JP5911350B2 - Exhaust gas purification method and apparatus - Google Patents

Description

  The present invention relates to an exhaust gas purification method and apparatus for supplying urea into a combustion exhaust gas as a reducing agent and catalytically reducing nitrogen oxides in the exhaust gas in the presence of a denitration catalyst.

  Nitrogen oxides (NOx) contained in the combustion exhaust gas are required to be reduced as much as possible because they may damage human health and the environment. As a method for removing nitrogen oxides from combustion exhaust gas, a method is known in which ammonia is added to the exhaust gas and passed through a catalyst in which vanadium is supported on a titanium oxide carrier, and nitrogen oxide and ammonia are reacted to detoxify nitrogen. (Patent Document 1). This method is widely used as a selective catalytic reduction method (ammonia denitration method) using ammonia as a reducing agent in thermal power plants and municipal waste incineration plants.

  Ammonia has a strong odor and is harmful, so if it leaks in large quantities, it may lead to a disaster. Therefore, a safer reducing agent that can be applied to small-scale facilities is required, urea denitration using urea as a reducing agent, which is stable in an aqueous solution and rapidly decomposes in high-temperature exhaust gas to produce ammonia. A method was developed (Non-Patent Document 1). As a result, a selective catalytic reduction method using ammonia as a reducing agent is becoming widespread even in distributed power generation devices such as cogeneration.

  Urea generates two molecules of ammonia and one molecule of carbon dioxide from one molecule of urea by the hydrolysis reaction of Formula 1. However, a temperature of 400 ° C. or higher is necessary in the gas phase in order for urea to be completely decomposed within a time allowed for practical use (Non-Patent Documents 1 and 2). It is known that the catalyst decomposes at a temperature lower than this, but at a low temperature, the rate is very slow and practical denitration performance cannot be obtained. The reality is that the system is stopped and denitration is not performed (Patent Document 2).

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ (Formula 1)

  In general, exhaust gas from an internal combustion engine rarely has an exhaust gas temperature of 200 ° C. or lower, whereas in an industrial furnace or the like, the exhaust gas temperature may be low depending on operating conditions. In recent years, exhaust heat recovery in industrial furnaces has progressed to the limit from the viewpoint of energy saving in the industrial field and reduction of carbon dioxide emission, and the exhaust gas temperature may be 200 ° C. or lower. The actual situation is that a method for applying the urea denitration method to such low-temperature exhaust gas has not been established.

  In the case of using ammonia as a reducing agent, in order to obtain an appropriate denitration effect at the time of start-up, a temperature sensor that measures the temperature of exhaust gas flowing into the denitration device, and a temperature measured by the temperature sensor are determined by A control device for a denitration apparatus that includes a processing means for starting the denitration apparatus when the temperature is equal to or higher than a performance temperature that is necessary for exhibiting performance is known (Patent Document 2). However, this document does not teach any countermeasure against the above-mentioned problems when performing denitration using urea as a reducing agent in a temperature range of 200 ° C. or lower.

  Based on the fact that ammonia is adsorbed on the denitration catalyst, several methods have also been proposed for controlling the amount of reducing agent supplied in consideration of the amount of adsorbed ammonia on the catalyst according to fluctuations in the exhaust gas amount and temperature. (Patent Documents 3 to 6). However, these are only based on the knowledge that the amount of adsorbed ammonia on the catalyst increases as the temperature decreases, and there are no problems regarding the above-mentioned problems and countermeasures when performing denitration using urea as a reducing agent in a temperature range of 200 ° C. or lower. Not teaching.

JP-A-50-51966 JP 2009-119306 A JP-A-9-75665 JP 2002-177741 A JP 2002-186833 A JP 2003-10645 A

Takahiro Tsuji, Akira Kato, Toshio Yamashita, Journal of the Chemical Society of Japan, No. 8, page 812 (1992) O. Kroecher, studies in surface science and catalysis, 171 261 (2007)

As described above, conventionally, a method for performing catalytic reduction of nitrogen oxides using urea as a reducing agent at a low temperature of substantially 200 ° C. or lower has not been established.
The inventor has advanced experimental and theoretical investigations. As a result, urea adsorbed on the catalyst is gradually decomposed even at a low temperature of 200 ° C. or lower, so that nitrogen oxides can be catalytically reduced using urea water. I found it. However, since the decomposition rate is extremely slow and the temperature dependency of the decomposition rate is large, there is a problem that leaked ammonia increases or the denitration rate decreases significantly under conditions where the temperature fluctuates.

