JPS63267422A - Removing method for nitrogen oxide - Google Patents

Abstract

PURPOSE:To remove NOX efficiently by mixing isocyanuric acid of 10 mesh or less particle diameter in the floating state with high-temperature exhaust gas NOX. CONSTITUTION:Isocyanuric acid of 10 mesh or less particle diameter in the floating state fed from feed opening 8, 8' is mixed with high-temperature exhaust gas of 330 deg.C or more containing NOX generated in a combustion section 1 in the range of 0.5-10mol. per NOX 1mol. Isocyanuric acid is thermally decomposed and dissociated into NH radicals and CO radicals, and NH radicals are reacted with NOX to generate N2 and CO2. In case exhaust gas is 700 deg.C or more, residual time should be 30sec or less. After NOX is removed, exhaust gas is exhausted out of a stack 4 through a heat exchanger 3.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for removing nitrogen oxides from flue gas generated when combustible substances are burned in boilers, incinerators, heating furnaces, internal combustion engines, etc. It is something.

BACKGROUND OF THE INVENTION A non-catalytic reduction method using ammonia and a catalytic reduction method are widely known for removing nitrogen oxides from combustion exhaust gas.

However, with the non-catalytic ammonia reduction method, the denitrification rate drops sharply when the treatment temperature drops below 900°C, and the ammonia reduction method that uses catalysts such as iron oxide, vanadium pentoxide, alumina, etc. It cannot be said that all of them are in a satisfactory state, as the operations are complicated.

Recently, in order to reduce nitrogen oxides in such combustion exhaust gas, a method has been proposed in which nitrogen oxides are brought into contact with isocyanate gas produced by thermal decomposition of isocyanuric acid. [Published by Macmiltan Journals, Nature
ure), Vol. 324, No. 6098, pp. 657-658], that is, according to this report, when treating combustion exhaust gas from an internal combustion engine, a heating bath in which isocyanuric acid has been accumulated is heated to a temperature of 360°C or higher. A method is disclosed in which isocyanate gas is generated by heating, and this gas is passed through a reaction bed at 450 to 900° C. filled with stainless steel bulbs together with combustion exhaust gas containing nitrogen oxides.

Problems that the invention aims to solve According to the method described in Nature magazine, the reaction proceeds at a much lower temperature than the non-catalytic ammonia reduction method, and nitrogen oxides are almost completely removed. .

However, this method targets a small amount of combustion exhaust gas generated from an internal combustion engine, and in particular requires that the gas to be reacted pass through a reaction layer filled with stainless steel bulbs. It is not suitable to adapt this type to the treatment of large amounts of combustion exhaust gas generated from boilers, etc.

Means for Solving the Problems The present inventor has repeatedly conducted various tests on a method for removing nitrogen oxides by reacting combustion exhaust gas containing nitrogen oxides with gas produced by thermal decomposition of isocyanuric acid. When thermal decomposition is performed in a fixed state such as in a heating bath, the generated isocyanate gas is overheated to a high temperature above the decomposition temperature, and its concentration increases locally, producing nitrogen oxides as a by-product. After discovering that a reaction occurs and conducting intensive studies to avoid this, we added isocyanuric acid with a particle size smaller than 10 mesh to the flow of combustion exhaust gas containing nitrogen oxides at a temperature higher than approximately 330"C. We have discovered a method that can substantially remove nitrogen oxides from combustion exhaust gas by mixing them in a suspended state without performing a secondary reaction treatment.

In carrying out the method of the present invention, isocyanuric acid having a particle size of less than 10 mesh is heated at a temperature of about 330 to 1500°C.
The combustion exhaust gas in the range of about 0.15 in the combustion furnace body
It should be introduced into the combustion furnace body or flue moving at a flow rate of ~20 m/sec, and in the flue at a flow rate of about 0.3-20 m/sec.

The isocyanuric acid used in the present invention is industrially produced by thermally decomposing urea, and may be either a purified product treated with a mineral acid or a crude product containing by-products such as ammeline and ammelide.

In the method of the present invention, if the particle size of isocyanuric acid is larger than the 1O mesh, it will not only be difficult to mix it with the combustion exhaust gas (also, the particle surface will be heated above the decomposition temperature when it comes into contact with the combustion exhaust gas, resulting in a locally high concentration). It produces isocyanic acid, with undesirable side reactions producing nitrogen oxides.

