Research on the Current Status of Waste Mineral Oil Management and Resource Utilization in China’s Railway Industry: A Case Study of the Beijing Railway Bureau

Abstract

In order to study the generation, management, and disposal status of waste mineral oil in China’s railway transport industry, this article takes the Beijing Railway Bureau and its subordinate Tangshan Locomotive Depot as the research objects and comprehensively applies the survey, case study, and statistical analysis methods to analyze the source of the generation of railway waste mineral oil, the distribution of the disposal enterprises and locomotive depots, the management mode, and the economic and environmental benefits of recycling waste engine oil. The results show that waste oil mainly originates from locomotive overhaul and maintenance. There is significant regional imbalance in the generation and disposal capacity of railway waste oil. The Beijing Railway Bureau can achieve the timely disposal of waste mineral oil and reduce transport risks. Waste mineral oil management integrates generation, storage, transfer, and disposal. If cooperation is initiated with waste oil disposal enterprises, the use of recycled oil can save up to RMB 178,600/year and reduce carbon emission by 76.42 tCO₂/year for this locomotive depot. In view of the current situation, the railway industry should improve the management and disposal deficiencies and explore the new model of waste oil reduction, reuse, and recycling.

1. Introduction

According to statistics, the production of waste mineral oil in China increased from 6.24 million tons in 2013 to 7.317 million tons in 2018 [1]. In recent years, China has produced approximately 7.8 million tons of waste mineral oil annually [2], reaching 8.577 million tons in 2022 [3]. The railway industry is a vital component of the global transportation network, and the maintenance and overhaul of locomotives generate significant amounts of waste mineral oil [4]. With the increasing electrification of the railway system and the implementation of policies such as the ‘Supervision and Management Measures for the Phase-out and Upgrade of Old-Type Railway Diesel Locomotives’ and the ‘Action Plan for the Large-Scale Upgrade of Transportation Equipment’ [5], the number of electric locomotives in operation has been increasing annually, from 13,665 units in 2019 to 14,634 units in 2023. Meanwhile, the number of diesel locomotives in operation has decreased year by year, from 8048 units in 2019 to 7751 units in 2023 [6] (Figure 1). The total amount of waste mineral oil generated has also shown a decreasing trend. However, with the sustained growth of China’s economy, the acceleration of urbanization, and the continuous advancement of infrastructure development, the amount of waste mineral oil generated by the national railway system remains significant and should not be underestimated.

Waste mineral oil contains various toxic and hazardous substances, including heavy metals, polycyclic aromatic hydrocarbons (PAHs), benzene homologues, and halogenated hydrocarbons, etc. Leakage can lead to the contamination of water, soil, and the atmosphere, posing significant risks to human health and the ecological environment [7,8]. Waste mineral oil is classified as a category of HW08 Waste Mineral Oil and Mineral Oil Containing Wastes in the Directory of National Hazardous Wastes (Version 2025), and it has obvious ignitability and toxicity [9]. Its disposal is subject to various constraints, including regulatory, legal, and technical factors, making it difficult to recycle waste mineral oil resources.

The disposal of hazardous waste in China’s railway sector is divided into two models: the self-disposal model and the outsourced disposal model [10]. Currently, the disposal of waste mineral oil is primarily carried out by specialized entities with the necessary qualifications. However, a comprehensive management system has yet to be established, and the technological approaches remain relatively limited. Furthermore, the resource utilization of waste mineral oil is still a distant goal. However, developed countries have established relatively comprehensive systems, standards, and regulations for the collection, disposal, and management of waste mineral oil [11]. Extensive research and discussions have been conducted on the treatment and disposal technologies for waste mineral oil. Common disposal methods include disposal and landfilling, road oiling, incineration, and regeneration into fuel oil and base oil, among others [12,13].

