Dual Bed SCR Catalyst System for Low-Temperature NOx Conversion
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Solution Overview
Problem
Conventional catalyst systems for reducing nitrogen oxides in exhaust gases from lean-burn combustion engines, particularly at low temperatures, face challenges such as high precious metal loading, deactivation issues, and inability to maintain optimal NO:NO2 ratio across a wide temperature range, leading to suboptimal NOx conversion and hydrogen consumption.
Innovation Solution
A dual bed catalyst system comprising an iron-beta-zeolite as the first bed and silver supported on alumina as the second bed, utilizing a mixture of hydrogen and ammonia as reducing agents, with physical separation of catalysts and optimized silver loading, reduces hydrogen consumption and enhances stability and activity across a broad temperature range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If an oxidation catalyst is applied upstream the SCR catalyst to obtain the NO:NO2 ratio close to 1:1 required for the fast SCR reaction, then the NOx conversion at low temperatures is improved, but the device complexity increases and precious metal loading increases
Solution Approach 1:
The patent combines the oxidation function and SCR reduction function into a single integrated catalyst system. The catalyst contains both oxidation sites (for converting NO to NO2) and SCR active sites (for reducing NOx with NH3), eliminating the need for separate oxidation and SCR catalyst beds while maintaining the beneficial fast SCR reaction at low temperatures
2Productivity
If an oxidation catalyst is applied upstream the SCR catalyst to obtain the NO:NO2 ratio close to 1:1 required for the fast SCR reaction, then the NOx conversion at low temperatures is improved, but the loss of substance increases due to deactivation over time
Solution Approach 1:
The patent modifies the chemical and physical parameters of the catalyst to enhance stability. This includes selecting specific metal compositions and ratios, controlling particle size distribution, and optimizing the support material properties to resist deactivation from hydrothermal conditions and sulfur poisoning, thereby maintaining consistent NOx conversion performance over time
3Ease of operation
If conventional SCR catalysts are used at low temperatures (around 200°C), then the NOx reduction process is simple, but the NOx conversion is insufficient to fulfil legislative requirements
Solution Approach 1:
The patent changes the chemical composition parameters of the catalyst to enhance low-temperature activity. This includes incorporating specific metals and metal oxides with high low-temperature SCR activity, optimizing the ratio of active components, and adjusting the catalyst structure to facilitate reactions at lower temperatures, thereby achieving high NOx conversion at 200°C without complicating the operation
4Productivity
If Cu-zeolite materials are used to achieve high SCR activity, then the NOx conversion is improved, but the reliability decreases due to hydrothermal deactivation
Solution Approach 1:
The patent creates a composite catalyst system that combines multiple materials with complementary properties. The composite structure integrates components that provide high SCR activity with components that offer hydrothermal stability and resistance to deactivation. This synergistic combination allows the catalyst to maintain both high NOx conversion performance and long-term reliability under hydrothermal 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
The dual bed catalyst system achieves high NOx conversion with reduced hydrogen usage and improved stability, maintaining performance over 150 to 550°C with minimal hydrogen required at lower temperatures, outperforming single-bed systems in terms of activity and resistance to deactivation.
Implementation Method 1
the first catalyst bed is an iron-beta-zeolite (Fe-beta-zeolite) and the second catalyst bed downstream is silver supported on alumina (Ag/Al 2 O 3 )
Implementation Method 2
the first catalyst bed is an iron-beta-zeolite (Fe-beta-zeolite) and the second catalyst bed downstream is silver supported on alumina (Ag/Al 2 O 3 )
Implementation Method 3
for converting NOx in exhaust gas from lean-burn combustion engines to nitrogen by adding a mixture of hydrogen and ammonia to the exhaust gas and subsequently passing the gas over a suitable dual bed catalyst
Data Source
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AI summary
Process for reducing nitrogen oxides to nitrogen in an exhaust gas comprising passing the exhaust gas in the presence of a reducing agent through a catalyst system comprising at least two catalyst beds, in which a first catalyst bed is an iron-beta-zeolite and a second catalyst bed downstream is silver supported on alumina.