A method for controlling the reaction capacity of a two-stage denox system by periodic ammonia storage state

Abstract

The application provides a method for improving the reaction capacity of a two-stage DeNOx system by periodic ammonia storage state control, comprising the following steps: S1. continuously monitoring the temperature, exhaust temperature and mass flow of Cat1 and Cat2, and judging whether the working condition is suitable for implementing the ammonia storage periodic control strategy; S2. judging whether the actual ammonia storage amount of Cat1 exceeds the upper limit value of the ammonia storage target, and if yes, the 1# nozzle enters the reduced injection state; S3. the 1# nozzle enters the reduced injection state while the 2# nozzle enters the injection state; S4. when the actual ammonia storage amount of Cat2 exceeds the upper limit value of the ammonia storage target, the 2# nozzle enters the reduced injection state; S5. after the conversion efficiency of Cat2 is monitored to start to decline, the 1# nozzle enters the injection state; S6. when the actual ammonia storage amount of Cat1 exceeds the upper limit value of the ammonia storage target, S2 is repeated, the 1# nozzle is switched to the reduced injection state, and a new round of ammonia storage periodic control is started. By using the method, the aftertreatment system under the same hardware performance condition can realize higher overall conversion efficiency and reducing agent energy efficiency ratio.

Description

Technical Field

[0001] This invention relates to the field of exhaust gas emissions, and more specifically, to a method for improving the reactivity of a two-stage DeNOx system by periodically controlling the ammonia storage state. Background Technology

[0002] Diesel engine exhaust contains a significant amount of nitrogen oxides (NOx) and particulate matter (PM). Existing emission regulations limit the amount of NOx and PM emissions and specify different limits. As emission regulations become more stringent, the weighting factor of the corresponding emission limits on cold start emissions increases significantly, prompting the development of aftertreatment systems towards a tighter coupling of size and function.

[0003] NOx is the product of the reaction between N2 and O2 in the air drawn into the cylinders of an engine at high temperatures, and its main components are NO and NO2. Urea Selective Catalytic Reduction (Urea-SCR) technology is a major technology for controlling NOx emissions in engines. The most common form of this technology involves using an aqueous solution of urea to decompose and produce ammonia (NH3). Under the action of the SCR catalyst, the ammonia undergoes a selective catalytic reduction reaction with NOx, generating nitrogen and water, which are then released into the atmosphere. By injecting different amounts of urea into the diesel engine exhaust, NOx emissions can be effectively controlled. The hydrolysis and pyrolysis reactions of urea cannot occur fully below 187℃. Furthermore, the reaction rate of the SCR reaction below 250℃ is significantly affected by the NO2 / NOx ratio, the real-time ammonia storage capacity, temperature, and space velocity.

[0004] Currently, the mainstream DeNOx system is the two-stage DeNOx system, with the first stage positioned as close as possible to the engine outlet to reach the efficient reaction temperature more quickly. There are several technical routes for two-stage DeNOx systems, among which the route of Oxidation Catalyst (DOC) + SCR-coated Particulate Filter (SDPF) + Selective Catalytic Reduction (SCR) + Ammonia Oxidation Catalyst (ASC) has significant advantages in terms of layout space and cost, and is widely used. However, this technical route has the following problems. Because the oxidation catalyst lacks selectivity and directly oxidizes NH3, the SDPF is usually only coated with the SCR catalyst. The SCR reaction preferentially consumes NO2 in the exhaust, severely weakening the passive regeneration capability of the SDPF and shortening the active regeneration interval, thus affecting fuel consumption and the lifespan of the aftertreatment system.

[0005] Meanwhile, in traditional SDPF+SCR two-stage injection systems, the urea distribution strategy often prioritizes the upstream SDPF, with the downstream SCR serving as a supplement when the upstream SDPF's reaction capacity is insufficient. In this case, a large amount of reducing agent reacts on the SDPF, leaving almost no NO2 residue. Consequently, the regeneration interval in the aftertreatment system tends to be too short. Furthermore, under high-temperature conditions, the traditional distribution strategy in two-stage urea distribution systems leads to greater urea oxidation due to the concentrated urea injection at the higher-temperature SDPF inlet, resulting in poor reducing agent efficiency.

