China's recycling potential of large-scale public transport vehicles and its implications

The continuous improvement of transport infrastructure through manufacturing transformation and technological upgrading necessitates understanding the material degradation and obsolescence of large-scale public transport vehicles (LPTV). China's rapid development of its LPTV industry, particularly rail and aviation equipment, has led to a substantial increase in resource consumption and the generation of waste LPTV. This waste stream presents both environmental risks and significant resource recycling opportunities. While research exists on the recycling potential of conventional end-of-life products like e-waste and end-of-life private vehicles, waste LPTV remains understudied. This study aims to quantitatively assess the recycling potential and economic benefits of LPTV in China, providing crucial data for promoting resource recycling and informing policy development. The study also explores the concept of industrial succession within the context of industrial ecology, observing the patterns of waste generation across different industries (e.g., consumer electronics, vehicles, and LPTV) to understand the life cycle and replacement of technical products. Several methods, including the possession coefficient method and the market supply A method, are employed to estimate waste LPTV generation, considering data availability and model robustness.

Previous research has extensively examined the recycling potential of conventional end-of-life products such as waste electrical and electronic equipment (WEEE or e-waste), end-of-life private vehicles (ELPV), and waste plastics. Studies have focused on estimating generation amounts, analyzing material composition, and assessing economic and environmental benefits of recycling these waste streams. However, research on waste LPTV, an emerging and significant waste stream, is lacking. This gap in knowledge highlights the need for a quantitative assessment of the recycling potential of LPTV, particularly in rapidly developing economies like China, where the growth of the transportation sector is driving increasing resource consumption and waste generation. The study draws upon existing methodologies employed in assessing other waste streams, adapting and refining them to the specific characteristics of LPTV. The concept of industrial succession, borrowed from ecology and applied in industrial ecology, provides a framework for understanding the dynamic nature of waste generation patterns across different industries over time.

This study employed two primary methods to estimate the generation of waste LPTV: the possession coefficient method and the market supply A method. The accuracy of the possession coefficient method was validated by comparing its results to those obtained using the market supply A method for railway equipment. The average difference between the two methods was found to be 15%, indicating reasonable agreement and justifying the use of the possession coefficient method for broader estimations. The possession coefficient method relies on the possession amount and the cumulative obsolescence ratio (C), which is set to 60% based on Weibull distribution. The market supply A method calculates obsolescence based on historical sales (production) amounts and lifespan distribution, using the Weibull distribution function to model obsolescence ratios. The future possession amount of LPTV was predicted using a logistic model, reflecting the expected saturation of LPTV numbers in China by 2050. Data on production amount, possession amount, and mass composition of LPTV were collected from various sources (Supplementary Tables 3-5). The study then analyzes the recycling potential and economic benefits of LPTV by focusing on four typical metals (Fe, Al, Ti, and Nd), determining their content in waste LPTV and assessing their market value. Carbon footprint calculations were conducted to assess the environmental benefits of recycling these metals, comparing emissions from primary mining with those from secondary material production. Sensitivity analysis was performed to evaluate the impact of variations in LPTV service life on the estimations, while uncertainty analysis using Monte Carlo simulation quantified the imprecision in predictions. The study also examines industrial succession by comparing the generation rates of waste LPTV with those of e-waste and ELPV, observing trends and patterns in waste generation over time to identify industry shifts and replacement patterns.

The study projects a significant increase in waste LPTV generation in China. The total obsolescence mass of railway equipment is predicted to increase from 0.5 Mt in 2000 to 73.6 Mt in 2050, while the obsolescence mass of aviation equipment will rise from 1.9 kt in 2015 to 61.5 kt in 2050. The overall obsolescence mass of LPTV is expected to grow from 0.5 Mt in 2000 to 73.7 Mt by 2050, with railway equipment accounting for over 99% of the total mass. Economic growth is shown to significantly accelerate the generation of waste LPTV. By 2050, waste LPTV is projected to contain substantial quantities of valuable metals: at least 72 million tons of steel, 838 kilotons of aluminum, 2539 tons of titanium, and 223 tons of neodymium. Recycling these metals offers significant economic potential, estimated to reach 32.9 billion by 2050, primarily driven by the recovery of iron and aluminum. Recycling LPTV also yields substantial environmental benefits through reduced carbon emissions. Proper recovery of the four key metals could reduce carbon emissions by approximately 80.6 Mt by 2050. Sensitivity analysis showed that variations in LPTV service life have a significant impact on short-term estimations but less of an effect on long-term projections. Uncertainty analysis, using Monte Carlo simulations, confirmed the robustness of the estimations. The study also reveals an industrial succession pattern: e-waste generation is declining as certain products become obsolete, while ELPV generation is increasing before potentially leveling off with the rise of electric vehicles. LPTV waste generation is rapidly increasing, with high-speed trains, large and medium aircraft, and general aviation aircraft projected to dominate the LPTV industry in the coming decades.

The findings highlight the substantial recycling potential and environmental benefits associated with recovering valuable materials from waste LPTV in China. The projected increase in waste LPTV necessitates the development of efficient and sustainable recycling infrastructure and policies. The economic potential of recovering metals from waste LPTV underscores the importance of investing in recycling technologies and creating a circular economy for this sector. The observed industrial succession underscores the need for proactive planning to manage the growing volume of waste LPTV. While the study focuses on China, the methodology and findings provide valuable insights for other countries facing similar challenges in managing the end-of-life phase of LPTV. The significant environmental benefits of recycling LPTV, through reduced carbon emissions, align with global efforts to mitigate climate change. The substantial quantities of steel and aluminum in waste LPTV make these metals priority targets for recycling. This study provides a science-based foundation for informing policies related to resource management, environmental protection, and sustainable development within the transportation sector.

This study provides a comprehensive assessment of China's waste LPTV generation and recycling potential, highlighting the significant economic and environmental opportunities associated with recovering valuable metals. The findings underscore the need for investments in recycling infrastructure and policy development to manage this growing waste stream. The identification of an industrial succession pattern emphasizes the importance of proactive planning and the need for innovative recycling technologies, particularly for railway equipment, the major contributor to waste LPTV. Future research could focus on developing more refined models to account for technological advancements and policy changes, while further investigations into the complexities of LPTV dismantling and recycling technologies are warranted.

The accuracy of the estimations depends on the reliability of the input data. Data availability for certain LPTV types and their composition may be limited. The study employs several assumptions including constant metal content in LPTV over their lifespan and constant market prices for the analyzed metals. Furthermore, technological advancements and policy changes may influence the future generation and recycling rates of waste LPTV. The study uses average values for metal content and market prices, which may introduce uncertainty into the results. The future may see the introduction of new materials or manufacturing processes that could alter the composition and recycling potential of LPTV.