Selective Catalytic Reduction System with Modulated DEF Dosing
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Solution Overview
Problem
Existing SCR systems face challenges in controlling ammonia production due to catalyst cooling by diesel exhaust fluid (DEF), leading to reduced NOx conversion and potential ammonia slip events, especially at low and medium temperature conditions.
Innovation Solution
A selective catalytic reduction system with a controller that modulates the DEF dosing frequency to less than or equal to 1 Hertz, allowing the hydrolysis catalyst to recover temperature and optimize ammonia production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If DEF is injected at high frequency (1-10 Hz) to maintain ammonia production, then ammonia availability for NOx reduction is improved, but catalyst temperature drops causing suppressed thermolysis and hydrolysis reactions
Solution Approach 1:
The patent applies periodic action by injecting DEF in pulsed sequences rather than continuous high-frequency injection. The controller is configured to inject DEF for a first period of time, then a second period of time with no injection, allowing the catalyst temperature to recover between injection events. This periodic injection pattern maintains ammonia production while preventing excessive catalyst cooling that would suppress the thermolysis and hydrolysis reactions.
2Productivity
If DEF injection frequency is increased to maximize ammonia release, then NOx conversion is improved, but ammonia slip events increase due to unreacted ammonia passing untreated
Solution Approach 1:
The patent implements feedback control where the controller monitors exhaust conditions and adjusts DEF injection timing and duration accordingly. By using feedback from temperature sensors and exhaust composition analysis, the system optimizes injection events to achieve complete ammonia utilization for NOx reduction, preventing ammonia slip events while maintaining high NOx conversion efficiency.
3Quantity of substance
If DEF is injected continuously at high flow rate, then ammonia storage in SCR catalyst is improved, but urea deposits form in exhaust passage due to prevented thermolysis
Solution Approach 1:
The periodic injection strategy allows sufficient time between DEF injection events for complete thermolysis of urea and hydrolysis of isocyanic acid. This prevents urea deposits from forming in the exhaust passage while still accumulating adequate ammonia storage in the SCR catalyst during injection periods, eliminating the trade-off between ammonia storage and deposit formation.
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 reduction capabilities and prevents ammonia slip events by ensuring effective ammonia production and storage in the SCR catalyst.
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 converts any nitrogen oxides (NCO) present in the exhaust gas into nitrogen and water
Implementation Method 3
the DEF cools the catalyst sufficiently to either slow down or effectively prevent thermolysis of urea
Implementation Method 4
The DEF contains urea which undergoes a hydrolysis and/or thermolysis within the exhaust passage whereby ammonia is produced
Data Source
AI summary
A selective catalytic reduction (SCR) system is provided for treating exhaust gas in an exhaust passage. The system comprises a hydrolysis catalyst located in the exhaust passage, and a diesel exhaust fluid (DEF) dosing unit configured to inject DEF onto the hydrolysis catalyst. A SCR catalyst is located in the passage downstream of the hydrolysis catalyst, and a controller controls DEF dosing by the dosing unit. The controller is configured to control the DEF dosing unit such that the DEF is injected at a modulated frequency of less than or equal to 1 Hertz. A method of treating exhaust gas in an exhaust passage using an SCR system is also provided.