Pathways to Carbon Neutrality: A Review of Life Cycle Assessment-Based Waste Tire Recycling Technologies and Future Trends

Abstract

Waste tires (WTs) pose significant environmental challenges due to their massive volume, with millions of tons generated globally each year. Improper disposal methods, such as illegal burning, further aggravate these issues by releasing substantial quantities of greenhouse gases (GHGs) and toxic pollutants into the atmosphere. To mitigate these impacts, the adoption of environmentally friendly resource recovery technologies and a thorough evaluation of their environmental benefits are crucial. Against this backdrop, this research reviews life cycle assessment (LCA)-based analyses of WT recycling technologies, focusing on their environmental performance and contributions to GHG emission reduction. Key recycling pathways, including pyrolysis, rubber reclaiming, and energy recovery, are evaluated in terms of their carbon emissions, alongside an in-depth analysis of carbon reduction opportunities across various stages of the recycling process. Based on these findings, this paper proposes feasible recommendations and identifies future trends for advancing WT resource recovery. The objectives are to (1) systematically review the existing LCA research findings and technological pathways for WT resource recovery; (2) evaluate the advantages and disadvantages of current technologies from the perspective of carbon emission reduction; and (3) explore future trends, proposing optimization pathways and recommendations for technological development.

1. Introduction

Amid the growing urgency of global climate change, achieving carbon neutrality has become a paramount goal for governments and international organizations worldwide. However, the production and disposal of waste tires (WTs) pose significant challenges to this objective. The global production of WTs is rising at an alarming pace and has become a major contributor to non-biodegradable municipal solid waste (MSW) [1]. Each year, approximately one billion WTs are produced globally from various vehicles, including trucks, buses, cars, motorcycles, and bicycles. This number is projected to rise further by 2030, particularly in rapidly developing emerging economies [2]. Traditional methods for managing WTs, such as landfilling and incineration, exert substantial environmental and resource pressures. Landfilling poses a threat by releasing toxic substances like polycyclic aromatic hydrocarbons (PAHs) and heavy metals, which degrade soil and groundwater quality. A study shows that the cumulative leaching rates of DOC (which can serve as a representative for PAH leaching) and common heavy metals (such as Al, Fe, Mn, etc.) from WTs reach 80–100% over a period of 30 days [3]. Landfilling also monopolizes significant land space over long periods, which restricts optimal land use. Incineration, on the other hand, directly emits significant quantities of carbon dioxide (CO₂), nitrogen oxides (NO_x), and toxic gases like dioxins, degrading air quality and hindering global carbon reduction goals. Moreover, the incineration process requires a substantial consumption of fossil fuels, further exacerbating the depletion of energy resources. In this context, the resource recovery and utilization of WTs offer a promising solution to these challenges. By converting WTs into high-value-added materials (e.g., reclaimed rubber, crumb rubber) or energy products (e.g., pyrolysis oil, pyrolysis gas), it is possible to simultaneously reduce dependence on virgin resources and substantially lower carbon emissions, thereby contributing to global carbon neutrality goals. For example, the pyrolysis of WTs generates fuel oil that can serve as a sustainable alternative to fossil fuels. Farhad M. [4] et al. demonstrates that a 10–20% blend of waste tire pyrolysis oil yields CO₂ emissions comparable to or slightly lower than those of conventional diesel, with a reduction of 1–2%. Furthermore, while maintaining similar engine performance, this blended fuel significantly reduces particulate matter (PM) and NO_x emissions by more than 30%, which highlights its promising environmental and overall performance advantages. Similarly, crumb rubber derived from physical recycling can be utilized for asphalt modification and the production of rubber products, significantly reducing the carbon footprint associated with these manufacturing processes. Moreover, WT resource recovery aligns seamlessly with the principles of the circular economy. Rooted in the core principles of “Reduce, Reuse, Recycle”, the circular economy not only reduces waste generation, extends material lifecycles, and improves resource recovery efficiency but also effectively reduces carbon emission intensity. This approach lays a strong foundation for transitioning to a low-carbon economy and presents a cost-effective pathway toward achieving carbon neutrality [5].

In the study of WT resource recovery, a lifecycle assessment (LCA) plays a pivotal role as an analytical tool. By comprehensively evaluating the environmental impacts of WT resource recovery technologies across their entire lifecycle, an LCA provides robust scientific evidence to optimize technologies and formulate effective carbon reduction strategies. It enables the quantification of the carbon footprint associated with various treatment methods, facilitates comparisons of the carbon reduction benefits across different technological pathways, and highlights key areas for technical improvement. Consequently, LCA-based research on WT resource recovery holds both significant theoretical and practical value in the pursuit of carbon neutrality. While numerous studies have utilized LCA methodologies to assess the current state of WT resource recovery technologies, there is, to the best of our knowledge, a notable gap in the literature.

Table 1. Comparison of review articles on LCA in the field of WT recycling technologies.

Field	Highlights	Carbon Reduction	Ref.
Solid waste (including WTs) mixture for concrete LCA comparison	Guidelines for a probabilistic LCA of solid waste mixtures for concrete have been proposed, indicating that the use of alternative materials (such as fly ash and recycled concrete aggregates) can significantly reduce CO2 emissions in concrete production. However, no detailed emission assessment has been conducted for other stages.	√	[6]
LCA study of natural rubber (WT raw material) production, use, and disposal management	A brief comparison of the environmental impacts of different waste disposal technologies is provided. The potential of sustainable treatment methods in reducing greenhouse gas emissions is highlighted. However, a deeper discussion is not provided.		[7]
LCA study of pyrolysis technology for WTs, plastics, and biomass	A comprehensive analysis of pyrolysis technology is conducted from the perspective of a circular economy. Although GHGs and other indicators of current technologies are discussed from an LCA perspective, there is a lack of specific guidance on future emission reduction strategies.	√	[8]
Comprehensive study of high-value WT recycling technologies	The review deeply explores high-value WT recycling technologies and outlines a sustainable assessment framework combining LCAs. However, it does not delve deeply into the field of CO2 emissions.		[9]
Technical review of gasification technologies	The review mainly introduces technological advancements in gasification. In the future outlook, it emphasizes the importance of LCAs in assessing the environmental impact of gasification emissions, but it does not delve further into this topic.		[10]
LCA of WT recycling technologies	A systematic review of the current CO2 emissions in WT recycling technologies has been conducted, and detailed recommendations for future carbon neutrality measures have been provided.	√	This work

organizes the latest review topics on the LCAs of WT pyrolysis, but it was found that systematic reviews and comprehensive evaluations of resource recovery technologies for WTs—particularly with a focus on carbon reduction goals—remain limited. This gap is most evident in the lack of thorough comparative analyses that address the latest technological advancements and their carbon reduction potential. To this end, there is an urgent need for a systematic review to offer clearer guidance and scientific support for the development and optimization of WT resource recovery technologies in alignment with carbon neutrality goals.

This review is organized into six sections. Following the introduction in Section 1, Section 2 provides a comprehensive overview of the latest advancements and practical developments in WT resource recovery, emphasizing both national-level strategies and technological innovations. Section 3 delves into the environmental impacts of WT resource recovery processes through LCA methods, including comparative analyses of different technological pathways. Section 4 examines the current state of carbon emissions associated with resource recovery technologies, detailing the contributions of each stage across a lifecycle and identifying opportunities for emission reductions and their underlying sources. Section 5 discusses the development trends in WT resource recovery, offering an in-depth analysis of emerging resource recovery technologies and the future potential of LCA methodologies. Additionally, it proposes targeted optimization strategies from a forward-looking perspective and investigates the macro-level integration of WT resource recovery with global carbon neutrality objectives. Finally, Section 6 presents the key conclusions of this review.

2. Overview of Waste Tire Recycling Technologies

2.1. State of National-Level Waste Tire Treatment

Waste tires (WTs) constitute a substantial portion of municipal solid waste (MSW) and possess distinctive chemical properties that set them apart from other MSW materials, including food waste, paper, plastics, and textiles. By comparing WTs with other MSW materials, key insights can be provided for life cycle assessment (LCA)-based recycling technologies. As illustrated in

Table 2. Elemental composition of waste tires and other MSW.

	Carbon (C) (%)	Hydrogen (H) (%)	Oxygen (O) (%)	Sulfur (S) (%)	Nitrogen (N) (%)	Metal (%)	Ash (%)	Moisture (%)	Ref.
WTs	70–85	5–10	10–20	1–3	0.5–1	1–2	8–10	1–3	[12]
Food Waste	40–55	5–7	30–50	0.2–0.5	1–4	1–3	5–10	40–80	[13]
Paper	35–45	5–6	30–45	<0.1	0.1–0.5	<1	5–10	5–15	[14]
Plastic	50–80	6–14	5–20	<0.1	<0.1	<1	<1	1–3	[15]
Textile	40–55	5–6	25–40	<0.2	15–20	<1	5–10	10–20	[16]

, WTs are characterized by a high carbon content (70–85%) and low moisture content (1–3%), making them excellent candidates for energy recovery and material recycling. Compared to food waste (40–80%) or textiles (10–20%), the low moisture content of WTs makes them more suitable for pyrolysis or thermal utilization processes. In contrast, high-moisture materials like food waste often require preprocessing or anaerobic digestion for effective utilization. The hydrogen content (5–10%) and oxygen content (10–20%) of WTs are comparable to those of other high-carbon MSW materials, such as plastics and textiles, further indicating their potential for conversion into fuels or chemicals. Moreover, a notable characteristic of WTs is their high sulfur content (1–3%), which is primarily attributed to the extensive use of vulcanized rubber in tire production [11]. Vulcanized rubber enhances the elasticity and durability of tires but also results in significantly higher sulfur concentrations in WTs compared to those in other MSW materials. Although the high sulfur content may pose environmental challenges, such as SO_x emissions during thermochemical processes, it also creates opportunities for sulfur recovery technologies, making WT recycling a distinctive area of research. Additionally, WTs contain small amounts of metals (1–2%), such as zinc and iron, which can also be extracted and reused during the recycling process, further enhancing resource utilization efficiency. Overall, it is crucial to adopt targeted strategies for waste management tailored to WTs compared with those for other MSW. Specific processing methods can harness their high-energy potential while mitigating sulfur- and metal-related environmental issues.

