SCR Catalyst NO2 to NOx Ratio Management
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Solution Overview
Problem
The efficiency of selective catalytic reduction (SCR) catalysts in reducing NOx emissions declines over time due to the deterioration of upstream diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) capabilities in converting NO to NO2, affecting the NO2 to NOx molar ratio at the SCR catalyst inlet.
Innovation Solution
A system and method that utilize a controller to manage the NO2 to NOx ratio at the SCR catalyst inlet by adjusting reductant dosing upstream of the close-coupled SCR catalyst, increasing NO2 production to maintain an optimal ratio, thereby extending the life of the aftertreatment system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If reductant dosing is increased upstream of the close-coupled SCR catalyst to maintain optimal NO2 to NOx ratio, then SCR catalyst efficiency is improved, but system complexity and cost increase
Solution Approach 1:
The system performs preliminary action by injecting reductant upstream of the close-coupled SCR catalyst before the exhaust reaches the main SCR catalyst. This advance dosing allows the close-coupled catalyst to convert NO to NO2, ensuring optimal NO2 to NOx ratio is established before the exhaust enters the main SCR catalyst, thereby maintaining high conversion efficiency throughout the aftertreatment system's operational life.
Solution Approach 2:
The aftertreatment system is segmented into two distinct SCR catalyst sections: a close-coupled SCR catalyst positioned upstream and a main SCR catalyst downstream. The close-coupled catalyst is specifically tasked with NO to NO2 conversion, while the main catalyst performs the primary NOx reduction. This segmentation allows each component to specialize in its optimal function, with the upstream catalyst compensating for aging effects in the DOC/DPF system.
2Reliability
If reductant dosing is increased to compensate for DOC/DPF aging effects, then NOx reduction efficiency is maintained, but ammonia slip increases
Solution Approach 1:
The close-coupled SCR catalyst acts as an intermediary component between the DOC/DPF system and the main SCR catalyst. It receives reductant dosing upstream and converts excess NO to NO2, thereby mediating the chemical composition of the exhaust before it reaches the main catalyst. This intermediary function ensures optimal NO2 to NOx ratio for the main catalyst while consuming reductant in a controlled manner, preventing excessive reductant carryover that would cause ammonia slip.
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 the NOx reduction efficiency of the SCR catalyst by maintaining an optimal NO2 to NOx ratio, even as the DOC/DPF system ages, thereby improving the overall performance and longevity of the aftertreatment system.
Implementation Method 1
a close-coupled selective catalytic reduction (SCR) catalyst positioned upstream of a diesel oxidation catalyst/DPF system
Implementation Method 2
these components tend to make NO2 from NO upstream of the SCR catalyst inlet
Data Source
AI summary
Systems and methods are provided for determining and controlling an NO2 to NOx ratio reference target in an exhaust conduit between a first SCR catalyst and a second SCR catalyst. The method includes determining a present NO2 to NOx ratio in the exhaust conduit between the first SCR catalyst and the second SCR catalyst, and providing a reductant doser command in response to a deviation of the present NO2 to NOx ratio from the NO2 to NOx ratio reference target.