  That is, the conventional exhaust gas purification method is considered to work effectively when applied to high temperature exhaust gas with high urea reactivity, but even if urea is added to low temperature exhaust gas with low urea reactivity, Immediately, effective denitration efficiency could not be obtained, and when the temperature of the exhaust gas rose, there was a problem that ammonia might be released unreacted.

  Therefore, the present invention provides a sufficient denitration rate even when operating conditions such as exhaust gas temperature and exhaust gas flow fluctuate when performing catalytic reduction of nitrogen oxides using urea as a reducing agent even at a low temperature of 200 ° C. or lower. It is another object of the present invention to provide an exhaust gas purification method and apparatus that can reduce ammonia (leak ammonia) that has been unreacted as much as possible.

  When the inventors proceeded with the analysis on the above-mentioned problems, the following situation became clear.

If the exhaust gas temperature (or the surface temperature of the catalyst) is constant, a certain amount of urea will be deposited on the catalyst, and the denitration rate will be constant when the amount of urea deposited reaches a balance between the reduction amount and the supply amount due to its decomposition. It becomes.
Here, when the temperature of the exhaust gas is low, the decomposition rate of urea decreases, so that the amount of ammonia that becomes a reducing agent is insufficient, and the denitration rate decreases.
If this state is maintained, the urea deposition amount increases to the amount where the decrease amount due to decomposition and the supply amount are balanced at the temperature after the decrease as time elapses.

  Therefore, even if the exhaust gas temperature decreases, the NOx removal rate will eventually recover due to the urea deposition on the catalyst. Therefore, sufficient denitration cannot be performed.

  On the other hand, when the exhaust gas temperature rises from a state in which the amount of urea deposited on the catalyst has increased, the urea decomposition rate begins to increase rapidly, and the urea deposited on the catalyst begins to decompose rapidly, More ammonia than necessary for this reaction is produced.

  That is, since the ammonia produced in this way does not contribute to the denitration reaction, the rise in the exhaust gas temperature has led to the generation of leaked ammonia.

Explaining with specific examples, the decomposition rate of urea in the case of using a commercially available ammonia denitration catalyst (supporting vanadium oxide and tungsten oxide as active components on a TiO ₂ carrier) is as shown in FIG. . At 250 ° C, urea decomposes within 2 minutes, but at 200 ° C it takes 11 minutes and at 170 ° C it takes about 45 minutes. That is, at 170 ° C. and 200 ° C., urea corresponding to the urea supply amount for 45 minutes and 11 minutes is deposited, and the denitration rate reaches equilibrium. Therefore, when the exhaust gas temperature rises from 170 ° C. to 200 ° C. within a few minutes, a large amount of ammonia leaks. On the other hand, when the exhaust gas temperature falls from 200 ° C. to 170 ° C. in a few minutes, the state in which the denitration rate is reduced continues for several tens of minutes.

[Configuration 1]
The characteristic configuration of the exhaust gas purification method of the present invention for achieving the object completed based on the above knowledge is as follows:
An exhaust gas purification method for supplying urea into a combustion exhaust gas as a reducing agent and catalytically reducing nitrogen oxides in the exhaust gas in the presence of a denitration catalyst,
When the exhaust gas temperature is in the range of 150 to 250 ° C., the urea supply amount is decreased as the time differential value of the exhaust gas temperature is larger with respect to the urea supply amount when the exhaust gas temperature is constant. The smaller the differential value is, the more the urea supply amount is controlled to increase.

[Operation effect 1]
According to this invention, since the exhaust gas temperature is in the low temperature range of 150 to 250 ° C., urea is likely to be deposited. When the time differential value of the exhaust gas temperature increases from this situation, the exhaust gas temperature tends to rise, so that urea deposited on the catalyst begins to decompose rapidly and the amount of ammonia produced starts to increase. If the urea supply amount is reduced at this timing, it is possible to offset the increase in ammonia generated from the urea deposited on the catalyst and the decrease in ammonia generated from the added urea. An increase in leaked ammonia due to an increase in the decomposition rate can be suppressed.

  Conversely, if the time derivative of the exhaust gas temperature decreases, the exhaust gas temperature tends to decrease, so the decomposition rate of urea decreases, urea begins to deposit on the catalyst, and ammonia is produced. The amount will decrease. If the amount of urea supplied at this timing is increased, the increase in the amount of ammonia produced from the added urea can compensate for the decrease in the amount of ammonia produced due to the decrease in the decomposition rate, thereby suppressing the decrease in the denitration rate. become able to.

