Flue gases from the drying/ignition zone are fed through the flame area of the combustion zone and to the flue gases leaving the combustion zone at least sec. air in individual jets is fed. Flue gases above the drying/ignition zone are collected and by at least one additional air jet, are mixed with hotter gases.The collected gases above the drying/ignition zone are pressed into the flame area of the combustion zone. The cover (11) of the combustion chamber is formed above the drying/ignition zone (17) of the fire-grate as gas collecting chamber (12) and in it at least one air nozzle (15,16) for mixing gas from the gas collection chamber (12) with hotter gases above the fire-grate (2) is provided.
