> 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 inhibits this reaction by changing the chemical make-up of the catalyst breaking down the substrate or ceramic. 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. Stacking too many logs in the firebox can elevate flames too close to the combustor.
> What causes thermal shock to the combustor?
A sudden temperature change or uneven temperatures to the combustor's substrate can cause cracking. In other words, two different substances are passing through the combustor at the same time. One of the main reasons for this happening is refueling with wood containing some form of moisture. This can be wood that has been exposed to snow or rain, or perhaps green wood that has not been stacked and seasoned for at least one year. The thermal shock comes when the moist smoke comes in contact with the combustor running at temperatures in excess of 1000°F (540 C). The cell walls will develop hairline cracks and eventually pieces will start to fall off the combustor. This loss of surface area means there will be less catalytic surface space and less efficiency of the combustor.
In regards to a steel catalyst, they may not crumble, but they are still susceptible to flame impingement. The impingement could lead to the destruction of the palladium coating.