Green hydrogen from waste biomass: Towards holistic bio-economy for a sustainable future

• Waste biomass valorization offers a renewable pathway for green hydrogen. • Pretreatment and conversion strategies influence hydrogen yield and efficiency. • Advanced catalysts and purification boost hydrogen selectivity and purity. • Techno-economic and life-cycle analyses validate sustainability of bio-refineries. • A holistic bio-economy model integrates waste, energy, and carbon management. The global energy transition toward net-zero emissions demands sustainable and carbon-neutral alternatives to fossil-derived hydrogen. Among the various green hydrogen pathways, waste biomass valorization presents a promising solution by simultaneously addressing energy security, waste management, and climate mitigation. This review critically examines the conversion of diverse waste biomass feedstocks, including agricultural residues, forestry waste, municipal solid waste, and industrial by-products into hydrogen through an integrated biorefinery framework. Key stages of the process are systematically discussed, beginning with feedstock characteristics and pretreatment methods, followed by conventional and advanced technologies for producing intermediates such as bioethanol, biogas, syngas, and bio-oil. The subsequent transformation of these intermediates into hydrogen is reviewed across established steam reforming, partial oxidation, and autothermal reforming routes, as well as emerging processes such as aqueous phase reforming, supercritical water gasification, and plasma-assisted reforming. Advanced hydrogen purification techniques, such as pressure swing adsorption, hybrid membranes, sorption-enhanced systems, and next-generation adsorbents are analyzed for their ability to deliver high purity hydrogen. Techno-economic analysis and life-cycle assessment highlight the competitiveness of biomass-derived hydrogen, with potential for negative emissions when integrated with carbon capture and utilization/storage. A conceptual holistic biorefinery model is proposed, emphasizing process integration, co-product valorization, and circular economy principles. The review identifies technical challenges, research gaps, and future directions in catalyst development, process intensification, and policy support. By bridging technological, economic, and environmental dimensions, this work underscores the role of waste biomass-derived green hydrogen as a cornerstone of the sustainable hydrogen economy.