JPH07265667A - Treatment of exhaust gas - Google Patents

Abstract

PURPOSE:To prevent increase in the leak of ammonia into an exhaust gas and to suppress the decrease in denitrification rate caused by the lowering of the temp. of the exhaust gas by changing the injecting quantity of ammonia with the change in the exhaust gas temp. CONSTITUTION:The exhaust gas is introduced into an orthogonal flow type moving bed reactor 3 through a line at 60-180 deg.C after ammonia is injected from a line 2. In this case, the quantity of ammonia injected from the line 2 is changed with the change in the exhaust gas temp. to decrease the quantity of ammonia leaking into the treated gas. The exhaust gas flows in a carbonaceous catalyst bed 4 moving downward in the reactor 3 and is brought into contact with the catalyst and is desulfurized and denitrificated. The inactivated carbonaceous catalyst is discharged from the bottom of the reactor 3, fed to a regenerator 6 and heated at 300-600 deg.C in an inert gas atmosphere to be regenerated. As a result, the decrease in denitrification rate is suppressed even if the exhaust gas temp. is lowered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating exhaust gas, and more particularly to an improved method for treating exhaust gas which can prevent ammonia leak and reduction of denitration rate due to temperature change of exhaust gas.

[0002]

2. Description of the Related Art A method for treating exhaust gas using a carbonaceous catalyst such as activated carbon or activated coke has a feature of a dry method that does not have problems such as securing water for use, wastewater treatment, and lowering of exhaust gas temperature as in a wet method. is doing. Further, in comparison with a general ammonia-catalyzed denitration method in which a vanadium catalyst is supported, not only sulfur oxide (SO _x ) but also nitrogen oxide (N _x
In addition to being able to remove O _x ), there is a great advantage that the catalyst can be easily regenerated by simple heat regeneration.

Specifically, this method is a reaction in which ammonia is injected into exhaust gas containing SO _X , NO _X , sintering furnace exhaust gas, refuse incinerator exhaust gas, etc., and a carbonaceous catalyst such as activated carbon or activated coke is filled. The exhaust gas is processed by passing through the chamber. In this method, the temperature is 130 to 150 ° C., which is the temperature of ordinary boiler exhaust gas,
This low temperature is also advantageous in preventing the carbonaceous catalyst from being consumed by oxygen.

SO _X in the exhaust gas is first adsorbed on the carbonaceous catalyst in the form of sulfuric acid. SO ₂ in the exhaust gas coexists with O ₂ ,
This is because it reacts with H ₂ O on the catalyst surface and is oxidized to H ₂ SO ₄ . If ammonia is injected here, N
H ₃ reacts with H ₂ SO ₄ produced on the catalyst to form acidic ammonium sulfate NH ₄ HSO ₄ or ammonium sulfate (NH ₄ ) ₂ SO ₄ . Most of the ammonia is used in the reaction with sulfuric acid and only part is NO
It will only be used to react with _X. That is,
Since the catalyst adsorbs sulfuric acid, the active adsorption point is reduced, so to speak, it is in a poisoned state, and ammonia also preferentially binds to sulfuric acid. Therefore, when the amount of ammonia is equivalent to NO _x , there is a balance. But there is a shortage of ammonia.

Therefore, if the concentration of SO _{x in} the flue gas is higher than that of NO _x , the denitration rate will decrease. As a solution to this problem, Japanese Patent Publication No. 63-50052 discloses that the exhaust gas is first passed through the first reactor to remove most of the SO _X in advance, and thereafter the exhaust gas is passed to the second reactor. A method has been proposed in which NO _x is removed exclusively by passing it through. According to this method, a denitration rate of 70 to 80% or more can be obtained even with exhaust gas having a high SO _X concentration.

However, the temperature of the exhaust gas is apt to change under the influence of the upstream side. Normally, the temperature is constant at around 140 ° C (depending on the boiler), but there is a temperature change around 20 ° C depending on the state of the boiler side. For example, when the temperature rises by 20 ° C., NH ₃ is adsorbed on the carbonaceous catalyst, so the adsorbed NH ₃ is desorbed all at once due to the temperature rise. Therefore, there arises a problem of ammonia leakage that the concentration of ammonia in the processing gas becomes extremely high.

This problem is especially caused by low temperature exhaust gas (100-1
It becomes remarkable at around 20 ° C). Many adsorption amount to the carbonaceous catalyst NH ₃ at low temperatures, a large amount of NH ₃ by a slight rise in temperature because of leaving.

