Abstract: Method of improving the operation and efficiency of a steam generating furnace fired with high moisture content fuel, comprises providing a drying shaft parallel and offset from the combustion chamber of the furnace; wet fuel is introduced into the upper portion of the shaft while a portion of the hot products of combustion gases from the upper portion of the combustion chamber are also permitted to flow into the upper portion of the shaft. The hot gases evaporate moisture from the fuel and are simultaneously cooled thereby so as to increase in density and fall by gravity within the shaft co-currently with the fuel for recirculation into the lower portion of the combustion chamber while the at least partially dried fuel is deflected from the bottom of the shaft into the lower portion of the combustion chamber along with the recirculated gases.

Abstract: A steam generating furnace which burns high moisture content fuel has a panel of heat absorbing tubes dividing it into two vertical interconnecting passageways: (a) a combustion chamber for burning the predried fuel and (b) a drying shaft for extracting moisture from the fuel. Wet fuel is introduced near the top of the drying shaft. As it falls it is dried by some of the hot gases diverted from the top of the combustion chamber into the top of the drying shaft. The cooling of the hot combustion gases due to evaporation of moisture from the wet fuel causes a difference in density between the gases in the two passageways, creates a natural, unidirectional circulation of part of the combustion gases from the top of the combustion chamber to the drying shaft. Predrying the wet fuel causes the fuel to burn faster and hotter thereby producing a stable and efficient combustion of the wet fuel at higher specific combustion rates.