Cooling system for post-mixed burner

A post-mixed burner having a cooling system which brings cooling water preferably from the area of the fuel tube, across the area of oxidant passages, proximate the burner face, and out of the burner preferably in the outermost conduit.

TECHNICAL FIELD 
This invention relates to a cooling system for a post-mixed burner having 
separate fuel and oxidant conduits which discharge into a furnace at the 
burner face. 
BACKGROUND ART 
Burners which operate in high temperature furnaces are cooled in order to 
preserve the structural integrity of the burner components and to retard 
the oxidation rate of hot metallic surfaces. In conventional burners using 
ambient air as the oxidant, adequate cooling of the burner is generally 
provided by the combustion air. 
Recently the use of oxygen or oxygen-enriched air has been gaining 
prominence as an oxidant for burners because its use is more energy 
efficient and less pollution generating than the use of air. However, use 
of oxygen or oxygen-enriched air as the burner oxidant has resulted in a 
number of problems with conventional cooling systems designed to cool a 
burner which uses air as the oxidant. 
First, as is well known, oxygen or oxygen-enriched air typically produces a 
hotter flame than that produced by air. Thus oxygen burners are exposed to 
higher heat flux from the flame. A second problem with oxygen burners 
results from the fact that the volume of the oxidant required to burn a 
unit amount of fuel is reduced significantly as compared with an air 
burner. Thus it is difficult to provide adequate cooling of the burner 
using the oxidant. 
Cooling of burners using oxygen or oxygen-enriched air is often provided by 
a separate cooling fluid. The most common cooling fluid is water. The 
amount of heat that cooling water is able to remove is a function of the 
conduction heat transfer from hot surfaces to water cooled surfaces and 
the convection heat transfer from water cooled surfaces to the water. It 
is generally desirable to provide cooling water as close to the hot 
surfaces as possible at a sufficient velocity to effectively transfer heat 
from the hot surfaces to water. 
For a post-mixed burner having separate fuel and oxidant conduits which 
discharge into a furnace or a small burner block at the burner face, it is 
desirable that both the fuel and the oxidant conduits as well as the 
burner exterior surfaces be directly cooled by water in order to provide 
effective cooling. 
One known method for providing cooling to a post-mixed burner is to provide 
cooling fluid in an incoming and outgoing annular stream between the fuel 
and annular oxidant conduits and in a separate incoming and outgoing 
annular stream on the outside of the oxidant conduit. Although such a 
system adequately cools the fuel and oxidant conduits and brings cooling 
fluid quite close to the burner face, it is disadvantageous because of the 
high fabrication costs required for the two separate cooling streams and 
also because it significantly increases the burner outside diameter. 
Another known cooling system for a post-mixed burner which can be employed 
when the oxidant and fuel are delivered to the burner face in separate, 
i.e. not concentric, tubes employs a number of oxidant tubes submerged in 
cooling water. Such a system effectively cools the burner but has the 
disadvantages of high fabrication costs, especially when the number of 
oxidant tubes is large, such as greater than four, and of high pressure 
drop in the oxidant tubes because of the small total cross-sectional area 
of the oxidant tubes. 
It is often desirable to outfit the discharge end of oxidant tubes with 
directional nozzles which direct the oxidant flow in a direction other 
than straight ahead. It is further desirable that such nozzles be 
replaceable to allow for a variety of flow directions. However, such 
replaceable nozzles require a solid portion proximate the burner face in 
order to provide a threaded seat to hold the nozzles. This portion is more 
susceptible to overheating because of its proximity to the burner face. 
The burner having a cooling system such as the one first described above 
is not applicable to this situation since it employs an annular oxidant 
tube. Replaceable nozzles are employed only on separate oxidant passages. 
The problem of cooling such a burner is made greater by the cooling 
requirements of the replaceable nozzles which can oxidize and seize in the 
threaded area thus rendering them incapable of removal. 
It is therefore an object of this invention to provide a burner having an 
improved cooling system. 
It is another object of this invention to provide a compact cooling system 
for a post-mixed burner which uses oxygen or oxygen-enriched air as the 
oxidant. 
It is a further object of this invention to provide an effective cooling 
system for a burner which employs replaceable nozzles. 
