Abstract: A shell steam boiler 2 comprises a furnace tube 10 having an entrance 32 which contains a gas burner 68 and also receives the hot exhaust gases from a small gas turbine 34 giving an output, on a power take off shaft 58 from a power turbine 56, of 0.5 MW or less which drives an electrical power generator 60. Hot heating gases from the gas turbine 34 and the burner 68 pass down the furnace tube 10 and then through a first group of pass tubes 16 and then through a second group of pass tubes 22 to an enclosure 26 comprising a flue 30. A driven fan 72 in the enclosure 26 acts to draw the heating gases through the boiler and promotes a good flow of those gases.