STSE_1_2_75-82

Introduction

End-of-life tyres accumulate as one of the most persistent forms of black pollution worldwide, a consequence of the chemical resistance of vulcanised rubber to natural degradation and of global tyre production that now exceeds two billion units annually (Han et al., 2023; Afash et al., 2023; Zerin et al., 2023). Landfilling and open burning the two disposal routes historically relied upon are increasingly constrained by environmental regulation and are, in any case, unsustainable both ecologically and economically. Thermochemical conversion, principally pyrolysis and gasification, has consequently emerged as the most promising route for recovering energy and material value from tyre waste, yielding pyrolysis oil, tire-derived char, and a combustible or synthesis-grade gas fraction (Han et al., 2023; Rogachuk and Okolie, 2024; Zerin et al., 2023).

The distribution of pyrolysis products is strongly governed by tyre formulation specifically the ratio of natural to synthetic rubber and by vehicle class. Tires from light passenger vehicles, richer in natural rubber, decompose over a comparatively narrow thermal window, whereas heavier truck and off-road tyres, formulated with a higher proportion of styrene–butadiene rubber (SBR), display a broader decomposition range and markedly different oil-to-gas ratios (Singh et al., 2018; Čepić et al., 2021). Raising the pyrolysis temperature above roughly 500–550 °C consistently increases gas yield at the expense of the liquid fraction, a consequence of secondary cracking of the primary volatiles (Singh et al., 2018; Nisar et al., 2018; Čepić et al., 2021; Zerin et al., 2023). The gas fraction itself is dominated by hydrogen, carbon monoxide, carbon dioxide, methane, and light C₁–C₅ hydrocarbons (Singh et al., 2018; Nisar et al., 2018).

Research attention has, over the past several years, shifted from simple product characterisation toward deliberately steering the process to favour high-value gases, in particular hydrogen and syngas. Catalytic pyrolysis studies employing Ni/ZSM-5 and Fe₂O₃ report marked improvements in H₂ and CH₄ selectivity through modification of the free-radical pathways responsible for gas formation (Li et al., 2022; Yu et al., 2022), while catalytic steam reforming of pyrolysis volatiles over a tire-char catalyst rather than a costly metal catalyst has achieved hydrogen yields of roughly 223 mmol g⁻¹, corresponding to about 74% of the theoretical maximum (Li and Williams, 2024). Solar-photocatalytic pyrolysis using TiO₂-based catalysts under concentrated sunlight has likewise raised gas yield to 40–41 wt%, compared with roughly 20 wt% for uncatalysed solar pyrolysis (Hijazi et al., 2018).

In parallel, numerical modelling and kinetic analysis thermogravimetric analysis (TGA) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) in particular have clarified the decomposition mechanisms and gas-formation pathways involved. These studies consistently show that increasing temperature raises the hydrogen fraction while reducing the share of C₁–C₄ hydrocarbons in the pyrolysis gas, with reported activation energies typically falling in the range of 100–190 kJ/mol depending on operating conditions and catalyst (Garder and Bogomolov, 2025; Menares et al., 2020; Osorio-Vargas et al., 2021). Recent comprehensive reviews confirm that product distribution oil, char, and gas is governed principally by temperature, reactor configuration, residence time, and catalyst type, with gas yield from thermal pyrolysis of tyres typically falling within a 5–20 wt% range (Han et al., 2023; Zerin et al., 2023; Hamzah et al., 2023, 2023a).

This review examines and compares the principal thermochemical and catalytic routes currently reported for converting waste tyre rubber into gaseous products, with emphasis on operating conditions, resulting gas composition, and the influence of catalyst type and process configuration on gas quality and yield. It further weighs the relative technical, economic, and environmental merits of each pathway and identifies the routes most likely to deliver high-value fuel gas or synthesis gas, supporting more advanced and sustainable management of end-of-life tyre waste.

