JPH0154089B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides sulfur oxides SOx and nitrogen oxides NOx.
Regarding the desulfurization and denitrification method for exhaust gas containing
It relates to a method of desulfurization and denitrification by processing at a low temperature range of ℃.

As a treatment method for exhaust gas containing SOx and NOx, such as exhaust gas from coal-fired or heavy oil-fired boilers and exhaust gas from sintering furnaces at steel plants, ammonia gas is injected into these exhaust gases, and this gas is injected into a packed bed of carbonaceous catalyst. A method of processing by passing gas is known (Japanese Patent Application Laid-open No. 92858/1983). This method uses
In addition to being able to remove SOx and NOx at the same time,
It has the advantage that the catalyst can be reused. However, in order to efficiently remove NOx with this method, the reaction temperature must be at least 200℃ or higher.
Preferably, it is necessary to maintain the temperature at about 220 to 250°C; NOx cannot be removed sufficiently at a temperature lower than this. However, when the reaction temperature exceeds 200° C., there is a problem that a part of the carbonaceous catalyst is consumed by oxygen coexisting in the exhaust gas, as in the case of C+O ₂ →CO ₂ . Furthermore, since the temperature of exhaust gas from a boiler or the like is generally around 150°C at the outlet of the air heater, there is the inconvenience that when processing this exhaust gas, it must be preheated to a temperature of 200°C or higher.

In contrast, a patented method has been developed that can efficiently remove not only SOx but also NOx even at temperatures of around 150℃.
Proposed in No. 1241899. This method uses two orthogonal moving bed reactors, and the flue gas injected with ammonia gas is first passed through the first moving bed reactor to remove most of the SOx, and then the resulting treated gas is Ammonia gas is mixed again, and this mixed gas is introduced into the second moving bed reactor for reprocessing.

This two-stage treatment method is excellent in that it can desulfurize and denitrate exhaust gas at low temperatures, but when treating exhaust gas with a relatively low SOx concentration (approximately 300 ppm or less), it tends to be overly exaggerated and may be used unnecessarily. It cannot necessarily be used because it uses a carbonaceous catalyst. because,
Since the SOx concentration in the exhaust gas is low, if ammonia gas is mixed with this exhaust gas and introduced into the moving bed reaction of the carbonaceous catalyst, theoretically, it can be done in one reactor without the need for two-stage treatment as in the above patent. This is because the exhaust gas can be sufficiently desulfurized and denitrated. Moreover, in this case, the SOx concentration in the exhaust gas is low and the amount of SOx adsorbed by the carbonaceous catalyst is small, so the residence time of the catalyst in the reactor can be increased. However, if the residence time is increased for economical reasons, even if the reaction temperature is as low as around 150°C, hot spots will occur in the catalyst layer due to the accumulation of heat of oxidation reaction in the carbonaceous catalyst, resulting in catalyst combustion accidents. There is a risk of inviting

Therefore, the present invention proposes an exhaust gas treatment that can desulfurize and denitrate even exhaust gas with a relatively low SOx concentration in a single stage process, and does not necessarily involve the occurrence of combustion accidents as described above.

That is, the present invention provides approximately
In the method of desulfurization and denitration treatment of exhaust gas at 100 to 180℃ by contacting it with a carbonaceous catalyst in the coexistence of ammonia gas, the exhaust gas is divided into two gas streams, and the first exhaust gas stream is Introduced into a first moving bed reactor filled with a carbonaceous catalyst to perform desulfurization and denitrification,
The second exhaust gas stream is introduced into a second moving bed reactor filled with a carbonaceous catalyst for desulfurization and denitrification, and the carbonaceous catalyst discharged from the outlet of the first moving bed reactor is subjected to a second moving bed reaction. The carbonaceous catalyst supplied to the inlet of the reactor and discharged from the outlet of the second moving bed reactor is heated and regenerated, and then supplied to the inlet of the first moving bed reactor.

Hereinafter, the present invention will be explained in more detail with reference to the accompanying drawings. First, in the embodiment shown in FIG.
The exhaust gas at a temperature of 100-180°C is mixed with ammonia injected from line 1 and then split into two gas streams, one exhaust gas stream passing through line 2 to the first straight gas stream filled with carbonaceous catalyst. It is introduced into the AC moving bed reactor 3. After the exhaust gas comes into contact with the carbonaceous catalyst layer 4 descending in the reactor 3 and undergoes desulfurization and denitrification treatment,
Reactor 3 is discharged into line 5. The other exhaust gas stream divided into two is introduced into the second cross-flow moving bed reactor 10 via line 9, contacts the carbonaceous catalyst bed 11 descending in the reactor, and undergoes desulfurization and denitrification treatment, It is discharged to line 6. The gases discharged from each reactor into lines 5 and 6 are combined and sent to a dust collector 7, where they are removed from dust and then discharged into the atmosphere from a line 8.