Currently, commonly employed resource recovery techniques primarily include the production of reclaimed rubber and crumb rubber, pyrolysis, and tire retreading. Different countries adopt varying waste tire approaches and trends in WT resource recovery. The comparative analysis of WT recycling practices across different countries provides valuable insights into global strategies for resource recovery and sustainable waste management. Taking the 2022 WT resource recovery industry structures of China and Japan as examples (Figure 1), a report by the China Rubber Industry Association (CRIA) [17] indicates that approximately 70% of WTs in China are processed through resource recovery methods. The predominant approach is the production of reclaimed rubber, with a total output of 734,351 tons, comprising 539,276 tons of ordinary reclaimed rubber, 71,309 tons of premium reclaimed rubber, and 123,766 tons of specialty reclaimed rubber. These products have been widely applied in the rubber industry (e.g., hoses, footwear) as well as in non-rubber industries such as building materials and municipal engineering. Secondary methods include crumb rubber production and pyrolysis, whereas tire retreading represents a relatively minor portion of the industry. According to statistics from the Japan Automobile Tire Manufacturers Association (JATMA) [18], Japan effectively utilized 776,000 tons of WTs in 2022, achieving an impressive utilization rate of 96%. Fuel utilization is the leading method of resource recovery, representing approximately 60% of total effective utilization, with a particularly significant role in the paper industry. Overall, the differences in WT recovery strategies between China and Japan arise from multiple factors, including economic structure, industrial demand, policy priorities, and technological capabilities. China, characterized as a resource-driven economy, prioritizes material recovery technologies, driven by relatively low labor costs and mature technologies. In contrast, Japan follows an energy-oriented model, leveraging WTs as an efficient fuel source, motivated by its energy demands and stringent environmental pollution control measures. These contrasting approaches represent distinct developmental pathways for WT resource recovery. European and American nations employ comparable technical approaches to Japan in tire recycling (41–53% material recovery, 33–43% energy conversion) [19], while their policy frameworks remain comparatively less cohesive than Japan’s integrated approach. Overall, they all align with the principles of the circular economy.

2.2. Primary Resource Recovery Technologies

2.2.1. Physical Recovery Technologies

From a technical classification perspective, the resource recovery of WTs can be broadly categorized into physical recovery and thermochemical treatment, with their main characteristics summarized in

Table 3. Main characteristics of WT resource technology.

	Main Technique	Key Products	Resource Utilization
Physical recovery	crushing, grinding, buffing, and other mechanical methods	crumb rubber, reclaimed rubber, and tread rubber	asphalt modifier, cushioning materials, rubber product manufacturing
Chemical (thermochemical) recovery	combustion	energy	energy production
	pyrolysis	oil, gas, and carbon black	fuel and chemical production
	gasification	syngas (H2 and CO)	clean energy production

. Physical recovery technologies primarily utilize mechanical processes to transform WTs into granules, powders, or reclaimed materials, preserving the chemical structure of the rubber molecules. These processes primarily involve crushing, grinding, and other size-reduction techniques, yielding notable products such as crumb rubber and reclaimed rubber or products for preparing rubber used in retreaded tires. The main steps of retreading include inspection, repairing, tread application, and curing [20]. This process, primarily through physical means, enables low-cost tire regeneration. However, the number of used tires meeting the required standards is relatively limited, and due to accumulated fatigue in the tire casing, the service life of retreaded tires is usually shorter, resulting in low adoption rates. Crumb rubber is produced through mechanical crushing, ambient grinding, or cryogenic grinding techniques to create rubber particles or powders of specific sizes. Its inherent properties, including elasticity, heat resistance, durability, wear resistance, and chemical stability, make it widely applicable across various fields. For instance, incorporating crumb rubber into asphalt significantly improves the elastic modulus and deformation resistance of asphalt mixtures, thereby boosting durability and overall road performance [21,22]. Zheng et al. [23] used 18 wt% of 80-mesh crumb rubber to prepare rubber-modified emulsified asphalt (E-CRMA). E-CRMA demonstrated excellent storage stability and rheological properties within a temperature range of 5 °C to 51.2 °C, making it suitable for bonding layers between road surfaces and base layers. In another application, Yang et al. [24] utilized crumb rubber from WTs in combination with polyvinyl alcohol (PVA), glutaraldehyde (GA), and xanthan gum (XG) to fabricate a three-dimensional porous rubber aerogel (RA) through a freeze-drying process. The resulting RA had a low density of 0.0697 g/cm³, an exceptionally high porosity of 94.36%, excellent compressive properties (Young’s modulus of 162.9 kPa), and outstanding thermal insulation, maintaining stability at temperatures below 200 °C. These characteristics highlight the potential of crumb rubber in applications requiring cushioning and thermal insulation. Furthermore, the production method for this WT rubber production has been shown to be scalable for industrial applications. Researchers at the National University of Singapore (NUS) have developed a process to convert waste tires into rubber aerogels [25]. This technology can transform waste rubber into materials with high absorbency and excellent thermal and acoustic insulation properties. The production cost of these aerogels is less than SGD 10 per square meter, and the preparation time is 12 to 13 h. Compared to crumb rubber, reclaimed rubber places greater emphasis on desulfurization to restore the plasticity and extensibility of rubber, enabling its use in producing higher value rubber products. While the initial processing of reclaimed rubber primarily relies on mechanical grinding and is thus classified as physical recovery, its core desulfurization stage often incorporates heat–mechanical, mechanochemical, or other physical and chemical processes, making it strictly a hybrid technique. In recent years, extensive research has been conducted on desulfurized reclaimed rubber. For instance, Guo et al. [26] employed low-temperature mechanochemical desulfurization (LTMD) to preserve the original additives in WTs, resulting in composite materials with excellent mechanical properties and aging resistance, suitable for manufacturing high-performance rubber products such as tires and shock absorbers. Similarly, Bockstal et al. [27] utilized microwave desulfurization to selectively break sulfur bonds (S–S and C–S) while retaining the primary carbon backbone of the rubber, enabling its application in epoxy resin composites. Asaro et al. [28] identified heat–mechanical desulfurization as the most practical method for industrial applications. By exposing rubber to high temperatures and shear forces while optimizing extruder parameters such as screw speed, temperature, and pressure, sulfur bonds can be selectively broken with minimal impact on the rubber backbone.

2.2.2. Thermochemical Recovery Technologies

In the field of chemical recycling, the high chemical stability of rubber molecules renders their decomposition at ambient temperatures exceedingly difficult. Consequently, thermochemical treatment has emerged as the predominant approach for processing WTs. As the term implies, thermochemical treatment encompasses techniques conducted under high-temperature conditions combined with specific chemical reactions. The principal technologies within this framework include combustion, pyrolysis, and gasification. Combustion refers to the process of burning WTs in the presence of excess oxygen. This process releases a significant amount of thermal energy, which can be utilized for power generation or industrial heating. Due to the high calorific value of WTs (approximately 30–40 MJ/Nm³) [29], combustion remains one of the most widely used disposal methods in many countries. However, from an environmental perspective, combustion poses considerable risks, as it emits a large quantity of pollutants, such as sulfur dioxide (SO₂), particulate matter, and dioxins, which can severely harm the environment. Therefore, advanced flue gas treatment systems are often required to reduce pollutant emissions. When modern flue gas desulfurization (FGD) systems are employed, SO₂ emissions can be reduced by more than 90%, depending on the efficiency of the scrubbing process [30]. With FGD systems, the SOx emissions from WT combustion can be reduced to as low as 37.8 ppm [31], which are significantly lower than those from coal combustion at 235 ppm. Through catalytic systems that employ V₂O₅, WO₃, and TiO₂, the dioxin levels can be lowered by over 90% as well [32]. Pyrolysis involves the thermal decomposition of WTs in an oxygen-deficient or oxygen-free environment. Under high temperatures (400–700 °C), rubber molecules break down into smaller molecules, with the main products being pyrolysis oil (usable as fuel or chemical feedstock), pyrolysis gas (a combustible gas), and solid carbon (which can be used to produce carbon black or activated carbon) [33]. Gasification can be seen as a derivative technology of pyrolysis. By introducing a controlled amount of oxygen and steam into the system, WTs undergo partial oxidation at high temperatures (600–1200 °C). Gasification converts the solid material into syngas, a mixture of hydrogen (H₂) and carbon monoxide (CO). Syngas can serve as a clean energy source or as a chemical feedstock for the production of methanol, ammonia, and other chemical products [34].

$$\mathrm{C}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to \mathrm{C}\mathrm{O}+\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}$$

(1)

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2$$

(2)

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$

(3)

