The present invention relates to an exhaust emission control device applied to an engine such as diesel engine.
Conventionally some diesel engines have been provided with selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a characteristic of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the selective reduction catalyst and is reacted with NOx (nitrogen oxides) in exhaust gas on the catalyst to thereby reduce a concentration of the discharged NOx.
Meanwhile, it is well known in a field of industrial flue gas denitration in a plant or the like that ammonia (NH3) has effectiveness as a reducing agent for reduction and purification of NOx. However, in a field of automobile, guaranteed safety is hard to obtain with respect to travel with ammonia itself being loaded, so that researches have been made nowadays on use of nontoxic urea water as reducing agent (see, for example, Reference 1).
More specifically, when added to the exhaust gas upstream of the selective reduction catalyst, the urea water is pyrolytically decomposed by heat of the exhaust gas into ammonia and carbon dioxide according to the following equation, and NOx in the exhaust gas on the catalyst is satisfactorily reduced and purified by the ammonia generated.
(NH2)2CO+H2O→2NH3+CO2
It has been experimentally ascertained that, by adding ammonia to selective reduction catalyst in such kind of exhaust emission control device, NOx reduction effect will be obtained, provided that exhaust temperature exceeds about 140° C.; however, pyrolytical decomposition of urea water into ammonia and carbon dioxide requires exhaust temperature of at least about 170-180° C. Thus, if an operating status with exhaust temperature of lower than about 200° C. continues (generally speaking, low-load operational areas are areas with low exhaust temperature), there is a problem that NOx reduction ratio is hardly to enhance since decomposition of urea water into ammonia does not proceed well. For example, in a vehicle such as city shuttle-bus with travel pattern of almost always traveling on congested roads, an operation with more than required exhaust temperature does not continue for a long time, and operational transitions occur with no chance of NOx reduction ratio being enhanced, failing in obtaining satisfactory NOx reduction effect.
The invention was made in view of the above and has its object to provide an exhaust emission control device which can obtain satisfactory NOx reduction effect even at exhaust temperature lower than that required conventionally therefor and even in a vehicle with travel pattern of continuing operational status with low exhaust temperature, which can effectively generate ammonia from urea water and which can enhance controllability in adding ammonia to the exhaust gas.
The invention is directed to an exhaust emission control device with selective reduction catalyst incorporated in an exhaust pipe, ammonia being added upstream of the catalyst so as to reduce and purify NOx, said exhaust emission control device comprising an ammonia generator with a vessel for holding urea water and with an electrode for generation of ammonia through action of plasma on the urea water in the vessel, the ammonia generated in the ammonia generator being fed upstream of the catalyst.
According to the above means, the ammonia generated through action of the plasma on the urea water in the ammonia generator is fed upstream of the selective reduction catalyst, so that a required amount of ammonia can be surely added to the exhaust gas even in an operational status with low exhaust temperature to thereby be effectively reacted with NOx in the exhaust gas on the selective reduction catalyst; as a result, NOx in the exhaust gas is satisfactorily reduced and purified even at exhaust temperature lower than that required conventionally therefor. Generation of ammonia can be easily and rapidly adjusted since the ammonia is generated through action of plasma on the urea water; and response in feeding the ammonia can be enhanced since the generated ammonia is added to the exhaust gas.
It is preferable in the exhaust emission control device that dielectric pellets are charged in the urea water in the vessel. Such charging of the dielectric pellets in the urea water brings about generation of plasma on surfaces of the pellets, thereby further effectively enhancing the action of generating ammonia from the urea water.
In the exhaust emission control device, ammonia gas may be taken out from the ammonia generator. Addition of such ammonia gas to the exhaust gas causes no trouble of lowering the exhaust temperature, so that NOx reduction effect of the selective reduction catalyst in an operational status with low exhaust temperature can be further enhanced.
In the exhaust emission control device, ammonia water may be taken out from the ammonia generator. Addition of such ammonia water to the exhaust gas substantially causes no trouble of lowering the exhaust temperature, though subtle heat may be taken upon evaporation of the water. Thus, NOx reduction effect of the selective reduction catalyst in an operational status with low exhaust temperature can be highly maintained.
In the exhaust emission control device, a pH meter may be arranged which detects concentration of ammonia taken out from the vessel and a controller may be arranged which outputs a command on amount of ammonia to be fed upstream of the selective reduction catalyst on the basis of detected value from the pH meter, whereby actual amount of ammonia to be fed to the exhaust gas can be controlled with high response.
The above-mentioned exhaust emission control device of the invention has effects and advantages. Ammonia is effectively generated through action of plasma on urea water in an ammonia generator and is fed upstream of the selective reduction catalyst so that, unlike the conventional supply of urea water, a required amount of ammonia can be surely added to exhaust gas without lowering in temperature of the exhaust gas; thus even in an operational status with low exhaust temperature, NOx can be effectively reduced by the selective reduction catalyst. Because of ammonia being generated through action of the plasma on the urea water, the generation of the ammonia can be easily and rapidly adjusted; because of the generated ammonia being added to the exhaust gas, response in feeding the ammonia to the exhaust gas can be enhanced.
