LNT-SCR Emissions System with CHA Molecular Sieve
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Solution Overview
Problem
Existing emissions treatment systems for lean burn engines, such as diesel engines, face challenges in efficiently reducing nitrogen oxides (NOx) due to the limited generation of ammonia (NH3) during rich operating periods, which can lead to insufficient NOx treatment during lean periods, and require external NH3 reservoirs and infrastructure for urea supply.
Innovation Solution
An emissions treatment system combining a lean NOx trap (LNT) catalyst with a downstream Selective Catalytic Reduction (SCR) catalyst, utilizing a molecular sieve with the CHA crystal structure, such as CuCHA or CuSAPO, and a diesel particulate filter, to generate and store ammonia internally, reducing NOx through alternating lean and rich exhaust gas operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a lean NOx trap catalyst is used to store and reduce NOx under alternating lean and rich conditions, then NOx conversion is improved, but ammonia is generated as a by-product which requires additional treatment
Solution Approach 1:
The patent converts the harmful ammonia by-product generated during rich period regeneration into a beneficial reductant for SCR catalyst operation. The ammonia stored in the molecular sieve during rich periods is subsequently used to reduce NOx during lean periods, transforming a harmful emission into a useful chemical resource for continuous NOx abatement.
Solution Approach 2:
The patent merges the LNT catalyst system with an SCR catalyst system into a single integrated emissions treatment device. The LNT catalyst performs NOx storage and periodic regeneration, while the SCR catalyst continuously reduces NOx using the ammonia generated and stored within the system, combining two different catalytic mechanisms to achieve superior overall NOx conversion.
2Reliability
If external urea reservoirs and supply infrastructure are used to provide ammonia for SCR treatment, then NOx reduction efficiency is improved, but device complexity and operational requirements increase
Solution Approach 1:
The patent implements a self-service ammonia generation and storage system where the LNT catalyst autonomously produces ammonia during rich period regeneration, and the integrated molecular sieve automatically stores and releases this ammonia for SCR operation. This eliminates the need for external urea reservoirs, injection systems, and supply infrastructure, as the system generates and manages its own reductant internally.
Solution Approach 2:
The patent introduces an integrated molecular sieve as an intermediary component that mediates between the LNT catalyst and SCR catalyst. The molecular sieve acts as an ammonia storage reservoir, absorbing ammonia during rich periods and releasing it during lean periods, thereby bridging the temporal gap between ammonia generation and consumption without requiring external storage or handling systems.
3Productivity
If ammonia is generated during rich pulse regeneration of the NSR catalyst, then the reduction function is improved, but ammonia breakthrough occurs which must be converted to innocuous species
Solution Approach 1:
The patent converts the harmful ammonia breakthrough into a beneficial resource by directing it to the SCR catalyst. Instead of treating ammonia as a harmful emission to be eliminated, the system uses it as a reductant for continuous NOx reduction, transforming a regulatory compliance issue into a performance enhancement opportunity.
Solution Approach 2:
The patent ensures continuous useful action by maintaining SCR catalyst operation throughout both lean and rich periods. While the LNT catalyst alternates between storage and regeneration modes, the SCR catalyst continuously reduces NOx using ammonia from the molecular sieve, eliminating interruptions in NOx abatement and preventing ammonia breakthrough to the atmosphere.
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 system effectively converts NOx to N2, reduces ammonia breakthrough, and provides flexible and efficient NOx abatement across varying operating conditions, eliminating the need for external NH3 sources and enhancing NOx reduction efficiency.
Implementation Method 1
NSR catalysts contain NOx sorbent materials capable of adsorbing or 'trapping' oxides of nitrogen under lean conditions
Implementation Method 2
The oxidation process does not stop here. Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen, is also a catalyzed reaction
Implementation Method 3
In an oxidizing environment, NO is oxidized to NO2
Implementation Method 4
The released NOx is then further reduced to gaseous N2 in a rich environment
Implementation Method 5
NOx Release M(NO3)2 + 2CO → MCO3 + NO2 + NO + CO2
Implementation Method 6
NOx is reduced with a reductant, e.g., NH3, to nitrogen (N2) over an SCR catalyst that is typically composed of base metals
Implementation Method 7
an SCR catalyst disposed downstream of the ammonia-generating component, wherein the SCR catalyst comprises a molecular sieve having the CHA crystal structure
Data Source
Figure 1A~2
Figure 3~4
Figure 5A~5B
AI summary
Provided are emissions treatment systems for an exhaust stream having an ammonia-generating component such as a NOx storage reduction (NSR) catalyst or a lean NOx trap (LNT) catalyst, and an SCR catalyst disposed downstream of the ammonia-generating catalyst. The SCR catalyst can be a molecular sieve having the CHA crystal structure, for example SSZ-13 or SAPO-34, which can be ion-exchanged with copper. The LNT can be layered, having an undercoat washcoat layer comprising a support material, at least one precious metal, and at least one NOx sorbent selected from the group consisting of alkaline earth elements, rare earth elements, and combinations thereof and a top washcoat layer comprising a support material, at least one precious metal, and ceria in particulate form, the top washcoat layer being substantially free of alkaline earth components. The emissions treatment system is advantageously used for the treatment of exhaust streams from diesel engines and lean burn gasoline engines.