In addition, China faces a significant shortage of petroleum resources, with an external dependency rate of 67.4% [14]. However, waste mineral oil holds substantial recycling potential, with only approximately 10–25% being degraded [15]. Research data indicate that the production of 1 L of mineral base oil requires 3–4 L of crude oil, whereas only about 1.6 L of waste mineral oil are needed [13,15]. Waste mineral oil can be regenerated into fuel oil, base oil, chemical raw materials, and so on by using scientific and reasonable technology, which is of great significance for saving resources and protecting the environment [16].

Given the current lack of research on the generation, disposal, and management of waste mineral oil in the railway transportation industry, the author selected the Beijing Railway Bureau and its subordinate Tangshan Locomotive Depot as the research objects. A combination of survey research, case study analysis, and statistical methods was employed to research, analyze, and summarize waste mineral oil in China’s railway industry. On this basis, the spatial and temporal distribution characteristics of waste mineral oil generation and disposal in the railway sector were analyzed. Additionally, the environmental and economic benefits of using recycled oil in the railway industry were examined.

2. Research Subjects and Methods

2.1. Overview of Research Subjects

Beijing Railway Bureau is one of the 18 Railway Bureau subsidiaries under the China State Railway Group Co., Ltd. (Beijing, China), with jurisdiction covering Beijing, Tianjin, and Hebei, the ‘two cities and one province’, as well as parts of Shandong, Henan, and Shanxi. It plays a pivotal role in the national railway network. The route of Beijing Railway Bureau is shown in Figure 2, with geographical coordinates ranging from 41°2′ to 34°7′ north latitude and 119°7′ to 113°08′ east longitude, situated in the temperate monsoon climate zone. There are 7 locomotive depots, 6 rolling stock depots and 6 high-speed train depots, 5 power supply depots, 15 public works depots, and 6 telecommunication depots and 6 communication depots. A total of 1678 locomotives are assigned to the company, with 630 being diesel locomotives and 1048 being electric locomotives [17].

Railway locomotive depots are responsible for the operation, maintenance, preparation, and rescue of locomotives, as well as the management of train drivers and high-speed trains. Tangshan Locomotive Depot has 82 diesel locomotives and 112 electric locomotives, which undertake the traction tasks of freight and some passenger trains between Fengtai West and Shanhaiguan of Beijing–Harbin line and Tianjin–Shanhaiguan of Jinshan line [17].

2.2. Research Methods

2.2.1. Survey Research Method

The survey research method involves directly obtaining relevant information by examining the actual situation of the subject, followed by the research and analysis of the collected data. To achieve a comprehensive understanding of the management and disposal practices of waste mineral oil within the railway sector, semi-structured interviews were conducted with the Beijing Railway Bureau and its affiliated locomotive depots [18]. The interviews encompassed the entire management chain, including the generation, classification, collection, temporary storage, transportation, and final disposal of waste mineral oil (see Supplementary Materials). Analysis of the interview transcripts enabled the identification of major deficiencies in the current management framework and provided insights into potential directions for optimization.

2.2.2. Case Study Method

To gain an in-depth understanding of the practical aspects of waste mineral oil management in the railway sector, this study selected the Tangshan Locomotive Depot, a subordinate unit of the Beijing Railway Bureau, as a representative case. The depot generates a relatively comprehensive range of waste mineral oil types, and its management procedures are considered representative within the system. Through on-site investigations and in-depth interviews, the study systematically analyzed the processes, operational standards, and supervisory mechanisms governing waste mineral oil from its generation to final disposal. This case study allowed for the identification of both common and depot-specific challenges in railway waste mineral oil management and, further, explored potential resource utilization pathways aimed at reducing operational costs in the railway industry [19].

2.2.3. Statistical Analysis Method

This study employed a comprehensive statistical analysis based on multiple data sources:

(1)

The number of enterprises qualified for HW08 (waste mineral oil) disposal was identified and verified using the official “List of Hazardous Waste Business License Enterprises” released by the environmental authorities of 31 provinces, autonomous regions, and municipalities in mainland China (excluding Hong Kong, Macao, and Taiwan). Their spatial distribution was further visualized using a Geographic Information System (ArcGIS 10.8) to reveal regional disparities in disposal capacity [20,21].

