SCR System Ammonia Storage Control via Variable DEF Dosing
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Solution Overview
Problem
Existing SCR systems face challenges in controlling ammonia storage and NOx conversion due to DEF cooling the catalyst, leading to delayed ammonia release and urea deposits, which result in reduced control over NOx conversion and potential ammonia slip events.
Innovation Solution
A method and system that predict and adjust the DEF dosing rate based on ammonia storage and NOx conversion rates, using sensors and a control module to optimize DEF injection on a hydrolysis catalyst, ensuring stable ammonia-equivalent storage and improved NOx conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DEF is dosed onto the hydrolysis catalyst to convert NOx, then ammonia is produced for NOx reduction, but the DEF cools the catalyst sufficiently to slow down or prevent thermolysis of urea and subsequent hydrolysis of isocyanic acid
Solution Approach 1:
The system performs preliminary action by dosing DEF in advance to build up ammonia storage on the hydrolysis catalyst before NOx conversion is needed. The control module monitors ammonia storage levels and adjusts DEF dosing rate proactively, allowing the catalyst to accumulate sufficient ammonia reservoir ahead of time, thus avoiding delays when conversion is required.
Solution Approach 2:
The control module implements feedback control by continuously monitoring ammonia storage on the hydrolysis catalyst and adjusting the DEF dosing rate accordingly. When ammonia storage reaches optimal levels, the module reduces or stops DEF dosing to prevent excessive cooling and maintain thermolysis/hydrolysis reactions, thereby preventing ammonia release delays while ensuring sufficient ammonia availability for NOx conversion.
2Productivity
If DEF dosing rate is increased to improve NOx conversion, then more ammonia is available for reduction, but urea deposits form in the exhaust passage and ammonia slip events occur
Solution Approach 1:
The control module uses feedback control to continuously monitor ammonia storage levels on the hydrolysis catalyst and adjust the DEF dosing rate in real-time. When ammonia storage reaches optimal levels, the module reduces or stops DEF dosing to prevent excessive ammonia production, urea deposit formation, and ammonia slip events, while maintaining sufficient ammonia availability for NOx conversion.
Solution Approach 2:
The system dynamically changes the DEF dosing rate parameter based on monitored ammonia storage levels and exhaust conditions. By adjusting this critical parameter, the system optimizes the balance between producing sufficient ammonia for high NOx conversion rates and preventing harmful side effects such as urea deposits and ammonia slip.
3Quantity of substance
If DEF is dosed to maintain ammonia storage, then NOx conversion is improved, but the catalyst temperature drops affecting subsequent ammonia release
Solution Approach 1:
The control module implements feedback control by monitoring ammonia storage levels and exhaust temperature, adjusting the DEF dosing rate to maintain ammonia storage while preventing excessive catalyst cooling. When temperature drops are detected or predicted, the module reduces DEF dosing to maintain thermolysis and hydrolysis reactions, ensuring continuous ammonia release capability.
Solution Approach 2:
The system dynamically adjusts the DEF dosing rate based on real-time conditions including ammonia storage levels and exhaust temperature. This dynamic control allows the system to optimize the balance between accumulating ammonia storage and maintaining catalyst temperature for subsequent ammonia release, adapting to changing operating conditions.
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 efficiency, reduces ammonia slip, and maintains stable ammonia storage, even under varying exhaust temperatures, improving engine start-up performance by managing ammonia-equivalent substances effectively.
Implementation Method 1
The DEF contains urea which undergoes a hydrolysis and/or thermolysis within the exhaust passage whereby ammonia is produced
Implementation Method 2
The ammonia passes into a SCR catalyst where it reacts with the exhaust gas, wherein any nitrogen oxides (NOx) present in the exhaust gas are converted to nitrogen and water
Implementation Method 3
When dosing DEF onto a hydrolysis catalyst in a SCR system under certain conditions the DEF cools the catalyst sufficiently to either slow down or effectively prevent thermolysis of urea and subsequent hydrolysis of isocyanic acid
Implementation Method 4
the DEF cools the catalyst sufficiently to either slow down or effectively prevent thermolysis of urea
Data Source
AI summary
A method of treating exhaust gas in an exhaust passage using a selective catalytic reduction system is provided. The system comprises a hydrolysis catalyst in the passage upstream of a SCR catalyst, and a diesel exhaust fluid (DEF) dosing unit for injecting DEF onto the hydrolysis catalyst at a variable DEF dosing rate. The method comprises the steps of predicting an initial DEF dosing rate for converting all nitrogen oxide (NOx) contained in the exhaust gas, and estimating an amount of ammonia stored on the SCR catalyst. The method further comprises the steps of measuring a NOx conversion rate for the system, and adjusting the initial DEF dosing rate based upon the ammonia storage estimate and the measured NOx conversion rate to produce a first adjusted DEF dosing rate. An amount of ammonia-equivalent stored on the hydrolysis catalyst is then estimated, and the first adjusted DEF dosing rate is adjusted based upon the ammonia-equivalent storage estimate to produce a second adjusted DEF dosing rate. DEF is then injected at the second adjusted DEF dosing rate.