WO2006082169A1

WO2006082169A1 - Flare stack

Info

Publication number: WO2006082169A1
Application number: PCT/EP2006/050517
Authority: WO
Inventors: Peter Janssens; Geert Dumortier
Original assignee: Nv Bekaert Sa; Bekaert Combustion Technology Nv
Priority date: 2005-02-02
Filing date: 2006-01-30
Publication date: 2006-08-10

Abstract

A flare stack for combustion of combustible fluids as subject of the invention comprises a gas feed pipe and at least one burner element for combustion of the combustible fluids. The burner element comprises a gas inlet being adapted to receive combustible gas from the gas feed pipe. The flare stack is characterized in that the gas inlet of each of the burner elements comprises a venturi and a gas injector nozzle for each of these venturis. The nozzle is aligned with the venturi for drafting primary combustion air with the combustible gas when injected by the nozzle in the venturi.

Description

FLARE STACK

Field of the invention

The present invention relates to flare stacks and more in particular to ground flare stacks for flaring combustible fluids.

Background of the invention

Flare stacks are widely used for combustion of combustible fluids such as waste gasses such as occurring at gas- or oil drilling sites, or liquids or process gasses at various chemical and petrochemical applications.

Most widely used flare stacks are of the open combustion type and combust fluids by means of a flame, where a burner assembly is mounted on top of a high stack. The combustion is done using open flames, possibly assisted by steam or compressed air for creating turbulent gas streams. Such combustion may cause not only incomplete combustion, but also may cause thermal nuisance, noise and/or light pollution. An example is provided in US5649820.

As an alternative, enclosed combustion may be used for flaring such waste or process fluids. As an example, NL1011009 describes such enclosed burner assembly for combustion of combustible gasses. Also JP53-98530 describes a flare stack using enclosed combustion of fluids.

The presently known flare stacks, either using open or closed combustion, have the disadvantage that the dimensions are determined to a large extent by the light emission and accompanying heat emission. In case of open flame combustion, the burner head is to be placed at significant height in order to avoid scorching of the surface beneath the flare stack. In case of closed combustion, the walls of the flare stack are to be dimensioned significantly large in order to avoid light emission and radiation in case the flare is operating under full capacity.

Summary of the invention

It is an object of the present invention to provide a flare stack which overcomes the disadvantages of the flare stack according to the presently known prior art.

It is an object of the present invention to reduce the light- and noise emission of flare stacks. It is also an object of the present invention to reduce the risk of dangerous temperature increases due to heat radiation from the flame of the flare stacks. It is an object of the present invention to reduce the height of the flare stacks. It is an object of the present invention to provide a more complete combustion of combustible gasses such as e.g. waste gasses or liquids or process gasses from various chemical and petrochemical processes, waste gasses of oil or gas drilling or biogas.

The above-mentioned advantageous effects are realized by a flare stack having the specific features set out in claim 1.

Specific features for preferred embodiments are set out in the dependent claims.

A flare stack of the invention comprises at least one, but preferably more than one burner element, which is provided with a partially mixed air-combustible gas mixture. This partially mixed air-combustible gas mixture is obtained by injecting combustible gas from the gas feed pipe using an injection nozzle, into the venturi at the gas inlet of the burner element. This mixture, which is preferably guided via a mixing chamber to a first side of a gas permeable combustion surface, is combusted at the opposite side of the gas permeable combustion surface.

As already a mixture of combustible gas and air is present at the moment of combustion, a blue flame combustion of the combustible gas is obtained to a large extent. Only in order to complete the combustion, a small amount of secondary combustion air is necessary from the vicinity, resulting in a minor amount of yellow flame combustion.

As a result, less or even no yellow flames occur, which directly results in less light emission to the environment. The flame length of the flare stack of the invention is much smaller as compared to prior art, which results in less high walls to be provided around the burner elements as compared to other enclosed burners used as flare stacks. Further, because less secondary air is to be provided to the combustion zone of the flare stack of the invention, as compared to the presently known flare stacks, the measures to be taken in order to provide sufficient air for combustion, are simplified. Because of the blue flame combustion of the combustible gas, the combustion is more complete and efficient. As less light is created by the combustion, the heat radiation by means of visible and infrared light is less. Surprisingly the noise emission of a flare stack of the invention is less than flare stacks as presently known.