The amount of isocyanuric acid supplied to the combustion exhaust gas containing nitrogen oxides is in the range of 0.5 to 10 mol, preferably 1 to 5 mol, per 1 mol of nitrogen oxides, and the amount of isocyanuric acid supplied is increased more than necessary. However, the effect remains almost unchanged, and unreacted isocyanuric acid produces white smoke.

Isocyanuric acid can be supplied by allowing it to fall naturally, by dropping it into the combustion furnace body or flue using a quantitative feeder, or by using a blower fan or spray gun to supply air, a carrier gas such as oxygen, ammonia, nitrogen, etc. A typical method is to spray and mix the mixture by entraining it with water.

The treatment conditions in the present invention are not particularly limited as long as the reaction temperature does not exceed about 700°C from the decomposition temperature of isocyanuric acid. If the gas is brought into contact with the gas, nitrogen oxides are produced as a by-product, so the gas temperature should be kept within 30 seconds by heat exchange or by mixing low-temperature gas.

Function Isocyanic acid produced by thermal decomposition of isocyanuric acid dissociates into N)1 radicals and CO radicals, and NH radicals react with nitrogen oxides to finally change into nitrogen and carbon dioxide gas.

Heat 11NcONII+CO−・−・−・−−−−・−・・
(1) NII+NO-> H+N, 0 −・−・−−
1...-... (2) Old +1NCO-1 → Ntb4CO
−−−−−−−−(3)011+CO−1 → lI+
CO, −1−−−・−・−−−−−−(4) N112+
NO→NZII+011-+Nz+HzO-・ (5)
N 2 II Nz + II −・
−−−−−−−−−−・・・・・−・ (6) However, when isocyanuric acid is thermally decomposed in a fixed state such as in a heating bath, the generated isocyanate gas is heated above the decomposition temperature. The temperature becomes high and its concentration locally increases,
A high-temperature reaction between NH radicals and oxygen occurs as shown in the following formula, and nitrogen oxides are produced.

N II + 0□ −−→ NO+011−−−−−
−−−−・−・・・・(7) On the other hand, when isocyanuric acid with small particle size is mixed in a suspended state in the flow of combustion exhaust gas and thermally decomposed, the amount of isocyanate gas generated is relatively low. It is assumed that the reaction that produces nitrogen oxides as a by-product hardly occurs due to the temperature.

Example 1 Combustion part 1.35nf and flue part approximately 0.3nf shown in the drawing
In a combustion furnace consisting of a heat exchange section of about 0.3n, kerosene is burned at an average rate of 14f per hour, and a powder metering machine is used from the supply port in the combustion section to remove 90% of particles smaller than 100 mesh. The powdered isocyanuric acid containing the above with a purity of 99.7% was continuously allowed to fall naturally.

Note that the average treatment temperature and residence time in this example are: approximately 1200°C for the combustion section, 12.9 seconds, and approximately 820°C for the flue section, 10 seconds.
The temperature of the heat exchange section was 510° C. for 10.9 seconds, and the concentration of nitrogen oxides was continuously measured using an atmospheric chemiluminescence NOx meter.

The results of this test were as shown in Table 1.

*Note 1: Expressed by the molar ratio of the supplied 4-isocyanuric acid to the nitrogen oxides in the untreated exhaust gas.

Example 2 In Example 1, the isocyanuric acid quantitative feeder was replaced with one equipped with a blower, and powdered isocyanuric acid was fed once per hour.
A similar test was conducted by spraying and mixing the mixture into the combustion section along with air blown at a rate of 00 Nrrf, and the results were as shown in Table 2.

The processing temperature and residence time in this example are approximately 1
000°C for 13.9 seconds, the flue section at about 730°C for 10.9 seconds, and the heat exchange section at about 460°C for 1.2 seconds.

Table 2 Reference Example 1 In Example 1, a similar test was conducted by supplying a predetermined amount of ammonia gas from the supply port in the combustion section.
The results are shown in Table 3.

The processing conditions in this example are approximately 1000"C
The temperature was 14.2 seconds, the flue section was at about 770°C for 0.9 seconds, and the heat exchange section was at about 480°C for 1.3 seconds.

Table 3 Example 3 A test was conducted in exactly the same manner as in Example 1 except that isocyanuric acid was supplied from the supply port of the flue section.

The average processing temperature and residence time in this example are as follows:
The temperature was 57°C for 0.8 seconds, and the heat exchange section was 488°C for 1.0 seconds.

The results of the test were as shown in Table 4.

Example 4 In Example 1, the isocyanuric acid quantitative feeder was equipped with a blower, and this was installed at the supply port of the flue section, and powdered isocyanuric acid was entrained in the air blown at a rate of 100 Nrrf per hour. A similar test was conducted by spraying and mixing the mixture into the flue.