On the contrary, the exhaust gas temperature may decrease depending on the condition on the boiler side. In this case, the amount of NH ₃ adsorbed on the carbonaceous catalyst increases, but the reaction rate of NO _X and NH ₃ decreases, so the NOx removal rate decreases. Therefore, excess NH ₃ remains adsorbed on the carbonaceous catalyst, and when the temperature next rises, more NH ₃ is released into the exhaust gas.

[0009] As one of the methods for suppressing such leakage of ammonia into exhaust gas, Japanese Patent Laid-Open No. 59-196719.
In the publication, a method is proposed in which a small amount of SO ₃ is added to the gas after the exhaust gas treatment and reacted with NH _3, and the sulfate solid content of the produced ammonia is collected by a dust collector such as a bag filter in the subsequent stage. ing.

Another method is disclosed in Japanese Patent Laid-Open No. 1-530.
In Japanese Patent Publication No. 86, a third reactor is provided together with a first reactor for desulfurization and a second reactor for denitrification, and the third reactor is provided with SO _{X in} the first reactor. There is described a method in which a carbonaceous catalyst having adsorbed thereinto and an ammonia-containing gas from a second reactor are introduced, the leak NH ₃ is reacted with SO _X, and finally this is removed as a sulfate of ammonia.

[0011]

SUMMARY OF THE INVENTION JP-A-59-196719 and JP-A-1-53086 described above solve the problem of ammonia leakage when the temperature of exhaust gas changes.
All of the conventional methods described in Japanese Patent Laid-Open Publication No. 2003-242242 are intended to treat the entire amount of the ammonia-containing gas, and each require a large dust collector and a third reactor, and in addition to the complexity of the method,
Economic problems arise.

An object of the present invention is to solve the ammonia leak problem by a simple method without using special large-scale equipment, and also to suppress a decrease in the denitration rate due to a decrease in exhaust gas temperature. The present invention provides a method of treating exhaust gas.

[0013]

Means for Solving the Problems The present invention achieves the above-mentioned object, in which ammonia is injected into exhaust gas containing nitrogen oxides or nitrogen oxides and sulfur oxides, and a carbonaceous catalyst is filled therein. Introduced into a moving bed reactor, which is subjected to denitration or desulfurization / denitration treatment, on the other hand, in a method for treating exhaust gas in which the carbonaceous catalyst deactivated by contact with exhaust gas is heated and regenerated, The gist is a method for treating exhaust gas, which is characterized in that it is changed according to a change in temperature.

The present invention will be described in detail below.

The point of the exhaust gas treatment method of the present invention is to adjust the amount of NH ₃ to be injected according to the temperature change of the exhaust gas. For example, when the exhaust gas temperature rises, by reducing the amount of ammonia to be injected, NH ₃ desorbed from the carbonaceous catalyst as the temperature rises can be consumed in the reaction with NO _{X in} the exhaust gas. It is possible to reduce the amount of ammonia leak into the exhaust gas. On the contrary, when the exhaust gas temperature decreases, the NH ₃ concentration can be increased by increasing the amount of ammonia to be injected, the denitration reaction rate can be increased, and a predetermined denitration rate can be obtained.

The exhaust gas treatment method of the present invention will be described in detail with reference to the drawings. In FIG. 1, the exhaust gas was injected with ammonia from the line 2 and then at the temperature of 60 to 180 ° C. in the line 1
And is introduced into the cross-flow type moving bed reactor 3 via. The exhaust gas flows in the carbonaceous catalyst bed 4 moving downward in the reactor 3, contacts the catalyst, and is desulfurized and denitrated. Reactor 3
The treated clean gas exiting from the above is discharged to the atmosphere from the line 5 directly or through a dust collector.

The deactivated carbonaceous catalyst in the reactor 3 is withdrawn from the lower part of the reactor 3 and sent to the regenerator 6. Here, 300 to 600 under an inert gas atmosphere
Regeneration is performed by heating to ℃. The regenerated catalyst is
It is passed through a separator 7 such as a vibrating screen to remove powdered catalyst and dust. After that, the regenerated catalyst is returned to the upper part of the reactor 3 through the line 8 by a conveyor or the like. The high-concentration SO ₂ gas recovered by the regenerator 6 is supplied to a subsequent stage, for example, a sulfuric acid, sulfur or gypsum manufacturing process via a line 9.

