OPTIMIZATION OF LOW-TEMPERATURE PYROLYSIS OF WASTE TIRES FOR THE PRODUCTION AND CHARACTERIZATION OF TIRE PYROLYSIS OIL

ABSTRACT

This study investigates the production and optimization of tire pyrolysis oil (TPO) from waste rubber materials through low-temperature pyrolysis. The effects of temperature, reaction time, and particle size on TPO yield were examined. The maximum yield of 42 wt.% was achieved at 500 °C, a reaction time of 50 min, and a particle size of 4 × 2 × 0.5 cm³. Fractional distillation separated TPO into seven boiling fractions ranging from 30 °C to over 300 °C. Desulfurization and decolorization were performed using a hydrogen peroxide and formic acid mixture (2:1), achieving up to 64.52% sulfur removal, with N,N-dimethylformamide as the most effective solvent. FTIR analysis confirmed the presence of alkenes, aromatics, ethers, carbonyl, and hydroxyl groups, indicating hydrocarbon-type fuel characteristics. The middle fractions (150–300 °C) exhibited improved combustion properties comparable to commercial diesel. Optimized TPO production presents a sustainable pathway for waste tire recycling, offering significant environmental and economic benefits.

АННОТАЦИЯ

В данном исследовании изучено получение и оптимизация пиролизного масла (ППМ), получаемого из отходов резиновых материалов методом низкотемпературного пиролиза. Рассмотрено влияние температуры, времени реакции и размеров частиц на выход ППМ. Максимальный выход 42 мас.% был достигнут при 500 °C, времени реакции 50 минут и размере частиц 4 × 2 × 0,5 см³. Фракционная перегонка разделила ППМ на семь фракций с диапазоном температур кипения от 30 °C до более 300 °C. Процессы десульфуризации и обесцвечивания проводились смесью перекиси водорода и муравьиной кислоты (2:1), при этом было удалено до 64,52 % серы; наиболее эффективным растворителем оказался N,N-диметилформамид. ИК-Фурье анализ подтвердил наличие алкенов, ароматических соединений, эфиров, карбонильных и гидроксильных групп, что указывает на углеводородную природу топлива. Средние фракции (150–300 °C) продемонстрировали улучшенные свойства сгорания, сопоставимые с коммерческим дизельным топливом. Оптимизированное получение ППМ является устойчивым методом переработки отходов шин с экологическими и экономическими преимуществами.

Keywords: Tire pyrolysis oil (TPO); waste rubber; low-temperature pyrolysis; desulfurization; fractional distillation; FTIR analysis; alternative fuel.

Ключевые слова: Пиролизное масло (ППМ); отходы резины; низкотемпературный пиролиз; десульфуризация; фракционная перегонка; ИК-Фурье анализ; альтернативное топливо.

Introduction.  The global number of motor transport vehicles has been steadily increasing every year, inevitably leading to the accumulation of large quantities of used and discarded tires. According to the European Tire Recycling Association, more than 9 million tons of end-of-life tires are generated annually in Europe. In the United States, approximately 1.5 million tons of worn-out tires are produced each year, while in the United Kingdom this figure reaches nearly 500 thousand tons, of which about 34% are recycled, 26% are retreaded, 15% are incinerated, and 6% are landfilled. Japan produces around 96 million waste tires annually, with 88.5% being recycled, whereas France and Germany generate 400–500 thousand tons per year, and Russia-over 1 million tons, of which less than 10% are processed [1].

Among the existing recycling techniques for waste rubber, thermal decomposition (pyrolysis) is recognized as one of the most efficient and sustainable methods. Various approaches, such as low-temperature pyrolysis, ozonation, catalytic cracking, and plasma treatment, have been explored; however, pyrolysis remains the most optimal due to its capability to recover valuable products including oils, gases, and carbonaceous solids. The utilization of used automobile tires as a secondary raw material is economically and environmentally beneficial, since tires contain 65–70% rubber, 15–25% carbonaceous material, and 10–15% metal cord. Among these components, the carbon-rich fraction plays a particularly significant role, and therefore, its physicochemical properties have been the subject of detailed investigation in this study [2].

Research Object The research object of this study is waste vulcanized rubber materials, primarily used automobile tires, which serve as the main raw material for low-temperature pyrolysis. These materials consist of a complex composite structure including natural and synthetic rubbers, carbon black, sulfur compounds, metal cord, and various additives used during tire manufacturing.

Methods and materials. The experimental work was carried out using samples of waste vulcanized rubber, mainly from used automobile tires. Prior to pyrolysis, the tires were mechanically cut into uniform pieces measuring approximately 4×2×0.5 cm³ to ensure consistent heat transfer during thermal processing. The samples were cleaned from metal cords and textile fibers before being subjected to pyrolysis [3-4].

