Multi-Stage SCR Control System for Catalyst Aging and Dosing Uncertainty
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Solution Overview
Problem
Current SCR systems face challenges in maintaining high deNOx efficiency due to uncertainties in dosing control, catalyst aging, and the difficulty in distinguishing between permanent and temporary failures, leading to increased system sensitivity and warranty costs.
Innovation Solution
A multi-stage SCR control system with a front and back SCR device configuration, using a DCU to adjust reductant dosing rates based on target deNOx efficiencies and reductant quality ratios, and employing a diagnostic system to differentiate between permanent and temporary catalyst failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If SCR catalyst with large storage capability is used together with AMOX catalyst to desensitize NOx control to NSR, then uncertainties in dosing system and sensors can be compensated through over-dosing, but aging issues occur since both SCR catalyst storage capability and AMOX selectivity are subject to aging effects
Solution Approach 1:
The system divides the SCR function into two separate catalysts: a first SCR catalyst for primary NOx reduction and a second SCR catalyst for secondary NOx reduction and ammonia storage. This segmentation allows each catalyst to have specialized functions, with the second catalyst specifically designed for storage capability to compensate for aging effects in the first catalyst.
Solution Approach 2:
The system dynamically adjusts the NSR (Normalized Stoichiometric Ratio) setpoints for dosing control based on operating conditions and catalyst aging states. By changing the dosing parameters adaptively, the system maintains optimal NOx reduction efficiency while managing the impact of catalyst aging over time.
2Reliability
If high deNOx efficiency is maintained to meet emission regulations, then NOx control performance improves, but system sensitivity to dosing uncertainties and catalyst aging increases
Solution Approach 1:
The dual-catalyst system segments the NOx reduction function to allow the first catalyst to operate at high efficiency while the second catalyst provides a buffer through ammonia storage. This reduces the overall system sensitivity to dosing uncertainties because the second catalyst can compensate for variations in ammonia availability.
Solution Approach 2:
The second SCR catalyst is specifically designed with high ammonia storage capability to provide a cushion against dosing uncertainties and catalyst aging effects. This beforehand cushioning allows the first catalyst to maintain high deNOx efficiency without being overly sensitive to variations in the dosing system or aging effects.
3Measurement precision
If feedback control is used to accurately control deNOx efficiency, then control precision improves, but difficulty in detecting catalyst failures increases due to effects of dosing system uncertainties and sensors
Solution Approach 1:
The system segments the SCR function into two independent catalysts with different roles. The second catalyst's ammonia storage function provides a detectable signal that can indicate system health status. When the second catalyst is fully saturated or when dosing uncertainties cause abnormal behavior, this segmentation allows for easier detection of failures compared to a single-catalyst system.
Solution Approach 2:
The second SCR catalyst acts as an intermediary element that can indicate system health status. Its ammonia storage capability provides a buffer that can be monitored to detect dosing system uncertainties and catalyst failures, serving as a mediator between the dosing system and the emission control function.
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
The system achieves robust deNOx efficiency control, reduces sensitivity to catalyst aging, and effectively detects failures, thereby improving overall system performance and reducing warranty costs.
Implementation Method 1
the reductant selectively reacts with NOx generating non-poisonous species, such as nitrogen, carbon dioxide, and water
Implementation Method 2
a dosing amount of reductant is injected into exhaust gas, and the result mixed gas flows into a SCR catalyst, where the reductant selectively reacts with NOx
Implementation Method 3
ammonia is obtained from a urea solution through thermolysis and hydrolysis
Implementation Method 4
ammonia is obtained from a urea solution through thermolysis and hydrolysis
Implementation Method 5
an aged AMOX tends to oxidize ammonia slip back to NOx
Data Source
AI summary
A multi-stage SCR control system including a front reductant dosing device, a front SCR device, a back reductant dosing device, a back SCR device, and a dosing controller. In normal control cycles, a NSR of the front SCR device is controlled below a stoichiometric reaction ratio to decrease system sensitivity to catalyst aging. In a diagnostic cycle, the NSR of the front or the back SCR device is controlled lower than the stoichiometric reaction ratio, and a reductant quality ratio, which is indicative of reductant quality and dosing accuracy, is calculated. In another diagnostic cycle, the NSR is controlled above the stoichiometric reaction ratio, and an average deNOx efficiency is calculated. The reductant quality ratio and the average deNOx efficiency values are further used in SCR feedback control and permanent catalyst damages can also be isolated from temporary catalyst poisons with these values after a thermal recovery event is completed.