In FIG. 2, the exhaust gas is 60 to 180 ° C.
At a temperature in the range of 1, the ammonia is injected directly or from line 2a and then introduced into first cross-flow moving bed reactor 3 via line 1. The exhaust gas flows through the carbonaceous catalyst bed 4 moving downward in the reactor 3 and comes into contact with the catalyst for mainly desulfurization, but also partial denitration.
Is discharged from the reactor 3. The gas leaving the first reactor is introduced into the second cross-flow moving bed reactor 10 via line 5. The gas in line 5 into which ammonia is injected from line 2b flows in the carbonaceous catalyst bed 11 moving downward in the second reactor 10, comes into contact with the catalyst, and undergoes partial desulfurization. Denitration is mainly performed. The clean gas thus obtained is discharged into the atmosphere through line 12 directly or through a dust collector.

On the other hand, the catalyst discharged from the lower part of the second reactor 10 is supplied to the upper part of the first reactor 3. The inactivated carbonaceous catalyst discharged from the lower part of the first reactor 3 is sent to the regenerator 6 and heated to 300 to 600 ° C. in an inert gas atmosphere to be regenerated. The regenerated catalyst is separated into a separator 7 such as a vibrating screen.
And powdered catalyst and dust are removed.
After that, the regenerated catalyst is returned to the upper portion of the reactor 10 via the line 8 by a conveyor or the like and is circulated and used.

The catalyst used in the present invention is generally obtained by carbonizing carbon-containing substances such as coal, pitch, or plant-derived substances such as wood and coconut shell, plastics and the like, and further activating them with steam or the like. Things are also used. Vanadium, manganese to increase the catalytic activity,
A metal oxide such as iron or copper may be supported or contained.

As described above, the point of the present invention is that FIG.
The amount of ammonia injected through the lines 2, 2a and 2b in the embodiment of FIG. 2 is changed according to the temperature of the exhaust gas to reduce the amount of NH ₃ leaking into the processing gas. Further, when this method is carried out, it is possible to obtain the effect of suppressing a decrease in the denitration rate even if the temperature of the exhaust gas is decreased.

Regarding the injection point of ammonia, in the embodiment of FIG. 2, the injection amount of ammonia is adjusted at each of the two injection points so as to correspond to the temperature change of the exhaust gas. However, when the exhaust gas contains a large amount of SO _X , desulfurization becomes the main component in the first reactor, and denitration is only additive. Therefore, even if there is a change in the temperature of the exhaust gas, the change in the amount of ammonia leakage is extremely small. From the viewpoint of ease of operation, it is often sufficient to adjust the ammonia injection amount only in front of the second reactor.

As described above, in the present invention, the injection amount of ammonia is changed in accordance with the exhaust gas temperature, but the adjustment of the amount of the exhaust gas at the point in line 1 is performed as shown in FIG. The temperature T is detected, and the ammonia supply valve 14 is controlled by the controller 13 based on the signal.
Is done by controlling.

In the embodiment shown in FIG. 2, the exhaust gas temperature Ta at the point in line 1 and the exhaust gas temperature Tb at the point in line 5 are detected, and the ammonia supply valve 14 is controlled by the controller 13 based on these signals.
The ammonia injection amount is changed by controlling the opening degrees of a and 14b.

The line 5 is short and NH ₃ is detected when Tb is detected.
If it is difficult to control the injection amount, Ta may be detected to control the injection amount of NH ₃ supplied from the line 2b. Now, the amount of ammonia adsorbed on the carbonaceous adsorbent (catalyst) q (expressed as gNH ₃ / Kg catalyst) is
It decreases as the temperature T of the exhaust gas rises, and increases with the concentration of ammonia in the exhaust gas. That is, it can be expressed mathematically as the following equation.

Q = k (C ^a / T ^b ) --- (A) where C = ammonia concentration (ppm) T = exhaust gas temperature (K) k, a, b = constant

FIG. 3 shows this relational expression when the temperature T is plotted on the abscissa and the parameter is expressed as the ammonia concentration C.

Now, it is assumed that the engine is normally operated at an exhaust gas temperature of 140 ° C. and an injected ammonia concentration of C ₀ (ppm). Next, when the exhaust gas temperature rises to 160 ° C., ammonia corresponding to Δq1 is released from the catalyst as shown in FIG. However, if the ammonia concentration is changed to C ₁ (ppm) from the relationship of the above formula (A), the desorbed ammonia will disappear. That is, the ammonia leak is controlled.

On the contrary, it is assumed that the exhaust gas temperature lowered to 120 ° C. after the operation at the exhaust gas temperature of 140 ° C. and the injected ammonia concentration C ₀ (ppm). In this case, the amount of adsorbed ammonia increases by Δq _2, so the ammonia concentration is changed to C ₂ (pp
m).

