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

Technological progress has produced a variety of products for human needs, enriching human life. Large-scale public transport vehicles (LPTV) include railways, aviation, and associated smart equipment. China has recently formulated plans and policies to promote the LPTV industry, aiming to upgrade high-tech manufacturing and revitalize equipment manufacturing. Rail and aviation equipment—railway locomotives (RL), railway passenger cars (RPC), railway wagons (RW), high-speed trains (HST), large and medium aircraft (LMA), and general aviation aircraft (GAA)—are the backbone of the modern transport system. With rapid LPTV development and infrastructure improvements, increasing metal resources are embedded in LPTV, raising concerns about resource scarcity. At end-of-life (EoL), waste LPTV presents both environmental risks and recycling opportunities, containing valuable metals such as steel, aluminum, titanium, and neodymium, forming significant anthropogenic mineral stocks. Estimating waste generation and recycling potential is fundamental but existing research has focused on e-waste (WEEE), end-of-life private vehicles (ELPV), and plastics, leaving LPTV understudied. This study quantitatively assesses the recycling potential and economic benefits of LPTV in China, and explores possible industry succession patterns by examining waste generation trends across WEEE, ELPV, and LPTV. Appropriate models are selected based on data availability and robustness. Future possession amounts are forecast with a logistic model; the possession coefficient method estimates waste generation and is validated against the market supply A method.

Prior work has extensively estimated generation and recycling potentials for conventional waste streams such as WEEE, ELPV, and plastics, including methods development (e.g., market supply approaches, stock-based models, IO analyses) and criticality/economic assessments of embedded metals. However, waste LPTV has received little attention despite rapid growth of the sector and significant embedded metal stocks. The paper situates LPTV within industrial ecology and succession paradigms, noting booms in consumer electronics around 2005 and vehicles around 2010, with LPTV industrialization advancing since ~2015. The authors review estimation methods (basic market supply, possession coefficient, stock-based, market supply A, Stanford method, consumption/use approach, MFA, time-series) and data needs (production, possession, mass composition), identifying a gap in applying and validating such methods for LPTV in China. They also draw on literature regarding metal criticality, recyclability, and environmental benefits of urban mining to motivate material prioritization (Fe, Al, Ti, Nd).

The study estimates China’s waste LPTV generation (railway and aviation) from 2000 to 2050 and assesses metal recycling and environmental benefits via the following steps: - Data collection: Historical production, possession, and mass composition data for LPTV categories (RL, RPC, RW, HST, LMA, GAA) compiled from national yearbooks and literature; parameters for lifespan distributions (Weibull), metal content and prices, and carbon footprints of primary vs. secondary production. - Forecasting possession amounts: A logistic regression model is fitted to past possession data to project future stocks under national policy targets (transportation power by 2035–2050). Logistic form: P(t) = Pmax / [1 + a·exp(−b(t − t0))], where Pmax is saturation possession, and a, b are growth parameters. - Waste generation estimation and validation: Two approaches are used. (1) Market supply A (distribution delay) method: obsolescence is computed by convolving historical sales/production with a Weibull lifespan distribution characterized by shape (β) and scale (η) parameters; f(x) and F(x) capture failure density and cumulative distribution. (2) Possession coefficient method (adopted for main estimates): assumes a fraction (C, set to 60% based on Weibull and literature) of the possession stock in year t will reach peak obsolescence within interval (a, b), distributing Q_{t+b} = C·P(t)/(b−a+1). The two methods are compared for RL, RPC, RW to validate feasibility (average difference ~15%). - Metal content and economic potential: For each year and LPTV type, metal mass Vi is computed as the product of waste mass Wj(z) and average metal content ej for key metals (Fe, Al, Ti, Nd). Economic potential is estimated by multiplying metal masses by average market prices over given periods and summing across metals (Fe and Al dominate value shares). - Environmental benefits: Carbon reduction potential is estimated by applying differential carbon footprints (primary minus secondary production) to the recoverable metal masses, aggregating to total CO2-equivalent savings by year. - Sensitivity and uncertainty: Sensitivity analysis varies service life by ±1 year to quantify short- and long-term impacts on waste generation. Uncertainty analysis applies Monte Carlo simulation (10^5 iterations) to mass, content, and related parameters (e.g., Fe and Nd stocks), reporting distributions (e.g., BetaPERT) for 2020 and 2050 totals to validate robustness of projections.

