U.S. patent number 3,837,788 [Application Number 05/295,249] was granted by the patent office on 1974-09-24 for reduction of gaseous pollutants in combustion fuel gas.
This patent grant is currently assigned to Aqua-Chem, Inc.. Invention is credited to Glenn D. Craig, David T. Feuling, Paul G. LaHaye.
United States Patent |
3,837,788 |
Craig , et al. |
September 24, 1974 |
REDUCTION OF GASEOUS POLLUTANTS IN COMBUSTION FUEL GAS
Abstract
Fuel is burned in a primary combustion chamber with less than
the air required for stoichiometric combustion so that the
combustion gases have a high carbon monoxide (CO) and a hydrocarbon
content and the temperature of the gases is held below that at
which significant nitrogen oxides (NO.sub.x) would be produced. The
combustion gases are then passed through a secondary combustion
zone in which more air is injected into the gas stream to oxidize
the CO and hydrocarbons to carbon dioxide (CO.sub.2). The secondary
burner comprises a plurality of foraminous tubes through which
secondary air is emitted. Combustion in the secondary zone is
maintained at a temperature below that at which nitrogen oxides
(NO.sub.x) will be produced in significant quantities.
Inventors: |
Craig; Glenn D. (Menomonee
Falls, WI), Feuling; David T. (Milwaukee, WI), LaHaye;
Paul G. (Cape Elizabeth, ME) |
Assignee: |
Aqua-Chem, Inc. (Milwaukee,
WI)
|
Family
ID: |
27393929 |
Appl.
No.: |
05/295,249 |
Filed: |
October 5, 1972 |
Current U.S.
Class: |
431/351;
431/10 |
Current CPC
Class: |
B01D
53/56 (20130101); F24H 9/1845 (20130101); F23C
13/00 (20130101); F23J 15/00 (20130101); F23C
6/04 (20130101); F23C 6/00 (20130101); F23B
5/00 (20130101); B01D 53/8631 (20130101); F24H
1/0045 (20130101); Y02A 50/20 (20180101); F23B
2700/018 (20130101); Y02A 50/2328 (20180101) |
Current International
Class: |
B01D
53/56 (20060101); F23J 15/00 (20060101); B01D
53/86 (20060101); F23C 6/04 (20060101); F23C
6/00 (20060101); F24H 9/18 (20060101); F23C
13/00 (20060101); F24H 1/00 (20060101); F23l
009/00 () |
Field of
Search: |
;431/351,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Favors; Edward G.
Claims
We claim:
1. Fuel burning apparatus constructed and arranged for minimizing
the discharge of nitrogen oxide in its exhaust gases
comprising:
means defining a primary combustion zone having an inlet for
primary combustion air and an outlet for gaseous combustion
products,
fuel delivery means for providing fuel to said primary combustion
zone, air delivery means coupled to said primary combustion zone
and constructed and arranged for delivering less than the
stoichiometric amount of air to said inlet to burn said fuel at a
temperature and under conditions whereby the combustion products
include significant residual amounts of unoxidized hydrocarbons and
carbon monoxide,
means defining a secondary combustion zone coupled to the outlet of
the primary combustion zone for receiving the stream of hot gaseous
combustion products from the primary combustion zone, the direction
of gas flow from said primary combustion zone to said secondary
combustion zone defining a downstream direction,
gas supply means coupled to a source of combustion supporting gas
and including a plurality of tube means extending across said
secondary combustion zone whereby said gaseous combustion product
stream is forced to flow between and around said tube means,
and means defining a plurality of spaced apart gas passages formed
in the downstream side of said tube means for injecting combustion
supporting gas into plural regions of said gas stream as the latter
passes around said tube means whereby to oxidize the residual
unoxidized hydrocarbon and carbon monoxide at a temperature below
which significant amounts of nitrogen oxides are formed.
2. The combustion device set forth in claim 1 wherein:
a. said tube means are comprised of porous material which is
refractory to the heat prevailing in the secondary combustion zone
and said openings are the pores of said material.
