Carbon emission model and evaluation analysis of vehicle waste tire different recycling processes based on reducing environmental pollution

Abstract

To scientifically assess and quantitatively evaluate the carbon emissions associated with different treatment methods and processes for end-of-life vehicle tires, the carbon emission factor method was employed. Carbon emission models and carbon emission-reduction inventories were developed for the tire production, transportation, use, and recycling stages. Specifically, models and inventories were constructed for four treatment methods: Tire retreading, reclaimed rubber production, reclaimed rubber powder production, and pyrolysis, encompassing eight distinct processes: Hot retreading, cold retreading, atmospheric continuous desulfurization, screw extrusion, ambient grinding, cryogenic grinding, atmospheric pyrolysis, and vacuum pyrolysis. The carbon emissions, carbon reduction, net carbon surplus, and carbon reduction rate for each treatment method and process were calculated. Furthermore, sensitivity and economic analyses were conducted on the material mass and energy consumption in the production stage, which impact the total carbon emissions. The results indicate that among the four recycling methods, tire retreading production demonstrates the highest carbon reduction efficiency, followed by reclaimed rubber and reclaimed rubber powder production, while pyrolysis shows the lowest efficiency. Among the eight processes, cold retreading exhibits the most significant carbon reduction effect, with a carbon reduction rate of 59.30%. The ranking of carbon reduction efficiency is as follows: Cold Retreading > Hot Retreading > Ambient Grinding > Atmospheric Continuous Desulfurization > Screw Extrusion >Atmospheric Pyrolysis > Vacuum Pyrolysis > Cryogenic Grinding. Sensitivity analysis reveals a positive correlation between the material mass and energy consumption required in the production stage and the total carbon emissions; however, the impact of material mass on total carbon emissions is significantly greater than that of energy consumption.

Introduction

Carbon emissions are one of the main causes of global warming, and reducing carbon emissions is crucial for protecting the Earth’s environment and human physical and mental health. Waste tires are not easy to degrade, and long-term accumulation can occupy a large amount of land resources. At the same time, chemicals in tires may seep into the soil, leading to soil and water pollution. When tires burn, they produce a large amount of toxic and harmful gases, such as sulfur dioxide. These gases not only pollute the atmospheric environment, but may also pose a threat to human health, potentially causing respiratory diseases and skin inflammation. Some chemicals in tires may cause damage to the human nervous system, affecting intellectual development and nervous system function. Establishing a sound green, low-carbon, and circular development economic system for waste tires and improving the efficiency of recycling waste tire resources are important components related to sustainable economic and social development^1,2. In December 2021, the State Council issued the "Comprehensive Work Plan for Energy Conservation and Emission Reduction in the 14th Five Year Plan" ([2021] No.33), which has listed the treatment of solid waste of waste tires and rubber as a key project for energy conservation and emission reduction. The green and low-carbon recycling of waste tires is one of the key tasks for China to build a resource-saving and environmentally friendly society and achieve sustainable economic development. In developed countries abroad, people’s awareness and attention to the potential environmental impact of the tire industry are increasing. Quantitative analysis of the environmental impact of tires throughout their entire lifecycle is crucial for clarifying the technological development trend of the tire industry^3,4,5. Extensive research has been conducted both domestically and internationally on the life cycle assessment of waste tire resource recycling, and certain achievements have been made. Eranki et al.⁶ conducted a study on the replacement of rubber in traditional tires with silver rubber chrysanthemum rubber based on the life cycle assessment method. Through data analysis, they evaluated the impact of silver rubber chrysanthemum tires on the environment and energy during the production and disposal process; Vaida Malikonyte et al.⁷ conducted an assessment study on the environmental impact of tire breakage and solid recycling fuel combustion based on the life cycle theory. Domestic scholars’ research on the life cycle assessment of waste tires mainly focuses on the following two aspects:firstly, analyzing the environmental impact of the entire life cycle of tires and identifying their carbon reduction potential. The second is to compare and analyze different recycling technologies for waste tires, and find the most environmentally beneficial recycling method. For example, Xin Chunlin et al.⁸ used the life cycle theory and the Yike eFoot print database to construct tire models for both carbon black and white carbon black systems, and conducted a comparative analysis of their full life cycle carbon footprint during production, transportation, and waste tire recycling stages; Cui Ning⁹ adopts a simplified life cycle assessment method to analyze the various environmental loads in the tire life cycle and discuss the significant influencing factors in the tire life cycle; Xu Jiefeng et al.¹⁰ established a lifecycle assessment model for radial tires and evaluated and analyzed the potential environmental impacts of radial tires throughout their lifecycle. Wang Ziyou¹¹ investigated the energy-saving and carbon reduction effects of technologies including intelligent sorting, low-temperature crushing, catalytic pyrolysis, waste heat recovery, and high-value utilization. QZ Wang et al.¹² proposed a triple exponential smoothing prediction model to forecast the supply of waste tires from 2019 to 2023, as well as the demand for these tires under three road construction scenarios. Additionally, a performance analysis was conducted to evaluate the carbon emissions generated during the production processes of rubber crumbs, modified asphalt, and styrene-butadiene-styrene (SBS) modified asphalt. The potential for reducing carbon emissions when using rubber crumbs in road construction was measured to promote the sustainable management of waste tires. Park S et al.¹³ employed microwave heating, leveraging the fact that waste tires are primarily composed of carbon black and rubber, to achieve selective and rapid heating. While rubber does not respond to microwaves, carbon black is an efficient microwave absorber. By utilizing this characteristic, microwave radiation can instantly heat the carbon black in waste tires and facilitate the carbonization process within one minute. J Fu et al.¹⁴ considered pyrolysis to be a highly practical, cost-effective, and environmentally friendly technology for waste tire treatment. They studied the properties of pyrolysis products and the distribution patterns of pollutants generated during different operational stages (start-up, stable operation, and shutdown). The findings provide guidance for reducing pollutant emissions and recycling pyrolysis products. Q Zhao et al.¹⁵ put forward feasible recommendations and identified future trends for advancing resource recovery of waste tires (WT). The objectives were to (1) systematically review existing life cycle assessment findings and technical pathways for WT resource recovery; (2) assess the strengths and weaknesses of current technologies from a carbon reduction perspective; and (3) explore future trends and propose optimized pathways and suggestions for technological development. K Slyusarsky et al.¹⁶ found that the oxidation characteristics under thermogravimetric analyzer conditions changed only slightly. In contrast, the addition of 15 wt% peat and sawdust additives under actual combustion conditions at 800°C reduced the ignition delay time by 42% and 78%, respectively, while SO2 emissions decreased by 73% and 52%, respectively. Additional sulfur was found to exist in the ash residue in the forms of CaS and CaSO4. The obtained results can be applied to integrate waste tire pyrolysis carbon mixtures with peat or sawdust into the energy sector.

