JP7537863B2 - Denitration equipment and method - Google Patents

Description

The present invention relates to a denitrification device and a denitrification method.

Exhaust gases emitted from factories, waste incinerators, thermal power plants, etc. contain nitrogen oxides (NOx), and measures to reduce NOx emissions are being taken daily.
One of the countermeasures is flue gas denitration technology. Currently, the dry selective catalytic reduction method is widely used, which enables a denitration rate of over 90%.
On the other hand, the non-catalytic selective reduction method has the advantage that the equipment cost and running cost are low because a catalyst is not required. However, the non-catalytic selective reduction method has a problem that the denitrification rate is about 30 to 50%, which is lower than that of the selective catalytic reduction method. In addition, the optimal temperature range for the non-catalytic selective reduction method is 850 to 1,000°C, which is higher than that of the selective catalytic reduction method.

To address these issues, for example, Patent Document 1 proposes an exhaust gas denitration device that performs two-stage combustion in a combustion melting chamber and a secondary combustion chamber to denitrify combustion exhaust gas in the optimal temperature range for non-catalytic selective reduction. The invention in Patent Document 1 aims to perform efficient denitration by injecting a denitrification reducing agent into the secondary combustion chamber.

Japanese Patent Application Publication No. 11-270814

However, in the invention of Patent Document 1, the temperature of the secondary combustion chamber is maintained at 850 to 1,000°C for denitration, which places a large burden on the environment. In addition, the invention of Patent Document 1 does not achieve a sufficient denitration rate.

Therefore, the present invention aims to develop a denitration device and method that can further reduce the environmental load and increase the denitration rate.

In order to solve the above problems, the present invention has the following aspects.
[1] A denitration apparatus comprising a decomposition section which causes nitrogen-containing organic matter and a gas containing nitrogen oxides to coexist and which sets the coexistence under subcritical conditions for water, oxidatively decomposes the nitrogen-containing organic matter and reductively decomposes the nitrogen oxides, the decomposition section having one decomposition tank, a treatment temperature of the decomposition tank being equal to or higher than the critical temperature of water and equal to or lower than 500°C, and the physical amount of nitrogen element contained in the nitrogen oxides in the decomposition tank being 0.6 to 1.0 mol per 1 mol of nitrogen element contained in the nitrogen-containing organic matter .

[2] A denitration method comprising a decomposition step of causing a nitrogen-containing organic matter to coexist with a gas containing nitrogen oxides, subjecting the resulting mixture to subcritical water conditions, oxidatively decomposing the nitrogen-containing organic matter, and reductively decomposing the nitrogen oxides, the decomposition step being carried out in one decomposition tank, the treatment temperature of the decomposition tank being equal to or higher than the critical temperature of water and equal to or lower than 500°C, and the physical amount of nitrogen element contained in the nitrogen oxides per mole of nitrogen element contained in the nitrogen-containing organic matter being 0.6 to 1.0 mol .
[3] The denitrification method according to [2], wherein the treatment temperature in the decomposition step is from the critical temperature of water to 450° C., and the treatment pressure is from 5 to 20 MPa.
[4] The denitration method according to [2] or [3], wherein the decomposition step is carried out in the presence of oxygen in an amount 1.0 times or more the stoichiometric amount of oxygen required for complete oxidation of the nitrogen-containing organic matter.
[5] The denitrification method according to any one of [2] to [4], wherein the treatment time in the decomposition step is 5 to 15 minutes.

The denitration device and method of the present invention can further reduce the environmental load and increase the denitration rate.

1 is a schematic diagram of a denitration device according to an embodiment of the present invention. 1 is a graph showing an example of the generation rate of ammoniacal nitrogen when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment. 1 is a graph showing an example of the effects of pressure and time on the conversion rate of nitrogen components to ammonia and nitrous oxide when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment. 1 is a graph showing an example of the generation rate of ammoniacal nitrogen when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment. 1 is a graph showing an example of the generation rate of ammoniacal nitrogen when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment. 1 is a graph showing an example of the generation rate of ammoniacal nitrogen when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment. 1 is a graph showing the residual nitrogen concentrations in Comparative Example 1, Example 1, and Example 2.

