Three-Way Catalyst Traps Volatile Vanadium and Tungsten
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Solution Overview
Problem
Vanadium and tungsten oxides used in SCR catalysts for diesel engines can vaporize at high temperatures, releasing toxic compounds and ammonia slip, posing emission risks, and existing systems struggle to effectively manage particulate matter and NOx removal efficiently while avoiding fuel penalties.
Innovation Solution
A method combining a vanadium and tungsten adsorbent with an ammonia oxidation catalyst downstream the SCR catalyst, using a three-way catalyst with oxides of vanadium, tungsten, and titanium, along with high surface metal oxides and zeolites, to adsorb volatile compounds and reduce NOx and ammonia slip, integrated with a catalyzed wall flow particle filter for efficient particulate matter filtration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If vanadium and tungsten oxides are used as SCR catalysts for effective NOx reduction, then NOx conversion efficiency is improved, but volatile toxic compounds are released at high temperatures
Solution Approach 1:
A third catalyst stage is introduced between the SCR catalyst and the exhaust system. This intermediate catalyst is specifically designed to adsorb and retain the volatile vanadium and tungsten compounds that escape from the SCR catalyst, preventing their release into the atmosphere while allowing the SCR process to continue effectively
Solution Approach 2:
The exhaust system is divided into three distinct functional zones with different catalyst compositions: the first stage for SCR, the second stage for adsorbing volatile compounds, and the third stage for additional NOx reduction. Each zone has optimized local properties to address specific problems at that position in the exhaust system
2Loss of energy
If passive soot regeneration is used to remove particulate matter, then fuel penalty is reduced, but temperature control below 550°C becomes critical to avoid vaporization
Solution Approach 1:
The intermediate catalyst stage acts as a thermal buffer and safety zone. It allows the system to operate at lower temperatures during passive regeneration without risking vaporization of the SCR catalyst components, while still enabling effective particulate matter removal through the wall flow filter
Solution Approach 2:
The third catalyst stage is pre-configured to handle volatile compound adsorption before they can be released into the exhaust stream. This preventive measure cushions against the harmful effects of temperature spikes during regeneration operations
3Productivity
If ammonia is added in over stoichiometric amounts for maximum NOx conversion, then NOx removal efficiency is improved, but ammonia slip into the atmosphere increases
Solution Approach 1:
The intermediate catalyst stage serves as a safety zone that captures excess ammonia and volatile compounds before they reach the exhaust system. This allows the SCR process to use higher ammonia concentrations for maximum NOx conversion without directly releasing the excess ammonia into the atmosphere
Solution Approach 2:
The exhaust system is divided into functional zones where the intermediate catalyst creates a localized area for capturing volatile substances. This local quality enhancement allows the overall system to achieve better NOx conversion while controlling ammonia slip through the specific properties of the intermediate catalyst zone
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 method effectively traps volatile vanadium and tungsten compounds and reduces NOx and ammonia slip, achieving high NOx conversion and ammonia oxidation while maintaining fuel efficiency and reducing emissions, as demonstrated by NOx conversion and ammonia selectivity data in the temperature range of interest.
Implementation Method 1
contacting the exhaust gas with an oxidation catalyst active in oxidation of volatile organic compounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide
Implementation Method 2
introducing a urea solution into the exhaust gas from step (a) and hydrolysing the urea solution to ammonia
Implementation Method 3
filtering off the particulate matter by passing the exhaust gas through gas permeable porous partition walls into a plurality of outlet channels of the wall flow particle filter
Implementation Method 4
removing the deposited particulate matter by reaction with the nitrogen dioxide contained in the exhaust gas and decreasing remaining amounts of NOx in the exhaust gas by contact with an SCR active catalyst in presence of the ammonia
Implementation Method 5
adsorbing volatile vanadium and tungsten compounds volatilized off the SCR active catalyst in step (d) in a three way catalyst downstream to step (d)
Data Source
Figure 1~2
AI summary
A method for the removal of volatile organic compounds, particulate matter and nitrogen oxides from exhaust gas of a compression ignition engine comprising the steps of contacting the exhaust gas with an oxidation catalyst active; introducing a urea solution into the exhaust gas; introducing the thus treated exhaust gas into a plurality of inlet channels of a catalyzed wall flow particle filter; removing the deposited particulate matter by reaction with the nitrogen dioxide contained in the exhaust gas and decreasing remaining amounts of NOx in the exhaust gas by contact with an SCR active catalyst in presence of the ammonia, the SCR active catalyst comprises oxides of vanadium and tungsten; removing remaining amounts of NOx and ammonia slip and trapping volatile vanadium and tungsten compounds volatilized off the SCR active catalyst in a three way catalyst.