The development of thermochemical treatment technologies reflects the continuous progression of research on WT management. In the 1970s, combustion technology became the mainstream due to its highly efficient thermal energy recovery and was widely adopted by various countries. However, it soon sparked significant environmental concerns due to the emission of pollutants. From the 1980s onward, as environmental awareness grew, research shifted towards cleaner technologies such as pyrolysis. By decomposing WTs at high temperatures, pyrolysis not only significantly reduces carbon dioxide emissions but also enables the simultaneous recovery of liquid fuels, gaseous fuels, and carbon black. This dual capability of energy recovery and pollution control has made pyrolysis an attractive solution. In recent years, researchers have delved deeply into the kinetics and thermodynamics of the pyrolysis process to optimize operational parameters and improve product quality. Advanced experimental and analytical techniques, such as thermogravimetric analysis (TGA) [35], gas chromatography–mass spectrometry (GC-MS) [36], thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) [37] and a gas chromatography–flame ionization detector (GC-FID) [38] have been employed. These methods have enabled more precise analysis of the products generated during pyrolysis, leading to significant progress at the micro- and molecular levels. For instance, Zhang et al. [39] provided a comprehensive summary of how parameters such as temperature, residence time, pressure, particle size, and heating rate affect product distribution. Additionally, Jiang et al. [40] explored the molecular mechanisms of carbon black formation, elucidating the processes of polymer chain cleavage, collision, and recombination during high-temperature reactions, thus laying the foundation for producing high-quality carbon black. Advancements in gasification technology have focused on controlling the production of specific syngas components. With limited oxygen supply, the partial oxidation of rubber carbon-based materials generates significant amounts of carbon monoxide (CO) and heat (Equation (1)) within the temperature range of 600–700 °C, while the reaction between carbon and steam produces both CO and hydrogen (H₂) via the water–gas reaction (Equation (2)) in the temperature range of 700–900 °C. This is a key pathway for hydrogen production during gasification. Under the same or higher temperature conditions (above 800°C), the addition of suitable catalysts, such as Ni/Al₂O₃ [41] and Pt-based composite catalysts (Pt-Ce/MgO) [42], can significantly enhance the water–gas shift reaction (Equation (3)), further increasing hydrogen yield. For example, bimetallic catalysts, such as Fe-promoted nickel catalysts, have demonstrated high hydrogen yields (up to 87%) due to their ability to balance the rates of methane decomposition and carbon deposit diffusion [43]. Copper-based catalysts, especially those promoted with ZrO₂, show high hydrogen yields (up to 12,600 mmol g⁻¹ h⁻¹) in steam reforming processes. ZnO₂ also has high stability, with some systems maintaining performance for over 200 h without a significant drop in activity [44]. This reaction is typically carried out in downstream gas purification and separation processes, while the major challenges of catalyst deactivation (e.g., coking, sintering) require attention in practical applications [45]. Moreover, the co-gasification of WTs with CO₂ has been shown to significantly improve hydrogen production and process efficiency while reducing CO₂ emissions [46]. These mechanisms demonstrate the potential of gasification in the field of clean energy. The syngas produced can be directly used for power generation, converted into liquid fuels via Fischer–Tropsch synthesis, or further processed to produce hydrogen or methanol for chemical applications. In summary, the evolution of thermochemical treatment technologies for WT recycling highlights a transition from single-purpose energy recovery to diversified, efficient, and environmentally friendly resource utilization approaches.

In summary, following an analysis of the current state of applications and technological advancements, the optimization strategies for the two traditional technologies can be outlined as follows: Regarding physical technologies, their strong resource retention and cost-efficiency make them ideal for recycling scenarios with minimal pollution requirements. Consequently, it is recommended to focus on enhancing the value of derived products, such as improving the purity and uniformity of rubber powder, or employing advanced techniques like mechanical–chemical desulfurization and microwave desulfurization. These methods can reduce energy consumption while enhancing the performance of regenerated rubber, driving the deep application of rubber powder as a high-performance engineering material in road construction, composite materials, and thermal insulation buffer materials. Concerning the optimization approach for thermochemical technologies, combustion and pyrolysis can efficiently reclaim the chemical energy embedded in WTs, thereby reducing their volume considerably. Gasification should be further explored, as countries with sufficient funding and technological support can harness its potential to achieve greater benefits.

3. LCA-Based Environmental Assessment of Recycling Technologies

3.1. Application of LCA in the Study of Waste Tire Resource Utilization

A life cycle assessment (LCA) is a systematic environmental evaluation method that quantifies the environmental impacts of products, processes, or services throughout their life cycle. In recent years, the LCA approach has been extensively applied to the study of WT valorization, serving as a tool to assess the environmental benefits and sustainability of various treatment technologies. Within the domain of WT valorization, the research framework of the LCA methodology can be categorized into three main stages: a goal and scope definition, life cycle inventory (LCI), and life cycle impact assessment (LCIA), as shown in Figure 2 [47].

The first stage, a goal and scope definition, involves clearly identifying the objectives of the LCA study, defining the system boundaries, and selecting functional units to provide a structured framework for subsequent LCI analysis. The functional unit serves as the foundation of an LCA, ensuring comparability between different scenarios. Commonly used functional units include processing one ton of WTs [48] or accounting for a certain mileage driven [49]. The system boundary defines the extent of the life cycle under consideration, with common boundary definitions including cradle-to-grave [50], cradle-to-gate [51], and gate-to-gate [52]. These boundaries are suitable for macro-level decision-making, the optimization of resource recovery processes, and localized process performance evaluations, respectively. However, the uncompleted boundaries such as cradle-to-gate only cover the stages from raw material extraction to product manufacturing, neglecting the product-use and end-of-life phases. This may lead to an incomplete evaluation of the environmental benefits of waste tire recycling, as the disposal methods in the end-of-life phase, such as incineration, landfill, or reuse, may significantly impact the environment [53,54]. Therefore, to ensure a comprehensive assessment of environmental burdens, a cradle-to-grave system boundary is recommended [55]. Ideally, the whole life cycle should cover the entire life cycle, starting from the extraction of raw materials (including rubber production) and continuing through manufacturing; usage; and recycling. Subsequently, the impact categories and evaluation indicators are preliminarily selected based on the research focus, such as greenhouse gas emissions, energy consumption, air pollutant emissions, or overall environmental impacts.

Table 4. The comparison of key LCA studies on WT recycling.

Scope	Impact Categories	Key Findings	Ref.
Comparative LCA of three types of hot mix asphalt (HMA) mixtures: standard, cellulose-reinforced, and WT-fiber-reinforced	Climate change, cumulative energy demand (CED), ReCiPe indicators	WT-fiber-reinforced HMA shows the best environmental performance, with a 30.8% reduction in CED and global warming potential (GWP) compared to standard HMA.	[57]
LCA of WT treatment methods (pyrolysis, energy recovery in a cement plant and in a dedicated incineration plant, production of infill)	Climate change, acidification, resource use, eutrophication, freshwater ecotoxicity	Pyrolysis offers lower environmental impacts in climate change and resource use compared to energy recovery pathways. Cement plant incineration shows better performance in energy recovery but higher impacts in some categories.	[58]
LCA of two thermochemical processes (pyrolysis and gasification and Fischer–Tropsch, GFT) for producing sustainable aviation fuel (SAF) from WTs	Climate change, human toxicity potential, terrestrial ecotoxicity, freshwater ecotoxicity	Pyrolysis has lower GWP but higher toxicity and photochemical pollution risks. Pyrolysis SAF is more expensive compared to GFT SAF (0.66 USD/l).	[59]
Prospective LCA of two emerging pretreatment technologies (supercritical swelling and supercritical decrosslinking) for incorporating crumb rubber from WTs into asphalt mixtures	Cumulative energy demand (CED), human health, ecosystem quality, mineral and resource scarcity	Supercritical swelling pretreated crumb rubber-modified asphalt shows the lowest environmental impact in most categories. The pretreatment process has high uncertainty but can be improved by reducing the use of dry ice and chemical reagents.	[60]

displays several examples of classical LCA research on WT recycling. As the table shows, the most common impact categories used in the WT recycling field encompass climate change, resource use, human health, and kinds of ecotoxicity [55,56]. The detailed research findings are also presented, and preliminary analysis suggests that WT recycling technologies, particularly those led by pyrolysis, demonstrate significant environmental benefits and exhibit promising research value.

In the second stage, it is essential to systematically collect and quantify the resource inputs and emission outputs associated with a product or system, as this forms the core of an LCI. Input data must be as comprehensive and reliable as possible to ensure the accuracy of the results. When necessary, LCA databases (such as Ecoinvent) can be utilized to supplement upstream and downstream data, covering aspects such as electricity generation and raw material extraction, while ensuring that the source of the data, temporal scope, and geographical relevance meet the research requirements [61,62]. Output data require further quantification and standardization to align with the functional unit, ensuring comparability across different systems and processes. Furthermore, many studies integrate tools such as Material Flow Analysis (MFA) [63], Energy Flow Analysis (EFA) [64], and Techno-Economic Analysis (TEA) [65] to create flowcharts that clearly illustrate material and energy flows, facilitating the identification of optimization opportunities. In the final LCIA stage, the inventory data from the LCI phase is converted into interpretable environmental impact results, focusing on specific impact categories. The first step involves selecting a method to determine the impact categories, with commonly used methods including CML [66], IPCC [67], and ReCiPe [57]. These are authoritative computational models supported by official databases and calculation standards. These methodologies each possess distinct characteristics and application scenarios. The CML method excels in its comprehensiveness and flexibility, enabling the coverage of multiple environmental impact categories while allowing regional adjustments. The IPCC methodology demonstrates high consistency and authority in climate change evaluation, which makes it ideal for scenarios focusing on greenhouse gas emissions. The ReCiPe methodology provides the most comprehensive integrated assessment, particularly suitable for complex scenarios requiring the consideration of multiple environmental impacts [68]. Consequently, the ReCiPe methodology is generally regarded as the most widely adopted approach in the field of WT recycling, as it offers the most holistic environmental impact assessment while simultaneously addressing multiple environmental indicators. The classification and characterization processes involve grouping inventory data into relevant categories and converting them into a unified unit (e.g., kg CO₂ eq) using GWP factors. Additionally, to facilitate comparisons across different systems, normalization and weighting methods are often employed. Normalization converts results from different impact categories into dimensionless values, while weighting assigns priority levels to impact categories based on policy or societal priorities. Finally, by analyzing the overall environmental impact of various treatment technologies, an LCIA enables the comparison of different approaches and their respective advantages and disadvantages. In summary, the comprehensive application of an LCA allows for a deeper understanding of the environmental impacts of WT valorization technologies across different stages, supporting decision-makers in selecting the most sustainable treatment technologies and providing scientific evidence for achieving more environmentally friendly waste management practices.

