Exhaust emission control device

Abstract

To provide an exhaust emission control device capable of stopping an engine at an optional timing without waiting for completion of decomposition treatment of ammonia.SOLUTION: An exhaust emission control device 1 includes: a first catalyst 4 having a function to oxidize and reduce NH3 and NOx contained in the exhaust gas flowing through an exhaust gas passage 3; a second catalyst 5 that is disposed downstream of the first catalyst 4 in the exhaust gas passage 3 and has a function to adsorb and decompose the NH3 that has passed through the first catalyst 4 and a function to oxidize the NH3 that has passed through the first catalyst 4; an air pump 7 for supplying air to the second catalyst 5; an initial determination unit 12 for determining whether or not the amount of NH3 adsorbed by the second catalyst 5 is equal to or less than a predetermined specified amount in a state where an ammonia engine 2 is stopped; and an air supplying control unit 16 for controlling the air pump 7 to supply air to the second catalyst 5 when the initial determination unit 12 determines that the amount of NH3 adsorbed by the second catalyst 5 is not equal to or less than the specified amount.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification device. [Background technology]

[0002] Known as an exhaust purification device is, for example, the technology described in Patent Document 1. The exhaust purification device described in Patent Document 1 is provided with a three-way catalyst that is disposed in an exhaust pipe connected to an ammonia engine and purifies ammonia, NOx, and hydrogen in the exhaust, and an SCR catalyst that is disposed in the exhaust pipe downstream of the three-way catalyst and purifies NOx in the exhaust using ammonia as a reducing agent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-167823 Summary of the Invention [Problem to be solved by the invention]

[0004] In some exhaust gas purification devices, a second catalyst having an ammonia adsorption function and an ammonia decomposition function is disposed downstream of a first catalyst, which is a three-way catalyst. In this case, when the temperature of the second catalyst is low, unburned ammonia that has passed through the first catalyst is adsorbed by the second catalyst. When the temperature of the second catalyst subsequently rises to its decomposition activation temperature, the ammonia adsorbed by the second catalyst is decomposed into hydrogen and nitrogen.

[0005] However, if the ammonia engine is stopped before the ammonia decomposition process in the second catalyst is completed, the next time the ammonia engine is started, ammonia will already be adsorbed in the second catalyst, and the unburned ammonia may not be adsorbed completely in the second catalyst. In this case, the ammonia will flow through the exhaust pipe and be discharged. Therefore, after the ammonia engine is started, it cannot be stopped until the ammonia decomposition process in the second catalyst is completed.

[0006] An object of the present invention is to provide an exhaust gas purification device that can stop the engine at any timing without waiting for the completion of the ammonia decomposition process. [Means for solving the problem]

[0007] (1) One aspect of the present invention is an exhaust purification device that purifies exhaust gas generated by an engine, comprising: a first catalyst disposed in an exhaust flow path connected to the engine and having a function of oxidizing and reducing ammonia and NOx contained in the exhaust gas flowing through the exhaust flow path; a second catalyst disposed in the exhaust flow path downstream of the first catalyst and having a function of adsorbing and decomposing ammonia that has passed through the first catalyst and a function of oxidizing ammonia that has passed through the first catalyst; an air supply unit that supplies air to the second catalyst; an initial determination unit that determines, when the engine is stopped, whether the amount of ammonia adsorbed on the second catalyst is equal to or less than a predetermined amount; and an air supply control unit that controls the air supply unit to supply air to the second catalyst when the initial determination unit determines that the amount of ammonia adsorbed on the second catalyst is not equal to or less than the predetermined amount.

