NOVELTY - Converting a biomass to hydrogen and carbon dioxide comprises mixing a dry solid or liquid or liquid hydroxide with a biomass to form a biomass mixture; heating the biomass mixture and water vapor until the hydroxide and the biomass react to produce hydrogen and carbonate; and calcining the carbonate or performing double replacement reactions of the carbonate to produce carbon dioxide and a hydroxide.    USE - The method is useful for converting biomass to hydrogen and carbon dioxide (claimed); and for co-generation of gaseous fuels, biochar, and fertilizer from biomass and biogenic wastes.    ADVANTAGE - In the method, the produced alkali carbonate can be calcined to recover the hydroxide while the pure stream of carbon dioxide formed can be easily sequestered via a number of carbon storage techniques.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a system for converting a biomass to hydrogen and carbon dioxide, comprising a pretreatment module for mixing at least one of a dry solid or liquid hydroxide and one or more catalysts with a biomass to form a biomass mixture; an alkaline hydrothermal treatment reactor for heating the biomass mixture and water vapor until the hydroxide and the biomass react to produce hydrogen, carbonate, and by-products such as at least one of a biochar and a fertilizer; and a hydroxide regeneration reactor for calcining the carbonate or performing double replacement reactions of the carbonate to produce carbon dioxide and a hydroxide; where the alkaline hydrothermal treatment reactor is in fluid communication with the pretreatment module and the hydroxide regeneration reactor is in fluid communication with the alkaline hydrothermal treatment reactor; and    (2) a method of converting a biomass to hydrogen and carbon dioxide, comprising mixing a dry solid or liquid alkali metal hydroxide and a nickel or iron catalyst with a biomass to form a biomass mixture; heating the biomass mixture and water vapor to about 200 to about 300 degrees C and at ambient pressure until the hydroxide and the biomass react to produce hydrogen, carbonate, and by-products such as at least one of a biochar and a fertilizer; and calcining the carbonate or performing double replacement reactions of the carbonate to produce carbon dioxide and hydroxide.