US20140329186A1

US20140329186A1 - Apparatus and method for minimizing smoke formation in a flaring stack

Info

Publication number: US20140329186A1
Application number: US14/266,992
Authority: US
Inventors: Donnie Dee McClain; Richard R. Martin; Jay D. Jennings
Original assignee: UOP LLC
Current assignee: Honeywell International Inc
Priority date: 2013-05-03
Filing date: 2014-05-01
Publication date: 2014-11-06

Abstract

An apparatus and method for minimizing smoke formation in the operation of a flaring stack. The apparatus includes a generally annular gas deflector having an outer surface for deflecting the waste gas therealong. A plurality of lobes extend radially from the deflector to provide improved mixing between the waste gas and combustion air during combustion to reduce smoke formation.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/819,192 filed May 3, 2013.

FIELD OF THE INVENTION

The subject application relates to an apparatus for minimizing smoke formation in a flaring stack.

BACKGROUND OF THE INVENTION

Flare apparatus have traditionally been utilized for burning and exhausting combustible gases. Flare apparatus are commonly mounted on flare stacks and located at production, refining, and processing plants for disposing of flammable waste gases or other flammable gas streams, which are diverted for any reason, including but not limited to venting, shut-downs, upsets, and/or emergencies. Primarily, flare stacks are used for venting unwanted waste gas streams from a facility.
It is generally desirable that flammable gas be burned without producing smoke, and reduction in smoke production during burning may be mandated by regulatory requirements.
One method that has been adopted for reducing smoke formation during burning includes mixing the waste gas stream to be burned with ambient air to maximize oxidation of the flammable waste gas to prevent the production of smoke. Another method that has been used includes supplying steam to the combustion zone, such as, for example, by an eductor to increase oxidation to restrict smoke formation. In some applications, ambient air and steam introduction are used together to further reduce smoke formation.
When sufficient ambient air or ambient air and steam is available to contact the combustible waste gas, the mixture can be smokelessly burned. For a typical flare apparatus, there is a limited quantity of air available for mixing with the waste gas and therefore a limited smokeless capacity.
A wide variety of apparatus and processes have been proposed to increase the smokeless burning of combustible gas from a flare. For example, U.S. Pat. No. 3,833,337 to Desty et al. and U.S. Pat. No. 8,337,197 to Poe et al. propose the use of a tulip shaped Coanda tip. Coanda tips have been used in flares with high flow rates and pressures to cause the adherence of the waste gas to the surface. The negative pressure and viscous forces caused by the Coanda effect cause the fluid to be drawn against the surface in a relatively thin film, which allows proximate fluid (e.g. ambient air) to be mixed efficiently with the fluid stream. Poe describes that to achieve a Coanda effect, the surface of the Coanda surface should be substantially smooth.
While current apparatus and methods have improved the smokeless combustion of waste gas streams, it is desirable to further reduce the amount of smoke formation based on regulatory and environmental considerations.

SUMMARY OF THE INVENTION

By one aspect, an apparatus is provided minimizing the formation of smoke in the operation of a flaring stack. The apparatus includes a generally annular gas deflector that has an outer surface for deflecting waste gas therealong. The apparatus also includes a plurality of lobes extending radially from the gas deflector for providing improved mixing between the waste gas and combustion air during combustion. According to one approach, the lobes include circumferentially spaced, generally vertical ribs. The gas deflector may include a tulip-shaped bowl having a Coanda surface.
By another aspect a method is provided for combusting a waste gas to reduce the formation of smoke. The method includes passing the waste gas along an outer surface of a generally annular gas deflector including a plurality of lobes extending radially from an outer surface thereof The method further includes drawing ambient air toward the outer surface for mixing with the waste gas. The method further includes igniting the waste gas to combust the waste gas with decreased smoke formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus including a plurality of support arms and a plurality of corresponding gas deflectors in accordance with various embodiments;

FIG. 2 is a perspective view of a support arm of the apparatus with a gas deflector in accordance with various embodiments;

FIG. 3 is a cross-sectional view of the support arm of FIG. 2 with the gas deflector supported thereon in a lowered position;

FIG. 4 is a partial cross-sectional view of the support arm of FIG. 2. with the gas deflector in a raised position;

FIG. 5 is a top view of the gas deflector of FIG. 2;

FIG. 6 is a side cross sectional view of the gas deflector of FIG. 5 taken along line A-A;

FIG. 7 is a side cross sectional view of the gas deflector of FIG. 6 taken along line B-B; and