3. The combustion device set forth in claim 1 wherein:
a. said supply means are a plurality of metal tubular elements
wherein said gas dispersing passages comprise openings located on
the downstream side thereof.
4. The combustion device set forth in claim 1 wherein:
a. said supply means are a plurality of tubular elements of
refractory material wherein said gas dispersing passages comprise
openings located on the downstream side thereof.
5. The combustion device set forth in claim 1 wherein each of said
tube means comprises:
a. an inner tubular element which is adapted for being connected to
said source of combustion supporting gas, said inner tubular
elements having said openings through their walls,
b. an outer tubular element surrounding each inner tubular element
to define a cooling fluid flow space therewith,
c. hollow means extending sealingly from said openings through said
cooling fluid space and said outer tubular element for permitting
the combustion supporting gas to discharge from said inner tubular
elements into said gas stream, and
d. means for coupling said outer tubular elements to a source of
cooling fluid.
6. The combustion device set forth in claim 1 wherein said
plurality of tube means are a plurality of spaced apart tubular
means, each said tubular means comprising:
a. an inner tubular element having at least two elements having
passageways extending radially therefrom and communicating with the
interior of said inner tubular element at one end and with said
secondary combustion zone at the other end so that combustion
supporting gas may flow to said zone,
b. an outer tubular element surrounding said inner element with
said passageway elements extending therethrough in a sealed manner,
said outer element and inner element defining a cooling fluid flow
space.
7. The device set forth in claim 6 wherein:
a. said passageway elements are elongated and substantially axially
coextensive with said tubular elements and the radially remote gas
discharge ends thereof are directed generally downstream of the
gaseous combustion product stream.
8. The device set forth in claim 7 wherein:
a. said passageway elements are substantially at an angle of about
120.degree. from each other and are disposed on the downstream side
of the tubular means equiangularly from a plane extending in the
general direction of gas flow and through the center of the tubular
elements.
9. The apparatus set forth in claim 1 wherein each of said tube
means comprises a tube extending across said secondary combustion
zone, said tubes being spaced apart in a direction generally
transverse to the direction of said gas flow, whereby gas flow
paths are defined between adjacent tubes, said passages for
combustion supporting gas being formed in each tube in the
downstream side thereof for injecting said combustion supporting
gas into said gas flow paths.
10. The apparatus set forth in claim 1 wherein each of said
plurality of tube means includes a first and a second group of tube
means, the tube means in each group being separated from each other
in a direction that is substantially transverse to the flow
direction of said gaseous combustion products, said second group of
tube means being located on the downstream side of said first group
of tube means.
11. The apparatus set forth in claim 10 wherein said tube means
from each group are spaced from each other and arranged in rows
which extend substantially transversely to the general flow
direction of said gaseous combustion products, the rows defined by
said second group of tube means being displaced in the downstream
direction from the row defined by said first group of tube means
and the tube means in each row being disposed in the path of
gaseous flow between the tube means in the other row.
12. The apparatus set forth in claim 1 wherein said tube means are
elongate, a plurality of gas dispersing openings formed
longitudinally in said tube means, a first portion of said openings
also being spaced longitudinally on said tube means relative to the
remainder of said openings.
13. The apparatus set forth in claim 12 wherein said openings are
arranged in at least a pair of longitudinal rows, said rows being
displaced from each other at an angle of about 120.degree. relative
to the longitudinal axis of said tube means.
14. The apparatus set forth in claim 13 wherein said tube means
have nozzle means extending therefrom, said nozzle means having
openings formed therein for dispersing said combustion supporting
gas.
15. The apparatus set forth in claim 14 and including cooling means
disposed adjacent said tube means for defining a space for
conducting a coolant in a heat exchange relationship with said tube
means.
16. The apparatus set forth in claim 15 wherein each of said
cooling means comprises tubular means embracing its respective said
tube means, said nozzles extending through said cooling tubes for
dispersing gas into said secondary combustion zone.
17. The apparatus set forth in claim 16 and including first header
means connected to each of said tube means for conducting
combustion supporting gas thereto and second header means connected
to said cooling tubular means for conducting a coolant thereto.
