I understand that wood is inefficient and a lot of heat is lost in the gases out the chimney in a normal fireplace or stove. These thermal mass heaters reclaim that heat and store in the stone and masonry. I've read on several sites online, people claiming a 90% reduction their wood consumption for heating their house. 3 or 4 cords a year down to 2/3's of a cord in one claim. From anyone who's had one, is that claim for real? It seems too good to be true.
So Franco B is saying the thermal mass stove is for wood because the aforementioned reclaimation of flue gas heat. Coal combustion gases don't have the temps and heat content of wood gases. That means most of the heat is conducted by the air and stove metal mass immediately surrounding and in contact with the fire chamber, which makes coal more efficient than wood. And it has a higher btu content anyway.
Ok so here's the next statement and question for Franco B to disprove
Given all of the above, there must be some benefit to storing the heat of a coal fire, even one that burns 24/7, in a thermal mass. Maybe that benefit is marginal, I don't know. But why can't we make a coal fire even more efficient by some means of thermal mass stove. To reclaim the heat from coal flue gas, maybe you need a smaller run of chambers and less reclaimation than you do with wood because the temperatures are lower, but there IS something there to reclaim if you tried. And, for the heat mass in the burn chamber, doesn't storage of that heat by surrounding it with a huge masonry mass do something beneficial? Perhaps you'd need a smaller coal fire going overnight as temperatures are dropping because the mass helps regulate things?