WO2006010693A1

WO2006010693A1 - Flare stack having enclosed flame combustion

Info

Publication number: WO2006010693A1
Application number: PCT/EP2005/053130
Authority: WO
Inventors: Geert Dumortier; Hans Verbauwhede; Marcel Corteville; Dirk Ten Hoeve
Original assignee: Nv Bekaert Sa
Priority date: 2004-07-26
Filing date: 2005-07-01
Publication date: 2006-02-02

Abstract

A flare stack for combustion of combustible fluids as subject of the invention comprises a burner assembly and a double walled exhaust tube having an upper end and a lower end. The double walled exhaust tube comprises an inner tube and an outer tube and means at the lower end of the exhaust tube to allow air flowing between the inner and the outer tube from the lower end to the upper end of the exhaust tube. The burner assembly is mounted in the inner tube for combusting the combustible fluids inside the inner tube and discharging combusted gasses by means of an exhaust opening from the inner tube at the upper end. The flare stack as subject of the invention is characterized in that at the upper end, the outer tube extends beyond the exhaust opening of the inner tube of the double walled exhaust tube.

Description

FLARE STACK HAVING ENCLOSED FLAME COMBUSTION

Field of the invention.

The present invention relates to flare stacks and more in particular to double walled flare stacks and ground flare stacks for flaring combustible fluids using enclosed flame combustion.

Background of the invention.

Flare stacks are widely used for combustion of combustible fluids such as waist gasses or liquids or process gasses at various chemical and petrochemical applications.

Most widely used are flare stacks which combust fluids by means of a flame, being a burner assembly 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 of such flare stack is described in US4468193.

As an alternative, enclosed combustion may be used for flaring such waist 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 enclosed combusting flare stacks have the disadvantage that, after being in operation for a certain time, the external walls of the flare stack become hot and radiate a severe amount of energy in the outer perimeter of the stack. Such radiation causes danger for the environment in the immediate neighborhood. Summary of the invention.

It is an object of the present invention to provide an enclosed combusting flare stack, having walls which does not obtain such elevated temperatures when burning over significant long period. It is further an object of the present invention to provide a flame enclosed flare stack producing significantly less noise during use.

A flare stack for enclosed flame combustion of combustible fluids as subject of the invention comprises a burner assembly and a double walled exhaust tube having an upper end and a lower end. The double walled exhaust tube comprises an inner tube and an outer tube and means at the lower end of the exhaust tube to allow air flowing between the inner and the outer tube from the lower end to the upper end of the exhaust tube. The burner assembly is mounted in the inner tube for enclosed flame combustion of the combustible fluids inside the inner tube and discharging combusted fluids by means of an exhaust opening from the inner tube at the upper end. A flare stack as subject of the invention is characterized in that at the upper end, the^*outer tube is extending beyond the exhaust opening of the inner tube of the double walled exhaust tube.

The outer tube is so-to-say higher than the inner tube. The exhaust gasses being discharged by the exhaust opening of the inner tube, creating an under pressure at the space between the outer and inner tube exhaust opening of the inner tube. This under pressure causes a higher amount of cooling air drafted between the inner and outer tube from the lower end of the double walled exhaust tube to the upper end. This amount of cooling air cools the inner tube to a larger extend, as well as the outer tube, and so prevents the outer tube to increase its temperature, which on its turn limits the thermal radiation from the outer tube to the environment in the direct neighborhood of the flare stack. It was found that preferably the outer tube extends at least 100mm beyond the exhaust opening of the inner tube. The length of the extension of the outer tube is however limited to the dimensions and performance of the burner assembly, this is, the length may not cause a pressure drop over the exhaust gasses to be overcome, which cannot be provided by the burner assembly. More preferred, the length of the outer tube extending beyond the exhaust opening of the inner tube is in between 100mm and 2000mm, most preferred between 100mm and 1200mm. The diameter of the inner and outer tube is depending on the burner assembly and more in particular to the maximum load to be foreseen for the burner assembly. The diameter of the inner tube may e.g. range between 120 and 1000mm.