FIG. 8 is a perspective view of a support arm of the apparatus with a gas deflector in accordance with another approach.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

The apparatus and method presented herein, in accordance with various aspects, relates to reducing smoke formation during combustion of a waste gas in a flare stack. The apparatus may be used with a flare stack, for example, at a refinery or production facility for flaring waste gas or other gas streams to the atmosphere. As used herein, the term “waste gas” refers to any combustible gas stream that is combusted by the flare stack, including, but not limited to undesired gas streams, product streams combusted during shutdown or emergency situations, and other streams.
Referring now to FIGS. 1 and 2, an apparatus 2 for the combustion of a waste gas stream in accordance with various aspects is provided. The apparatus 2 includes a gas deflector 4 for deflecting waste gas along a surface 6 thereof The apparatus 2 may also include a support arm 8 for supporting the gas deflector 4 thereon. The waste gas may be passed through the support arm 8 to the gas deflector 4. In this regard, the support arm 8 may have a waste gas passageway 10 formed therein as illustrated in FIG. 3 for facilitating the flow of the waste gas therethrough. A waste gas outlet 12 is provided for introducing the waste gas from the waste gas passageway 10 to the gas deflector 4. As illustrated in FIGS. 3- 4 and described further below, the outlet 12 may include an annular opening 14 between the support arm 8 and the gas deflector 4 so that the waste gas flows through the opening 14 and along the gas deflector outer surface 6.
The waste gas deflector includes a plurality of lobes 16 that extend radially therefrom. In this regard, as waste gas flows along the outer surface of the gas deflector 4, the gas flows over and between the lobes 16. It has been identified that including radially extending lobes 16 on the gas deflector 4 improves mixing of the waste gas stream and ambient air during operation of the flare stack resulting in a reduction in the amount of smoke that is produced during combustion. It has further been identified that including radially extending lobes 16 as described herein provides a lower flame temperature and reduced emissions of unwanted by-products into the atmosphere, such as NO_xemissions. By one aspect, the lobes include a plurality of generally vertically oriented ribs 18 spaced circumferentially about the gas deflector 4 such that the gas flows along the ribs and through channels 20 formed between adjacent ribs 18.
According to various aspects, the support arm 8 is provided for supporting the gas deflector 4 thereon. The support arm 8 may also include a gas passageway 10 for passing the waste gas to be combusted from a gas source to the gas deflector 4. In one approach, as illustrated in FIG. 1, the apparatus 2 may include a plurality of support arms 8 supporting a plurality of gas deflectors 4. In this manner, the size of each gas deflector 4 may be decreased as opposed to having a single large gas deflector. This may improve the ability for smoke free combustion by increasing the amount of air available for mixing with the gas at each of the plurality of gas deflectors 4 as opposed to a single larger gas deflector.
The support arm 8 may extend from a central plenum 22 as illustrated in FIG. 1. As shown, one support arm 8 extends upwardly from the top of the plenum 22 while additional support arms 8 extend at inclined angles from side portions 24 of the plenum 22 and extend generally vertically at bent portions 26 thereof. In one example, the vertical portions 28 of the support arms 8 extend vertically at an angle of less than about 5 degrees from the vertical, less than about 3 degrees from the vertical axis in another example, and at less than about 1 degree from the vertical in yet another example. In yet another example, vertical portions 28 of the support arms 8 extend vertically.
The support arm 8 may include the gas passageway 10 as illustrated in FIG. 3 for passing the waste gas through the support arm 8 toward the gas deflector 4. In one approach, as shown in FIG. 3, the gas passageway 10 may include a hollow passageway through the support arm 8. In this regard, the support arm 8 may be formed by a generally hollow tube providing the passageway 10. The tube may be cylindrical as illustrated in FIG. 3 or other suitable configurations.
According to one aspect, the support arm 8 includes an upper seating portion 30 for supporting the gas deflector 4 thereon. The upper seating portion 30 by one approach includes a rim or flange 32 for supporting the gas deflector 4. As illustrated in FIG. 3, the flange 32 may include a generally annular flange extending radially outwardly from the support arm upper seating portion 30 to provide an upper seating surface 34 that may also serve to direct the flow of gas along the deflector 4, as described further below.
As mentioned previously, the apparatus 2 according to various aspects includes a gas deflector 4. In one preferred form, the gas deflector 4 includes a gas deflector bowl 36 having a Coanda surface 38 as illustrated in the figures. The Coanda bowl 36 may have a tulip-shaped configuration as illustrated in FIG. 7 having a generally horizontal or slightly inclined lower portion 40, a vertical or inclined upper portion 42, and a convex portion 44 between the lower portion 40 to the upper portion 42. The remainder of the description will be made with reference with use of the Coanda bowl 36 as the gas deflector. Coanda bowls are generally known and understood by those of skill in the art, and are known to produce a “Coanda effect”, wherein gases flowing along the outer surface thereof tend to follow the surface and draw in surrounding gas or air. In one approach, the Coanda bowl has a generally round cross-section taken along a plane orthogonal to a longitudinal axis 46 of the bowl, although the bowl 36 may also include other suitable cross-sectional configurations, for example oval or polygonal.