18. The apparatus set forth in claim 17 wherein each of said
plurality of tube means includes a first and a second group of tube
means, the tube means in each group being separated from each other
in a direction that is substantially transverse to the flow
direction of said gaseous combustion products, said second group of
tube means being located on the downstream side of said first group
of tube means.
Description
BACKGROUND OF THE INVENTION
Oxides of nitrogen and carbon monoxide are gaseous pollutant
products of the combustion of hydrocarbon fuels. As pollution
control standards become more stringent, the reduction or
elimination of these products becomes a serious problem.
SUMMARY OF THE INVENTION
The invention comprises a combustion method and apparatus which is
characterized by burning carbonaceous or hydrocarbon fuel in a
primary combustion zone with less than the stoichiometric amount of
air required for complete combustion. Generally up to about 75 or
80 percent of the stoichiometric amount is supplied to the primary
zone. Incomplete combustion results in the temperature of the
combustion gases remaining below 2700.degree. F, a temperature
above which significant quantities of NO.sub.x would be produced.
Incomplete combustion in the primary zone results in the gaseous
combustion products containing a high percentage of CO, unburned
hydrocarbons and carbonaceous materials. All of the hot gases from
the primary zone are then passed through a secondary combustion
zone where air is injected in the gas stream for oxidizing the CO,
unburned hydrocarbons and carbonaceous materials to innocuous
CO.sub.2 under such conditions that a temperature is never exceeded
at which nitrogen from the air or from the fuel might be oxidized
to NO.sub.x in significant quantities.
The secondary combustion zone includes a plurality of foraminous
tubes over which the gaseous combustion products exiting the
primary combustion zone are constrained to pass. Air at positive
pressure is fed into the tubes from any suitable source. The tubes
have foramina of some kind such as pores or perforations for
emitting air into the gaseous combustion product stream. The
secondary air mixes with the gases to support a low temperature
combustion process which oxidizes the CO, unburned hydrocarbons and
carbonaceous materials to CO.sub.2 under temperature conditions
which minimize production of NO.sub.x.
The invention is further characterized by controlling the total air
required for combustion of the fuel at prevailing feed rates in the
usual way. More specifically, a primary damper is provided for
controlling air flow to the primary combustion zone. This primary
damper operates coordinately with the fuel feed control device in
response to the thermal demand of the system. A secondary damper is
also provided for automatically regulating the secondary air flow
in response to the CO level in the flue or stack gas.
A primary object of this invention is to reduce air pollution by
reducing NO.sub.x, CO, hydrocarbon and particulate content of the
exhaust gases from carbonaceous and hydrocarbon fuel burners.
A further object of this invention is to provide a combustion
system and method in which combustion conditions are so controlled
that consequential quantities of NO.sub.x are not produced, thereby
obviating the need for removing any NO.sub.x from the flue
gases.
A still further object is to minimize NO.sub.x production without
adversely affecting the thermal efficiency of the combustion
apparatus.
Another object is to provide a device for reducing air pollutants,
expecially NO.sub.x which device can be readily adapted to various
types of boilers and other fuel burning devices as well.
How the foregoing and other more specific objects of the invention
are achieved will appear in the detailed description of an
illustrative embodiment of the invention which will be set forth
shortly hereinafter in reference to the drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical side elevation, partly in section, of a boiler
incorporating the invention;
FIG. 2 is a front elevation view of the boiler shown in the
preceding figure;
FIG. 3 is a front elevation view of the secondary burner as viewed
in the direction of the arrows 3--3 in FIG. 1;
FIG. 4 is a top view of the secondary burner assembly shown in the
preceding figure;
FIG. 5 is a front elevation view of an alternative type of
secondary burner, with some parts broken away and in section;
FIG. 6 is a transverse section taken on the line 6--6 in FIG. 5;
and
FIG. 7 is a section of one of the burner tube assemblies taken on
the line 7--7 in FIG. 5.