In this specification, the subcritical condition of water includes either a state in which water exists as a gas (water vapor) below the critical temperature of water (374° C.) and below the critical pressure of water (22 MPa), or a state equal to or higher than the critical temperature of water and below the critical pressure of water.
In this specification, the above-mentioned oxidative decomposition of nitrogen-containing organic matter under subcritical water conditions is referred to as subcritical water oxidation treatment.

<Denitrification equipment>
The denitration device of the present invention has a decomposition section that causes nitrogen-containing organic matter and a gas containing nitrogen oxides to coexist and that subjects the coexistence to subcritical water conditions, oxidatively decomposes the nitrogen-containing organic matter, and reduces and decomposes the nitrogen oxides.
Hereinafter, one embodiment of the denitration apparatus of the present invention will be described in detail with reference to FIG.

As shown in FIG. 1 , the denitration device 1 of this embodiment includes a sludge supplying section 10 , an exhaust gas supplying section 12 , and a decomposition section 20 .
The sludge supply unit 10 and the decomposition unit 20 are connected by a pipe L1. The exhaust gas supply unit 12 and the decomposition unit 20 are connected by a pipe L2.

The decomposition section 20 includes a decomposition tank 21, a heater 22, a measurement section 23, a high-pressure pump P1, and a pressure adjustment valve B1. The measurement section 23 is connected to the decomposition tank 21. A pipe L3 is connected to the decomposition tank 21. The high-pressure pump P1 is provided in the pipe L1. The pressure adjustment valve B1 is provided in the pipe L3.

The sludge supply unit 10 supplies nitrogen-containing organic matter to the decomposition tank 21 of the decomposition unit 20. The sludge supply unit 10 may be any unit capable of supplying nitrogen-containing organic matter, such as a portion of a drainage pipe in a sewage treatment facility, a tank capable of temporarily storing organic sludge, or a vehicle capable of carrying organic sludge.

The exhaust gas supply unit 12 supplies a gas (exhaust gas) containing nitrogen oxides to the decomposition tank 21 of the decomposition unit 20. The exhaust gas supply unit 12 may be any unit capable of supplying exhaust gas, such as a tank capable of temporarily storing exhaust gas discharged from a waste incineration plant or the like, or a pipe through which exhaust gas flows.

The decomposition tank 21 may be, for example, a pressure-resistant vessel made of a metal such as stainless steel or a nickel alloy.

The heater 22 may be any heater capable of heating the inside of the decomposition tank 21, such as a steam heater that passes high-temperature steam through it or a gas boiler.

The measuring unit 23 may be capable of measuring the temperature, pressure, and nitrogen oxide concentration inside the decomposition tank 21, and examples of such measuring units include known thermometers, pressure gauges, and concentration meters.

The high-pressure pump P1 is required to pump nitrogen-containing organic matter from the sludge supply section 10 to the decomposition tank 21 of the decomposition section 20, and may be a high-pressure liquid delivery pump or a compressor.

Examples of the pressure adjustment valve B1 include a known valve or pressure adjustment valve. The pressure adjustment valve B1 may also function as an opening and closing valve.

It is preferable that the heater 22, the measuring unit 23, the high-pressure pump P1, and the pressure adjustment valve B1 are all controlled by an external control unit (not shown) to turn them on, off, open, and closed.

The pipe L1 may be a pipe made of a metal such as stainless steel.
The pipes L2 and L3 may be the same as the pipe L1. The pipes L2 and L3 may be different from each other or may be the same as the pipe L1. Furthermore, the pipes L2 and L3 may be different from each other or may be the same as the pipe L1.

<Denitrification method>
The denitration method of the present invention includes a decomposition step of causing nitrogen-containing organic matter and a gas containing nitrogen oxides to coexist and subjecting the resulting mixture to subcritical water conditions, thereby oxidatively decomposing the nitrogen-containing organic matter and reductively decomposing the nitrogen oxides.
A denitration method using a denitration device 1 will be described with reference to FIG.

