JP3029512B2 - Method for removing nitrous oxide from combustion gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for removing nitrous oxide from a combustion gas.

[0002]

2. Description of the Related Art Fluidized bed combustion using coal, heavy oil, petroleum coke, industrial waste, or the like as fuel is carried out at a low temperature of about 800 ° C., so that nitrogen monoxide (a pollutant gas) is used. The emission of nitrogen oxides (hereinafter referred to as NO) and nitrogen dioxide (hereinafter referred to as NO ₂ ) is small, but the amount of nitrous oxide (hereinafter referred to as N ₂ O) that causes global warming and destruction of the ozone layer Therefore, there is a need to develop measures to reduce it.

[0003] reduction of N ₂ O, the N ₂ O contained in the combustion gas discharged from the combustion furnace to which 300-400
Development of a method of decomposing at a low temperature of ° C. using a catalyst, plasma discharge, or the like has been advanced. However, its practical use still has problems such as simplification of the system, development of a good catalyst, and reduction of plant cost. The biggest problem with this method is that a new N ₂ O removal plant must be incorporated into the existing plant. It can be said that it is difficult to add a new N ₂ O removal system to an existing plant that has the best system layout in terms of the plant installation area.

Japanese Patent Application Laid-Open No. 63-12328 discloses a method in which particles containing alumina as a main component are sprayed on a freeboard portion of a combustion furnace to denitrify a combustion gas. However, this patent publication mainly describes the removal of NO, and does not mention at all the removal of N ₂ O which causes global warming and destruction of the ozone layer. In this patent publication, 2NO → N ₂ + O ₂ (1) is cited as a NO decomposition reaction equation, but 4NO → 2N ₂ O + O ₂ (2) is cited as another decomposition reaction equation that occurs simultaneously. According to the latter reaction equation (2), N decomposing NO causes global warming and ozone layer destruction.
₂ O will be newly generated.

[0005] Further, as shown in the following equation,
Some of the NO is described also produce oxidized NO _2. 2NO + O ₂ → 2NO ₂ (3) As described above, the denitration method described in JP-A-63-12328 is effective for removing NO, but is ineffective for removing N ₂ O. It is clear that N ₂ O is newly generated by the decomposition of the compound, so that it cannot be adopted as a method for removing N ₂ O.

An object of the present invention is to solve the above-mentioned disadvantages of the prior art and to provide a method capable of effectively removing N ₂ O in a combustion gas.

[0007]

SUMMARY OF THE INVENTION The above-mentioned JP-A-63-123 is disclosed.
The denitration method described in Japanese Patent Publication No. 28 uses alumina, but according to the study of the present inventor, the oxidation reaction shown in the above (3) is inherent in the case where alumina other than γ-alumina is used. This is a reaction, and it was concluded that the denitration method described in the above patent publication uses alumina other than γ-alumina.
As a result of further study, α-alumina was N ₂ O under fluidized bed combustion temperature conditions (for example, 700 to 900 ° C.).
Is not effective for the decomposition removal of γ, but γ is used instead of α-alumina.
It has been found that when -alumina is used, it is possible to decompose and remove N ₂ O under fluidized bed combustion temperature conditions which could not be achieved when α-alumina was used. Also, when a mixture of γ-alumina and a metal compound is used, N ₂ O can be decomposed and removed, and a higher N ₂ O selective decomposition activity can be obtained than in the case of using γ-alumina alone. I found something.

[0008] The present invention has been completed based on these findings, and the nitrous oxide (N ₂ O) of the combustion gas of the present invention has been developed.
The removal method includes charging γ-alumina or a mixture of γ-alumina and a metal compound into a fluidized bed combustion furnace, and using the mixture of γ-alumina or a mixture of γ-alumina and a metal compound in the combustion furnace. It is characterized in that N ₂ O in the combustion gas is decomposed by processing.

