Note: Descriptions are shown in the official language in which they were submitted.
106807~
The present invention relates to a process for avoiding air pollu-
tion and more particularly to a method for converting gases which contain
oxides of nitrogen ~NOx) into gases which may be vented safely to the
surrounding atmosphere. -~
More particularly, the process of this invention not only removes ~- -
objectionable NQx gases, but also objectionable gases such as carbon monoxide
~CO) which are vented substantially smokelessly into the surrounding atmos-
phere.
Several industrial processes produce waste gases containing oxides
of nitrogen in quantities too great for dischar~e into the air. Various
methods for the disposing of waste gases containing oxides of nitrogen have
been proposed and one provides for their absorption in water and the
recovery of nitric acid therefrom. Such processes require large investments
and often do not provide sufficient acid to justify coping with the many
corrosion problems encountered in the complicated processes required to ~`
produce the acid at an economic concentration. It also has been proposed
to subject such waste gases to a reducing flame where the oxides
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106B075
of nitrogen are reduced to nitrogen such as disclosed in United States patent
2,673,141. In such a process the amount of solid combustible material sus-
pended in the gas exceeds the oxygen available from the oxides of nitrogen
with the result that smoke emission becomes a problem as a result of unburned
carbon in the gas. Further, said patent does not provide a process to reduce
carbon monoxide concentration to values that will eliminate contamination.
Accordingly, it is an object of the present invention to provide a
method for converting waste gases into gases which can be safely vented to
the surrounding enYironment by providing a method for reducing oxides of
nitrogen in waste gases to nitrogen without introducing the problem of ex-
cessive smoke emission.
According to the invention there is provided a process of convert-
ing gases containing oxides of nitrogen into gases which may be vented safely
to the surrounding environment in which fuel is burned in a first zone in
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the presence of less than one hundred percent of said fuel~s stoichiometric
~s requirements of oxygen while mixing said gases containing oxides of nitrogen
with the combustion products resulting from said burning in a ratio which
provides an excess of oxidizable combustion products over oxygen available
from said oxides, first cooling the aforesaid combustion mixture to maintain
a temperature that is within the range between 649 to 1093 C, mixing, in a
, .
` second zone, the oxidizable combustion products remaining after substantially
all of the oxides of nitrogen have been reduced, with sufficient oxygen to
convert substantially all or the oxidizable combustion products remaining to
carbon dioxide and water without reformation of oxides of nitrQgen and
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second cooling the resultant gas products, the invention characterized in
controllably using said cooled resultant gas product for maintaining the
proper temperature in any one or plurality of the first zone, first cooling
step, second zone or second cooling step.
In the accompanying drawings:
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1068~75
Figure 1 is a schematic representation of an apparatus used to
perform the basic process of this invention; and
Figure 2 is a schematic flow diagram of another embodiment utiliz-
ing the process of this invention.
Before explaining the present invention in detail, it is to be
understood that the invention is not limited in its application to the de-
tails of construction and arrangement of parts illustrated in the accompany-
ing drawings, since the invention is capable of other embodiments and of
being practiced or carried out in various ways. Also, it is to be understood -
that the phraseology or terminology employed herein is for the purpose of
description and not of l;m;tation.
Referring now to Figure 1, a furnace having a steel shell 10 lined
with refractory material is shown with a flame 11 burning within the lower -
portion thereof.
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A hydrocarbon such as methane is supplied through a
pipe 12 to a burner assembly 14 while air is admitted
through the burner assembly at a rate to provide about
sixty-five percent of the stoichiometric requirements
; of oxygen for complete combustion of the hydrocarbon.
In some instances the air may be air contaminated with
NOx, C0, etc. For example, a waste gas containing
I about 20,000 parts per million of mixed oxides of
nitrogen such as nitrous oxide, nitric oxide, nitrogen - ~-
trioxide, nitrogen peroxide and nitrogen pentoxide is
fed through a conduit 16 and into the zone 17 adjacent
the flame 11. It is to be understood that nitrogen
oxide or oxides alone or nitrogen oxide or oxides with
other gases may be fed through conduit 16. The combustible
products of the methane include methyl alcohol, formaldehyde,
carbon monoxide and hydrogen which become mixed in
¦ combustion zone 17. The temperature in combustion zone 17 ` ~ -
j is preferably maintained at eighteen hundred to two
' thousand degrees Fahrenheit by introducing a cooling
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, 20 fluid through tubes 18 and 19. As the gaseous mixture
flows upwardly through zone 17 toward zone 12, the
combustible products are oxidized by oxygen stripped
i from the oxides of nitrogen reducing them to nitrogen.
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The fuel flow to the burner is regulated to insure there
is always an excess of combustible products, predominantly
carbon monoxide and hydrogen in zone 17 so there is a
demand for all the oxygen available from the oxygen in
the oxides of nitrogen.
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1068075
The mixture of nitrogen and excess carbon
monoxide, hydrogen and the other oxidizable materials
are mixed with air admitted through pipes 22 and 23 as
the gaseous mixture leaves zone 17. Air flow is
: controlled to insure oxidation of substantially all
combus~ible material to avoid venting of undesirables
such as carbon monoxide and hydrogen and particulate
matter such as carbon particles from zone 21. A cooling
fluid flows into zone 21 through the pipe 24 to absorb
heat developed during the oxidation in zone 21. The ~.
I gas leaving the zone 21 is substantially free from carbon, ~:
carbon monoxide and hydrogen and NOx so as to be vented
safely into the surrounding environment. The volu~e of : :
air introduced through the pipes 22 and 23 is regulated ~
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~ preferably to produce less than one percent oxygen in ~:
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excess of the stoichiometric requirements of the ¦ :
.l combustibles such as carbon, carbon monoxide and hydrogen
j compounds, etc. in the gas to prevent substantial ..