[0006] It is evident that current mainstream SCR systems employ urea injection control strategies based on a relatively constant ammonia-to-nitrogen ratio (ANR) as feedforward control, supplemented by feedback values ​​from downstream NOx concentration or catalyst support ammonia storage as closed-loop control. These strategies generally maintain the urea injection rate at a level relatively stable to upstream NOx emissions. However, performance studies of selective catalytic conversion systems indicate that a constant injection rate cannot simultaneously maximize catalyst performance and reductant efficiency under stable inlet conditions. Only by varying the reductant supply according to the optimal dynamic parameter variation curve under current operating conditions—that is, adopting a periodic reductant supply strategy—can the relevant catalyst parameters be maintained at a high activity level within a controllable range, thereby achieving higher conversion efficiency and energy efficiency ratio. In single-stage DeNOx systems employing a periodic reductant supply strategy, the actual ammonia storage capacity of the support limits the time required to reach high NH3 or NOx storage levels. The relatively short peak NOx conversion efficiency window often fails to cover the NOx conversion efficiency decline during this storage period, resulting in poor overall application performance. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for improving the reaction capacity of a two-stage DeNOx system through periodic ammonia storage state control, effectively improving the NOx conversion efficiency and reducing agent energy efficiency ratio of the post-treatment system under the same post-treatment hardware performance conditions.

[0008] To achieve the above-mentioned technical effects, the present invention adopts the following technical solution:

[0009] A method for improving the reactivity of a two-stage DeNOx system by periodically controlling the ammonia storage status includes the following steps:

[0010] S1. Continuously monitor the temperature of Cat1 carrier, the temperature of Cat2 carrier, the exhaust temperature, and the exhaust mass flow rate, and determine whether the current operating conditions are suitable for implementing the ammonia storage periodic control strategy.

[0011] S2. When the judgment condition is met, the system enters the ammonia storage periodic control state. First, it is judged whether the actual ammonia storage amount of Cat1 carrier exceeds its target upper limit value. If the actual ammonia storage does not exceed this limit, the urea injection amount of No. 1 urea nozzle is adjusted to make the ammonia storage level on Cat1 carrier increase rapidly. When the actual ammonia storage exceeds this limit, No. 1 urea nozzle enters the reduced injection state.

[0012] While the S3.1# urea nozzle enters the reduced spray state, the 2# urea nozzle enters the spray state, and the spray volume of the 2# urea nozzle is controlled.

[0013] S4. When the actual ammonia storage capacity of Cat2 carrier exceeds its target upper limit for ammonia storage, urea nozzle #2 enters a reduced spraying state.

[0014] S5. After monitoring that the real-time DeNOx conversion efficiency of Cat2 carrier begins to decline, No. 1 urea nozzle enters the injection state and sets its injection volume control target as follows: while maximizing the ammonia storage rate of Cat1 carrier, the NH3 leakage concentration at the outlet of Cat1 carrier is controlled within the calibration limit; under this state, the control target of NH3 supply of No. 1 urea nozzle = theoretical NH3 reaction consumption rate calculated by the original NOx sensor + maximum ammonia storage rate of Cat1 carrier + ammonia leakage rate limit.

[0015] S6. When the actual ammonia storage capacity of Cat1 carrier exceeds the target upper limit of ammonia storage, repeat step S2, switch the No. 1 urea nozzle to the reduced spray state, and start a new round of ammonia storage periodic control.

[0016] S7. During the periodic control of ammonia storage, if the temperature and exhaust conditions no longer meet the conditions for implementing the periodic control strategy, the periodic control mode of ammonia storage will be exited.

[0017] Preferably, the implementation conditions of the control strategy require determining suitable temperature and space velocity windows through small-scale catalyst experiments.

