End-of-life tires present a significant waste management challenge globally, with millions of tons generated annually. Converting this waste into hydrogen through pyrolysis and gasification offers a sustainable solution while addressing energy demands. These processes thermally decompose tires in an oxygen-controlled environment, yielding hydrogen-rich syngas alongside byproducts like carbon black and oil. The approach aligns with circular economy principles, turning waste into a valuable energy carrier.

Pyrolysis involves heating tires in the absence of oxygen, typically at temperatures between 400°C and 800°C. This breaks down the rubber polymers into smaller molecules, producing gas, liquid, and solid fractions. The gas fraction contains hydrogen, methane, and other hydrocarbons, while the liquid fraction includes oils and tars. The solid residue consists mainly of carbon black and inorganic materials like steel and ash. Gasification, on the other hand, introduces a limited amount of oxygen or steam at higher temperatures (700°C to 1,500°C), converting the tire material into syngas composed of hydrogen, carbon monoxide, carbon dioxide, and methane. The hydrogen yield depends on process parameters such as temperature, heating rate, and reactor configuration.

One of the primary challenges in hydrogen production from tires is the sulfur content. Tires contain sulfur as a vulcanizing agent, which converts to hydrogen sulfide (H₂S) during pyrolysis or gasification. H₂S is corrosive and poisons catalysts used in downstream processes like water-gas shift reactions, which enhance hydrogen purity. Desulfurization techniques, such as scrubbing with amine solutions or adsorption on metal oxides, are necessary to remove H₂S before further processing. Failure to address sulfur contamination can lead to equipment degradation and reduced hydrogen quality.

Carbon black, a major byproduct, accounts for approximately 30-35% of the tire mass. This material has commercial value as a reinforcing agent in rubber products or as a pigment. However, the quality of pyrolysis-derived carbon black depends on the process conditions. High-temperature treatments improve its purity by reducing volatile content, making it suitable for industrial applications. Alternatively, carbon black can be gasified to produce additional syngas, increasing overall hydrogen yield. Effective utilization of this byproduct improves the economic viability of waste tire-to-hydrogen systems.

Reactor design plays a critical role in optimizing hydrogen production. Rotary kilns are commonly used due to their ability to handle heterogeneous feedstocks like shredded tires. These reactors provide continuous processing, uniform heating, and efficient mixing, which enhance thermal decomposition. Fluidized bed reactors offer another option, with better heat transfer and shorter reaction times, but they require finer feedstock preparation. Fixed-bed reactors are simpler but less efficient for large-scale operations. Each design has trade-offs in terms of capital cost, operational complexity, and hydrogen output efficiency.

The energy balance of the process is another consideration. Pyrolysis and gasification are energy-intensive, requiring external heat input, especially for high-temperature operations. Autothermal gasification, where partial oxidation of the feedstock provides the necessary heat, can reduce external energy demand but may lower hydrogen concentration in the syngas. Integrating waste heat recovery systems improves overall efficiency, making the process more sustainable.

Economic feasibility depends on scaling up operations and minimizing costs associated with feedstock preparation, reactor maintenance, and gas cleaning. While small-scale plants may struggle with profitability, larger facilities benefit from economies of scale, especially when coupled with byproduct valorization. Policy support, such as subsidies for waste-derived hydrogen or carbon pricing, could further enhance competitiveness against conventional hydrogen production methods.

Environmental benefits include reducing tire stockpiles, which pose fire hazards and leaching risks, while displacing fossil-derived hydrogen. However, emissions from auxiliary energy use and byproduct management must be carefully controlled to ensure net environmental gains. Life cycle assessments indicate that waste tire-to-hydrogen pathways can achieve lower carbon footprints compared to steam methane reforming, provided clean energy sources power the process.

Technological advancements are needed to address remaining challenges. Improved sulfur-tolerant catalysts could simplify gas cleaning steps, while advanced reactor designs may enhance heat transfer and reduce energy consumption. Research into hybrid systems, combining pyrolysis with plasma gasification, shows promise for higher hydrogen yields and cleaner byproducts.

In summary, hydrogen production from end-of-life tires via pyrolysis and gasification offers a dual benefit of waste reduction and clean energy generation. Overcoming technical and economic barriers will be crucial for large-scale adoption, but the potential for sustainable hydrogen within a circular economy framework is significant. Continued innovation and supportive policies will determine the role of this pathway in future energy systems.