DESCRIPTION OF A PREFERRED EMBODIMENT
Although the invention is applicable to various fuel burning
apparatus it will be discussed for purposes of illustration in
connection with a steam or hot water boiler.
FIGS. 1 and 2 show a boiler which is somewhat schematically
represented and which incorporates the new pollutant reduction
system. The illustrative boiler comprises a housing 10 in which
there is an upper steam or hot water drum 11 that connects with a
lower feed water drum 12 by means of a group of water filled tubes
13. The tubes have webs 14 welded between them to enlarge the heat
absorption surface and to confine the flue gases to flow in a
predetermined path. There is a group of tubes 13 on the other side
of the boiler also extending from upper drum 11 to lower drum 12.
These tubes together with drums 11 and 12 define a space in which
heat is absorbed by radiation from the combustion devices and from
the hot combustion gases that flow through the boiler. Ultimately,
the combustion gases reach an adapter 15 from which the gases are
piped to a stack, not shown, for discharge to the atmosphere.
The boiler has a primary combustion chamber 20 comprising a
cylindrical refractory shell 21 which is continuous with a conical
extension 22 and which defines an internal volume 24 which is
herein called a primary combustion zone. Gaseous or vaporized
liquid fuel may be burned in the primary combustion zone. The fuel
is injected with a nozzle 25 which has a pipe 26 leading back to a
burner block 27 where a fuel line connection 28 is made thereto.
Nozzle 25 is supported on its feed pipe 26 centrally within a
hollow cylindrical element 29 which has a plurality of openings 30
that act as a diffuser for air which is supplied for combustion in
primary combustion zone 24. The burner assembly may be any
conventional type that is suitably adapted for burning gas or
liquid fuel.
Supported on the front of the boiler is a motor 31 on whose shaft
32 there is mounted a fan 33. In a conventional manner, rotation of
fan 33 causes generation of pressurized air in a compartment 34.
The pressurized air is supplied both to the primary combustion zone
24 and to a secondary combustion zone 61 and, in some designs in
accordance with the invention, to a tertiary combustion zone, not
shown, as will be explained shortly hereinafter.
In the depicted embodiment, there is a main duct 35 directing
combustion air to two subdividing ducts, a primary combustion air
duct 41 and a secondary combustion air duct 40. Primary air duct 41
has a damper 46 mounted in it for rotation on a shaft 47. Damper 46
may be turned to regulate air flow through primary air duct 41.
Shaft 47 is driven by a motor 39 as indicated by the dashed line
38. Also driven by motor 39, as indicated by the dashed line 38',
is a cam 36 whose follower 37 operates a fuel flow control valve,
not shown. By conventional means which are not shown, motor 39 is
driven bidirectionally to operate damper 46 and control primary
combustion air flow in response to boiler operating conditions
including steam or hot water load demand. Thus, damper 46 and fuel
control cam 36 are operated coordinately to maintain the desired
fuel-to-air ratio in the primary combustion zone 24 throughout the
entire range of boiler operating conditions. In accordance with the
invention, less than the amount of air for complete combustion is
normally supplied to primary combustion zone 24.
In general, from about 75 to 80 percent of the air delivered by the
fan 33 is furnished to the primary combustion zone 24 and the
balance is furnished to the secondary combustion device 42 which is
in secondary zone 61. Primary air duct 41 leads to a compartment 43
from which air flows through diffuser ports 30 into primary
combustion zone 24 where the air enables the fuel injected by
nozzle 25 to be burned incompletely, in accordance with the
invention, by suitably predetermining the fuel-to-air ratio
throughout the operating range of the boiler.
The smaller secondary air duct 40 has a damper 44 in it which is
mounted for rotation on a shaft 45. Shaft 45 is turnable
bidirectionally by a motor 49 in response to a condition such as
the CO level which prevails in the flue gas stack 15 leading from
the boiler. Motor 49 is controlled by a servo-controller 50 which
in turn responds to CO level in the stack as sensed by a suitable
sensor 51. Changes in CO level result in secondary damper 44
altering the amount of air delivered to the secondary combustion
device 42 by way of duct 40.