[0018] Preferably, in step S2, the specific injection control target of the reduced injection state of the No. 1 urea nozzle needs to be determined based on the real-time responsiveness of the Cat2 carrier. When the real-time responsiveness of the Cat2 carrier is weak, the injection volume of the No. 1 urea nozzle needs to be appropriately increased to control the NOx level entering the Cat2 carrier within its convertible range. When the real-time responsiveness of the Cat2 carrier is sufficient, the injection volume of the No. 1 urea nozzle can be set to 0.

[0019] Preferably, in step S3, the injection volume of the 2# urea nozzle after entering the injection state needs to be calculated using the chemical reaction kinetics model and adsorption-desorption rate model of the Cat2 carrier: NH3 target supply of 2# urea nozzle = NH3 theoretical reaction consumption rate calculated by the measurement value of the NOx sensor at the inlet of the Cat2 carrier + maximum ammonia storage rate under the current state calculated by the adsorption-desorption rate model of the Cat2 carrier + maximum value of NH3 oxidation reaction rate calculated by the ASC reaction kinetics model.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] This invention provides a method for periodic control of ammonia storage in a two-stage DeNOx system. This method, through periodic full-load and no-load control of the ammonia storage levels of the front and rear stage DeNOx catalyst supports, can achieve a significant increase in system reaction capacity three times within a single injection control cycle. Using this method, the aftertreatment system can achieve higher overall conversion efficiency and reductant efficiency ratio under the same hardware performance conditions. Furthermore, the wide-range ANR environment provided by this method offers better diagnostic conditions for NOx sensor signal diagnosis and catalyst performance diagnosis, facilitating more comprehensive and accurate online monitoring (OBM) functionality in the aftertreatment system. Attached Figure Description

[0022] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0023] Figure 1 This is a schematic diagram of the vehicle aftertreatment system suitable for matching the two-stage DeNOx system described in Example 2;

[0024] Figure 2 This is a flowchart illustrating the method for improving the reactivity of a two-stage DeNOx system by periodically controlling the ammonia storage status as described in Example 1.

[0025] Figure 3 This is a schematic diagram of the system state changes during the periodic control process of ammonia storage described in Example 1. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0027] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0028] Descriptions using terms such as "first" and "second" in the application are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated.

[0029] Example 1

[0030] like Figure 2 As shown, this embodiment provides a method for improving the reactivity of a two-stage DeNOx system by periodically controlling the ammonia storage state. The method includes the following steps:

[0031] 1) The control system continuously monitors the temperatures of Cat1 and Cat2 carriers, exhaust temperature, and exhaust mass flow rate to determine whether the conditions for implementing the periodic ammonia storage control strategy are met. At lower temperatures, both the ammonia adsorption / desorption rate and the SCR reaction rate are lower, resulting in a longer cycle for achieving a single conversion efficiency gain. When the entire cycle is so long that the single conversion efficiency gain is insufficient to compensate for the decrease in conversion efficiency during other periods within the cycle, implementing the periodic ammonia storage control strategy under such conditions will not achieve the desired effect. Simultaneously, when the temperature is too high, the ammonia storage capacity of the carrier is very limited, making the implementation of the periodic ammonia storage control strategy less meaningful and instead increasing the control difficulty of the system. The implementation conditions of the control strategy need to be determined through small-scale catalyst experiments to identify suitable temperature and space velocity windows.

[0032] Once the judgment conditions are met, the ammonia storage periodic control state is entered. Since the reducing agent supply strategy before entering this state is generally based on closed-loop control of the target ammonia storage amount, Cat1 is usually at a high ammonia storage level. At this point, the calculated actual ammonia storage amount of Cat1 is compared with the target upper limit under the ammonia storage periodic control state. Once the actual ammonia storage exceeds this limit, urea nozzle #1 enters a reduced-spray state to obtain a short-term boost in reaction capacity. Simultaneously, urea nozzle #2 enters a spray state to allow Cat2 to quickly reach the target upper limit of ammonia storage. After urea nozzle #1 enters the reduced-spray state, because the NOx / (NH3+NOx) ratio in the Cat1 inlet exhaust is very high, and most of the active sites on the Cat1 carrier are occupied by NH3, the probability of collisions between NH3 and NOx molecules at the active sites increases under this condition, thus giving Cat1 a short-term boost in reaction capacity. The reduction in the spray volume of urea nozzle #1 can quickly empty the ammonia stored on Cat1. The continuation of this state allows NOx to be continuously stored on the Cat1 carrier.

