If the temperature is not too high, copper tubing with an extruded Teflon coating is available. Other coating types may work as well depending on the specifics of your application. I do not have any direct experience with coal-fired furnaces or coatings for this kind of application so I cannot make any specific recommendations. Another possibility would be to replace the copper with a more corrosion resistant alloy. Literature related to the energy industry and coal-fired plants might be a good source of further information.