Through the analysis of the current research status of tire lifecycle carbon emissions at home and abroad, most studies focus on the impact of waste disposal or recycling processes on the environment, while there is relatively little research on the carbon emissions of the entire lifecycle process of tires (raw material blending process, tire production process, tire transportation process, tire use process, and waste tire recycling process), which cannot systematically and accurately reflect the carbon emissions of tires at each stage of their lifecycle. Through an analysis of key stages such as pretreatment, pyrolysis conversion, and recycled rubber production, and by integrating case studies and technical assessment methods. At present, there are four main ways to recycle and reuse waste tires, including tire retreading, recycled rubber, recycled rubber powder, and thermal cracking^17,18,19. Tire retreading mainly includes thermal and cold retreading processes, while recycled rubber mainly includes atmospheric pressure continuous desulfurization process and screw extrusion process. Recycled rubber powder mainly includes ambient temperature crushing process and low-temperature crushing process, and thermal cracking mainly includes atmospheric pressure cracking process and vacuum cracking process^20,21,22. Although these four waste tire treatment methods and eight different processes can greatly save rubber raw materials, the impact of different treatment methods and process forms on promoting energy and environmental protection, as well as on the environment and corporate carbon emissions, has not yet been systematically and scientifically evaluated and quantitatively assessed. Therefore, the research on the life cycle carbon emissions assessment of waste tire resource utilization will have important theoretical significance and practical application value.

Carbon emission model for the tire life cycle

The primary methods for carbon emission accounting include the carbon emission factor approach, mass balance method, and direct measurement method^23,24,25,26. The carbon emission factor approach is currently the most commonly used method for quantifying carbon emissions. The fundamental equation for carbon emission calculation is provided by the IPCC, namely: Carbon Emissions = Activity Data × Corresponding Emission Factor. This study employs the carbon emission factor approach to develop carbon emission models for the tire life cycle, encompassing four recycling methods—tire retreading, reclaimed rubber production, reclaimed rubber powder production, and pyrolysis—across eight different processes.

Carbon emissions throughout the tire life cycle primarily originate from four stages: tire production, transportation, use, and recycling. Each stage is accompanied by a certain amount of carbon emissions, which are unavoidable by-products of the tire life cycle, reflecting the environmental impact of the entire process from tire manufacturing and use to end-of-life recycling. The carbon emission model for the tire life cycle is established based on the First Law of Thermodynamics and the Law of Mass Conservation. The model is expressed as Eq. (1).

$$AC = AC_{1} + AC_{2} + AC_{3} + AC_{4}$$

(1)

In the formula, $AC$ represents the total carbon emissions throughout the entire lifecycle of a tire; $AC_{1}$ is the carbon emissions of the tire during the production stage; $AC_{2}$ is the carbon emissions of the tire during transportation; $AC_{3}$ is the carbon emissions of the tire during its usage phase; $AC_{4}$ is the carbon emissions during the stage of tire recycling after waste disposal.

Carbon emission model during tire production stage

The carbon emission model during the tire production stage is shown in equation (2).

$$AC_{1} = \sum\limits_{i} {(PNM_{i} \times PNMI_{i} )} + \sum\limits_{j} {(PNE_{j} \times PNEI_{j} )}$$

(2)

In the formula, $AC_{1}$ represents the carbon emissions during the tire production stage; $PNM_{i}$ is the consumption of raw material $i$ during the tire production stage; $PNMI_{i}$ is the carbon emission coefficient of raw material $i$ in the tire production stage; $PNE_{j}$ is the consumption of energy $j$ during the tire production stage; $PNEI_{j}$ is the carbon emission coefficient of energy $j$ during the tire production stage.