The obtained products were characterized by FTIR spectroscopy (IRTracer-100, Shimadzu) to identify functional groups, and by elemental analysis for carbon, hydrogen, sulfur, and oxygen content. The solid residue (char) was studied using X-ray diffraction (XRD-6100, Shimadzu) and scanning electron microscopy (SEM, JEOL JSM-6390) to evaluate its morphology and structure. The yield of each product was determined gravimetrically, and the energy content of the liquid fractions was estimated calorimetrically [5].

Results and discussion. The optimization of tire pyrolysis oil (TPO) production depended on temperature, reaction time, and the particle size of the tire sample. In our previous study, it was reported that the maximum TPO yield reached 42 wt.% at a temperature of 500 °C, a reaction time of 50 minutes, and a tire particle size of 4 × 2 × 0.5 cm³. Low-temperature and prolonged pyrolysis promotes the formation of secondary products, resulting in a higher yield of solid char and a lower yield of liquid oil. Conversely, pyrolysis at higher temperatures (above 500 °C) increases the calorific value of the volatile components but decreases the yield of TPO due to enhanced secondary cracking reactions, which lead to an increased production of gaseous fractions. Tire particles larger than 4 × 2 × 0.5 cm³ restrict uniform heating, preventing full carbonization of the rubber core and thereby reducing TPO yield while increasing gas and char formation.

Table 1.

Presents the yields of different pyrolysis products obtained at various reaction times at a constant temperature of 500 °C and tire particle size of 4 × 2 × 0.5 cm³

For desulfurization, a mixture of hydrogen peroxide (H₂O₂) and formic acid (HCOOH) in a 2:1 ratio was used. Solutions of different concentrations (10%, 15%, 20%, 25%, and 30% of TPO) were applied to oxidize sulfur-containing compounds. The oxidized components were extracted using various solvents, such as acetone, ethanol, methanol, and N,N-dimethylformamide (DMF), among which DMF proved to be the most effective. A maximum 64.52% sulfur removal was achieved using 25% H₂O₂ + HCOOH mixture in TPO.

Fractional distillation separated TPO into seven distinct fractions according to their boiling point ranges: 30–80 °C, 81–140 °C, 141–180 °C, 181–220 °C, 221–260 °C, 261–300 °C, and the residue above 300 °C. Approximately 40% of the total distillate was collected below 180 °C, 17% between 180–220 °C, and 20% between 220–300 °C. The distillates obtained in the 150–300 °C range exhibited improved sprayability and better ignition properties, which facilitate combustion initiation at lower temperatures.

The FTIR (Fourier Transform Infrared) spectroscopy method was used to determine the functional groups present in different distillation fractions of TPO. The absorption peaks confirmed the presence of alkenes, aromatic compounds, ketones, alcohols, and aldehydes. The characteristic absorption bands for alkene (=C–H) groups were observed between 730–770 cm⁻¹, specifically at 731.02 cm⁻¹, 756.10 cm⁻¹, and 752.24 cm⁻¹.

Figure 1. FTIR spectra of various TPO fractions obtained at different temperatures

Stretching vibrations of C–O bonds appeared in the range of 1300–1100 cm⁻¹, with distinct peaks at 1039.03 cm⁻¹, 1045.42 cm⁻¹, 1026.13 cm⁻¹, 1092.07 cm⁻¹, and 1274.95 cm⁻¹, indicating the presence of ether and related functional groups. Additionally, peaks at 1778.37 cm⁻¹, 1897.95 cm⁻¹, and 1899.88 cm⁻¹ corresponded to carbonyl (C=O) groups within the TPO fractions. The presence of C–H, C=C, and O–H functional groups in TPO and its fractions indicates that the obtained liquid possesses hydrocarbon characteristics, making it suitable for use as a fuel component.

Conclusion

The conducted research demonstrates that low-temperature pyrolysis is an effective and environmentally sustainable method for converting waste rubber materials, particularly used automobile tires, into valuable products such as tire pyrolysis oil (TPO), gas, and carbonaceous char. The optimization of process parameters—temperature, reaction time, and particle size—allowed achieving the maximum oil yield of 42 wt.% at 500 °C, 50 minutes, and 4 × 2 × 0.5 cm³ tire fragments.

The post-treatment processes, including desulfurization and decolorization using a hydrogen peroxide–formic acid system, effectively removed up to 64.52% of sulfur compounds and improved the fuel quality of TPO. Fractional distillation and FTIR characterization confirmed the presence of hydrocarbon functional groups, indicating that the obtained oil has properties comparable to conventional diesel fuel.

Overall, the optimized production and purification of TPO provide a sustainable and economically viable solution for waste tire recycling. The process not only reduces environmental pollution but also contributes to the development of alternative fuels and circular economy principles in industrial applications.

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