(Decomposition process)
First, a slurry mixture of waste containing nitrogen-containing organic matter and water (also called organic sludge or sludge) is supplied from the sludge supply unit 10 to the decomposition tank 21 via the high-pressure pump P1.
Next, a gas containing nitrogen oxides (also referred to as exhaust gas) is supplied from the exhaust gas supply unit 12 to the decomposition tank 21 .
As a result, organic sludge and exhaust gas coexist in the decomposition tank 21 .
There is no particular limitation on the order in which the organic sludge and the exhaust gas are supplied to the decomposition tank 21. The exhaust gas may be supplied to the decomposition tank 21 first, or the organic sludge and the exhaust gas may be supplied to the decomposition tank 21 simultaneously.

The ratio of organic sludge to exhaust gas is expressed, for example, by the ratio of the amount of substance of nitrogen element contained in the organic sludge to the amount of substance of nitrogen element contained in the exhaust gas (hereinafter also referred to as "coexistence ratio"). The coexistence ratio is expressed, for example, by the amount of substance of nitrogen element contained in the exhaust gas per mole of nitrogen element contained in the organic sludge. The reduction reaction between nitrogen oxides (also referred to as NOx) in the exhaust gas and ammonia ( _NH3 ) derived from the organic sludge is expressed by any of the following formulas.
4NO+ _4NH3 + _O2 → _4N2 + _6H2O ...(1)
NO+ _NO2 + _2NH3 → _2N2 + _3H2O ...(2)
6NO ₂ +8NH ₃ → 7N ₂ +12H ₂ O...(3)
The amount of substance of the nitrogen element contained in the exhaust gas relative to 1 mole of the nitrogen element contained in the organic sludge is, for example, preferably 0.6 to 1.0 mole, and more preferably 0.6 to 0.75 mole. When the amount of substance of the nitrogen element contained in the exhaust gas relative to 1 mole of the nitrogen element contained in the organic sludge is equal to or greater than the above lower limit, a sufficient amount of nitrogen oxides can be denitrified efficiently without increasing the amount of remaining ammonia. When the amount of substance of the nitrogen element contained in the exhaust gas relative to 1 mole of the nitrogen element contained in the organic sludge is equal to or less than the above upper limit, the amount of ammonia for reducing NOx is sufficient, and a decrease in the denitrification rate can be suppressed.

Nitrogen oxides is a general term for oxides of nitrogen, such as nitric oxide (NO), nitrogen dioxide (NO ₂ ), nitrous oxide (dinitrogen monoxide, N ₂ O), and dinitrogen trioxide (N ₂ O ₃ ).
In this specification, nitrate nitrogen is gasified under subcritical conditions of water to become _NO2 , and _NO2 dissolves in water by cooling. Furthermore, _NO2 becomes nitrate nitrogen and nitrite nitrogen by reacting with water as described below.
3NO ₂ +H ₂ O → 2HNO ₃ +NO ... (4)
2NO ₂ +H ₂ O → HNO ₃ +HNO ₂ ...(5)

In the decomposition step, an oxidation reaction of nitrogen-containing organic matter and a reduction reaction of nitrogen oxides (hereinafter, simply referred to as "reaction") occur.
In this specification, the denitrification rate is expressed by the following formula (6).
Denitrification rate (%) = 100 - {(mass (mg) of nitrate nitrogen after reaction + mass (mg) of nitrite nitrogen after reaction) / (mass (mg) of nitrate nitrogen before reaction + mass (mg) of nitrite nitrogen after reaction)} × 100 ... (6)
The mass of nitrate nitrogen can be determined, for example, by ion chromatography.
The mass of nitrite nitrogen can be determined, for example, by ion chromatography.

Nitrate nitrogen refers to nitrogen that exists in the form of nitrate ions (NO ₃ ⁻ ).
Nitrite nitrogen refers to nitrogen that exists in the form of nitrite ions (NO ₂ ⁻ ).
Ammoniacal nitrogen refers to nitrogen present in the form of ammonium ion (NH ₄ ⁺ ).
The sum of nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen is called inorganic nitrogen.

After the organic sludge and exhaust gas are supplied to the decomposition tank 21, the decomposition tank 21 is heated by the heater 22 and pressurized by the high-pressure pump P1, so that the organic sludge and exhaust gas are brought into subcritical water conditions.
Ammonia is generated by coexisting organic sludge and exhaust gas and bringing them (organic sludge and exhaust gas) to subcritical water conditions. In the presence of ammonia, nitrogen oxides are reduced (denitrification) to produce a mixed fluid containing nitrogen, water, and carbon dioxide.
The organic sludge and the exhaust gas may be supplied to the decomposition tank 21 at regular intervals or continuously.
The generated ammonia may be separated and used as an energy carrier or energy source.

