Note: Descriptions are shown in the official language in which they were submitted.
Disposal of Oxides of Nitrogen and Heat
Recovery in a Single Self-Contained Structure
BACKGROUND_OF THE INVENTION
This invention lies in the field of NOx reduction.
More particularly, i-t concerns a self-contained single
structure in which gases which contain oxides of nitrogen
(referred to herein as "NOx") are reduced by the burning
of fuel under selected conditions, and including heat
recovery means, all within the single enclosure.
In the prior art, apparatus has been shown, in which
NOx gases are reduced in a somewhat similar chemical
procedure but in entirely separate and isolated structures.
Any heat recovery from such a structure has been in the
orm of a separate boiler structure for heat recovery. In
view of the great interest at the present time in removal
of nitrogen oxides from effluent gases, it is important to
provide a simplified unitary structure of compact construc-
tion and efficient operation.
Advantages of this type of struc-ture are multiple.
First of all, there is a reduction in cost due to elimina-
tion of outer walls of separate structures, and there is
reduced volume which requires less floor space in the over-
all operation. Also, means are conveniently provided for
recovering high temperature heat in the furnace se-ction, as
well as low temperature heat in the heat recovery section,
with internal connections whenever possible.
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SUMMARY OF THE INVENTION
_ . .
It is the primary object of this invention to provide
a unitary compact structure in which fuel can be burned
for the reduction of gases, and complete high-temperature
and low-temperature heat recovery can be accomplished in the
same structure.
These and other objects are reali~ed and the limita-
tions of the prior art are overcome in this invention by
breaking up the total longitudinal flow of gases into
shorter sections so that the successive sections can be
- changed in direction so that common walls can be provided
between two sections and a smaller overall volume required
for the total structure.
In one embodiment fuel, gases and primary air are
passed through a b~lrner into a reducing section, in which
heat is extracted by a fire tube boiler, which is in the
form of an annular volume surrounding the cylindrical
furnace reducing section. The flame and hot gases pass
downstream into a second section into which cooled stack
gases are injected so as to reduce the temperature of the
hot gases while in a reducing atmosphere, to reduce the
probability of reoxidizing the nitrogen. The flow of
cooled gases is then directed in the opposite direction,
through an annular passage which forms a third section,
into which secondary air is also injected, to provide a
reoxidation section, in which complete combustion of the
partially-burned fuel is accomplished, butat a lower flame
temperature. The effluent from the reoxidation section
then passes through fire tubes of the boiler, which surrounds
the reduction section. Thereafter, the cooled products of
combustion are conducted to a stack in a conventional man-
ner.
This type of construction reduces the overall length
of the structure to approximately one half, with only a
3~ nominal increase in diameter, thereby providin~ for a more
con~enient shape and size for a combination NOx disposal
and waste heat recovery system.
~3~
In another and preferred embodiment, the four operating
sections, namely the furnace or reducing section, the cooling
section the reoxidation section and the waste heat recovery
section are arranged in three adjacent parallel zones, with two
complete reversals from the first direction of the entering
gases, fuel, and air, which pass into the reducing section. In
the second section the cooled stack gases are introduced and
mixed with the hot products of combustion from the first section.
At the same time the direction of flow of gases is reversed.
After injection of the secondary air the gases pass into a re~
oxidation zone, where the final combustion of all combustible
material takes place. On the output of the reoxidation section
the gases are again reversed in direction into their original,
first direction, as they pass into the heat recovery section,
after which the cooled products of combustion flow to the stack.
