The generator (10) has two combustion hearths (F1,F2) in which a fuel is burnt with an oxidant. A combustion hearth has a boiler (12) and a vaporization zone (V) including a vaporization exchanger (20). The other hearth has another boiler (24) and an over heat zone (S1) including an over heat exchanger (30). An exit of over heat zone communicates with an envelope having a preheat that includes an economizer.