The internal temperature of the decomposition tank 21 in the decomposition step (hereinafter also referred to as the processing temperature) can be measured by the measuring unit 23.
The treatment temperature is equal to or higher than the critical temperature of water (374° C.), and is preferably 374° C. to 500° C., and more preferably 400° C. to 450° C. When the treatment temperature is equal to or higher than the lower limit, the nitrogen-containing organic matter can be sufficiently oxidized and decomposed. When the treatment temperature is equal to or lower than the upper limit, the conversion rate to ammonia is easily improved, and energy required for heating with the heater 22 is easily saved. This makes it easier to reduce the environmental load.
A preferable treatment temperature can be estimated from the results of the experimental examples described later.

The internal pressure of the decomposition tank 21 in the decomposition step (hereinafter also referred to as the treatment pressure) can be measured by a measuring unit 23 .
The treatment pressure is less than the critical pressure of water (22 MPa), and is preferably 5 MPa to 20 MPa, more preferably 10 MPa to 20 MPa, and even more preferably 10 MPa to 15 MPa. When the treatment pressure is equal to or higher than the lower limit, the nitrogen-containing organic matter can be sufficiently oxidatively decomposed. When the treatment pressure is equal to or lower than the upper limit, the load on the decomposition tank 21 can be easily reduced.
A preferable treatment pressure can be estimated from the results of the experimental examples described later.

Nitrogen-containing organic matter refers to organic matter that contains nitrogen components. Examples of nitrogen-containing organic matter include ammonia-containing digestive liquid discharged from the methane fermentation process, food waste, livestock waste, concentrated sewage sludge, digested sludge, organic sludge, and other biomass waste.

Nitrogen-containing organic matter contains moisture and is usually dehydrated before being incinerated.
In the denitration device 1 of this embodiment, the decomposition tank 21 is kept at high temperature and pressure to oxidatively decompose nitrogen-containing organic matter under subcritical water conditions, so that a prior dehydration step is not required.
The solid content of the nitrogen-containing organic matter produced in the decomposition step can be separated from gas components containing water vapor using a discharge pump (not shown) and discharged to the outside of the decomposition tank 21. In this case, the dehydration step and the incineration step are not required.

The moisture content of the nitrogen-containing organic matter is preferably 90% by mass or more. If the moisture content of the nitrogen-containing organic matter is equal to or greater than the lower limit, the flowability from the sludge supply section 10 to the decomposition tank 21 is excellent, and the denitrification device 1 can be operated continuously, making it easier to increase the denitrification rate.

In the decomposition step, the oxidizing agent may be air, oxygen in the air, hydrogen peroxide, etc. It is preferable that the oxidizing agent supplies oxygen in an amount in excess of the amount required for the oxidative decomposition reaction of the nitrogen-containing organic matter, thereby completely oxidizing and decomposing the nitrogen-containing organic matter.
The decomposition step is preferably carried out in the presence of oxygen at 1.0 times or more the stoichiometric amount of oxygen required for complete oxidation of the nitrogen-containing organic matter. Here, the ratio of oxygen to the stoichiometric amount of oxygen required for complete oxidation of the nitrogen-containing organic matter is referred to as the "oxygen ratio". The oxygen ratio is preferably 1.0 or more, more preferably 1.2 to 2.5, even more preferably 1.2 to 2.0, and particularly preferably 1.2 to 1.5. When the oxygen ratio is equal to or more than the lower limit, the conversion rate to ammoniacal nitrogen contained in the organic sludge (the generation rate of ammoniacal nitrogen) is easily improved. Therefore, it is easy to reduce nitrogen oxides in the exhaust gas and to further increase the denitrification rate. When the oxygen ratio is equal to or less than the upper limit, the excess supply of the oxidizing agent can be suppressed.
A preferred oxygen ratio in the decomposition step can be estimated from the results of the experimental examples described below.