Preferably the flare stack of the invention comprises more than one burner element, such as two, three, four, five, six, seven, eight, nine, ten or even more burner elements. These burner elements each comprise a gas inlet which is provided with a venturi and a gas permeable combustion surface, which gas inlet and combustion surface are coupled by means of a mixing chamber. The number of injector nozzles provided in the gas feed pipe is identical to the number of Venturis of the gas inlets provided by the burner elements. Each injector nozzle is aligned with one of the gas inlets of one of the burner elements. When gas is injected to each of the Venturis by means of one injector nozzle, in each mixing chamber of a burner element, a mix of combustible gas and primary air is obtained, this is discharged to the combustion zone by means of the gas permeable combustion surface. When the mixture is discharged through the gas permeable combustion surface, the mixture is ignited and combusted resulting in a blue flame combustion.

Preferably the burner elements of the flare stack are all aligned with each other. Preferably they are mounted coplanar. Even more preferred, they are mutually equidistant and substantially parallel.

Preferably the burner elements are tubular burner elements. Such tubular burner elements may have a first side at which the gas inlet according to the invention is provided, the other side of the tubular burner is closed in a gastight way. A tubular volume couples the gas inlet and gastight opposite end of the tubular burner and functions as a mixing chamber, in which the primary air and the combustible gas, injected in the tubular burner by means of the injector nozzle and venturi, mixes. The tubular burner comprises a surface zone which functions as the gas permeable combustion surface and which is coupled to the gas inlet by this tubular volume. The gas permeable combustion surface may be obtained by perforating the tubular surface, or by replacing part of the tubular surface by a metal fiber membrane, possibly supported by a metallic grid. It is understood that tubular burners may have many different cross sectional shapes such as round, oval, square or rectangular.

Preferably the tubular burner elements of the flare stack are all aligned with each other. Preferably they are coplanar mounted, most preferred in a substantially horizontal plane. Even more preferred, they are mutually equidistant and substantially parallel. Preferably the gas permeable combustion surfaces substantially are facing away from each other and most preferred, the combustion surfaces are provided in a way that during combustion, the flame front is pointing vertically upwards.

This applies as well to non-tubular burner elements as part of the flare stack as subject of the invention. Preferably the gas permeable combustion surfaces are coplanar mounted, most preferred in a substantially horizontal plane. Even more preferred they are mutually equidistant and substantially parallel. Preferably the gas permeable combustion surfaces substantially are facing away from each other and most preferred, the combustion surfaces are provided in a way that during combustion, the flame front is pointing vertically upwards

The gas permeable combustion surface may be provided in many different ways. It is of importance that the combustion surface comprises apertures for allowing combustible gas through the surface, which apertures are small enough to prevent the combustible gas to inflame at the gas-side of the combustion surface. Such apertures are known as e.g. small slots or perforations provided in a metal plate or tube, such as e.g. in DE29510374U1. Alternatively a metal fiber burner membrane may be used, as e.g. a woven or knitted metal fiber membrane from WO 97/04152 or a sintered and perforated metal fiber membrane from WO 93/18342.

An advantage of the flare stack of the invention over the presently known flare stacks is that both the gas pressure and the Wobbe-index of the combustible gas may vary over a large range, meanwhile providing a substantially complete combustion.

Wobbe indices of the combustible gas provided by the gas feed pipe may vary over a range of 42 to 85 MJ/Nm³. The gas pressure may vary from 0 mbar to about 35 mbar, preferably, however, less than or equal to 30 mbar. Also the dimensions of the flare stack of the invention compared to the existing flare stacks are significantly reduced for combustion of comparable amounts of gas.

The gas permeable combustion surface is preferably made of a temperature resistant stainless steel alloy such as Aluchrome®- or Fecralloy® -alloys.