In one operating mode the secondary air damper 44 may be wide open
when the boiler is being started. This results in substantially
complete combustion in the secondary zone 61 of the residual
carbonaceous solids, other particular matter, hydrocarbons and CO
which are not completely oxidized in the primary combustion zone
24. Then the small damper 44 may be gradually closed until CO is
sensed in the stack gas. The system then goes on automatic
operation to maintain the CO level near zero or below a preset
minimum substantially by controlling secondary combustion air flow
through regulation of damper 44. The primary combustion air flow
is, during normal operation of the boiler, regulated coordinately
with the proper fuel ratio in accordance with the thermal load on
the boiler and on other conditions.
The total amount of air supplied for combustion in the primary and
secondary combustion zones is generally slightly greater than the
stoichiometric requirements for complete combustion of the
combustible components of the fuel, but it will be understood that
stoichiometric combustion conditions are not approached in the
primary zone 24, in accordance with the invention, because gas
temperature in the primary zone under these conditions could reach
2700.degree. F and cause much NO.sub.x to be produced which is
contrary to the invention. For the purposes of the invention, the
boiler is operated so that gases in the primary combustion zone are
maintained well below 2700.degree. F and some incompletely oxidized
products result.
In an alternative form of the invention, a main damper, not shown,
is installed in main duct 35 preceding the ducts 40 and 41. This
main damper may be driven by motor 39 which also drives fuel
control cam 36 and damper 46 in response to boiler demands as in
the illustrated embodiment. The smaller secondary air control
damper 45 is then used for fine control in response to CO level in
the stack gas. Control over the composition of the effluent
combustion products may also be achieved with another alternative
in which the main damper, not shown, in main duct 35 and the fuel
control cam 36 are controlled by motor 39 in response to demand on
the boiler while the primary damper 46 in duct 41 and secondary
damper 45 in duct 40 are jointly controlled by CO level responsive
motor 49.
Attention is now invited to FIGS. 1, 3 and 4 for a more detailed
description of the new secondary combustion device 42. As indicated
heretofore, the secondary combustion device is situated in a
secondary combustion zone 61 at the outlet end of the primary
combustion chamber 20 so that all gases of combustion must flow
through or near the device 42. Basically, the secondary combustion
device 42 comprises two parallel rows of foraminous tubes, the
tubes in one row being marked 54 and the tubes in the other row
being marked 53. The purpose of the tubes is to diffuse or inject
secondary combustion air uniformly into the stream of gaseous
combustion products flowing from primary combustion chamber 20 to
promote mixing and insure complete combustion without an excessive
amount of secondary air. Thus, in this embodiment, a tube such as
54 is provided with two longitudinally extending rows of small
holes 55 and 56 through which air may emerge into the gaseous
combustion product stream. In this case, the tubes are supported in
a header 57 over which there is a cap 58 to prevent evolution of
undispersed air from the ends of the tubes into the gas stream. The
lower ends of the tubes are also in a header 59 but the lower ends
of the tubes communicate with the secondary air duct 40 which is
under control of small damper 44. Air evolving from tubes 53 and 54
through the rows of small holes 55 and 56 effectuates combustion of
CO and unburned substances such as hydrocarbons emanating from the
primary combustion chamber. The tubes are mounted in a supporting
structure 60 which maintains their position in the gaseous
combustion product stream. The temperature in the secondary
combustion zone is dependent upon the temperature and quantity of
air discharged from tubes 53 and 54 and the radiation of heat away
from this zone. The boiler tubes to which the secondary combustion
device 42 is exposed insures that the secondary combustion occurs
at a temperature of 2500.degree. F or below which is low enough to
minimize oxidizing nitrogen from the fuel or the combustion air to
nitrogen oxides. In this embodiment, tubes 53 and 54 may be
comprised of a suitable refractory or stainless steel or other
material which will not degrade at prevailing temperatures. It is
desirable to locate and arrange the secondary combustion device 42
in such manner that it can radiate heat to the boiler tubes so that
the secondary combustion air flowing through the perforated tubes
will not be significantly preheated before it emerges.
