Reforming reactor for the conversion of a process fluid into hydrogen comprising: a reforming section (4) which is contained within a closed volume (8), a boiler section (3) and a combustion section (2), in which said reforming section contains one or more catalyst tubes (20) filled with reforming catalyst, said boiler section is provided with one or more flow channels (11) for the passage of flue gas from the combustion section and said combustion section is provided with at least one burner (1), wherein the heat exchanging medium required for the reforming of said process fluid in the one or more catalyst tubes is a gas-liquid mixture that self-circulates and is encapsulated inside said closed volume (8) and wherein at least a portion (9) of said closed volume protrudes inside said boiler section.