Effect of operating conditions and tire composition on gas yield and composition

Operating temperature, residence time, and tyre type constitute the principal variables governing the quantity and composition of gas evolved during pyrolysis and gasification of tyre rubber. Elevated temperatures intensify secondary cracking of the primary devolatilisation products, raising the yield of light hydrocarbons (C₁–C₅) and hydrogen, while feedstocks rich in SBR rubber are disproportionately associated with higher hydrogen output. Deliberate manipulation of these two variables temperature and tyre type therefore provides a practical means of steering the process toward either a fuel-gas or a synthesis-gas product slate, according to the intended industrial application. These findings, established primarily through fixed-bed and batch-reactor studies (Singh et al., 2018; Nisar et al., 2018; Čepić et al., 2021), underpin the process-design choices examined in the sections that follow (Hamzah, 2025).

Role of catalysts and process integration in enhancing gas production

Catalysts markedly alter both the quantity and the selectivity of gas evolved from tyre pyrolysis. Ni/ZSM-5 and iron oxide (Fe₂O₃) have both demonstrated a pronounced capacity to raise gas yield while steering product selectivity toward hydrogen and methane, and pyrolysis char generated in situ performs a comparable function at substantially lower cost (Sridevi et al., 2024, 2026; Varma et al., 2024). Photocatalysts operating under solar irradiation have likewise produced sizeable increases in gas output while reducing reliance on fossil energy input for process heat. Process integration offers a complementary route to gas-quality improvement: Coupling pyrolysis with steam reforming of the resulting volatiles particularly when the reforming step is catalysed by tire-derived char rather than an externally supplied catalyst extends the potential for producing H₂- and CO-rich synthesis gas, although the approach carries appreciable energy and steam demand that must be weighed against the value of the hydrogen recovered (Wang et al., 2020; Li et al., 2022; Yu et al., 2022; Hijazi et al., 2018; Li and Williams, 2024).

Comparative analysis of thermochemical and catalytic conversion technologies

Conventional thermal pyrolysis in batch or fixed-bed reactors remains the principal technological basis for gas recovery from end-of-life tyres. The process is typically operated between 400 and 750 °C under an inert atmosphere, using shredded tyre crumb or granulate as feedstock. Gas yield rises with temperature, particularly above 500 °C, as a consequence of secondary cracking, and the resulting gas stream consists predominantly of H₂, CO, CO₂, CH₄, and light C₁–C₅ hydrocarbons (Singh et al., 2018; Nisar et al., 2018; Čepić et al., 2021). The technology’s principal attraction lies in its simplicity and industrial maturity; its principal limitation is a comparatively modest gas yield of 5–20 wt%, with substantial quantities of oil and char generated concurrently (Čepić et al., 2021; Zerin et al., 2023; Hamzah et al., 2026; Hamzah et al., 2026a). Table 1 and Figure 1 summarise these figures alongside the remaining routes discussed below.

Studies examining the influence of tyre type on gaseous products indicate that the combination of tyre formulation and operating temperature can be used deliberately to tune gas composition: Tyres with a higher SBR content generate proportionally more hydrogen at elevated temperature (Singh et al., 2018). Detailed fraction-separation experiments coupled with gas-chromatographic analysis confirm that increasing temperature raises the light-hydrocarbon (C₁–C₅) share of the product gas at the expense of the liquid fraction, providing the quantitative basis needed to design units targeting a specific gas composition (Nisar et al., 2018; Husam et al., 2024).

Table 1: Comparative summary of thermochemical and catalytic routes for gas production from waste tyre rubber.

