Abstract: The enclosure of a generator comprises a combustion zone 1 surrounded at least partially by a heat exchange zone 2, the combustion zone comprising a premix chamber 25 having means for supplying with air at 12 and with fuel 10,11 and inert recycling particles coming from zone 2.The premix chamber has mounted thereover a vertical duct 21 filled with a refractory stack 6 and comprises above the stack an opening 22 serving as overflow communicating with a vertical duct 4 situated outside duct 21 and opening into zone 2. The heat exchange zone 2 is formed as a dense fluidized bed zone and contains fluid circulation tubes 3. The vertical duct 21 serving as combustion hearth is filled with a fixed structure formed by a stack of several layers of spaced parallel bars, offset angularly from one layer to the next.