Next, the present invention will be described in detail in terms of chemical reaction. The injected NH ₃ has the following formulas (1), (2),
According to (3), (4) and (5), it is consumed by the denitration reaction, the desulfurization reaction and the adsorption on the carbonaceous catalyst. Nitrogen and water are produced in the reaction with NO _x, and acidic ammonium sulfate and ammonium sulfate are produced in the reaction with sulfuric acid produced by the oxidation of SO _x on the surface of the catalyst, and further, they are physically adsorbed on the catalyst surface. is there.

Denitration reaction NO + NH ₃ + 1 / 4O ₂ → N ₂ + 3 / 2H ₂ O (1) NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2) Reaction with sulfuric acid H ₂ SO ₄ + NH ₃ → NH ₄ HSO ₄ * (3) NH ₄ HSO ₄ + NH ₃ → (NH ₄ ) ₂ SO ₄ * (4) Adsorption on carbonaceous catalyst NH ₃ + C → C ... NH ₃ * (5) *: C indicating adsorption state : Represents a carbonaceous catalyst

The amount of NH ₃ adsorbed on the carbonaceous catalyst shown in formula (5) increases as the NH ₃ concentration increases, and the temperature of the carbonaceous catalyst decreases as shown in formula (A). Many.
Therefore, when the exhaust gas temperature rises, NH ₃ adsorbed on the carbonaceous catalyst will be rapidly desorbed and leak into the exhaust gas.

In the flue gas denitration method, of course, the purpose is to remove NO _x , but leaking ammonia is not permitted not only in terms of chemical loss but also in terms of the environment.
Controlling the injection amount of NH3 is an essential technique. In the conventional method, it was general to detect the NH ₃ concentration at the outlet of the reactor and to control the injection amount of NH ₃ at the inlet of the reactor by using this as a signal. However, with such a feedback system, if desorption of ammonia occurs rapidly, NH
_{3 Even} if a signal was sent after detecting the concentration to reduce the amount of ammonia injected at the inlet, the effect was small and desired control was difficult.

On the other hand, according to the present invention, according to the formula (A) found by the present inventors, when the exhaust gas temperature rises, the ammonia injection amount can be reduced without waiting for the NH ₃ concentration at the outlet to rise. Since the signal to be operated is obtained, it is possible to promptly respond to the rapid desorption of ammonia. Even if the injection amount is reduced, NH ₃ will be released to some extent by desorption, but since this is an amount used for the reaction with NO _X , NH ₃ leaking into the processing gas is kept at a very low level. be able to.

Further, even when the exhaust gas temperature suddenly drops, the insufficient ammonia required to cope with the increased adsorption amount of ammonia is compensated according to the formula (A) without waiting for the change in the NO _x outlet concentration. Therefore, extremely accurate control can be performed.

[0036]

Embodiments of the present invention according to the configuration of FIG.
The details will be described in comparison with a comparative example using a conventional technique.

Example 1 140 containing 50 ppm SO _x and 200 ppm NO _x
The boiler exhaust gas at ℃ is taken out at 1000 Nm ³ / h,
After injecting 250 ppm of NH ₃ into this, activated carbon 1.67
The mixture was introduced into a cross-flow type moving bed reactor filled with m ³ and brought into contact with the activated carbon catalyst bed descending in the reactor to perform desulfurization and denitration simultaneously. The residence time of activated carbon in the reactor is 3
The moving speed was set to be 0 hour.

After continuing the operation under the above conditions, the exhaust gas temperature rose from 140 ° C to 160 ° C. The ammonia concentration corresponding to this temperature change was calculated to be 200 ppm according to the above formula (A), and the NH ₃ injection amount was immediately increased to 2 ppm.
It was changed from 50 ppm to 200 ppm. The obtained results are shown in FIG. As shown in FIG. 4, the exhaust gas temperature is 16
Even if the temperature was raised to 0 ° C., there was no abnormality in the ammonia leak concentration, and it was kept at several ppm or less as before.

Comparative Example 1 Under the same conditions as described in Example 1, the operation was continued at 140 ° C., but thereafter the exhaust gas temperature rose to 160 ° C.
However, the NH ₃ injection amount is 250 even if the exhaust gas temperature changes.
It was kept constant at ppm. The obtained results are shown in FIG. As shown in FIG. 5, in response to the rise in the exhaust gas temperature, the leak concentration of ammonia increased abnormally to 40 ppm or more from the previous several ppm.

Example 2 140 Containing 50 ppm SO _X and 200 ppm NO _X
The boiler exhaust gas at ℃ is taken out at 1000 Nm ³ / h,
After injecting 250 ppm of NH ₃ into this, activated carbon 1.67
The mixture was introduced into a cross-flow type moving bed reactor filled with m ³ and brought into contact with the activated carbon catalyst bed descending in the reactor to perform desulfurization and denitration simultaneously. The residence time of activated carbon in the reactor is 3
The moving speed was set to be 0 hour.