3.2. Comparison of LCA Results of Technical Routes

3.2.1. Comparison of Environmental Impacts Across Different Technological Pathways

The diversity in WT valorization technologies and their lifecycle performance leads to significant complexity in evaluating their environmental impacts. Each technological pathway exhibits distinct characteristics in terms of resource consumption, emissions, and environmental impact across its lifecycle. For example, mechanical grinding primarily involves physical processes, resulting in relatively low energy consumption. However, additional processing steps are often required to enhance the performance of recycled rubber [69]. In contrast, pyrolysis utilizes high temperatures to break down WTs, enabling the recovery of fuel oil and carbon black, but it requires substantial energy input and poses potential environmental challenges due to the need for emissions management. Similarly, incineration with energy recovery effectively harnesses the calorific value of WTs, yet the high-temperature combustion process can release significant amounts of pollutants. One of the key features of an LCA is the ability to analyze resource consumption and pollutant emissions at different lifecycle stages. Comparing the LCA results of various WT valorization technologies helps address several important questions: Which technology is more environmentally friendly? At what lifecycle stages do the main differences between technologies arise? Which stages—such as raw material production and transportation, processing, usage, or end-of-life disposal—contribute the most to the overall environmental impact? What are the critical points for improving environmental performance? To explore these questions, several representative studies were analyzed. For instance, Clauzade et al. [70] compared nine common WT recycling technologies and identified cement kiln coprocessing and molded product manufacturing as having the highest environmental benefits. The environmental advantages of cement kilns and molded products are primarily concentrated in the stages of energy substitution and raw material replacement. Specifically, molded product manufacturing achieves significant reductions in mineral resource consumption (−26 kg Sb eq.) by directly replacing virgin rubber and plastics. Meanwhile, the cement kiln’s advantages are mainly due to fuel substitution, significantly reducing the use of coal and petroleum coke, which leads to notable energy savings (−21 kg Sb eq.). Building on this, Ortiz-Rodriguez et al. [71] conducted an LCA study in Colombia’s Valle del Cauca region to evaluate the performance of three technologies: tire retreading, cement kiln incineration, and mechanical grinding. Their findings revealed that mechanical grinding and incineration were more environmentally friendly. Mechanical grinding demonstrated the largest reductions in GWP (−1320 kg CO₂ eq.) and acidification potential (AP) (−5.26 kg SO₂ eq.), while incineration reduced environmental burdens primarily through fuel substitution (GWP −1110 kg CO₂ eq.). A detailed comparison of the resource consumption and emission characteristics at various lifecycle stages revealed distinct patterns for each technology. In mechanical grinding, the environmental benefits were primarily concentrated in the byproduct recovery stage, with key secondary products including rubber granules, steel, and fibers. Among these, the use of rubber granules in flooring manufacturing proved to be the most environmentally beneficial application, delivering significant environmental gains across almost all impact categories. In contrast, the primary environmental benefits of incineration stemmed from fuel substitution during the usage phase, replacing coal with energy recovered from tire combustion. However, for tire retreading, the highest environmental burdens were linked to the production of synthetic rubber from raw materials. It is worth noting that, despite their environmental advantages, certain valorization and recycling processes can still result in considerable environmental impacts. For example, modified asphalt production can generate high GWP and human toxicity potential (HTP) values, reaching 667 kg CO₂ eq. and 102 kg 1,4-DCB eq., respectively. To address these challenges, the authors proposed several measures to improve environmental performance. These include enhancing the energy recovery efficiency of incineration to reduce coal demand, controlling NO_x and SO_x emissions to mitigate acidification potential, and further improving the utilization rates of steel and fiber in mechanical grinding processes.

Many studies have focused on improving specific technologies while exploring their environmental benefits. The detailed environmental indicators of various comparative cases are summarized in

Table 5. The environmental performance of improved waste tire resource recovery technologies.

Case	Environmental Indicators		Scenario 1 (Blank)	Scenario 2 (Improvement)	Ref.
Independent system vs. integrated system	GWP 1	kg CO2 eq.	382	354	[72]
	ODP 2	kg CFC-11 eq.	0.025	0.02
	POCP 3	kg C2H4 eq	1.5	1.3
	AP 4	kg SO2 eq.	13.8	12.4
Molten salt heating tire pyrolysis vs. molten salt heating tire pyrolysis with product substitution	GWP	kg CO2 eq.	224	−1292.2	[73]
	AP	kg SO2 eq.	0.22	−3.2
	HTP 5	kg 1,4-DCB eq.	127	−373.86
	ADP 6-fossil	kg oil eq.	15.1	−1152.1
	ADP-mineral	kg Sb eq.	15.1	−26.0
ELT-fiber-reinforced HMA8 vs. standard HMA	GWP	kg CO2 eq.	62.6	43.3	[57]
	AP	kg SO2 eq.	0.091	0.062
	POCP	kg C2H4 eq	0.034	0.025
	CED 7	MJ	2781	1924
	ReCiPe endpoint	EcoPt	0.116	0.08
High-value vs. conventional utilization pyrolysis process	GWP	kg CO2 eq.	8.76 × 10−10	2.21 × 10−11	[74]
	ODP	kg CFC-11 eq.	4.21 × 10−11	2.51 × 10−12
	HTP	kg 1,4-DCB eq.	7.73 × 10−11	5.73 × 10−12
	ADP-fossil	kg oil eq.	3.69 × 10−11	5.73 × 10−12
Physical modification (M1)/nitric acid modification (M2)/plasma modification (M3) pyrolysis vs. conventional physical recycling (C1)/chemical desulfurization (C2)/conventional pyrolysis (C3)	GWP	kg CO2 eq.	M1: 1.16 × 104, M2: 1.22 × 104 M3: 1.16 × 104	C1: 1.24 × 104, C2: 1.21 × 104 C3: 1.21 × 104	[50]
	ODP	kg CFC-11 eq.	M1: 1.80 × 10−3, M2: 1.74 × 10−3 M3: 1.65 × 10−3	C1: 2.43 × 10−3, C2: 2.28 × 10−3 C3: 2.28 × 10−3
	HTP	kg 1,4-DCB eq.	M1: 1.27 × 104, M2: 1.30 × 104 M3: 1.28 × 104	C1: 1.69 × 104, C2: 1.68 × 104 C3: 1.68 × 104
	AP	kg SO2 eq.	M1: 4.32 × 101, M2: 4.49 × 101 M3: 4.31 × 101	C1: 5.09 × 104, C2: 4.92 × 104 C3: 4.94 × 104

¹ GWP = global warming potential, ² ODP = ozone depletion potential, ³ POCP = photochemical ozone creation potential, ⁴ AP = acidification potential, ⁵ HTP = human toxicity potential, ⁶ ADP = abiotic depletion potential, ⁷ CED = cumulative energy demand, ⁸ HMA = hot mix asphalt.

. Some studies concentrate on technological advancements in valorization systems. For instance, one study integrated all subprocesses, including tire pyrolysis, the refining of tire pyrolysis oil, and the modification of pyrolytic carbon black, into a unified system. By directly utilizing byproducts like syngas generated during the pyrolysis process to power the system, this integrated approach achieved a high energy recovery efficiency (ERE) of 88.1%. Additionally, by reducing the environmental burden associated with external energy consumption and logistics, the system lowered GWP by approximately 7.33% and ozone depletion potential (ODP) by 20% [72]. Qi et al. [73] explored the advantages of using carbonate-based molten salts as both a heat transfer medium and a reaction carrier in molten salt pyrolysis, comparing it with WT combustion for power generation. Through product substitution, they further reduced the consumption of fossil fuels and mineral resources. The authors highlighted that the use of molten salts not only enhanced heat transfer efficiency but also effectively captured sulfur compounds and other pollutants produced during pyrolysis, significantly reducing gas emissions. Furthermore, the liquid environment minimized coke formation, leading to higher yields of carbon black and pyrolysis oil. This process demonstrated far greater efficiency and added more value than combustion. The authors also emphasized emission optimization during stages like pyrolysis and carbon black modification, which accounted for 80.5% of total emissions. By adopting renewable energy sources, the study achieved significant environmental improvements, including reductions of 40.5% in GWP and 42.6% in AP.

On the other hand, some studies focused on the environmental benefits of downstream product modifications and reuse. For example, Landi et al. [57] investigated the use of WT fibers as asphalt reinforcements. These fibers enhanced the crack resistance and fatigue performance of asphalt mixtures, extending the lifespan of road surfaces under heavy loads or harsh conditions. WT-fiber-reinforced hot mix asphalt (HMA) not only demonstrated superior mechanical performance but also delivered environmental benefits. By recycling fibers, emissions were reduced by 19.3 kg CO₂ eq/t, and energy consumption was lowered by 857 MJ/t compared to that of standard HMA. Overall, the environmental impact decreased by 30.6%, thanks to reductions in fossil fuel consumption and greenhouse gas emissions. Wu et al. [74] focused on high-value pyrolysis processes, an optimized pyrolysis technique designed to maximize product value. In this approach, carbon black was utilized as a filler for high-performance rubber products or as a pigment, pyrolysis oil was refined into diesel or lubricants, and all pyrolysis gases were fully recovered and repurposed for process heating, thereby minimizing external energy demands. This process achieved reductions of over 90% in ODP and HTP, and more than 84% in abiotic depletion potential (ADP). However, the main environmental burden of high-value pyrolysis stemmed from the deep processing of carbon black, contributing 61.98% of fossil ADP and 75.55% of ODP. In comparison, traditional environmental burdens were primarily concentrated in the pyrolysis stage. By optimizing electricity and natural gas usage during deep processing, indicators such as GWP, fossil ADP, and HTP could be further reduced. Moreover, replacing coal-based electricity with renewable energy generation helped significantly lower the overall environmental impact. The same research team [50] further evaluated the environmental performance of carbon black modification methods, comparing three approaches—physical modification, nitric acid modification, and plasma modification—against traditional methods such as physical or chemical desulfurization and thermal pyrolysis. They also analyzed the economic performance of these methods, offering a comprehensive study of the advanced value-added benefits of pyrolytic carbon black. The results showed that physical modification demonstrated the best resource and environmental benefit to cost ratio (RBECR), while plasma-modified carbon black exhibited the highest economic and performance efficiency to cost ratio (EPECR). Nitric acid-modified carbon black, however, performed the worst, particularly in terms of environmental impact, due to the high mineral resource consumption associated with the nitric acid modification process.