Carbon emission model during tire transport stage

The carbon emission model during tire transportation is shown in equation (3).

$$AC_{2} = ANE_{j} \times ANEI_{j}$$

(3)

In the formula, $ANE_{j}$ represents the energy consumption during the transportation phase; $ANEI_{j}$ is the carbon emission coefficient of energy during the transportation phase.

Tire use stage carbon emission model

The carbon emission model during the tire usage stage is shown in equation (4).

$$AC_{j} = MNE_{j} \times MNEI_{j}$$

(4)

In the formula, $MNE_{j}$ represents the energy consumption during the usage phase; $MNEI_{j}$ is the carbon emission coefficient of the energy used during the usage phase.

Carbon emission model for tire reuse stage

Tire refurbishment carbon emission model

• (1).  Carbon emission model using hot flipping process

The carbon emission model using the hot flip process is shown in equation (5).

$$AC_{4} = \sum\limits_{i} {(RNM_{i} \times RNMI_{i} )} + \sum\limits_{j} {(RNE_{j} \times RNEI_{j} )}$$

(5)

In the formula, $AC_{4}$ represents the carbon emissions generated by the hot flip process; $RNM_{i}$ is the consumption of raw material $i$ using the hot flipping process; $RNMI_{i}$ is the carbon emission coefficient of raw material $i$ using the hot flipping process; $RNE_{j}$ is the consumption of energy $j$ using the thermal flipping process; $RNEI_{j}$ is the carbon emission coefficient of energy $j$ using the thermal flipping process.

• 1. Carbon reduction model

Using the hot rolling process for tire retreading will generate various new products or new energy sources. Calculate the carbon reduction using the hot flipping process according to equation (6).

$$RC = \sum\nolimits_{i} {(RCP_{i} \times RCPI_{i} )} + \sum\nolimits_{j} {(RCE_{j} \times RCEI_{j} )}$$

(6)

• 2.  Net carbon surplus

The relationship between carbon reduction and carbon emissions using the hot flipping process can be expressed as net carbon surplus, as shown in equation (7).

$$RCY = RC - AC_{4}$$

(7)

• 3. Carbon reduction rate

The carbon reduction rate using the hot flipping process can be calculated according to equation (8).

$$RCE = \frac{RC}{{AC_{1} + AC_{4} }} \times 100\%$$

(8)

• (2).  Carbon emission model using cold flipping process

The carbon emission model using the cold flip process is shown in equation (9).

$$BC_{4} = \sum\limits_{i} {(LNM_{i} \times LNMI_{i} )} + \sum\limits_{j} {(LNE_{j} \times LNEI_{j} )}$$

(9)

In the formula, $BC_{4}$ represents the carbon emissions generated by the cold flip process; $LNM_{i}$ represents the consumption of raw material $i$ using the cold flipping process; $LNMI_{i}$ is the carbon emission coefficient of raw material $i$ using the cold flipping process; $LNE_{j}$ is the consumption of energy $j$ using the cold flipping process; $LNEI_{j}$ is the carbon emission coefficient of energy $j$ using the cold flip process.

• 1. Carbon reduction model

Calculate the carbon reduction model using the cold flip process according to equation (10).

$$LC = \sum\limits_{i} {(LCP_{i} \times LCPI_{i} )} + \sum\limits_{j} {(LCE_{j} \times LCEI_{j} )}$$

(10)

In the formula, $LC$ represents the carbon reduction amount using the cold flipping process; $LCP_{i}$ is the output of product $i$ using the cold flipping process; $LCPI_{i}$ is the carbon emission coefficient of product $i$ using the cold flipping process; $LCE_{j}$ is the output of energy product $j$ using the cold flip process, and $LCEI_{j}$ is the carbon emission coefficient of energy product $j$ using the cold flip process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using the cold flip process can be expressed as net carbon surplus, as shown in equation (11)

$$LCY = LC - BC_{4}$$

(11)

• 3. Carbon reduction rate

The carbon reduction rate using the cold flipping process can be calculated according to equation (12).

$$LCE = \frac{LC}{{AC_{1} + BC_{4} }} \times 100\%$$

(12)

Carbon emission model for regenerated rubber

• (1). Carbon emission model using atmospheric continuous desulfurization process

The carbon emission model of the atmospheric pressure continuous desulfurization process is shown in equation (13).

$$DC_{4} = \sum\limits_{i} {(WNM_{i} \times WNMI_{i} )} + \sum\limits_{j} {(WNE_{j} \times WNEI_{j} )}$$

(13)

In the formula, $DC_{4}$ represents the carbon emissions of the atmospheric continuous desulfurization process; $WNM_{i}$ represents the consumption of raw material $i$ using the atmospheric pressure continuous desulfurization process; $WNMI_{i}$ is the carbon emission coefficient of raw material $i$ using atmospheric continuous desulfurization process; ${\text{WNE}}_{j}$ represents the energy consumption of process $j$ using atmospheric pressure continuous desulfurization method; $WNEI_{j}$ is the carbon emission coefficient of energy $j$ using the atmospheric pressure continuous desulfurization process.