  When the exhaust gas temperature has a larger time differential value, the urea supply amount is decreased. When the exhaust gas temperature has a smaller time differential value, the urea supply amount is increased. The supply amount may be linearly increased or decreased according to a linear relational expression, or may be changed in a quadratic or higher curve. Further, it may be varied step by step. Further, the relational expression may be varied according to the temperature region, the positive / negative of the time differential value, the absolute value, or the like.

[Configuration 2]
The characteristic configuration of the exhaust gas purifying apparatus of the present invention is as follows:
An exhaust gas purification device that supplies urea as a reducing agent into combustion exhaust gas, and catalytically reduces nitrogen oxides in the exhaust gas in the presence of a denitration catalyst,
It is equipped with a means for detecting the exhaust gas temperature and a urea water supply device for adding urea water to the exhaust gas,
When the exhaust gas temperature is in the range of 150 to 250 ° C., the time derivative of the exhaust gas temperature is calculated, and the time derivative of the exhaust gas temperature based on the urea supply amount when the exhaust gas temperature is constant according to this value When the value is large, the urea supply amount is decreased, and when the time differential value of the exhaust gas temperature is small, a calculation means for obtaining the urea supply amount so as to increase the urea supply amount is provided ,
The urea water supply device is configured to add urea water of the urea supply amount obtained by the calculation means to the exhaust gas .

[Operation effect 2]
Since the exhaust gas purification device includes the exhaust gas temperature detection means and the urea water supply device that adds urea water to the exhaust gas, the supply amount of urea water can be set based on the exhaust gas temperature.

  At this time, when the exhaust gas temperature is in the range of 150 to 250 ° C., a time differential value of the exhaust gas temperature is calculated, and according to this value, the exhaust gas temperature is based on the urea supply amount when the exhaust gas temperature is constant. When the time differential value is large, the urea supply amount is decreased, and when the exhaust gas temperature time differential value is small, the urea water supply amount is set so as to increase the urea supply amount. The exhaust gas purification method can be executed, and the exhaust gas purification method can be performed following the change in the temperature of the combustion exhaust gas by determining and controlling the supply amount by the calculation means.

[Configuration 3]
The denitration catalyst may be a denitration catalyst in which vanadium and tungsten are supported on a titanium oxide support.

[Operation effect 3]
The denitration catalyst has a high ability to catalytically reduce nitrogen oxides in exhaust gas using urea as a reducing agent, and easily exhibits high denitration efficiency by the exhaust gas purification method.

  Therefore, even if it is exhaust gas in a low temperature region, when performing catalytic reduction of nitrogen oxides using urea as a reducing agent, when operating conditions such as exhaust gas temperature and exhaust gas flow fluctuate, a sufficient denitration rate is obtained, Leak ammonia can be reduced.

Graph showing temperature dependence of urea decomposition rate Schematic diagram of exhaust gas purification device Graph showing exhaust gas temperature change of combustion equipment The graph which shows the denitration rate change by the urea water supply amount non-control with respect to the exhaust gas temperature change in FIG. The graph which shows the urea water supply amount control form with respect to the exhaust gas temperature change in FIG. The graph which shows the denitration rate change at the time of performing urea water supply amount control of FIG. 5 with respect to the exhaust gas temperature change in FIG. Graph showing exhaust gas temperature change of combustion equipment Graph showing change in denitration rate by urea water supply control with respect to exhaust gas temperature change, broken line shows change in denitration rate without control, and solid line shows change in denitration rate when control of FIG. 5 is performed Graph showing denitration rate change without control of urea water supply with respect to exhaust gas temperature change Graph showing denitration rate change when urea water supply control is performed against exhaust gas temperature change

  The exhaust gas purification method and apparatus of the present invention will be described below. In addition, although suitable examples are described below, these examples are described in order to more specifically illustrate the present invention, and various modifications can be made without departing from the spirit of the present invention. The present invention is not limited to the following description.