3.2.2. Analysis of Key Environmental Factors

In the assessment of the environmental impacts of valorization technologies, it is essential not only to compare different processes and evaluate the environmental differences across internal subprocesses but also to examine the longitudinal effects of external conditions and process parameters on environmental performance. Changes in environmental parameters can directly compare the environmental factors between different scenarios. A more comprehensive and quantitative approach is to employ sensitivity analysis, particularly when there are interdependencies with economic and energy consumption factors [75]. Sensitivity analysis is a quantitative method used to evaluate how changes in input variables affect the outputs of a model or system. By analyzing the degree to which fluctuations in input variables influence the target variables, it identifies the key factors that significantly impact system performance or decision-making outcomes, assesses the robustness of results, and optimizes technological pathways. If the model output is represented as Y = f(X₁, X₂, …, X_n), where X₁, X₂, …, X_n are the input variables, the sensitivity index, S_i, of X_i can be calculated using the following standard Equation (4) [76].

$${\mathrm{S}}_{\mathrm{i}}={\displaystyle \frac{∆\mathrm{Y}/\mathrm{Y}}{∆{\mathrm{X}}_{\mathrm{i}}/{\mathrm{X}}_{\mathrm{i}}}}$$

(4)

In the context of WT valorization, sensitivity analysis typically follows these steps: First, identify the environmental performance indicators to be assessed, such as greenhouse gas emissions, energy efficiency, or economic benefits. Indirect factors affecting environmental performance, such as the unit price of energy, grid carbon intensity, or the market price of recycled materials, are also considered. Next, determine the input variables within the system that may significantly impact the outputs. These variables may include external factors such as energy prices; the carbon intensity of electricity; the market prices for recycled materials; and process parameters like reaction temperature, pressure, and energy consumption. Sensitivity analysis is then conducted within a defined range of input variable fluctuations (commonly based on actual data variability or assumed ranges, such as ±10% or ±20%) [77], using models like an LCA to simulate the effects of input changes on environmental performance indicators. Single-factor or multi-factor sensitivity analyses are performed to assess these impacts. Finally, based on the findings, strategies are proposed to reduce emissions and improve environmental performance.

Numerous studies have employed this approach to analyze complex environmental variables. For example, Wu et al. [74] used the profit–cost ratio (PCR) as a key sensitivity analysis indicator to evaluate high-value utilization pathways for pyrolysis technology. The study identified that production costs, product prices, and environmental taxes were critical factors for improving the performance of high-value tire pyrolysis. Among these, production costs had the largest impact, with a ±25% change in costs leading to a ±194.95% change in the PCR. Additionally, the authors combined LCA results to show that replacing coal-based electricity with renewable energy significantly reduced fossil fuel consumption (ADP-fossil) and greenhouse gas emissions (GWP). They recommended adopting renewable energy sources, such as wind or nuclear power, to greatly enhance environmental performance. In another study, Wu et al. [72] conducted a more detailed sensitivity analysis of the high-value utilization pathways for pyrolysis products. In chemical modification processes, they focused on the recycling of nitric acid, finding that the number of recycling cycles directly influenced the reduction in environmental burdens. Sensitivity analysis revealed that doubling the number of nitric acid recycling cycles reduced the ERR of ADP-mineral by 49.6%, while the ERR for HTP and eutrophication potential (EP) exceeded 30%. Similarly, improvements in the physical modification process significantly reduced environmental burdens, with the ERR for ADP-mineral reaching nearly 100%, demonstrating the maximization of environmental benefits. A detailed sensitivity analysis was also performed on the refining pathways of pyrolysis oil, particularly evaluating the impacts of sulfur conversion rates and desulfurization catalyst efficiency on environmental performance. The analysis showed that improving sulfur conversion efficiency by adjusting pyrolysis conditions (e.g., varying the pyrolysis temperature by ±10 °C) significantly affected the final oil quality and desulfurization costs. Economically, when the market price of pyrolytic carbon black fluctuated by ±20%, the physical modification pathway maintained a high level of profitability, though the desulfurization efficiency of high-sulfur carbon black showed some economic sensitivity. A combined sensitivity analysis of physical modification and optimized desulfurization processes confirmed similar conclusions: the cost of WTs was identified as one of the most critical factors, with a sensitivity coefficient of 42%. Based on these findings, Wu et al. recommended prioritizing the combined improvement pathway of physical modification and tire pyrolysis oil desulfurization refining in the future to further reduce resource consumption and environmental burdens, while maintaining high economic returns even amidst fluctuations in tire and oil prices. Additionally, the development of more efficient desulfurization catalysts and lower cost modification methods was recommended to further optimize pathway performance. In summary, the multidimensional quantitative evaluation provided by sensitivity analysis offers a theoretical foundation for the optimization and promotion of WT valorization technologies.

In conclusion, through the comparison and summary of the current WT valorization technologies discussed above, the initial questions can be effectively addressed. Accordingly, the key LCA results corresponding to different technologies are summarized in

Table 6. The key LCA results of different technologies for WT recycling.

Technology	Environmental Impact Level	Energy Recovery Potential	Environmental Deterioration Phase	Environmental Benefit Phase
Incineration	High	High	Combustion processing	Fuel substitution
Cement kiln coprocessing (improved incineration)	Medium or low	High	Combustion processing	Fuel substitution, energy recovery
Pyrolysis	High, low after modification	High	Combustion processing, production modification	Energy recovery, material and energy substitution, product repair
Mechanical recycling (grinding, moulded product manufacturing and retreading)	Low	Low	Product manufacturing	Raw material replacement and product reuse

. Specifically, mechanical technology demonstrates a lower environmental influence in the byproduct recovery stage, making it one of the most environmentally friendly technologies. Pyrolysis and incineration with energy recovery technologies offer significant advantages in energy recovery but have notable environmental impacts due to high energy consumption and emission management issues, but after codification as well as fuel or material substitution. Cement kiln coprocessing significantly reduces energy consumption and greenhouse gas emissions by substituting coal and increasing the utilization of WTs. The environmental impacts of these technologies are primarily concentrated in the stages of processing, energy recovery, and emission management, while the recovery and utilization of byproducts are keys to improving environmental benefits. The current research reduces the overall environmental impact through both the technological improvements and modifications of downstream products. Meanwhile, attention is given to key sensitivity analysis indicators, such as economic factors (e.g., costs and product prices), energy and resource utilization, technical performance, and environmental impact. These efforts help identify the influence of various variables on the economic and environmental performance of WT valorization technologies, which enables the optimization of technology selection and processes to improve overall resource efficiency and reduce environmental burdens.

4. Current Status of Carbon Emission Reduction in Waste Tire Valorization

4.1. Contributions of Lifecycle Stages to Carbon Emissions

In the face of escalating global climate challenges, carbon emission control and reduction have become shared goals for countries worldwide. The Paris Agreement explicitly requires that global temperature increases be limited to within 2 °C, or ideally 1.5 °C, above pre-industrial levels [78]. In response, countries have introduced policies aimed at achieving carbon peaking and carbon neutrality. The WT valorization sector, characterized by high energy consumption and the extensive use of fossil-based raw materials, has come under increasing scrutiny for its carbon emissions. Consequently, research on carbon emissions and reduction strategies within the WT valorization field holds significant importance. Unit process analysis (UPA) is a key method in an LCA. It decomposes each stage into independent unit processes, quantifying their energy use and carbon emissions. This approach not only identifies the sources of carbon emissions but also provides a clear direction for optimizing resource utilization and technical processes [79]. The complete process flow of different valorization technologies can be summarized into four stages: raw material production, transportation and logistics, WT valorization, and final waste disposal. During the tire production process, the main contributors to carbon emissions are the production of natural rubber, synthetic rubber, and fillers. Indirect emissions arise from the land-use changes associated with natural rubber cultivation and the consumption of petrochemical energy during synthetic rubber production. In the transportation and logistics stage, direct carbon emissions result from fuel consumption during the transportation of WTs from collection points to processing facilities [80]. The WT valorization stage is the most complex, as it involves a variety of processing technologies. For example, in pyrolysis technology, the focus lies on analyzing the energy demands of the pyrolysis process and the carbon footprint contributions from pyrolysis gas emissions. In reclaimed rubber production, particular attention is paid to carbon emissions during the desulfurization and processing stages [69], while for coprocessing in cement kilns, the study focuses on the reduction in emissions achieved by substituting part of the coal combustion with WTs. In the final waste disposal stage, the potential climate impacts of landfilling or incinerating residual materials, such as carbon black residues and non-recyclable waste, are explored.

In practice, many studies tend to focus on the valorization stage (i.e., gate-to-gate analysis) using a single functional unit. However, a rigorous and comprehensive assessment should ideally consider the entire lifecycle, from cradle to grave, including each stage in sequence. For instance, Wu et al. [50] examined the environmental burdens of both the WT valorization process and the production of new tires. As shown in Figure 3, three cases were analyzed: physical modification (M1), nitric acid chemical modification (M2), and plasma modification (M3). During the new tire production phase, synthetic rubber was identified as the largest contributor to GWP, accounting for 46–47% of the total emissions. Steam production and usage were the second-largest contributors, responsible for 23–26%, followed by electricity, which contributed approximately 15–16%, highlighting its direct link to energy efficiency. In the WT valorization phase, M1 resulted in the lowest carbon emissions. In contrast, M2 showed significantly higher emissions due to the energy-intensive use of nitric acid. M3 achieved the lowest GWP through optimized energy usage, reducing its GWP contribution to just 2.9%. Buadit et al. [55] focused on incorporating the raw material production stage into their LCA environmental analysis, as shown in Figure 4a. In the production stage, high-energy processes (electricity and steam) contributed 69% of GWP, while carbon-intensive raw materials such as carbon black and synthetic rubber contributed 18% and 8%, respectively. Combining these insights with Figure 4b, the “Global Warming” row highlights the GWP contributions of each stage. The GWP contribution from the production stage was relatively small, as approximately 55.13 kg CO₂ eq, whereas the use phase contributed the largest share, accounting for 92% of the total lifecycle GWP, approximately 509 kg CO₂ eq. Additionally, it is important to note that in the environmental assessment of carbon emissions, excluding transportation processes is not recommended, as vehicle exhaust emissions often contribute significantly to greenhouse gas emissions, leading to substantial evaluation errors [81]. However, due to external factors such as complex geographical conditions, many studies struggle to incorporate detailed transportation processes into their analyses. Developing universal transportation modeling approaches to optimize LCAs, as well as exploring the sensitivity relationship between transportation distances and carbon emissions, remains a promising direction for future research [82].

