Why is flame impingement bad for the catalyst?

This is the first of a series of articles on understanding the catalytic combustor a little better.

Why is flame impingement bad for the catalyst?

Direct flame contact is death to the catalyst. A catalyst burns the byproducts in the smoke. The gases such as CO, HC and O2 ignite with each other in a chemical reaction in the presence of the catalyst (while passing through the honeycomb configuration). Direct flame inhabits this reaction by changing the chemical make-up of the catalyst breaking down the substrate or ceramic.

Today's modern wood burning stoves are designed so that flame impingement is unlikely. However, it is not impossible. A strong fast draft can pull the flames into the catalyst. A hot fire with all the primary air controls wide open or perhaps the firebox door or ash pan door ajar are other ways the catalyst might receive flame impingement.