[0008] In this exhaust gas purification device, during normal engine operation, ammonia that passes through the first catalyst is adsorbed by the second catalyst, and the ammonia adsorbed by the second catalyst is decomposed when the second catalyst is heated. When the engine is stopped, it is determined whether the amount of ammonia adsorbed by the second catalyst is equal to or less than a specified amount. If the amount of ammonia adsorbed by the second catalyst is greater than the specified amount, air is supplied from the air supply unit to the second catalyst. This causes the second catalyst to oxidize the ammonia. The ammonia oxidation process starts at a lower temperature than the ammonia decomposition process. Therefore, the second catalyst performs the ammonia oxidation process before the ammonia decomposition process begins. In this way, even if the decomposition process of the ammonia adsorbed by the second catalyst is not completed when the engine is stopped, the ammonia adsorbed by the second catalyst is removed by supplying air to the second catalyst to oxidize it. This allows the engine to be stopped at any time without waiting for the ammonia decomposition process to be completed.

[0009] (2) In the above (1), the exhaust purification device may further include a heating unit that heats the second catalyst, a temperature detection unit that detects the temperature of the second catalyst, a temperature determination unit that determines whether the temperature of the second catalyst detected by the temperature detection unit is equal to or higher than an oxidation activation temperature at which ammonia can be oxidized, and a heating control unit that controls the heating unit to heat the second catalyst when the temperature determination unit determines that the temperature of the second catalyst is not equal to or higher than the oxidation activation temperature, and the heating control unit controls the heating unit to stop heating the second catalyst when it is determined that the temperature of the second catalyst is equal to or higher than the oxidation activation temperature after the heating unit starts heating the second catalyst, and the air supply control unit controls the air supply unit to supply air to the second catalyst when it is determined that the amount of ammonia adsorbed on the second catalyst is not equal to or lower than a specified amount and the temperature of the second catalyst is equal to or higher than the oxidation activation temperature.

[0010] In this configuration, when the temperature of the second catalyst is lower than the oxidation activation temperature, the second catalyst is heated by the heating unit, and the temperature of the second catalyst is immediately raised. Then, when the temperature of the second catalyst reaches the oxidation activation temperature, air is supplied to the second catalyst from the air supply unit, and the ammonia is oxidized in the second catalyst. Therefore, the ammonia adsorbed on the second catalyst is removed in a short time. Furthermore, by supplying air to the second catalyst from the air supply unit after the temperature of the second catalyst reaches the oxidation activation temperature, unnecessary operation of the air supply unit is avoided. This contributes to power savings.

[0011] (3) In the above (1) or (2), the exhaust purification device may further include a treatment completion determination unit that determines whether the oxidation process of the ammonia adsorbed on the second catalyst has been completed after the air supply unit has started to supply air to the second catalyst, and the air supply control unit may control the air supply unit to stop the supply of air to the second catalyst when the treatment completion determination unit determines that the oxidation process of the ammonia adsorbed on the second catalyst has been completed.

[0012] In this configuration, when the oxidation process of the ammonia adsorbed on the second catalyst is completed, the supply of air to the second catalyst is stopped, thereby eliminating the needless operation of the air supply unit, thereby achieving further power savings.

[0013] (4) In the above (3), the treatment completion determination unit may determine that the oxidation treatment of ammonia adsorbed in the second catalyst is completed when a specified time has elapsed since the air supply unit started to supply air to the second catalyst.

[0014] In this configuration, it is possible to determine whether the ammonia oxidation process has been completed by a simple process by determining whether a predetermined specified time has elapsed since the air supply unit started to supply air to the second catalyst.

[0015] (5) In the above (3), the exhaust purification device may further include a NOx detection unit that detects the concentration of NOx present downstream of the second catalyst in the exhaust flow path, and the processing completion determination unit may determine that the oxidation processing of ammonia adsorbed on the second catalyst is completed when the concentration of NOx detected by the NOx detection unit becomes equal to or less than a predetermined value.

