JPH0367728B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a denitrification method, and in particular, to a dry denitrification device for removing nitrogen oxides (hereinafter referred to as _NOx ) from exhaust gas, for example, ammonia (NH ₃ ) is used as a reducing agent.
This relates to a dry denitrification method that reduces _NO [Prior art] In recent years, in Japan, due to the tight supply of heavy oil, in order to correct the dependence on oil, the fuel is being changed from the conventional heavy oil-burning to coal-burning. Large-capacity thermal power plants are being constructed. However, since coal fuel has poor combustibility compared to petroleum fuel, _NOx and unburned substances contained in exhaust gas are likely to be generated. Measures to reduce NO _x include flame splitting, exhaust gas recirculation, two-stage combustion, and in-furnace denitration to achieve slow combustion and reduce NO _x . In this coal-fired thermal power plant, the boiler load is rarely operated at full load (100% load), but the load may be increased or decreased to 80% load, 50% load, 25% load. , thermal power plants are shifting to thermal power plants that take on intermediate loads such as shutting down operations. On the other hand, for intermediate-load thermal power generation, there are so-called combined cycle plants that combine a gas turbine with good startup characteristics and an exhaust heat recovery boiler.
A system that operates only during the day when electricity demand is high and shuts down at night is about to be built. However, this coal-fired intermediate load boiler,
In addition to _conventional combustion improvements, NH ₃
An increasing number of plants are installing dry catalytic reduction and denitrification equipment that performs denitrification in the presence of a catalyst using a reducing agent. In coal-fired boilers, the coal has poor combustibility, so the amount of _NO
Since it contains _NOx , a denitrification device 8 as shown in FIG. 1 is installed in the plant. FIG. 1 is a diagram showing a typical flue system of a boiler in which this denitrification device 8 is installed. As shown in the figure, the combustion air introduced into the air duct 1 is pressurized by the forced draft fan 2, heated by the high-temperature exhaust gas flowing through the exhaust gas duct 4 by the air preheater 3, and then heated by the high temperature exhaust gas flowing through the exhaust gas duct 4. , Windbox 5
The water is then supplied to the boiler 6. On the other hand, the exhaust gas burned in the boiler 6 is mixed with NH ₃ from the NH ₃ injection pipe 7 in the exhaust gas duct 4.
Denitration is performed by the catalyst 9 in the denitrification device 8 located downstream of the exhaust gas, and _NOx in the exhaust gas is removed.Then, the pressure is increased by the air preheater 3, dust collector 10, and induced draft fan 11 to create an air atmosphere. released to. However, although the reaction temperature of the denitrification device 8 varies somewhat depending on the type of catalyst 9, the temperature range with the highest efficiency is a relatively high temperature of 300 to 400°C, and the temperature range is quite narrow. Therefore, in boilers for intermediate load thermal power plants and gas turbines that are always operated at partial load, there is a problem that the exhaust gas temperature fluctuates due to load fluctuations and goes outside the usable range of the catalyst 9. be. In this case, if the exhaust gas temperature is too high, the quality of the catalyst 9 will change and its function as a catalyst will be impaired. Furthermore, if the exhaust gas temperature is too low, it will react with sulfuric anhydride (SO ₃ ) present in the exhaust gas, resulting in deterioration of the catalyst function. Therefore, the exhaust gas at these partial loads is:
The problem of denitrification during partial load is being solved by temperature control using gas bypass systems, afterburning, etc., or by improving catalyst performance. [Problems to be Solved by the Invention] However, the drawback of the denitrification equipment for intermediate-load thermal power boilers and gas turbines is that these boilers and gas turbines repeatedly start and stop, and immediately after startup, A large amount of NO _X is generated,
Moreover, since the gas temperature is low, it takes several hours after NH ₃ is injected until the progress of the denitrification reaction reaches a steady state. Therefore, reducing _NOx , which is emitted immediately after startup, becomes a new issue. Conventionally, in order to perform stable denitrification,
An invention as described in Publication No. 1858 has been proposed. In this invention, the maximum adsorption amount of NH ₃ of the catalyst to be used is determined in advance, and the amount of NO _X in the exhaust gas emitted from the denitrification device during exhaust gas ventilation is monitored using a gas analyzer, so that the amount of _NO When this happens, the solenoid valve is opened and excess NH ₃ within the maximum adsorption amount of NH ₃ is injected from the NH ₃ injection pipe for a predetermined period of time. and _NH3
By closing the solenoid valve when the injection of
Stop the _NH3 injection. After this solenoid valve is closed, the denitrification reaction starts with the excess adsorbed on the catalyst.
The mechanism is such that NH ₃ leaves the catalyst and participates in the denitrification reaction. In other words, the excess NH ₃ adsorbed on the catalyst causes fluctuations in the flow rate of exhaust gas, _NO
It serves as a buffer that compensates for fluctuations in concentration, fluctuations in the ratio of NO and NO ₂ , and response delays of the analyzer. As mentioned above, in this method, excess NH ₃ is injected during exhaust gas ventilation, a portion of which is directly involved in the denitrification reaction of the exhaust gas, and the remaining NH ₃ is adsorbed by the catalyst. However, when flowing a large amount of exhaust gas to be treated, injecting excess NH ₃ and allowing some of it to participate in the denitrification reaction while allowing the excess NH ₃ gas to be adsorbed by the catalyst is practically impossible. is almost impossible. That is, since a large amount of exhaust gas is flowing together with NH ₃ gas, the gas flow rate is increasing. Therefore, when injected, excess NH ₃ gas that does not participate in the reaction is washed away by the exhaust gas before being adsorbed by the catalyst, and is hardly adsorbed by the catalyst. In that case, at the denitrification equipment outlet
Not only does NH ₃ gas leak and cause secondary pollution, but after the solenoid valve is closed and the supply of NH ₃ gas is stopped, almost no denitrification reaction takes place.
_NOx will be emitted directly into the atmosphere. Furthermore, even if NH ₃ gas is flowed together with the exhaust gas,
In order to cause NH ₃ to be adsorbed by the catalyst, the gas flow rate within the denitrification device may be slowed down to lengthen the adsorption time. However, in order to do so, it is necessary to take up a large space within the denitrification apparatus, which inevitably increases the size of the apparatus, which is not desirable from a design and economical point of view. The present invention solves these drawbacks of the conventional technology, and its purpose is to provide a denitrification method that can efficiently denitrify immediately after startup, even in plants that frequently start and stop. It's about doing. [Means for Solving the Problems] In order to achieve the above object, the present invention provides a denitration device containing a catalyst in an exhaust gas duct, and an injection means for injecting a reducing agent upstream of the denitration device, In a dry denitrification method in which a reducing agent is injected into an exhaust gas duct to reduce nitrogen oxides in the exhaust gas to be treated, the injection means By injecting an excess reducing agent into the exhaust gas duct, this excess reducing agent is adsorbed on the catalyst in advance, and then the exhaust gas to be treated is vented into the exhaust gas duct to reduce nitrogen oxides in the exhaust gas. There are some features. [Operation] As described above, in the present invention, an excess reducing agent is injected before the denitrification device is started up, and this excess reducing agent is adsorbed on the catalyst in advance before the exhaust gas is vented. Since the reducing agent is injected before startup in this way, sufficient time can be taken for the reducing agent to be adsorbed. Furthermore, the flow rate is much slower than in conventional proposals, in which the reducing agent was flowed together with a large amount of exhaust gas to be treated. For this reason, the reducing agent can be sufficiently adsorbed on the catalyst, and the reducing agent can be kept in a radical state even before venting the exhaust gas. This is especially important at startup. That is, for example, when a boiler or the like is started up, the _NOx content in the exhaust gas is especially high and the exhaust gas temperature is relatively low, which is the worst condition for denitration. Under such conditions, if the reducing agent can be kept in a radical state before venting the exhaust gas as in the present invention, the denitrification reaction can proceed efficiently. [Example] Next, an example of the present invention will be described, but first, an experimental example conducted by the present inventors will be described. In the NH ₃ catalytic reduction denitrification method, the present inventors
Changes in NH ₃ adsorption amount and denitrification rate after NH ₃ injection were measured, and the results are shown in FIGS. 2 and 3. As is clear from these figures, it takes several hours for the denitrification rate to reach a steady value after the start of NH ₃ injection. As a result of various studies on the cause of this from the viewpoint of the reaction mechanism, we came to the following conclusion. Note that these experimental conditions were a gas temperature of 200℃,
Area velocity 6m/h, gas is kerosene combustion gas, NO value
200ppm, NH ₃ injection amount is 200ppm. As is clear from these figures, as shown in Fig. 3
The denitrification rate after the start of NH ₃ injection and the change over time in the amount of NH ₃ adsorbed on the catalyst shown in Figure 2 are in good agreement. Based on this, the denitrification rate
When examining the relationship with the NH ₃ adsorption amount Q, the following relationship is established. _{ln 1/1 _} It turns out that this is because it takes time for the amount of NH ₃ adsorbed on the catalyst to reach a steady value because the amount of NH 3 supplied is small. On the other hand, in order to increase the denitrification rate X, it is sufficient to increase the amount of NH ₃ adsorbed on the catalyst. As a result of investigating the properties of this adsorbed NH ₃ , it was found that the adsorbed NH ₃ is extremely stable, and once the NH ₃ is adsorbed on the catalyst, there is no change even during the period when the denitrification equipment is stopped, and it remains adsorbed on the catalyst. It also became clear that The present invention applies this surprising stability of adsorbed NH ₃ to a denitrification device. In other words, if the injection of NH ₃ is started before the denitrification equipment is started, and a large amount of NH ₃ is adsorbed on the catalyst, that is, an amount of NH ₃ that is excessive compared to the amount of NH ₃ required for the denitrification reaction, it will be NH ₃ can be maintained in a radical state, and even in equipment that frequently starts and stops, such as gas turbines and intermediate-load boilers, it can efficiently denitrify immediately after startup. [Example] Next, the present invention will be explained in detail with reference to Examples. Example 1 A Ti-based catalyst formed and sized into 10 to 20 meshes was
Fill the reaction tube of the denitrification equipment. The gas shown in Table 1 below was heated to 350°C, and the gas was passed through the reaction tube at a space velocity of 50,000 h ^-1 , and the aeration state was maintained for 30 minutes. Then, increasing the _NH3 concentration to 300ppm
After maintaining the same conditions for 30 minutes, gas ventilation was stopped, and the sample was immediately taken out into the atmosphere and left as it was for 12 hours. The catalyst that has adsorbed NH ₃ was packed into the reaction tube again, and the temperature was raised from room temperature to 350°C as shown in Figure 4 while passing gas with the composition shown in Table 1. Changes in the denitrification rate at that time were measured. It was measured. This result is shown by the two-dot chain curve A in FIG. As shown in this curve, the denitrification rate increases from the time of startup, and injecting an excess amount of NH ₃ before venting the exhaust gas to the denitrification equipment is the most effective way to increase the denitrification rate at restart. It can be seen that it is.

