firebed

noun

1. A layer of fuel that burns under a heat source

   “Another factor of great importance in everyday service is the degree to which the gas/steam ratio is influenced by growing firebed resistance, and this is again a matter in the hands of the designer if he kbows what to do about it. [...] poorer grades of coal require more air per unit of heat released as well as offering greater firebed resistance. [...] (Dr.) A. Giesl-Gieslingen, Vienna.”