a mixing section to mix an oxygen containing gas with a hydrocarbon containing gas and a reaction section connected to the mixing section to react the mixture of the oxygen containing gas and the hydrocarbon containing gas to produce a product;

said mixing section including a mixing chamber having an inlet for the hydrocarbon containing gas, an oxygen injector located within the mixing chamber for injecting the oxygen containing gas into the hydrocarbon containing gas and a flame arrestor located at least below the oxygen injector;

the flame arrestor being formed of a mass of porous material that permits mixing in both radial and axial directions of said mixing chamber to promote mixing of the oxygen containing gas and the hydrocarbon containing gas and the flame arrestor in contact with the walls of said mixing chamber such that flow of said oxygen containing gas and said hydrocarbon containing gas is constrained to pass through said flame arrestor before entering the reaction section; and

said reaction section including an inner chamber positioned to receive the mixture of the oxygen containing gas and the hydrocarbon containing gas, a catalyst located within said inner chamber to promote the reactions involving said mixture, an outer pressure vessel, thermal insulation between said inner chamber and said outer pressure vessel and an outlet penetrating the outer chamber and in communication with the inner chamber to discharge a product gas containing the product.