Recycling of Low-Quality Carbon Black Produced by Tire Pyrolysis

Abstract

Pyrolysis is a promising way to reuse of waste tires. However, the carbon black generated in the process is often contaminated with various pyrolysis products. This study aims to recycle low-quality recycled carbon black (rCB) from waste tire pyrolysis, addressing the challenges posed by organic residues (up to 5 wt% bituminous substances, 112.2 mg/kg PAH). This causes the agglomeration of particles and decreases the active specific surface area. Cavitational vortex milling (both wet and dry) emerges as a promising method to valorize contaminated rCB, allowing for a significant reduction in the concentration of contaminants. This novel method allows for the generation of hydrophilic and hydrophobic black pigments. In parallel experiments, low-quality rCB is incorporated into solid biofuel to enhance its calorific value. The addition of 10 wt% rCB) to peat residues significantly elevates the calorific value from 14.5 MJ/kg to 21.0 MJ/kg. However, this improvement is accompanied by notable increases in CO₂ and SO₂ emissions. This dual effect underscores the necessity of considering environmental consequences when utilizing recycled carbon black as a supplement to solid biofuels. The findings provide valuable insights into the potential of cavitational vortex milling for carbon black valorization and highlight the trade-offs associated with enhancing biofuel properties through the addition of rCB.

1. Introduction

Carbon black stands as a fundamental product within the chemical industry, playing a versatile role across various sectors of the material industry [1]. Defined as micro- or nano-sized carbon powder, carbon black is predominantly sourced from natural gas through combustion under oxygen-deficient conditions or derived from natural oil, such as through the process of cracking. This manufacturing process is energy-intensive, relying heavily on fossil fuels, which is environmentally undesirable amid the climate crises and escalating concerns about the depletion and rising costs of natural gas and oil resources [2].

To address these challenges, the scientific community seeks alternatives to virgin carbon black, turning their attention to recycled carbon black (rCB), primarily sourced from waste car tires. Some tire producers have already transitioned their entire production to incorporate rCB. In European Union countries, the landfill disposal of waste tires has nearly ceased, with almost 100% of these tires being recycled. Certain US states have also prohibited tire landfilling, resulting in the cleanup of many tire disposal sites since the early 2000s [3]. However, at a global scale, the recycling rate remains significantly lower, hovering at around 10% [4]. Many discarded tires are incinerated alongside hazardous waste, emitting substantial quantities of toxic gases (sulfur compounds, PAHs, etc.). Additionally, tire landfilling persists as a prevalent disposal method, with the worst-case scenario being abandoned tires littering the landscape [5].

This study introduces a pioneering approach—cavitational vortex milling—to valorize low-quality rCB produced through a low-temperature tire pyrolysis process. To the best of our knowledge, this innovative process has not been previously employed to enhance the quality of carbon black. Earlier, several methods were developed for the improvement of the properties of low-quality rCB, e.g., patent WO 2013/095145 Al [6]. This patent describes the use of additional high-temperature pyrolysis to remove impurities from low-quality carbon black.

The primary objective of this project was to develop three potential products (hydrophobic and hydrophilic black pigments and solid fuel with elevated calorific value) and obtain their certifications. The certification process involves the data derived from laboratory experiments; however, it is acknowledged that certain indicators may vary in real industrial processes.

Recycled carbon black (rCB) derived from waste tires holds significant potential as a raw material for various valuable products. To unlock this potential, it is imperative to devise straightforward, economically viable, and environmentally sustainable technologies that are capable of eliminating undesirable contaminants from the recycled carbon black powder. The composition of rCB differs from that of virgin carbon black due to the substantial presence of additives used in rubber production. The high impurity content in rCB precludes its direct reuse in the rubber industry, as these impurities significantly diminish the specific surface area of carbon black.

Key additives include sulfur (for vulcanization), and ZnO, SiO₂, and TiO₂ (for enhancing rubber’s mechanical properties). For instance, the addition of TiO₂, even in small concentrations, enhances the resistance of tire mixtures to solar UV radiation, O₃, and other oxidizers [7]. During the pyrolysis process, metal oxides react with sulfur, forming sulfides. Similarly, steel wire in tires reacts with sulfur, resulting in FeS. Optimal pyrolysis conditions lead to a solid residue with minimal organic impurities, while inadequate temperatures result in rCB with elevated bituminous substances and undesirable organic impurities, such as PAHs, causing particle agglomeration and reduced specific surface area [8].

