Heating boiler with a heat engine

The boiler (1) has a combustion chamber (2) for combustion of solid fuel i.e. biomass. A flame tube (4) includes an inflow region facing the combustion chamber, and an outflow region facing a flue gas duct (3) for discharging flue gases. A heat engine i.e. Stirling engine (5) for converting heat energy into mechanical energy is coupled to a heat exchanger (6). The heat exchanger is provided in the outflow region of the flame tube, where the heat exchanger includes a leading face (7) formed by a tapering in the direction of a tapered section (8) of the heat exchanger.