Burning bit coal in the old Kewaunee boiler--- well I recall every summer helping clean it then I recall I was the one that had to crawl in the fire box and clean it -- but the point is once swept out there always were fire brick to replace. But the vessel was not effected....
Fire brick are cheep - so what if several need to be annually replaced
your sort of making me think about this hand stoking thing
But I will need one about 50 % larger