SUMMARY OF THE INVENTION 
The above and other objects which will become apparent to those skilled in 
the art upon a reading of this disclosure are attained by: A burner 
comprising: 
(a) a fuel tube having its discharge end at the burner face; 
(b) an annular oxidant passageway circumferentially around said fuel tube 
and axially along said fuel tube to a point short of the burner face so as 
to define a space between said point and the burner face; 
(c) a plurality of oxidant passages passing through said defined space, 
connected to and communicating with said annular oxidant passageway at 
said point and having their discharge end at the burner face; 
(d) a second annular passageway circumferentially around said fuel tube and 
extending along said fuel tube into said defined space, adapted for flow 
of cooling fluid and positioned between said fuel tube and said annular 
oxidant passageway; 
(e) a third annular passageway circumferentially around both said fuel tube 
and said oxidant passageway, extending into said defined space and adapted 
for flow of cooling fluid; and 
(f) at least one connecting conduit, connecting said second and third 
annular passageways, in said defined space.

DETAILED DESCRIPTION 
The burner of this invention will be described in detail with reference to 
the drawings. As indicated above, the drawings depict one preferred 
embodiment of the burner of this invention. The numerals for FIG. 1 and 2 
are identical for the common elements of burner 9. 
Fuel is delivered to the furnace through fuel tube 1 and is discharged into 
the furnace at the burner face 2 which is essentially perpendicular to the 
flow direction of fuel through fuel tube 1. The burner may be flush with 
the furnace wall or recessed a short distance in a burner block as is well 
known to those skilled in this art. Oxidant annulus 3 is circumferentially 
around fuel tube 1 and extends axially along the fuel tube to a point 10. 
At this point the oxidant annulus is connected to and communicates with a 
plurality of oxidant passages 11 which extend from the oxidant annulus to 
burner face 2 and discharge into the furnace. The burner comprises a 
relatively solid portion 12 from the burner face to point 10. This portion 
is commonly referred to as the burner head. It is preferred that the 
burner head be a unitary piece as this will facilitate heat transfer 
better than a piece which has been welded or otherwise fastened together. 
The plurality of oxidant passages 11 extend through portion or space 12 
from the oxidant annulus to the burner face essentially parallel to fuel 
tube 1. Space 12 may conveniently also contain threaded seats for the easy 
attachment and removal of replaceable nozzles. 
The embodiment of FIGS. 1 and 2 is a preferred embodiment wherein there are 
eight oxidant passages equispaced around one central fuel tube. Each 
oxidant passage is equipped with a nozzle 4 which is threaded for easy 
removal and replacement. The illustrated preferred embodiment also has a 
small annular conduit 5 for the delivery of annular oxidant to the fuel 
stream in order to stabilize the flame. Such a small annular conduit is 
particularly useful when the oxidant is oxygen. 
Cooling fluid is preferably provided to the burner through second annular 
passageway 6 which is positioned axially along and radially around the 
fuel tube 1. This second annular passageway extends into space 12 and 
preferably extends as close to burner face 1 as possible. The cooling 
fluid is preferably removed from the burner through third annular 
passageway 7 which is positioned axially along and radially around both 
fuel tube 1 and annular oxidant passageway 3 and extends into space 12. 
Preferably third annular passageway 7 extends as close to burner face 2 as 
does second annular passageway 6. Annular passageways 6 and 7 are 
connected to one another by at least one connecting conduit 8. The 
illustrated embodiment depicts a preferred arrangement wherein there are 
eight connecting conduits 8, each between two different oxidant passages 
11. Each connecting conduit 8 being parallel to the burner face and 
connecting both the second and third annular passageways at their 
respective points most proximate burner face 2. As mentioned, it is 
preferable that cooling fluid be provided to the burner through passageway 
6 and removed from the burner through passageway 7. However, if desired, 
the roles of these passages may be reversed, i.e., the cooling fluid could 
be provided to the burner through passageway 7 and withdrawn from the 
burner through passageway 6. 
In operation, fuel which is generally coke oven gas or natural gas, and 
oxidant flow in their separate conduits and are discharged through the 
discharge end of each conduit into the furnace at the burner face. 
Combustion occurs upon mixture of the fuel and oxidant. Due to the intense 
flame created proximate to the burner face, the burner components are 
subject to high heat flux resulting in heating of the burner components. 
Cooling fluid, generally preferably water, is brought to the hot area 
preferably through second annular passageway 6. The cooling water flows to 
the end of passageway 6 inside space 12 where it is directed radially 
outward through conduit 8 and into third annular passageway 7, through 
which the warmed cooling water is removed from the burner. 
