This study comprehensively investigates the production of hydrogen-rich syngas from waste through pyrolysis-catalytic steam reforming in a two-stage fixed-bed reactor system. Various char catalysts, including tire char, biochar, refuse-derived fuel char, and modified carbon catalysts, were utilized to explore the catalytic behavior under different feedstocks and operating conditions.
Tire char was used as a sacrificial catalyst, simultaneously participating in both catalytic steam reforming and carbon-steam gasification. Optimized process parameters-such as high reforming temperatures (up to 1000 °C), steam space velocities (6-10 g h-1 g-1), and high catalyst: plastic ratios-led to significant improvements in hydrogen and syngas yields from HDPE, with hydrogen yields reaching up to 223 mmol g-1. The presence of inherent metals such as Zn, Fe, Ca, and Mg in tire char contributed to its catalytic activity.
Further investigations on single plastics (e.g., HDPE, LDPE, PP, PS, PET) revealed that polyolefin plastics exhibited the highest hydrogen yields (~130 mmol g-1) due to their favorable decomposition behavior. Additionally, real world mixed waste plastics from drink bottles, household packaging, construction waste plastics, agricultural waste plastics and mixed municipal solid waste plastics were investigated. Theoretical maximum hydrogen yields based on elemental compositions were calculated and compared with experimental results for different feedstocks. Biochar and RDF char proved effective for pyrolysis-catalytic steam reforming of plastics, with RDF char offering higher hydrogen potential at high temperatures owing to its higher inorganic metal content. Ashes derived from tires and RDF were also evaluated as catalytic alternatives, where metal contents played key roles in catalytic performance.
Tire char was further used in the pyrolysis-catalytic steam reforming of waste tires. The higher temperature and steam space velocity increased H2 and CO yields. Elemental analysis, surface morphology and pore structure of the used tire char provided insights into tire char consumption in the reaction. Prolonged reaction time allowed for more thorough reactions between the pyrolysis volatiles and tire char, promoting the production of H2. At a reaction time of 2 h, the H2 yield reached 223 mmol g-1, representing 74 wt.% of the maximum hydrogen yield.
In addition, biochar derived from sawdust was tested for steam reforming of both biomass components (cellulose, hemicellulose, lignin) and real biomass. Among the components, the lignin showed the highest H2 and syngas yields, at around 110 mmol g-1 and 140 mmol g-1, respectively, while mixtures demonstrated synergistic effects. Moreover, the incorporation of K and Ca significantly promoted carbon conversion, increased hydrogen production, and inhibited methane generation. Real biomass showed different behavior from model mixtures, indicating complex interactions beyond simple additive effects.
Finally, the properties of pure carbon were modified via acid treatments, metals doping, and steam gasification. The influence of metal species, char surface area and char acidity on hydrogen yield, syngas quality and catalyst stability were systematically investigated.
This work highlights the potential of waste-derived char catalyst for converting plastic tires and biomass waste into valuable hydrogen-rich syngas. The results provide insights into feedstock-catalyst interactions, catalyst design strategies, and the optimization of process conditions for efficient waste-to-energy conversion.