(2)

Information on the number and geographical distribution of locomotive depots nationwide was obtained from the official websites of railway bureaus and their annual reports.

(3)

The average annual generation of waste mineral oil at representative locomotive depots in Beijing was estimated through a combination of field investigations and publicly available data released by local environmental protection bureaus.

(4)

Price differentials between new diesel engine oil and re-refined oil were analyzed using procurement price lists of railway enterprises, industry reports, and the Lubricating Oil Price Network. Furthermore, based on the IPCC (2019) [22] carbon emission factors and relevant literature, the environmental and economic benefits of substituting re-refined oil for virgin oil were quantitatively assessed.

2.2.4. Data Sources

Table 1. Primary data sources for the study.

Data Category	Specific Data Items	Source
Case Data	Spectroscopic records of lubricating oil	Derived from field investigations at locomotive depots affiliated with the Beijing Railway Bureau
Macro Data	Number and spatial distribution of locomotive depots across China	Extracted from official websites of railway bureaus and their annual reports (detailed statistics are provided in the Supplementary Materials)
	Number and spatial distribution of licensed HW08 disposal enterprises	Compiled from publicly available lists released by provincial, regional, and municipal environmental protection authorities (detailed statistics are provided in the Supplementary Materials)
	Annual waste oil generation at Tangshan Locomotive Depot	Obtained from publicly released data of the Tangshan Municipal Bureau of Ecology and Environment
Market Data	Price of virgin diesel engine oil	Calculated as the average values from lubricating oil procurement price lists of railway enterprises (covering three major product types: multigrade Generation IV, multigrade Generation V, and monograde Generation V)
	Price of re-refined diesel engine oil	Estimated from the Lubricating Oil Price Network (Mysteel), indicating that re-refined oil is priced at approximately 75% of virgin oil
Environmental Data	Carbon emission factors of virgin and re-refined oils	Derived from the IPCC 2019 Guidelines for National Greenhouse Gas Inventories and related literature [22,23,24,25,26]; the emission factor for virgin oil was set at 3.5 tCO2/t, and that for re-refined oil at 1.1 tCO2/t

provides the main data sources for the study, including case data, macro data, market data, and environmental data.

3. Results and Analysis

3.1. Analysis of Waste Mineral Oil Generation in Railway

Through investigation and research, it was found that the generation of railway waste mineral oil involves multiple stages, with a complex variety and large quantities produced. The sources of railway waste mineral oil include waste mineral oil and mineral oil-containing waste generated during the transportation of mineral oils; waste mineral oil and oil sludge produced during the dismantling of internal combustion locomotives; waste oils generated during the cleaning of metal components; waste lubricants produced during the dismantling of locomotives, vehicles, and track maintenance machinery; oil, scum, and sludge generated from the treatment of oily wastewater (excluding sludge from wastewater biological treatment); waste hydraulic oils produced during the maintenance, replacement, and dismantling of hydraulic equipment and transformers; other waste mineral oils and oil-containing containers, packaging, and waste generated during maintenance, production, and use; and waste mineral oil and oil sludge produced during honing, grinding, and polishing processes.

As the core department of the railway transportation system, the locomotive depots are primarily responsible for the dispatch management and traction operation of diesel/electric locomotives. Additionally, they are in charge of the hierarchical overhaul and maintenance of locomotives, during which hazardous wastes containing mineral oil are produced. This process represents the main source of waste mineral oil within the industry. The main stages and categories of waste mineral oil generated at the locomotive depots are presented in

Table 2. Main generation processes and categories of waste mineral oil in locomotive depots (based on locomotive maintenance regulations and industry standards).