[Exhaust gas purification equipment]
As shown in FIG. 2, the exhaust gas purification device is provided in an exhaust gas passage 2 for discharging exhaust gas from the combustion device 1, and a urea water supply device 3 that supplies urea water to the exhaust gas flowing through the exhaust gas passage 2. And a denitration catalyst 4 for detoxifying ammonia produced by the decomposition reaction of urea contained in the urea water with nitrogen oxides contained in the exhaust gas, and the exhaust gas supplied to the denitration catalyst 4 A temperature sensor 5 as a detecting means for detecting the temperature, and a time differential value of the exhaust gas temperature is calculated based on the output from the temperature sensor 5, and according to this value, the urea supply amount when the exhaust gas temperature is constant is calculated. As a reference, calculating means for obtaining a urea supply amount that decreases the urea supply amount as the exhaust gas temperature time differential value increases and increases the urea supply amount as the exhaust gas temperature time differential value decreases. Provided with a 1, a control device 6 for controlling the supply amount of urea water based on the urea supply amount determined.

[Combustion device]
The combustion exhaust gas targeted by the present invention is not limited to the type of fuel or the type of the combustion apparatus 1, but is preferably the exhaust gas of the combustion apparatus 1 that uses gaseous fuel such as natural gas or liquefied petroleum gas as fuel. This is because when performing a denitration reaction in a low temperature region of 250 ° C. or lower, if the SOx concentration in the exhaust gas is high, clogging of the catalyst due to precipitation of acidic ammonium sulfate (NH ₄ HSO ₄ ) may occur. This is because gaseous fuel has less S in the fuel and less concern.
The SOx concentration in the combustion exhaust gas needs to be 3 ppm or less, preferably 1 ppm or less.
If the oxygen concentration in the combustion exhaust gas is too low, the progress of the denitration reaction becomes slow, so that it is preferably 1% or more on a volume basis, and is usually in the range of 2 to 15%.

[Denitration catalyst]
The denitration catalyst 4 used in the present invention is preferably a denitration catalyst formed by supporting vanadium and tungsten as active metals on a titanium oxide support. In addition, various well-known things are applicable about a support | carrier, an active metal, and the additional component added additionally. The shape of the catalyst may be any shape such as a pellet or a honeycomb, but a honeycomb shape is preferable in order to reduce pressure loss. As such a denitration catalyst 4, a commercially available catalyst for ammonia denitration can be used.

[Urea water supply device]
The urea water supply device 3 includes a urea water tank 31 that stores urea water, a spray unit 32 that sprays urea water on the exhaust gas passage 2, and urea water is supplied from the urea water tank 31 to the spray unit 32. A supply valve 34 is provided in the supply supply path 33, and the supply valve 34 is configured to be capable of opening degree adjustment control based on a control signal from the control device 6.

Urea used as the reducing agent is supplied to the exhaust gas flow path upstream of the denitration catalyst 4 in the form of urea water. If the urea concentration in the urea water is too high, it may solidify and block the flow path. If it is too low, the required amount will increase and the economic efficiency will decrease, so it should be about 10-40% on a mass basis. preferable.
The reference urea supply amount (supply amount when there is no fluctuation in exhaust gas temperature) A (mole / second) is a nitrogen oxide flow rate B per unit time determined from the exhaust gas flow rate and the nitrogen oxide concentration in the exhaust gas ( Mol / second) to a value of 30 to 45%. As this value is increased, the denitration rate is improved, but there is a concern that the amount of ammonia leakage increases.

[Detection means]
The exhaust gas temperature can be measured by a temperature sensor 5 such as a thermocouple inserted in the exhaust gas passage. The temperature sensor 5 is preferably provided as close to the denitration catalyst 4 as possible, and may be inserted into the denitration catalyst 4, but may be provided immediately before or immediately after the layer of the denitration catalyst 4.

〔Control device〕
The control device 6 receives the output of the temperature sensor 5 and calculates the time differential value of the exhaust gas temperature from the change over time, and the urea is calculated from the relational expression between the time differential value of the exhaust gas temperature stored in advance and the urea water supply amount. The calculation means 61 for obtaining the water supply amount is provided, and the control is performed to adjust the opening degree of the supply valve 34 based on the obtained urea water supply amount.

  Hereinafter, the effect of suppressing variation in the denitration rate by the change control of the urea water supply amount will be described.

[Control of urea water supply]
In the following example, it is assumed that the amount of the denitration catalyst 4 is sufficient to complete the denitration reaction, and nitrogen oxides are quantitatively reduced according to the amount of ammonia generated by urea decomposition. In addition, it is assumed that excess ammonia larger than the amount of nitrogen oxides is released as leaked ammonia, and the decomposition rate of urea follows FIG.