Embodiments of the invention will be described in conjunction with drawings.
Exhaust gas discharged from the respective cylinders of the engine 1 is fed via an exhaust manifold 8 to a turbine 2b of the turbocharger 2. The exhaust gas 7 thus having driven the turbine 2b is discharged via an exhaust pipe 9 to outside of the vehicle.
Incorporated in the exhaust pipe 9 through which the exhaust gas 7 flows is a selective reduction catalyst 10 encased by a casing 11. The selective reduction catalyst 10 is in the form of a flow-through type honeycomb structure and has a feature capable of selectively reacting NOx with ammonia even in the presence of oxygen.
In the above construction, the exhaust pipe 9 is provided with a spray nozzle 14 upstream of the casing 11, said nozzle injecting ammonia 13 generated in an ammonia generator 12 to add the same to the exhaust gas 7.
In
In the embodiment shown in
Inserted into and opened in the vessel 15 adjacent to the bottom thereof is a lower end of a urea water feed pipe 20 which serves to feed urea water 23a in a urea water tank 23 arranged above the vessel 15 into the vessel 15 via a urea water feed valve 21.
The electrode 16 is connected with power wire 25 which in turn is connected to a power source 24 such as battery. The power wire 25 is provided with a controller 26 for control of voltage, driving pulse and the like. Thus, energization of the electrode 16 by the power source 24 generates plasma between the electrode 16 and casing 17, such plasma acting on the urea water 23a for decomposition into ammonia and carbon dioxide.
Opened to space 27 in the vessel 15 and above a liquid level of the urea water 23a is an ammonia feed pipe 28 which is connected via a pump 29 and an ammonia feed valve 30 to the spray nozzle 14. Thus, in this embodiment, the ammonia gas 13a generated in the space 27 of the vessel 15 is taken out through the ammonia feed pipe 28 and fed to the spray nozzle 14. In this connection, if the space 27 is low in volume, the ammonia gas 13a may have difficulty to stably feed; therefore, as shown in
In
The controller 32 outputs an electricity control command 36 to control the controller 26 such that the electricity fed to the electrode 16 has predetermined voltage and drive pulse.
Further inputted to the controller 32 is a detected pH value 38 from a pH meter 37 which detects pH of the urea water 23a in the vessel 15 (pH adjacent to the liquid level of the urea water 23a). Thus, depending upon the detected pH value 38 from the pH meter 37, the controller 32 outputs ammonia feed command 39 to control an opening degree of the ammonia feed valve 30 to control the flow rate of the ammonia gas 13a fed to the spray nozzle 14. More specifically, the controller 32 and an engine control computer (ECU: Electronic Control Unit) (not shown) exchange data such as revolution speed and load of the engine 1, detected temperatures of inlet and outlet temperature sensors 42a and 42b for the selective reduction catalyst 10 and intake air amount; on the basis of a current operational status judged from such data, an amount of NOx generated is presumed. An amount of the ammonia gas 13a to match the presumed generation amount of NOx is calculated so that the required amount of ammonia gas 13a is added to the exhaust gas 7.
Next, mode of operation of the above embodiments will be described.
As shown in
(NH2)2CO+H2O→2NH3+CO2
into ammonia and carbon dioxide. As a result, the urea water 23a in the vessel 15 is changed into ammonia water while the ammonia gas 13a is generated in the upper space 27 of the vessel 15, the ammonia gas 13a containing carbon dioxide and evaporated water.
As mentioned above, in the ammonia generator 12, the plasma acts on the urea water 23a for decomposition into ammonia gas 13a, so that the ammonia gas 13a can be generated easily and quickly.
Further, in this connection, when the dielectric pellets 19a made of material with high dielectric constant such as titania, barium titanate or alumina are charged in the urea water 23a in the vessel 15, plasma is generated on respective surfaces of the pellets 19a, which substantially enhance decomposition reaction of the urea water 23a, resulting in effective generation of the ammonia gas 13a. With the plural ammonia generators 12 being arranged as shown in
In the above, by driving the pump 29, the ammonia gas 13a generated in the vessel 15 is taken out through the ammonia feed pipe 28 and is injected by the spray nozzle 14 upstream of the selective reduction catalyst 10 to be added to the exhaust gas 7 in the exhaust pipe 9.
