You would be better served by shielding the combustibles with a sheet of steel than using double wall pipe from your coal appliance to the thimble.
See my problem isn't the single wall pipe to the wall, it's the single wall pipe to the ceiling and the 18 inch clearance. I was thinking of going with a power vent since it's cheaper to set up right now then going with a SS chimney, but with the ceiling height it seems I would barely have enough room in between the two tee's to have any pipe in there. I was looking at one tee for the baro and one tee used instead of an elbow for clean out.
If I place a sheet of metal on the ceiling to get the 8 inch clearance could it just be any type of metal plate or does it need to be a certain thickness and type? Would one of those thin metal sheets you can buy at the local hardware store work?