EP1224422A1

EP1224422A1 - AIR STAGED LOW-NOx BURNER

Info

Publication number: EP1224422A1
Application number: EP00989709A
Authority: EP
Inventors: Stephen P. Pisano; Anthony G. Fennell; John R. Hemmerlin; Glenn W. Baggley; John C. Dormire
Original assignee: Bloom Engineering Co Inc
Current assignee: Bloom Engineering Co Inc
Priority date: 1999-10-27
Filing date: 2000-10-26
Publication date: 2002-07-24
Also published as: RU2242674C2; WO2001035022A1; CZ20021421A3

Abstract

An apparatus and method for using staged air combustion. The apparatus includes a burner body (10) secured to a port block (42), and a fuel passageway (12) extending through the burner body (10), terminating in a fuel nozzle (22), which injects fuel into the burner throat (40). Primary air jets (20) are configured to inject primary air into a primary combustion region (24), which is normally in the burner throat (40). A dish with a dish surface (28) is connected to the burner throat (40); the dish surface (28) extending in a divergent angle with respect to a burner centerline (35). Secondary air jets (34) are connected to the air passageway (14) and extend through the port block (42). The secondary air jets (34) inject secondary air into a secondary combustion region (38), which may be at the dish surface (28) or the hot face (30) of the burner.

Description

AIR STAGED LOW-NOx BURNER

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to low-NOx burners, and, in particular, to

air-staged low-NOx burners.

2. Description of the Prior Art

Oxides of nitrogen (NOx) are produced from the burning of fuels during

the normal operation of a typical burner. These oxides combine with hydrocarbons in the

atmosphere, creating "smog", which, when inhaled, may cause injury. Further, the U.S.

Environmental Protection Agency, as well as state and local air pollution agencies, have

passed certain environmental laws providing limitations and technological standards on

the amount of NOx a facility may emit. These standards are continuing to become more

and more stringent, creating a technological need for low-NOx burners.

Decreasing the NOx emissions from a burner is a well-known need. For

example, U.S. Patent No. 4,004,875 to Zink et al. (hereinafter "the Zink patent") discloses

a low-NOx burner concept that introduces secondary air to the hot face of the burner in

addition to the primary air. In the Zink patent, primary air is provided in an amount that

is insufficient to completely combust the fuel. The secondary air is introduced in a

second stage to complete the combustion process. Overall, the use of staged air in this

manner leads to reduced NOx emissions from the burner unit. Likewise, U.S. Patent No.

4,347,052 to Reed et al. discloses the use of primary, secondary and tertiary air in

predetermined stoichiometric proportions in order to stage combustion and, thus, reduce

the production of NOx from the burner. Finally, U.S. Patent No. 4,983,118 to Hovis et

al. describes the use of air staging to reduce the production of NOx from a regenerative burner. The introduction of secondary or tertiary air in all of these burner concepts demonstrates the well-known usage of incomplete combustion to retard the production

of NOx from the burner. This retardation occurs due to the overabundance of carbon

dioxide, water vapor and methane in the burner mix at the initial stage.

As the environmental laws tighten, there is still considerable room in the

art for technology that further reduces the production of NOx from industrial burners.

While the above-referenced patents, among others, use incomplete combustion to reduce

NOx, improvements over this design concept are in need.

SUMMARY OF THE INVENTION

The present invention uses staged air combustion to reduce the production

of NOx from a burner and includes a burner body adjacent a port block. The present

invention also includes a fuel passageway connecting a fuel source to a burner throat.

Primary air jets are connected to an air source and inject air into a primary combustion

region. This primary combustion region is in the burner throat. The primary air jets can

be configured such that air is introduced into the primary combustion region in a swirling

manner. A dish surface is located in the port block; the dish surface extending in an

angle divergent with respect to a centerline extending through the burner throat. Finally,

the present invention utilizes secondary air jets connected to an air source. These

secondary air jets extend through the port block and inject secondary air into a secondary

combustion region located downstream from the primary combustion region.

