JP5840976B2 - Freezing prevention method for urea SCR system - Google Patents

Description

  The present invention relates to a method for preventing freezing of a urea SCR system that can reduce and purify NOx (nitrogen oxide) in exhaust gas using urea water as a reducing agent.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust system through which exhaust gas flows, and the selective reduction type There is a type in which a necessary amount of a reducing agent is added upstream of the catalyst and the reducing agent is allowed to undergo a reduction reaction with NOx in exhaust gas on the selective reduction catalyst, thereby reducing the NOx emission concentration. .

On the other hand, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method for reducing and purifying NOx using ammonia (NH ₃ ) as a reducing agent is already widely known. Since it is difficult to ensure safety with respect to traveling with ammonia itself, in recent years, it has been proposed to use non-toxic urea water as a reducing agent.

  That is, if urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is decomposed into ammonia and carbon dioxide under a temperature condition of about 170 ° C. or higher, and the exhaust gas is exhausted on the selective catalytic reduction catalyst. The NOx contained therein is reduced and purified well by ammonia.

  When urea water is used as a reducing agent in this way, urea water is sent to the upstream side of the selective catalytic reduction catalyst from a urea water tank mounted on the vehicle via a pipe. This type of urea water is about -11. Because it freezes at temperatures below ℃, it is necessary to take some measures against freezing so that the piping, pumps, injectors, etc. will not be damaged by freezing pressure. After-run control for sucking urea water back to the urea water tank side is performed.

  Incidentally, as prior art document information related to freezing countermeasures in this type of urea SCR system, there is the following Patent Document 1 and the like.

JP 2008-138583 A

  However, in the conventional urea SCR system as described above, urea crystals are precipitated at the tip of the injector under special conditions such as when the engine is suddenly stopped from an operation state where the exhaust temperature is high, and the precipitated urea crystals cause the injector to If the after-run control is performed with the injector tip clogged, excessive negative pressure may be generated in the path due to urea water sucked back toward the urea water tank. After the end of the after-run control, the urea water in the urea water tank flows back to the path side and freezes in the path, and the freezing pressure may damage the piping, the pump, the injector, and the like.

  The present invention has been made in view of the above circumstances, and when the after-run control is performed, the urea water is sucked back without causing excessive negative pressure in the path, and the back-flow of the urea water after the end of the after-run control is performed. The purpose is to ensure that it can be prevented.

The present invention includes a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen in the middle of an exhaust system, and urea water as a reducing agent in the exhaust system upstream of the selective reduction catalyst. A method for preventing freezing of a urea SCR system having an injector for adding a fuel, wherein after the key-off, the pump is driven to wait for the exhaust temperature to fall below a predetermined temperature, and after the urea water is increased to a predetermined pressure by the pump, the injector And after-running the urea water in an appropriate number of times, after-run control is performed in which the urea water in the path is sucked back into the urea water tank by a pump.

  Thus, in this way, even when urea crystals are deposited at the tip of the injector when the engine is suddenly stopped from an operating state with a high exhaust temperature, the urea water is injected as many times as necessary after the key-off. Since the urea crystals deposited at the tip of the injector are dissolved in new urea water and removed, if after-run control is performed immediately after that, the urea water in the path is sucked back into the urea water tank, It is possible to suck the urea water in the path back into the urea water tank without difficulty while maintaining the open state of the injector tip.

In addition , there is a risk that hot exhaust gas may be sucked into the passage when after-run control is performed, or that new urea water injected to dissolve urea crystals at the tip of the injector may be exposed to the hot exhaust gas. In addition, the urea water is surely injected at a predetermined pressure or higher.

  According to the antifreezing method of the urea SCR system of the present invention described above, various excellent effects as described below can be obtained.

  (I) According to the first aspect of the present invention, the urea water can be sucked back without generating an excessive negative pressure in the path when the after-run control is performed. Therefore, it is possible to prevent the urea water from flowing back and prevent the urea water from freezing in the path, so that it is possible to prevent the piping, pump, injector and the like from being damaged by the freezing pressure. it can.