Regarding the carbonaceous catalyst, the catalyst discharged from the lower outlet of the first reactor 3 is discharged from the line 1
The catalyst fed via line 13 to the upper inlet of the second reactor 10 and discharged from the lower outlet of the reactor 10 is led via line 13 to the regenerator 14 . In this regenerator, the catalyst is regenerated in a high temperature inert gas atmosphere at about 300-600°C, and the regenerated catalyst is returned to the upper inlet of the first reactor 3 through line 15 for reuse. And high concentration SO ₂ generated in the regenerator 14
The gas is taken out to the line 16, purified directly or by means such as washing with water, and then supplied to a by-product recovery process to obtain sulfuric acid, ammonium sulfate, sulfur, and the like.

In the embodiment shown in FIG. 1, ammonia is injected and then the exhaust gas is divided into two gas streams, but ammonia may be injected into each gas stream after the exhaust gas is divided. Also, lines 12, 13,
15 may be provided with means for separating dust, catalyst powder, etc., such as a vibrating screen, if necessary.

In the embodiment shown in FIG. 2, after dividing the exhaust gas into two gas streams, ammonia is injected only into the exhaust gas supplied to the first reactor 3, and into the exhaust gas supplied to the second reactor 10. There is no difference from the embodiment in Figure 1 except that ammonia is not injected, but in the embodiment in Figure 2, it is possible to reduce the amount of ammonia in the highly concentrated SO ₂ gas recovered during heating and regeneration of the catalyst. can.

As the carbonaceous catalyst of the present invention, activated coke and activated carbon obtained by heat treating coal or activating it with steam are generally used. Of course, you can also use it.

By the way, SOx in exhaust gas is adsorbed and removed as sulfuric acid through contact with a carbonaceous catalyst as follows.

SO ₂ +1/2O ₂ +H ₂ O→H ₂ SO ₄ (1) SO ₃ +H ₂ O→H ₂ SO ₄ (2) In this case, if ammonia is made to coexist in the exhaust gas in advance, the adsorbed sulfuric acid will turn into ammonium salt. converted.

H ₂ SO ₄ +NH ₃ →NH ₄ HSO ₄ (3) NH ₄ HSO ₄ +NH ₃ → (NH ₄ ) ₂ SO ₄ (4) In other words, the carbonaceous catalyst releases sulfuric acid and its ammonium salt by contact with the exhaust gas. As the amount of SOx adsorbed increases, it becomes gradually inactivated, but the rate of this becomes faster as the concentration of SOx in the exhaust gas increases. Therefore, when the SOx concentration is high, in order to maintain the desired desulfurization rate,
It is necessary to shorten the residence time of the catalyst in the moving bed reactor. On the other hand, when the SOX concentration in the exhaust gas is low, it is possible to lengthen the residence time of the catalyst. There is a risk that hot spots will occur in the layer and a catalyst combustion accident will occur.

However, in the present invention, the exhaust gas is divided into two and each is introduced into a separate reactor, so the residence time of the catalyst in each reactor can be shortened, and the problem of hot spots can therefore be avoided. can. In addition, in the present invention, the catalyst used in the first reactor can be used in the second reactor without being regenerated, so there is an advantage that the catalyst can be used effectively.

Furthermore, in the present invention, the dust and catalyst powder collected in the first reactor can be removed using a separator such as a vibrating screen, which has the advantage of avoiding problems such as increased pressure loss and blockage in the reactor. be. For example, when processing dust-containing exhaust gas of 100 to 200 mg/Nm ³ or more in one reactor, if the catalyst transfer speed is slow and the residence time is long, the amount of dust in the catalyst bed will increase, resulting in an increase in pressure loss. Problems such as blockage of the catalyst layer arise.

In addition, _NH3 is added to the exhaust gas containing SOx and NOx.
When denitrifying by injecting NH 3 , the denitrification reaction is shown as NH ₃ + NO + 1/4O ₂ →N ₂ + 3/2H ₂ O (5), but at a reaction temperature of around 150°C, most of the injected NH ₃ is carbon. SO ₂ adsorbed on quality catalyst
(actually adsorbed as sulfuric acid) [Equations (3) and (4) above]. especially reaction
Since (3) is fast, the amount of NH ₃ injection required for denitrification reaction (5) is
The amount consumed in reaction (3) must be taken into consideration when adding. (When the reaction temperature is lower than about 100℃, it is necessary to take into account the amount consumed by reaction (4).) In other words, the relationship between the denitrification rate and the amount of NH ₃ injected is roughly as follows. There is.

(NH ₃ injection amount, ppm) = (SOx concentration, ppm × desulfurization rate) × (0.5 to 1.5) + (NOx concentration, ppm × denitrification rate) + (NH ₃ leakage amount, ppm) In other words, it is used for desulfurization and denitrification. The ratio of NH ₃ and SO ₂ adsorbed on the carbonaceous catalyst discharged from the reactor (NH ₄ ⁺ /SO ₄ ^2- ) is close to 1, and in thermal regeneration of the catalyst in this state, The decomposition rate of ammonium salts to nitrogen is low, and a large amount of NH ₃ is mixed into the recovered SO ₂ , causing problems such as blockages and reduced product purity when recovering H ₂ SO ₄ , S, and other byproducts. In addition, when purification is performed by washing with water, NH ₄ HSO ₃ is released into the wastewater.
A loss occurs because a large amount is dissolved as such.

