JPH0582246B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an ammonia injection amount control device,
In particular, the present invention relates to an ammonia injection amount control device that injects ammonia (NH ₃ ) into a dry denitrification device that removes nitrogen oxides (NOx) from exhaust gas.

[Background of the invention]

In recent years, in Japan, due to the tight supply of heavy oil, in order to correct the dependence on oil, fuels have been changed from conventional heavy oil-only combustion to coal-only combustion and LNG (liquefied natural gas)-only combustion, especially for commercial use. As for boilers, large-capacity thermal power plants with exclusively coal-fired or LNG-fired boilers are being constructed.

However, coal fuel has poor fuel properties compared to petroleum fuel and gas fuel, so NOx contained in exhaust gas
In order to reduce NOx, flame division, exhaust gas recirculation, two-stage combustion, in-furnace denitration, etc. are adopted to achieve slow combustion and reduce NOx. is also being carried out.

In these coal-fired power plants and LNG-fired power plants, there are few cases in which the boiler load is always operated at full load, and the load is increased and decreased to 75% load, 50% load, and 25% load. So-called high-frequency startup/shutdown (Daily
Start Stop (abbreviated simply as DSS) operation is now underway, and the transition is underway to thermal power plants that handle intermediate loads.

On the other hand, in addition to this thermal power generation boiler, so-called combined plants, which combine a gas turbine with good startup characteristics and an exhaust heat recovery boiler, are also used for intermediate load thermal power generation, and they are used to perform DSS operation during the daytime when electricity demand is high. A new system is being built that will only operate at night and shut down at night.

However, due to stricter regulations on NOx emission concentration in these coal-fired and LNG-fired intermediate-load boilers and gas turbines, in addition to conventional combustion improvements, denitration is performed in the presence of a catalyst using NH ₃ as a reducing agent. An increasing number of plants are installing dry catalytic reduction denitrification equipment.

This is because coal-fired boilers have poor fuel combustibility, which increases the amount of NOx, and LNG-fired boilers,
Because gas turbine plants burn at high temperatures with a high amount of oxygen, the exhaust gas contains a large amount of NOx, similar to coal-fired boilers, so a denitrification device as shown in Figure 3 is installed. .

Figure 3 shows a typical flue system of a boiler equipped with a denitrification device.

The combustion air in the air dust 1 is pressurized by the forced draft fan 2, heated by the exhaust gas from the exhaust gas duct 4 in the air preheater 3, and then transferred to the wind box 5.
The water is then supplied to the boiler 6.

On the other hand, the exhaust gas combusted in the boiler 6 is denitrified in the exhaust gas duct 4 by NH 3 from the NH ₃ injection pipe 7, and the denitrification is promoted in the catalyst ₉ in the denitrification device 8 disposed downstream. NOx is removed, the pressure is increased by the air preheater 3, the dust collector 10, and the induced draft fan 11, and the air is released into the atmosphere.

However, although the reaction temperature range of the denitrification device 8 varies somewhat depending on the type of catalyst 9, the temperature range with the highest denitrification efficiency is a relatively high temperature range of 300 to 400°C.
The temperature range is quite narrow, so in boilers for medium-load thermal power plants and those that are constantly operated by DSS, such as in a combined cycle, the exhaust gas temperature will constantly fluctuate due to load fluctuations, and the exhaust gas temperature will constantly fluctuate due to load fluctuations. There is a drawback.

In this case, if the temperature of the gas used in the catalyst 9 is too high, the structure of the catalyst 9 will change and the function of the catalyst 9 will be impaired, and if the temperature of the gas used is too low, the sulfuric anhydride (SO _{3 )} present in the exhaust gas will change. ), and the function of the catalyst 9 deteriorates.

On the other hand, boilers for thermal power generation that are constantly operated by DSS,
In the combined cycle, the amount of exhaust gas and the NOx concentration fluctuate, which has the disadvantage that the followability of the denitrification performance deteriorates.

This is because the exhaust gas amount and NOx concentration due to the reaction mechanism of NOx and NH ₃ on the catalyst 9 increase at startup.
This is because when the load fluctuates, such as when the load changes, the denitrification performance cannot follow the load fluctuation even if the NH ₃ injection amount is changed in accordance with the load fluctuation.

To avoid these problems, conventional _NH3
A typical example of the injection amount control device is shown in FIG.

