JPS63198713A - Method for removing nitrogen oxides from diesel engine - Google Patents

Abstract

PURPOSE:To make it unnecessary to install and store NH3 as a toxic gas and enable the efficient reduction and removal of NOx in an exhaust gas by mixing the exhaust gas with an NH3 gas generated with reaction between urea and water, and causing the exhaust gas to come in contact with a TiO2 catalyst. CONSTITUTION:In the removal of NOx from a diesel engine exhaust gas, an NH3 gas generated with the reaction of water with urea is mixed with the exhaust gas. Also, the exhaust gas is made in contact with a TiO2 catalyst. According to the aforesaid system, NOx in the exhaust gas is efficiently reduced and removed. In this case, NH3 of a reducing agent is generated by the reaction of urea with water, and therefore it is unnecessary to install and store NH3 as a toxic gas.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention provides a method for removing nitrogen oxides from a diesel engine;
More specifically, the present invention relates to a method for removing nitrogen oxides (NOx) from a diesel engine, which can effectively remove nitrogen oxides (NOx) from the exhaust gas of a diesel engine using a catalyst.

[Technical background of the invention]

Nitrogen oxides in diesel engine exhaust gas less than cN
As a method of reducing M (referred to as Ox), there is a known method of preventing the generation of NOx by slightly delaying the fuel injection timing of a diesel engine and combusting the fuel (so-called timing retard). That is,
Since a large amount of NOx is generated when fuel is combusted at a high temperature, by performing timing retard as described above, the fuel is combusted at a lower temperature to suppress the generation of NOx.

However, according to this timing retard method, N
Although the generation of Ox is suppressed, there are disadvantages in that not only the combustion efficiency deteriorates but also the generation of black smoke increases.

To remove NOx as described above, a method of reducing and removing NOx generated by dust using a catalyst as in a gasoline engine may be considered.

However, compared to gasoline engines, diesel engines have more residual oxygen in their exhaust gas, which cannot be reduced using the reducing agents used in gasoline engines, such as hydrocarbons and carbon oxides. There were drawbacks.

In other words, in a gasoline engine, it is possible to burn the air-fuel at a stoichiometric mixing ratio, and the oxygen in the exhaust gas is almost zero, but in the case of a diesel engine, even at full engine load, the air-fuel equivalent is The ratio is about 1.3:1
Therefore, a large amount of residual oxygen is mixed in the exhaust gas. For this reason, even if a normal reducing agent, such as hydrogen or carbon oxide, is added to the exhaust gas in the presence of a catalyst, it will react with oxygen first, making it impossible to reduce NOx. . For this reason,
At present, a method for removing exhaust gas from a diesel engine using a catalytic reaction has not been put to practical use.

Recently, as a method for removing NOx from combustion gas from thermal power generation, a method has been developed in which NH3 is used as a reducing agent to reduce NOx from oxygen-rich combustion gas in the presence of a catalyst.

However, NO in diesel engine exhaust gas
This technique cannot be directly used to reduce x. That is, (1) in the thermal power generation, the temperature of the combustion gas is almost constant in order to reduce the combustion gas in a steady state, but in the case of a diesel engine, the temperature of the exhaust gas changes significantly depending on the engine load; ■Since the thermal power generation is in a steady state, the amount of NOx is almost constant, but in the case of a diesel engine, the amount of NOx changes depending on the engine load; ■The NHa is toxic and has a strong pungent odor. Therefore, it is necessary to strictly control the amount of NH3 in accordance with the amount of NOx so that no NH3 remains in the exhaust gas.
This is because there are various conditions such as the fact that 1 (3) is a toxic gas, so it is not desirable to have it installed in a car equipped with a diesel engine, for example.

[Summary of the invention]

The present invention was made in view of the above points, and is based on Ni13
This is a method for reducing and removing NOx from a diesel engine by using CO2 as a reducing agent and using a catalyst, and in particular removes the drawbacks of storing +013 on board among the aforementioned drawbacks. The purpose is to provide a method.

Therefore, the method for removing nitrogen oxides from a diesel engine according to the present invention can remove nitrogen oxides from diesel engine exhaust gas.
This method is characterized in that NH3 gas formed by reacting urea with water is mixed and brought into contact with a TlO2 catalyst to reduce the NOx.

According to the present invention, by using NH3 as a reducing agent and TiOs, which is active at relatively high temperatures, as a catalyst, NOx in the oxygen-rich exhaust gas of a diesel engine can be reduced and removed. It has the advantage of being a practical method for removing NOx from engines, and since the reducing agent, Nl+3, is generated by the reaction between urea and water, NH, which is a toxic gas, is
There is an advantage that there is no need to store 3 on board.

[Detailed description of the invention]

The present invention will be explained in more detail.

The method for removing nitrogen oxides from a diesel engine according to the present invention preferably detects the amount of NOx in the exhaust gas and adds a controlled amount of NH3.

The method of detecting the amount of NOx in the exhaust gas is not fundamentally limited in the present invention. For example, a map may be prepared by measuring the amount of NOx generated under changes in engine speed and load in advance, and the amount of NOx generated may be estimated and detected by measuring the engine speed and load. Good too. Furthermore, it is also possible to know the amount of NOx by detecting the fuel injection amount by taking advantage of the fact that about 2% by weight of fuel is oxidized to NO. is also possible.

