NH3-SCR Catalyst Downstream of Lean NOx Trap
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Solution Overview
Problem
Lean NOx traps face challenges with low NOx conversion rates, NOx breakthrough, and ammonia emissions, particularly under lean operating conditions, which are exacerbated by stringent emissions standards and the need for improved fuel efficiency.
Innovation Solution
A catalyst system combining a lean NOx trap with an ammonia selective catalytic reduction (NH3-SCR) catalyst, where the NH3-SCR catalyst is placed downstream to adsorb and react with ammonia emissions from the lean NOx adsorber, enhancing net NOx conversion and reducing ammonia emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a lean NOx trap is used to reduce NOx emissions under lean operating conditions, then fuel efficiency is improved, but ammonia emissions and NOx breakthrough occur
Solution Approach 1:
An NH3-SCR catalyst is introduced as an intermediary component between the lean NOx trap and the exhaust system. This catalyst mediates the harmful ammonia emissions by facilitating the reaction between ammonia and NOx to produce nitrogen and water, thereby converting the harmful ammonia byproduct into a beneficial NOx reduction mechanism
Solution Approach 2:
The invention converts the harmful ammonia emissions generated by the lean NOx trap during rich operation into a beneficial resource. The ammonia, which was previously a pollutant, is now utilized as a reductant in the NH3-SCR catalyst to reduce NOx emissions during lean operation, transforming a harmful byproduct into a useful chemical agent
2Reliability
If a lean NOx trap is used to reduce NOx emissions, then NOx conversion is improved during rich operation, but NOx breakthrough occurs during extended lean operation
Solution Approach 1:
The NH3-SCR catalyst serves as an intermediary that handles NOx emissions throughout the entire operating cycle. During lean operation, it compensates for the lean NOx trap's inability to reduce NOx by providing continuous NOx reduction through ammonia-based SCR reactions, thereby eliminating NOx breakthrough
Solution Approach 2:
The invention ensures continuous NOx reduction across both lean and rich operating conditions. While the lean NOx trap only reduces NOx during rich pulses, the NH3-SCR catalyst maintains continuous NOx reduction capability during lean operation by utilizing stored ammonia, thereby achieving uninterrupted NOx conversion
3Reliability
If the air excess ratio is decreased to improve NOx reduction, then NOx conversion increases, but ammonia production increases
Solution Approach 1:
The invention converts the ammonia produced during rich operation (when air excess ratio is decreased) into a beneficial reductant. The NH3-SCR catalyst utilizes this ammonia to reduce NOx emissions during subsequent lean operation, transforming the harmful ammonia byproduct into a useful chemical agent for NOx control
Solution Approach 2:
The system operates in periodic lean-rich cycles. During rich pulses, ammonia is generated and stored; during lean operation, the stored ammonia is consumed to reduce NOx. This periodic generation and consumption of ammonia allows the system to tolerate higher ammonia production during rich operation without compromising overall emissions performance
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 combination significantly improves net NOx conversion from 55% to 80% and eliminates ammonia spikes, reducing NOx breakthrough and emissions, while allowing for lean operation without additional fuel economy penalties.
Implementation Method 1
The lean NOx trap functions by adsorbing NOx when the engine is running under lean conditions
Implementation Method 2
an ammonia selective catalytic reduction (NH3-SCR) catalyst, which stores the ammonia formed in the lean NOx trap during rich air/fuel operation and then reacts the stored ammonia with nitrogen oxides to improve NOx conversion to nitrogen
Implementation Method 3
a short rich pulse of reductants, carbon monoxide, hydrogen and hydrocarbons reduces the NOx adsorbed by the trap during the lean cycle
Data Source
AI summary
This catalyst system simultaneously removes ammonia and enhances net NOx conversion by placing an NH3-SCR catalyst formulation downstream of a lean NOx trap. By doing so, the NH3-SCR catalyst adsorbs the ammonia from the upstream lean NOx trap generated during the rich pulses. The stored ammonia then reacts with the NOx emitted from the upstream lean NOx trap-enhancing the net NOx conversion rate significantly, while depleting the stored ammonia. By combining the lean NOx trap with the NH3-SCR catalyst, the system allows for the reduction or elimination of NH3 and NOx slip, reduction in NOx spikes and thus an improved net NOx conversion during lean and rich operation.