• 1. Carbon reduction model

Calculate the carbon reduction model using the atmospheric pressure continuous desulfurization process according to equation (14).

$$ZC = \sum\limits_{i} {(ZCP_{i} \times ZCPI_{i} )} + \sum\limits_{j} {(ZCE_{j} \times ZCEI_{j} )}$$

(14)

In the formula, $ZC$ represents the carbon reduction amount obtained through the atmospheric pressure continuous desulfurization process; $ZCP_{i}$ is the output of product $i$ using the atmospheric pressure continuous desulfurization process; $ZCPI_{i}$ is the carbon emission coefficient of product $i$ using atmospheric continuous desulfurization process; $ZCE_{j}$ is the output of energy product $j$ using atmospheric continuous desulfurization process, and $ZCEI_{j}$ is the carbon emission coefficient of energy product $j$ using atmospheric continuous desulfurization process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using atmospheric continuous desulfurization process can be expressed as net carbon surplus, as shown in equation (15)_.

$$ZCY = ZC - DC_{4}$$

(15)

• 3. Carbon reduction rate

The carbon reduction rate of the atmospheric pressure continuous desulfurization process can be calculated according to equation (16).

$$ZCE = \frac{ZC}{{DC_{1} + DC_{4} }} \times 100\%$$

(16)

• (2).  Carbon emission model using screw extrusion process

The carbon emission model of the screw extrusion process is shown in equation (17) Show.

$$JC_{4} = \sum\limits_{i} {(WGM_{i} \times WGMl_{i} )} + \sum\limits_{j} {(WGE_{j} \times WGEI_{j} )}$$

(17)

In the formula, $JC_{4}$ represents the carbon emissions generated by the screw extrusion process; $WGM_{i}$ is the consumption of raw material $i$ using screw extrusion process; $WGMI_{i}$ is the carbon emission coefficient of raw material $i$ using screw extrusion process; $WGE_{j}$ represents the energy consumption of the screw extrusion process $j$; $WGEI_{j}$ is the carbon emission coefficient of energy $j$ using screw extrusion process.

• 1. Carbon reduction model

The carbon reduction model using screw extrusion process is calculated according to equation (18)

$$LG = \sum\limits_{i} {(LGP_{i} \times LGPI_{i} )} + \sum\limits_{j} {(LGE_{j} \times LGEI_{j} )}$$

(18)

In the formula, $LG$ represents the carbon reduction achieved through screw extrusion process; $LGP_{i}$ is the output of product $i$ using screw extrusion process; $LGPI_{i}$ is the carbon emission coefficient of product $i$ using screw extrusion process; $LGE_{j}$ is the output of energy product $j$ using screw extrusion process, and $LGEI_{j}$ is the carbon emission coefficient of energy product $j$ using screw extrusion process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using screw extrusion process can be represented by net carbon surplus, as shown in equation (19).

$$LGY = LG - JC_{4}$$

(19)

• 3. Carbon reduction rate

The carbon reduction rate of the screw extrusion process can be calculated according to equation (20).

$$LE = \frac{LG}{{DC_{1} + JC_{4} }} \times 100\%$$

(20)

Carbon emission model for recycled rubber powder

• (1).   Carbon emission model using room temperature crushing process

The carbon emission model using room temperature crushing process is shown in equation (21).

$$WC_{4} = \sum\limits_{i} {(CLM_{i} \times CLMI_{i} )} + \sum\limits_{j} {(CLE_{j} \times CLEI_{j} )}$$

(21)

In the formula, $WC_{4}$ represents the carbon emissions generated by the ambient temperature crushing process; $CLM_{i}$ is the consumption of raw material $i$ using room temperature crushing process; ${\text{CLM}}I_{i}$ is the carbon emission coefficient of raw material $i$ using room temperature crushing process; ${\text{CLE}}_{j}$ is the consumption of energy $j$ using room temperature crushing process; $CLEI_{j}$ is the carbon emission coefficient of energy $j$ using room temperature crushing process.

• 1. Carbon reduction model

Calculate the carbon reduction model using the room temperature crushing process according to equation (22).

$$CF = \sum\limits_{i} {(CFP_{i} \times CFPI_{i} )} + \sum\limits_{j} {(CFE_{j} \times CFEI_{j} )}$$

(22)

In the formula, $CF$ represents the amount of carbon reduction achieved through room temperature pulverization process; $CFP_{i}$ is the output of product $i$ using room temperature crushing process; $CFPI_{i}$ is the carbon emission coefficient of product $i$ using room temperature crushing process; $CFE_{j}$ is the output of energy product $j$ using room temperature crushing process, and $CFEI_{j}$ is the carbon emission coefficient of energy product$j$ using room temperature crushing process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using room temperature crushing process can be expressed as net carbon surplus, as shown in equation (23).

$$CFY = CF - WC_{4}$$

(23)

• 3. Carbon reduction rate

The carbon reduction rate using room temperature crushing process can be calculated according to equation (24).

$$CT = \frac{CF}{{WC_{1} + WC_{4} }} \times 100\%$$

(24)

• (2).  Carbon emission model using low-temperature crushing process

The carbon emission model using low-temperature crushing process is shown in equation (25).

$$FC_{4} = \sum\nolimits_{i} {(DFM_{i} \times DFMI_{i} )} + \sum\nolimits_{j} {(DFE_{j} \times DFEI_{j} )}$$

(25)

In the formula, $FC_{4}$ represents the carbon emissions generated by the low-temperature crushing process; $DFM_{i}$ is the consumption of raw material $i$ using low-temperature crushing process; $DFMI_{i}$ is the carbon emission coefficient of raw material $i$ using low-temperature crushing process; $DFE_{j}$ is the consumption of energy $j$ using low-temperature crushing process; $DFEI_{j}$ is the carbon emission coefficient of energy $j$ using low-temperature crushing process.