As mentioned above, the amount of NOx is detected in the exhaust gas.
It is preferable to add NH3 in accordance with the amount of fJx. This N
The amount of ll3 gas added is 0.3 to 0 per mol of NOx.
．． Preferably it is 9 moles. In other words, if the amount of NH3 added is less than 0.3 mol, NOx may not be reduced sufficiently, while if it is more than 0.9 mol, unreacted Nl
This is because too much l3 gas will result in a large amount of NH3 being released into the atmosphere.

In addition, the amount of NOx generated and the reduction efficiency of the catalyst change depending on the fuel combustion temperature of the diesel engine and the temperature of the reduction reaction system, respectively.In other words, they change depending on the temperature of the exhaust gas, and are expressed as a function of the exhaust gas temperature. Since this is possible and the temperature function can be determined experimentally, the amount of NH3 added can also be determined from the following formula.

C≧Ci −Co /Ci E Here, C is the equivalent ratio of Nll3 to NOx, Ci is the NOx concentration in the exhaust gas, Co is the unreacted NOx concentration,
E is the catalyst efficiency.

As mentioned above, Ci and E can be determined experimentally as functions of the exhaust gas temperature T. According to the results of experiments conducted by the present inventors, the above-mentioned Ct and E were determined by the following experimental formula under a constant amount of NH3 added and a constant rotational speed.

If NH3 is added according to the above formula, NOx can be effectively removed.

In this way, by adding Nll3 to the exhaust gas and passing it through the catalyst, NOx is reduced and removed according to the reaction formula below.

4 NO+4 NH3 +Ot →4N t +61
120...fl)6NO* +8 Nll3
7N Il+12H20-(2) The above NH3 is
In the present invention, urea is generated by reacting water with urea without being installed in a vehicle equipped with a diesel engine. Therefore, there is no need to store and carry NH3 in a car or the like.

CO(Nib)! I+Il 20 2 Nl
l3 +C02-(31 Urea is highly hygroscopic at room temperature,
It is a non-toxic granular material and dissolves well in water. Then, by heating the aqueous solution, NH3 and CO2 are generated.

By adding this gas mixture as it is to exhaust gas, it can be used as a reducing agent. In this case, exhaust heat from a diesel engine can be used as a heating source for the urea aqueous solution.

Furthermore, since the reaction of formula (3) can also occur in the presence of a dilute acid or a dilute alkali, a small amount of acid or alkali may be added to the aqueous solution.

As mentioned above, the generated NH3 contains CO2, but it is clear that this CO2 does not need to be separated in the present invention and can be mixed with the exhaust gas as it is. In this case, as is clear from the above formula (3), the NH3 ti is about 2/3 of the generated gas, and is added to the exhaust gas taking this ratio into consideration.

In the present invention, a TiO2-based catalyst is used as a catalyst for causing such a reaction. For example, noble metal catalysts such as Pi and Pd also cause the above reaction, and such catalysts are characterized by having good activity at relatively low temperatures of 200 to 300°C. However, at such low temperatures, the amount of NOx in the exhaust gas is limited (if the fuel is burned at a relatively low temperature, the amount of NOx produced is small), and there is little need to reduce and remove NOx.

Therefore, in the present invention, a TiO2-based catalyst which is inexpensive and active at relatively high temperatures is used.

This is because when the exhaust gas is at a relatively high temperature, even if carbon is adsorbed on the TiOg-based catalyst, it will be burned by heat and will not reduce the activity of the catalyst.

Such Tjo 2 catalyst is active at relatively high temperatures of 200 to 500°C. Therefore, the temperature of the exhaust gas treated in the catalyst is between 250 and 500.
Adjust to ℃. If the temperature of the exhaust gas is less than 250℃, the activity of the catalyst is low and there is a risk that the NOx cannot be reduced sufficiently.On the other hand, if the temperature exceeds 500℃, the catalyst may deteriorate and lose its activity. It is from.

In the case of a diesel engine, the exhaust gas is 70
Although the temperature may reach as high as 0°C, in this case, the temperature is lowered by means such as mixing air, etc., and the exhaust gas is converted to TiO1! Contact with catalyst. On the other hand, when the exhaust gas temperature is less than 200℃, the amount of NOx is
Since the concentration is relatively low, it may be discharged into the atmosphere without contacting the TiO2 catalyst.

〔Effect of the invention〕

As explained above, according to the method for removing nitrogen oxides from a diesel engine according to the present invention, by using TiO2 as a catalyst and NHa as a reducing agent,
This method has the advantage of being able to effectively remove nitrogen oxides from diesel engine exhaust gas, which was previously impossible. Therefore, there is no need to delay the injection timing as in the conventional method, and the fuel can be burned efficiently, and there are also advantages in that the generation of black smoke can be suppressed.

Furthermore, according to the present invention, since NH3 is generated by basically using non-toxic urea and decomposing it in an aqueous solution, there is no need to store and carry NH3 in a tank, etc., and the reduction system It has the advantage of being easier and safer to handle.

Claims (1)

[Claims] (1) NH_3 gas formed by reacting urea with water is mixed into the exhaust gas of a diesel engine, and
A method for removing nitrogen oxides from a diesel engine, which comprises bringing the NOx into contact with a TiO_2 catalyst to reduce the NOx.