SCR Catalyst Ammonia Slip Control During High Temperature Transitions
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Solution Overview
Problem
Selective Catalytic Reduction (SCR) systems face challenges in minimizing ammonia slip, which is both a regulatory concern and an efficiency issue due to momentary ammonia spikes caused by desorption at high temperatures, leading to unreacted ammonia emissions.
Innovation Solution
The method involves detecting engine operation transitions, increasing NOx concentration in the exhaust gas stream, and decreasing reductant dosing to the SCR catalyst, allowing the desorbed ammonia to react with available NOx and reducing ammonia slip by optimizing engine control and reductant injection strategies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the SCR catalyst is exposed to high temperature conditions, then ammonia desorption occurs which can be used for NOx reduction, but momentary ammonia spikes result in increased ammonia slip and wasted ammonia
Solution Approach 1:
The control system detects engine operation transitions and anticipates high temperature conditions before they fully develop. By proactively adjusting reductant dosing and increasing NOx concentration in advance of the temperature rise, the system prepares the exhaust composition to capture desorbed ammonia as it occurs, preventing ammonia slip while maintaining NOx reduction efficiency during the transition phase
Solution Approach 2:
The system continuously monitors engine operating conditions and exhaust parameters to detect transitions. Based on this feedback, the control module dynamically adjusts reductant dosing rates and modulates engine NOx output to match the actual ammonia desorption rate, creating a closed-loop control system that adapts to changing thermal conditions and prevents ammonia slip
2Productivity
If reductant dosing is increased to maintain NOx reduction during high temperature transitions, then NOx conversion is maintained, but ammonia slip increases due to desorption
Solution Approach 1:
The system dynamically adjusts reductant dosing rates based on real-time detection of engine operation transitions and predicted ammonia desorption rates. Rather than using a fixed dosing strategy, the control module modulates the reductant injection rate to match the varying thermal and chemical conditions, optimizing the balance between maintaining NOx conversion and preventing ammonia slip during transient high temperature events
Solution Approach 2:
The control system changes key operational parameters including reductant dosing rate and engine NOx concentration in response to detected transitions. By modulating these parameters dynamically rather than maintaining constant values, the system adapts to changing thermal conditions and ammonia desorption rates, achieving both sustained NOx conversion and reduced ammonia slip
3Temperature
If engine operation transitions to high temperature conditions, then thermal energy is available for NOx reduction, but ammonia desorption causes momentary ammonia spikes and emissions
Solution Approach 1:
The system converts the potentially harmful effect of ammonia desorption into a beneficial resource by capturing the desorbed ammonia through increased NOx concentration. The thermal energy that causes ammonia release is thus productively utilized - the desorbed ammonia is immediately consumed in NOx reduction reactions rather than being emitted, transforming what would be a harmful slip event into an opportunity for enhanced NOx conversion
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 effectively reduces ammonia slip, minimizes fuel consumption, and enhances fuel economy by ensuring that desorbed ammonia is utilized, thereby meeting regulatory standards and improving SCR system efficiency.
Implementation Method 1
The ammonia then reacts with NOx in the presence of the SCR catalyst to reduce NOx to nitrogen and water
Implementation Method 2
Most SCR catalysts adsorb ammonia at low temperature conditions (less than about 300 °C) which may then be used for NOx reduction
Implementation Method 3
Ammonia can be desorbed when the SCR catalyst substrate is exposed to high temperature (greater than about 350 to about 400 °C)
Implementation Method 4
An ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst may be needed to convert most of the ammonia slipping past the SCR catalyst to N2 and other less harmful compounds
Data Source
AI summary
Method, apparatus, and system to control ammonia slip in a selective catalytic reduction (SCR) system. The method includes detecting an engine operation transition event, increasing NOx to the SCR catalyst, and decreasing a reductant dose. The system includes a controller to receive an engine transition signal, anticipate a temperature transition at the SCR catalyst in response to the engine transition signal, and cause a NOx increase in the exhaust gas stream to the SCR catalyst and a decrease of reductant to the SCR catalyst coinciding with the temperature transition at the SCR catalyst. The apparatus includes a NOx increase module to cause the NOx increase in the exhaust gas stream to the SCR catalyst in response to the engine transition signal. The apparatus also includes a reductant shut-off module configured to cause the reductant addition to the exhaust gas stream to turn off in response to the engine transition signal.