Sources of Waste Mineral Oil Generation in Locomotive Depots	Main Waste Categories	Basis for Waste Mineral Oil Generation	Reference Standards
Diesel locomotive maintenance	Waste engine oil	Oil changes and the identification of waste mineral oil are performed based on the maintenance protocols for different regions and locomotive models (DF-series locomotives for medium overhaul, minor repairs, and temporary repairs; ND-series locomotives for monthly, quarterly, semi-annual, annual overhauls, and temporary repairs; HX-series locomotives for C1–C3, C4, and C5 repairs), as well as the performance specifications of different lubricants.	Oil-change criterion of diesel locomotive engine oils (Q/CR 182-2014) Diesel locomotive hydraulic transmission oil change index(Q/CR 218-2014) Air compressor lubricating oil for railway locomotive (Q/CR 332-2014) Lubricating grease for railway locomotive wheelset rolling bearings (Q/CR 290-2022) Rolling Stock Special Lubricating Oil Silicone Oil for Brake Valve(Q/CR 761-2020) Rolling Stock Special Grease Traction Motor Bearing Grease (Q/CR 881-2022) Maintenance procedures for various types of electric locomotives, diesel locomotives, etc.
	Waste gear oil, etc.
Electric locomotive maintenance	Waste gear oil, etc.

Note: The waste mineral oil categories listed in this table represent the primary oil residues generated in locomotive depots, mainly during routine maintenance and major overhauls. The criteria for determining waste oils are primarily based on locomotive-specific maintenance procedures and physicochemical performance indicators of lubricants.

.

Waste mineral oil from locomotive depots mainly refers to waste oil residues from the oil sumps of internal combustion locomotive lubrication systems, as well as various types of waste oil generated during maintenance, cleaning, and other operations of locomotive equipment. Its components include oil–water mixtures formed by mixing with sewage, sludge, or other impurities, as well as high-molecular-weight oils containing highly toxic substances (such as transformer oil and other insulating oils containing polychlorinated biphenyls, PCBs) [10]. Waste oil is mainly the expired lubricating oil discharged from the diesel engine lubrication system during regular maintenance or major and medium-sized repairs of diesel locomotives because the physical and chemical properties such as viscosity, acid value, and alkali value of the oil do not meet the requirements for continued use. (based on the Oil-change criterion of diesel locomotive engine oils (Q/CR 182-2014) [27]).

According to the spectral analysis records of the regular testing of the internal combustion engine lubricating oil in use obtained from the research, as shown in

Table 3. Spectral analysis data of in-service lubricating oil from diesel locomotives (selected samples).

Locomotive Type	Cu	Al	Cr	Ni	Si	Na	Sn	Pb
HXN3B	2.936	2.7	0.4	0	4.3	2.2	16.5	11.1
HXN3B	0.005	2.0	0.2	2.9	14.9	2.6	5.6	0.1
HXN3B	0.005	0.5	0.5	4.6	13.8	5.9	4.5	0
HXN3B	0.005	0.3	0.3	0	0	6.9	3.7	0
HXN3	6.646	1.9	0.7	2.5	5.4	38.0	11.4	14.4
HXN3	0.005	4.0	0.3	1.6	13.0	4.8	11.0	4.3
HXN3	2.694	0	0.5	0.1	0	15.8	13.4	11.8

Note: All values are expressed in ppm. The data were obtained from emission spectrometric analysis of periodic oil sampling in locomotive depots. The measured elements include metallic wear particles (Cu, Al, Cr, Ni, Sn, and Pb) and contaminants (Si and Na), which indicate engine wear conditions and lubricant degradation. When the concentrations exceed normal thresholds, the lubricants must be replaced promptly to prevent potential equipment failures.

, it can be found that the internal combustion engine lubricating oil produces metal abrasive debris in use, which is found to contain copper, aluminum, nickel, tin, silicon, sodium, and other metal elements. In addition to this, it also contains carbon particles and oxidation products.

The locomotive depot uses spectroscopy analysis to establish oil change criteria. By comparing and analyzing laboratory test results, it determines whether the content of each metal element in various types of engine oil falls within the normal range. If the content reaches an abnormal range, the lubricating oil must be replaced promptly. Additionally, the viscosity, flash point, and moisture content of the engine oil in use are tested to ensure they meet the specified standards (Diesel engine oils for railway diesel locomotives (TB/T 2956-2009) [28]). If they do not meet the requirements, they must be replaced promptly.