Thus this invention provides a self-contained combina-
tion NOx disposal and heat recovery system characterized by:
(a) a burner means and means to supply to said burner
means;
NOx gases to be reduced
gaseous fuel at selected fiow rate sufficient to main-
tain combustion, and
primary air in less-than-stoichiometric flow rate;
~b) a reducing section in which said NOx gases, pri-
mary air and fuel are burned after flowing through said burner
means in a first direction;
(c) water/steam tubes positioned in said reducing
section to receive heat from the flame therein;
(d) means to inject cooled stack gases into the hot
products of combustion as they leave said reducing section;
(e) outlet means from said reducing section positioned
such that the gases flowing out of said reducing section after
the injection of said cooled stack gases are caused to flow in a
second direction opposite to said first direction;
(f) means to inject secondary air into said gases
after the injection of said cooled stack gases whereby the total
of primary plus secondary air is greater-than-stoichiometric
value thus creating a reoxidation section for the combustion pro-
cess in said second direction, and to output its products of re-
oxidation in sald first direction; and
(g) means to recover heat from the resulting NOx-
reduced products of combustion as they pass to said stack~
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BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention
and a better understanding of the principles and details of
the invention will be evident from the following description
taken in conjunctioll with the appended drawings in which:
FIGURE 1 represents the prior art construction.
FIGURE 2 represents one embodiment of this invention.
FIGURES 3, 4 and 5 illustrate separate views of a
preferred embodiment of the invention.
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DESCP~IPT ION OF TI~E PREFERRED E~IBODIMENT
Referriny now to the drawings and, in particular,
to FIGURE 1, there is shown a conventional construction of
a system for reduction of gases. These are in separate
structures and include a separate waste-heat boiler and
finally the stack.
The overall structure is indicated generally by the
numeral 10. The burner is indicated generally by the
numeral 12. The radiant reducing section is indicated
generally by the numeral 1~, with the heat recovery from
the radiant section indicated generally by the numeral 16.
At the output of the radiant section, the hot gases
are cooled in a irst zone 18 by injection of cooled stack
gases. Thereafter additional combustion air, namely
secondary air, is injected to reoxidize in a reoxidation
section for the purpose of completiny the combustion of all
combustible materials. The heat produced by combustion in
the reoxidation section 22 is then recovered in a waste
heat boiler indicated generally by the numeral 24, of con-
ventional design. Thereafter the cooled products of com-
bustion go to a stack indicated generally by the numeral 26.
All of the various sections of the system are con-
nected in series and the gas flow is in a single direction.
At the start, fuel 28 is injected along with primary air
30 and bearing gases 46 to be reduced.
The amount of primary air is approximately 80% of
the total air supply so as to provide less-than stoichio-
metric quantity of air. This provide~ a reducing atmosphere
in section 14, and oxygen is given up by the nitrogen oxides
for burning the ~uelO In the presence of high temperature
gases the nitrogen would tend to reoxidize but, in view of
the lack of oxygen, there is no recombination and the hot
gases are then cooled b~ the injection of cooled stack
gases. This reduces the temperature of the combustion
~ases from a high temperature in the region of 2,000 F
to a more moderate temperatureof 1,200 F. At the lower
temperature there is only limited tendency for the nitrogen
to reoxidize. The recycled skack gases are taken to a
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recycle blower 34 and by conduit 35 through a control valve
36 and line 37 are injected into the ~low of hot gases from
the radiant reduci.ng section 14. The rate of flow of cool-
ing gases is controlled by the v~lve 36, which is under con-
5 trol of thermocouples 38 sensitive to the temperature ofthe cooled gases.
The secondary air is then introduced through conduit
32 ak the second zone 20, and all of the remaining com-
bustibles, such as carbon, carbon monoxide, hydrogen,
10 formaldehyde, or others, that might be present, are burned
in the reoxidation section 22. This again raises the tem-
perature so that it becomes economical to recover the waste
heat in the effluent gases. This is done by means of the
waste heat boiler 24, which produces steam output by line
15 40. Incidentally, feed water is supplied by line 42, and
goes through line 44 to the radiant heat recovery in the
reduciny section, and the steam output goes by line 47 to
the waste heat boiler 24. Feed water also goes by line 43
into the waste heat boiler. The outflow from the waste
20 heat boiler is now in the neighborhood of 300 and passes
up the stack 26 in accordance with arrow 45.
Referring now to FIGURE 2, there is shown one embodi-
ment of the invention, which is a system which operates
suhstantially along the lines of the conventional system of
25 FIGURE 1 but on a more compact and efficient type of
operation.