The components of burner 9 for which cooling is most important are the 
burner face, the oxidant nozzles and the fuel tube. Cooling is very 
important for the burner face because it is the component closest to the 
combustion reaction thus receiving more heat than other burner components. 
Cooling is very important for the oxidant nozzles because high 
temperatures will increase the oxidation rate and possibly result in the 
threaded area seizing, rendering the nozzles unremovable. Cooling is very 
important to the fuel tube because due to the small annular oxidant 
conduit, the fuel tube surface is not directly water cooled. 
The cooling system of the burner of this invention successfully addresses 
each of these concerns. First, preferably the cooling water flows closest 
to the fuel tube when it is in its coldest condition thus facilitating 
heat removal from the fuel tube by radiation heat transfer even though 
there is no direct contact between the hot fuel tube and a water cooled 
surface. Second, the cooling water flows completely around or across the 
oxidant passages within space 12 and proximate the burner face. This 
facilitates heat removal from larger portions of the oxidant nozzles than 
is possible with conventional designs. Third, the cooling water flows 
across a larger area proximate the burner face because it flows in from 
close to the fuel tube on the inside of the oxidant passages, across the 
oxidant passages, and out on the outside of the oxidant passages. The 
large area proximate the burner face where the cooling were flows across 
the oxidant passages through connecting conduits 8 greatly improves the 
heat removal from the burner face. 
The following examples serve to further illustrate the benefits of the 
burner of this invention of demonstrate the advantages of the burner of 
this invention over the cooling available with use of a conventional 
cooling arrangement. 
EXAMPLE 1 
A burner similar to that depicted in FIGS. 1 and 2 was extended into a hot 
furnace and cooled by flowing cooling water through the burner at the rate 
of 8.1 gallons per minute (gpm). The cooling fluid flowed in the preferred 
direction of toward the burner face in passageway 6, radially outward 
through conduits 8 and away from the burner face through passageway 7. At 
steady state conditions the furnace temperatures was 2397.degree. F., the 
temperature of the fuel tube at the discharge end was 1901.degree. F. and 
the temperature of the oxidant nozzles was 232.degree. F. The heat carried 
away by the cooling water, calculated based on the rise in water 
temperature and the flowrate was 0.073 million BTU per hour. The 
temperature of the incoming water was 61.degree. F. and the temperature of 
the outgoing water was 79.degree. F. 
The cooling water flowrate was then reduced to 4.1 gpm and at steady state 
the temperature of the fuel tube discharge end was 1902.degree. F., the 
temperature of the oxidant nozzles was 246.degree. F. and the heat removal 
was at a rate of 0.066 million BTU per hour. The incoming water 
temperature was 62.degree. F. and the outgoing water temperature was 
94.degree. F. 
For comparative purposes a post-mixed burner was extended into a hot 
furnace and cooled using cooling water flowing through a conventional 
cooling system wherein cooling water is supplied through an annular cavity 
radially outward from the annular oxygen passageway, and is removed by 
directing the water flow 180 degrees into another annular cavity radially 
outward the first. The cooling water flowrate was 8 gpm. The temperature 
of the furnace was 2326.degree. F., the temperature of the fuel tube at 
the discharge end was 1994.degree. F. and the temperature of the oxidant 
nozzles was 490.degree. F. Heat removal was at a rate of only 0.040 
million BTU per hour. The incoming water temperature was 52.degree. F. and 
the outgoing water temperature was 62.degree. F. 
It is thus demonstrated tat the cooling system of the burner of this 
invention produces significantly improved cooling over that attainable be 
conventional cooling systems for post-mixed burners. 
As indicated earlier, the advantages of the burner of this invention are 
more apparent when oxygen or oxygen enriched air is the oxidant. Other 
advantages of this invention, in addition to those discussed earlier, are 
ease of manufacture due to a much smaller burner head with no internal 
threads, and ease of water distribution. 
By the use of the burner and cooling system of this invention one can 
employ replaceable oxidant nozzles at the burner face and yet adequately 
cool the burner face and the portion of the burner proximate the burner 
face which is needed to support the nozzles. The cooling is accomplished 
by bringing cooling fluid toward the burner face preferably close to the 
inner fuel tube and on the inside of the major oxidant annulus. The 
cooling fluid travels past the end of the major oxygen annulus into the 
space through which pass the plurality of oxidant passages. In this space 
the cooling fluid is able to travel across the plurality of oxidant 
passages and proximate the burner face. From this point the cooling fluid 
travels out away from the burner face preferably on the outside of the 
major oxidant annulus.