• 1. Carbon reduction model

Calculate the carbon reduction model using the low-temperature crushing process according to equation (26).

$$ZD = \sum\limits_{i} {(ZDP_{i} \times ZDPI_{i} )} + \sum\limits_{j} {(ZDE_{j} \times ZDEI_{j} )}$$

(26)

In the formula, $ZD$ represents the amount of carbon reduction achieved through low-temperature crushing technology; $ZDP_{i}$ is the output of product $i$ using low-temperature crushing technology; $ZDPI_{i}$ is the carbon emission coefficient of product $i$ using low-temperature crushing technology; $ZDE_{j}$ is the output of energy product $j$ using low-temperature crushing process, and ${\text{ZDEI}}_{j}$ is the carbon emission coefficient of energy product $j$ using low-temperature crushing process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using low-temperature crushing technology can be expressed as net carbon surplus, as shown in equation (27).

$$ZDY = ZD - FC_{4}$$

(27)

• 3. Carbon reduction rate

The carbon reduction rate using low-temperature crushing process can be calculated according to equation (28).

$$ZG = \frac{ZD}{{WC_{1} + WC_{4} }} \times 100\%$$

(28)

Thermal cracking carbon emission model

• (1).  Carbon emission model using atmospheric cracking process

The carbon emission model using atmospheric cracking process is shown in equation (29).

$$YC_{4} = \sum\limits_{i} {(LJM_{i} \times LJMI_{i} )} + \sum\limits_{j} ( LJE_{j} \times LJEI_{j} )$$

(29)

In the formula, $YC_{4}$ represents the carbon emissions generated by the atmospheric pressure cracking process; $LJM_{i}$ is the consumption of raw material $i$ using atmospheric pressure cracking process; $LJMI_{i}$ is the carbon emission coefficient of raw material $i$ using atmospheric pressure cracking process; $LJE_{j}$ is the consumption of energy $j$ using atmospheric pressure cracking process; $LJEI_{j}$ is the carbon emission coefficient of energy $j$ using atmospheric pressure cracking process.

• 1.  Carbon reduction model

Calculate the carbon reduction model using the atmospheric pressure cracking process according to equation (30).

$$CL = \sum\limits_{i} {(CLP_{i} \times CLPI_{i} )} + \sum\limits_{j} {(CLE_{j} \times CLEI_{j} )} ,$$

(30)

In the formula, $CL$ represents the amount of carbon reduction using atmospheric pressure cracking process; $CLP_{i}$ represents the yield of product $i$ during the atmospheric pressure cracking process stage; ${\text{CLPI}}_{i}$ is the carbon emission coefficient of product $i$ using atmospheric pressure cracking process; $CLE_{j}$ is the yield of energy product $j$ using atmospheric pressure cracking process, and $CLEI_{j}$ is the carbon emission coefficient of energy product $j$ using atmospheric pressure cracking process.

• 2. Net carbon surplus

The relationship between the reduction of carbon emissions and the use of atmospheric cracking process can be expressed as net carbon surplus, as shown in equation (31).

$$CLY = CL - YC_{4}$$

(31)

• 3. Carbon reduction rate

The carbon reduction rate using atmospheric pressure cracking process can be calculated according to equation (32).

$$YT = \frac{CL}{{YC_{1} + YC_{4} }} \times 100\%$$

(32)

• (2).  Carbon emission model using vacuum cracking process

The carbon emission model using vacuum cracking process is shown in equation (33).

$$KC_{4} = \sum\limits_{i} {(CHM_{i} \times CHMI_{i} )} + \sum\limits_{j} {(CHE_{j} \times CHEI_{j} )}$$

(33)

In the formula, $KC_{4}$ represents the carbon emissions generated by the vacuum cracking process; $CHM_{i}$ is the consumption of raw material $i$ using vacuum cracking process; $CHMI_{i}$ is the carbon emission coefficient of raw material $i$ using vacuum cracking process; $CHE_{j}$ is the consumption of energy $j$ using vacuum cracking process; $CHEI_{j}$ is the carbon emission coefficient of energe $j$ using vacuum cracking process.

• 1. Carbon reduction model

Calculate the carbon reduction model using the vacuum cracking process according to equation (34).

$$HL = \sum\nolimits_{i} {(HLP_{i} \times HLPI_{i} )} + \sum\nolimits_{j} {(HLE_{j} \times HLEI_{j} )}$$

(34)

In the formula, $HL$ represents the amount of carbon reduction achieved through vacuum cracking process; $HLP_{i}$ is the yield of product $i$ using vacuum cracking process; $HLPI_{i}$ is the carbon emission coefficient of product $i$ using vacuum cracking process; $HLE_{j}$ is the output of energy product $j$ using vacuum cracking process, and $HLEI_{j}$ is the carbon emission coefficient of energy product $j$ using vacuum cracking process.

