JPS6114406B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention involves burning waste gas or (and) waste liquid in a reducing atmosphere that contains nitrogen oxides or nitrogen compounds that may be converted into nitrogen oxides by combustion, either singly or simultaneously, and whose flow rate and composition vary.
The present invention relates to a method for detoxifying waste gas and liquid by stably converting nitrogen atoms in nitrogen oxides and nitrogen compounds into harmless nitrogen gas. Generally, regarding the detoxification treatment of waste gas or (and) waste liquid (hereinafter simply abbreviated as waste gas body) containing nitrogen oxides (hereinafter abbreviated as NOx) or nitrogen compounds (hereinafter abbreviated as N compounds). Various treatment methods have been used to prevent environmental pollution. For example, as a method for detoxifying waste gas containing NOx by burning it, a method is disclosed in US Pat. No. 2,673,141 in which the waste gas is burned together with fuel in a reducing atmosphere to reduce and decompose Nox into harmless _N2 . It is described in Japanese Patent Application Laid-open No. 50-121159 and Japanese Patent Application Laid-Open No. 51-104470.
In these inventions, the present inventors discovered that residual N remains after combustion in a reducing atmosphere.
No consideration or measures have been taken to prevent the reconversion of compounds to NOx. As a result, NOx
The decomposition rate varies greatly depending on the NOx concentration in the waste gas, and the treated gas has the disadvantage that a considerable amount of NOx remains in the treated gas if it cannot be directly discharged into the surrounding atmosphere. On the other hand, as a detoxification treatment by incineration of waste gas containing N compounds, for example,
As a combustion method for suppressing NOx conversion, a well-known staged combustion method such as two-stage combustion can be applied. Therefore, the NOx suppression effect was insufficient. In order to eliminate the above-mentioned drawbacks, the present inventors have already combusted a waste gas body containing NOx and N compounds alone or together in a reducing atmosphere, and reduced and decomposed the NOx and N compounds into harmless _N2. He completed an invention related to a combustion treatment method and filed a patent application on the same day. The present invention provides that the amount of oxygen supplied for combustion to the combustible components contained in the waste gas body and the fuel supplied as necessary is 95% of the stoichiometric amount necessary to oxidize the entire amount of combustible components including the fuel. % or less, the waste gas is combusted in a reducing combustion atmosphere with a combustion temperature within the range of 700℃ to 1100℃, and the NOx and N compounds contained are detoxified, and most of the NOx and N compounds are converted to _N2 . After decomposition, the incompletely combusted gas obtained is supplied with oxygen up to the stoichiometric amount necessary to oxidize all remaining combustible components, and the combustion temperature is maintained between 700℃ and 1100℃. It is burned in a reducing combustion atmosphere within a certain range, and the residual N compounds are detoxified and decomposed into almost _N2 .
After that, the incompletely combusted gas is supplied with more than a stoichiometric amount of oxygen capable of oxidizing all remaining combustible components to complete combustion, thereby rendering the waste gas harmless. in the waste gas body
This combustion treatment method is characterized by a high decomposition rate of NOx and N compounds. In general, the flow rate and composition of waste gas often vary over time, but in putting this invention into practical use,
There is a problem in that it is extremely difficult to instantly detect these fluctuation amounts and control the amount of oxygen supplied in each reducing combustion atmosphere. Therefore, it is desired to employ a combustion method that can relatively easily control fluctuations in the flow rate and composition of the waste gas body. The present invention has been made in response to such demands, and it decomposes NOx and N compounds contained in the waste gas into _N2 at a high decomposition rate, and also reduces the flow rate of the waste gas.
The object of the present invention is to provide a combustion treatment method that can sufficiently cope with variations in composition and renders waste gas harmless to the extent that it can be released directly into the surrounding atmosphere. The present invention deals with waste gas (or flue gas) from NOx generation sources such as nitric acid plants, boilers, heating furnaces, and incinerators, and NOx and N from organic synthetic chemical plants.
