Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1062884
Because of the great interest in reducing the
pollution of the atmosphere it becomes increasingly important that
any nitrogen oxide gas which might result from chemical or refinery
operation must be reduced to basic nitrogen before they are
permitted to go into the atmosphere. There are a large number
of nitrogen oxides, such as N20, N0, N02, (N0)2, N203, (N02)2,
N205, etc., and referred to herein as "NOx". Several of these
such as N02, (N02)2 and N203 help form a large part of the brownish
haze which is seen in smog in the larger cities. Most nitrogen
oxide gases can support combustion at suitable temperatures.
However, it is necessary to have a reducing atmosphere before
the nitrogen oxides are reacted to free the nitrogen.
The prior art devices have a serious shortcoming that
is overcome in the present invention, in that all of them rely on
a burner operation with less than stoichiometric air for generation
of combustibles to reduce the nitrogen oxides.
It is more convenient to operate the burner or heat
source at stoichiometric air supply for the fuel which creates a
hot gaseous atmosphere, and then cause the atmosphere to be reducing
through separate addition to it of a hydrocarbon-steam mixture,
or H2, C0, or hydrocarbon. The steam-hydrocarbon mixture is
preferred because of typical rsforming reaction as:
CH4 + H20 = C0 + 3H2
This reaction, which is endothermic, is caused to occur by heat
available from the hot furnace atmosphere to cause the atmosphere
to become reducing. It is also possible to cause the furnace
atmosphere to become reducing through, again, separate introduction
of an air-hydrocarbon mixture in which the hydrocarbon-air mixture
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contains less than stoichiometric air for the hydrocarbon but
enough air to prevent the presence of free carbon.
When the NOx are introduced into the region of reduc-
ing atmosphere the nitrogen is freed and the hydrogen and carbon
monoxide are partially burned. With the nitrogen freed, the
gases must be cooled below a temperature, above which, reoxida-
tion of the nitrogen will occur. It is therefore necessary to
move the gases from the first chamber into a second contiguous
chamber where the temperature is dropped as rapidly as possible.
This can be done by the introduction of cooling means, to reduce
the temperature rapidly to below 2,000F and preferably to about
1,800F. This is well above the auto-ignition temperature for
hydrogen and carbon monoxide so they will automatically ignite
and be burned before the gases issue into the atmosphere, and
it is well below the temperature at which substantial amounts of
nitrogen will automatically oxidize.
According to one aspect of the present invention there
is provided a method to diminish NOx in burning gases comprising
the steps of:
2Q burning a fuel with a substantially stoichiometric amount
of combustion supporting gas or air in a first refractory lined
chamber;
introducing first gases containing said NOx into said first
chamber;
introducing a mixture of hydrocarbon gas and steam into
said first chamber to provide and substantially maintain therein
a reducing atmosphere;
passing the resulting gas effluent at temperature above
2,000F into a second refractory lined chamber;
rapidly cooling and maintaining said resulting effluent
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to a temperature within the range of 1,250F to 2,000F; and
introducing air into said second chamber to burn excess
combustibles.
According to another aspect of the present invention
there is provided a furnace system for the reduction of nitrogen
oxldes, comprislng:
a) a refractory lined furnace having a first and second
chamber of substantially the same diameter; the first end of
said first chamber being a plane surface;
b) first axial means for introducing gaseous fuel and
stoichiometric air for burning said fuel into said first chamber
at said first end, and including ignition means for igniting
said fuel;
c~ second means to radially inject combustible hydrocarbon
and steam at a plane close to said first end of said first
chamber to create a reducing atmosphere in said first chamber;
d) third means for radially introducing nitrogen oxide
containing gases into the reducing atmosphere in said first
chamber at the first end of said first chamber whereby said
0 nitrogen oxides will be reduced;
e~ passage means of diameter less than said chamber dia-
meter at the second end of said first chamber leading to the
first end of said second chamber for the flow of gases from
said first chamber to said second chamber, said passage means
having transverse parallel walls;
f) fourth means for radially introducing cooling means
into said second chamber at its said first end to reduce the
temperature of the gases to within the range of 1250F to
2000F. entering from said first chamber to a temperature low
0 enough to prevent thermal regeneration of nitrogen oxides; and
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g) fifth means downstream of said fourth means near the
second end of said second chamber for radially introducing air
into said second chamber for burning combustibles remaining in
the gases in said second chamber.
The drawing illustrates in schematic form a vertical
axis, two-chambered burner suitable for the reduction of
nitrogen oxide gases.
Referring to the drawing, the numeral 10 indicates
generally the combustion furnace. Numeral 12 indicates the
first chamber of the furnace and numeral 14 the second chamber.
Numeral 16 indicates generally the fuel and air supply to provide
the proper temperature for the reduction operation. The burner
system comprises a gaseous fuel line 18 and burner 20 with a
plenum and air pipe 22 for supplying air for the combustion.
The supply of air is controlled stoichiometrically so that there
will not be excess oxygen in the first chamber 26. The purpose
of the co~bustion of fuel 18 is to provide a suitable temperature
within the first chamber so that a reducing atmosphere can be
generated by introduction of a mixture of hydrocarbon and steam.
This is accomplished through inlet pipe 28 wherein steam enter-
ing through line 30 and hydrocarbon through line 32 are mixed
and injected into the heated zone in chamber 26.
Having provided in the chamber 26 the proper reducing
atmosphere, which means an excess of hydrogen and CO, the nitro-
gen oxide containing gases are introduced into the chamber 26
through pipe 34. Alternatively, the mixture of hydrocarbon and
steam can be introduced into the pipe 34 through the branch line
36 so that the nitrogen oxides, hydrocarbon and steam are all
mixed prior to entry into the heated chamber 26. Alternatively,
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instead of utilizing steam and hydrocarbon to provide the
reducing atmosphere, combustibles, such as hydrogen, carbon
monoxide or methane can be introduced into the branch pipe 36 and
mixed with thè nitrogen oxides in pipe 34 to provide the reduction
zone needed to reduce the nitrogen oxides.
After a sufficient residence time in the chamber 26 in the
reduction zone the gases, including the nitrogen now free of its
oxygen, pass through opening 46 in the area wall 44 into the second
chamber 14. Here the gases are rapidly cooled, by means of a coolant
entering through line 50, to a temperature in the range of 1,250F to
2,000F. This cooling should be as rapid as possible and conveniently
can be accomplished by any cooling medium introduced in a turbulent
manner so that it quickly contacts, cools and dilutes the gases issuing
from the first chamber 26. The dashed line 48 indicates this zone of
quenching which is required to keep the temperature so low that the
nitrogen will not recombine with oxygen.
However, there are now excess combustibles, mainly hydrogen
and CO which must be removed from the gas outflow 54 by burning with
air which can be conveniently introduced through line 52. On the
other hand, if cool air is used as the coolant it will also provide the
oxygen for combustion of the remaining combustibles. Since heat will
be provided by the combustion of the hydrogen and carbon monoxide the
cooling in zone 56 must be such that even with this heating the effluent
gasesJ shown by arrows 54, will still be below the temperature of
2,000 at which nitrogen will recombine with oxygen. The effluent
gases now contain an excess of air plus water, carbon dioxide and
nitrogen.
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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 arrangement of components. It
is understood that the in~ention is not to be limited to the specific
embodiment set forth herein by way of exemplifying the invention, but
the invention is to be limited only by the scope of the attached claim
or claims, including the full range of equivalency to which each element
or step thereof is entitled.