The global management of waste tires has become a pressing issue, given the exponential increase in tire production each year. At present, over 1.5 billion tires are manufactured annually, equating to an estimated 17 million metric tons of used tires [5,9]. Projections for the year 2025 anticipate a production of up to 2.5 billion new tires [10].

1.1. Pyrolysis

Pyrolysis, a thermochemical process, involves the decomposition of complex macromolecular organic compounds in the absence of external oxygen, resulting in different products, such as oil, gas, water phase, and char. This transformation occurs within a temperature range of 400–800 °C. Pyrolysis methods vary based on operating parameters and can be categorized as atmospheric, vacuum, catalytic, fast, ultrafast (flash), and slow [3,11].

Chemical transformations initiate at temperatures exceeding 150 °C. The rubber degradation process occurs between 250 and 500 °C, yielding the majority of pyrolysis oil and gas. The peak release of volatiles occurs in the 450–500 °C range. Beyond 550 °C, the release of volatiles from char diminishes, and tire rubber undergoes near-complete decomposition. The secondary cracking of larger primary pyrolysis products into smaller molecules occurs in the 600–850 °C range [12]. Under fast or ultrafast pyrolysis conditions, rubber particles rapidly heat up and degrade, instantly releasing volatile pyrolysis products into the gas phase. This rapid release helps prevent secondary chemical reactions involving the pyrolysis products. Generally, the oil yield reaches a maximum above 550 °C and decreases after this, while the gas yield increases, and char yield decreases with rising temperature [3,11].

1.2. Pyrolysis Char as a Source of Carbon Black

Among the three primary pyrolysis products, the solid residue—pyrolysis carbon black—has received relatively limited attention, despite constituting 30–40 wt% of the pyrolysis products. The properties and output of pyrolysis products are influenced by the tire class that is processed, as well as the pyrolysis reactor configuration and conditions (temperature, pressure, and heating rate). Despite the substantial production of rCB in tire pyrolysis, a straightforward and cost-effective recycling technology for this material remains elusive. Therefore, the valorization of rCB stands as a pivotal criterion for the cost-effectiveness and environmental friendliness of tire pyrolysis.

Carbon black from pyrolysis is frequently incinerated with other hazardous wastes, presenting a significant obstacle to the widespread adoption of tire pyrolysis technology globally. Given the global quantity of waste tires, developing an economically and ecologically sustainable technology for processing and utilizing rCB could result in substantial savings in primary raw materials, such as oil and natural gas, as sources of carbon black.

According to the classification standard D1754 [13], carbon black grades are designated based on their surface area and structure, which are essential for identifying specific carbon black properties. The ASTM standard D3053 [14] establishes carbon black grades for tire manufacturing industries, distinguishing between hard blacks (e.g., N110–N330) with a high BET surface area and structure, used for tire treads, and soft blacks (e.g., N550–N774) with a lower surface area and structure, employed in tire carcasses. Specialty products often require very high-quality carbon black.

In an ideal scenario, rCB would be reused for new tires. However, this application is hindered by the abundance of inorganic impurities in rCB, mainly resulting from rubber mixture additives during tire manufacturing, such as fillers (e.g., silica fume, TiO₂), vulcanizing agents (e.g., sulfur, ZnO), and other auxiliary compounds. These impurities, along with non-volatilized volatiles like pyrolysis oil and PAH adsorbates, diminish the free BET surface area, lower rCB structure, and compromise other quality standards [8].

1.3. Possible Uses of the rCB

Options for Recycling Pyrolysis Carbon Black:

• Filler in the Rubber and Plastic Industry:

The most logical approach for recycling rCB is its regeneration for use as a filler in the production of new rubber or plastic products. While cheaper quality classes of carbon black have relatively low requirements for surface area and adsorption properties, a significant challenge lies in the presence of metal oxides that persist in the composition of regenerated carbon black. Ongoing studies are exploring laboratory conditions that could effectively remove these impurities [15].

2.