The present invention can be applied to the detoxification treatment of various waste gas bodies (or exhaust gas bodies) such as exhaust gases (or exhaust gas bodies, waste liquids (or waste liquids)) from single or simultaneous sources of compounds. When burning and decomposing a waste gas body that contains nitrogen oxides (NOx) or nitrogen compounds (N compounds) that may be converted into NOx by combustion, either singly or simultaneously, and whose flow rate and composition fluctuate, The combustion air is divided into two parts while controlling the oxygen supply amount to 95% or less, preferably 90% or less of the stoichiometric amount required to oxidize the entire amount of combustible components contained in the combustible components and fuel supplied as necessary. The waste gas body and the fuel are heated to a combustion temperature within the range of 700°C to 1100°C,
The combustion is preferably performed in a reducing combustion atmosphere within the range of 850°C to 1050°C, and the combustion reaction is allowed to proceed sufficiently, thereby decomposing most of the NOx and N compounds into _N2 . After that, in the incomplete combustion gas obtained, N compounds remain along with combustible components such as carbon monoxide (CO), hydrogen (H ₂ ), and hydrocarbons (HC), so the other branched gas is That is, the remaining air is supplied to remove the combustible components remaining in the combustion gas at a combustion temperature within the range of 700°C to 1100°C, preferably 850°C.
The remaining N compounds are mostly decomposed into N ₂ by burning while maintaining the temperature within the range of ~1050°C and allowing the combustion reaction to proceed sufficiently. The combustion temperature in the combustion and the combustion is in an appropriate temperature range to suppress the addition of new heat-generated NOx and to enable complete combustion decomposition of NOx and N compounds. In addition, in order to further complete the detoxification reaction of the residual N compounds, oxygen is supplied to the combustion gas in an amount that does not exceed the stoichiometric amount required to oxidize all combustible components to raise the combustion temperature. This can be done by allowing the combustion reaction to proceed while maintaining the temperature, and then repeating the combustion reaction in the same manner by supplying oxygen. After that, for the incomplete combustion gas obtained,
Oxygen in a stoichiometric amount or more capable of oxidizing the entire amount of combustible components remaining in the combustion gas is supplied to combust the combustible components, and by completing the combustion, the waste gas body is completely rendered harmless. In the present invention, "oxygen" is used for ease of explanation, but
Air is used as the oxygen supply source. The present inventors aimed to establish a detoxification combustion treatment method for waste gases that simultaneously contain NOx and N compounds and whose flow rates and compositions fluctuate. The present invention was achieved as a result of repeated experiments and research on the reductive decomposition process of NOx, and the process of converting N compounds to NOx in staged combustion methods such as two-stage combustion. In these experiments and studies, the following conclusions were obtained as the main points of the structure of the present invention. That is,
(1) In a reducing combustion atmosphere, most of NOx is decomposed into N ₂ by reducing agents such as hydrocarbons, but some of it is converted into N compounds such as hydrogen cyanide (HCN), and this combustion gas is used in conventional methods. (U.S. Patent No.
2673141, JP-A-50-121159, and JP-A-51-104470), when the residual combustible components are directly oxidized and burned, the converted N compound is reconverted to NOx. In order to decompose this converted N compound into _N2 , combustion in a reducing atmosphere is effective. The reason for this is clear in (2) below. (2) In the reaction space where combustion occurs in a reducing atmosphere, some N compounds are oxidized and converted to NOx in the reaction area until the oxygen is completely consumed by oxidation of combustible components; In the reaction part where the entire amount of NOx is consumed and a reducing atmosphere is created, the generated NOx is decomposed into _N2 by the remaining combustible components such as hydrocarbons. This once generated
To complete the decomposition of NOx, it is important to allow the combustion reaction to proceed sufficiently until the decomposition is completed. Furthermore, even if the decomposition of the generated NOx is completed, the converted N compounds mentioned in (1) above are still produced, and unoxidized N compounds also remain.