The present invention also includes a method of reducing NOx emissions

from a burner, wherein fuel is taken from a fuel source and injected into a burner throat

via a fuel passageway, and primary air is injected from an air source into a primary

combustion region in the burner throat. Further, this primary combustion is conducted

in a fuel-rich highly vitiated environment which consumes available oxygen, limiting flame temperature and thermal NOx. Fuel is fed into the burner and proceeds to the throat

where the primary air and fuel mix together to form the initial stage of combustion. A

combustion reaction is initiated in the burner throat. The preferable convergent, angled

introduction of the air through the primary air jets creates a swirling cyclone pattern that

hugs the walls of the port block and pulls and mixes the fuel and recirculated products

of combustion into the cyclone. After the primary combustion step, the air/fuel mixture

then enters a secondary combustion region. Air is introduced into the secondary

combustion region so as to allow the combustion process to complete. Products of combustion are drawn into a vortex created by the swirling mixture of fuel and air during

the combustion process. The overall NOx production is thereby reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of a single stage burner design according to the prior

art;

Fig. 2 is a side view of a first embodiment according to the present

invention;

Fig. 3 is a side view of a second embodiment according to the present

invention;

Fig. 4 is a side view of a third embodiment according to the present

invention;

Fig. 5 is a front view of the present invention illustrating a secondary air

jet hole configuration in a dish surface on a burner;

Fig. 6 is a front view of the present invention illustrating a further

secondary air jet configuration in a hot face of the burner; Fig. 7 is a front view of the present invention illustrating a still further

secondary air jet configuration in a hot face of the burner;

Fig. 8 is a side view of the present invention illustrating the use of

multiple air supplies as applied to a non-regenerative burner;

Fig. 9 is a front view of the present invention illustrating a swirling

primary air jet configuration;

Fig. 10 is a side view of the present invention illustrating a two direction

gas nozzle configuration;

Fig. 11 is a table illustrating the NOx emissions of the present invention

versus conventional Coanda burners; and

Fig. 12 is a side view of a further embodiment according to the present

invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in Fig. 1, the design of a typical prior art burner includes a burner

body 10, which houses an air passageway 14 and a fuel passageway 12. The air

passageway 14 may have an optional heat storing media 18 area, depending upon the

application. Fuel is introduced into the fuel passageway 12, which directs the fuel

through the burner body 10, and flows out through a fuel nozzle 22. All required

combustion air enters through an air entrance 16, runs through the air passageway 14 and

enters a combustion region through primary air jets 20. The burner body 10 is fixed to

a port block 42. The fuel and air initially mix in a burner throat 40 of the burner.

Combustion occurs in the burner throat 40 and continues into cup 26 and from these to

a space surrounded by a dish surface 28. The present invention is an apparatus and method directed to an air-staged

low-NOx burner. The first embodiment is illustrated in Fig. 2. A liquid or gaseous fuel

is introduced into the burner body 10 through the fuel passageway 12 where it proceeds

through the fuel nozzle 22 into the burner throat 40 in a primary combustion stage 24.

The air enters through the air entrance 16 where it may or may not pass through the heat

storing media 18. The air flows through the air passageway 14 and is split into primary air (i.e., the first air to be introduced to the fuel), which exits through the primary air jets

20, and secondary air, which exits through secondary air jets 34.

Due to the jet action and an angular orientation of the primary air jets 20,

the air enters the throat 40 in a swirling manner, illustrated as line 21 in Fig. 2. This

swirling pattern is created by tangential forces and causes the swirling air to travel along

the dish surface 28 of the port block 42. This swirling and sticking phenomena (line 21)

is called the "Coanda effect", which also creates a negative vortex within the center of

the air swirl. This negative vortex pulls the fuel stream and recirculated products of

completed combustion into the swirling air 21, mixing the components together. A

preferable angular orientation of the primary air jets 20 is illustrated in Fig. 9.

The combustion process is initiated by spark, pilot flame or another

suitable method. Upon ignition, combustion occurs in the primary combustion region 24.