By the way, Figure 3 shows that after passing various amounts of NH ₃ into SO ₂ -containing gas through an activated carbon layer,
_This is a measurement of the decomposition rate of adsorbed NH ₃ to N ₂ when heated and regenerated _at
As the amount increases, the rate of decomposition to _N2 decreases. A large amount of NH ₃ adsorption and a decrease in the NH ₃ decomposition rate mean that the NH ₃ concentration in the recovered SO ₂ gas becomes significantly high.

According to the method of the present invention shown in FIG. 2, the adsorption ratio (molar ratio) of NH ₃ /SO ₂ on the catalyst in the first reactor is about 1, and this catalyst is Since almost the same amount of SO ₂ (actually H ₂ SO ₄ ) is adsorbed again, NH ₃ /SO ₂ is approximately 0.5. It can be understood that the NH ₃ concentration in the desorbed gas is therefore significantly reduced.

In the present invention, it is suitable to divide the exhaust gas into two halves because it is actually convenient to manufacture the same reactor, but the embodiment shown in FIG. In general, the amount supplied to the first reactor where NH ₃ injection is performed can be about 1/4 to 2/3 of the total amount of exhaust gas.

Example 10000Nm ³ /h of exhaust gas at 120℃ containing 150ppm sulfur oxides, 250ppmm nitrogen oxides, 10% moisture, and 10% oxygen is divided into 5000Nm ³ /h each, and one exhaust gas is charged with NH ₃ After injecting 350ppm of
It was passed through a first cross-flow moving bed reactor packed with 8.3 m ³ of granular activated carbon. In this case, the residence time of the activated carbon in the reactor was set to 40 hours. The desulfurization rate of this reactor is 99.9%, the denitration rate is 80%, and the leakage
NH ₃ was 16 ppm.

On the other hand, the remaining 5000Nm ³ /h of exhaust gas is directly 8.3
It was passed through a second cross-flow moving bed reactor packed with m ³ activated carbon. The residence time of activated carbon in the reactor in this case is set to 40 hours. The desulfurization rate of this reactor was 95%, and the denitrification rate was 6%.

When the gas discharged from the first and second reactors above was combined, the denitrification rate relative to the original exhaust gas was found to be
A desulfurization rate of 43% and a desulfurization rate of 97.5% were obtained. Also the leak _NH3 is
It was 8ppm.

On the other hand, the used activated carbon discharged from the second reactor via the first reactor is supplied to the regenerator,
It was heated and regenerated at 400°C in an inert carrier gas atmosphere. By adjusting the amount of carrier gas to the regenerator, 15% high concentration SO ₂ gas was recovered. As a result of measuring the NH ₃ concentration in this SO ₂ gas, NH ₃
The concentration was 0.95%.

Comparative example As a comparative example, without dividing exhaust gas
After injecting 175 ppm NH ₃ into the exhaust gas at 10000 Nm ³ /h, it was introduced into a cross-flow moving bed reactor filled with 16.6 m ³ of granular activated carbon. In this case, the residence time of activated carbon is 80
The time is set. The desulfurization rate of the exhaust gas discharged from the reactor was 97%, and the denitrification rate was 20%. Also, the leaked NH ₃ was 5 ppm.

In this case, the NH ₃ concentration in the highly concentrated SO ₂ gas (15%) recovered by the regenerator was 4.5%.

As another comparative example, in order to obtain the same denitration rate as in the example, the NH ₃ injection amount was changed to 250 ppm, resulting in a desulfurization rate of 98.5% and a denitration rate of 43%. Also, the leaked NH ₃ was 8 ppm. In this case, the _NH3 concentration in the highly concentrated _SO2 gas (15%) obtained in the regenerator is
It rose significantly to 5.2%.

[Brief explanation of drawings]

Figures 1 and 2 are flow diagrams showing one embodiment of the present invention, and Figure 3 shows the relationship between the molar ratio of _NH3 / _SO2 adsorbed on activated carbon and the decomposition rate of adsorbed _NH3 . This is a graph showing. 3, 10... Cross flow moving bed reactor, 7... Dust collector, 14... Regenerator.

Claims (1)

[Claims] 1 Approximately 100 ~ containing sulfur oxides and nitrogen oxides
In the method of desulfurization and denitration treatment of exhaust gas at 180℃ by contacting it with a carbonaceous catalyst in the coexistence of ammonia gas, the aforementioned exhaust gas with a sulfur oxide content of about 300 ppm or less is divided into two gas streams. the first exhaust gas stream is introduced into a first moving bed reactor filled with a carbonaceous catalyst for desulfurization and denitrification, and the second exhaust gas stream is introduced into a second moving bed reactor filled with a carbonaceous catalyst. The carbonaceous catalyst discharged from the outlet of the first moving bed reactor is supplied to the inlet of the second moving bed reactor, and the carbonaceous catalyst discharged from the outlet of the second moving bed reactor is A method for treating exhaust gas, comprising supplying a catalyst to an inlet of a first moving bed reactor after heating and regenerating the catalyst.