Technology/ Study	Key operating conditions	Gas yield and composition	Advantages and limitations	Primary application	References
Conventional thermal pyrolysis (batch/fixed-bed)	400–750 °C, inert N₂, tire crumb/granulate feed	5–20 wt% gas, rising above 500 °C; H₂, CO, CO₂, CH₄, C₁–C₅	Simple, industrially mature; substantial co-production of oil and char	General fuel gas / process heat	Singh et al., 2018; Nisar et al., 2018; Čepić et al., 2021; Zerin et al., 2023
Tire-type-controlled pyrolysis	650–750 °C; light/medium/heavy tyre classes, batch reactor	Same gas species; SBR-rich tyres favour higher H₂ at elevated T	Enables gas-composition tuning via feedstock/temperature selection	Design basis for targeted gas composition	Singh et al. 2018
Fraction-resolved pyrolysis with GC analysis	Stepped temperature program; separated gas/liquid/tar/char fractions	Gas enriched in C₁–C₅ with rising T, at oil's expense	Provides quantitative design data for composition-specific gas units	Light fuel-gas (C₁–C₅) production	Nisar et al. 2018
Hot-char-catalysed pyrolysis (two-stage)	Two sequential stages, 500–550 °C; hot tyre char as in-situ catalyst	Gas yield raised from 3 to 10.5 wt%	Low-cost in situ catalyst; reduces PAHs and carbon deposition; needs a suitable char supply.	Improved gas/oil quality at low catalyst cost	Wang et al. 2020
Ni/ZSM-5-catalyzed pyrolysis for H₂	Catalytic pyrolysis with Ni, ZSM-5, Ni/ZSM-5; MD-simulation-supported	Major gases H₂, CH₄, C₂H₄; Ni/ZSM-5 gives highest H₂ content	High H₂ selectivity, lower required temperature; costly metal catalyst	Dedicated hydrogen-rich gas production	Li et al. 2022
Fe₂O₃-catalyzed pyrolysis with mechanistic analysis	Fixed-bed + MD/DFT simulation; commercial Fe₂O₃	Main gases CH₄, H₂, C₂H₄; Fe₂O₃ raises CH₄/H₂, lowers C₂H₄	Enables gas-ratio tuning via catalyst choice and radical-pathway control	Tailored CH₄/H₂-rich fuel gas	Yu et al. 2022
Solar-photocatalytic pyrolysis	550–570 °C, 950–1050 W/m² solar flux; TiO₂, Pd/Pt–TiO₂, Bi₂O₃/SiO₂/TiO₂	Gas 20 wt% uncatalyzed; 27% with TiO₂; 32% with Bi₂O₃/SiO₂/TiO₂; 40–41% with Pd/Pt–TiO₂	Harnesses solar energy, strong yield gains; complex, weather- and catalyst-sensitive	Fuel-gas production with reduced fossil energy input	Hijazi et al. 2018
Ni/SiO₂-catalyzed pyrolysis (liquid-focused)	350–450 °C; analytical reactor + TGA-FTIR + Py-GC/MS	Catalyst redirects pathways toward aromatics; lowers activation energy, accelerates volatile release	Improves vapor/gas formation at reduced severity; primary focus is oil quality	Product-distribution optimisation at reduced severity	Osorio-Vargas et al. 2021
Co-pyrolysis with waste plastics (HDPE, LDPE, PP, PS, PET)	Fixed batch-bed reactor; mixed tire/plastic feed ratios	Main gases H₂, CH₄, C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₄; synergistic yield above additive expectation	Raises and tunes gas yield via plastic co-feed; added feedstock-logistics complexity	Fuel-gas production with tailored oil aromaticity	Alzahrani et al. 2024
Gasification + steam reforming with tire-char catalyst	Two-stage: pyrolysis, then steam reforming of volatiles over tyre char, 700–1000 °C	H₂-/CO-rich syngas; H₂ yield ≈ 223 mmol g⁻¹ (≈74% of theoretical maximum) at 2 h	Combines char gasification with vapour reforming; high H₂/CO yield; high steam/energy demand.	Hydrogen and syngas for chemical/fuel use	Li and Williams 2024
Air-plasma gasification of rubber powder	Air-plasma, ≈1750 K, rubber-powder feed	44.6% syngas (19.1 vol% H₂, 25.5 vol% CO); 95.6% carbon conversion	High-purity, high-yield syngas; no notable harmful impurities; high energy consumption	H₂/CO syngas for fuel or chemical feedstock	Messerle and Ustimenko 2024
Conventional gasification (review)	Steam, air, or CO₂ gasifying agent; ≈900–1060 K	Gas rich in H₂, CO, CH₄, C₂H₄, C₃H₆; composition set by agent and equivalence ratio	Flexible gas composition; more difficult process control and emissions management	Syngas/fuel-gas recovery at scale	Han et al. 2024
Kinetic/numerical modelling of pyrolysis	Finite-element simulation, fixed dense bed, rubber chips (3 mm), 350–650 °C.	Rising T increases H₂, decreases C₁–C₄ share; activation energy ≈188.6 kJ/mol	Design and optimisation tool; reduces experimental burden	Reactor design and operating-condition optimisation	Garder and Bogomolov 2025

