What is the emissivity of a kiln's shell? I ask this because i need to know it to take infrared thermography of the surface shell of a kiln.

The emissivity of the rough oxidised kiln shell at 400 degrees C is approximately 0.9. According to my notes emissivity is dimensionless.

I would like to know how to get rid of the rings that occurs around upper transition zone without mechanically breaking by stopping the kilns.

There are various options, but if you cannot avoid the problem by better control of the kiln feed chemistry or burning conditions in the kiln then there are two possibilites: (i) install the equipment to fire Cardox charges in the region where you have persistent problems, and (ii) by an industrial gun and shoot the rings out.