However, the fuel to primary air ratio is adjusted to ensure this combustion occurs under

a highly vitiated fuel-rich condition. The fuel-rich condition allows the combustion

process to consume all available oxygen, disallowing complete combustion and

preventing creation of excess thermal NOx. Combustion under fuel-rich conditions,

coupled with the recirculated products of combustion pulled through the vortex, limits

flame temperature and reduces the amount of thermal NOx produced. Further, the "Coanda effect" causes the combusted mixture to continue along the surface of the burner

throat 40, the cup 26, and along the dish surface 28. This also provides a uniform temperature and rotating flame within the port block 42. The dish surface 28 extends in

a divergent manner with respect to a centerline 35 running through the longitudinal axis

of the burner throat 40. Specifically, in the case of a planar or flat dish surface 28, this

angle of divergence between the dish surface 28 and centerline 35 may be between

about 25° and 89° (i.e., ±5° on either end of the range) with the preferred angle between

about 25° and about 50° (i.e., ±5°).

It is also envisioned that the dish surface 28 may have a continuously

shifting angle of divergence α, resulting in a trumpet-like shape to the dish surface 28.

As shown in Fig. 12, the angle of divergence , measured between the centerline 35 and

a line tangential to the rounded, bell-shaped dish surface 28, is continuously shifting. The

trumpet-like shaped dish surface 28 of Fig. 12 still allows for the required Coanda effect,

with enhancement of the Coanda effect by the secondary air jets 34.

As the combusted mixture rides out of the cup 26 and into the dish

surface 28, the negative vortex continues to pull the products of combustion through the

mixture from a furnace atmosphere into which the burner is firing. This mixture then

encounters the secondary air jets 34, which open into the dish surface 28. In a preferred

embodiment, these secondary air jets 34 are oriented in a divergent manner. As

illustrated in Figs. 2 and 3, the secondary air jets 34 are divergent with respect to the

centerline 35 running through the longitudinal axis of the burner throat 40. The angle of

divergence β between the secondary air jets 34 and centerline 35 may be between 1° and

89°, however the optimal range is between about 15° and about 50° (i.e., ±5°). Larger

angles could be beneficial to flame shape, but become difficult from a construction standpoint. It is envisioned that the burner throat 40, as well as the fuel passageway 12 extend perpendicularly to the port block 42 in a normal burner configuration. The

divergent orientation of the secondary air jets 34 encourages the same "Coanda effect",

further maintaining the negative vortex. Again, this negative vortex continues to pull the

air/fuel/products of combustion together into a homogenous mixture. This homogenous

mixture, created by the use of the secondary air jets 34, controls the combustion process

and limits the flame temperature, thereby limiting the amount of thermal NOx produced

in a secondary combustion region 38.

The primary air jets 20 and the secondary air jets 34 are controlled as to

both velocity and air split ratio. Both of these characteristics control the flame geometry,

combustion pattern and the amount of emissions emitted from the burner. Specifically,

it is envisioned that the air split ratio be within the limits of 40/60 (primary air/secondary

air) to 75/25 (primary air/secondary air). As shown in Fig. 11, using a 58% primary

air/42% secondary air split ratio together with the above described invention, the burner

NOx emissions are significantly reduced. However, this air split ratio can vary according

to the use of ambient air and other variable factors.

Another embodiment of the present invention is illustrated in Fig. 3. This

embodiment operates in substantially the same manner as the first embodiment described

above. However, as opposed to the secondary air jets 34 entering the dish surface 28 in

a divergent orientation, the secondary air jets 34 open at a hot face 30 in a divergent

orientation. In this embodiment, the secondary combustion zone 38 is moved further into

the furnace. The swirling pattern and negative vortex are created due to the angular entry

of primary air. The flame geometry and overall combustion process are altered in the

new orientation. The mixing of the secondary air with uncombusted partially-reacted fuel is further delayed (relative to Fig. 2), yielding further NOx reduction and increased flame diameter.

The third embodiment of the present invention is illustrated in Fig. 4.

This embodiment operates in substantially the same manner as the first embodiment

described above. However, as opposed to the secondary air jets 34 entering the dish

surface 28 in a divergent orientation, the secondary air jets 34 enter the hot face 30 in an

orientation parallel to the centerline 35 extending through the longitudinal axis of the

burner throat 40. The flame geometry and overall combustion process are altered in the

new orientation. The flame will be more stable and produce only slightly higher NOx

(relative to the first and second embodiments).