  (II) According to the invention described in claim 2 of the present invention, it is possible to avoid burning and corrosion of piping, pumps, injectors and the like due to high-temperature exhaust gas being sucked into the path, and the injector It is possible to reliably avoid the situation that even the new urea water injected to dissolve the urea crystal at the tip of the crystal is crystallized, and the urea crystal can be formed by reliably injecting the urea water at a predetermined pressure or more. It can also be removed more effectively.

It is a systematic diagram which shows an example of the form which implements this invention. It is a systematic diagram explaining operation which sucks back urea water with the pump of FIG. It is a flowchart which shows the specific control procedure in the control apparatus of FIG.

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

  1 to 3 show an example of an embodiment of the present invention. Reference numeral 1 in FIG. 1 denotes a urea water tank that stores urea water 2, and 3 denotes a suction line 4 from the urea water tank 1. 1 shows a pump that sucks up the urea water 2 through the pressure and sends the urea water 2 to the injector 6 through the pressure line 5.

  Here, the pump 3 that supplies the urea water 2 to the injector 6 includes a flow path switching valve 7 that can switch between two forward and reverse positions, and a pump body 8 that substantially takes in and discharges the urea water 2. The urea water 2 in the pressure line 5 can also be sucked back by the pump body 8 by switching the position of the flow path switching valve 7 from the state of FIG. 1 to the state of FIG.

  That is, when the engine is stopped, after the key is turned off, the flow path switching valve 7 is switched to a reverse position (the position shown in FIG. 2), and the urea water 2 is sucked back to the urea water tank 1 side by driving the pump body 8. Measures are taken to prevent freezing of urea water 2 while the engine is stopped on the ground or the like.

  Further, a pressure sensor 9 for detecting the pressure of the urea water 2 in the pressure line 5 is provided in the pump 3, and an exhaust gas is disposed at an appropriate position of the exhaust system passage 10 in which the injector 6 is disposed. A temperature sensor 12 for detecting the temperature of the gas 11 is provided, and the detection signals 9a and 12a of the pressure sensor 9 and the temperature sensor 12 control the operation of the injector 6, the flow path switching valve 7 and the pump body 8 as a control signal 6a. , 7a, 8a is input to the control device 13 controlled.

  The controller 13 also receives an on / off signal 14a from the key switch 14, and even after the key-off is confirmed by the on / off signal 14a, the controller 13 is maintained in an operating state without being stopped (shut down). After the key-off, after-run control of returning the urea water 2 in the path to the urea water tank 1 after the injection of the urea water 2 (once in this embodiment) is performed.

  That is, a flowchart of a specific control procedure in the control device 13 is as shown in FIG. 3. First, when key-off is confirmed based on the on / off signal 14a from the key switch 14 in step S1, the next step S2 is performed. Then, the determination is repeated until the exhaust temperature falls below the predetermined temperature T based on the detection signal 12a of the temperature sensor 12, and when the exhaust temperature falls below the predetermined temperature T, the process proceeds to the next step S3 and the pump 3 (pump main body 8). A control signal 8a for driving is output, and thereby the pump 3 (pump main body 8) is driven.

  When the pump 3 is driven in this way, in the next step S4, the determination is repeated based on the detection signal 9a of the pressure sensor 9 until the pressure of the urea water 2 in the pressure line 5 becomes equal to or higher than the predetermined pressure P. When the pressure becomes equal to or higher than the predetermined pressure P, the process proceeds to the next step S5, where a control signal 6a for opening the injector 6 is output, whereby the injector 6 performs the injection of the urea water 2.

  Next, in step S6, a control signal 7a for switching the position of the flow path switching valve 7 from the state of FIG. 1 to the state of FIG. 2 is output, whereby the position of the flow path switching valve 7 is changed from the state of FIG. When the state is switched, the urea water 2 in the path is sucked back into the urea water tank 1 and the after-run control is performed.

