SCR Catalyst Reducing Agent Dosing for High Load NOx Conversion
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Solution Overview
Problem
Two-stage SCR catalytic converter systems face challenges in achieving sufficient NOX conversion rates, especially at high load situations due to limited NOX conversion at the close-coupled SCR exhaust gas cleaning device and high exhaust gas temperatures leading to ammonia desorption and reducing agent slip.
Innovation Solution
A method for operating the reducing agent metering system that switches to a high-load operating mode by reducing the reducing agent fill level in the upstream SCR exhaust gas cleaning device at high exhaust gas mass flows, allowing the downstream SCR catalytic converter to be charged with released reducing agent, ensuring complete conversion and preventing overload.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the close-coupled SCR exhaust gas cleaning device is used to quickly heat up and enable early reducing agent metering, then the NOX conversion is improved at low loads, but the NOX conversion rate becomes insufficient at high load situations due to limited catalytic coating volume
Solution Approach 1:
The SCR system is divided into two separate catalytic converters: a close-coupled SCR device for quick warm-up and early reducing agent metering, and a downstream SCR catalyst for handling high load situations. This segmentation allows each component to be optimized for its specific operating range, resolving the contradiction between early activation and sufficient conversion capacity at high loads.
Solution Approach 2:
Each SCR component is assigned a specific functional role based on operating conditions: the close-coupled device handles low-load operations with its rapid warm-up capability, while the downstream catalyst handles high-load operations with its larger volume and higher conversion capacity. This local optimization of function to condition resolves the volume limitation contradiction.
2Productivity
If the downstream SCR catalytic converter is sufficiently loaded with reducing agent, then the NOX conversion capacity is ensured, but the reducing agent slip occurs at high exhaust gas temperatures due to ammonia desorption
Solution Approach 1:
The reducing agent metering is dynamically adjusted based on real-time exhaust gas temperature and mass flow conditions. At high temperatures and high loads, the metering rate is reduced to prevent excessive ammonia storage that would lead to desorption and slip. This dynamic control allows the system to maintain adequate reducing agent levels for NOX conversion while preventing harmful slip under extreme conditions.
Solution Approach 2:
The controlling parameters for reducing agent metering are changed based on operating conditions: at high exhaust gas temperatures and high mass flows, the target ammonia filling level is reduced to prevent desorption and slip, while at lower temperatures and loads, higher filling levels are maintained to ensure conversion capacity. This parameter adaptation resolves the contradiction between conversion capacity and slip prevention.
3Quantity of substance
If the reducing agent fill level in the upstream SCR device is maintained at high levels, then the reducing agent storage is ensured, but the NOX conversion efficiency decreases at high exhaust gas mass flows due to high space velocities and low contact times
Solution Approach 1:
The target reducing agent fill level in the close-coupled SCR device is dynamically adjusted based on exhaust gas mass flow conditions. At high mass flows, the target fill level is reduced to improve space velocity management and contact time, thereby maintaining conversion efficiency. At lower mass flows, higher fill levels are maintained to ensure adequate reducing agent storage. This dynamic adjustment resolves the contradiction between storage and efficiency.
4Device complexity
If a single reducing agent metering device is used to charge both SCR components, then the system simplicity is maintained, but the precise control of reducing agent distribution to optimize both components' performance becomes difficult
Solution Approach 1:
The single reducing agent metering device is controlled to provide locally optimized reducing agent distribution to each SCR component based on their specific needs. The close-coupled device receives reducing agent at rates optimized for its small volume and quick warm-up function, while the downstream catalyst receives reducing agent at rates optimized for its large volume and high-capacity function. This local optimization through intelligent control resolves the contradiction between system simplicity and performance optimization.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances NOX conversion rates by optimizing reducing agent distribution and utilization, ensuring effective conversion at high load situations and preventing reducing agent slip, thereby meeting stringent emission requirements.
Implementation Method 1
catalytic converter systems, which work according to the principle of selective catalytic reduction (SCR for selective catalytic reduction). These systems include at least one SCR catalytic converter, which converts the nitrogen oxides in the exhaust gas into nitrogen and water in the presence of a reducing agent
Implementation Method 2
The reducing agent can be stored in the SCR catalytic converter. The reducing agent is metered in at least two operating modes, with either only the first SCR component being charged and charged with reducing agent or with a single reducing agent metering device both SCR components being charged and charged with reducing agent
Implementation Method 3
high exhaust gas temperatures leading to ammonia desorption and reducing agent slip
Data Source
AI summary
The invention relates to an SCR catalysts system and to a method for operating a reducing agent dosing of an SCR catalyst system (14) of an internal combustion engine (10), wherein the SCR catalyst system (14) has a first SCR exhaust gas purification device (16) and an SCR catalyst (18) downstream thereof, as well as a reducing agent dosing device (20) for dosing a reducing agent into the exhaust gas flow, downstream of the first SCR exhaust gas purification device (16). According to the invention, if an exhaust gas mass flow (m_Abgas) is greater than a predetermined exhaust gas mass flow threshold (m_max), and a temperature (T_SCR) of the downstream SCR catalyst (18) is greater than a predetermined minimum temperature (T_SCR_min), the reducing agent dosing changes into a high-load operating mode (III), in which a desired reducing agent filling level (NH3_SPF_soll) of the first SCR exhaust gas purification device (16) is reduced or lowered to zero.