Environmental Issue: Approximately 1.5 billion tires are produced each year, eventually entering the waste stream and representing a significant environmental problem. Vehicle tires contain long-chain polymers (butadiene, isoprene, and styrene-butadiene) cross-linked with sulfur, giving them excessive resistance to degradation. One common method of tire disposal is landfilling. However, tires are bulky, with 75% of the space they occupy being void, which makes landfilling problematic. Waste tires require considerable space because their volume cannot be compacted. Additionally, tires tend to float or rise to the surface of landfills. The void space inside waste tires captures gases such as methane, which can suddenly combust and cause large explosions.

When waste tires are discarded carelessly, rainwater can accumulate in them, creating an ideal breeding ground for mosquitoes and bacteria, which in turn can cause various harmful diseases to humans. Direct burning of scrap tires in brick kilns or other incineration plants releases harmful gases like CO₂, CO, SOₓ, and NOₓ, contributing to environmental pollution. Moreover, the burning of these tires emits pollutants that are detrimental to human health, including polyaromatic hydrocarbons (PAHs), benzene, styrene, butadiene, and phenol-like substances.

Conversion of Waste Tires to Energy: Converting waste tires to energy through pyrolysis is a modern technology that not only helps minimize waste disposal but also provides an alternative fuel for various applications. Pyrolysis is generally defined as the thermal decomposition of organic waste in the absence of oxygen at moderate temperatures, typically around 450°C. One advantage of the pyrolysis process is its ability to handle waste tires effectively.

Studies show that pyrolysis oil from automobile tires contains approximately 85.54% carbon, 11.28% hydrogen, 1.92% oxygen, 0.84% sulfur, and 0.42% nitrogen. Pyrolysis is also non-toxic and does not emit harmful gases, unlike incineration. Tire pyrolysis oil has a high calorific value, ranging from 41–44 MJ/kg, which makes it a suitable replacement for diesel fuel if properly distilled. Therefore, waste tires should be utilized for producing new, clean energy sources.

Although various disposal methods for used automotive tires exist, the problem persists due to the inexhaustible quantity of waste tires. Pyrolysis offers a way to create value-added products, including pyrolysis oil, pyrolysis gas, and char.

What is Pyrolysis: Pyrolysis is a thermochemical process that involves the decomposition of complex macromolecular organic compounds in the absence of external oxygen, producing oil, gas, a water phase, and char. This transformation occurs within a temperature range of 400–800°C. Pyrolysis processes vary based on operational parameters and can be classified as atmospheric, vacuum, catalytic, fast, ultrafast (flash), and slow.

Chemical transformations begin at temperatures above 150°C. The degradation of rubber occurs between 250–500°C, producing the majority of pyrolysis oil and gas, with the peak release of volatiles between 450–500°C. At temperatures exceeding 550°C, the release of volatiles from char decreases, and the tire rubber undergoes near-complete decomposition. Secondary cracking of larger primary pyrolysis products into smaller molecules occurs between 600–850°C. This rapid release prevents secondary chemical reactions involving pyrolysis products. Generally, oil yield reaches a maximum at temperatures above 550°C, while gas yield increases and char yield decreases with rising temperatures.