Catalytic intervention whether through hot tyre char or metal-oxide catalysts such as Fe₂O₃ has consistently improved both gas quality and gas yield while suppressing PAH formation and carbon deposition; Ni/ZSM-5 in particular produces a hydrogen-rich gas stream, albeit at the cost of an expensive metal catalyst (Wang et al., 2020; Li et al., 2022; Yu et al., 2022). Solar-photocatalytic pyrolysis using TiO₂ and its derivatives has separately demonstrated a substantial increase in gas yield while harnessing solar energy input, although the approach introduces additional system complexity and sensitivity to catalyst formulation and weather conditions (Hijazi et al., 2018), as illustrated in Figure 1.

Co-pyrolysis of tyre rubber with various plastic wastes (HDPE, LDPE, PP, PS, PET) exhibits a synergistic effect that raises gas productivity and modifies gas composition according to the plastic co-feed, at the cost of greater complexity in feedstock logistics (Alzahrani et al., 2024). Combining pyrolysis with steam reforming of the char has separately achieved high yields of hydrogen and synthesis gas, supporting the use of these gases in chemical or industrial-fuel applications, notwithstanding the elevated energy and steam demand this coupling entails (Li and Williams, 2024).

Numerical modelling provides an effective tool for optimising reactor design and operating conditions and for predicting product gas composition without recourse to costly repeated experimentation (Garder and Bogomolov, 2025; Akpan et al., 2026). Comprehensive recent reviews confirm that reactor type, temperature, catalyst, and residence time are the decisive factors governing both gas yield and gas composition, pointing toward the process refinements needed to extract maximum value from tyre recycling (Han et al., 2023; Gao et al., 2022; Zerin et al., 2023).

Beyond pyrolysis, gasification-based routes offer a further means of producing hydrogen- and CO-rich synthesis gas directly. Plasma gasification of tire-rubber powder, operated at a mass-average temperature of approximately 1750 K under an air plasma, converts the great majority of feedstock carbon (95.6%) into a syngas stream containing 19.1 vol% H₂ and 25.5 vol% CO — a 44.6% syngas yield essentially free of harmful impurities though at considerable energy cost (Messerle and Ustimenko, 2024). Conventional gasification using steam, air, or CO₂ as the gasifying agent, typically operated between roughly 900 and 1060 K, offers greater flexibility in tailoring gas composition (H₂, CO, CH₄, C₂H₄, C₃H₆) through the choice of gasifying agent and equivalence ratio, but presents greater challenges in process control and emissions management (Han et al., 2024). Figure 2 situates these gasification routes within the broader set of conversion pathways surveyed in this review, and Table 2 collects the corresponding quantitative performance benchmarks.

Table 2: Quantitative performance benchmarks for the principal conversion routes.