Although the operation was continued under the above conditions, the exhaust gas temperature dropped to 120 ° C. The ammonia concentration corresponding to this temperature change is set to 300 p according to the above formula (A).
pm was calculated, and the NH ₃ injection amount was immediately changed from 250 ppm to 300 ppm. The obtained results are shown in FIG. As shown in FIG. 6, even if the exhaust gas temperature was lowered, there was no abnormality in the ammonia leak concentration, and it was kept at several ppm or less as before. Moreover, the decrease in the denitration rate was slight.

Comparative Example 2 Under the above-mentioned conditions of Example 2, the operation was continued at 140 ° C. first, but thereafter the exhaust gas temperature dropped to 120 ° C. However, the amount of NH ₃ injected is 250 pp even if the exhaust gas temperature changes.
It was kept constant at m. The obtained results are shown in FIG.
Shown in. As shown in FIG. 7, in response to the decrease in exhaust gas temperature, the ammonia leak concentration slightly decreased, but on the other hand, the denitrification rate decreased abnormally, from 80% to 65%.
It has dropped below.

[0043]

As described above, according to the present invention, in the flue gas denitration method or flue gas desulfurization / denitrification method using a carbonaceous catalyst-filled moving bed reactor, the amount of ammonia injected is adjusted to the temperature of the exhaust gas. The amount of NH ₃ leaking into the processing gas can be reduced by changing the amount of NH ₃ into the process gas. Also,
According to the present invention, it is possible to obtain an advantage that the decrease in the denitration rate can be suppressed even if the temperature of the exhaust gas decreases.

[Brief description of drawings]

FIG. 1 is a diagram showing one example of the configuration of a desulfurization / denitration apparatus for carrying out the method of the present invention.

FIG. 2 is a diagram showing another example of the configuration of a desulfurization / denitration apparatus for carrying out the method of the present invention.

FIG. 3 is a diagram showing the amount of ammonia adsorbed on a carbonaceous catalyst, with temperature T as the horizontal axis and ammonia concentration C as a parameter.

FIG. 4 is a graph showing the response of NOx removal rate and ammonia leak when the NH ₃ injection amount is decreased according to the rise of exhaust gas temperature in Example 1 of the present invention.

FIG. 5 is a graph of Comparative Example 1 in which N
It is a diagram showing the response of denitrification rate and ammonia leak if that had been with H ₃ injection amount constant.

FIG. 6 is a diagram showing a response of a NOx removal rate and an ammonia leak when an NH ₃ injection amount is increased according to a decrease in exhaust gas temperature in Example 2 of the present invention.

FIG. 7 is a graph showing a denitration rate and an ammonia leak response when the NH ₃ injection amount is kept constant even when the exhaust gas temperature is lowered in Comparative Example 2.

[Explanation of symbols]

 1 Exhaust gas line 2, 2a, 2b Ammonia injection line 3 Moving bed reactor 4 Carbonaceous catalyst bed 5 Line 6 Heat regenerator 7 Separator 8 Line 9 Line 10 Second moving bed reactor 11 Second carbonaceous catalyst bed 12 Line 13 Controller 14, 14a, 14b Ammonia control valve

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/77 53/56 53/74 53/60 53/86 ZAB 53/96 B01D 53/34 125 B 129 E 132 Z 53/36 ZAB 102 E

Claims (4)

[Claims] 1. Denitration or desulfurization / denitration treatment by injecting ammonia into exhaust gas containing nitrogen oxides or nitrogen oxides and sulfur oxides and introducing the ammonia into a moving bed reactor filled with a carbonaceous catalyst. On the other hand, in the method for treating exhaust gas in which the carbonaceous catalyst inactivated by contact with the exhaust gas is heated and regenerated, the amount of ammonia to be injected is changed in accordance with the change in exhaust gas temperature. . 2. The method according to claim 1, wherein the amount of injected ammonia is decreased when the exhaust gas temperature rises, and conversely, the amount of injected ammonia is increased when the exhaust gas temperature decreases. 3. The injection amount of ammonia into the exhaust gas is changed so that the ammonia concentration in the exhaust gas is determined by the relational expression of the temperature of the exhaust gas, the ammonia concentration in the exhaust gas, and the adsorption amount of ammonia on the catalyst. The method according to claim 2, wherein 4. The method according to claim 3, wherein a decrease in the denitration rate is suppressed by increasing the amount of ammonia injected when the exhaust gas temperature decreases.