Hereinafter, the present invention will be described in detail. N ₂ O of the present invention
The removal method targets combustion gas generated in a fluidized bed combustion furnace. Fluidized bed forming material (fluid medium: sand,
Limestone, coal ash, etc.) and air that contributes to combustion and particle flow from the lower part of the furnace, and burns fuel (coal, heavy oil, petroleum coke, industrial waste, etc.) and the heat collector in the furnace In a fluidized-bed combustion furnace such as a normal pressure type, a pressurization type, a bubbling type, or a circulation type, in which the temperature inside the furnace is controlled to 700 to 900 ° C. by the heat balance, combustion contributes to global warming and destruction of the ozone layer. N ₂ O is generated. This N ₂ O
Is different from NO or NO _2, which is generated by various combustion reactions and whose emission is regulated as an air pollutant.

[0010] N ₂ O is produced in the gas phase as a nitrogen component contained in the fuel as NCO or NHi chemical species upon combustion, and simultaneously reacts with NO generated by combustion to produce N ₂ O.
, But the details are not yet clear. NCO + NO → N ₂ O + CO NHi + NO → N ₂ O + i / 2H ₂ 50 to 160 ppm of N ₂ O in a general fluidized bed combustion furnace
Is released.

In the method of the present invention, the above-mentioned N ₂ O is decomposed and removed with γ-alumina, which is one kind of alumina, or a mixture of γ-alumina and a metal compound. Alumina is an industrially important inorganic material and its use is wide. Therefore, research on its properties, structure, and the like is ongoing, but the details are not yet clear. However, industrially roughly classified, there are stable α-alumina obtained by dehydrating alumina hydrate and metastable γ-alumina which is an intermediate product thereof. When γ-alumina is classified in detail, κ, θ,
Although it is divided into seven types, δ, γ, η, χ, and ρ, these are collectively referred to as γ-alumina. Alumina has low activity as a decomposition catalyst in flue gas (300 to 400 ° C.) in flue gas (300 to 400 ° C.) and has little catalytic effect, but it is said that its activity becomes high at high temperatures. It has been discovered that N ₂ O has a high selective decomposition activity under combustion temperature conditions (700 to 900 ° C.).

Further, alkali metals such as Na and K, alkaline earth metals such as Mg and Ca, C
copper group metals such as u, Ag and Au; zinc group metals such as Zn; titanium group metals such as Ti and Zr; vanadium group metals such as V and Ta; iron group metals such as Fe, Co and Ni;
Compounds of metals such as chromium group metals such as r and manganese group metals such as Mn, for example, oxides, carbonates, halides (chlorides, bromides, etc.), sulfates, sulfides, nitrates, nitrites, and organic acid salts (Formate, acetate, benzoate, etc.)
It has been discovered that the use of a mixture of γ-alumina and a metal compound, which is obtained by mixing the above, can further enhance the selective decomposition activity of N ₂ O. Here, the mixture of γ-alumina and the metal compound is preferably a mixture mainly composed of γ-alumina and containing a small amount of the metal compound.

In the method of the present invention, N ₂ in the combustion gas is used.
For decomposition and removal of O, γ-alumina or a mixture of γ-alumina and a metal compound is charged into a combustion furnace, and combustion gas is mixed with γ-alumina or a mixture of γ-alumina and a metal compound in the combustion furnace. It is achieved by processing. The charging of γ-alumina or a mixture of γ-alumina and a metal compound into a fluidized-bed combustion furnace includes the steps of: (A) transferring γ-alumina or a mixture of γ-alumina and a metal compound to a freeboard in a fluidized-bed combustion furnace It is preferably carried out by charging the mixture into (B) γ-alumina or a mixture of γ-alumina and a metal compound as a fluidized medium of a fluidized bed in a fluidized bed combustion furnace. Then, the charging methods (A) and (B) of γ-alumina or a mixture of γ-alumina and a metal compound will be described in detail below.