The heating and pressurizing time in the decomposition step (hereinafter also referred to as the processing time) is preferably 1 minute or more and 30 minutes or less, more preferably 1 minute or more and 20 minutes or less, and even more preferably 1 minute or more and 15 minutes or less. When the processing time is equal to or more than the above lower limit, the nitrogen-containing organic matter can be sufficiently oxidized and decomposed. When the processing time is equal to or less than the above upper limit, the conversion rate to ammonia nitrogen contained in the organic sludge is easily improved, and the denitrification rate is easily increased. In addition, when the processing time is equal to or less than the above upper limit, it is easy to save energy when heating with the heater 22, and the environmental load is easily reduced.
A preferable treatment time can be estimated from the results of the experimental examples described later.

The fluid (mixed fluid) produced in the decomposition process is released to the outside of the denitrification device 1 via pipe L3 by opening pressure control valve B1.

According to the denitration device 1 of the present embodiment, the nitrogen components contained in the nitrogen-containing organic matter can be rapidly converted to ammoniacal nitrogen by the subcritical water oxidation treatment. The ammoniacal nitrogen functions as a reducing agent for the nitrogen oxides in the exhaust gas. Therefore, it is easy to increase the denitration rate of the exhaust gas.
According to the denitration device 1, since denitration is performed under subcritical conditions of water, it is possible to denitrify exhaust gas at a lower temperature than in the conventional method (e.g., 850° C.), thereby further reducing the environmental load.
The denitrification device 1 can treat both organic sludge and exhaust gas in a single decomposition tank, which reduces the burden of capital investment and enables efficient treatment of both organic sludge and exhaust gas.

Although the denitration device and the denitration method of the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified as appropriate without departing from the spirit and scope of the present invention.
For example, all or a part of the nitrogen-containing organic matter may be converted into oxidative decomposition products upstream of the sludge supply section, and then the products may be supplied to the decomposition section.
For example, a reaction tank capable of further denitrifying the mixed fluid may be provided downstream of the decomposition section, to produce a cleaner fluid.
The number of sludge supply units may be two or more, rather than one.
The number of exhaust gas supply parts may be two or more, rather than one.

In the decomposition process of this embodiment, the treatment temperature is equal to or higher than the critical temperature of water, and the treatment pressure is lower than the critical pressure of water, but the treatment temperature and pressure may be such that the subcritical conditions of water are satisfied. An example of a combination of temperature and pressure that satisfies the subcritical conditions of water is a combination of a treatment temperature lower than the critical temperature of water, and a treatment pressure lower than the critical pressure of water.

The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

[Experimental Example 1]
The generation rate of ammoniacal nitrogen from digested sewage sludge (organic sludge) was measured at a temperature of 400°C, a pressure of 10 MPa (subcritical conditions for water), and an oxygen ratio of 1.2. The generation rate of ammoniacal nitrogen was calculated by dividing the mass of ammonium nitrogen after the subcritical water oxidation treatment by the total mass of nitrogen components contained in the organic sludge before the subcritical water oxidation treatment (the sum of organic nitrogen and inorganic nitrogen in the liquid portion and nitrogen components in the solid portion).
The mass of total nitrogen in the liquid was determined by measuring the concentration using a total organic carbon meter (TOC-V/TN, manufactured by Shimadzu Corporation).
The mass of inorganic nitrogen in the liquid was determined by measuring the concentration using ion chromatography (Dionex ICS-1600, manufactured by Thermo Fisher Scientific). The mass of organic nitrogen in the liquid was determined by subtracting the mass of inorganic nitrogen from the mass of total nitrogen.
The mass of the nitrogen component in the solid content was determined by measuring the concentration using a fully automatic elemental analyzer (2400II, manufactured by PerkinElmer).
Subcritical water oxidation treatment was performed for treatment times ranging from 0 to 15 minutes, and the generation rate of ammoniacal nitrogen was measured. The results are shown in Figure 2.

As shown in Figure 2, the generation rate of ammonia nitrogen increases up to 5 minutes of treatment time. The generation rate of ammonia nitrogen does not change significantly between 5 and 10 minutes of treatment time, but decreases slightly at 15 minutes of treatment time.