  In the next step S7, the elapsed time of the after-run control is measured by a timer (not shown) built in the control device 13, and the determination is repeated until the elapsed time reaches a predetermined time t or longer. When the time reaches the predetermined time t or longer, the process proceeds to the next step S8, where a control signal 8a for stopping the pump 3 (pump main body 8) is output, whereby the pump 3 (pump main body 8) is stopped. .

  When the pump 3 is stopped in this way, the control signal 6a for closing the injector 6 is output in the next step S9, the injector 6 is closed, and the control device 13 is stopped (shut down) in the next step S10. Will do.

  In FIG. 1 and FIG. 2, reference numeral 15 denotes a selective reduction catalyst that is disposed downstream of the injector 6 in the exhaust passage 10 and reduces and purifies NOx in the exhaust gas 11. The urea water 2 added therein is supplied as a reducing agent, and the catalyst performance is such that ammonia produced from the urea water 2 and NOx in the exhaust gas can be selectively reacted even in the presence of oxygen.

  Thus, in this way, even when urea crystals are deposited at the tip of the injector 6 when the engine is suddenly stopped from an operating state with a high exhaust temperature, the urea water 2 is injected after the key-off (in this embodiment). 1), the urea crystals deposited at the tip of the injector 6 are dissolved and removed in the new urea water 2, and immediately after that, after-run control is performed to remove the urea water 2 in the path. If sucked back into the water tank 1, the urea water 2 in the path can be sucked back into the urea water tank 1 without difficulty while maintaining the open state of the tip of the injector 6.

  In addition, after the key-off, the pump 3 is driven after waiting for the exhaust gas temperature to fall below the predetermined temperature. After the urea water 2 is boosted to a predetermined pressure by the pump 3, the injector 6 is opened to inject the urea water 2 (this (In the embodiment, once), since after-run control is performed, high-temperature exhaust gas 11 may be sucked into the path, or a new spray injected to dissolve the urea crystal at the tip of the injector 6 The possibility that the urea water 2 is exposed to the high-temperature exhaust gas 11 is avoided in advance, and the urea water 2 is reliably injected by a pressure higher than a predetermined pressure.

  Therefore, according to the above embodiment, the urea water 2 can be sucked back without generating an excessive negative pressure in the path when the after-run control is performed, and the back flow of the urea water 2 after the end of the after-run control is reduced. Since the freezing in the path of the urea water 2 can be avoided with certainty, the freezing pressure may damage the piping such as the suction line 4 and the pressure line 5, the pump 3, the injector 6 and the like. Can be prevented in advance.

  Further, it is possible to avoid burning and corrosion of the piping such as the suction line 4 and the pressure line 5, the pump 3, and the injector 6 due to the high temperature exhaust gas 11 being sucked into the passage, and the tip of the injector 6. It is possible to reliably avoid the situation where even the new urea water 2 injected to dissolve the urea crystals of the crystallization is crystallized, and the urea crystals 2 can be formed by reliably injecting the urea water 2 at a predetermined pressure or higher. It can also be removed more effectively.

  In addition, the freeze prevention method of the urea SCR system of this invention is not limited only to the above-mentioned example, Of course, various changes can be added within the range which does not deviate from the summary of this invention.

1 Urea water tank 2 Urea water 3 Pump 4 Suction line (path)
5 Pressure line (path)
6 Injector 10 Exhaust System 11 Exhaust Gas 13 Control Device 15 Selective Reduction Catalyst

Claims (1)

Injector comprising a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen in the middle of the exhaust system, and adding urea water as a reducing agent into the exhaust system upstream of the selective reduction catalyst A method for preventing freezing of a urea SCR system comprising:
After the key-off, the pump is driven after the exhaust temperature falls below the predetermined temperature, and after the urea water is boosted to the predetermined pressure by the pump, the injector is opened and the urea water is injected as many times as necessary. A method for preventing freezing of a urea SCR system, wherein after-run control is performed to suck the urea water in the tank back into the urea water tank by a pump.

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