Dual SCR Catalyst System for Low-Temperature NOx Conversion
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Solution Overview
Problem
Existing SCR catalyst systems for diesel engine exhaust gas purification struggle with 'kick-off' behavior at low temperatures and high nitrogen oxide conversion over a wide temperature range, often requiring toxicologically questionable vanadium compounds and being costly due to zeolite-based systems.
Innovation Solution
A dual SCR catalyst system is implemented, where the first catalyst, composed of tungsten oxide and cerium-zirconium mixed oxide, oxidizes hydrocarbons and catalyzes nitrogen oxide reduction between 300° C. and 500° C., followed by a vanadium-free copper-exchanged zeolite catalyst that operates between 150° C. and 400° C., effectively storing excess ammonia and maintaining high nitrogen oxide conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single SCR catalyst formulation is used, then the catalyst structure is simple and cost-effective, but nitrogen oxide conversion is only optimal in a narrow temperature range
Solution Approach 1:
The patent divides the catalyst system into two distinct catalyst beds: a first SCR catalyst optimized for high temperatures (300-500°C) and a second SCR catalyst optimized for low temperatures (150-400°C). This segmentation allows each catalyst to specialize in a specific temperature range, achieving broad overall coverage while maintaining simple individual catalyst formulations.
2Productivity
If vanadium-containing SCR catalysts are used, then nitrogen oxide conversion is improved, but toxicological concerns arise due to vanadium compound volatility
Solution Approach 1:
The patent replaces toxic vanadium-containing catalysts with alternative catalyst formulations that, while potentially having shorter lifespan or requiring more frequent replacement, eliminate the toxicological concerns associated with vanadium compounds. The alternative catalysts achieve comparable nitrogen oxide conversion without the harmful volatility issue.
3Productivity
If zeolite-based SCR catalysts are used, then nitrogen oxide conversion is improved, but system cost increases
Solution Approach 1:
The patent applies different catalyst formulations to different locations (temperature zones) in the exhaust system. By matching catalyst type to local temperature conditions, the system achieves high nitrogen oxide conversion where needed while using cost-effective catalyst formulations in each specific zone, avoiding the need for expensive zeolite-based catalysts throughout the entire system.
4Quantity of substance
If SCR catalysts operate at low temperatures, then ammonia storage capability is improved, but nitrogen oxide conversion efficiency decreases
Solution Approach 1:
The patent segments the catalyst system into two temperature-optimized zones: a low-temperature zone (150-400°C) where ammonia storage and nitrogen oxide conversion both occur effectively, and a high-temperature zone (300-500°C) where conversion efficiency is maximized. This segmentation resolves the trade-off by allowing each zone to operate in its optimal temperature range.
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 configuration achieves improved 'kick-off' behavior at low temperatures, maintains high nitrogen oxide conversion across a wide temperature range, avoids toxic vanadium compounds, and reduces costs compared to zeolite-based systems, with flexible ammonia metering adapted to dynamic engine conditions.
Implementation Method 1
a first SCR catalyst, which effectively catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 300° C. and 500° C.
Implementation Method 2
at least partially oxidizing the hydrocarbons contained in the exhaust gas
Implementation Method 3
a second, downstream SCR catalyst, which effectively catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 150° C. and 400° C.
Implementation Method 4
at the same time stores excess ammonia
Data Source
AI summary
An exhaust gas purification system (method and device) for the treatment of diesel exhaust gases containing nitrogen oxides and hydrocarbons is disclosed, which comprises the addition of ammonia or of a compound decomposable to ammonia into the exhaust gas stream and the subsequent leading of the exhaust gas stream over two successively arranged SCR catalysts with different properties and compositions. Both SCR catalysts are free of vanadium compounds and only the downstream SCR catalyst contains zeolite compounds. The exhaust gas purification system according to the invention is characterized by good “kick-off” behavior at low temperatures and a simultaneously high conversion performance over a wide temperature range.