My father took a large piece of asbestos / cement board back in the 70's, cut a 6" hole in it for the pipe, lined it with rope gasket, and bolted it to the brick mantle. We burned wood with that setup for 30 years. When he switched to coal, he made the mistake of getting the town inspector involved. That cost him several hundred dollars in materials that this clown wanted in place in order to pass.
He's got the same setup, minus the asbestos board (some type of fire proof board in it's place) ... but the guy made him run stainless
behind the fireboard, AND made him seal off the built-in chimney damper completely, and run the SS pipe through that. A recipe for hard-to-repair pipe corrosion if you ask me.
What he had worked for 30 years with wood .... but get the state involved and they'll screw your whole setup up!
Inspectors have ZERO knowledge of coal use. I'll bet that pipe is perforated right now .... but I digress.