In FIGURE 2 fuel is introduced in accordance with
arrow 128. NOx gases are introduced in accordance with
arrows 146 and/or 146A with the primary air, which is
30 less-than-stoichiometric, being about 80 of the re~uired
air. Primary air flows in accordance with arrow 130 into
the burner system and into the combustion space 114, in
accordance with arrows 152. The fuel and air entry system
is indicated generally by the numeral 112. This flows
35 through a burner tile 154, into a combustion chamber or
furnace 114, which is called a reducing zone, inasmuch as
it will have a reducing atmosphere. The wall 158 of the
reducing section 114 is generally in a circular cylindrical
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form, and is a separating wall between the furnace space
114 and a fire tube boiler 124. The radiant heat from the
flame in tne space 114 is transferred through the wall 158
to the water 160 and, thus, provides steam which flows
through an outlet conduit 153 in accordance with the arrow
140. Feed water is brought in throu~h an inlet 151 in
accordance with arrow 142.
At the position 147, where the reducing section 114
terminates, the hot gases 162 ~low into a cooling section
118. This i9 substantially the same diameter as that of the
reducing section 114. However, it will have a wall 139 of
heat-resisting material, because, unlike wall 158, it is not
in contact with cooling water.
There is an axial tubular conduit 121 through which
cooled stack gases are flowed in accordance with arrows
135 and 136. These gases flow out through a series of cir-
cumferential ports 170 in accordance with arrows 137 to mix
with the hot gases 162, so that their temperature will be
reduced to a selected value. The cooled gases 164 continue
to flow in the annular zone between the axial tube 121 and
the wall 139. There is a transverse end wall 155, which
incidentally supports the axial tube 121. The wall 155
is spaced downstream from the end 147 of the wall 139 pro-
viding a region in which the flow of gases 16~ can be
reversed from a first direction of the fuel and air, to a
reversed direction, 180 different.
Secondary air illustrated by arrows 132 is injected
through ports 172 in the wall 155, in accordance with arrows
174, to mix with the cooled gases 164 so that they will
continue to burn utilizing the remaining combustibles in
the gases 164. This combustion takes place in the reoxi-
dation zone 122 in the annular space 182 between the wall
139 and the outer wall 161 of the reoxidation zone.
~dditional combustion takes place in the annular
space 182 so that, by the time the gases reach the position
147 in accordance with arrows 176, all the combustion has
been completed. The hot products of combustion then pass
through fire tubes 178 in the heat recovery section 124,
where they are cooled to a temperature in the region of
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300 F. They flow in accordance with arrows 179 and 145 to
a conventional stack for emission into the atmosphere.
As shown by arrow 169, part of the cooled combustion
gases at the outlet of the fire tubes 178, are carried by
dashed line to a recycle blower 134, and delivered in
accordance with dashed line 135 to the arrows 135-136, to
be injected into khe cooling section 118 as previously
mentioned.
While the system as shown in FIGURE 2 has all of the
apparatus shown in FIGURE 1, there is a great saving in
overall size since it is, in a sense, folded once so that
the longitudinal dimension is reduced by about half. Fur-
thermore, there are common walls between certain of the
various sections so that, there is less cost and space
require~, while permitting the transfer of heat through
one wall such as 158, as is desired.
Referring now to FIGURES 3, 4 and 5, three views are
shown of a second preferred embodiment of this invention.
FIGURE S shows a side view of the structure. FIGURE 4 is
an end view of the structure taken across plane 4-4 of
FIGURE 5. FIGURE 3 is a cross-section taken across hori-
zontal plane 3-3 of FIGURE 5. FIGURE 3 shows more clearly
than any of the others the construction and operation of
the preferred system of this invention.
As in the case of FIGURE 3, there is a burner system
indicated generally by the numeral 212, into which is
supplied NOx gases in accordance with arrows 246 and 246A.
Fuel in selected quantity is supplied in accordance with
arrow 228. Primary air is supplied in accordance with
arrow 230. The three sets of gases meet and pass through
the tile 254 of the burner, into the interior 256 of a
furnace section, or reducing section 214. Here combustion
is permitted to take place at high temperature under re-
ducing conditions, which are provided by the act that the
amount of primary air 230 i5 less-than-stoichiometric,
being in the range of 60-80~ of the total requirements.