While the current air supply of primary and secondary air is described as

emanating from a common air source, it is also anticipated that a second air source can

be used to supply the secondary air jets 34. For example, the air may be supplied through

direct connections to passageways in the port block 42. Using alternate air supplies allow

further control of the flame geometry and combustion characteristics through

stoichiometric variation. As seen in Fig. 8, with application to a non-regenerative burner

configuration, the secondary air jets 34 can be supplied through a different air source. For

example, a secondary air inlet 46 can be utilized, allowing secondary air to flow through

a secondary air passageway 44 into the secondary air jets 34. This would allow the use

of air with different qualitative and quantitative variations than the primary air, yielding further control over the process. Still further, each of the secondary air jets 34 may have

identical or different air sources from each other, allowing even greater control of the

process. In another variation, the number and location of secondary air jets 34 may

be changed, affecting the flame geometry and combustion process. Fig. 5 shows a first

secondary air jet configuration, using four secondary air jets 34 equally spaced around the dish surface 28. Fig. 6 shows a second secondary air jet configuration, using four

secondary air jets 34 equally spaced around the hot face 30. Fig. 7 shows a third

secondary air jet configuration, using six secondary air jets 34 equally spaced around the

hot face 30. It will be apparent to those skilled in the art that the number of secondary

air jets 34 used, and their relative location, can vary. The preferred arrangement is with

equally spaced secondary air jets 34, however, non-uniformly spaced jets will function

with minor change in NOx emissions.

Another arrangement for adjusting flame stability is seen in Fig. 10.

Specifically, using a two-direction fuel nozzle 48 more evenly distributes the fuel into the fuel/primary air mixture. This optional addition would create an even more homogenous

mixture of fuel and air.

It will be evident to those of ordinary skill in the art that various changes

and modifications may be made to the present invention without departing from the spirit

and scope thereof. For example, the swirling effect in the burner throat 40 could be

accomplished by swirling the fuel, instead of swirling the primary combustion air, as

described above. It is therefore intended that the invention be limited only by the

attached claims, and equivalents thereof.

Claims

We claim:

1. An air-staged low-NOx burner, comprising:

a burner body adjacent a port block;

a fuel passageway for connecting a fuel source to a burner throat in the burner body;

at least one primary air jet configured to inject air provided by an air

source into a primary combustion region located in the burner throat, the at least one

primary air jet configured to produce a swirling effect in the burner throat; a dish surface in the port block, the dish surface diverging with respect to

a centerline extending through the burner throat;

at least one secondary air jet extending through the port block, the at least

one secondary air jet configured to inject secondary air into a secondary combustion

region located downstream from the primary combustion region.

2. The air-staged low-NOx burner of claim 1, wherein the at least one

secondary air jet is configured to inject secondary air into one of the dish surface and a

hot face.

3. The air-staged low-NOx burner of claim 1, wherein the at least one

secondary air jet extends at an angle divergent with respect to the centerline extending

through the burner throat.

4. The air-staged low-NOx burner of claim 3, wherein the angle of

divergence of the at least one secondary air jet is from about 15° to about 50°.

5. The air-staged low-NOx burner of claim 1, wherein the at least one

secondary air jet extends at an angle parallel with respect to the centerline extending

through the burner throat.

6. The air-staged low-NOx burner of claim 1, further comprising a heat

storing media located in the burner body.

7. The air-staged low-NOx burner of claim 1, wherein the fuel

passageway terminates in a fuel nozzle.

8. The air-staged low-NOx burner of claim 7, wherein the fuel nozzle is

a two-directional fuel nozzle.

9. The air-staged low-NOx burner of claim 1, wherein a dish surface angle

of divergence is between about 25° and about 50°.

10. The air-staged low-NOx burner of claim 1, wherein a dish surface

angle of divergence is continuously shifting, with respect to the centerline, resulting in a trumpet-shaped dish surface.

11. The air-staged low-NOx burner of claim 1 , further comprising a

discrete secondary air source, the secondary air source providing secondary air to the at

least one secondary air jet via a secondary air passageway.

12. The air-staged low-NOx burner of claim 1, further comprising four

secondary air jets having secondary air jet openings equally spaced around and extending to the dish surface.

13. The air-staged low-NOx burner of claim 1 , further comprising four

secondary air jets having secondary air jet openings equally spaced around and extending to a burner hot face on the port block.