[0016] In this configuration, when the oxidation process of the ammonia adsorbed on the second catalyst is completed, the NOx present downstream of the second catalyst in the exhaust flow path decreases. Therefore, by determining whether the concentration of NOx present downstream of the second catalyst in the exhaust flow path is equal to or less than a specified value, it is possible to determine with high accuracy whether the oxidation process of the ammonia is completed. [Effects of the Invention]

[0017] According to the present invention, the engine can be stopped at any timing without waiting for the completion of the ammonia decomposition process. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram showing the configuration of an exhaust gas purification device according to an embodiment of the present invention; [Figure 2] 2 is a block diagram showing a control system of the exhaust purification device shown in FIG. 1. FIG. [Figure 3] 3 is a flowchart showing a processing procedure executed by the ECU shown in FIG. 2. [Figure 4] FIG. 10 is a block diagram showing a control system of an exhaust gas purification device according to another embodiment of the present invention. [Figure 5] 5 is a flowchart showing a processing procedure executed by the ECU shown in FIG. 4. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0020] Fig. 1 is a schematic diagram showing the configuration of an exhaust gas purification device according to one embodiment of the present invention. In Fig. 1, the exhaust gas purification device 1 of this embodiment is mounted on a vehicle (not shown). The exhaust gas purification device 1 is a device that purifies exhaust gas generated from an ammonia engine 2.

[0021] The ammonia engine 2 is an engine that uses ammonia (NH3) as fuel. At this time, hydrogen (H2) may be mixed with the ammonia as a combustion improver to make the flame-retardant ammonia more combustible. An exhaust flow path 3 is connected to the ammonia engine 2. The exhaust flow path 3 is a flow path through which exhaust gas generated from the ammonia engine 2 flows.

[0022] The exhaust purification device 1 includes a first catalyst 4, a second catalyst 5, an electric heater 6, an air pump 7, temperature sensors 8 and 9, a NOx sensor 10, and an ECU (Electronic Control Unit) 11.

[0023] The first catalyst 4 is disposed in the exhaust flow path 3. The first catalyst 4 is a catalyst that has the function of oxidizing and reducing NH3 and NOx (nitrogen oxides) contained in the exhaust gas. The first catalyst is, for example, a three-way catalyst.

[0024] The second catalyst 5 is disposed downstream of the first catalyst 4 in the exhaust flow path 3. The second catalyst 5 has the function of adsorbing and decomposing NH3 that has passed through the first catalyst 4, and the function of oxidizing NH3 that has passed through the first catalyst 4. The second catalyst 5 oxidizes NH3 in an oxidizing atmosphere. The catalytic species of the second catalyst 5 may be, for example, ruthenium (Ru), rhodium (Rh), cobalt (Co), iron (Fe), or nickel (Ni).

[0025] The electric heater 6 is disposed upstream of the second catalyst 5. The electric heater 6 is a heating unit that heats the second catalyst 5.

[0026] The air pump 7 is an air supply unit that supplies air to the second catalyst 5. The air pump 7 introduces air into a portion of the exhaust flow path 3 between the first catalyst 4 and the second catalyst 5. Therefore, the air from the air pump 7 flows through the exhaust flow path 3 and is supplied to the second catalyst 5.

[0027] The temperature sensor 8 is a sensor that detects the temperature of a portion of the exhaust flow path 3 between the first catalyst 4 and the second catalyst 5. Specifically, the temperature sensor 8 detects the temperature of a portion of the exhaust flow path 3 between the portion where air from the air pump 7 is introduced and the second catalyst 5. In other words, the temperature sensor 8 is a sensor that detects the temperature on the inlet side of the second catalyst 5.

[0028] The temperature sensor 9 is a sensor that detects the temperature of a portion of the exhaust passage 3 downstream of the second catalyst 5. In other words, the temperature sensor 9 detects the temperature of the outlet side of the second catalyst 5.

[0029] The NOx sensor 10 is a sensor that detects the concentration of NOx contained in the exhaust gas that flows downstream of the second catalyst 5 in the exhaust flow path 3. The NOx sensor 10 is a NOx detection unit that detects the concentration of NOx that exists downstream of the second catalyst 5 in the exhaust flow path 3.

[0030] The ECU 11 is composed of a CPU, RAM, ROM, an input / output interface, etc. As shown in Fig. 2, the ECU 11 has an initial determination unit 12, a temperature estimation unit 13, a temperature determination unit 14, a heating control unit 15, an air supply control unit 16, and a processing completion determination unit 17.