[0033] The specific injection control target for the reduced injection state of the No. 1 urea nozzle needs to be determined based on the real-time response capability of Cat2. When the real-time response capability of Cat2 is weak, the injection volume of the No. 1 urea nozzle needs to be appropriately increased to control the NOx level entering Cat2 within its convertible range; when the real-time response capability of Cat2 is sufficient, the injection volume of the No. 1 urea nozzle can be set to 0.

[0034] The injection rate of the No. 2 urea nozzle after entering the injection state needs to be calculated using the chemical reaction kinetics model and adsorption-desorption rate model of Cat2: The target NH3 supply of the No. 2 urea nozzle = the theoretical NH3 reaction consumption rate calculated by the measurement value of the NOx sensor at the Cat2 inlet + the maximum ammonia storage rate under the current state calculated by the adsorption-desorption rate model of Cat2 + the maximum value of the NH3 oxidation reaction rate calculated by the ASC reaction kinetics model. The purpose of setting this target supply is to quickly establish the ammonia storage level of Cat2 to the target value so as to ensure that the overall DeNOx efficiency of the system is always at a high level while controlling ammonia leakage within the conversion capacity of ASC.

[0035] 2) Before the calculated value of the actual ammonia storage in Cat2 exceeds the upper limit of the ammonia storage target, urea nozzles #1 and #2 maintain the current injection control state. Once it is detected that the calculated value of the actual ammonia storage in Cat2 exceeds the upper limit of the ammonia storage target, urea nozzle #2 enters the reduced injection state to obtain a short-term boost in reaction capacity. Since Cat1 is still in the reduced injection state at this time, the actual conversion efficiency of Cat2 will drop rapidly after the short-term efficiency peak. Before the actual conversion efficiency of Cat2 starts to drop, urea nozzle #1 should enter the injection state.

[0036] The time interval between the reduced spray state of urea nozzle #2 and the start of spraying from urea nozzle #1 is a key parameter directly affecting the overall conversion efficiency. If this time interval is set too short, the NOx ratio at the Cat2 inlet will not have a significant advantage over NH3, making it difficult for urea nozzle #2 to achieve an efficiency improvement. If this time interval is set too long, the decrease in Cat2 conversion efficiency when urea nozzle #1 is not spraying will directly lead to a decrease in overall DeNOx efficiency. Therefore, there is an optimal value for the time interval between the reduced spray state of urea nozzle #2 and the start of spraying from urea nozzle #1. This optimal value can be obtained by referring to the optimal spray interval calibration table based on the inlet NOx mass flow rate and the real-time ammonia storage of Cat2. To avoid emission risks caused by calibration deviations of the optimal spray interval, the real-time DeNOx efficiency of the Cat2 carrier needs to be monitored simultaneously. Once a significant decrease in the real-time DeNOx efficiency of the Cat2 carrier is detected within a short period, urea nozzle #1 should be immediately opened to supply reducing agent.

[0037] 3) After the No. 1 urea nozzle is opened, Cat1 will experience a short-term boost in responsiveness due to the high NOx storage level. Once the ammonia storage level in Cat1 reaches the target value, a new round of periodic ammonia storage control will begin.

[0038] 4) After entering the ammonia storage periodic control state, the control system will still monitor the temperature of Cat1 carrier, the temperature of Cat2 carrier, the exhaust temperature, and the exhaust mass flow rate. Once the conditions for entering the state are not met for a certain period of time, the system will exit the ammonia storage periodic control state and enter the normal reducing agent supply control state.