14. The air-staged low-NOx burner of claim 1, further comprising six

15. The air-staged low-NOx burner of claim 1, wherein the at least one

primary air jet extends at an angle convergent with respect to the centerline extending

through the burner throat.

16. An air-staged low-NOx burner, comprising:

a burner body adjacent a port block;

a fuel passageway terminating in a fuel nozzle for connecting a fuel

source to a burner throat;

at least one primary air jet configured to inject primary air into a primary

combustion region located in the burner throat; a dish surface in the port block, the dish surface diverging with respect to

a centerline extending through the burner throat;

at least one secondary air jet extending through the port block, the at least

one secondary air jet configured to inject secondary air into a secondary combustion region located downstream from the primary combustion region.

17. The air-staged low-NOx burner of claim 16, wherein at least one of

the primary air jet and the fuel nozzle is configured to produce a swirling effect in the

burner throat.

18. The air-staged low-NOx burner of claim 16, wherein the at least one

hot face.

19. The air-staged low-NOx burner of claim 16, wherein the at least one

through the burner throat.

20. The air-staged low-NOx burner of claim 19, wherein the angle of

21. The air-staged low-NOx burner of claim 16, wherein the at least one

through the burner throat.

22. The air-staged low-NOx burner of claim 16, further comprising a heat

storing media located in the burner body.

23. The air-staged low-NOx burner of claim 16, wherein the fuel nozzle is a two-directional fuel nozzle.

24. The air-staged low-NOx burner of claim 16, wherein a dish surface

angle of divergence is between about 25° and about 50°.

25. The air-staged low-NOx burner of claim 16, wherein a dish surface

angle of divergence is continuously shifting, with respect to the centerline, resulting in

a trumpet-shaped dish surface.

26. The air-staged low-NOx burner of claim 16, further comprising a

least one secondary air jet via a secondary air passageway.

27. The air-staged low-NOx burner of claim 16, further comprising four

secondary air jets having secondary air jet openings equally spaced around and extending

to the dish surface.

28. The air-staged low-NOx burner of claim 16, further comprising four

29. The air-staged low-NOx burner of claim 16, further comprising six

30. A method of reducing NOx emissions from a burner comprising the

steps of:

(a) injecting fuel from a fuel source into a burner throat via a fuel passageway;

(b) injecting primary air into a primary combustion region located in the

burner throat, the ratio of fuel to primary air being such as to create a fuel rich mixture

of fuel and primary air;

(c) inducing a swirling effect upon the mixture of fuel and primary air in

the burner throat;

(d) combusting the mixture of fuel and primary air;

(e) passing the swirling mixture of fuel and primary air to a port block

wherein, due to a Coanda effect, at least part of the mixture of fuel and primary air

remains adjacent a dish surface in the port block;

(f) injecting secondary air into a secondary combustion region located

downstream from the primary combustion region in an amount at least sufficient to

complete the combustion of the fuel; (g) drawing products of combustion into a vortex created by the swirling

mixture of fuel and air during the combustion process, thereby reducing NOx produced

in the combustion process.

31. The method of claim 30, wherein the swirling effect in the burner

throat is induced by configuration of at least one primary air jet.

32. The method of claim 30, further comprising the step of passing the

primary and secondary air through a heat storing media.

33. The method of claim 30, wherein the secondary air is injected through

at least one secondary air jet diverging with respect to a centerline extending through the

burner throat.

34. The method of claim 33, wherein the angle of divergence of the at

least one secondary air jet is between about 15° and about 50°.

35. The method of claim 31, wherein the secondary air is injected to the

dish surface.

36. The method of claim 30, wherein the secondary air is injected to a

burner hot face on the port block.

37. The method of claim 30, wherein the secondary air is provided from a discrete secondary air source, which connects to at least one secondary air jet via a

secondary air passageway.

38. The method of claim 30, wherein the fuel is injected through a fuel nozzle on an end of the fuel passageway.

39. The method of claim 38, wherein the fuel nozzle is configured to

inject the fuel in more than one direction.

40. The method of claim 38, wherein the fuel is caused to swirl in the

burner throat via the fuel nozzle.

41. The method of claim 30, wherein the air split ratio of primary air to

secondary air is within the range 40/60 to 75/25.