed by the fixa tion of atmospheric nitrogen available in the combustion supporting air, which is a function of the flame temperature. When the flame temperature exceeds 2800F, the amount of 100 fixed nitrogen removed from the combustion supporting air rises exponentially with in creases in the temperature. This condition leads to the production of high levels of nitrogen oxides in the final combustion pro ducts, which causes severe air pollution prob lems.

Nitrogen oxides are also formed from the fuel bound nitrogen available in the fuel itself, which is not a direct function of the flame temperature, but is related to the quantity of available oxygen during the combustion pro cess.

In view of the foregoing, attempts have been made to suppress the burner and flame temperatures and reduce the quantity of avail able oxygen during the combustion process and thus reduce the formation of nitrogen oxides. Attempted solutions have included techniques involving two stage combustion, flue gas recirculation, the introduction of an oxygen-deficient fuel-air mixture to the burner and the breaking up of a single large flame into a plurality of smaller flames. However, although these attempts singularly may pro duce some beneficial results they have not resulted in a reduction of nitrogen oxides to minimum levels. Also, these attempts have often resulted in added expense in terms of increase construction costs and have lead to other related problems such as the production of soot and the like.

According to the present invention, a burner assembly comprises inner and outer coaxial tubular members defining an annular passage; an inlet located at one end of the annular passage for receiving fuel, an outlet located at the other end of said passage for the discharge of fuel therefrom; a plurality of blocks in circurnferentially spaced relationship in said annular passage, one end of each block extending in the outlet, and the other end having a curved surface for directing fuel between the blocks for splitting up the discharge of fuel so that upon ignition thereof, a plurality of flame patterns are formed, the blocks extending between the tubular members and being tapered in a direction from the outer to the inner tubular member. The invention enables the surface area of the flame per unit volume to be increased which results in a greater flame radiation, a lower flame temperature, and a shorter residence time of the gas component within the flame at maximum tem- perature. The assembly may thus operate in a manner which considerably reduces the production of nitrogen oxides in the combustion of fuel without significant increase in cost or other related problems.

Stoichiometric combustion of the fuel in a burner assembly of the invention can be regulated to reduce the quantity of available oxygen during the combustion process and achieve an attendant reduction in the formation of nitrogen oxides. The provision of secondary air is also contemplated, such secondary air being directed toward the burner outlet in two parallel paths with register means being disposed in each path for indivi- dually controlling the flow of air through each path.

Reference is directed to parent Application No: 81 17154 as early published Specification No: 2 079 925A. The entire disclosure of

Specification No: 2 079 925A is incorporated herein by reference. Application No: 81 17154 describes and claims a burner assembly comprising means defining an annular passage; an inlet located at one end of the passage for receiving fuel, means disposed in the passage for dividing fuel passing therethrough into two radially separate streams, an outlet located at the other end of the passage for the discharge of such separate fuel streams therefrom; means disposed within said annular passage for splitting up one such fuel stream discharging from the outlet so that upon ignition thereof, a plurality of flame patterns are formed.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings wherein:

Figure 1 is a sectional view depicting a burner assembly according to the invention; 2 GB 2 146 112A 2 Figure 2 is a partial perspective view of a component of the burner assembly of Figure 1; Figure 3 is an enlarged elevational view, partially cut-away, of the burner portion of the 70 assembly of the present invention; and Figure 4 is a perspective view of a compc nent of the burner portion of Figure 3.

In Figure 1, reference numeral 10 refers in general to a burner assembly which is dis posed in axial alignment with a through open ing 12 formed in a front wall 14 of a conven tional furnace. It is understood that the fur nace includes a back wall and side walls of an appropriate configuration to define a combus- 80 tion chamber 16 immediately adjacent the opening 12. Also similar openings are pro vided in the furnace front wall 14 for accom modating additional burner assemblies identi cal to the burner assembly 10. The inner surface of the wall 14 as well as the other walls of the furnace are lined within an appro priate thermal insulation material 18 and, while not specifically shown, it is understood that the combustion chamber 16 can also be lined with vertically extending boiler tubes through which a heat exchange fluid, such as water, is circulated in a conventional manner for the purposes of producing steam.