• 2. Net carbon surplus

The relationship between carbon reduction and carbon emissions using vacuum cracking process can be represented by net carbon surplus, as shown in equation (35).

$$HLY = HL - CH_{4}$$

(35)

• 3. Carbon reduction rate

The carbon reduction rate using vacuum cracking process can be calculated according to equation (36).

$$HY = \frac{HL}{{YC_{1} + YC_{4} }} \times 100\%$$

(36)

Carbon emission Inventory

A tire with the specification 305/90R22.5 was selected for this analysis. Assuming an average tire lifespan of 2-3 years and a mileage of 70,000 km, the tire weight was estimated using a commonly adopted weight calculation formula (Eq. 37) in the tire industry. The weight of a single tire was calculated as 80 kg. The analysis is based on a functional unit of 10 tires, equivalent to 0.8 tonnes. The data used in the inventory were primarily sourced from References⁵ and^{27,28,29,30,31,32,33,34}. The methodology for estimating tire weight was derived from References^{35,36,37,38,39,40,41,42}.

$$M_{g} = W_{g} \, \cdot H_{g} \, \cdot R_{g} \, \cdot n^{2}$$

(37)

In the equation, $M_{g}$ represents the tire weight in kg; $W_{g}$ represents the tire width in mm; $H_{g}$ represents the rim diameter in inches; $n$ represents the estimation coefficient based on the average density of the tire material.

Carbon emission inventory for the tire production stage

The production of a 305/90R22.5 tire requires the consumption of natural rubber, carbon black, steel cord, and other auxiliary materials. Based on a calculated weight of 80 kg per tire, the composition is as follows: 25% is natural rubber⁴³, with a net weight of approximately 20 kg; 23% is carbon black^44,45,46, with a net weight of approximately 18.4 kg; and 18% is steel cord, with a net weight of approximately 14.4 kg. Auxiliary materials such as active zinc oxide and sulfur, due to their relatively small proportion, are considered negligible.

The carbon emissions for the tire production stage are calculated using Formula (2). The carbon emission inventory for this stage is presented in Table 1. From the data, the total carbon emissions for the tire production stage amount to 1368.725 kg.

Table 1 Carbon emission inventory during tire production stage

Carbon emission inventory for the tire transportation stage

Approximately 1 liter of diesel is equivalent to 0.86 kg. The diesel consumption required for transportation per kilometer is 0.20 liters, and the transportation distance is set at 100 km. The carbon emission factor for a 12-tonne truck is 2.63 kg/(t·km).

The carbon emissions for the tire transportation stage are calculated using Formula (3). The carbon emission inventory for this stage is presented in Table 2. The results indicate that the carbon emissions for the tire transportation stage amount to 1931.58 kg.

Table 2 Carbon emission inventory during tire transport stage

Carbon emission inventory for the tire use stage

The carbon emissions generated during a tire’s service life primarily originate from the fuel consumed by the vehicle throughout the period from tire installation to end-of-life, covering the entire operational lifespan. The average service life of a tire is set at 70,000 km. Considering a fuel contribution factor of 0.25 and a diesel consumption of 0.20 liters per kilometer, the total diesel consumption over the complete life cycle of a single tire is calculated to be approximately 14,000 liters. Applying the fuel conversion factor of 0.25, the total diesel consumption for 10 tires is further calculated as 140,000 L × 25% = 35,000 L. Given that the mass of 1 liter of diesel is approximately 0.86 kg, 35,000 liters of diesel equates to 35,000 L × 0.86 kg/L = 30,100 kg.

The carbon emissions for the tire use stage are calculated using Formula (4). The corresponding carbon emission inventory is presented in Table 3. The results indicate that the carbon emissions for the tire use stage amount to 79,163 kg.

Table 3 Carbon emission inventory during tire use stage

Carbon emission inventory for the tire recycling stage

Carbon emission inventory for tire retreading

• (1).  Carbon emission-reduction inventory for the hot retreading process

The hot retreading process involves applying raw rubber, formulated according to the tread compound recipe, onto the used tire casing, followed by revulcanization. During the tire remanufacturing process, the preparation of the tread compound is a critical step. Its raw material composition primarily includes natural rubber, styrene-butadiene rubber (SBR), and carbon black. A new batch of 200 kg of tread compound raw materials needs to be prepared in specific proportions: 45% natural rubber, 25% SBR, and 30% carbon black. The casing section utilizes the original end-of-life tire casing; thus, in the carbon emission calculation, the inputs for the used casing and water are considered to have zero emissions.

The carbon emissions for the hot retreading process are calculated using Equation (5), the carbon reduction is calculated using Equation (6), the net carbon surplus is calculated using Equation (7), and the carbon reduction rate is calculated using Equation (8). The carbon emission-reduction inventory for the hot retreading process is presented in Table 4.

Table 4 Carbon emissions reduction inventory using the hot flip process

• (2).  Carbon emission-reduction inventory for the cold retreading process

The cold retreading process involves directly applying a pre-vulcanized and pre-fabricated tread onto the used tire casing. This method has a lesser thermal impact on the tire but is correspondingly more time-consuming. During the preparation process, the compound is formulated using 45% natural rubber, 25% styrene-butadiene rubber, and 30% carbon black. The original end-of-life tire casing is used as the carcass material. In the carbon emission calculation, the inputs for the used casing and water are considered to have zero emissions.