Under the high temperature and reduced air supply the
nitrogen oxides are reduced.
The fuel is burned in accordance with the inflow of
_ 9 _
arrows 246, 228 and 252, in the space 256, which is lined
with fire brick or other insulating material, and the
temperature is allowed to be high enough to reduce the
NOx. Radiant heat is absorbed by the water/steam pipes
216, with water entering in accordance wi~h arrow 244, and
steam leaving in accordance with arrow 247. In g~neral,
the cross-section of the furnace is maintained in a rec-
tanyular form between the walls 280 and the floor and roof.
As shown in FIGU~E 4, there are two sets of conduits
10 247 for recycling of cool stack gases in accordance with
lines 235 from b].ower 234. There are two vertical pipes
247 from which, by suitable nozzles or openings, the
cooled stack gases are injected in accordance with arrows
237, into the hot products of combustion in the space 256.
The cooled gases from space 256 go into a second
section, space 266, where they are reversed in direction,
in accordance with arrow 264, and flow in a second direc-
tion, which is opposite to the first direction of the
incoming ~uel, air and NOx.
.After the cooled stack gases are thoroughly mixed
with the hot products of combustion from the reduction
zone 214, secondary air 232 supplied by vertical pipes
292 is injected into the gas stream in accordance with
arrows 236. This is supplied through overhead pipes and
25 vertical pipes 292.
The injection of secondary air brings up the total
air supply to a customary value of about 110% of stoich-
iometric value. There is, therefore, sufficient air now
for complete combustion of all combustibles, which takes
place in the reoxidation zone 282, 286. The combustion
continues as the gases flow in accordance with arrow 276
through a second 180 bend~ through the space 286, and
into a water tube heat exchanger for heat recovery from
the products of combustion 276. In the heat recovery
section 224 the water tubes 284 absorb heat from the
gases and they leave in accordance with arrows 279 at a
sufficiently low temperature, of about 300, that they
can be passed through the stack for emission to the
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atmosphere in accordance with arrows 245.
Part of the effluent gases 279, after cooling, are
passed to a recycle blower 234 and hy conduit shown by
dashed line 235. These cooled stack gases are passed to
the pipes 247 to supply the cooling gases 237 or cooling
the output of the reduction section.
Because of the hiyh temperature of the gases through-
out the entire passage through the apparatus, all walls 280
of the structure are made of heat resisting material, such
10 as is well-known in the art.
Because of the manner of introducing the cooled stack
gases through the pipes 247 and the secondary air through
the pipes 292, the cross-sections for flow of combustion
gases are reduced, providing a more rapid mixing of the
15 injected gases into the flow of combustion pro~ucts. Also,
the design illustrates that the injection of the cooling
gases and the secondary air are from opposite sides into
the continuing flow of hot gases.
FIGURE 4 shows the outer wall 288 of the structure
20 and the outlet conduit 287 from the heat exchanger includ-
ing the water tubes 284.
FIGURE 5 shows, in addition, the plenum 250 and the
inlet pipes 228, 246 for the fuel and the NOx gases, and
the inlet pipe for the primary air. In addition, it shows
25 the steam chamber 290 having outlets 291, all of which
are conventional. Also shown is the feed water entry 242,
243 and the pipe 244 to the furnace section. The pipe
244 as input to the steam pipes 216 can come from the
boiler feed water 242, as in FIGURE 5, or from the outlet
30 240 of the waste heat boiler 224 over dashed lines 244
of FIGURE 3.
It ~ill be clearly seen from examination of FIGURE 3,
in comparison with FIGURES 1 and 2, that, although they
are not drawn to scale, it would require a much smaller
35 floor space than either of FIGURE 1 or FIGURE 2.
While the invention~has been described with a certain
degree of particularity, it is manifest that many changes
may be made in the details of construction and the
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arranyement of components without departing frorn the spirit
and scope of ~he disclosure. It is understood that the
invention is not limited to the embodiments set forth
herein for purposes of exemplification, but is to be limited
only by the scope of the attached claims, includiny the
.full range of equivalency -to which each element or step
thereof ls entitled.