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.~ conversion or reformation of nitrogen to NOx.
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~ 20 By controlling the volume of air supplied to burner
.: assembly 14 and properly regulating the volume of gas
. through the conduit 16 and maintaining the temperature
.1 preferably between about eighteen hundred degrees Fahrenheit
.l and about two thousand degrees Fahrenheit, and furnishing
the proper quantity of air through pipes 22 and 23, the
oxides of nitrogen can be substantially reduced to ~
.! nitrogen and the fuel oxidized to harmless carbon dioxide ..
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j and water thus converting the undesirable gas flowing
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1068075
through conduit 16 into a harmless and smokeless
gas substantially-free from free particulate matter
or smoke flowing from zone 21.
Although any fuel which can be substantially
completely oxidized to carbon dioxide and/or water
such as coal, oil, liquid hydrocarbons and the like
are contemplated, best results are obtained with
gaseous fuels such as methane or natural gas containing ~-
lower molecular weight hydrocarbons which normally burn
with a clear flame substantially free from carbon.
Referring to FIGURE 2, nitrogen oxide containing
gases from one or a plurality of sources are caused to
flow into one or a plurality of burners 27 which are
directed into a furnace 30 via conduit 32 and/or directly
to furnace 30 via conduit 34. Air is supplied and
controlled in amounts to support combustion yet less than
the stoichiometric amount required to support full
combustion of the fuel injected as heretofore defined.
Fuel is injected into the burners via conduits 36.
Suitable pilot gas conduits 38 and supporting ~ir
conduits 40 are provided to initiate and/or maintain
combustion.
A quench fluid such as the cooled inert gas
composed primarily of nitrogen (N2), carbon dioxide (C02)
¦ and water (H20) is injected via conduits 46A and 46B
into a plenum space 42 subsequent to the initial burner
operation. The quench and the subsequent combusted fuel
enters the furnace 30 under such conditions that the
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1068~7S
oxides of nitrogen are reduced, the quench acting to
control the temperature to prevent reoxidation of the
nitrogen (N2) to NOx. Subsequently thereto and still :
within the furnace 30 the excess fuel is oxidized by
injecting secondary air, supplied by a blower 48 via -~
one or more conduits 50, the control of such air being
a function of the characteristics (temperature and
composition) of the inert gases to vent stack 52 via
outlet 54. The effluent from the furnace is directed, as
for example,through a conduit 56, to a means of utilizing ~-~
the heat as for example a waste heat boiler and economizer
58 in which relatively cool feed water entering via
conduit 60 is converted to useful energy (steam) via :
conduit 62. The effluent of inert gases in conduit 54 -
has all or substantially all of the oxides of nitrogen
removed at least to an acceptable level. The gases are
then passed through the vent stack 52 and/or portions
thereof recycled by way of conduit 64 to the intake of :
fan or blower 66 via conduit 46 to the quench plenum 42.
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;. 20 A control valve 68 operates to supply proper quantity of
quench fluid in accordance with a temperature sensing
` means 70. In some instances it may be desirable to `
introduce quench fluid via line 72 into the furnace as
'!,, further control over the temperature therein preventing :~
~,~ reformation of nitrogen oxides. This control is
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~................ accomplished by valve 74 which is further controlled as
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a function of the temperature within the effluent 56
, from the nitrogen oxide reduction furnace 30.
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It is important that the temperature within -
the combustion zone be maintained within the above
specified limits to insure proper oxidation and
reduction while avoiding substantial damage to the
refractory material. Any suitable liquid or gas
substantially free from available oxygen such as
nitrogen, carbon dioxide, water, etc. may be used as
; the coolant.
- The following examples illustrate the process
of this invention. ~;
Bunker C or No. 6 fuel oil was injected into a
, furnace by steam atomization. Air for the fuel was
obtained from a ventilation source and contained up to
.4% by volume of NOx. The amount of oxygen available in
the air stream was insufficient to give complete
combustion. Into the burning zone gases containing .2%
NOx plus 3% 2~ and 96.8~ N2 (all percentages by volume)
were injected to mix with the products of the fuel
combustion. A portion of the vent stack 600F. gas,
containing small quantities Of 2 with the remainder
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being N2, C02 and water vapor, was recycled into the
; combustion chamber to maintain the temperature therein
at about 1,800 - 2,000F. After about two seconds
residence time, air was injected to burn the combustibles.
The resulting mixture was then fed to a waste heat
boiler to generate steam, to cool the resultant gas
mixture to about 600F. for discharge into a vent stack
~¦ and recycle blower. The resultant vent gases were
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1068075
smokeless and colorless being substantially reduced
in NOx and C0 and thus safely vented into the surrounding
environment.
Hot waste gases containing approximately 14% -
2 and 1.2% NOx (by volume) were injected into a
combustion chamber using natural gas as fuel and some -
air. The total oxygen available for the fuel combustion
was approximately 70% of the stoichiometric needs to
completely convert the fuel to C02 and H20. After
burning and mixing the resultant gas, containing some ~-
NOx and combustibles such as C0, H2, etc., passed into a
two stage reduction zone for the purpose of converting
NOx compounds to nitrogen. Between the reduction zone
stages recycle vent gases at approximately 600F. were
admitted to the system for temperature control in the
subsequent burning (reoxidation) of the remaining combustibles
to prevent reformation of NOx compounds from the available
oxygen and nitrogen~ These gases pass through a waste-
heat boiler to a vent stack with a portion thereof being
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recycled as mentioned. The following Table gives results
` of the test:
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1068~75
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