The flare stack as subject of the invention may further comprise a housing, in which the burner elements and possibly other elements of the flare stack are mounted. The housing may comprise a wall, being mounted substantially vertically and a base section, preferably being a substantially horizontal plane. The gas permeable combustion surface of the burner elements are enclosed by this wall and the base. The combustion takes place inside the housing. Preferably the gas permeable combustion surfaces of the burner elements are aligned coplanar, the plane being substantially perpendicular to the vertical wall and parallel to the base. The upper side of the wall is defining an open area, which open area may be covered with a roof. The wall may be provided out of metal plate. The base may be provided with apertures to allow additional secondary air to enter the housing. In a preferred embodiment the wall is made of a corrugated metal plate which allows thermal expansion of the wall without creating tension on the assembly of the wall and the base of the flare stack. In a further preferred embodiment, the vertical wall, whether corrugated or not, is enclosed by a second wall made out of metal plate. The space between this second wall and the vertical wall, which is open at the bottom and at the top of the plates, creates an air- insulation layer around the inner corrugated plate, together with a cooling of the corrugated plate because of the airflow that is created by the convective movement of the insulating air layer between the outer metal plate and the inner corrugated plate. A further advantage of this outer plate is that the outer temperature of the flare stack is reduced resulting in a safer system for the environment, e.g. preventing burns or even melting of materials which were put in the direct proximity of the flare stack.

Between roof and the upper side of the wall, sufficient open space is left to allow the exhaust gasses from the combustion to escape.

Such flare stack is provided with combustible gas via its gas feed pipe. At the gas inlets, the gas is injected into the burner elements, drafting primary air along into the burner element. This mixture is combusted at the gas permeable burner surfaces of the burner elements. Because of the combustion, exhaust gasses are created which may leave the housing via the open space between roof and upper side of the wall. This exhaust gas movement causes a draft of secondary air into the housing via the apertures which are present below the burner elements. This secondary air may be used to complete the combustion, in case the amount of primary air was not sufficient for complete combustion of the combustible gas. The wall of the housing protects the combustion at its inner side from extinguishing because of natural movement of air such as wind. The roof of the housing protects the combustion at its inner side from extinguishing because of rain, snow or other natural precipitation.

The apertures in the base may be provided as large opening below the burner elements. The apertures may be provided with a flame arrestor.

Such flame arrestor may be a metal grid or a metal fiber woven or knitted fabric such as described in e.g. WO 97/04152.

In order to prevent the combustion from extinguishing because of natural movement of air such as wind via the apertures in the base, the apertures may be covered by means of a baffle plate. The Venturis and the injectors may be protected from influence of natural movement of air such as wind, by mounting the inlet sides, Venturis and injectors, together with a part of the gas feed pipe in which the injectors are located, in a box. This box is, however, provided with openings to allow the primary air to enter into the box. The openings may be provided with a flame arrestor and a baffle plate, similar as used at and for the same reasons as they are used for the apertures at the base section of the housing.

Preferably, the construction elements such as the housings of the flare stacks are provided in steel, more preferred out of stainless steel such as AISI 304 stainless steel.

It is further understood that the flare stack of the invention may additionally comprise other elements such as means for ignition of the combustible gas, pilot flames, means for flame monitoring, and many more. It is understood that the flare stack of the invention may be carried out without having any electrical component to be used once the flare stack is put into use.

Further advantages and embodiments of the present invention will become apparent from the following description and drawings.

Brief description of the drawings

The invention will now be described into more detail with reference to the accompanying drawings wherein

Figure 1 is a schematic side view of the flare stack of the invention; Figure 2 is a cross section according to plane AA' of this flare stack of Figure 1 ;

Figure 3 is a cross section according to plane BB' of this flare stack of Figure 1 ;

Figure 4 is a schematic 3D-view of an embodiment of the flare stack of the invention (roof not shown);

Figure 5 is a preferred embodiment of the flare stack of the invention.