In FIG. 4, an inlet NOx signal 13 detected by an inlet NOx concentration detector 12 and an air flow signal 15 detected by an air flow rate detector 14 are converted by a function converter 16, and this converted signal 17 is calculated. The total NOx amount signal 19 is calculated by the unit 18. On the other hand, from the inlet NOx signal 13 and the set NOx signal 21 set by the outlet NOx setting device 20, the necessary molar ratio (ratio of NOx amount and NH ₃ amount) is determined by the required molar ratio setting device 22.
The signal 23 is calculated, and an adder 26 corrects it with the actually measured outlet NOx signal 25 detected by the outlet NOx concentration detector 24, and calculates this correction signal 27 and the total NOx amount signal 19 mentioned earlier. The required NH ₃ flow rate signal 29 is calculated by the device 28 .

A comparator 32 compares this required NH ₃ flow rate signal 29 and an actual NH ₃ flow rate signal 31 detected by an actual NH ₃ flow rate detector 30 to calculate a deviation signal 33, which is used by a proportional integrator 34 to open the valve. convert it into a control signal 37 by an electro-pneumatic converter 36, open the NH ₃ flow control valve 39 of the NH ₃ pipe 38,
close

In this way, in the conventional NH ₃ injection amount control device, when the NOx amount at the inlet of the denitrification equipment becomes less than the planned outlet NOx amount due to DSS operation, fuel conversion, etc.
That is, when the required NH ₃ flow rate signal 29 becomes smaller than the measured NH ₃ flow rate signal 31, the NH ₃ flow rate control valve 39 is fully closed and NH ₃ is not injected into the denitrification device.

Therefore, if the conventional NH ₃ injection amount control device shown in Figure 4 is adopted for the denitrification equipment of a plant designed to reduce NOx, the drawback is that there will be a delay in denitrification response when the load increases in the case of DSS operation. There is.

In Figure 5, curve A is the boiler load, curve B is
is the NOx amount at the inlet of the denitrification device, the broken line C is the planned NOx amount at the denitrification device's outlet, the curve D is the NH ₃ injection amount, the dashed line E is the zero point of the NH ₃ injection amount, and the curve F is the measured NOx amount at the denitrification device outlet.

As shown in Figure 5, when the boiler load decreases to points G, H, and I on curve A, NOx at the inlet of the denitration equipment
The quantity decreases to points G, H, and I as shown by curve B,
The amount of NH ₃ injected also decreases to points G, H, and I as shown by curve D, and between points H and I, the amount of NH ₃ injected becomes zero as shown by curve D.

When the boiler load increases from point I to point J on curve A, the amount of NOx at the inlet of the denitration equipment increases from point I to J on curve B, and the amount of _NH3 injection also increases from point I to point J on curve D. Rise.

However, as shown by curve D in Figure 5, point H
The amount of NH ₃ injected from point I to point I is zero, and even if the amount of NH ₃ injection is increased from point I to point J as the boiler load increases, most of the NH ₃ in the initial stage of injection is adsorbed by the catalyst. Therefore, the actually measured NOx amount at the outlet of the denitrification device exhibits a response delay as shown at points K, L, and M on the curve F in FIG. 5, which is undesirable.

[Purpose of the invention]

The present invention attempts to eliminate such conventional drawbacks, and its purpose is to eliminate the response delay when the load increases, and to eliminate NOx during restart and at low loads.
It is intended to reduce the amount.

[Summary of the invention]

In order to achieve the above-mentioned object, the present invention provides a comparison means for comparing the required ammonia flow rate signal and the measured ammonia flow rate signal, and adjusts the ammonia flow rate regulating valve based on the deviation signal from the comparison means to supply the denitration catalyst to the denitrification catalyst device. The present invention is directed to an ammonia injection amount control device for a denitrification catalyst device that controls the ammonia injection amount.

A low signal selection means is provided between the comparison means and the ammonia flow rate adjustment valve, and as a result of the comparison between the required ammonia flow rate signal and the measured ammonia flow rate signal, the ammonia flow rate is such that the adjustment of the ammonia flow rate adjustment valve becomes unstable. When the valve opening signal of the regulating valve becomes small or when the valve opening signal of the ammonia flow regulating valve becomes zero, the low signal selection means selects the valve opening so that a predetermined amount of a small amount of ammonia can be injected. It is characterized in that it is composed of.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a system diagram of an NH ₃ injection amount control device according to an embodiment of the present invention, and FIG. 2 is a characteristic curve diagram of the control system of FIG. 1.

In FIG. 1, numerals 14 to 39 are the fourth
Shows the same thing as the one in the figure.