Production of Activated Carbon:

The activation of rCB to produce activated carbon can be achieved through conventional methods [16,17,18], with activation using reflux gas emerging as a particularly promising technology. The surface of rCB tends to be contaminated with various organic substances, including aromatic compounds, and activation serves to eliminate these, thereby reducing material toxicity. The activated carbon produced from rCB can find applications in which a notable amount of insoluble impurities are acceptable. The activation process can also be coupled with the removal of metal oxides. In various applications, especially those where the adsorption capacity and/or the content of toxic compounds are crucial, partial surface activation becomes a necessary consideration.

3.

As Black Pigment:

The utilization of rCB as a pigment hinges on the permissible concentrations of toxic substances and particle fineness [19]. Fine grinding, achieved through a disintegrator or cavitation vortex mill, is essential for rCB. Post-processing, the material can potentially be integrated into mortars and building mixtures. However, for pigments in close human contact, such as printing ink and toner, the additional removal of toxic substances through thermal treatment (e.g., gas pyrolysis) or other technologies may be necessary.

4.

As an Additive to Solid Fuels:

With a very high calorific value (>8 kWh/kg), rCB can serve as an additive to solid fuels, including biofuels and waste fuels, to enhance their calorific value. The high sulfur content complicates the incineration of rCB alone, necessitating caution due to the potential emissions of dangerous compounds during combustion.

5.

In the Composition of Mortars and Construction Mixtures:

Activated carbon black, when added at a ratio of 2–5% of the mortar’s mass, significantly enhances compressive strength and structural uniformity. Microcement, a popular binder globally, benefits from the activated carbon black, offering advantageous properties such as self-leveling and a high compressive strength. It finds applications in underwater concrete casting, tunnel walls’ strengthening, and the production of thin-walled construction elements [20,21,22,23,24].

6.

As an Additive to Asphalt Concrete:

Carbon black, including rCB, has been effectively employed to enhance the temperature stability of bitumen in asphalt concrete. This addition increases bitumen stiffness, durability against cracking, and resistance to rut formation at varying temperatures. Previous studies have indicated that incorporating 5–15% carbon black into asphalt concrete can boost wear resistance by up to 30%, particularly at higher temperatures [25,26].

In terms of properties, rCB from tire pyrolysis closely resembles the carbon black produced from oil or natural gas through conventional methods. The demonstrated benefits in various applications underscore the potential for sustainable recycling and reutilization of rCB in diverse industries.

2. Materials and Methods

2.1. Carbon Black from Pyrolysis of Waste Tires

The samples of raw carbon black from the pyrolysis of waste tires provided for the studies by Biopower OÜ (Kunda, Estonia) consisted of black powder whose particles were agglomerated into large clumps with a diameter of up to a few millimeters (Figure 1). The carbon black was produced from shredded waste tires using the apparatus of Jinan Youbang Hengyu Science and Technology Development Ltd. (Jinan, China) As the production of carbon rCB had ceased when we started our study, we could not determine the immediate causes of process disturbances and the resulting low-quality product, but as the solid product contained up to 5% high-molar-mass organic impurities (bituminous substances, etc.) we assume that the operating mode was unstable and the temperature was insufficient for an effective pyrolysis process. The pyrolysis temperature was set at approximately 350–450 °C and the reaction time in the pyrolysis reactor was about 1 h, using an alumosilicate-based catalyst with added ZnO, as provided in the patent EP 2 103 668 A1. After cessation of production, the previous owner left more than 3000 tons of low-quality rCB on the site. Our study aimed to reuse this rCB as its deposition in the landfill was prohibited.

2.2. Methods

2.2.1. Measurements

Various analyses and measurements were conducted on the raw material, employing a comprehensive approach. The techniques included thermogravimetry, incineration tests, XRD/XRF, SEM, simple chemical analyses, and the determination of specific surface area. The elemental composition of carbon black was assessed using X-ray fluorescence spectrometry with the Bruker Tracer S5 XRF instrument (Bruker, Billerica, MA, USA). XRD and XRF are excellent nondestructive techniques that correspondingly provide detailed information about the crystallographic structure and elemental composition of the studied objects. The composition of polyaromatic hydrocarbons in carbon black was determined at the Estonian Environmental Research Centre through gas chromatographic methods with mass spectrometric detection (GC-MS), following ISO 18287:2006 [27]. Thermogravimetric analyses were conducted with a Netzsch STA 449 F3 Jupiter thermal analyzer (AZom, Riyadh, Saudi Arabia). Observations of the carbon black microstructure were carried out using a Hitachi TM-3000 (Hitachi High-Technologies Corporation, Tokyo, Japan) scanning electron microscope (SEM). Scanning electron microscopy (SEM) was used to characterize the elemental composition and geometrical structure of the surface of materials. The calorific value of solid fuel briquettes made of rCB-peat mixtures (Section 3.4) was examined following EN 14918 standard [28] methods. Concrete test specimen compressive strength with added carbon black was tested using a cone apparatus according to standard methods (ASTM C803) [29]. The method is indicative, but sufficient for the purposes of the current study.