4.2. Identifying Key Sources of Carbon Reduction Benefits

To achieve comprehensive carbon reduction across the full lifecycle, it is imperative to conduct an in-depth analysis of the emission characteristics and optimization potential at each stage. Each process stage contributes to emissions through its use of technology, resources, and energy. By leveraging technological innovations, system optimization, material substitution, and energy restructuring, the carbon footprint can be significantly minimized. Firstly, technological advancements are an effective means to achieve carbon reductions at root. Taking the pyrolysis process as an example, the current applications of tire pyrolysis oil (TPO) and pyrolytic carbon black (rCB) face significant limitations due to their properties, making direct utilization difficult. For instance, the high sulfur content and viscosity of TPO, as well as the high ash content and low microporosity of rCB, necessitate further upgrading, leading to additional processing emissions. To reduce the carbon footprint of the pyrolysis process and enhance resource utilization efficiency, it is essential to promote the high-value utilization of resources. For example, Aydın et al. [83] explored desulfurization technologies for TPO, such as hydrodesulfurization, to improve fuel quality. They found that optimizing the process can significantly lower sulfur content while also reducing carbon emissions throughout the fuel’s lifecycle. Jiang et al. [40] explored the mechanism of rCB formation and revealed that operating at a temperature of 1300 °C with a residence time of 2–4 s maximizes carbon black yield while enhancing the structural order and graphitization, offering valuable insights for the production of high-quality rCB. System optimization refers to improving resource utilization efficiency and carbon reduction benefits from an operational perspective across the entire process chain. Common approaches include circular economy models [84] and industrial symbiosis [85]. In a circular economy model, WTs are treated as resource inputs, recycled through processes such as pyrolysis or reclaimed rubber production, and reused in tire manufacturing. This creates a closed-loop system of production, consumption, and regeneration, achieving a truly “zero waste” model. Through industrial symbiosis, the byproducts from WT processing are supplied to other industrial sectors, enabling cross-industry resource sharing and maximizing resource utilization efficiency. Alternatively, waste materials from other industries (e.g., plastics and biomass) can be coprocessed with WTs [86,87], improving pyrolysis reaction efficiency while reducing disposal costs and emissions.

Material substitution in WT valorization has become a key area of research. Preliminary solutions for material substitution include using reclaimed rubber to replace virgin rubber [88], rCB to replace commercial carbon black [83], and TPO as a substitute for engine-blend oil [89]. Additionally, waste tire chips [90] or pyrolysis gas [91] can be reused as fuel. These approaches not only reduce the demand for virgin material production but also effectively avoid the energy and chemical inputs required for manufacturing new materials. Research indicates that incorporating reclaimed rubber can reduce the GWP of self-healing rubber by 28% [92]. Analogously, asphalt mixtures produced with crumb rubber derived from WTs achieve a 43–46% lower GWP and 43–45% lower gross energy requirement (GER) compared to those of traditional dense-graded mixtures [93]. By improving the physical or chemical modification of rCB, it can replace up to 50% of commercial carbon black, thereby reducing reliance on virgin carbon black [94]. Additionally, energy use is often a significant contributor to GWP in the lifecycle of WT processing [50,74]. As such, energy optimization is another critical approach to enhancing resource utilization and reducing environmental burdens. This primarily involves substituting traditional fossil fuels, such as coal, oil, and natural gas, with clean energy sources and improving overall energy efficiency. Increasing the share of renewable energy in the energy mix for WT processing fundamentally reduces the GWP associated with energy consumption. Research shows that incorporating solar power and wind energy into production processes not only reduces dependence on fossil fuels but also lowers the system’s GWP by 15–30%. Additionally, utilizing combustible gases generated during the thermal treatment of WTs as reactor fuel is another effective energy optimization strategy. For instance, Subramanian and colleagues studied a multi-product system for WT recycling, where combustible gases like methane and ethane generated through pyrolysis or gasification were used for combined heat and power (CHP) generation. In their 2021 study in 2021 [95], the gasification of WTs for CHP reduced CO₂ emissions by 20–30%, while in 2022 they further optimized a mixed-feed system, achieving more than 40% carbon emission reductions under high-carbon-tax scenarios [96]. Overall, these measures not only act as effective carbon reduction strategies but also align seamlessly with circular economy principles. They help mitigate environmental burdens, reduce operational costs, and even create economic value, thereby supporting the sustainability of the entire industrial value chain. Bi et al. [97] quantitatively assessed the economic benefits of energy self-sufficiency in WT pyrolysis plants. By using TPO as a substitute for light liquid fuel (valued at 3000 RMB/ton) and converting pyrolytic carbon black into activated carbon (valued at 4000 RMB/ton), a pyrolysis plant with an annual capacity of 50000 tons could achieve annual profits of up to RMB 248.13 million.

In summary, to achieve comprehensive carbon reduction throughout the entire lifecycle, it is necessary to thoroughly analyze the emission characteristics of each stage. Depending on the technology, context, and boundary settings, the sources of carbon emissions vary to some extent but are primarily concentrated in the production and usage stages of raw materials and products. For these stages, several carbon reduction measures have been proposed, encompassing improving process technologies, implementing resource-sharing optimization systems, using recycled materials to replace raw materials, and substituting clean energy for traditional fossil fuels. In addition to the aforementioned measures, achieving carbon reduction benefits from WT valorization requires the integration of strategies, such as digital management, logistics optimization, diversified policy support, and consumer behavior initiatives. Particularly in the era of Industry 4.0, the development of data-driven models [98,99] plays a crucial role. The combination of these strategies can greatly reduce the carbon footprint of WT processing and utilization, while driving the progress of a global low-carbon economy.

5. Future Trends in Carbon Reduction for Waste Tire Valorization

While current WT valorization technologies have achieved notable progress in carbon reduction, significant challenges remain, including inadequate pollution control, high energy consumption, suboptimal lifecycle carbon management, and insufficient policy and technological support. To address these issues, trends in emission reduction from other sectors have also been integrated to offer insights into the future directions of WT resource utilization. This includes improved LCA and integrated assessment models under different policy scenarios, informed by research on emerging WT utilization technologies. The discussion highlights three key areas: the development of innovative technologies, the adoption of advanced LCA models for scientific evaluation, and strategic planning, with the goal of achieving a seamless integration of resource valorization, carbon reduction, and neutrality.

5.1. Prospects for Emerging Valorization Technologies

In Section 2.2, the commonly used traditional physical and chemical techniques have been summarized. However, as previously discussed, many conventional resource recovery technologies rely on high-temperature processes, which are typically powered by fossil fuels or electricity. This leads to significant energy consumption and substantial indirect carbon emissions, thereby diminishing the overall carbon reduction benefits of resource recovery. Moreover, the products derived from certain WT resource recovery methods, such as carbon black and fuel oil, often exhibit inconsistent quality. This variability hinders their high-value utilization and limits their potential to fully replace virgin materials. The limited market demand for these low-value-added products further extends the lifecycle of fossil resource consumption, ultimately constraining the effectiveness of carbon reduction efforts. In recent years, research on WT resource recovery technologies has increasingly focused on emerging processes, such as microwave-assisted depolymerization [100], photothermal pyrolysis [101], and supercritical fluid technology [102]. These technologies show great potential in improving energy efficiency, pollution control, and producing high-value products. Specifically, microwave pyrolysis is a technology that utilizes microwave electromagnetic waves to rapidly heat materials and facilitate pyrolysis reactions. This is achieved by inducing oscillations in the dipoles of polar molecules within tire rubber under the influence of the high-frequency electromagnetic field, thereby generating frictional heat. Additionally, certain ionic compounds in WTs such as carbon black or metal components undergo conductive motion in the microwave field, further generating Joule heat. This direct energy conversion method makes microwave heating faster and more uniform compared to conventional pyrolysis, which relies on heat conduction [103]. A typical system setup is shown in Figure 5a. The microwave absorber bed at position 5, typically made of silicon carbide (SiC) particles, is used to absorb microwaves and increase the system temperature to ensure efficient heating. The thermocouple at position 6 monitors the temperature of the reaction zone in real time, ensuring the precise control of reaction conditions and higher selectivity. The catalyst bed at position 7 is loaded with catalysts to catalytically crack and optimize the reaction pathways of volatile products generated during pyrolysis. For example, Chen et al. [100] used the microwave-assisted pyrolysis of WTs co-pyrolyzed with microalgae, supplemented by a 15% NiO catalyst, achieving a bio-oil yield of 28.4%, with hydrocarbons accounting for 58.3% of the total. Additionally, Song et al. [104] found that the microwave pyrolysis of WTs produced a higher L-limonene yield compared to that of traditional pyrolysis, with the limonene mass fraction in the pyrolysis oil reaching 23.4%. The optimal processing parameters were a microwave power of 15 W/g and a weight hourly space velocity (WHSV) of 3.75 h⁻¹. Photothermal pyrolysis, inspired by the combination of photothermal energy in nature, is a multifunctional cross-disciplinary technology. It relies on clean and renewable solar energy, significantly reducing dependence on fossil fuels. Its system, as shown in Figure 5b, maintains an inert atmosphere using nitrogen flow. The photothermal source provides concentrated heat energy, which is transmitted to the WT samples through quartz glass, causing the primary components to decompose and form free radicals. These free radicals undergo intramolecular cyclization, dimerization, and isomerization reactions, resulting in the generation of target products. Silva et al. [101] developed a photothermal pyrolysis technology using high-intensity xenon lamp flashes to simulate solar energy effects. This technology was able to convert WTs into a hydrogen-rich syngas (36%) within just a few seconds. The process achieved an energy recovery of 23.1 kWh, which exceeded the input energy by 8.8 kWh. Supercritical pyrolysis utilizes the drastic changes in the physical and chemical properties of fluids under supercritical conditions (temperature ≥ 374 °C, pressure ≥ 22 MPa), such as a significant reduction in the density, dielectric constant, and ionic product, making it an efficient reaction medium [105]. Taking supercritical water treatment as an example, as shown in Figure 5c, water is pressurized to supercritical conditions using a high-temperature pump and then introduced into the reactor through a pipeline. The reactor is heated in a molten salt bath. After the reaction, the reactor is cooled through a cooler, and the products are separated into three phases: gas, liquid, and solid. Yan et al. [106] further explored the mechanism of element migration during supercritical pyrolysis using ReaxFF and DFT characterization. Taking sulfur migration during the pyrolysis of WTs in supercritical water as an example, Figure 5d shows that the sulfur in tires mainly exists in the form of sulfur cross-linked bonds (C–S bonds and C–S–S bonds). Under supercritical conditions, water molecules exist in two forms: hydrogen-bonded molecular clusters and single water molecules. Both types of water molecules interact with sulfur atoms through the attractive force of oxygen atoms or through •OH radicals released during pyrolysis. This ultimately leads to the cleavage of sulfur bonds, resulting in oxidation and the formation of inorganic sulfur compounds (e.g., H₂SO₄, H₂S₂O₃) or the migration of H₂S to the gas phase. It is worth noting that both photothermal pyrolysis and supercritical technologies demonstrate excellent performance in improving hydrogen yield due to their high-energy reaction environments, which effectively excite hydrogen free radicals [107,108]. This feature shows promising prospects in energy applications such as the syngas production and hydrogen supply for fuel cells. Furthermore, many other emerging technological pathways, such as liquefaction [109], plasma gasification [110], low-temperature pyrolysis [111], and various synergistic resource recovery solutions using catalytic materials [112], merit further exploration.