40 is a low signal selector interposed between the comparator 32 and the NH ₃ flow control valve 39;
0 is also a limiter for the amount of NH ₃ injected.

Further, symbols A to J in FIG. 2 refer to those in FIG. Points O and P indicate the amount of NH ₃ injected.

In such a structure, the difference between the NH ₃ injection amount control device shown in FIG. 4 and the NH ₃ injection amount control device of the present invention shown in FIG. 40, and the valve opening signal 35 becomes zero or the required NH ₃ injection amount signal 29
If _the deviation between the actual measured NH ₃ injection amount signal 31 and the deviation between the NH _{3 3} A flow rate adjustment valve 39 is opened to inject NH ₃ at a predetermined flow rate.

In other words, the NH ₃ injection amount control according to the present invention requires that the difference between the inlet NOx amount shown by curve B in FIG. 2 and the outlet NOx amount shown by broken line C is small, or the inlet NOx amount changes the outlet NOx amount from point H If the valve opening signal 35 from the lower proportional integrator 34 slightly opens the NH ₃ flow control valve 39 or fully closes it as shown in I, the low signal selector 40 adjusts the NH ₃ flow rate. Valve 3
The opening degree of 9 is shown in Figure 2 from point O to point P on curve D.
The specified flow rate (second flow rate) is maintained constant as shown in
The NH ₃ (hatched area in the figure) is continuously injected.

In this way, even when the load is low, the amount of excess NH ₃ is supplied to the denitrification equipment and adsorbed by the catalyst between point O and point P on curve D in Figure 2, so the boiler load is reduced by the curve in Figure 2. Even if there is a sudden rise from point I to point J at A, this surplus NH ₃ injected at low load
contributes to the denitrification reaction, and the response delay of the denitrification device is eliminated, as shown by curve F in FIG.

In addition, in the conventional NH ₃ injection amount control device,
If the deviation between the required NH ₃ injection amount signal 29 and the measured NH ₃ injection amount signal 31 is small, the adjustment of the NH ₃ flow rate control valve 39 becomes unstable, but in the NH ₃ injection amount control device of the present invention, both Even when the deviation between the signals 29 and 30 is small, the predetermined flow rate (the shaded area in FIG. 2) is always flowing, so the adjustment of the NH ₃ flow rate control valve 39 is stable.

〔Effect of the invention〕

As described above, the present invention provides a low signal selection means between the comparison means and the ammonia flow rate adjustment valve, and as a result of the comparison between the required ammonia flow rate signal and the measured ammonia flow rate signal, the adjustment of the ammonia flow rate adjustment valve becomes unstable. When the valve opening signal of the ammonia flow regulating valve becomes small to a certain degree, or when the valve opening signal of the ammonia flow regulating valve becomes zero, the valve opening is such that a predetermined amount of a small amount of ammonia can be automatically injected. Since the low signal selection means is configured in such a manner, there is less ammonia leakage, and the responsiveness of the denitrification device is excellent, so that the amount of NOx can be reduced even when the load increases, restarting, etc.

[Brief explanation of the drawing]

Fig. 1 is a control system diagram of the NH ₃ injection amount control device according to the embodiment of the present invention, Fig. 2 is a characteristic curve diagram of Fig. 1, and Fig. 3 is a typical boiler equipped with a denitrification device. FIG. 4 is a control system diagram of a conventional NH ₃ injection amount control device, and FIG. 5 is a characteristic curve diagram of FIG. 4. 29... Required NH ₃ flow rate signal, 31... Actual measurement
_NH3 flow rate signal, 32...Comparator, 39... _NH3
Flow control valve, 40...Low signal selector.

Claims (1)

[Claims] 1. Comparing means for comparing the required ammonia flow rate signal and the measured ammonia flow rate signal is provided, and the ammonia flow rate regulating valve is adjusted based on the deviation signal from the comparison means to control the amount of ammonia injected into the denitrification catalyst device. In the control device, a low signal selection means is provided between the comparison means and the ammonia flow rate adjustment valve, and as a result of the comparison between the required ammonia flow rate signal and the measured ammonia flow rate signal, adjustment of the ammonia flow rate adjustment valve becomes unstable. When the valve opening signal of the ammonia flow rate adjustment valve becomes small, or when the valve opening signal of the ammonia flow rate adjustment valve becomes zero, the valve opening is such that a predetermined amount of a small amount of ammonia can be injected. A device for controlling the amount of ammonia injected into a denitrification catalyst device, characterized in that a low signal selection means is configured.