[0031] The initial determination unit 12 determines whether the amount of NH3 adsorbed on the second catalyst 5 is equal to or less than a predetermined specified amount while the ammonia engine 2 is stopped. The specified amount may be, for example, zero or a numerical value close to zero.

[0032] The temperature estimation unit 13 estimates the temperature of the second catalyst 5 based on the detection values ​​of the temperature sensors 8 and 9. The temperature estimation unit 13 constitutes a temperature detection unit that detects the temperature of the second catalyst 5 in cooperation with the temperature sensors 8 and 9.

[0033] The temperature determination unit 14 determines whether the temperature of the second catalyst 5 estimated by the temperature estimation unit 13 is equal to or higher than the oxidation activation temperature. The oxidation activation temperature is the temperature at which NH3 can be oxidized, and differs depending on the catalyst type of the second catalyst 5.

[0034] When the temperature determination unit 14 determines that the temperature of the second catalyst 5 is not equal to or higher than the oxidation activation temperature, the heating control unit 15 controls the electric heater 6 to heat the second catalyst 5. After the electric heater 6 starts heating the second catalyst 5, when the temperature determination unit 14 determines that the temperature of the second catalyst 5 is equal to or higher than the oxidation activation temperature, the heating control unit 15 controls the electric heater 6 to stop heating the second catalyst 5.

[0035] When the initial determination unit 12 determines that the amount of ammonia adsorbed in the second catalyst 5 is not equal to or less than a specified amount, the air supply control unit 16 controls the air pump 7 to supply air to the second catalyst 5. Specifically, when it is determined that the amount of ammonia adsorbed in the second catalyst 5 is not equal to or less than a specified amount and the temperature of the second catalyst 5 is determined to be equal to or higher than the oxidation activation temperature, the air supply control unit 16 controls the air pump 7 to supply air to the second catalyst 5.

[0036] In addition, when the treatment completion determination unit 17 described later determines that the oxidation treatment of the ammonia adsorbed in the second catalyst 5 has been completed, the air supply control unit 16 controls the air pump 7 to stop the supply of air to the second catalyst 5.

[0037] The treatment completion determination unit 17 determines whether or not the oxidation process of NH3 adsorbed to the second catalyst 5 is completed after the air pump 7 starts to supply air to the second catalyst 5. Specifically, the treatment completion determination unit 17 determines that the oxidation process of NH3 adsorbed to the second catalyst 5 is completed when a predetermined specified time has elapsed since the air pump 7 started to supply air to the second catalyst 5.

[0038] 3 is a flowchart showing the processing procedure executed by the ECU 11. This processing is executed when an engine stop signal is input to the ECU 11, for example, by a user turning off the ignition switch. At the start of this processing, the heater flag is set to 0. The heater flag is a flag for determining whether or not to turn off the electric heater 6.

[0039] 3, the ECU 11 first determines whether or not the process of removing NH3 adsorbed on the second catalyst 5 has been completed (step S101). The process of removing NH3 is a process of decomposing and oxidizing NH3. This step corresponds to the step of determining whether or not the amount of NH3 adsorbed on the second catalyst 5 is equal to or less than a specified amount.

[0040] For example, the amount of NH3 adsorbed on the second catalyst 5 is estimated based on the temperature on the inlet side of the second catalyst 5 and a mixture ratio of the exhaust gas obtained separately, and the time required for the NH3 to be removed is estimated from the amount of NH3. Therefore, the ECU 11 determines whether the process of removing the NH3 adsorbed on the second catalyst 5 has been completed by determining whether a certain time has elapsed.

[0041] When the ECU 11 determines that the process of removing NH3 adsorbed in the second catalyst 5 has been completed, the ECU 11 ends this process. When the ECU 11 determines that the process of removing NH3 adsorbed in the second catalyst 5 has not been completed, the ECU 11 acquires the detection values ​​of the temperature sensors 8 and 9 (step S102). Then, the ECU 11 estimates the temperature of the second catalyst 5 based on the detection values ​​of the temperature sensors 8 and 9 (step S103).