In order to decompose these N compounds into N ₂ , oxygen is supplied to the combustion gas, which is then burned again in a reducing atmosphere to allow the combustion reaction to proceed sufficiently. The N compounds are once converted into NOx and then N ₂ It is necessary to decompose it into The above considerations and measures are not taken in conventionally known stage combustion methods such as two stage combustion.
(3) In order to efficiently perform the decomposition reaction of NOx and N compounds, when implementing the present invention, it is necessary to take the residence time necessary for the decomposition reaction and to promote the reaction by improving the mixing of gases during the reaction. , it is effective to partition the boundaries of the reaction parts in order to maintain the unique combustion form of each reaction part. As described above, the basic configuration of the present invention is that the combustion chamber in the reducing atmosphere on the upstream side and the combustion chamber in the oxidizing atmosphere on the downstream side are connected in series. Therefore, if the combustion conditions on the upstream side change due to the influence of the flow rate and composition of waste gas, the combustion conditions on the downstream side will also change accordingly, and it is necessary to separate both combustions as independent. I can't think like that. If combustion air is supplied independently for each combustion, there may be a time lag in the supply of combustion air that corresponds to fluctuations in the flow rate and composition of the exhaust gas, causing interference between the two combustions. This makes stable low NOx operation difficult. In the present invention, in order to prevent such damage, the air supplied through the same duct is divided into two parts and used, thereby avoiding interference between the two types of combustion and enabling stable low NOx operation. Examples of the present invention will be described below, but the present invention is not limited to these Examples. The various gases in the examples were analyzed by the following methods. All analytical values were based on dry volume. NOx: Chemiluminescence method HCN: Cyan ion electrode method All analyzes of other compounds were performed by gas chromatography. Example The apparatus shown in FIG. 1 was used. In FIG. 1, 1 is a fuel conduit, 2 is a waste gas conduit, and 3 and 4 are air conduits.The air conduit 3 is divided into two air conduits 3a and 3b downstream of the control damper 5,
b are connected to the combustion chamber of the combustion furnace via fixed dampers 6a and 6b, respectively. The combustion furnace has a fireproof and insulated structure (cylindrical shape with an inner diameter of 350 mm and a length of 4000 mm), and includes a hot air generating furnace 7, reduction combustion chambers 8 and 9, and oxidation combustion chamber 10.
It is divided into. Grate-type partition walls 11a, 11b, and 11c were used as a means of division. 1
2 is a combustion gas passage connected to the oxidation combustion chamber, 13
is the control signal line connected to the control damper. In the apparatus configured as described above, propane gas 0.8Nm ³ /H is first supplied into the hot air generation furnace 7 through the fuel conduit 1 and combusted to generate high temperature hot air gas, and the hot air gas is transferred to the reduction combustion chamber. 8. The main purpose of this hot air gas is to maintain the combustion temperature of the reduction combustion chambers 8 and 9 within the range of 700°C to 1100°C and to supply oxygen to the reduction combustion chamber 8. At the same time, the waste gas generated in the chemical plant (composition: N compounds =
550 ppm, NOx = 120 ppm, CO = 1.2%, hydrocarbons = 1.4%, O ₂ = 2%, others are N ₂ ) were introduced through the 45 Nm ³ /H waste gas conduit 2 and combusted. Propane gas and waste gas combustion air were supplied into the hot air generating furnace 7 through the air conduit 3a.
