CA1068075A - Process for disposal of oxides of nitrogen - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for converting gases containing oxides of nitrogen into gases which may be safely vented to the atmosphere wherein the gases containing the oxides of nitrogen (NOx) are mixed with excess combustible products obtained by burning a hydrocarbon fuel with less than its stoichiometric requirements of oxygen, the mixture thus obtained is cooled to avoid temperatures substantially above 2,000°F. but not below about 1,200°F., the combustible material remaining after substantially all of the oxides of nitrogen have been reduced to nitrogen are oxidized, so that the resulting gas, substantially free of NOx and carbon monoxide (CO) may be vented to the surrounding atmosphere without contamination of the environment and without smoke or other particulate matter.

Description

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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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 ~.
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 ~"' .
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:
., , ~' L : . ' , , . ~ : : ' 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 i , , 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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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.
~: j ~ 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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.. . , -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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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 ,, . ~
~................ accomplished by valve 74 which is further controlled as r ` .~ ~
a function of the temperature within the effluent 56 , from the nitrogen oxide reduction furnace 30.

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' 106~ 75 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 . ~ .
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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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 ... .... .
recycled as mentioned. The following Table gives results ` of the test:
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of converting 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 the presence of less than one hundred percent of said fuel's stoichiometric 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 remain-ing after substantially all of the oxides of nitrogen have been reduced, with sufficient oxygen to convert substantially all of the oxidizable combustion products remaining to carbon dioxide and water without reformation of oxides of nitrogen and 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.