A process and system for generating hydrogen gas are described, in which water is electrolyzed to generate hydrogen and oxygen, and a feedstock including oxygenate(s) and/or hydrocarbon(s), is non-autothermally catalytically oxidatively reformed with oxygen to generate hydrogen. The hydrogen generation system in a specific implementation includes an electrolyzer arranged to receive water and to generate hydrogen and oxygen therefrom, and a non-autothermal segmented adiabatic reactor containing non-autothermal oxidative reforming catalyst, arranged to receive the feedstock, water, and electrolyzer-generated oxygen, for non-autothermal catalytic oxidative reforming reaction to produce hydrogen. The hydrogen generation process and system are particularly advantageous for using bioethanol to produce green hydrogen.