The carbon emissions for the cold retreading process are calculated using Equation (9), the carbon reduction is calculated using Equation (10), the net carbon surplus is calculated using Equation (11), and the carbon reduction rate is calculated using Equation (12). The carbon emission-reduction inventory for the cold retreading process is presented in Table 5.

Table 5 Carbon emissions reduction inventory using cold flip process

Carbon emission inventory for reclaimed rubber

• (1).  Carbon emission-reduction inventory for the atmospheric continuous desulfurization process

The primary products of the atmospheric continuous desulfurization process are reclaimed rubber and steel wires. The carbon emissions associated with water input are considered zero.

The carbon emissions for the atmospheric continuous desulfurization process are calculated using Equation (13), the carbon reduction is calculated using Equation (14), the net carbon surplus is calculated using Equation (15), and the carbon reduction rate is calculated using Equation (16). The carbon emission-reduction inventory for the atmospheric continuous desulfurization process is presented in Table 6.

Table 6 Carbon emissions reduction inventory for continuous atmospheric pressure desulfurization process

• (2).  Carbon emission-reduction inventory for the screw extrusion process

The carbon emissions for the screw extrusion process are calculated using Equation (17), the carbon reduction is calculated using Equation (18), the net carbon surplus is calculated using Equation (19), and the carbon reduction rate is calculated using Equation (20). The carbon emission-reduction inventory for the screw extrusion process is presented in Table 7.

Table 7 Carbon emission reduction inventory for screw extrusion process

Carbon emission inventory for reclaimed rubber powder

• (1).  Carbon emission-reduction inventory for the ambient grinding process

When processing end-of-life tires using the ambient grinding process, electric energy and fuel oil are indispensable energy sources during the production process. To meet production demands, relevant equipment must be fitted with steel cutters. In the carbon emission calculation, the carbon emission associated with water input is considered zero.

The carbon emissions for the ambient grinding process are calculated using Equation (21), the carbon reduction is calculated using Equation (22), the net carbon surplus is calculated using Equation (23), and the carbon reduction rate is calculated using Equation (24). The carbon emission-reduction inventory for the ambient grinding process is presented in Table 8.

Table 8 Carbon emission reduction inventory using room temperature crushing process

• (2). Carbon emission-reduction inventory for the cryogenic grinding process

The cryogenic grinding process requires a refrigeration system for cooling, resulting in relatively high energy consumption. It requires electricity, nitrogen, natural gas, and steel wires to power the equipment. The carbon emission associated with water input is treated as zero.

The carbon emissions for the cryogenic grinding process are calculated using Equation (25), the carbon reduction is calculated using Equation (26), the net carbon surplus is calculated using Equation (27), and the carbon reduction rate is calculated using Equation (28). The carbon emission-reduction inventory for the cryogenic grinding process is presented in Table 9.

Table 9 Low temperature Crushing Carbon Emissions Reduction Inventory

Carbon emission inventory for pyrolysis

• (1). Carbon emission-reduction inventory for the atmospheric pyrolysis process

The primary pyrolysis products generated by the atmospheric pyrolysis process include pyrolysis gas, pyrolysis oil, carbon black, and steel wires. The yield distribution is approximately 40% for pyrolysis oil, 35% for carbon black, 15% for steel wires, and about 10% for pyrolysis gas.

The carbon emissions for the atmospheric pyrolysis process are calculated using Equation (29), the carbon reduction is calculated using Equation (30), the net carbon surplus is calculated using Equation (31), and the carbon reduction rate is calculated using Equation (32). The carbon emission-reduction inventory for the atmospheric pyrolysis process is presented in Table 10.

Table 10 Carbon emissions reduction inventory using atmospheric pressure cracking process

• (2). Carbon emission-reduction inventory for the vacuum pyrolysis process

The vacuum pyrolysis process requires lower pyrolysis temperatures and pressures, which can more effectively restrict the occurrence of side reactions. The yield of pyrolysis oil is generally higher than that from atmospheric pyrolysis, resulting in approximately 55% pyrolysis oil, 35% solids (20% carbon black and 15% steel wires), and 10% gas.

The carbon emissions for the vacuum pyrolysis process are calculated using Equation (33), the carbon reduction is calculated using Equation (34), the net carbon surplus is calculated using Equation (35), and the carbon reduction rate is calculated using Equation (36). The carbon emission-reduction inventory for the vacuum pyrolysis process is presented in Table 11.

Table 11 Carbon emissions reduction inventory using vacuum cracking process

Results and analysis

Carbon emission analysis

The life cycle carbon emission-reduction inventories for the eight processes (hot retreading, cold retreading, atmospheric continuous desulfurization, screw extrusion, ambient grinding, cryogenic grinding, atmospheric pyrolysis, vacuum pyrolysis) under the four recycling methods (tire retreading, reclaimed rubber, reclaimed rubber powder, pyrolysis) for 1.2 tonnes of end-of-life tires are presented in Tables 12, 13, 14, and 15.