In the field of carbon science and technology, carbon capture has garnered significant interest. Carbon capture and storage (CCS) technologies encompass a range of methods for capturing CO₂ from emission sources, which are then stored or utilized to reduce greenhouse gas emissions and mitigate climate change. These methods include pre-combustion capture, post-combustion capture, and oxy-fuel combustion capture [115]. In fact, research has long focused on using the activated carbon derived from WTs as an adsorbent material for post-combustion capture, including serving as a chemical catalyst [116] with KOH or a photocatalytic agent carrier [117] for CO₂ capture or conversion. This is due to its high specific surface area and excellent adsorption properties, with the process generally being proven to be physical adsorption [118]. A recent study has conducted thermodynamic evaluations of the activation process and identified an optimal solution with a CO₂ capture capacity of up to 1.42 mmol/g through an orthogonal experimental design [119]. Furthermore, recent research reported decarbonization technologies in the resource recovery processes themselves. For example, in the gasification process, CO₂ is often required to assist in driving the water–gas shift reaction [120]. However, many studies overlook its emission. In this regard, Liu et al. [121] set up an MEA (Monoethanolamine)-based CO₂ capture unit in the gasification process to minimize carbon emissions. MEA is an efficient solvent that selectively absorbs CO₂ from the syngas generated during plasma gasification. This process reduces the amount of CO₂ released into the atmosphere. Al-Qadri et al. [122] achieved a 26% reduction in specific CO₂ emissions through an integrated steam gasification and steam methane reforming system. The whole setup is projected to yield a net present value (NPV) of EUR 18.14 million over 30 years, highlighting its economic viability. Looking ahead, as the mentioned technologies evolve, there may be an opportunity to integrate different stages and facilitate the gradual achievement of carbon capture. For instance, pre-combustion capture could be considered by enriching CO₂ during the gasification process at medium to low temperatures. Such insights can be drawn from applications in natural gas power plants [123,124].

In summary, the above discussion elaborates on several representative emerging technologies for WT recycling from the perspective of technical feasibility and highlights their potential combined with carbon capture and storage technologies. However, it should be noted that the development of these new technologies remains in its infancy. Most emerging technologies have only reached laboratory stages [100,101], with both environmental and economic assessments lacking. While the potential of additional carbon capture technologies has been explored, environmental metrics such as climate change mitigation are rarely quantified in practice. Compared to more conventional methods like pyrolysis and gasification [125], the environmental benefits of most emerging technologies, such as carbon savings, remain largely unverified and require further investigation. Additionally, many emerging technologies rely on inputs such as photothermal energy, catalysts, and chemical reagents, which are likely to increase costs, yet this aspect is seldom considered. The WT recycling market itself faces challenges such as significant price volatility and long investment payback periods, making economic viability a critical concern. Therefore, both environmental and economic assessments should be prioritized as key directions for future research.

5.2. Development of LCA Models

Traditional LCA systems for WTs are typically based on static evaluations conducted at specific time points under known environmental conditions. However, this evaluation approach has several limitations. For example, it cannot accurately capture the temporal dynamics of emissions or adequately reflect regional differences and the potential for future technological advancements. By integrating various novel LCA models, a more comprehensive and precise assessment can be achieved across both temporal and spatial dimensions. The characteristics of these models and their applications in WT resource recovery are summarized in

Table 7. Characteristics of the novel LCA models and their reference application scenarios.

Classification Dimension	Category	Characteristics	Application Scenarios
Temporal perspective	DLCA	Combines the time dimension to analyze the dynamic changes in carbon emissions.	Analyzing the dynamic changes in carbon emissions during the resource recovery process of WTs.
	PLCA	Predicts the potential environmental impacts of emerging technologies before large-scale commercialization.	Evaluating the carbon reduction potential of emerging treatment technologies such as supercritical pretreatment.
	FLCA	Focuses on the cumulative environmental impacts of technological pathways over a long-term time frame.	Assessing the trend of long-term carbon emissions and resource consumption for different waste tire treatment methods.
Geographical perspective	GE-LCA	Combines geographical and regional characteristics to analyze the distribution and impacts of carbon emissions.	Comparing the differences in carbon emissions from transportation, recycling, and treatment in urban and rural areas.
	MR- LCA	Cross-regional supply chain analysis to evaluate the distribution of resource flows and carbon emissions across regions.	Evaluating cross-regional carbon emissions from the generation site to the recycling site; analyzing the carbon footprint of recycled products used in multiple regions.
Functional perspective	Different FU application	Considers different functional units in the analysis to evaluate the environmental impacts of diverse goals.	When considering different target needs or requirements for more comprehensive consideration.

. From a temporal perspective, the recommended improvements include a dynamic LCA (DLCA), prospective LCA (PLCA), and future LCA (FLCA). The DLCA [126] approach captures the changes in carbon emissions over time, including long-term cumulative emissions and time-sensitive mitigation measures. It serves as a valuable tool for long-term carbon neutrality planning by enabling the identification and precise targeting of high-emission periods and critical process steps, thereby providing robust scientific evidence to inform and optimize emission reduction strategies. For example, Shimako [127] used the DLCA method to analyze carbon emissions over a 30-year operational cycle of a wastewater treatment plant. The DLCA revealed significant short-term fluctuations in emissions of CO₂, CH₄, and N₂O during the wastewater treatment process. These fluctuations were closely related to diurnal and seasonal variations in wastewater flow, with noticeable emission peaks during periods of high load at treatment peaks. The results showed that the traditional LCA methods overestimate the long-term climate impact of emissions, as they assume all emissions occur at time zero, whereas a DLCA reflects the actual temporal distribution of emissions. Shorter time intervals (e.g., 0.5 days or 1 day) allow for the more precise capture of the dynamic characteristics of carbon emissions. A prospective LCA (PLCA) focuses on the potential environmental impacts of emerging technologies, new materials, or future systems, and it offers a predictive perspective. Relevant studies already exist in the field of WT resource recovery. For instance, Li et al. [60] used a PLCA to evaluate two emerging supercritical pretreatment technologies—supercritical swelling pretreatment and supercritical decrosslinking pretreatment—for producing crumb rubber-modified asphalt mixtures. Under simulated future industrial application scenarios, the supercritical swelling pretreatment technology had the lowest life cycle carbon emissions, while the supercritical decrosslinking pretreatment technology had the highest. This was primarily due to the significant carbon footprint introduced by the use of chemical decrosslinking agents. These comparative results were based on the potential for technological iteration, such as the optimization of dry ice and chemical reagent consumption (a 50% reduction) under industrial conditions, providing a predictive outlook on resource and emission characteristics. An FLCA examines the potential lifecycle environmental impacts of external environments under long-term future scenarios, placing emphasis on scenario construction and uncertainty analysis. For example, Guo et al. [128] studied the impact of clean energy transitions on carbon emissions across 850 industrial parks in China by 2030. The study compared baseline scenarios, which maintain current structures and emission patterns, with mitigation scenarios that eliminate coal-fired facilities and replace them with on-site energy, grid decarbonization, and natural gas heating. It was found that greenhouse gas emissions under the mitigation scenario were reduced by 41% (approximately 768 million tons of CO₂ eq), equivalent to 7% of China’s total carbon emissions in 2014. This underscores the urgency of clean energy transitions.

From a geographical perspective, novel LCA approaches include a geographically explicit LCA (GE-LCA) and multi-regional LCA (MR-LCA). A GE-LCA incorporates spatial differences into environmental impact analysis, using geographic information system (GIS) data along with region-specific emission factors, energy structures, and resource utilization efficiencies to visually reflect environmental impacts across different areas. Guo et al. [129] further utilized a GE-LCA to summarize and propose differentiated waste disposal strategies for various regions. In the economically developed eastern regions, the high calorific value of waste results in incineration carbon emissions of approximately 500–700 kg CO₂ per ton of waste, with significantly improved energy recovery rates. In the southwestern regions, where hydropower dominates, the carbon emission intensity of the grid electricity replaced by waste incineration is relatively low, leading to weaker emission reduction benefits. Moreover, in remote or sparsely populated areas, transportation emissions account for 5–10% of total emissions due to dispersed waste collection, highlighting the need for optimized incineration plant siting and waste collection routes. To address biases arising from reliance on regional averages, an LCA can be coupled with multi-regional input–output (MRIO) models to highlight interregional cooperation and material flows, especially in cases where carbon emissions depend on cross-regional energy inputs. This approach is known as a multi-regional LCA (MR-LCA). Hong et al. [130] selected three typical building clusters in China (a mixed-use residential–commercial complex in Guangdong, a residential project in Sichuan, and an office building in Guangdong) to quantify embodied carbon emissions across regions. In the Guangdong projects, materials such as steel and cement were predominantly sourced from neighboring provinces, whereas construction in Sichuan relied more on localized resources, reducing long-distance transportation emissions. The results indicated that green technologies and optimized material supply chains are key to lowering construction carbon emissions, while regional energy structure optimization can further enhance emission reduction benefits. The integration of an LCA and MRIO allows environmental assessments to combine micro-level details with a macro-level system perspective. An LCA provides a detailed analysis of the full lifecycle of buildings, identifying emission hotspots and the emission reduction potential of green technologies at a micro-level. Meanwhile, MRIO quantifies the flow of resources and energy across regions and their environmental impacts, offering a macro-level system analysis of interregional cooperation and emission reduction potential [131]. This multi-scale integration provides a novel perspective for the field of WT resource recovery. It is also worth noting that in LCA studies, although most adopt a single functional unit (FU), research has shown that the selection of functional units is generally driven by specific research requirements [53]. For example, Hugo et al. [132] demonstrated in an LCA study on the construction industry that comparing three types of FUs for assessing the environmental performance of typical Brazilian residential buildings revealed significant deviations in results caused by FU selection. They recommended using at least two FUs to gain a more comprehensive understanding of building performance. From an environmental assessment perspective, studies using “per ton of waste tires” as the functional unit remain predominant [72,74]. However, when evaluating the efficiency and environmental impacts of WTs as an energy source, it is advisable to adopt “per unit of energy output” as the functional unit. Therefore, in comprehensive assessments, calculating LCAs from different functional perspectives offers flexibility and the advantage of comparative analysis, enabling a more holistic evaluation.

