SCR vs. 3-way Catcon - NOx Emission Reduction

[KarthikM]

Hello

My question is regarding SCR and 3 way catalytic converter. In a traditional car, we have 3 way catalytic converter with oxidizing agents (Platinum, Palladium) to reduce HC, CO emissions and reducing agent (Rhodium/ Ruthenium) to reduce NOx emissions. However, in heavy duty diesel engines, we go for Selective Catalytic Reduction (SCR) technology to reduce NOx emissions.

I am wondering if it is feasible to use 3-way catalytic converter in heavy duty diesel engines to reduce NOx emissions. Some possible reasonsmight be: lean mixtures favoring HC/CO oxidation than NOx reduction, unfavorable chemical composition for NOx reduction, bigger size of the unit, higher cost in using Rhodium etc.

Please let me know if the above reasons are correct and share your other thoughts. Also let me know if there is any material (journal/conference papers) or discussion addressing this topic.

Thanks
Karthik M

[wam]

Three-way catalysts must operate in a narrow air-to-fuel ratio window, near stoichiometric. They are ineffective for NOx reduction under lean conditions, typical for the diesel engine.

[pallavkarsameer]

Dear Karthik
You have raised the right concerns. The conventional TWC catalytic converter is ineffective in the temperature range of the diesel engine exhaust, for one that the temperatures are low, and secondly, the presence of excess oxygen. Promoting reduction reactions in such lean environment is a daunting task. There are few reports/publications reporting Pt/Pd based catalysts showing high De-NOx activity but their temperature window is too narrow for the diesel engine exhaust!

Conversely, if you can somehow (hypothetically) prolong, enhance this temperature window for the conventional TWC catalyst, and provide sustained heating/energy, the catalyst may exhibit both reduction and oxidation reactions for this expanded temperature window.

I suggest you read articles pertaining to Lean NOx Trap especially during the regeneration phase which gives a very good picture of how this technology brings about >90% NOx reduction. Its demerits are high catalyst loading (2 to 10% of Pt.) and complex engine control (lean/rich cycle of trapping and regeneration phase).

Another take on this subject matter is to see if your conventional TWC catalyst can demonstrated high De-NOx activity when NH3 is used as the reducing agent? Much of it also depends on the type of reducing agent used.

At this stage, NH3 based SCR has demonstrated high De-NOx activity under all driving conditions and oxygen content!!!

Some of the authors I would recommend regarding Lean NOx trap (for publications) Dr. W. Epling, Dr. S. Mulla (I think this person is in Johnson Mathey now!) Dr. M. Harold/Dr. R. Muncrief

You can also look for articles by Dr. Shelef, and Dr. R. Burch for Lean NOx catalyst.

Also do check a patent by Dr. Paul Fanson in association with Toyota Motor Co., they are using Microwave-Assisted De-NOx catalytic system for NOx abatement using H2 as the reducing agent! (But has shown high power requirement and a De-NOx activity of about 10 mins.)

Let me know if I could guide you in any way regarding NOx abatement technology. Feel free to email me at smpallavkar@my.lamar.edu

All the best

[KarthikM]

Thank you Sameer and Wam. I appreciate your response.