  When the ratio of the urea supply amount A to the nitrogen oxide flow rate B contained in the exhaust gas is 35%, the denitration rate in the steady state is 70%. Here, even if the exhaust gas flow rate and the nitrogen oxide concentration in the exhaust gas are constant, if the exhaust gas temperature varies as shown in FIG. 3, the denitration rate varies greatly as shown in FIG. This is because the decomposition rate of urea accumulated on the catalyst is slowed in the process where the exhaust gas temperature is lowered, so that the production amount of ammonia is reduced, and in the process where the exhaust gas temperature is raised, it is accumulated on the catalyst. This is because the production rate of ammonia increases because the decomposition rate of urea increases.

  On the other hand, the case where the urea supply amount is changed as shown in FIG. 5 according to the fluctuation of the exhaust gas temperature will be considered. The solid line in FIG. 5 shows the urea supply amount with respect to the time differential value of the temperature change. Usually, the urea supply amount is constant over the entire range (0.35: When the exhaust gas temperature is constant (the time differential value of the temperature change is In the case of 0), a value that achieves 70% as the denitration rate) is maintained, and it is assumed that the urea supply amount is controlled so as to increase as the time differential value of the temperature change decreases and decrease as the time differential value increases. To do. That is, when the exhaust gas temperature decreases at a rate of 2 ° C. per minute, the ratio (molar ratio) of urea supply to the nitrogen oxide flow rate in the exhaust gas is set to 52%, and the exhaust gas temperature increases at a rate of 2 ° C. per minute. In this case, the ratio (molar ratio) of the urea supply amount to the nitrogen oxide flow rate in the exhaust gas is set to 18%.

  It should be noted that the change rate of the urea supply amount is changed small (the inclination in FIG. 5 is small), the effect of suppressing the reduction of the denitration rate when the exhaust gas temperature is lowered tends to be insufficient, and the leakage is increased when the exhaust gas temperature is increased. The suppression effect of ammonia tends to be insufficient. Conversely, the greater the change (the greater the slope in FIG. 5), the lower the denitration rate when the exhaust gas temperature decreases, but if it is too large, the urea adsorption amount on the catalyst will increase rapidly, When the exhaust gas temperature is restored, the amount of ammonia generated increases rapidly, so that the problem of leaked ammonia is likely to occur. Therefore, the change rate of the urea supply amount is ensured to be a sufficient change amount for suppressing the decrease in the denitration rate when the change rate of the urea supply amount is negative, and the leakage ammonia is changed when the change rate of the urea supply amount is positive. It is desirable to set a change amount that is difficult to generate.

  Note that FIG. 5 shows an example, and the slope (the fluctuation range of the urea supply amount with respect to the time differential value of the exhaust gas temperature) may be changed according to the exhaust gas temperature. In that case, the higher the exhaust gas temperature is, It is appropriate to reduce the inclination.

  When the urea supply amount is changed with respect to the time differential value of the temperature change shown in FIG. 5, the change of the denitration rate is as shown in FIG. From this result, in the case of no control (FIG. 4), the denitration rate may be lower than 40%, but even if the denitration rate is lowered, it hardly falls below 40%, even when the exhaust gas temperature is lowered. It can be seen that a sufficient denitration rate can be maintained. In addition, leak ammonia (denitration rate exceeding 100% at 40 minutes in the graph corresponds to leak ammonia) is hardly generated even when the exhaust gas temperature is restored. From these, it was found that the exhaust gas properties can be maintained satisfactorily according to the control method of the present invention.

  Even when the exhaust gas temperature changes more complicatedly, the method of the present invention is effective. FIG. 8 shows the change in the denitration rate when the exhaust gas temperature is changed as shown in FIG. 7 and when the urea supply amount is controlled to be constant and when the exhaust gas temperature is changed as shown in FIG. 5 according to the method of the present invention. When the urea supply amount is kept constant (broken line in the figure), when the exhaust gas temperature rises, the leak ammonia concentration increases and reaches a maximum of 50% of the nitrogen oxide concentration in the exhaust gas. Further, when the exhaust gas temperature was lowered, the denitration rate was reduced to 27% at the lowest point. On the other hand, when the urea supply amount is controlled according to the method of the present invention (solid line in the figure), the leaked ammonia concentration is at most 3% of the nitrogen oxide concentration in the exhaust gas, and the denitration rate is also below 38%. It never happened. From this, it was found that according to the control method of the present invention, even when the exhaust gas temperature is unstable, it can be sufficiently dealt with.