Reference list of used numbers in the figures

100 flare stack

101 gas feed pipe

102 injector nozzle 120 burner element

121 gas inlet 122 venturi

123 gas permeable combustion surface

124 mixing chamber

130 primary combustion airflow 131 secondary airflow 140 blue flame front

150 housing

151 vertical wall

152 upper end of vertical wall 153 open area

154 roof

155 base

156 large opening

157 metal fiber knitted fabric 158 baffle plate

160 open space between roof 154 and upper end of vertical wall 152

190 box

191 opening in box 190

192 opening in box 190 193 baffle plate

194 flame arrestor

195 apertures in vertical wall 151

201 total height of the flare stack

202 height of the roof 203 height of open area 160 between roof 154 and upper end of vertical wall 152

204 height of wall 151

205 leg height

206 distance of tubular burner elements to base 155

207 width of the flare stack 208 roof width

209 roof length

210 length of the flare stack

300 second wall around vertical wall 151

301 airflow between vertical wall 151 and second wall 300

Description of the preferred embodiments of the invention Figure 1 shows schematically the front view of a flare stack 100 of the invention. Figure 2 shows a schematic inside view of the flare stack according to the plane AA'. Figure 3 shows a schematic inside view of the flare stack according to the plane BB'.

A flare stack 100 comprises a gas feed duct 101 , comprising a number of injector nozzles 102. Each of the burner elements 120 comprises a gas inlet 121 which is provided with a venturi 122. The gas permeable combustion surface 123 is coupled to the gas inlet 121 by means of a mixing chamber 124.

In the embodiment shown in Figure 1 , Figure 2, Figure 3 and Figure 4, six tubular burner elements are shown.

The flare stack 100 of the invention further comprises a housing 150, which housing comprises a substantially vertical wall 151, which upper end 152 defines an open area 153 which is covered by a roof 154. The housing 150 further comprises a base 155 which is substantially horizontal. The wall 151 and the base 155 encompass the gas permeable combustion surfaces 123 of the burner elements 120.

The base 155 is provided with an aperture, which is a large opening 156 and which, on its turn, is covered by a metal fiber knitted fabric 157, which functions as a flame arrestor. The large opening 156 is provided as well with a baffle plate 158, which prevents the flame front at the burners to extinguish due to e.g. blasts of wind. Also the vertical wall 151 is provided with apertures 195, also provided with a baffle plate, in the lower part of the wall 151.

When combustible gas is injected by the injector nozzle 102 into the venturi 122, primary combustion air 130 is drafted via the venturi into the mixing chamber 124. This mixture is ignited and combusted at the combustion surface 123, providing a blue flame front 140. The exhaust gas provided by the combustion is evacuated via the open area 153 and the open space 160 between the roof and the upper end 152 of the wall 151. These exhaust gasses draft additional secondary air, indicated by arrows 131 , into the housing 150, where it completes the combustion of the combustible mixture.

As shown in Figures 1 , 2, 3 and 4, the Venturis, injectors and gas feed pipe are mounted in a box 190. The box is provided with two openings 191 and 192, for allowing the primary air 130 used by the Venturis and injectors, to enter into the box. The openings 191 and 192 are provided with a baffle plate 193 to avoid that the functioning of the Venturis and injectors is influenced by e.g. blasts of wind. The openings are also provided with a metal grid or a woven or knitted metal fiber fabric which functions as a flame arrestor 194.

A preferred embodiment of the flare stack as shown in Figures 1, 2 and 3 has the dimensions of

• Total height 201 of 3050 mm

• Height 202 of roof being 200 mm;

• Height 203 of open area between roof and upper end of wall being 600 mm

• Wall height 204 being 1050 mm • Leg height 205 of 1200 mm

• The six injector nozzles being coaxial with the Venturis and tubular burner elements being mounted on a distance 206 from the base being 150 mm

• Roof width 208 in direction perpendicular to burner tubes being 1000 mm

• A wall width 207 in direction perpendicular to burner tubes being 800 mm • Roof length 209 in direction parallel to burner tubes being 820 mm

• A wall length 210 in direction parallel to burner tubes being 600 mm

• The injector nozzles having a diameter of 2.7 mm

• The tubular burners having a length of 425 mm, a diameter of 50 mm and a combustion zone being 38 mm width by 382 mm long, the latter in axial direction of the tube. The combustion zone comprises substantially rectangular slots of 4 mm to 6 mm by 0.5mm.

Twelve groups of 30 slots are arranged over the combustion zone. The slots are provided in such a way that the combustion zone is facing the open area 153 defined by the wall 151.

• The open area 156 at the base 155 of the housing 150, being located under the burner elements having dimensions of 506 mm by 336 mm.

