Sulfur Management Strategies in Tyre Pyrolysis

The pyrolysis of end-of-life tyres offers a sustainable route for recovering valuable hydrocarbons, carbon black, and steel, yet it introduces a complex challenge—sulfur control. Tyres typically contain 1–2% sulfur, introduced primarily through vulcanization agents such as zinc oxide and sulfur compounds. During thermal decomposition, these compounds undergo fragmentation, producing sulfur-bearing gases, tars, and solids that, if unmanaged, can cause severe corrosion, catalyst poisoning, and secondary pollution. Efficient sulfur management is therefore an essential technical pillar for any modern tyre pyrolysis plant.

Sulfur Distribution in Tyre Pyrolysis

In a pyrolysis process, sulfur distributes among three main product streams: the gas phase, the liquid oil, and the solid char. Typically, 35–45% of the total sulfur migrates into the gas fraction as hydrogen sulfide (H₂S) and carbonyl sulfide (COS), 30–40% dissolves in the pyrolytic oil as organosulfur species, and the remainder becomes chemically bound to the solid char as inorganic sulfides and sulfates.

The distribution ratio depends on parameters such as feedstock composition, reactor temperature, and heating rate. A high-temperature pyrolysis environment (>600°C) tends to promote the release of volatile sulfur species into the gaseous stream, while lower temperatures favor retention in the char. Understanding these transformation dynamics is essential for designing effective sulfur control systems.

Reactor Design and Temperature Optimization

The core of sulfur mitigation lies in the design and operation of the pyrolysis reactor. In a continuous tyre pyrolysis plant, maintaining an optimal temperature range between 500°C and 650°C minimizes secondary reactions that promote the formation of complex thiophenic structures in the oil. Above this threshold, sulfur-containing aromatics can recombine into refractory compounds that are difficult to remove downstream.

Temperature uniformity within the reactor is equally critical. Uneven heating or localized hot zones cause partial cracking, leading to inconsistent sulfur partitioning. Advanced temperature mapping and automated control systems allow for fine-tuned heat distribution, preventing undesirable side reactions and improving gas purity.

Catalytic Desulfurization Within the Reactor

Integrating catalysts directly into the pyrolysis reactor provides a first line of sulfur control. Metal oxides such as calcium oxide (CaO), magnesium oxide (MgO), and iron oxide (Fe₂O₃) have shown strong affinities for sulfur species, forming stable sulfides at high temperatures. These sorbents can be introduced as reactor bed materials or mixed with the feedstock before pyrolysis.

The reaction between CaO and H₂S, for example, produces calcium sulfide (CaS), effectively capturing sulfur from the vapor phase. Similarly, iron-based sorbents promote the conversion of volatile sulfur into iron sulfide, which remains in the char. This in-situ desulfurization approach reduces the sulfur burden in both the pyrolysis oil and gas, simplifying downstream treatment.

Post-Pyrolysis Gas and Oil Treatment

Despite effective in-reactor control, the gaseous effluent from a pyrolysis plant still contains trace sulfur compounds. These are commonly removed using wet scrubbing, adsorption, or catalytic oxidation.

Wet scrubbing employs alkaline solutions, such as sodium hydroxide or ammonia, to neutralize H₂S and COS, producing non-volatile sulfates or sulfides. Activated carbon beds and zinc oxide filters provide a dry alternative, selectively adsorbing sulfur-bearing molecules at ambient or elevated temperatures.

For the liquid fraction, hydrodesulfurization (HDS) remains the most robust technique. Under hydrogen-rich conditions and moderate pressures, organosulfur compounds are converted into H₂S, which is subsequently separated and treated. Emerging plasma-assisted and photochemical methods also show promise in achieving deep desulfurization without excessive hydrogen consumption.

Sulfur Stabilization in Char and Byproducts

The solid char derived from tyre pyrolysis contains both fixed carbon and inorganic residues, including sulfur in various chemical forms. Stabilizing these sulfur species prevents their release during char utilization in construction materials or activated carbon production. Thermal post-treatment or controlled oxidation can convert unstable sulfides into stable sulfates, reducing odor and leaching potential.

Additionally, incorporating sorbents such as limestone during pyrolysis not only captures gaseous sulfur but also improves char stability. This dual benefit enhances both environmental safety and material performance of the recovered solid fraction.

Toward Cleaner Tyre Pyrolysis Systems

Advancements in process integration and emission monitoring are transforming sulfur control from a reactive step to a proactive design principle. Real-time sensors now allow continuous tracking of H₂S concentration, enabling dynamic adjustment of reactor parameters or sorbent dosing.

Through a combination of optimized thermal regimes, catalytic capture, and advanced post-treatment, sulfur emissions from tyre pyrolysis can be reduced to minimal levels. As the global emphasis on circular economy and low-emission technologies intensifies, these sulfur management strategies ensure that pyrolysis remains a technically sound and environmentally compliant solution for end-of-life tyre recycling.