[0042] Next, the ECU 11 determines whether the temperature of the second catalyst 5 is equal to or higher than the oxidation activation temperature (step S104). The oxidation activation temperature is the temperature at which the oxidation process of NH3 starts. The oxidation activation temperature is lower than the decomposition activation temperature, which is the temperature at which the decomposition process of NH3 starts.

[0043] When the ECU 11 determines that the temperature of the second catalyst 5 is not equal to or higher than the oxidation activation temperature, it controls the electric heater 6 to be ON (step S105). Then, the ECU 11 sets the heater flag to 1 (step S106) and executes the above step S102 again. When the electric heater 6 is turned ON, the second catalyst 5 is heated by the electric heater 6.

[0044] When the ECU 11 determines in step S104 that the temperature of the second catalyst 5 is equal to or higher than the oxidation activation temperature, it determines whether the heater flag is 0 (step S107). When the ECU 11 determines that the heater flag is 0, it controls the air pump 7 to turn on (step S108). When the ECU 11 determines that the heater flag is 1 and not 0, it controls the electric heater 6 to turn off (step S109) and controls the air pump 7 to turn on (step S108).

[0045] When the air pump 7 is turned on, air is supplied from the air pump 7 to the second catalyst 5, thereby oxidizing the NH3 adsorbed on the second catalyst 5. Therefore, the NH3 is removed from the second catalyst 5.

[0046] Next, the ECU 11 determines whether a specified time has elapsed since the air pump 7 was turned on (step S110). The specified time is, for example, the time it takes for the oxidation process of NH3 to be completed in the second catalyst 5. The specified time is determined in advance through experiments or the like.

[0047] Next, when the ECU 11 determines that a specified time has elapsed since the air pump 7 was turned on, it turns off the air pump 7 (step S111) and ends this process.

[0048] Here, the initial judgment unit 12 executes step S101. The temperature estimation unit 13 executes steps S102 and S103. The temperature judgment unit 14 executes step S104. The heating control unit 15 executes steps S105 to S107 and S109. The air supply control unit 16 executes steps S108 and S111. The processing completion judgment unit 17 executes step S110.

[0049] In the exhaust purification device 1 as described above, when the ammonia engine 2 is started, exhaust gas generated from the ammonia engine 2 flows through the exhaust flow path 3. Immediately after the ammonia engine 2 is started, the first catalyst 4 is not sufficiently heated, and therefore unburned NH3 contained in the exhaust gas passes through the first catalyst 4 and is adsorbed by the second catalyst 5. Even during steady operation of the ammonia engine 2, fluctuations in the air-fuel ratio (A / F) of the NH3 gas and air supplied to the ammonia engine 2 can cause unburned NH3 contained in the exhaust gas to pass through the first catalyst 4 and be adsorbed by the second catalyst 5.

[0050] When the temperature of the second catalyst 5 reaches the decomposition activation temperature, a decomposition reaction of the unburned NH3 adsorbed on the second catalyst 5 occurs, and the unburned NH3 is decomposed into hydrogen and nitrogen. As a result, the unburned NH3 is removed from the second catalyst 5.

[0051] However, if the ammonia engine 2 is stopped early after it has been started, before the decomposition process of unburned NH3 is completed, the following problem occurs: If NH3 has already been adsorbed by the second catalyst 5 when the ammonia engine 2 is next started, the amount of unburned NH3 adsorbed by the second catalyst 5 will decrease, and the unburned NH3 that is not adsorbed by the second catalyst 5 will flow through the exhaust flow path 3 and be emitted into the atmosphere.

[0052] Therefore, when the ammonia engine 2 is stopped early, the air pump 7 is turned on to supply air to the second catalyst 5. Specifically, when the temperature of the second catalyst 5 is equal to or higher than the oxidation activation temperature, the air pump 7 is turned on. When the temperature of the second catalyst 5 is lower than the oxidation activation temperature, the electric heater 6 is turned on to heat the second catalyst 5. Thereafter, when the temperature of the second catalyst 5 reaches equal to or higher than the oxidation activation temperature, the air pump 7 is turned on.