The test results were as shown in Table 5.

The processing conditions in this example are: 554°C 10.9°C in the flue section;
The heat exchange section was heated at 385° C. for 1.1 seconds.

Example 5 In Example 4, granular isocyanuric acid with a particle size of 12 to 35 mesh (purity 9) was used instead of powdered isocyanuric acid.
When a similar test was conducted using 9.7%), the test results were as shown in Table 6.

Note that the processing conditions in this example are: flue part 687°C 10.8°C;
The temperature was 11.0 seconds at 510° C. in the heat exchange section.

Table 6 Example 6 In Example 4, except for using 70% pure isocyanuric acid containing ammeline and ammelide (the particle size contains 90% or more of particles smaller than 100 mesh), all (the same tests were conducted) The results were as shown in Table 7.

The processing conditions in this example were: 697°C in the flue for 10.8 seconds, and 517°C in the heat exchange section for 1.0 seconds.

Table 7 Example 7 In Example 3, when heavy oil was burned instead of kerosene, the nitrogen oxides in the exhaust gas were 95 PPm. As a result of conducting a similar test at a temperature of about 750° C. and a residence time of 1 second in the flue, the concentration of nitrogen oxides in the exhaust gas decreased to 5PPa+. (The removal rate of nitrogen oxides was 94.7%.) Reference Example 2 In Example 1, instead of floating-mixing powdered 1-socyanuric acid using a quantitative feeder, 30 g of powdered isocyanuric acid was mixed in a wire mesh container. When a similar test was conducted by suspending this in a combustion section with a gas temperature of approximately 1200°C, the nitrogen oxide 4° content in the exhaust gas was 62 PPm in a blank test without using isocyanuric acid. ,
When using isocyanuric acid, nitrogen oxides in the exhaust gas increased to 80PPra.

Reference Example 3 In Reference Example 2, a similar test was conducted by suspending a wire mesh container containing 30 g of powdered isocyanuric acid in the flue section showing a gas temperature of approximately 720°C; a blank test without isocyanuric acid was performed. The nitrogen oxides in the exhaust gas were 52PPm, and when isocyanuric acid was used, the nitrogen oxides in the exhaust gas increased to 68PPm.

Effects of the Invention According to the present invention, there is an excellent nitrogen oxide removal effect in combustion exhaust gas at a relatively low temperature exceeding the decomposition temperature of isocyanuric acid, and isocyanuric acid is added to the flow of combustion exhaust gas containing nitrogen oxides. Since it is sufficient to carry out floating mixing, the processing equipment can be manufactured easily and inexpensively, and it can be easily integrated into existing combustion equipment, which has great practical effects.

[Brief explanation of the drawing]

The drawing shows an example of a combustion apparatus suitable for carrying out the method of the present invention, in which (1) shows a combustion part, (2) shows a flue part, and (
3) is the heat exchange part, (4) is the chimney, (5) is the burner, (
6) is a combustion supply pipe, (7) is an air supply pipe, (8) and (
8”) is -isocyanuric acid supply port, (9) and (9°)
is a blower fan, flat material ε (Shoji Ho (spontaneous), June 1988, g. 1, display of the case.Relationship with the case of Patent Application No. 101376 of 1988: Patent applicant 4, of the specification to be amended. Detailed Description of the Invention Column and Brief Description of Drawings Column 5, Contents of Amendment (1) In the beginning of line 10 on page 36 of the specification, the phrase “gas temperature by” has been changed to “by a temperature range of approximately 700°C or more.” (2) Correct the combustion supply pipe J to "r!l!, t'+supply pipe" as stated in the beginning of line 9 on page 16 of the specification.

Claims (4)

[Claims] (1) A method for removing nitrogen oxides, which comprises mixing isocyanuric acid having a particle size of less than 10 mesh in a suspended state into a stream of combustion exhaust gas containing nitrogen oxides and having a temperature higher than about 330°C. (2) The method according to claim (1), wherein isocyanuric acid is mixed with a carrier gas by spraying into a stream of combustion exhaust gas containing nitrogen oxides. (3) The method according to claim (1), wherein the temperature of the combustion exhaust gas containing nitrogen oxides is controlled to be higher than the decomposition temperature of isocyanuric acid and not to exceed about 700°C. (4) The method according to claim (1), wherein the combustion exhaust gas containing nitrogen oxides is treated with isocyanuric acid or its thermal decomposition product so that the residence time in a temperature range of about 700°C or higher is 30 seconds or less. .