It is understood that the term 'tube' is not to be understood as limited to circular shaped concentric or coaxial shapes.

The term "double walled tube" is to be understood as comprising two distinct tubular shaped objects, of which one object (the inner object) is present substantially encompassed by the other object (the outer object).

Preferably, burner assembly is mounted in the double walled tube in such a way that at the lower side of the double walled tube, the lower end of the outer tube end is located below the flame front on the burner assembly.

By the term "enclosed flame combustion" is to be understood that the flames obtained by combustion of the combustible gasses when the flare stack is run on maximum capacity, does substantially not extend beyond the exhaust opening of the inner tube.

At the lower end of the exhaust tube, means are present to allow cooling air to flow between the two tubes from the lower to the upper end of the tube. Such means may be e.g. openings in the outer tube. -A-

Alternatively the outer tube may be mounted in such a way that the radial open space between the inner and outer tube is freely accessible for cold air. Preferably, it was found that the minimum distance between the inner and the outer tube is at least 12mm, more preferred, the distance is between 15mm and 75mm, such as between 20mm and

55mm.

The term "burner assembly" is to be understood as any type of burner suitable to combust the combustible fluids, being mounted at the lower end of the exhaust tube in order to combust the combustible fluids inside the inner tube. Preferably the burner assembly is a premix burner assembly, which may be mounted in such a way that it closes the inner tube at its lower end completely. The dimensions of the burner assembly are determined by the load range, this is the amount of combustible gas the burner assembly is to be able to combust per time unit. As the burner assembly is mounted at the inner side of the inner tube, the diameter of the inner tube is dependent on the burner assembly dimensions. The diameter of the inner tube may vary however over a large range, preferably between 100mm and 1500mm.

The length of the inner tube is determined by the height of the flame front which may be generated when the burner assembly is burning under its largest load. Usually, but not restrictively, the height of the inner tube extends at least 150mm above the burner assembly, this is above the combustion point of fluids being combusted by the burner assembly.

In order to further improve the heat exchange between the inner tube and the cooling air passing between the inner and outer tube, the inner tube may be foreseen of walls which are corrugated.

To further improve the amount of cooling air flowing between the inner and outer tube, preferably the outer tube is foreseen of additional openings, located near the exhaust opening of the inner tube. An additional advantage of the presence of such additional openings, is the reduction of noise which may occur when no such openings are present. Preferably the openings in the outer tube are located at the location of the exhaust opening of the inner tube plus or minus 10 times the distance between the two tubes.

Most preferred, the openings are present in the outer tube at the location of the exhaust opening of the inner tube, discharging the combusted gasses at the inside of the outer tube.

The additional openings may e.g. be provided as circular perforations, but may as well be provided in many other shapes (e.g. elliptic slots or rectangular slots) or may be foreseen by using an outer tube, which is provided as two tubes, which two parts are spaced from each other at the location of the additional openings.

Preferably, the additional openings represents al least 8% of the zone of the outer tube wall, said zone being bound by the highest and lowest point of the additional openings. Preferably, the additional openings represent between 10% and 30% of this zone.

Several details will be described hereinafter by means of some preferred embodiments.

Brief description of the drawings.

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

FIGURE 1, FIGURE 2 and FIGURE 3 shows schematically some embodiments of flare stacks as subject of the invention. Description of the preferred embodiments of the invention.

A flare stack 100 for combustion of combustible fluids is shown in FIGURE !

The flare stack 100 comprises a burner assembly 101 and a double walled exhaust tube 110 having an upper end 111 and a lower end 112. The double walled exhaust tube comprises an inner tube 120 and an outer tube 130 and means 140 at the lower end 112 of the exhaust tube 110 to allow air flowing between the inner and the outer tube from the lower end to the upper end of the exhaust tube as indicated with arrow 102. In this embodiment, the means 140 are provided by mounting the outer tube 130 to the inner tube 120 by means of some studs 141, in such a way that the lower end of the outer tube 130 is open for air to enter. The outer tube 130 envelopes the burner assembly 101. The means 140 to enter cooling air is provided below the burner assembly 101.