By one aspect, the Coanda bowl 36 includes a plurality of lobes 16 extending radially outwardly from its outer surface 38. As illustrated in the figures, the lobes 16 may include a plurality of generally vertical ribs 18 spaced circumferentially about the bowl outer surface 38. In one approach, the ribs extend radially outwardly from the Coanda bowl outer surface 38 (or floors 20 of the channels). As used herein, the phrase “total outer surface” refers to the outer surface formed along all outer surface of the gas deflector, including by one example along the outer surfaces of the Coanda bowl 36, ribs 18, and channels 20, such that the “total outer surface” of a ribbed portion of the Coanda bowl 36 has a larger surface area than the outer surface of a corresponding Coanda bowl would have without ribs.
According to one approach, the ribs 18 extend generally vertically along the Coanda bowl outer surface 38. It should be understood that as described herein, the ribs 18 extend generally vertically as viewed head-on and that where the upper portion 42 of the bowl 36 is inclined as illustrated in FIG. 6, the vertically extending ribs may similarly be inclined toward the longitudinal axis 46 of the bowl 36 when viewed from profile (i.e. 90 degrees from head-on as shown by the side-cross section of FIG. 6). With this in mind, by one approach, the ribs have a generally vertical axis 48 when viewed head-on as shown in FIG. 2 that is less than about 5 degrees from vertical in one example, less than about 2 degrees in another example, and less than about 1 degree from vertical in yet another example.
The ribs are circumferentially spaced so that a plurality of corresponding channels 20 are formed between adjacent ribs 18 as illustrated in FIG. 2. The channels 20 extend generally vertically between the ribs 18 and can have a variety of different shapes and configurations. The channels 20 include a channel floor 50 at a base thereof. The channel floor may be flush with the Coanda bowl outer surface 36, or may be raised or indented relative thereto.
The ribs 18 may have a generally constant radial profile (i.e. distance the ribs extend from the bowl outer surface 36 and/or channel floor 50). Alternatively, the ribs 18 may have a varying radial profile as illustrated in FIG. 6. By one approach, as seen in FIGS. 2 and 6, the ribs 18 are tapered from a lower rib portion 52 to raised rib portion 54. In this regard, the tapered lower portion 52 may be slightly elevated with respect to, or flush with, the bowl surface 36 to provide a smooth transition surface over which gas traveling upwardly therealong can flow. The ribs 18 may also include a tapered upper rib portion 56 to provide for smooth flow of the waste gas and combustion air mixture as it exits the Coanda surface. It should be understood that the radially extending ribs may be radially extending relative to an outer surface of a Coanda bowl and/or relative to channels. In this regard, the ribs may be formed, for example by providing ribs along the outer surface of a Coanda bowl, or by forming channels or indentations in a Coanda bowl so that the ribs are formed above the channels.
The ribs 18 may have a constant circumferential width or a varying width about the perimeter of the Coanda bowl 36 as illustrated in FIGS. 2 and 5. Similarly, the channels 20 may have a constant or varying circumferential width. Typically, where the Coanda bowl includes an inwardly tapered upper portion 42 as illustrated, at least one of the ribs and channels will have a varying width to account for the upwardly decreasing circumference.
By one aspect, the ribs 18 may have inclined sidewalls 58 extending between rib top portions 60 and the channel floors 50 as best seen with reference to FIG. 5. The inclined sidewalls 58 can be generally flat, or may be curved or formed in other manners. The inclined side walls provide a smooth surface over which the gas can flow by reducing the amount of sharp angles between the ribs and the channels.
Without intending to be bound by theory, it is believed that the addition of ribs 18 to the Coanda bowl 36 increases the total surface area of the Coanda bowl 36 to improve waste gas/combustion air mixing without providing a corresponding increase in outer diameter of the bowl. In this manner, the Coanda bowl 36 can advantageously be kept relatively small while providing sufficient surface area for drawing in combustion air for mixing with the waste gas and reducing smoke formation.
To this end, by one aspect, the ribbed Coanda bowl has a relatively high ratio of a perimeter (as shown in FIG. 5) to an outer radius 62. As used herein, outer radius refers to the distance between the bowl longitudinal axis 46 and the rib top portions 60. For example, a traditional un-ribbed Coanda bowl has a ratio of perimeter (circumference) to outer radius of 2πr/r=2π. In one example, the ratio of the perimeter to the outer radius of the ribbed bowl described herein is greater than 2π. In another example, the ratio of perimeter to outer radius is between about 6.5 and about 20, between about 7.5 and about 16 in another example, and between about 8.5 and about 12 in yet another example.
According to one aspect ribs 18 may be formed along the entire outer surface 38 of the Coanda bowl 36. In this regard, the surface area of the entire bowl 36 is increased such that mixing between the waste gas and the combustion air is improved along the total outer surface as described above.