2.2.2. Dry and Wet Grinding

For cavitational vortex milling of carbon black, a laboratory apparatus was constructed, as illustrated in Figure 1. The heart of the system is a Flexicone VM150 vortex device (Energystore Group Pty Ltd., Wetherill Park, Australia), featuring a capacity of 150 kg/h and operating at a frequency of 33 kHz. The system was powered by an industrial diesel compressor with a pressure range of 2–3 bar and a flow rate of 3 m³/min. The airflow rate was visually determined to ensure the best quality of the milled product (absence of agglomerated particles; checked using an optical microscope LEICA ICC50 HD (Leica, Wetzlar and Mannheim, Germany), magnification 400 times). The flow rate was adjusted using the control panel of the compressor.

In this apparatus, a very fast rotation of air is induced, creating cavitational waves. These cavitational waves lead to extremely high forces that cause solid particles to break up.

Our apparatus is suitable for both the wet and dry milling of carbon black. In the wet processing of carbon black, a mixture of carbon black and water at a 1:3 ratio is conveyed to a cavitation vortex mill using a peristaltic pump. The carbon black was mixed with a minimal amount of water to ensure the pumpability of the obtained slurry.

When scaling up into the industrial scale, a small cavitational vortex mill is replaced with a larger one.

2.2.3. Preparation of Concrete Test Specimens

Concrete test specimens were prepared by mixing one part of Portland cement with four parts of dry construction sand. The amount of water added to the mixture was half that of Portland cement. Some test specimens were added with rCB, that were 1%, 2%, 5%, and 10% of the total weight of cement and sand; for comparison, test specimens were prepared without the addition of rCB. After 7 days of setting and hardening, a cone test was performed, as described in Section 2.2.1.

2.2.4. Preparation of Peat-rCB Mixtures for the Determination of Calorific Value

The rCB was mixed with well-decomposed peat sourced from the lower strata of the Elva–Rannu deposit (H8-H9) at natural moisture content at mass ratios of 1:1, 1:4, and 1:9, respectively. Then, the mixtures were briquetted and used for calorific value tests, as described in Section 2.2.1.

3. Results and Discussion

3.1. Carbon Black

The observed tendency toward agglomeration is presumably attributed to the migration of bituminous matter, waxes, and other adhesive compounds from the internal pores of ground particles to their surface layer. This process leads to the adherence of smaller particles, culminating in the formation of larger clumps (see Figure 2). These clumps are enveloped by a layer of very fine-grained amorphous carbon black. While the top surface of these agglomerates exhibits a minimal presence of contaminants, the disruption of these clumps allows for the extraction of up to 5 wt% of bituminous organic substances, including predominant polycyclic aromatic hydrocarbons, such as naphthalene, phenanthrene, fluorene, and pyrene, constituting up to 0.5 wt% of the total mass. Some samples also displayed an elevated moisture content. Alongside carbonized material, a noteworthy quantity of wire fragment scraps, ranging from 2 to 20 mm in length and originating from tire reinforcement, was identified in the carbon black pyrolysis samples, indicative of an insufficient separation process.

Table 1. Concentration of PAHs (mg/kg) found in raw carbon black.

PAH	mg/kg
Naphthalene	24
Fluorene	13
Phenanthrene	13
Acenaphthene	12
Pyrene	11
Fluoranthene	6.7
Anthracene	5.7
Benzo(g,h,i)perylene	5
Benzo(a)pyrene	4.4
Benzo(a)anthracene	4.2
Chrysene	4.1
Benzo(b)fluoranthene	2.6
Acenaphthylene	2
Indeno(1,2,3-cd)pyrene	2
Benzo(k)fluoranthene	1.6
Dibenzo(a,h)anthracene	0.94
Total PAH	112.2

presents the concentrations of polycyclic aromatic hydrocarbons (PAHs) in the raw pyrolysis carbon black, expressed in milligrams per kilogram (mg/kg).