Method (A) In a fluidized bed combustion furnace, a freeboard portion exists above a fluidized bed formed by a fluidized medium (usually, sand, limestone, coal ash, or the like is used). This method (A)
According to the method, γ-alumina or a mixture of γ-alumina and a metal compound is charged into the free boat portion. The charged γ-alumina or the mixture of γ-alumina and the metal compound comes into contact with the combustion gas generated by the combustion of the fuel in the combustion furnace, and decomposes and removes N ₂ O in the combustion gas. In this case, the input amount of the mixture of γ-alumina or γ-alumina and the metal compound is such that γ-alumina or γ-alumina and the metal compound which stay in the combustion furnace with respect to N ₂ O generated when not charged are added. Is preferably present in a molar ratio in the range of 1 to 100, whereby a de-N ₂ O rate of, for example, 90% or more can be achieved.

Method (B) In this method (B), γ-alumina or a mixture of γ-alumina and a metal compound is charged as a fluidized medium of a fluidized bed in a fluidized bed combustion furnace. The γ-alumina or the mixture of γ-alumina and the metal compound in the fluidized bed comes into contact with the combustion gas generated by the combustion of the fuel in the combustion furnace to decompose and remove N ₂ O in the combustion gas. In this case, the filling amount of γ-alumina or the mixture of γ-alumina and the metal compound is an amount necessary for fluidization of the fluidized bed, and is much larger than an amount required for decomposition of N ₂ O. . Therefore the generated N
₂ O is almost quantitatively decomposed and removed by γ-alumina or a mixture of γ-alumina and a metal compound in the fluidized bed. The fluidized bed is preferably formed only of γ-alumina or a mixture of γ-alumina and a metal compound. However, if necessary, γ-alumina and a normal fluid medium (sand, limestone, coal ash, etc.) may be used. Can also be formed.

[0016]

The present invention will be further described with reference to the following examples. Example 1 As a model experiment, an N ₂ O decomposition experiment was performed using γ-alumina alone, and a fluidized bed combustion temperature condition (700 to 900) was used.
C)), the N ₂ O decomposition activity of γ-alumina was measured,
It was compared with the N ₂ O decomposition activity of α-alumina under the same conditions measured by a similar experiment. Details are as follows.

[0017] Using reagents γ- alumina (particle size 0.2~1.0mm particles) alpha-alumina (particle size 0.2~1.0mm particles) experiment This experiment using the apparatus shown in FIG. 1 . The apparatus shown in FIG. 1 includes a quartz tube 1 having an inner diameter of 6 mm, an electric furnace 2 surrounding a central portion in the longitudinal direction of the quartz tube 1, an N ₂ O gas introducing tube 3 connected to the bottom of the quartz tube 1, and N ₂ O gas introduction pipe N ₂ O gas flow meter 4 provided in the middle of 3, a gas discharge pipe 5 connected to the top of the quartz tube 1 was provided in an intermediate portion of the gas exhaust pipe 5 N ₂ O analyzer 6.

Each of the above-mentioned reagents (γ-alumina or α-alumina) was filled into a quartz tube 1 so as to have a filling length of 100 mm to form a reagent layer 7. At the upper end of the reagent layer 7, a three-layer packing body composed of a thin quartz wool layer 9 a ₁ , a silica sand layer 8 a having a filling length of 30 mm and a thin quartz wool layer 9 a ₂ , and at the lower end of the reagent layer 7, , A thin quartz wool layer 9b ₁ , a silica sand layer 8b having a filling length of 30 mm, and a thin quartz wool layer 9b ₂ are formed to hold and fix the reagent layer 7.