[0053] When air is supplied from the air pump 7 to the second catalyst 5, an oxidation reaction of unburned NH3 occurs in the second catalyst 5, and the unburned NH3 adsorbed on the second catalyst 5 is oxidized. Generally, in low temperature ranges, oxidation reactions are more active than decomposition reactions. Therefore, even in low temperature ranges where the decomposition reaction of NH3 does not proceed sufficiently, the oxidation reaction of NH3 is easily initiated. Therefore, unburned NH3 is removed from the second catalyst 5.

[0054] Furthermore, when an oxidation reaction of NH3 occurs, the heat of oxidation raises the temperature of the second catalyst 5. Therefore, if unburned NH3 remains in the second catalyst 5, when the temperature of the second catalyst 5 reaches the decomposition activation temperature, a decomposition reaction of the unburned NH3 occurs in the second catalyst 5, and the unburned NH3 adsorbed on the second catalyst 5 is decomposed. Therefore, the unburned NH3 is reliably removed from the second catalyst 5.

[0055] When a specified time has elapsed since the air pump 7 was turned on, the oxidation reaction of NH3 is completed. Then, the air pump 7 is turned off, and the supply of air from the air pump 7 to the second catalyst 5 is stopped.

[0056] As described above, in this embodiment, during normal operation of the ammonia engine 2, NH3 that has passed through the first catalyst 4 is adsorbed by the second catalyst 5, and the NH3 adsorbed by the second catalyst 5 is decomposed when the temperature of the second catalyst 5 is raised. When the ammonia engine 2 is stopped, it is determined whether the amount of NH3 adsorbed by the second catalyst 5 is equal to or less than a specified amount. If the amount of NH3 adsorbed by the second catalyst 5 is greater than the specified amount, air is supplied from the air pump 7 to the second catalyst 5. Then, the second catalyst 5 performs an oxidation process for NH3. The oxidation process for NH3 is started at a temperature lower than that of the decomposition process for NH3. Therefore, the second catalyst 5 performs an oxidation process for NH3 before the decomposition process for NH3 is performed. In this way, even if the decomposition process of NH3 adsorbed by the second catalyst 5 is not completed when the ammonia engine 2 is stopped, the NH3 adsorbed by the second catalyst 5 is removed by supplying air to the second catalyst 5 to oxidize the NH3 adsorbed by the second catalyst 5. This makes it possible to stop the ammonia engine 2 at any timing without waiting for the completion of the NH3 decomposition process.

[0057] Furthermore, in this embodiment, when the temperature of the second catalyst 5 is lower than the oxidation activation temperature, the second catalyst 5 is heated by the electric heater 6, thereby immediately raising the temperature of the second catalyst 5. Thereafter, when the temperature of the second catalyst 5 reaches the oxidation activation temperature, air is supplied from the air pump 7 to the second catalyst 5, and the second catalyst 5 performs an oxidation process of NH3. Therefore, the NH3 adsorbed on the second catalyst 5 is removed in a short time. Furthermore, by supplying air from the air pump 7 to the second catalyst 5 after the temperature of the second catalyst 5 reaches the oxidation activation temperature, unnecessary operation of the air pump 7 is avoided. Therefore, further power savings can be achieved.

[0058] Furthermore, in this embodiment, when the oxidation process of NH3 adsorbed on the second catalyst 5 is completed, the supply of air to the second catalyst 5 is stopped, thereby eliminating the needless operation of the air pump 7. Therefore, power consumption can be reduced.

[0059] In addition, in this embodiment, by determining whether a specified time has elapsed since the air pump 7 started to supply air to the second catalyst 5, it is possible to determine whether the oxidation process of NH3 has been completed through a simple process.