Table 2. Elemental composition of raw rCB.

Element	wt%
C	74.2
S	6.0
Zn	5.3
Si	4.3
Ca	1.7
Fe	1.0
K	0.1
Co	0.05
Sr	0.026
Hg	0.018
Pb	0.0016

displays the ultimate analysis of rCB, showcasing the elemental composition of the crystalline phases in weight percentages (wt%). Predominantly composed of carbon (74%), the material also contains significant quantities of silicon, sulfur, zinc, and iron. Additionally, minor components such as potassium and calcium are present. The determination of metals and silicon was conducted through XRD-XRF analyses (Bruker, Billerica, MA, USA). Elements susceptible to volatilization during incineration were computed based on incineration tests.

Metals in the sample predominantly exist as sulfides, with elemental sulfur also present in the raw carbon black. The average ash content recorded in the sample was 19.7%. The heavy metal content was analyzed, suggesting that the pyrolysis process parameters might not have been optimal. Possible factors influencing this include a potentially low pyrolysis oven temperature, leading to the inadequate separation of heavier organics from the pyrolysis mass. Alternatively, insufficient mixing of the mass could yield similar results.

The primary challenge in recycling rCB from tire pyrolysis lies in its structure and substantial organic content, hindering effective adhesion with other materials when used as a filler or pigment [30]. Moreover, the concentration of polycyclic aromatic compounds surpasses permissible limits (see Table 1). The limits depend on the nature of the final product and the ratio of rCB to total mass [31]. In most products, this must not exceed 0.2 mg/kg.

While it is theoretically feasible to regenerate raw rCB into a form suitable for the rubber industry, this process is deemed impractical due to its high energy intensity, necessitating reheating to at least 600–700 °C and releasing significant sulfurous gases [32,33]. Alternatively, rCB can be chemically modified using either acids or alkaline agents. Sulfur recovery needs further research; a sustainable means to achieve this purpose is application of biological methods [34]. Thus, the economically viable reprocessing of rCB involves transforming it into less energy-intensive products.

3.2. Hydrophobic Pigments/Fillers for Use in Paints, Plastics, Low-Quality Rubber, etc.

The initially chosen method for recycling rCB material involves cavitation processing utilizing an ultrasonic vortex mill, as described in Section 2.2.2. This process yields a powder with an average grain diameter of less than 10 μm (refer to Figure 3). The ultrasonic vortex processing effectively releases a significant portion of water and adsorbed organic matter during the procedure.

Ultrasonic effects and the very fast movement of air during the cavitational vortex milling process presumably also facilitate desorption of the contaminants; moreover, the fine pores capped by bituminous substances are opened to mass transfer with the ambient air.

Recycled carbon black is suitable for use as a pigment in mortar and paints, as well as use as a filler in the rubber and plastic industry. However, caution is advised regarding its use in the production of mechanically demanding rubber items, especially tires, due to its low specific surface area. Moreover, it is not recommended for domestic applications.

Residual compounds, such as zinc, sulfur, and silicon dioxide, in the product do not hinder its utilization since these additives are commonly integrated into rubber mixtures in standard manufacturing procedures [35].

The fundamental characteristics of ultrasonic vortex-treated rCB closely parallel those of commercially available recycled carbon blacks worldwide. The composition and properties of the hydrophobic pigment, determined through proximate analysis, are outlined in

Table 3. Main ingredients, and chemical and physical properties (proximate analysis) of hydrophobic rCB.

Property/Ingredient	Unit	Value
Carbon black, C	wt%	74.2
Silicon dioxide, SiO2	wt%	9.2
Zink sulfide, ZnS	wt%	7.9
Sulfur, S	wt%	2.8
Calcium carbonate, CaCO3	wt%	1.7
Iron sulfide, FeS	wt%	1.6
Surface area	m2/g	7–9
Oil absorption number	cm3/100 g	105
Pour density	kg/m3	440
Particle size > 10 μm	%	<1
pH	-	6.8
Volatiles content	wt%	<3
Moisture content	wt%	<1
Ash content	wt%	27
PAH content	ppm	<120

.