This flare stack is able to process gas having a Wobbe-index between 43.8 MJ/Nm³ and 81.1 MJ/Nm³, at gas pressures up to 35 mbar.

Another preferred embodiment is depicted in figure 5. Here the wall 151 of the housing 150 is made of a corrugated plate, preferably a metal plate, which, because of these corrugations, allows thermal expansion of the wall without creating tension in the housing 150 of the flare stack 100. In another preferred embodiment, the vertical wall 151 is enclosed by a second wall 300 made out of plate, preferably a metal plate. The space between this second wall 300 and the vertical wall 151, which is open at the bottom and at the top of the walls (151 and 300), creates an air- insulation layer around the inner vertical wall 151, together with a cooling of the system because of the airflow 301 that is created by the convective movement of the insulating air layer between the outer wall 300 and the inner vertical wall 151. A further advantage of this outer wall 300 is that the outer temperature of the flare stack 100 is reduced resulting in a safer system for the environment, e.g. preventing burns or even melting of materials which were put in the direct proximity of the flare stack.

Claims

1. A flare stack (100) for combustion of combustible fluids, said flare stack comprising a gas feed pipe (101) and at least one burner element (120) for combustion of said combustible fluids, said burner element comprising a gas inlet (121), said gas inlet being adapted to receive combustible gas from said gas feed pipe (101), characterized in that said gas inlet

(121) of each of said burner elements (120) comprises a venturi (122), said gas feed pipe (101) comprises a gas injector nozzle (102) for each of said venturi (122), said nozzle (102) being aligned with said venturi (122) for drafting primary combustion air (130) with said combustible gas when injected by said nozzle (102) in said venturi (122).

2. A flare stack (100) according to claim 1 , wherein said stack comprises more than one burner element (120).

3. A flare stack (100) according to one of the claims 1 to 2, wherein each of said burner elements (120) comprises a gas permeable combustion surface (123) and a mixing chamber (124), said gas inlet (121) being coupled to said gas permeable combustion surface (123) by means of said mixing chamber (124).

4. A flare stack (100) according to one of the claims 1 to 3, wherein said burner elements (120) are coplanar.

5. A flare stack (100) according to one of the claims 1 to 4, wherein said burner elements (120) are substantially parallel.

6. A flare stack (100) according to one of the claims 1 to 5, wherein said burner elements (120) are tubular burner elements.

7. A flare stack (100) according to claim 6, wherein said burner elements (120) are mutually equidistant.

8. A flare stack (100) according to one of the claims 1 to 7, wherein said flare stack further comprises a housing (150), said burner elements (120) being mounted in said housing (150).

9. A flare stack (100) according to claim 8, wherein said housing (150) has a substantially vertically mounted wall (151) and a base (155), characterized in that said burner elements (120) are mounted in said housing (150) substantially parallel to said base (155).

10. A flare stack (100) according to one of the claims 8 to 9, wherein said housing (150) further comprises a roof (154), said wall (151) is defining an open area (153) at the opposite side of said base (155), said open area (153) being covered by said roof (154).

11. A flare stack (100) as in one of the claims 8 to 10, wherein said base (155) comprises apertures (156) for allowing secondary combustion air (131) to enter in the housing (150).

12. A flare stack (100) as in claim 11, wherein said apertures (156) are covered by means of a flame arrestor (157).

13. A flare stack (100) as in one of the claims 1 to 12, wherein said gas permeable combustion surface (123) of said burner elements (120) is a perforated metal plate.

14. A flare stack (100) as in one of the claims 1 to 12, wherein said gas permeable combustion surface (123) of said burner elements (120) is a metal fiber burner membrane.

15. A flare stack (100) according to any one of the claims 1 to 14 wherein said vertically mounted wall (151) of said housing (150) is made of a corrugated plate.

16. A flare stack (100) according to any one of the claims 1 to 14 wherein said vertically mounted wall (151) of said housing (150) is made of a corrugated metal plate.

17. A flare stack (100) according to any one of the claims 1 to 16 wherein said vertically mounted wall (151) of said housing is surrounded by a second wall (300).

18. A flare stack (100) according to claim 17 wherein said second wall (300) is a metal plate.