N ₂ O concentration of 500 pp
m N ₂ O gas is supplied from the N ₂ O gas introduction pipe 3 to the bottom of the quartz tube 1 at a flow rate of 1 liter / min (normal temperature, normal pressure) via the N ₂ O gas flow meter 4, and the electric furnace 2 The N ₂ O gas was allowed to pass through the reagent layer 7 while changing the temperature of the reagent layer 7 in the above. Then, the gas discharged from the top of the quartz tube 1 to the gas discharge pipe 5 is led to the N ₂ O analyzer 6, where the N 2
_{The 2} O concentration was analyzed to determine the N ₂ O removal rate for each reagent. The measurement results are shown in FIG. As shown in FIG. 2, γ-alumina showed a satisfactory high N ₂ O decomposition activity under fluidized bed combustion temperature conditions (700 to 900 ° C.). The decomposition products were N ₂ and O ₂ gas, and generation of NO or NO ₂ gas was not confirmed. On the other hand, it was confirmed that α-alumina did not show any N ₂ O decomposition activity under fluidized bed temperature conditions (700 to 900 ° C.).

Using the apparatus shown in FIG. 1, NO or NO ₂ gas was introduced into and contacted with the reagent layer 7 filled with γ-alumina, and its decomposition activity was examined at each temperature. It was confirmed that it did not decompose under the layer combustion temperature condition (700 to 900 ° C.). Therefore, it is clear that γ-alumina used in the present invention has a fundamentally different decomposition reaction from α-alumina in the decomposition of nitrogen oxides, and γ-alumina has the ability to selectively decompose only N ₂ O. It became.

Example 2 A small bubbling fluidized bed combustion furnace shown in FIG. 3 was used to remove N ₂ O from combustion gas. The combustion furnace shown in FIG. 1 includes a quartz tube 11 having an inner diameter of 40 mm and a length of 700 mm. Silica sand, which is a fluid medium, is placed on a porous plate 12 provided below the quartz tube 11 so that the bed height becomes 60 mm. Filled. The air supply tube 1 provided at the bottom of the quartz tube 11
From 3, air was supplied to the quartz tube 11 at 25 liters / minute to flow the fluid medium to form the fluidized bed 14. Coal as fuel (particle size is 0.5 to 1.0 mm, components are 69.0% of carbon, 6.4% of hydrogen, 9.6% of oxygen, and 1.
160 g from the fuel supply pipe 15 inserted from the top of the quartz tube 11 into the quartz tube 11.
/ Hr continuously and burned while maintaining the temperature in the fluidized bed 14 at 850 ° C. by the heater 16 provided on the outer periphery of the quartz tube 11 (the temperature in the fluidized bed is a thermocouple. 17). At the same time, γ-alumina was continuously supplied at a rate of 10 g / hr from the reagent supply tube 18 inserted into the free board portion 19 of the quartz tube 11 from the top of the quartz tube 11. The gas after combustion was discharged from a gas discharge pipe 20 provided at the top of the quartz tube 11.

When the N ₂ O concentration of the exhaust gas was measured, it was O ppm. On the other hand, γ which is an N ₂ O remover
The N ₂ O concentration of the exhaust gas when no alumina was supplied was 105 ppm. From this, it became clear that by supplying γ-alumina to the free board portion, γ-alumina completely decomposed N ₂ O, and a 100% N ₂ O removal rate was achieved. The NO concentration is
Approximately 160 pp, regardless of the supply of γ-alumina
m.

Comparative Example 1 The procedure of Example 2 was repeated except that α-alumina was supplied to the free board portion 19 of the quartz tube 11 instead of γ-alumina. As a result, the N ₂ O concentration of the exhaust gas was 102 p.
pm, which was almost the same as the N ₂ O concentration of the exhaust gas of 105 ppm when α-alumina was not supplied, and it became clear that α-alumina was ineffective in decomposing N ₂ O.

Example 3 As the fluidizing medium of the fluidized bed, instead of the silica sand used in Example 2, γ-alumina as an N ₂ O remover was used.
Example 2 was carried out in the same manner as in Example 2 except that a fluidized bed was formed with alumina and γ-alumina was not supplied to the free board portion of the quartz tube. As a result, when silica sand is used as the fluid medium, the N ₂ O concentration in the exhaust gas is 1
The N ₂ O concentration in the exhaust gas in this example was O ppm, whereas the N ₂ O concentration was 05 ppm, and a 100% N ₂ O removal rate was achieved. The NO concentration in the exhaust gas was almost the same (about 160 ppm) when using γ-alumina and when using silica sand.