3.3. Hydrophilic Pigment for Use in Mortars and Construction Materials

In the initial stage, rCB underwent a dry process similar to that of hydrophobic pigment/filler. However, storage induced the reagglomeration of ground material, leading to rCB parameter deterioration and rendering it unsuitable for use as a pigment due to the uneven particle mixing. Furthermore, rCB contained a notable amount of steel wire fragments (up to 1 wt%) with lengths of 3–20 mm, necessitating sieving through a 10–15 mesh vibrosieve or, alternatively, magnetic separation.

To address reagglomeration issues, a system was devised based on cavitation wet processing, as described in Section 2.2.2. The resulting pulp swiftly separated into a solid paste and water, with the concentrated paste containing approximately 50% water.

Concrete test specimens, incorporating varying amounts of rCB into the cement mortar, were produced (depicted in Figure 4). Carbon black proves suitable for application as a pigment in cement and lime-based mortars, construction mixtures, paints, and elements with a concrete binder (paving stones, blocks, tiles, etc.). The product can withstand alkaline environments well but is unsuitable for acidic settings. The addition of up to 10% pigment to the mixture weight has no significant impact on the final product’s mechanical properties.

3.4. Enhancing Calorific Value of Natural Fuels

Given the exceptionally high calorific value of pure carbon (32.8 MJ/kg), the augmentation of natural fuels’ calorific value is achievable through the incorporation of rubber-derived carbon black. In our experiments, well-decomposed peat sourced from the Elva-Rannu deposit (H(-H9)) served as a representative natural fuel. The highly decomposed (“black”) peat is a mining residue that remains in the deposits due to low demand. As the water table in the peat mining areas is lowered, the remaining peat is exposed to air and thus undergoes decomposition, causing significant greenhouse gases emissions. Due to its high acidity, it inhibits the recovery of the natural state of peatland. For this reason, it is recommended to remove these layers in the depleted peat mines for any purpose, including their use as fuel [36]. The initial calorific value of peat (dry matter) stood at 14.5 MJ/kg, with an approximate moisture content of 55%. rCB was introduced to peat in ratios of 1 + 9, 1 + 4, and 1 + 1.

Various mixtures were pressed into briquettes, and their combustion properties were meticulously examined at the Estonian University of Life Sciences’ biofuels laboratory following standard methods (EN 14918) [28]. Briquettes with a 1 + 9 ratio demonstrated robust cohesion, while others tended to disintegrate upon mold removal. The comprehensive results (on a dry matter basis) are presented in

Table 4. Parameters of the briquettes made of different rCB/peat mixtures.

rCB to Peat Ratio Parameter	Unit	1 + 9	1 + 4	1 + 1
Moisture content,	wt%	49.3	32.8	23.4
Ash content of dry matter	wt%	11	13	18
Calorific value	MJ/kg	20.975	21.108	21.783
CO2 emission	kg/kg	1.9	1.98	2.08
SO2 emission	kg/kg	20	32	46

.

Notably, carbon black achieves complete combustion when included in the briquette mixture at a 1 + 9 ratio, significantly enhancing the resulting briquette’s calorific value. This is evident both in the table and visually through the resulting ash. However, a higher rCB content leads to incomplete carbon combustion, leaving unburned residues in the ash composition. Moreover, briquettes with an elevated rCB content lack structural integrity and exhibit a notable increase in sulfur emissions. The briquettes may break while reloading; thus, we recommend not using an rCB-to-peat ratio higher than 1 + 4.

4. Conclusions

When waste tire pyrolysis occurs under non-optimized conditions, the resulting raw carbon black poses health concerns due to the elevated concentrations of PAHs, limiting its practical applications. Additionally, the presence of significant bituminous substances leads to a reduced specific surface area and particle agglomeration.

Employing gravitational dry or wet grinding effectively breaks down these agglomerates, with wet grinding proving particularly effective in reducing harmful contaminants and preventing re-agglomeration. This processed material is suitable for use as a pigment in applications such as mortars and concrete mixtures, exhibiting excellent coloring properties and resistance to basic environments.

Incinerating raw carbon black from tire pyrolysis is challenging due to the high SOx content in flue gases, overloading the conventional DeSOx equipment (Babcock Power Environmental Inc., Worcester, MA, USA). However, the developed method provides a solution, enabling the utilization of low-quality recycled carbon black in high-calorific-value solid fuel compositions. This innovative approach enhances the viability and versatility of carbon black from waste tire pyrolysis.