Subsequently, combustion air was supplied into the reduction combustion chamber 9 through the air conduit 3b for further combustion. After the above combustion, air was supplied into the oxidation combustion chamber 10 through the air conduit 4, and the combustible components flowing into the oxidation combustion chamber 10 were completely oxidized to complete combustion. In this embodiment, assuming that the total amount of air supplied through the air conduits 3 and 4 is constant, the air conduits 3a,
Fig. 2 shows the experimental results when changing the amount of air supplied from 3b. In FIG. 2, the vertical axis shows the NOx concentration (ppm) in the treated combustion gas, and the horizontal axis shows the theoretical oxygen ratio (the calculation formula will be described later) in the reduction combustion chamber 8. From FIG. 2, if the stoichiometric oxygen ratio in the reduction combustion chamber 8 is 0.95 or less, the change in NOx concentration in the treated combustion gas is relatively small;
It can be seen that this can be suppressed to a low level. Therefore, if the stoichiometric oxygen ratio in the combustion chamber 9 is 0.95 or less, the stoichiometric oxygen ratio in the combustion chamber 8 will necessarily be 0.95 or less, and in this case, even if the flow rate and composition of the waste gas vary, The NOx concentration in the treated combustion gas can be maintained at a low level. That is, if the amount of air supplied through the air conduit 3 is controlled by the control damper 5 so that the stoichiometric oxygen ratio in the combustion chamber 9 is 0.95 or less, and then this air is divided into two parts and supplied through the air conduits 3a and 3b, Even if the stoichiometric oxygen ratio in the combustion chambers 8 and 9 varies over a wide range, in other words, the stoichiometric oxygen ratio in the combustion chambers 8 and 9 varies due to fluctuations in the flow rate and composition of the waste gas, NOx and N compounds can be decomposed at a high rate. It can be decomposed and rendered harmless. In addition, in FIG. 2, each curve shows the case where the theoretical oxygen ratio of the reduction combustion chamber 9 is 1.34, 1.00, 0.89, and 0.78 from the top. The theoretical oxygen ratio in the reduction combustion chambers 8 and 9 was calculated using the following formula.

[Table] Stoichiometric amount of oxygen required for

[Table] The stoichiometric amount of oxygen required for On the other hand, it can be seen that the theoretical oxygen ratio in the reduction combustion chambers 8 and 9 should be controlled to 0.95 or less. As this control method, as shown in this embodiment, the amount of air supplied through the air conduit 3 is controlled by the control damper 5 so that the stoichiometric oxygen ratio in the reduction combustion chamber 9 does not exceed 0.95, and the air flow divided into two parts is It can be seen that the amount may be set to an appropriate amount by the fixed dampers 6a, 6b provided in the air conduits 3a, 3b. As explained above, the present invention provides a method that allows relatively easy control of the waste gas when the flow rate and composition of the waste gas changes over time.
It has the excellent effect of being able to decompose and render harmless NOx and N compounds at a high decomposition rate.

[Brief explanation of the drawing]

Fig. 1 is a systematic explanatory diagram showing an example of a detoxification combustion treatment apparatus for waste gas and waste liquid for carrying out the method of the present invention, and Fig. 2 shows the stoichiometric oxygen ratio in the reduction combustion chamber and the stoichiometric oxygen ratio in the treated combustion gas. It is a graph showing the relationship with NOx concentration. 1...Fuel pipe, 2...Waste gas pipe, 3,3
a, 3b, 4... air conduit, 5... control damper,
6a, 6b... fixed damper, 7... hot air generating furnace,
8, 9... Reduction combustion chamber, 10... Oxidation combustion chamber, 1
1a, 11b, 11c... partition walls.

Claims (1)

[Claims] 1 Nitrogen oxides, nitrogen compounds that may be converted into nitrogen oxides by combustion, singly or simultaneously, are contained in waste gas or waste liquid when the waste gas or waste liquid whose flow rate and composition fluctuate is decomposed by combustion. 95% of the stoichiometric amount required to oxidize all the combustible components and fuel supplied as required.
% or less, combustion air is divided into two streams and supplied, and one of the divided airs is used to heat the waste gas or waste liquid and the fuel at a temperature of 700°C to 1100°C.
Combustion is performed in a reducing combustion atmosphere within the temperature range of 700°C to 1100°C by supplying the other branched air to the resulting incompletely combusted gas to maintain the temperature within the range of 700°C to 1100°C. After that, the incompletely combusted gas is supplied with a stoichiometric amount or more of oxygen capable of oxidizing the entire amount of combustible components remaining in the combustion gas to combust the combustible components. A detoxification combustion treatment method for waste gas and liquid.