5.3. Integration with Carbon Neutrality Goals

The integration of WT resource recovery with carbon neutrality involves incorporating the process of WT recycling and reuse into the global framework for greenhouse gas reduction and carbon neutrality. Through technological optimization, policy support, and comprehensive evaluation systems, this approach aims to maximize economic, social, and environmental benefits in synergy. Fundamentally, it represents a systematic strategy that promotes coordinated development across multiple sectors. Under the global backdrop of carbon neutrality, the selection of waste tire resource recovery technologies should prioritize processes with high emission reduction potential. Processes with a lower carbon footprint should receive focused policy support, such as technologies that utilize a higher proportion of renewable energy or reduce dependence on fossil fuels through innovations. Through technological advancements, it is possible to efficiently recover and reuse the high-value byproducts from waste tires, thereby enhancing the overall efficiency of resource recovery. In addition, government policies based on international carbon neutrality commitments can provide enterprises with support and subsidies to promote WT resource recovery. For instance, the EU’s “Fit for 55” initiative aims to reduce greenhouse gas emissions by at least 55% compared to 1990 levels by 2030. The EU strictly restricts the landfilling of waste tires and emphasizes the circular economy, promoting their utilization through remanufacturing, material recycling, and energy recovery to reduce greenhouse gas emissions [8]. Through programs like Horizon Europe, the EU has allocated nearly EUR 1 billion in funding to support research and development (R&D) in waste valorization technologies for materials such as WTs [133]. Australia has implemented targeted plans to phase out landfills [134] and announced a transformative investment of approximately AUD 1 billion in its waste and recycling sector [135]. China’s 2060 carbon neutrality target represents a significant commitment to addressing climate change, with WT valorization playing a pivotal role. The Chinese government actively guides enterprises in developing advanced technologies to resolve challenges in the WT industry, establishing a green, low-carbon, closed-loop industrial chain of “resources-products-waste-recycled resources” [136]. A series of supportive measures have been introduced to facilitate the R&D and application of WT valorization technologies. These include a value-added tax (VAT) refund-upon-collection policy, offering a 50% VAT refund for sales of rubber powder, retreaded tires, and reclaimed rubber produced from waste tires and rubber products, while granting a 70% refund rate for regenerated oil and pyrolysis carbon black [137]. These financial subsidies can encourage enterprises to adopt low-carbon and efficient resource recovery technologies, particularly by supporting investment in pyrolysis processes that coproduce fuel oil and carbon black. Simultaneously, establishing and improving carbon trading market systems can impose higher costs on high-emission processes, driving technological innovation and selection toward more sustainable options.

Notably, the selection of WT resource recovery technologies should avoid a “one-size-fits-all” approach. Instead, emphasis should be placed on aligning specific technological pathways with precise policy objectives. Figure 6a illustrates various technological pathways for addressing climate change, evaluated from the dual dimensions of “Mitigation” and “Adaptation” for their multiple benefits [138]. The mitigation dimension focuses primarily on reducing greenhouse gas emissions or increasing carbon sinks to address the root causes of climate change. In contrast, the adaptation dimension emphasizes enhancing the resilience of systems and societies to the impacts of climate change. Drawing inspiration from this within the context of waste tire utilization, if the short-term goal is to improve energy efficiency, WT combustion may be a more practical option, provided strict pollutant emission control measures are implemented. On the other hand, if the objective is resource valorization, technologies with high byproduct recovery efficiency should take precedence, such as pyrolysis processes that coproduce high-quality fuel oil and carbon black. A critical point to consider in aligning WT resource recovery with carbon neutrality goals is avoiding the potential “environmental trade-off effect” [139]. This effect refers to scenarios where optimizing certain environmental metrics inadvertently exacerbates other environmental issues. To mitigate this risk, constructing a comprehensive evaluation system is essential to balance policy objectives with multiple environmental benefits.

Aligned with carbon neutrality goals, a potential assessment model for waste tire resource recovery could be developed under policy support. For carbon emission forecasting, many evaluation models across multiple dimensions are worth referencing. At the national level for emission reduction analysis, an example is provided by Xu et al. [140] who employed a forecasting model with 27 different scenarios based on the following combinations: economic growth rates and energy consumption intensities, which are classified into high, medium, and low levels. Meanwhile, the low-carbon optimized energy consumption structure is divided into three types: positive, baseline, and negative. Based on this, the model offers a detailed projection of the impact of variations in economic growth and energy consumption patterns on the timing of the carbon peak between 2021 and 2060 in China, as depicted in Figure 6b. Among these scenarios, Scenario 1 performs the best, where the carbon peak is expected to be reached around 2026 and carbon neutrality ultimately achieved by 2056. Scenario 1 relies on slowing economic growth, advancing low-carbon energy transitions, improving energy efficiency, enhancing technological innovation, and increasing policy support. The authors also identified through regression modeling that actively optimizing the low-carbon energy consumption structure is the key pathway to achieving low-carbon goals. Calderón et al. [141] used four integrated models (GCAM, TIAM-ECN, Phoenix, and MEG4C) to assess different emission reduction scenarios for Colombia. Each model varies in the emission reduction pathways it proposes. The GCAM model primarily relies on biomass and CCS technologies, while the TIAM-ECN model places more emphasis on wind and hydropower. The Phoenix model achieves emission reductions through the CCS technology applied to coal and natural gas. The core of the MEG4C model is to analyze CO₂ reductions by evaluating the response of the entire economic system. Figure 6c shows the trend of CO₂ intensity in energy consumption, which refers to the amount of carbon dioxide emissions per unit of energy consumed from 2005 to 2050 according to four models. The GCAM and Phoenix models predict an increase in CO₂ intensity. This trend is mainly due to their assumptions of a higher share of coal and natural gas in energy production. In contrast, the TIAM-ECN and MEG4C models predict a decrease, primarily due to their assumptions of reduced carbon intensity in the residential and transport sectors; an increase in the share of electric vehicles; and greater use of hydropower, wind energy, and biomass. The results from all four models consistently emphasize the impact of carbon reduction policies, particularly in the electricity sector, where a significant reduction in CO₂ emissions can be achieved by increasing the adoption of renewable energy. Although implementing a carbon tax may have negative impacts on GDP and consumption, these economic costs can be alleviated through appropriate tax revenue redistribution measures. Moreover, the application of scenario analysis can be utilized not only for macro-level national analyses but also for being further applied to specific sectors. For instance, Dou et al. [142] predicted several carbon emission scenarios for electric vehicle (EV) battery recycling based on different recycling strategies, including business as usual (BAU), enhanced collection (COL), balanced development (BAL), regeneration priority (REG), and second-use priority (B2U). As shown in Figure 6d, the B2U strategy demonstrates the most significant emission reduction effect, with GHG emissions from LFP batteries reduced by 272% compared to BAU, and from NCM batteries by 15%. For individual NCM batteries, REG yields the best emission reduction results, with emissions reduced by 34% compared to BAU. These scenarios help predict GHG emissions and metal resource demands, providing different solutions for various strategies. Beyond the commonly used forecasting models, optimization models and prediction factor selection models [143] can also be employed. These models optimize and extract predictive factors for accurate carbon emission forecasting through algorithms such as programming, and their performance can be further enhanced with machine learning [144,145]. Moreover, such models and scenarios can be further applied to carbon neutrality scenarios for WT recycling. For example, scenario simulations could analyze the impact of varying subsidy levels or carbon pricing on the adoption rates of different technological pathways and their overall carbon reduction benefits. This would provide scientific evidence to inform policymaking decisions.

Overall, multi-dimensional breakthroughs in emission reduction, from technology to assessment, are discussed in the context of the waste tire sector. The emerging technologies mentioned above leverage advantages such as selective thermal effects, efficient heat transfer, and high solubility to achieve high-value and efficient resource recovery from WTs. Furthermore, in combination with carbon capture technologies, deeper post-combustion capture and the future exploration of pre-combustion capture should be further pursued. In the environmental pollution assessment phase, the application of LCA models with time and space dimensions should be recommended as an extension of traditional models. Additionally, based on insights from other carbon prediction methods, recommendations for multi-dimensional models to predict carbon emissions are proposed. By predicting across various scenarios, these models can more effectively help identify critical pathways, thereby providing theoretical support for policy decisions on prioritizing development and balancing trade-offs.

6. Conclusions

In light of the above analysis, this review provides a comprehensive evaluation of the research progress on LCA-based waste tire recycling technologies. It provides valuable insights into the application of waste tire recycling technologies aligned with carbon neutrality goals, while also presenting constructive recommendations for the future development of valorization technologies and the optimization of carbon reduction pathways. The technical comparisons of the systems indicate that many LCA evaluation metrics represented by carbon emissions vary depending on the technologies and operational conditions, while most originate from production and usage phases. Enhanced thermochemical technologies, such as material and energy substitution, demonstrate better environmental benefits. For the research direction of environmental evaluations through an LCA, although the application of WT valorization technologies has yet to establish unified global standards and their LCAs remain in developmental stages, it is imperative to expand the scope of environmental evaluations by focusing on analyses of different dynamic and functional dimensions. Future LCAs should encompass more lifecycle phases, aiming to advance environmental optimization through the deeper exploration of emission control and optimization in transportation and production processes. For the future trends in carbon reduction through waste tire valorization, potential directions may involve the following:

• Improving mass and heat transfer, as well as catalytic performance in technological processes.
• Developing more detailed and multidimensional evaluation models.
• Establishing systematic approaches that balance policy objectives with environmental benefits within the framework of carbon neutrality.

These efforts will pave the way for a more sustainable and low-carbon future in waste tire recycling.