[0060] In addition, in this embodiment, the temperature of the second catalyst 5 can be detected with high accuracy by estimating the temperature of the second catalyst 5 based on the temperature of the inlet side of the second catalyst 5 and the temperature of the outlet side of the second catalyst 5.

[0061] Fig. 4 is a block diagram showing a control system of an exhaust gas purification device according to another embodiment of the present invention, and corresponds to Fig. 2. In Fig. 4, the exhaust gas purification device 1 of this embodiment includes an ECU 11A instead of the above-mentioned ECU 11. The ECU 11A includes an initial determination unit 12, a temperature estimation unit 13, a temperature determination unit 14, a heating control unit 15, an air supply control unit 16, and a process completion determination unit 17A.

[0062] The treatment completion determination unit 17A determines whether or not the oxidation process of NH3 adsorbed to the second catalyst 5 is completed after the air pump 7 starts to supply air to the second catalyst 5. Specifically, the treatment completion determination unit 17A determines that the oxidation process of NH3 adsorbed to the second catalyst 5 is completed when the concentration of NOx detected by the NOx sensor 10 becomes equal to or less than a predetermined specified value.

[0063] Fig. 5 is a flowchart showing a modified example of the processing procedure executed by the ECU 11A, and corresponds to Fig. 3. In Fig. 5, steps S101 to S109 are the same as those in Fig. 3.

[0064] After executing step S108, the ECU 11A acquires the detection value of the NOx sensor 10 (step S121). Then, the ECU 11A determines whether the concentration of NOx present downstream of the second catalyst 5 is equal to or less than a specified value based on the detection value of the NOx sensor 10 (step S122). Steps S121 and S122 are executed by the processing completion determination unit 17A.

[0065] When an oxidation reaction of NH3 occurs in the second catalyst 5, a small amount of NOx is generated, but when an oxidation reaction of NH3 does not occur in the second catalyst 5, NOx is unlikely to be generated. For this reason, the specified value is preset to a value at which it is determined that the oxidation treatment of NH3 has been completed.

[0066] When the ECU 11A determines that the concentration of NOx present downstream of the second catalyst 5 is not equal to or less than the specified value, it executes the above-described step S121 again. When the ECU 11A determines that the concentration of NOx present downstream of the second catalyst 5 is equal to or less than the specified value, it controls the air pump 7 to turn OFF (step S111) and ends this process.

[0067] In this embodiment, when the oxidation process of NH3 adsorbed on the second catalyst 5 is completed, the amount of NOx present downstream of the second catalyst 5 in the exhaust flow path 3 decreases. Therefore, by determining whether the concentration of NOx present downstream of the second catalyst 5 in the exhaust flow path 3 is equal to or less than a specified value, it is possible to determine with high accuracy whether the oxidation process of NH3 has been completed.

[0068] The present invention is not limited to the above embodiment. For example, in the above embodiment, when a specified time has elapsed since the air pump 7 was turned on or when the concentration of NOx downstream of the second catalyst 5 is equal to or less than a specified value, it is determined that the oxidation process of NH3 in the second catalyst 5 is completed, and the supply of air from the air pump 7 to the second catalyst 5 is stopped. However, the present invention is not particularly limited to such an embodiment.

[0069] For example, it may be determined that the oxidation process of NH3 in the second catalyst 5 is complete when the temperature on the outlet side of the second catalyst 5 detected by the temperature sensor 9 is equal to or lower than a predetermined specified temperature. Alternatively, it may be determined that the oxidation process of NH3 in the second catalyst 5 is complete when the difference between the temperature on the inlet side of the second catalyst 5 detected by the temperature sensor 8 and the temperature on the outlet side of the second catalyst 5 detected by the temperature sensor 9 is equal to or lower than a predetermined threshold.

[0070] In the above embodiment, when the temperature of the second catalyst 5 is lower than the oxidation activation temperature, the second catalyst 5 is heated by the electric heater 6, but this is not a particular limitation. For example, Ru, Rh, Co, Fe, and the like used in the second catalyst 5 generate oxidation heat when exposed to air supplied from the air pump 7. Therefore, as long as the oxidation heat can be used to heat the second catalyst 5 to the oxidation activation temperature, the electric heater 6 may not be necessary.