Then, the controller 32 and the engine control computer (not shown) exchange data such as revolution speed and load of the engine 1, detected temperatures by the inlet and outlet temperature sensors 42a and 42b for the selective reduction catalyst 10 and intake air amount to thereby detect the current operational status, so that a generation amount of NOx is presumed on the basis of the detected operational status. An amount of the ammonia gas 13a to match the presumed generation amount of NOx is calculated and the ammonia feed valve 30 is controlled by the ammonia feed command 39 so as to feed the required amount of ammonia gas 13a. Since the detected pH value 38 from the pH meter 37 in the vessel 15 is inputted into the controller 32, the controller 32 can calculate ammonia concentration depending upon the detected pH value 38 of the pH meter 37 to compensate, on the basis of such ammonia concentration, the ammonia feed command 39 for control of the opening degree of the ammonia feed valve 30.
Since the liquid level in the vessel 15 is gradually lowered due to the fact that the ammonia gas 13a decomposed from the urea water 23a is taken out as mentioned above and due to evaporation of the water, the urea water feed valve 21 is controlled by the controller 32 on the basis of the liquid level signal 34 from the liquid level meter 33 so as to feed, to the vessel 15, the urea water 23a adjusted to a predetermined concentration in the tank 23, whereby the amount of the urea water 23a in the vessel 15 is kept constant.
According to the above embodiment, irrespective of the temperature of the exhaust gas 7, the ammonia generator 12 generates the ammonia gas 13a by the action of the plasma, the ammonia gas 13a being injected into and fed to the exhaust gas 7 in the exhaust pipe 9. As a result, in comparison with the conventional feed of urea water, a required amount of ammonia can be surely added to the exhaust gas 7 even if the temperature of the exhaust gas 7 is low; thus, even in a vehicle with travel pattern of continuing the operational status with low exhaust temperature, a sufficient NOx reduction effect can be exhibited even at exhaust temperature lower than that conventionally required therefor. Since the ammonia gas 13a causes no problem of lowering the exhaust temperature upon addition to the exhaust gas 7, NOx reduction effect can be further highly maintained in the operational status with low exhaust temperature.
In fact, according to experimental results effected by the inventors as shown in the graph of
Since the ammonia gas 13a is generated through the action of the plasma on the urea water 23a, the generation of the ammonia gas 13a can be easily and rapidly adjusted. Since the generated ammonia gas 13a is added to the exhaust gas 7, the amount of ammonia gas 13a to be fed to the exhaust gas 7 can be controlled with high response.
When the ammonia water 13b generated in the ammonia generator 12 is added in this manner to the exhaust gas 7, ammonia in the ammonia water 13b is reacted with NOx and NOx reduction effect can be obtained just like the above. Even in a vehicle with travel pattern of continuing operational status with low exhaust temperature for a long time, a sufficient NOx reduction effect can be obtained even at exhaust temperature lower than that conventionally required therefor. More specifically, when the ammonia water 13b is added to the exhaust gas 7, subtle heat may be taken upon evaporation of the water; however, endotherm required for evaporation of the water is lower than heat required in a conventional pyrolytical decomposition of the urea water 23 into ammonia and carbon dioxide in utilization of heat of the exhaust gas 7. Thus, lowering in temperature of the exhaust gas 7 is subtle; therefore, according to the invention, also in a case where the ammonia water 13b is fed to the exhaust gas 7, the NOx reduction effect can be highly maintained even in an operational status with low exhaust temperature.
Further arranged in space 27 within the vessel 40 above the liquid level of the urea water 23a is an ammonia feed pipe 28 similar to that in
In
In the
Thus, in the ammonia generator 12 of
Since the ammonia water 13b generated in the ammonia generator 12 is added in this manner to the exhaust gas 7, ammonia in the ammonia water 13b is reacted with NOx to obtain NOx reduction effect just like the above. Thus, even in a vehicle with travel pattern of continuing operational status with low exhaust temperature for a long time, a satisfactory NOx reduction effect can be obtained with exhaust temperature lower than that required conventionally therefor. More specifically, when the ammonia water 13b is added to the exhaust gas 7, subtle heat may be taken upon evaporation of the water; however, lowering in temperature of the exhaust gas 7 is subtle in comparison with a conventional pyrolytical decomposition of the urea water 23a into ammonia and carbon dioxide in utilization of heat of the exhaust gas 7. Thus, also in a case of feeding the ammonia water 13b, the NOx reduction effect can be highly maintained even in an operational status with low exhaust temperature.
It is to be understood that an exhaust emission control device of the invention is not limited to the above embodiments and that various changes and modifications may be made without departing from the scope of the invention.
An exhaust emission control device of the invention can be effectively utilized in effectively generating ammonia from urea water and in enhancing controllability in ammonia addition for obtaining a satisfactory NOx reduction effect even in a vehicle with travel pattern of continuing operational status with exhaust temperature lower than that required conventionally therefor.
Number | Date | Country | Kind |
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2004-334578 | Nov 2004 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP05/21108 | 11/17/2005 | WO | 00 | 5/15/2007 |