FIG. 3 shows FIG. 6 described in Japanese Patent No. 4,838,013.
As shown in FIG. 3, under the treatment conditions of 450° C., pressure of 10 MPa, and oxygen ratio of 1.5, the conversion rate to ammonia decreases after a treatment time of 20 minutes.
2 and 3, it appears that the generation rate of ammoniacal nitrogen decreases as the treatment time increases.

[Experimental Example 2]
Subcritical water oxidation treatment (temperature 400° C., treatment time 5 minutes, oxygen ratio 1.2) was carried out on the same sample as in Experimental Example 1, but under different pressure conditions. The generation rate of ammoniacal nitrogen is shown in FIG.
4, the generation rate of ammoniacal nitrogen was 80% or more at a treatment pressure of 5 to 20 MPa. The generation rate of ammoniacal nitrogen reached a maximum value (87%) when the treatment pressure was 10 MPa.

[Experimental Example 3]
Subcritical water oxidation treatment (pressure 10 MPa, treatment time 5 minutes, oxygen ratio 1.2) was carried out on the same sample as in Experimental Example 1, but under different temperature conditions. The generation rate of ammoniacal nitrogen is shown in FIG.
As shown in FIG. 5, when the treatment temperature was 350 to 450° C., the generation rate as ammoniacal nitrogen reached a maximum value (87%) when the treatment temperature was 400° C.

[Experimental Example 4]
Subcritical water oxidation treatment (temperature 400° C., pressure 10 MPa, treatment time 5 minutes) was carried out on the same sample as in Experimental Example 1, with the oxygen ratio changed. The generation rate of ammoniacal nitrogen is shown in FIG.
As shown in Fig. 6, there was a tendency for the generation rate of ammoniacal nitrogen to increase as the oxygen ratio increased. When the oxygen ratio was 1.2 or more, the generation rate of ammoniacal nitrogen plateaued, and when the oxygen ratio was 1.2, the generation rate of ammoniacal nitrogen was 87%.

From the results of Experimental Examples 1 to 4 above, in the case of sewage organic sludge as nitrogen-containing organic matter, suitable conditions for increasing the generation rate of ammoniacal nitrogen include a treatment temperature of 350 to 450°C, a treatment pressure of 5 to 20 MPa, an oxygen ratio of 1.0 or more, and a treatment time of 5 to 15 minutes. More suitable conditions for increasing the generation rate of ammoniacal nitrogen include a treatment temperature of 380 to 420°C, a treatment pressure of 5 to 15 MPa, an oxygen ratio of 1.2 to 1.5, and a treatment time of 5 to 10 minutes.

[Comparative Example 1, Examples 1 to 2]
Sodium nitrate (approximately 1,000 mg-N/L) was subjected to subcritical water oxidation (temperature 400°C, pressure 10 MPa, treatment time 5 minutes) assuming that only exhaust gas-derived NOx ( _NO2 ) was present, and the nitrogen concentration (mg-N/L) before and after the subcritical water oxidation was determined. At the same time, the state of the nitrogen component (respective nitrogen concentration (mg-N/L) of nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen) before and after the subcritical water oxidation was determined (Comparative Example 1).
The nitrogen concentration was determined by measurement using ion chromatography (Dionex ICS-1600, manufactured by Thermo Fisher Scientific).

Assuming that ammoniacal nitrogen derived from nitrogen-containing organic matter and NOx ( _NO2 ) derived from exhaust gas coexist, ammonium nitrate (approximately 2,000 mg-N/L) was subjected to subcritical water oxidation treatment under the same conditions as in Comparative Example 1 (amount of ammoniacal nitrogen derived from nitrogen-containing organic matter: amount of nitrogen element derived from exhaust gas = 1:1), and the nitrogen concentration and nitrogen component state were determined before and after subcritical water oxidation treatment in the same manner as in Comparative Example 1 (Example 1).