- Method validation: For 2020 obsolescence amounts of railway equipment, the possession coefficient method vs. market supply A method yielded RL: 759 vs. 664 units, RPC: 1992 vs. 2180 units, RW: 39,539 vs. 32,092 units; average difference ~15%, indicating no significant discrepancy and supporting use of the possession coefficient method. - Railway waste generation: Obsolescence mass grew from ~0.5 Mt in 2000 to 33.2 Mt in 2020, projected 73.6 Mt by 2050. RW dominates by mass, contributing ~92.8–93.7%; RPC contributes ~4.1–6%, RL ~0.2–0.6%; HST is lowest by mass. - Aviation waste generation: Average obsolescence mass was 1.9 kt in 2015, 4.2 kt in 2020, and is projected to reach 61.5 kt in 2050, with average annual growth of ~1.7 kt. LMA:GAA mass ratio remains ~9:1. - Total LPTV waste: Total obsolescence mass rises from ~0.5 Mt (2000) to 33.2 Mt (2030) and 73.7 Mt (2050), an average annual growth rate of ~10.6%, with railway equipment accounting for >99% of mass. - Economic linkage: Scrap per capita increased by ~12 kg from 2010 to 2020 with strong correlation to GDP per capita (R^2 = 0.98), indicating economic growth drives LPTV deployment and subsequent waste flows. - International benchmarking: Projected 2050 per capita possessions in China—RL 2.5 kg, RPC 91.6 kg, RW 635.3 kg, HST 65.5 kg—are close to 2019 levels in developed countries (Japan, Germany, UK), supporting plausibility of forecasts. - Metal recycling potential: Encapsulated metals in waste LPTV increased from 2020 to 2050: Fe 32.5 Mt → 71.9 Mt; Al 350.5 kt → 838.2 kt; Ti 172.9 t → 2538.7 t; Nd 2.4 t → 223.1 t (rapid Nd growth linked to HST development). Fe and Al together constitute ~99% of economic value. - Economic value: Aggregate economic potential of Fe, Al, Ti, Nd was ~0.2 billion (2000), 14.8 billion (2020), and projected 32.9 billion (2050) (currency as implied in source; Fe and Al dominate shares). - Carbon reduction: Proper recovery by 2050 could avoid approximately: Fe 76.2 Mt CO2, Al 4.4 Mt CO2, Ti 2.1 kt CO2, Nd 1.6 kt CO2; total average reduction ~80.6 Mt CO2. - Sensitivity and uncertainty: A ±1-year shift in service life changes 2010 waste generation by −16.8%/+17.4%, but deviations after 2015 are within ~±4.3%, indicating long-term robustness. Monte Carlo results for 2020 and 2050 total waste masses and Fe/Nd stocks align well with deterministic forecasts, validating accuracy.

The study addresses the core questions of how much waste LPTV China will generate and what recycling potential it represents by integrating validated estimation methods and forecasting possession trajectories. Findings reveal a rapid rise in LPTV waste—dominated by railway equipment—creating substantial opportunities for resource recovery, economic value, and carbon mitigation. The strong GDP–scrap linkage underscores macroeconomic drivers of LPTV deployment and subsequent waste flows. The metal composition analysis highlights Fe and Al as priority recycling targets by both mass and value, while Nd’s accelerating growth reflects the expansion of HST and points to strategic importance for critical materials management. Placing LPTV dynamics within an industrial ecology succession framework, the paper shows that while many WEEE categories have plateaued or are declining in growth, and ELPV streams are transitioning with EV rises, LPTV—particularly HST, LMA, and GAA—will continue to expand and become leading waste streams by 2050. This suggests an evolving focus for circular economy policies and infrastructure, shifting toward large-scale transport assets. However, practical constraints (complex dismantling, toxic components, limited dismantling capacity, immature management systems) currently hinder effective LPTV recycling. The results imply that targeted policy, technology development, and capacity building—especially for railway equipment—are essential to realize the identified circularity and climate benefits.

This work provides the first comprehensive quantification of China’s waste LPTV generation (2000–2050), validating estimation methods and projecting substantial recycling potential. By 2050, waste LPTV will encapsulate roughly 71.9 Mt Fe, 838.2 kt Al, 2.54 kt Ti, and 223.1 t Nd, yielding significant economic value and enabling an estimated ~80.6 Mt CO2 mitigation if properly recycled. Railway equipment contributes >99% of the waste mass, making it the primary focus for circular strategies. The analysis situates LPTV within broader industrial succession trends, indicating future prominence of HST, LMA, and GAA in waste flows. The study suggests policy and practical directions: prioritize development and deployment of advanced dismantling and recovery technologies, expand licensed dismantling capacity, and establish a national waste LPTV resource information platform to coordinate stakeholders and accelerate technology transfer. Future research should refine metal content heterogeneity across LPTV types, track real-world dismantling yields and losses, integrate dynamic price and policy scenarios, and extend environmental assessments beyond CO2 to include energy, pollutants, and toxicity impacts.

- Data and model assumptions: Future possession is forecast via logistic models calibrated to historical data and policy targets, which may not capture unforeseen technological or policy shifts. The possession coefficient method assumes a cumulative obsolescence ratio C = 60% and a fixed peak interval (a, b); deviations in actual lifetimes could alter results. - Lifespan and content uncertainty: Lifespan distributions are modeled with Weibull parameters from literature; metal contents use average values due to limited type-specific data. Real-world heterogeneity and technology changes (e.g., material substitutions) may affect metal estimates. - Recovery and losses: The analysis assumes negligible losses of metals during service life and does not explicitly model dismantling, pre-processing, and recycling process yields; actual recoverable quantities may be lower. - Scope: Environmental benefits focus on CO2 reductions for selected metals; broader environmental impacts (e.g., energy use, other emissions, toxicity) and economic variability (price volatility) are not fully explored. Current industrial capacity constraints and informal sector dynamics may limit near-term applicability of projections.