[Table] Example 2 Instead of 300 ppm NH ₃ concentration in Example 1
A similar test was conducted with the concentration being 400 ppm and the other conditions being the same as in Example 1 above. The change in the denitrification rate at this time was measured, and the results are shown by the dashed line curve B in FIG. Example 3 Instead of 300 ppm NH ₃ concentration in Example 1
A similar test was conducted with the concentration being 600 ppm and the other conditions being the same as in Example 1 above. The change in the denitrification rate at this time was measured, and the results are shown by the solid curve C in FIG. As is clear from Examples 2 and 3, a high denitrification rate was obtained from the time the denitrification device was started. Comparative Example 1 A similar test was conducted without performing the high NH ₃ concentration (300 ppm) adsorption treatment shown in the previous example. Changes in the denitrification rate at this time were measured, and the results are shown by the dotted curve D in FIG. As is clear from FIG. 5, in the case of the comparative example, the denitrification rate at startup is worse than curves A, B, and C.
It is not possible to obtain a high denitrification rate immediately after venting the exhaust gas at startup. Example 4 Eight Ti-based plate-shaped catalysts having a thickness of 1 mm, a width of 40 mm, and a length of 100 mm were packed into a 40 mm square glass reaction tube at a spacing of 5 mm to form a catalyst layer. Then, as shown in FIG. 8, diluted 1000 ppm NH ₃ was passed through the catalyst layer at a rate of 6 N/min for 20 minutes to adsorb NH ₃ on the catalyst. Then, use the simulated gas shown in Table 2 below.
Aerated at a rate of 6 N/min with 300 ppm of NH ₃ . As shown in Figure 6, this simulated gas is heated in a preheater for one hour to simulate the startup of the turbine.
The temperature was raised to 500°C, and the inlet _NOx was set to 300 ppm as shown in Figure 7.