In FIG. 4, the rows of holes 55 and 56 are circumferentially spaced
apart so as to intercept a central angle of about 120.degree.. It
will be understood that the rows of holes may be angularly closer
or farther from each other as well without defeating the purposes
of the invention. An angle of about 120.degree. is, however,
desirable since it enhances turbulence and mixing of the injected
air and stream of gaseous combustion products in which case more
complete combustion is promoted. With this angle, turbulence and
good mixing are obtained because the air emitted from the small
holes at just about the point where the gas stream flowing past the
tubes begins to separate therefrom to form vortices.
Various kinds of hollow air dispersing means may be substituted for
the perforated metal tubes 53 and 54 which were described. For
instance, the hollow means may be made sintered metal or ceramic or
other refractory which is perforated or porous partially or
entirely around their perimeters. The means may also be provided
with narrow continuous or interrupted longitudinal slots for
emitting air instead of being provided with many small holes or
pores. The tubes may be made of any material that withstands the
conditions that prevail in the vicinity of the secondary combustion
device, 42. There may also be more or fewer air supply tubes in a
row or more or fewer than the two rows illustrated depending upon
requirements of the system.
A modified form of secondary combustion device will now be
described in reference to FIGS. 5-7. This embodiment is
distinguished by its having means for keeping the air emitting
tubes cool and for precooling or, at least preventing, preheating
of the incoming secondary air.
In FIG. 5, the modified secondary combustion device is generally
designated by the reference numeral 70. It comprises a frame 71
which supports a water feed header 72 and a water discharge header
73. Beneath the lower water header 72 is a secondary air feed
header 74 that is supplied through secondary air duct 40 under the
control of damper 44. As can be seen in FIG. 6, in this exemplary
embodiment there are again two rows of water cooled, air emitting
tube structures, the structures in the back row being marked 75 and
those in the front row 76. As in the previous embodiment, it will
be understood that the number of tube structures in each of the
rows and their size and geometry will depend on the gas quantities
handled in a particular boiler size.
One of the air emitting tube structures 75 will be described since
they may all be the same. Referring to FIG. 5, one may see that the
structure 75 comprises a central secondary air conducting tube 80
which has two longitudinally extending rows of small holes such as
81 and 82 through which secondary combustion air may emerge into
the gaseous combustion products stream. As can be seen particularly
well in the cross sectional view of one of the tube structure 75 in
FIG. 7, the rows of holes 81 and 82 are aligned with angularly
diverging longitudinally disposed hollow flutes 83 and 84,
respectively. Both flutes have the same geometry. For instance,
flute 84 extends radially from tube 80 and has a longitudinally
extending open ended slot 85 which conducts the secondary air
emitted through the row of holes 82 to the gaseous combustion
product stream surrounding the secondary combustion device. As can
be seen in FIG. 7, the tips of the slotted flutes are beveled so
that the slots 85 open substantially exclusively on the leeward
side of gaseous combustion product flow.
The center tube 80 in this embodiment is capped at its upper end 86
so as to constrain all of secondary combustion air to flow through
the orifices or small holes 81 and 82. The lower ends of center
tubes 80 are connected with header 74 through which secondary
combustion air is supplied through duct 40 under the control of
small damper 45. The inner tube is surrounded by a concentric outer
tube 87 which defines a water jacket 89 around the inner tube.
Water flows axially in the segments of the jacket between the
flutes 83 and 84. This results from the fact that the lower end 88
of outer tube 87 connects into water feed head 72 as is
particularly evident in FIG. 5 in the lower broken away portion of
the tube structure. The inner and outer tubes are suitably welded
or otherwise sealed where they pass through or into their
respective headers. Water flowing axially through the water jacket
area 89 emerges at the top end of the structure and continues its
flow path through a cavity 90 and through a hole 91 in upper water
exit header 73. Of course, the inner air tube 80 may be adapted to
extend through upper water exit header 73 to another secondary air
header, not shown, or the air inlet header may be arranged to feed
the tubes 80 from the top instead of the bottom or there may even
be two independent air headers feed tubes from the top and bottom.