Assuming that ammoniacal nitrogen derived from nitrogen-containing organic matter and NOx ( _NO2 ) derived from exhaust gas coexist, ammonium nitrate (approximately 2,000 mg-N/L) and the eluate of digested sludge (nitrogen-containing organic matter) (ammoniacal nitrogen approximately 1,200 mg-N/L) were mixed in a 1:1 ratio (ratio of nitrogen substance amounts), and the mixed solution was subjected to subcritical water oxidation treatment under the same conditions as in Comparative Example 1 (amount of ammoniacal nitrogen derived from nitrogen-containing organic matter: amount of nitrogen element derived from exhaust gas = 2:1), and the nitrogen concentration and nitrogen component state were determined before and after subcritical water oxidation treatment in the same manner as in Comparative Example 1 (Example 2).

The results of Comparative Example 1 and Examples 1 and 2 are shown in FIG.
As shown in Fig. 7, when the subcritical water oxidation treatment was performed on sodium nitrate (Comparative Example 1), the concentration of the nitrogen component derived from nitrate nitrogen hardly changed. In other words, it was confirmed that denitrification was not performed unless in the presence of nitrogen-containing organic matter.

When ammonium nitrate was subjected to subcritical water oxidation treatment (Example 1), the remaining nitrogen concentration was greatly reduced. This is believed to be because the NOx derived from the nitrate nitrogen in ammonium nitrate was reduced by ammonia and converted to nitrogen gas. The denitrification rate in Example 1 was 73%.

When the mixed solution of the eluate from digested sludge and ammonium nitrate was subjected to subcritical water oxidation treatment (Example 2), the concentration of remaining nitrogen decreased. This is considered to be due to the fact that NOx derived from nitrate nitrogen in the mixed solution was reduced by ammonia and converted to nitrogen gas. The denitrification rate in Example 2 was 86%.
Moreover, as shown in FIG. 7, in Example 2, almost no nitrogen components derived from nitrate nitrogen remained.

These results show that in the presence of exhaust gas-derived NOx and nitrogen-containing organic matter-derived ammonia, the denitrification rate can be increased, even at a lower treatment temperature, compared to the denitrification rate (30-50%) achieved by the conventional non-catalytic selective reduction method.
From the above, it was found that when nitrogen-containing organic matter is subjected to subcritical water oxidation treatment, by introducing a gas (exhaust gas) containing nitrogen oxides, the nitrogen oxides in the exhaust gas can be more efficiently denitrified using ammoniacal nitrogen derived from the nitrogen-containing organic matter.

1...Denitrification device, 10...Sludge supply section, 12...Exhaust gas supply section, 20...Decomposition section, 21...Decomposition tank, 22...Heater, 23...Measurement section, P1...High pressure pump, B1...Pressure adjustment valve, L1-L3...Pipes

Claims (5)

A denitration apparatus comprising a decomposition section which causes nitrogen-containing organic matter and a gas containing nitrogen oxides to coexist and which subjects the coexistence to subcritical water conditions, oxidatively decomposes the nitrogen-containing organic matter and reductively decomposes the nitrogen oxides, the decomposition section having one decomposition tank, a treatment temperature of the decomposition tank being equal to or higher than the critical temperature of water and equal to or lower than 500°C, and in the decomposition tank, the physical amount of nitrogen element contained in the nitrogen oxides is 0.6 to 1.0 mol per 1 mol of nitrogen element contained in the nitrogen-containing organic matter . The method includes a decomposition step of causing a nitrogen-containing organic substance and a gas containing nitrogen oxides to coexist and subjecting the resulting mixture to a subcritical water condition, thereby oxidatively decomposing the nitrogen-containing organic substance and reducing and decomposing the nitrogen oxides,
The denitration method includes the steps of: performing the decomposition step in one decomposition tank; a treatment temperature in the decomposition tank is equal to or higher than the critical temperature of water and equal to or lower than 500°C; and a physical amount of nitrogen element contained in the nitrogen oxides is 0.6 to 1.0 mol per 1 mol of nitrogen element contained in the nitrogen-containing organic matter . The denitrification method according to claim 2, wherein the treatment temperature in the decomposition step is between the critical temperature of water and 450°C, and the treatment pressure is between 5 and 20 MPa. The denitrification method according to claim 2 or 3, wherein the decomposition step is carried out in the presence of oxygen at least 1.0 times the stoichiometric amount of oxygen required for complete oxidation of the nitrogen-containing organic matter. The denitrification method according to any one of claims 2 to 4, wherein the treatment time in the decomposition step is 5 to 15 minutes.

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