[0039] System state changes during the periodic control process of ammonia storage, such as Figure 3As shown, the ammonia storage periodic control is carried out under suitable temperature and exhaust conditions. After entering the ammonia storage periodic control state, the system first confirms whether the actual ammonia storage amount of Cat1 has reached the upper limit of the ammonia storage target under this control state. If the upper limit is reached, the No. 1 urea nozzle enters the reduced spray control state to improve the primary conversion efficiency. While the No. 1 urea nozzle reduces spray, the No. 2 urea nozzle starts spraying. Its spray control objective is to make Cat2 reach the upper limit of the ammonia storage target as soon as possible. Therefore, it is necessary to continuously calculate the maximum NH3 adsorption rate of Cat2, the NOx mass flow rate at the Cat2 inlet, and the real-time ammonia storage amount under this operating condition.

[0040] As this state continues, the ammonia storage level in Cat1 will continue to decrease. After decreasing to a certain level, the NOx storage will begin to rise because there is not enough NH3 to react with the NOx in the exhaust gas. At this time, the NOx storage level in Cat2 remains close to 0 due to the continuous urea injection at the inlet.

[0041] When the actual ammonia storage capacity of Cat2 reaches the upper limit of the ammonia storage target under this control state, the No. 2 urea nozzle enters the reduced spray control state to improve the primary conversion efficiency. At this time, the No. 1 urea nozzle is still in the stopped spray state. The NOx concentration at the Cat2 inlet is very high, and the actual ammonia storage level of Cat2 will decrease rapidly.

[0042] Once the efficiency peak of Cat2 is reached, and a decrease in Cat2 efficiency is detected, the No. 1 urea nozzle is immediately opened. At this point, a large amount of NOx has been adsorbed on the Cat1 carrier. The adsorbed active NOx molecules and the NH3 molecules generated by the No. 1 urea nozzle have a higher collision probability, thus generating a conversion efficiency peak. As the reaction proceeds, the NOx stored on Cat1 is rapidly reacted off, and the remaining NH3 begins to be stored on Cat1, causing the ammonia storage level of Cat1 to gradually increase. When the actual ammonia storage capacity of Cat1 reaches the upper limit of the target ammonia storage under this control state, a new round of ammonia storage periodic control begins.

[0043] Example 2

[0044] like Figure 1As shown, this embodiment provides a vehicle aftertreatment system suitable for matching a two-stage DeNOx system. Engine exhaust sequentially passes through DOC, Cat1, and Cat2 before entering the atmosphere. DOC oxidizes reducing gases such as CO and HC in the exhaust, while Cat1 and Cat2 consume NOx through SCR reactions. When the carrier temperature of Cat1 exceeds the critical temperature for urea injection, urea nozzle #1 begins to inject urea. The injected urea undergoes pyrolysis and hydrolysis under the heating effect of the exhaust, generating NH3. NH3 reacts rapidly with NOx under the action of the SCR catalyst on Cat1, generating N2 and H2O. If the urea injection amount is insufficient, the remaining NOx will continue to flow downstream; if the urea injection amount is excessive, the remaining NH3 will flow downstream. Cat2, as a redundant DeNOx system, handles the remaining NOx conversion, and urea nozzle #2 supplies it with the necessary reducing agent. The reducing agent supply system of the entire aftertreatment system uses a 32.5% (w / w) urea aqueous solution as the reducing agent. The ECU and DCU can be independent hardware structures or combined into a single control unit. The ECU and DCU collect signals from engine speed, fuel injection quantity, intake air temperature, intake air pressure, intake air mass flow rate, EGR valve opening, coolant temperature, upstream temperature sensor of the DOC catalyst, Cat1 inlet temperature sensor, Cat2 inlet temperature sensor, downstream temperature sensor of the SCR catalyst, upstream NOx concentration sensor of the DOC catalyst, Cat2 inlet NOx concentration sensor, Cat2 outlet NOx concentration sensor, and urea level sensor. Based on the periodic control target of the ammonia storage levels of Cat1 and Cat2, the corresponding control function modules calculate the precise amount of reductant supplied to the two-stage injection system, thereby improving the overall conversion efficiency and reductant energy efficiency ratio of the two-stage DeNOx system.

