Reforming reactor for the conversion of a process fluid into hydrogen comprising: a reforming section and a boiler section which are both contained within a common volume and a combustion section, in which said reforming section contains one or more catalyst tubes filled with reforming catalyst, said boiler section is provided with one or more tubes carrying flue gas from the combustion section and said combustion section is provided with at least one burner, 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 common volume containing said reforming and boiler sections.