The burner assembly 101 is mounted in the inner tube 120 for combusting the combustible fluids inside the inner tube 110 and discharging combusted gasses by means of an exhaust opening 121 from the inner tube 120 at the upper end. At the upper end 111, the outer tube 130 is extending beyond the exhaust opening 121 of the inner tube 120 of the double walled exhaust tubei 10.

The outer tube 130 is a stainless steel tube with circular cross section, having a diameter (132) of 250mm. The inner tube 120 is a stainless steel tube with circular cross section, having a diameter (122) of 200mm. The tubes are mounted coaxially, so the distance (123) between inner and outer tube is substantially identical and equal to 25mm.

The outer tube 130 has a length (134) of 600mm. The inner tube has a length (124) of 485mm. The lower ends of the inner tube 120 and outer tube 130 are coplanar. The inner tube 120 is coupled to the combustible gas supply 150 comprising a fan 151, which takes combustible gas (indicated 152) via valve 153 and air (indicated 154), and discharges the combustible gas mixture (indicated 155) to the burner assembly 101.

The burner assembly 10I₁ comprising a circular metal fiber burner membrane with diameter (156) of 135mm, is mounted in the inner side of the inner tube 120, at a distance (157) of 310mm from the lower end of both inner and outer tube. The burner assembly comprises a metal fiber burner membrane being a knitted fabric out of metal fibers, such as described in EP839221B1. The shaved fibers may be obtained by coil shaving such as described in EP319959B1. The alloy out of which the metal fibers are provided, is preferably a thermally resistant Iron- Chromium-Aluminum- alloy such as e.g. Fecralloy ® or Aluchrome®.

The an exhaust opening 121 from the inner tube 120 at the upper end is located on a distance (158) of 175mm above the burner assembly. Exhaust gasses (indicated 159) are discharged to the inner side of the outer tube 130 via this exhaust opening 121. The outer tube 130 extends 115mm beyond the exhaust opening 121 of the inner tube 120.

It is understood that the lower part of the inner tube, this is the part between the combustible gas supply 150 and the burner assembly 101 is used as a mixing chamber 161. At the upper part 162 of the inner tube 120, the flame front 163 resulting from combustion of the combustible gasses is enveloped by the inner tube 120.

In order to further improve the draught of cooling air (102) between the inner and outer tube, additional openings 171 are provided in a zone

172 defined by the exhaust opening 121 of the inner tube 120 plus or minus 10 times the distance between the inner and outer tubes. Preferably the additional openings 171 are located at the height of the exhaust opening 121 of the inner tube 120. The openings as shown in FIGURE 1 are twenty-six circular perforations of the outer tube with diameter (173) of 10mm. This represents 10.7% of the zone 173 being bound by the highest and lowest point of the additional openings 171. additional cold air may be drafted via these additional openings from the outside of the outer tube inwards the outer tube, as indicated with arrow 174. This additional cold air prevents the upper part of the outer tube to be heated by the thermal energy of the exhaust gasses being discharged by means of the exhaust opening from the inner tube. This results in thermal differences between the exhaust gasses and the outer tube, which is believed to be the reason of the improvement of the draught of cooling air from the lower side of the double walled tube to the upper side of this tube. Further it was found that noise created by the combustion of the gasses, was reduced by adding the additional openings.

In an alternative embodiment, the outer tube has a length of 1500mm, and is replacing the outer tube as the one described in FIGURE 1. The outer tube extends 1015mm beyond the inner tube.

It was found that the flare stack 100 may be used to combust several gasses, varying from rich waist or process gasses from chemical or petrochemical plants, to relatively poor waist gasses from other sources. Preferably the burner assembly and hence the flare stack is loaded with combustible gasses, generating during combustion less than 15OkW₁ more preferred less than 120 kW.