[Table] Changes in the denitrification rate at this time were measured, and the results are shown by the solid curve E in FIG. Comparative Example 2 At the same time as passing the above-mentioned simulated gas through the catalyst layer, NH ₃
was injected at 300 ppm (equimolar to NO). The change in the denitrification rate at this time was measured, and the results are shown by the broken line curve F in FIG. As is clear from this figure, the denitrification rate at startup in Example 4 is much higher than that in Comparative Example 2. Although the above embodiment describes the denitrification of gas turbines, the present invention is not limited to denitrification of gas turbines, but is also effective for a wide range of other denitrations. [Effects of the Invention] According to the present invention, a high denitrification rate can be obtained immediately after starting the denitrification equipment, especially in equipment that frequently starts and stops, such as intermediate load boilers and gas turbine exhaust heat recovery boilers. This is particularly effective in systems that generate a large amount of _NOx when starting up.

[Brief explanation of drawings]

Figure 1 is a typical flue system diagram of a boiler, Figures 2 and 3 are characteristic curve diagrams showing the correlation between the amount of NH ₃ adsorption and the denitrification rate, Figures 4, 6, and FIG. 7 is a characteristic curve diagram showing changes in exhaust gas temperature and inlet _NO
The figure shows the time to start injection of reducing agent and the amount of injection. Figures 5 and 9 are characteristic curve diagrams showing the relationship between denitrification rate and time. Figure 10 shows the start of injection of reducing agent. FIG. 2 is an explanatory diagram showing time points. 4...Exhaust gas duct, 7... _NH3 injection pipe, 8
...Denitrification equipment, 9...Catalyst.

Claims (1)

[Claims] 1. A denitration device containing a catalyst is provided in an exhaust gas duct, and an injection means for injecting a reducing agent is provided upstream of the denitration device, and the reducing agent is injected into the exhaust gas duct for treatment. In a dry denitrification method for reducing nitrogen oxides in exhaust gas, an excess reducing agent is injected into the exhaust gas duct by the injection means before the exhaust gas ventilation to the denitrification device is stopped and before the start of the next denitrification device at the latest. A dry denitrification method characterized in that this excess reducing agent is adsorbed in advance on a catalyst, and then the exhaust gas to be treated is vented into an exhaust gas duct to reduce nitrogen oxides in the exhaust gas.