SCR Feed Ratio Oscillation for Low-Temperature NOx Conversion
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Solution Overview
Problem
Existing SCR systems in lean-running internal combustion engines face challenges in achieving high NOx conversion efficiency due to varying operating conditions, ammonia precursor decomposition issues, and ammonia emissions, particularly under conditions of low exhaust gas temperature and uneven distribution, leading to undesirable deposits and reduced catalyst performance.
Innovation Solution
A method involving periodic variation of raw NOx emissions to adjust the NH3 to NOx ratio (feed ratio α) by altering engine operating parameters, such as injection timing and air-fuel ratio, to optimize ammonia storage and utilization on the catalyst surface, even under suboptimal conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the amount of ammonia supplied exceeds the amount of nitrogen oxides converted, then NOx conversion is maximized, but unused ammonia is emitted causing toxicity issues
Solution Approach 1:
The patent applies periodic action by varying the ammonia supply amount in a periodic manner rather than maintaining a constant supply. The control unit periodically adjusts the reducing agent supply based on detected NOx levels, creating cyclic variations in ammonia concentration that prevent both insufficient NOx conversion and excessive ammonia accumulation, thereby eliminating harmful ammonia emissions while maintaining high conversion rates
2Loss of energy
If urea decomposition is performed at temperatures below 350°C, then energy consumption is reduced, but solid deposits and clogging occur
Solution Approach 1:
The patent applies parameter changes by modifying the temperature parameter dynamically. The control unit adjusts the exhaust gas temperature parameter in response to detected NOx levels and operating conditions, allowing the system to operate at lower temperatures when possible while preventing deposits by adjusting temperature parameters to maintain optimal decomposition conditions and avoid catalyst clogging
3Productivity
If the feed ratio α is increased to improve NOx conversion, then conversion efficiency increases, but ammonia storage on catalyst increases leading to potential emissions
Solution Approach 1:
The patent applies feedback by continuously detecting NOx levels and using this information to control the reducing agent supply. The control unit receives feedback from the NOx detector and adjusts the ammonia supply accordingly, creating a closed-loop system that prevents excessive ammonia accumulation on the catalyst while maintaining optimal conversion efficiency through real-time parameter adjustment
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
Enhances NOx conversion rates and reduces ammonia deposits, improving catalyst efficiency and fuel consumption by dynamically managing ammonia storage and utilization, even at low temperatures and uneven distributions.
Implementation Method 1
The use of SCR catalysts to reduce nitrogen oxides in the exhaust stream of an internal combustion engine is well known. In the selective catalytic reduction (SCR) process carried out with these SCR catalysts, a directly reducing substance, such as ammonia or a precursor that only releases reducing substances in the exhaust gas, is added to the exhaust stream.
Implementation Method 2
In connection with the decomposition of urea to ammonia, it is known that this occurs in two stages under optimal conditions, i.e., temperatures above 350°C. After (NH2)2CO → NH3 + HNCO (1) First, thermolysis occurs, i.e., the thermal decomposition of urea.
Implementation Method 3
Then, according to HNCO + H2O → NH3 + CO2 (2), hydrolysis takes place, i.e., the catalytic decomposition of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2).
Implementation Method 4
One mole of ammonia is required to react one mole of nitric oxide. 4NO + 4NH3 + O2 → 4N2 + 6H2O (3) The ratio between NH3 and NOx is called the feed ratio α.
Data Source
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AI summary
A method for use in conjunction with an exhaust aftertreatment system operated on an internal combustion engine running with excess air, wherein the reduction of nitrogen oxides is achieved by adding an ammonia-releasing reducing agent to the exhaust stream upstream of a catalyst loaded with a catalyst material for the selective catalytic reduction of nitrogen oxides. The method provides that the NH3 to NOx ratio (feed ratio α) is varied phasewise by changing the raw nitrogen oxide emissions of the internal combustion engine, such that the feed ratio α oscillates around a predetermined value in each phase.