  The effects of the present invention will be shown by specific experimental examples below.

Comparative Example 20% urea water was added at a flow rate of 1.6 g / min to the exhaust gas (flow rate: 135 Nm ³ / h, NOx concentration 120 ppm, oxygen concentration 8%) of a city gas fuel combustion furnace, and vanadium and tungsten were added to the titania carrier. Was passed through 45 liters of a denitration catalyst (SCN. 207 manufactured by Sakai Chemical Industry).

  The exhaust gas temperature measured at the catalyst inlet was initially 200 ° C. (0 to 8 minutes), but decreased to 170 ° C. (8 to 28 minutes) over about 20 minutes, and then maintained at around 170 ° C. for about 20 minutes ( 28 to 48 minutes), and then returned to 200 ° C. over about 20 minutes (48 to 68 minutes). The transition of the exhaust gas temperature and the NOx concentration at the catalyst outlet at this time is shown in FIG. Initially, the NOx concentration, which was a little over 10 ppm, gradually increased, and while the exhaust gas temperature was 170 ° C., it slightly changed, but remained at about 40 ppm (denitration rate of about 67%). When the exhaust gas temperature started to rise, the NOx concentration rapidly decreased.

Example A catalyst was prepared by adding 1.6 g / min of urea water as in the comparative example except that the flow rate of 20% urea water was 2.4 g / min during the temperature drop and 0.8 g / min during the temperature rise. The NOx concentration on the outlet side was measured. Specifically, at the initial stage when the exhaust gas temperature was stable at 200 ° C., urea water of 1.6 g / min was added, and then the exhaust gas temperature gradually decreased to 170 ° C. for 8 minutes to 28 minutes. 2.4 g / min, the exhaust gas temperature is stabilized at 170 ° C., from 28 minutes to 48 minutes, 1.6 g / min, and then the exhaust gas temperature rises to return to 200 ° C. 48 minutes to 68 minutes In the meantime, the control of adding urea water at 0.8 g / min was performed. The results are shown in FIG. Unlike the comparative example, the NOx concentration remained at 30 ppm (denitration rate of about 75%) even at the peak, indicating that the denitration efficiency was maintained high. Further, at this time, leaked ammonia is not detected, and it is obvious that the effect of stably maintaining the denitration rate can be obtained by controlling the urea supply amount according to the method of the present invention.

  The exhaust gas purification method of the present invention can be used, for example, as an exhaust gas purification device for reducing nitrogen oxides contained in exhaust gases such as gas engines and boilers.

DESCRIPTION OF SYMBOLS 1: Combustion apparatus 2: Exhaust gas path 3: Urea water supply apparatus 31: Urea water tank 32: Spray part 33: Supply path 34: Supply valve 4: Denitration catalyst 5: Temperature sensor 6: Control apparatus 61: Calculation means

Claims (3)

An exhaust gas purification method for supplying urea into a combustion exhaust gas as a reducing agent and catalytically reducing nitrogen oxides in the exhaust gas in the presence of a denitration catalyst,
When the exhaust gas temperature is in the range of 150 to 250 ° C., the urea supply amount is decreased as the time differential value of the exhaust gas temperature is larger with respect to the urea supply amount when the exhaust gas temperature is constant. An exhaust gas purification method for controlling the urea supply amount so as to increase the urea supply amount as the differential value is smaller. An exhaust gas purification device that supplies urea as a reducing agent into combustion exhaust gas, and catalytically reduces nitrogen oxides in the exhaust gas in the presence of a denitration catalyst,
It is equipped with a means for detecting the exhaust gas temperature and a urea water supply device for adding urea water to the exhaust gas,
When the exhaust gas temperature is in the range of 150 to 250 ° C., the time derivative of the exhaust gas temperature is calculated, and the time derivative of the exhaust gas temperature based on the urea supply amount when the exhaust gas temperature is constant according to this value the higher the value, to reduce the urea supply amount, the more the time differential value of the exhaust gas temperature is low, an arithmetic means for obtaining the urea supply amount so as to increase the urea supply amount,
An exhaust gas purification device configured such that the urea water supply device adds urea water of the urea supply amount obtained by the calculating means to the exhaust gas.   The exhaust gas purification apparatus according to claim 2, wherein the denitration catalyst is formed by supporting vanadium and tungsten on a titanium oxide support.

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