Abstract: A horizontal shell boiler has a horizontal furnace section which communicates with a flue gas transfer section at its end which transfers the flue gases from the furnace section to flue tubes extending horizontally through the space between the furnace section and the shell of the boiler. The furnace section is designed for fluidized bed combustion and has a flue gas transfer tube extending horizontally through its upper portion. The tube has hollow walls connected to the boiler fluid surrounding the furnace section and has apertures formed in its hollow wall through which furnace gases can enter the tube. The furnace gases after entering the flue gas transfer tube travel horizontally and do not entrain material of the fluidized bed.