FIGURES 2, 3 and 4 show alternative burner assemblies which may be used for a flare stack as subject of the invention.

To combust gasses presented at high pressure (e.g. 6 bar) a combustible gas supply 250 as shown in FIGURE 2 may be used. Gasses (via gas duct indicated 201) may be provided to a ventury system 202, which draft primary combustion air (indicated 203) at specified ratio gas/air and present the premixed combustible gas to the burner assembly and flare stack, identical as for the flare stack 100 shown in FIGURE 1. In case of rich gas 201 , additional primary air 204 may be provided by means of a fan 205.

In case of varying pressure on the gas to be combusted, a combustible gas supply 350 as shown in FIGURE 3 may be used. Gasses (via gas duct as indicated 301) may be provided to a mixing chamber 304 by means of an injector 303, located at the smallest diameter of a conical mixing chamber 304. Primary air (indicated 305) on a predetermined ratio is provided to the mixing chamber 304 by means of a fan 306. The amount of primary air is coupled to the gas volume by means of a temperature sensor 307, measuring the temperature of the exhaust gasses. The measures temperature is compared with a set-value by a control unit 308, which may set the fan speed in order to obtain a higher or lower exhaust gas temperature. The mixing chamber 304 present the premixed combustible gas to the burner assembly and flare stack, identical as for the flare stack 100 shown in FIGURE 1.

It is understood that a burner assembly for combustion of liquid combustible fluids may be provided as well. Such burner assembly may be mounted in the inner side of the inner tube in substantially identical way.

Claims

1. A flare stack for enclosed flame combustion of combustible fluids, said flare stack comprising a burner assembly and a double walled exhaust tube having an upper end and a lower end, said double walled exhaust tube comprising an inner tube and an outer tube and means at said lower end of said exhaust tube to allow air flowing between said inner and said outer tube from said lower end to said upper end of said exhaust tube, said burner assembly being mounted in said inner tube for enclosed flame combustion of said combustible fluids inside said inner tube and discharging combusted gasses by means of an exhaust opening from said inner tube at said upper end, characterized in that at said upper end, said outer tube is extending beyond said exhaust opening of said inner tube of said double walled exhaust tube.

2. A flare stack as in claim 1, wherein said outer tube extends at least 100mm beyond said exhaust opening of the inner tube.

3. A flare stack as in any one of the claims 1 to 2, wherein said outer tube extends between 100mm and 2000mm beyond said exhaust opening of the inner tube.

4. A flare stack as in any one of the claims 1 to 3, wherein the minimum distance between said inner and said outer tube is at least

12mm.

5. A flare stack as in claim 4, wherein the minimum distance between said inner and said outer tube is between 15mm and 75mm.

6. A flare stack as in any one of the claims 1 to 5, wherein the walls of said inner tube being corrugated.

7. A flare stack as in any one of the claims 1 to 6, wherein said outer tube has openings at a zone of said outer tube at the location of the exhaust opening of the inner tube plus or minus 10 times the distance between the two tubes.

8. A flare stack as in claim 7, wherein said openings are present at the height of said exhaust opening of said inner tube.

9. A flare stack as in any one of the claims 1 to 8, wherein the inner tube extends al least 150mm above the burner assembly.

10. A flare stack as in any one of the claims 1 to 9, wherein said means at said lower end of said exhaust tube to allow air flowing between said inner and said outer tube from said lower end to said upper end of said exhaust tube are located below said burner assembly.

11. A flare stack as in any one of the claims 1 to 10, wherein represents al least 8% of the zone of the outer tube wall, said zone being bound by the highest and lowest point of the additional openings.

12. A flare stack as in any one of the claims 1 to 11 , wherein burner assembly is premix burner assembly.

13. A flare stack as in any one of the claims 1 to 12, wherein burner assembly is a gas burner assembly.

14. A flare stack as in any one of the claims 1 to 13, wherein said tubes having circular cross section