Technology	Operating temperature	Gas / Syngas yield	Reference
Conventional thermal pyrolysis	400–750 °C	5–20 wt%	Čepić et al. 2021; Zerin et al. 2023
Hot-char-catalysed pyrolysis	500–550 °C	3–10.5 wt%	Wang et al. 2020
Solar pyrolysis, uncatalysed	550–570 °C	20 wt%	Hijazi et al. 2018
Solar pyrolysis + TiO₂	550–570 °C	27 wt%	Hijazi et al. 2018
Solar pyrolysis + Bi₂O₃/SiO₂/TiO₂	550–570 °C	32 wt%	Hijazi et al. 2018
Solar pyrolysis + Pd/Pt–TiO₂	550–570 °C	40–41 wt%	Hijazi et al. 2018
Tire-char-catalysed steam reforming	700–1000 °C	H₂: ≈223 mmol g⁻¹ (≈74% of theoretical max.)	Li and Williams 2024
Air-plasma gasification	≈1750 K (≈1477 °C)	44.6 wt% syngas	Messerle and Ustimenko 2024

Prospects and challenges for industrial-scale deployment

Despite substantial progress in understanding and optimising these conversion routes, their translation to industrial scale remains constrained by energy efficiency, the complexity of the required process systems, and the cost of high-performance catalysts. Numerical modelling offers a promising means of reducing dependence on costly experimental campaigns while improving reactor design and operating strategy. Looking forward, integrating plastic co-feeds with tyre rubber, together with developing low-cost, environmentally benign catalysts, represents a particularly promising research direction for improving the economic and environmental viability of these technologies. Equally important are systematic techno-economic assessments, extended catalyst-durability trials under continuous operation, and closer integration of tire-derived char with co-pyrolysis or chemical-looping gasification schemes priorities that would substantially strengthen the evidence base needed for industrial deployment.

Conclusion

This review demonstrates considerable diversity among available technologies for producing gas from waste tyre rubber, each carrying distinct advantages and limitations in terms of gas yield, by-product quality, energy demand, and operating cost. Catalytic and integrated process configurations including catalytic pyrolysis, char-based gasification, and photocatalyst-assisted routes consistently achieve higher hydrogen and synthesis-gas productivity with reduced pollutant formation relative to uncatalysed thermal pyrolysis, though the complexity of these systems and the cost of catalysts remain obstacles to industrial-scale expansion. The evidence assembled here underscores the importance of continued development in numerical modelling and process integration to achieve cleaner gas production with improved economic and environmental efficiency, opening promising prospects for managing tire rubber waste and converting it into value-added resources.

Acknowledgements

The authors acknowledge the Department of Computer Engineering, Al-Farabi University, and the Applied Sciences Department, University of Technology–Baghdad, for providing the academic and library resources that supported the preparation of this review.

Novelty Statement

Unlike previous reviews that examine tyre pyrolysis or gasification in isolation, this work provides an integrated, quantitative comparison of five conversion routes conventional thermal pyrolysis, catalytic pyrolysis, solar-photocatalytic pyrolysis, catalytic steam reforming, and plasma/conventional gasification under a single evidence framework (Tables 1–2, Figures 1–2). It further links operating parameters, catalyst chemistry, and tyre composition directly to gas yield and hydrogen selectivity, and identifies techno-economic assessment, catalyst durability, and co-pyrolysis/chemical-looping integration as the priority gaps limiting industrial deployment.

Author’s Contribution

Almuntadher Alwhelat: Conceptualisation, literature investigation, formal analysis, writing original draft, writing review and editing, corresponding author. Supervision, validation, writing review and editing.

Funding

No funding received.

Generative AI and AI-assisted technology statement

Generative AI tools were used solely for language polishing and grammar refinement of the manuscript text. No AI tool was used to generate scientific data, analysis, interpretations, or references. The authors reviewed, verified, and take full responsibility for the accuracy and integrity of the final content.

Conflict of interest

The authors has declared no conflict of interest.

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