It is also understood that a vertical wall is disposed in a spaced parallel relationship with the furnace wall 14 in a direction opposite that of the furnace opening 12 along with correspondingly spaced top, bottom and side walls to form a plenum chamber, or wind box, 100 for receiving combustion supporting air, corn monly referred to as -secondary air", in a conventional manner.

The burner assembly 10 includes a nozzle 20 having an inner tubular member 22 and an outer tubular member 24. The outer tubu lar member 24 extends over the inner tubular member 22 in a coaxial, spaced relationship thereto to define an annular passage 26 which extends towards the furnace opening 12.

A tangentially disposed iniet 28 communi cates with the outer tubular member 24 for introducing a stream of fuel into the annular passage 26 as will be explained in further detail later.

A pair of spaced annular plates 30 and 32 extend around the burner 20, with the inner edge of the plate 30 terminating on the outer tubular member 24. A liner member 34 ex tends from the inner edge of the plate 32 and in a general longitudinal direction relative to the burner 20 and terminates adjacent the insulation material 18 just inside the wall 14.

An additional annular plate 38 extends around 125 the burner 20 in a spaced, parallel relation with the plate 30. An air divider sleeve 40 extends from the inner surface of the plate 38 and between the liner 34 and the nozzle 20 in substantially parallel relation to the burner 20130 and the liner 34 to define two air flow passages 42 and 44.

A plurality of outer register vanes 46 are pivotally mounted between the plates 30 and 32 to control the swirl of secondary air from the wind box to the air flow passages 42 and 44. In a similar manner a plurality of inner register vanes 48 are pivotally mounted between the plates 30 and 38 to further regu- late the swirl of the secondary air passing through the annular passage 44. It is understood that although only two register vanes 46 and 48 are shown in Figure 1, several more vanes extend in a circumferentially spaced relation to the vanes shown. Also, the pivotal mounting of the register vanes 46 and 48 may be done in any conventional manner, such as by mounting the vanes on shafts (shown schematically in Figure 1) and jour- nailing the shafts in proper bearings formed in the plates 30, 32 and 38. Also, the position of the vanes 46 and 48 may be adjustable by means of cranks or the like. Since these types of components are conventional they are not shown in the drawings nor will be described in any further detail.

The quantity of air flow from the wind box into the register vanes 46 is controlled by movement of a sleeve 50 which is slidably disposed on the outer periphery of the plate 32 and is movable parallel to the longitudinal axis of the burner nozzle 20. An elongated worm gear 52 is provided for moving the sleeve 50 and is better shown in Figure 2. The worm gear 52 has one end portion suitably connected to an appropriate drive means (not shown) for rotating the worm gear and the other end provided with threads 52a. The worm gear 52 extends through a bushing 54 (Figure 1) which is attached to the plate 30 to provide rotatable support. The threads 52a of the worm gear 52 mesh with appropriate apertures 56 formed in the sleeve 50 so that, upon rotation of the worm gear, the sleeve moves longitudinally with respect to the longitudinal axis of the burner 20 and across the air inlet defined by the plates 30 and 32. In this manner, the quantity of combustion supporting air from the wind box passing through the wind box passages 42 and 44 can be controlled by axial displacement of the sleeve 50. A perforated air hood 58 extends between the plates 30 and 32 immediately downstream of the sleeve 50 to permit the air flow to the burner 20 to be independently determined by means of static pressure differential movements, in a conventionel manner.

Figure 3 depicts the details of the burner nozzle 20. The end portion of the outer tubular member 24 and the corresponding end portion of the inner tubular member 22 are tapered slightly radially inwardly toward the furnace opening 12. A plurality of divider blocks 60 are circumferentially spaced in the 3 GB 2 146 112A 3 annular space between the tubular members 22 and 24 in the outlet end portion of the burner. Four such blocks are spaced at 90 intervals and extend from the outlet to a point approximately midway the tapered portions of the members 22 and 24 and as shown, the radially inner boundaries of the blocks 60 are spaced from the inner tubular member 22.

The side portion of the blocks 60 are curved to complement the corresponding curved surf faces of the tubular members 22 and 24 and the blocks are tapered radially inwardly. As shown in Figure 4, the leading end portion of each block 60 is configured in a curved relationship so that the fuel flowing in the 80 passage 26 and impinging against the leading ends of the blocks 60 will be directed into the adjacent spaces defined between the blocks to facilitate the splitting of the fuel stream into four separate streams.