[0071] Furthermore, in the above embodiment, the first catalyst 4 is a three-way catalyst, but the first catalyst 4 is not particularly limited to a three-way catalyst, and may be any catalyst that has the function of oxidizing and reducing NH3 and NOx.

[0072] Furthermore, although the exhaust purification device 1 in the above embodiment is mounted on a vehicle, it is not particularly limited to this form, and may be mounted on, for example, a ground-based power generation device or the like. [Explanation of symbols]

[0073] 1...exhaust gas purification device, 2...ammonia engine (engine), 3...exhaust flow path, 4...first catalyst, 5...second catalyst, 6...electric heater (heating section), 7...air pump (air supply section), 8...temperature sensor (temperature detection section), 9...temperature sensor (temperature detection section), 10...NOx sensor (NOx detection section), 12...initial judgment section, 13...temperature estimation section (temperature detection section), 14...temperature judgment section, 15...heating control section, 16...air supply control section, 17...processing completion judgment section.

Claims

1. An exhaust purification device that purifies exhaust gas generated from an engine, a first catalyst disposed in an exhaust passage connected to the engine and having a function of oxidizing and reducing ammonia and NOx contained in exhaust gas flowing through the exhaust passage; a second catalyst disposed downstream of the first catalyst in the exhaust flow path, the second catalyst having a function of adsorbing and decomposing ammonia that has passed through the first catalyst and a function of oxidizing ammonia that has passed through the first catalyst; an air supply unit that supplies air to the second catalyst; an initial determination unit that determines whether or not the amount of ammonia adsorbed on the second catalyst is equal to or less than a predetermined specified amount while the engine is stopped; an air supply control unit that controls the air supply unit to supply air to the second catalyst when the initial judgment unit determines that the amount of ammonia adsorbed on the second catalyst is not equal to or less than the specified amount.

2. a heating unit that heats the second catalyst; a temperature detection unit that detects the temperature of the second catalyst; a temperature determination unit that determines whether the temperature of the second catalyst detected by the temperature detection unit is equal to or higher than an oxidation activation temperature at which the ammonia can be oxidized; a heating control unit that controls the heating unit to heat the second catalyst when the temperature determination unit determines that the temperature of the second catalyst is not equal to or higher than the oxidation activation temperature, the heating control unit controls the heating unit to stop heating the second catalyst when it is determined that the temperature of the second catalyst is equal to or higher than the oxidation activation temperature after the heating unit starts heating the second catalyst; 2. The exhaust purification device according to claim 1, wherein the air supply control unit controls the air supply unit to supply air to the second catalyst when it is determined that the amount of ammonia adsorbed on the second catalyst is not less than the specified amount and when it is determined that the temperature of the second catalyst is not less than the oxidation activation temperature.

3. a treatment completion determination unit that determines whether or not an oxidation treatment of ammonia adsorbed on the second catalyst is completed after the air supply unit starts supplying air to the second catalyst, 2. The exhaust purification device according to claim 1, wherein the air supply control unit controls the air supply unit to stop supplying air to the second catalyst when the treatment completion determination unit determines that the oxidation treatment of the ammonia adsorbed on the second catalyst has been completed.

4. 4. The exhaust purification device according to claim 3, wherein the treatment completion determination unit determines that the oxidation treatment of ammonia adsorbed in the second catalyst is completed when a predetermined specified time has elapsed since the air supply unit started to supply air to the second catalyst.

5. a NOx detection unit that detects the concentration of NOx present downstream of the second catalyst in the exhaust passage, 4. The exhaust purification device according to claim 3, wherein the treatment completion determination unit determines that the oxidation treatment of ammonia adsorbed on the second catalyst has been completed when the concentration of NOx detected by the NOx detection unit becomes equal to or less than a predetermined value.

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