According to another aspect, the ribs 18 may extend along one or more portions of the Coanda bowl 36, but less than the full outer surface 38 thereof, such that a portion of the gas deflector is unribbed and provides a relatively smooth surface for gas flow. For example, as illustrated in FIG. 7, the lower portion 40 and/or the intermediate portion 44 of the Coanda bowl 36 may be unribbed, while an upper portion 42 includes ribs. In this regard, gas may better flow along the lower portion 40 of the Coanda bowl 36, along the convex intermediate portion 44, and to the ribbed upper portion 42 before flowing over and between the ribs 18. Further, having the lower portion 40 and/or the intermediate portion 44 of the Coanda bowl unribbed provides a lower seating portion of the Coanda bowl 36 so that when the bowl 36 is in a seated position, as illustrated in FIG. 3, the Coanda bowl seating portion is in generally close contact with the support arm upper seating portion 30 to reduce the amount of waste gas flowing therethrough. In one example, between about a bottom 5 to 50 percent of the Coanda bowl is unribbed with an upper portion including ribs. In another example between about a bottom 10 to 40 percent of the Coanda bowl is unribbed with an upper portion including ribs. In another example, as illustrated in FIG. 8 a bottom portion may include ribs with at least an intermediate portion and/or a top portion being unribbed.
As illustrated in FIGS. 2 and 8, different numbers and sizes of ribs 18 may be included on the Coanda bowl to maximize the air/waste gas mixing. For example, it may be beneficial to select the number of ribs extending circumferentially about the Coanda bowl 36 to provide increased surface area and the associated improvement in gas/air mixing, while still ensuring that the gas will flow smoothly over the total surface area during operation. FIG. 2 illustrates an example of a Coanda bowl that includes a smaller number of relatively wider ribs while FIG. 8 illustrates another example where a larger number of narrower ribs 18 is used. With this in mind, in one example a ratio of a combined circumferential width of the one or more ribs 18 to a combined circumferential width of a plurality of channels 12 between the ribs 18 is between about 0.5 and about 5 and between about 1 and about 3 in another example. In another example, a ratio of a rib radial height above the channel floor to the outer radius of the bowl is between about 0.01 and about 0.2 in one example and between about 0.03 and about 0.2 in another example.
By one aspect, the gas outlet 12 is provided for introducing the waste gas toward the outer surface of the Coanda bowl. As illustrated in FIGS. 2-4, the gas outlet 12 may include a generally annular opening 14 of the waste gas passageway 10 formed about the outer surface 38 so that the waste gas can flow through the opening and along the outer surface. The annular opening 14 may include a relatively round shape, or another shape, such as an oval or polygon. By one approach, the annular opening includes a single annular opening, but may also include a plurality of openings formed about the Coanda bowl 36. The annular opening 14 may be formed by a gap between the support arm upper seating portion 30 and the Coanda bowl lower portion 40, such that waste gas flowing through the gas passageway 10 exits through the opening 14 and flows along the outer surface 38.
In one approach, the waste gas is provided at a relatively high pressure and flowrate. The apparatus disclosed herein may be well suited to waste gases flowing at high flowrates as they will pass over the Coanda surface 38 and the ribs 18 and draw in a large amount of combustion air for mixing and reduced smoke formation.
In one approach, the Coanda bowl 36 is shiftable between a seated position as illustrated in FIG. 3 and a raised position as illustrated in FIG. 4. In the seated position, the Coanda bowl seating portion contacts the support arm seating portion 30. As mentioned previously, in the seated position, the Coanda bowl 36 and support arm 8 are in close contact so that the annular opening 14 is in a closed position and the flow of gas therethrough is restricted. In the raised position, the Coanda bowl seating portion is raised relative to the support arm seating portion 30 to form a gap therebetween to provide the annular opening 14 to allow the flow of waste gas therethrough.
By one approach, the Coanda bowl 36 is biased toward the closed position, however high pressure waste gas contacts the Coanda bowl 36, causing it to lift into the raised position shown in FIG. 4 so that the waste gas is able to pass through the annular opening 14. The Coanda bowl 36 may be biased toward the closed position by a spring 64 as illustrated in FIG. 3. A rod 66 is connected to the Coanda bowl 36 and the spring 64, such that the spring 64 urges the rod 66, and accordingly the Coanda bowl 36, toward the seated position.
According to various aspects, during operation, the waste gas to be combusted flows through the gas passageway and through the annular opening 14. Where the Coanda bowl 36 is shiftable, the waste gas may shift the Coanda bowl to the raised position so that the gas can exit the annular opening 14 and flow along the total outer surface of the Coanda bowl 36. As the waste gas flows along the outer surface, combustion air (for example ambient air) is drawn toward the waste gas and mixed therewith. The waste gas passes over the ribs 18 and through the channels 20 therebetween. The waste gas is ignited and combusted with reduced smoke formation.
The above description and examples are intended to be illustrative of the invention without limiting its scope. While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

