Kilns Question 1

I don't think there is a minimum velocity in the downcomer duct. You must draw the exhaust gases out of the kiln, but provided you do that adequately it does not matter at what velocity they flow down the downcomer duct. However, I'm not sure what you are going to achieve by this modification. The pressure drop across the downcomer duct is unlikely to be a limiting factor on your kiln. That limiting pressure drop will be elsewhere in the system.

I witnessed a phenomenon at a plant here in Europe, which could not be explained by anybody there. At one plant I found ring formation at the upper transition zone in a rotary kiln at three different areas. The plant is burning alternative fuels like house trash, plastics and bone meal. I was told, that all actions taken with regards to the process, have not been successful. My questions are:
1. Is there anything you could recommend apart of the standard procedures to avoid coating formation?
2. Do you know, if there is any refractory manufacturer who does supply are fractory solution for this matter and what should be the technical data for such a refractory?

The rings you describe must be ash rings formed where the ash from the different alternative fuels combine with the charge in the kiln. It is not uncommon for coating and rings to form at that point in the kiln. As there are three distinct rings I suppose than the different alternative fuels have different aerodynamic properties in the kiln, or different delivery systems injecting them into the kiln and this is causes the ash to combine at different positions. If that is the case I would adjust the different fuel injection velocities to try to even out the coating. I am not aware of are fractory solution to this type of problem in the lower transition zone. I suggest you contact David Shepherd of Ceram Research who will know if such a solution exists, david.shepherd@ceram.co.uk.