In operation of the burner assembly of the present invention, the movable sleeve 50 as sociated with each burner is adjusted during initial start up to accurately balance the air to each burner. After the initial balancing, no further movement of the sleeves 50 are needed since normal control of the secondary air to the burners is accomplished by oper ation of the outer register vanes 46.

Fuel, preferably in the form of pulverized coal suspended or entrained within a source of primary air, is introduced into the tangential inlet 28 where it swirls through the annular chamber 26 and is ignited by suitable igniters (not shown) appropriately positioned with respect to the burner nozzle 20. The stream of fuel and air encounters the blocks 60 at the end portion of the nozzle 20 whereby the stream is split into four equally spaced streams which, upon ignition, form four separate flame patterns. The igniters are shut off after steady state combustion has been achieved and secondary air from the wind box is admitted through the perforated hood 58 and into the inlet between the plates 110 and 32. The axial and radial velocities of the air is controlled by the register vanes 46 and 48 as it passes through the air flow passages 42 and 44 and into the furnace opening 12 for mixing with the fuel from the burner 20.

Several advantages result from use of the burner assembly described herein. For example, since the pressure drop across the perforated air hoods 58 associated with burner assemblies can be equalized by balancing the secondary air flow to each burner by initially adjusting the sleeves 50, a substantially uniform gas distribution can be obtained across the furnace. This also permits a com- mon wind box to be used and enables the unit to operate at lower excess air with signifi cant reductions in both nitrogen oxides and carbon monoxides. Also, the provision of se parate register vanes 46 and 48 for the outer130 and inner air flow passages 42 and 44 enables secondary air distribution as well as flame shape to be independently controlled resulting in a significant reduction of nitrogen oxides, and a more gradual mixing of the primary air coal stream with the secondary air since both streams enter the furnace on parallel paths with controlled mixing.

Further, the provision of multiple flame pat- terns results in a greater flame radiation, a lower average flame temperature and a shorter residence time of the gas components within the flame at a maximum temperature, all of which, as stated above, contribute to reduce the formation of nitric oxides.

Also, the use of the curved surface 60a on the blocks results in a more streamline flow of the fuel stream before it discharges from the outlet of the nozzle 20. Still further, the provision of the tangential inlet 26 provides excellent distribution of the fuel around the annular space 26 in the burner 20 resulting in more complete combustion and reduction of carbon loss and making it possible to use individual burners with capacities significantly higher than otherwise could be used.

Finally, since the arrangement permits the admission of air at less than stoichmetric, overfire air ports or the like can be provided as needed to supply air to complete the combustion.

Claims (7)

1. A burner assembly comprising inner and outer coaxial tubular members defining an annular passage; an inlet located at one end of the annular passage for receiving fuel, an outlet located at the other end of said passage for the discharge of fuel therefrom; a plurality of blocks in circumferentially spaced relationship in said annular passage, one end of each block extending in the outlet, and the other end having a curved surface for directing fuel between the blocks for splitting up the discharge of fuel so that upon ignition thereof, a plurality of flame patterns are formed, the blocks extending between the tubular members and being tapered in a direction from the outer to the inner tubular member.

2, A burner assembly according to Claim 1 wherein the radially inner boundaries of the blocks are spaced from the inner tubular member.

3. A burner assembly according to Claim 1 or Claim 2 including an enclosure extending around the tubular members for receiving air, means for directing air from the enclosure towards the outlet in two parallel paths extending around said tubular members, and register means respectively disposed in each path for regulating the flow of air along the paths.

4. A burner assembly according to Claim 3 including means for regulating the flow of air into the enclosure.

4 GB 2 146 112A 4

5. A burner assembly according to Claim 4 wherein the regulating means comprises a sleeve movable across an inlet to the enclo sure to vary the size of said inlet.

6. A burner assembly according to any preceding Claim including means for directing fuel through the inlet and into the annular passage in a tangential direction relative to the passage.

7. A burner assembly substantially as de scribed herein with reference to the accom panying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935. 1985. 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A I AY, from which copies may be obtained.