Claims

1. A method for combusting a waste gas to reduce the formation of smoke, the method comprising:

passing the waste gas along an outer surface of a generally annular gas deflector including a plurality of lobes extending radially from an outer surface thereof;

drawing ambient air toward the outer surface for mixing with the waste gas; and

igniting the waste gas.

2. The method of claim 1, wherein passing the waste gas along the outer surface includes passing the waste gas over a plurality of circumferentially spaced generally vertical ribs extending radially from the outer surface.

3. The method of claim 2, further comprising passing the waste gas through channels between the vertical ribs.

4. The method of claim 3, further comprising passing the waste gas along inclined sidewall surfaces extending from the channels to rib top portions.

5. The method of claim 1, further comprising passing the waste gas over a Coanda surface of the gas deflector.

6. The method of claim 5, wherein passing the waste gas over the Coanda surface includes passing the waste gas over a plurality of circumferentially spaced generally vertical ribs extending radially from the Coanda surface.

7. The method of claim 5, further comprising passing the waste gas over a lower unribbed portion of the Coanda surface and then over an upper ribbed portion of the Coanda surface.

8. The method of claim 1, wherein passing the waste gas over the outer surface includes passing the waste gas over an outer surface having a ratio of a perimeter of a ribbed portion of the gas deflector to an outer diameter of the ribbed portion of the gas deflector of between about 6.5 and about 20.

9. The method of claim 1, wherein passing the waste gas over the outer surface includes passing the waste gas over an outer surface having a ratio of a perimeter of a ribbed portion of the gas deflector to an outer diameter of the ribbed portion of the gas deflector of between about 7.5 and about 16.

10. A method for combusting a waste gas to reduce the formation of smoke, the method comprising:

passing a waste gas through an inner passageway of a support arm;

passing the waste gas through an annular gas passage between the support arm and a generally annular gas deflector;

passing the waste gas along an outer surface of the gas deflector and over lobes extending radially from the annular gas deflector; and

igniting the waste gas.

11. The method of claim 10, further comprising drawing ambient air toward the gas deflector to mix with the waste gas and reduce smoke formation during combustion of the waste gas.

12. The method of claim 10, wherein the gas deflector includes a generally tulip- shaped Coanda bowl and passing the waste gas along the outer surface includes passing the waste gas over a Coanda surface of the Coanda bowl.

13. The method of claim 10, further comprising shifting the gas deflector from a lower seated position to an raised open position to provide the annular opening to allow the waste gas to pass through the annular opening.

14. The method of claim 13, further comprising shifting the gas deflector from the raised open position to the lower seated position to close the annular opening.

15. The method of claim 14, wherein shifting the gas deflector to the lower seated position includes contacting a smooth un-ribbed seating portion of the gas deflector with an upper seating portion of the support arm to provide generally close contact therebetween.

16. The method of claim 14, wherein shifting the gas deflector from the raised open position to the lowered closed position includes biasing the gas deflector toward the lowered position with a spring and reducing a waste gas pressure in the waste gas passageway.