[0045] The specific embodiments of the present invention have been described above. Based on the above description, those skilled in the art can make various changes and modifications without departing from the technical concept of the present invention.

Claims

1. A method for enhancing the reactivity of a two-stage DeNOx system by periodically controlling the ammonia storage state, characterized in that, Specifically, the following steps are included: S1. Continuously monitor the temperature of Cat1 carrier, the temperature of Cat2 carrier, the exhaust temperature, and the exhaust mass flow rate, and determine whether the current operating conditions are suitable for implementing the ammonia storage periodic control strategy. S2. When the judgment condition is met, the system enters the ammonia storage periodic control state. First, it is judged whether the actual ammonia storage amount of Cat1 carrier exceeds its target upper limit value. If the actual ammonia storage does not exceed this limit, the urea injection amount of No. 1 urea nozzle is adjusted to make the ammonia storage level on Cat1 carrier increase rapidly. When the actual ammonia storage exceeds this limit, No. 1 urea nozzle enters the reduced injection state. While the S3.1# urea nozzle enters the reduced spray state, the 2# urea nozzle enters the spray state, and the spray volume of the 2# urea nozzle is controlled. S4. When the actual ammonia storage capacity of Cat2 carrier exceeds its target upper limit for ammonia storage, urea nozzle #2 enters a reduced spraying state. S5. After monitoring that the real-time DeNOx conversion efficiency of Cat2 carrier begins to decline, No. 1 urea nozzle enters the injection state and sets its injection volume control target as follows: while maximizing the ammonia storage rate of Cat1 carrier, the NH3 leakage concentration at the outlet of Cat1 carrier is controlled within the calibration limit; under this state, the control target of NH3 supply of No. 1 urea nozzle = theoretical NH3 reaction consumption rate calculated by the original NOx sensor + maximum ammonia storage rate of Cat1 carrier + ammonia leakage rate limit. S6. When the actual ammonia storage capacity of Cat1 carrier exceeds the upper limit of the ammonia storage target, repeat step S2, switch the No. 1 urea nozzle to the reduced spray state, and start a new round of ammonia storage periodic control. S7. During the periodic control of ammonia storage, if the temperature and exhaust conditions no longer meet the conditions for implementing the periodic control strategy, the periodic control mode of ammonia storage will be exited.

2. The method for improving the reaction capacity of a two-stage DeNOx system by periodically controlling the ammonia storage state according to claim 1, characterized in that, The implementation conditions for the control strategy need to be determined by conducting small-scale catalyst tests to identify suitable temperature and space velocity windows.

3. The method for improving the reaction capacity of a two-stage DeNOx system by periodically controlling the ammonia storage state according to claim 1, characterized in that, In step S2, the specific injection control target of the reduced injection state of the No. 1 urea nozzle needs to be determined based on the real-time responsiveness of the Cat2 carrier. When the real-time responsiveness of the Cat2 carrier is weak, the injection volume of the No. 1 urea nozzle needs to be appropriately increased to control the NOx level entering the Cat2 carrier within its convertible range. When the real-time responsiveness of the Cat2 carrier is sufficient, the injection volume of the No. 1 urea nozzle can be set to 0.

4. The method for improving the reaction capacity of a two-stage DeNOx system by periodically controlling the ammonia storage state according to claim 1, characterized in that, In step S3, the injection rate of the No. 2 urea nozzle after entering the injection state needs to be calculated using the chemical reaction kinetics model and adsorption-desorption rate model of the Cat2 carrier: NH3 target supply rate of No. 2 urea nozzle = NH3 theoretical reaction consumption rate calculated by the NOx sensor measurement value at the inlet of the Cat2 carrier + maximum ammonia storage rate under the current state calculated by the adsorption-desorption rate model of the Cat2 carrier + maximum value of NH3 oxidation reaction rate calculated by the ASC reaction kinetics model.

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