Waste-to-hydrogen conversion: A mini review of feedstocks, processes, and hydrogen production

Abstract

Waste-to-hydrogen (WtH₂) technologies can address two persistent challenges simultaneously: the accumulation of heterogeneous wastes and the need for lower-emission hydrogen. However, route-centered reviews often compare gasification, pyrolysis–reforming, dark fermentation, or microbial electrolysis independently, obscuring the more fundamental question of how feedstock properties determine reaction pathways and achievable hydrogen performance. This review develops a feedstock–process framework that links moisture, ash, elemental composition, volatile matter, polymer chemistry, carbohydrate accessibility, and biodegradability to thermochemical and biological conversion routes. Dry, carbon-rich residues and plastics generally favor high-temperature gasification or staged pyrolysis–reforming, whereas wet, readily biodegradable wastes more naturally match dark fermentation and bioelectrochemical upgrading. Sewage sludge and heterogeneous municipal wastes occupy an intermediate region in which pretreatment, hydrothermal processing, co-conversion, or hybrid systems become particularly important. Across thermochemical routes, hydrogen formation is governed by devolatilization, cracking, steam reforming, char gasification, and water–gas shift reactions; catalyst stability, tar control, steam-to-carbon ratio, and heat integration are therefore decisive. In biological systems, hydrolysis, acidogenic metabolism, hydrogen partial pressure, pH, microbial ecology, and suppression of hydrogen-consuming pathways control net recovery. The analysis shows that hydrogen yield cannot be interpreted independently of feedstock basis, gas composition, carbon conversion, external energy input, and downstream purification. Hybrid concepts-such as dark fermentation followed by microbial electrolysis, or pyrolysis followed by catalytic steam reforming and water–gas shift- offer a pathway toward more complete carbon and energy recovery. The review proposes standardized reporting metrics and a route-selection matrix and identifies impurity-tolerant catalysts, wet-waste integration, carbon management, modular reactors, and harmonized techno-economic/life-cycle boundaries as priorities for scalable WtH₂ systems.