Exhaust Gas Aftertreatment Split-Stream Catalyst System
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Solution Overview
Problem
Existing exhaust gas aftertreatment systems for internal combustion engines face challenges in efficiently reducing nitrogen oxides and particulates, particularly due to difficulties in metering reducing agents like ammonia under varying operating conditions, and issues with catalyst deactivation and filter clogging, which lead to incomplete reactions and additional pollutant formation.
Innovation Solution
The system introduces a thermolysis catalyst near the engine to vaporize water from an aqueous urea solution, followed by a hydrolysis catalyst to convert isocyanic acid to ammonia, with a split-stream configuration ensuring thorough mixing and avoiding trimerization, combined with an oxidation catalyst to produce nitrogen dioxide for particulate reaction, optimizing nitrogen oxide and particulate reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a particle filter is used to reduce solid particulates, then particulate removal is achieved, but the filter becomes clogged increasing exhaust gas back pressure and reducing engine output
Solution Approach 1:
The patent implements continuous regeneration of the particle filter by utilizing nitrogen dioxide produced in the oxidation catalyst to react with and remove deposited soot particles. This converts the accumulated particulates into gaseous products (CO, CO2, N2, NO) that can be discharged, thereby recovering the filter's capacity without requiring manual intervention or system shutdown.
Solution Approach 2:
The system establishes a continuous particle oxidation process where nitrogen dioxide generated upstream continuously reacts with soot particles in the filter. This eliminates the need for periodic regeneration cycles with uncombusted hydrocarbons, maintaining continuous engine operation at optimal output while constantly cleaning the filter.
2Productivity
If V2O5 is used as the active material for the SCR catalyst, then nitrogen oxide conversion is achieved, but catalyst deactivation occurs when exhaust gas temperature exceeds 650°C
Solution Approach 1:
The patent transitions from V2O5-based SCR catalysts to iron or copper zeolites, representing a fundamental change in catalyst material parameters. This substitution enables the catalyst to maintain stability and activity at higher exhaust gas temperatures exceeding 650°C, where V2O5 would deactivate, thereby expanding the operational temperature window.
Solution Approach 2:
The system employs composite zeolite structures containing iron or copper as active materials, combining the benefits of high-temperature stability with effective SCR activity. These composite materials resist deactivation at elevated temperatures while maintaining nitrogen oxide conversion efficiency.
3Productivity
If the reducing agent is metered to maximize nitrogen oxide conversion, then NOx reduction is improved, but unconsumed ammonia is emitted
Solution Approach 1:
The system incorporates an ammonia-blocking catalyst downstream of the SCR catalyst that acts as a feedback mechanism. This catalyst selectively oxidizes excess ammonia that passes through the SCR catalyst, converting it to nitrogen and water vapor. This feedback loop ensures that ammonia emissions are minimized while maintaining high nitrogen oxide conversion efficiency.
4Object-affected harmful factors
If an oxidation catalyst is used to produce nitrogen dioxide for particulate reaction, then continuous particulate removal is achieved, but the nitrogen dioxide is consumed by the SCR catalyst and not available for particulate oxidation
Solution Approach 1:
The patent divides the exhaust gas stream into two separate streams: one stream passes through the oxidation catalyst to generate nitrogen dioxide for particulate oxidation, while the other stream bypasses the oxidation catalyst. This segmentation allows nitrogen dioxide to be produced and utilized for particulate removal without being completely consumed by the SCR catalyst, ensuring sufficient availability for both functions.
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 ensures complete urea decomposition at low temperatures, prevents clogging, and maintains high conversion rates of nitrogen oxides and particulates, reducing ammonia slip and problematic byproducts, thus optimizing the exhaust gas aftertreatment process.
Implementation Method 1
a thermolysis catalyst is located near the engine in the exhaust gas split stream downstream of the supply point of the reducing agent. At an exhaust gas temperature above 135° C., this thermolysis catalyst vaporizes the water component of the aqueous urea solution.
Implementation Method 2
The hydrolysis catalyst contains a catalyst material that converts the isocyanic acid formed during the thermolysis to ammonia and carbon dioxide with the aid of the water vapor formed in the thermolysis catalyst.
Implementation Method 3
an oxidation catalyst, which usually a catalyst that contains platinum as the active material and is located upstream of the particle separator or particle filter, oxidizes the nitric oxide in the exhaust gas with the aid of the residual oxygen that is also present to form nitrogen dioxide
Implementation Method 4
the nitrogen oxides are reduced by means of an SCR catalyst... In this regard, 1 mole of ammonia is required for the reaction of 1 mole of nitric oxide. 4NO+4NH3+O2→4N2+6H2O
Implementation Method 5
the oxidation catalyst, which is usually a catalyst that contains platinum as the active material and is located upstream of the particle separator or particle filter, oxidizes the nitric oxide in the exhaust gas with the aid of the residual oxygen that is also present to form nitrogen dioxide, which in turn reacts with the carbon particulates in the particle separator or particle filter to form CO, CO2, N2, and NO
Data Source
AI summary
Exhaust gas aftertreatment system for internal combustion engines operated with a lean mixture, wherein nitrogen oxides are reduced by an SCR catalyst, and particulates are removed by a particle separator or filter. A thermolysis catalyst is located near the engine in the exhaust gas split stream downstream of the supply point of the reducing agent. At temperatures above 135° C., this thermolysis catalyst vaporizes the water component of the aqueous urea solution. It contains a catalyst material that is thermally stable at exhaust gas temperatures occurring near the engine and preferentially reacts with the urea to form isocyanic acid. A hydrolysis catalyst is located in the exhaust gas stream downstream of the return of the split stream into the main stream of exhaust gas. The hydrolysis catalyst converts the isocyanic acid formed during thermolysis to ammonia and carbon dioxide using water vapor formed in the thermolysis catalyst.