Table 12 Tire refurbishment life cycle carbon emissions reduction inventory

Table 13 Carbon emissions reduction inventory for the life cycle of recycled rubber

Table 14 Carbon emissions reduction inventory for the life cycle of recycled rubber powder

Table 15 Carbon emissions reduction inventory for thermal cracking life cycle

As shown in Tables 12, 13, 14, and 15, the total carbon emissions throughout the life cycle for 0.8 tonnes of tires across the production, transportation, and use stages are approximately 82,463.305 kg. Among these stages, the tire use stage generates the most significant carbon emissions, amounting to 79,163 kg and accounting for 96% of the total. This is followed by the transportation stage, with carbon emissions of 1,931.58 kg, representing approximately 2.01% of the total. The production stage contributes 1,638.725 kg of carbon emissions, constituting 1.99% of the total, as illustrated in Figure 1.

Fig. 1

Proportion of carbon emissions during tire production, use, and reuse stages

Carbon emission assessment

Figure 2 shows the net carbon surplus for the eight tire recycling processes, and Figure 3 illustrates their carbon reduction rates. Among the four typical tire recycling methods, the ranking of carbon reduction effectiveness is as follows: Tire Retreading > Reclaimed Rubber > Reclaimed Rubber Powder > Pyrolysis. For the eight specific processes, the ranking of carbon reduction effectiveness is: Cold Retreading > Hot Retreading > Ambient Grinding > Atmospheric Continuous Desulfurization > Screw Extrusion >Atmospheric Pyrolysis > Vacuum Pyrolysis > Cryogenic Grinding.

Fig. 2

Net carbon surplus of 8 reuse methods and processes

Fig. 3

Carbon reduction rates of 8 reuse methods and processes

Sensitivity and economic analysis

Due to space constraints, this paper focuses solely on the sensitivity analysis of carbon emissions in the production stage, as illustrated in Figure 4. Figure 4 demonstrates that when the mass of materials required in the tire production stage increases from 90% to 130%, the total carbon emissions rise from 1276 kg to 1678 kg, representing an increase of approximately 31.5%. Conversely, when the energy consumption in the production stage increases from 90% to 130%, the total carbon emissions increase from 1344 kg to 1469 kg, an increase of about 9.3%. Both material mass and energy consumption in the production stage show a positive correlation with total carbon emissions; however, the impact of material mass on total carbon emissions is significantly more pronounced than that of energy consumption.

Fig. 4

Material quality/energy consumption sensitivity test curve during tire production stage

The economic study indicates that the total energy consumption throughout the entire tire life cycle is approximately 357,968 MJ. Among these stages, the manufacturing phase consumes the most energy, accounting for about 42.77% of the total. This is followed by the material procurement stage, which consumes approximately 33.23% of the energy. The tire use stage accounts for about 23.58% of the energy consumption. The energy used for tire transportation is the smallest, representing only about 0.33% of the total energy.

Among the four recycling processes—retreading, rubber powder reclaiming, rubber reclaiming, and pyrolysis—the retreading process demonstrates the highest energy recovery rate, while rubber reclaiming process is the second most effective. The energy recovery rate for the rubber powder reclaiming process is 22.76%, whereas that for the pyrolysis is 21.74%. Overall, among the four recycling methods, tire retreading exhibits the highest energy recovery efficiency, making it the most effective approach for recycling end-of-life tires.

Conclusion

• (1) A carbon emission calculation model for the tire life cycle was constructed. Carbon emission-reduction inventories for the production, transportation, use, and recycling stages were established, enabling the calculation of carbon emissions, carbon reduction, net carbon surplus, and carbon reduction rate for each stage.

• (2) An evaluation and analysis of the carbon reduction effectiveness were conducted for eight processes under the four recycling methods: tire retreading, reclaimed rubber, reclaimed rubber powder, and pyrolysis. The results indicate that the ranking of carbon reduction effectiveness for the four recycling methods is: Reclaimed Rubber > Tire Retreading > Reclaimed Rubber Powder > Pyrolysis. Specifically, the ranking for the eight processes is: Cold Retreading > Hot Retreading > Ambient Grinding > Atmospheric Continuous Desulfurization > Screw Extrusion >Atmospheric Pyrolysis > Vacuum Pyrolysis > Cryogenic Grinding. Retreading consistently delivers a higher carbon surplus than any material recovery or recycling process as we preserve large part of tyre carcass .

• (3) The cold retreading process used in reclaimed rubber production demonstrates the most significant carbon reduction effect, with a carbon reduction rate of 59.30%, making it the preferred pathway for energy saving and emission reduction in end-of-life tire recycling. This is followed by the hot retreading process, with a carbon reduction rate of 54.50%. Therefore, tire retreading also stands out as a highly efficient and environmentally friendly method for recycling end-of-life tires.

• (4) The findings of this study can provide a necessary theoretical underpinning and guidance for the low-carbon green transition of the waste tire recycling industry, policy formulation in the tire sector, strategic development of tire enterprises, and regulatory planning by national authorities. Additionally, it offers a theoretical basis for the establishment of corporate carbon emission standards by the state. Due to space limitations, the sensitivity analysis presented in this paper focuses primarily on the production stage in terms of material quality and energy consumption. A more in-depth investigation into economic feasibility analysis is reserved for a separate, as-yet-unpublished manuscript by the research team, with only brief mention made herein. Owing to constraints in data sources, some of the findings may be subject to certain deviations. Future research will aim to enhance the accuracy of the computational results. deviations. Future research will aim to enhance the accuracy of the computational results.