The design will depend on the quantity of primary gaseous
combustion products to be burned and upon meeting the condition
that secondary combustion should be well below 2700.degree. F such
as at about 2100.degree. F to oxidize the CO and hydrocarbons
completely and yet inhibit NO.sub.x production.
In FIG. 7, an incremental filament of the gaseous combustion
product stream is indicated by the arrowed line 95. It will be
noted that this typical incremental stream deflects off of the
periphery of the outer tube 87 at an angle such that the stream
will intersect with the secondary air stream emerging from the
slotted flutes 83, 84 in which case turbulence is maximized. This
promotes oxidation of the residual hydrocarbons and carbon monoxide
and any other burnable matter in the gaseous combustion product
stream from the primary combustion chamber 20. The fact that the
tube structure is water cooled not only prevents its thermal
degradation but it also results in precooling of the incoming
secondary combustion air which aids in suppressing the temperature
of the secondary combustion products to well below the 2700.degree.
F at which nitrogen oxide might be formed. It is also desirable to
position the secondary combustion device in relation to the heat
absorbing surfaces of the boiler such that the device will be
cooled by radiating to the surfaces. The existence of a fairly high
CO level in the secondary combustion gases also tends to inhibit
formation of nitrogen oxide because the air reacts preferentially
with the carbon monoxide rather than with the nitrogen derived from
the air or the fuel.
In the previously described embodiments, there are primary and
secondary combustion zones. For example, in the secondary
combustion device 42 shown in FIGS. 1, 3 and 4 the rows of
foraminous tubular elements 53 and 54 are both supplied from the
same secondary air duct 40 in which case there is actually a single
secondary combustion zone in the vicinity of device 42. Similarly,
in the FIGS. 5-7 embodiment, the foraminous tubes such as 80 are
all connected into a common header 74 which is supplied from the
secondary air duct 40 in which case all of the tubes contribute air
to a single secondary combustion region. Although specific
structure is not shown, those skilled in the art may readily infer
from what has been disclosed that a tertiary combustion zone may
also be provided. This can be done by connecting the leading rows
of tube structures 54 or 75 in the FIG. 1 or FIG. 6 embodiments,
respectively, to one secondary air supply. The other rows of tubes
53 or 76 in the respective embodiments, may then be connected into
a third pressurized air supply, not shown. Thus, primary, secondary
and tertiary combustion zones are created. The zone ahead of the
rows of tubes 53 or 76 becomes the secondary zone and the zone in
front of the rows of tubes 54 or 75 become the tertiary combustion
zone. In this arrangement, the gaseous combustion products which
are rich in CO and unburned hydrocarbons from primary combustion
chamber 20 undergo further burning in two stages in the secondary
and tertiary combustion zones so that the probability of elevating
the gas temperature to that above which NO.sub.x might be formed is
further reduced. In this arrangement, it is easier to keep the
tubes cool since the flame surrounding each of the rows is not as
intense.
During operation of the illustrated embodiment, changes in the CO
level of the effluent stack gas are sensed and used to control the
small damper 44 in the secondary sir supply duct 40. If CO in the
stack increases, secondary air is increased by automatic increased
opening of damper 44 in which case the combustion products from the
primary zone are more effectively oxidized in the secondary zone
and the CO level goes down again. A decrease in stack gas CO level
brings about converse action. The essence of the system is to
assure that gases passing through the secondary combustion device
are oxidized but to minimize the intensity of flame in that region
so that the temperature will not be increased to the point where
nitrogen oxides would be produced. Usually, the CO level of the
stack gases will reach a steady state as long as load requirements
on the boiler are fairly constant. However, if there is greater or
lesser load, the fuel and air to the primary combustion zone change
accordingly in which case the CO level in the flue gases may change
and effectuate automatic readjustment of the secondary air.
Although embodiments of the invention have been described in
considerable detail, such description is to be considered
illustrative rather than limiting for the invention may be
variously embodied and is to be limited only by interpretation of
the claims which follow.
* * * * *