Comparative Example 2 The same procedure as in Example 3 was carried out except that α-alumina was used instead of γ-alumina as a fluid medium. As a result, the N ₂ O concentration in the exhaust gas was 103 ppm, and it became clear that α-alumina was almost ineffective in decomposing N ₂ O.

Example 4 0.3 wt% of SiO ₂ as an impurity as an N ₂ O remover
% Of γ-alumina and a mixture of γ-alumina, chromium oxide and magnesium oxide [γ-Al ₂ O ₃
/ Cr ₂ O ₃ / MgO = 85/12/3 (wt / wt / wt)]
The N ₂ O decomposition experiment was performed in the same manner as in Example 1 except that each was used. FIG. 4 shows the result of measuring the N ₂ O removal rate when the temperature was changed. FIG. 4 shows that the mixture of γ-alumina, chromium oxide and magnesium oxide has a higher N _{2 value} than the case of using almost pure γ-alumina.
The rise of the O removal rate curve is sharp, and the removal of N ₂ O
The activity was found to be high.

[0027]

As described above, according to the present invention, a method for effectively decomposing and removing N ₂ O (nitrous oxide) from a combustion gas is provided.

[Brief description of the drawings]

FIG. 1: N ₂ O with γ-alumina and α-alumina
Schematic diagram of the equipment used in the model experiment to investigate decomposition characteristics

FIG. 2: γ-alumina and α when temperature is varied
- a graph showing the N ₂ O decomposition characteristics of Alumina

FIG. 3 is a schematic view of an apparatus for performing the method for removing N ₂ O from combustion gas of the present invention.

FIG. 4 is a graph showing N ₂ O decomposition characteristics of substantially pure γ-alumina and a mixture of γ-alumina, chromium oxide and magnesium oxide when the temperature is changed.

[Explanation of symbols]

1 quartz tube 2 electric furnace 3 N ₂ O gas introduction pipe 4 N ₂ O gas flow meter 5 gas discharge pipe 6 N ₂ O analyzer 7 reagent layer 8a, 8b silicate sand _{9a 1, 9a 2, 9b 1} , 9b 2 quartz Wool layer 11 Quartz tube 12 Perforated plate 13 Air supply tube 14 Fluidized bed 15 Fuel supply tube 16 Heating heater 17 Thermocouple 18 Sample supply tube 19 Free board unit 20 Gas exhaust tube

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) F23C 10/00 F23G 5/30 ZAB B01D 53/86 ZAB B01D 53/94 B01J 21/04 ZAB

Claims (4)

(57) [Claims] 1. A γ-alumina or γ-alumina in a fluidized bed combustion furnace.
It is characterized by charging a mixture of alumina and a metal compound, treating the combustion gas with γ-alumina or a mixture of γ-alumina and a metal compound in the same furnace, and decomposing nitrous oxide in the combustion gas. A method for removing nitrous oxide from a combustion gas. 2. The free board portion in the fluidized bed combustion furnace has γ-
The method according to claim 1, wherein alumina or a mixture of γ-alumina and a metal compound is introduced. 3. The method according to claim 1, wherein γ-alumina or a mixture of γ-alumina and a metal compound is filled as a fluidized medium of the fluidized bed in the fluidized bed combustion furnace. 4. The metal compound is an oxide or carbonate of an alkali metal, alkaline earth metal, copper group metal, zinc group metal, titanium group metal, vanadium group metal, iron group metal, chromium group metal or manganese group metal. , A halide, a sulfate, a sulfide, a nitrate, a nitrite or an organic acid salt.
4. The method according to any one of the above items 3.