The main supply areas for waste mineral oil from railways are concentrated in the four major economic regions of East China, North China, South China, and Central China, which have dense transportation networks. These four regions together account for more than 80% of the total supply of waste mineral oil [29]. According to the distribution map of national railway locomotive depots and HW08 hazardous waste disposal and reuse enterprises (Figure 3), it can be seen that the railway locomotive depots are usually concentrated along major railway lines (such as in economically active regions like North China, East China, and South China); their maintenance and overhaul activities will generate a large amount of waste mineral oil. The weight of waste mineral oil produced in these regions is significantly higher than in less developed central and western regions, and accordingly, waste mineral oil treatment and utilization enterprises are more densely distributed in these areas, with stronger treatment capacities. However, economically and transportation-wise underdeveloped regions in the southwest and northwest have relatively insufficient disposal capabilities and need to rely on cross-regional transportation and processing, which increases costs and environmental risks.

There is a significant regional imbalance in the generation and disposal capacity of railway waste mineral oil. East China has a better match between supply and demand, while central and western China need to strengthen infrastructure construction and policy support.

As of December 2024, according to the list of hazardous waste enterprises given by the Ecology and Environment Bureaus of each province, the query found that there are 60 waste mineral oil disposal and reuse enterprises in Hebei Province, 14 in Tianjin, and 6 in Beijing (detailed statistics are provided in the Supplementary Materials). As a traditional heavy industrial province, Hebei Province generates large amounts of waste mineral oil during production, resulting in high demand for disposal and utilization. Additionally, its location in the heart of the Beijing–Tianjin–Hebei region has established it as a regional disposal hub. Tianjin’s industrial structure is becoming more high-end, with relatively limited waste mineral oil production and strict environmental standards. Companies need to undergo costly technological upgrades to meet standards, which has suppressed the number of new enterprises. As the capital city, Beijing has the most stringent environmental protection requirements, restricting the establishment of high-polluting enterprises and retaining only a small number of large-scale, technologically advanced disposal enterprises.

As can be seen from the distribution of waste mineral oil disposal and reuse enterprises in the locomotive depots and jurisdictions under the Beijing Railway Bureau (Figure 4), there are seven locomotive depots (Huairou North, Beijing, Fengtai, Tianjin, Tangshan, Shijiazhuang Electric Power, and Handan locomotive depots) covering the Beijing–Tianjin–Hebei urban agglomeration and major railway trunk lines, forming a railway network centered on Beijing and radiating across North China. The distribution of locomotive depots is closely related to regional economic activities and railway transportation demand. The Beijing, Fengtai, Tianjin, and Shijiazhuang Electric Power locomotive depots are located near waste mineral oil disposal companies, with short transport distances, low transport costs, minimal transport-related carbon emissions, and lower transport risks, enabling the efficient disposal of the generated waste mineral oil. However, Huairou North, Handan, and Tangshan locomotive depots are far from disposal enterprises, requiring longer transport distances. Some transport needs to be transferred across provincial borders, which increases transport risks. At the same time, the carbon emissions required for transport are relatively higher. Waste mineral oil may face potential risks such as untimely disposal and environmental pollution.

3.2. Current Status of Management and Disposal of Waste Mineral Oil in China’s Railway

The whole process of the identification, storage, transfer, disposal, and management of waste mineral oil-based hazardous waste in the railway industry must be in accordance with relevant laws, regulations, standards, norms, and management practices.

Table 4. Relevant content to be complied with in the whole process management of waste mineral oil hazardous wastes.

Serial Number	Implementation Time	Name	Main Content
1	2011	Technical Specifications for Pollution Control of Used Mineral Oil Recovery, Recycle and Reuse	Regulate the collection, transportation, storage, utilisation, and disposal of waste mineral oil to prevent environmental pollution caused by waste mineral oil. This includes specifying pollution control technologies and environmental management requirements for the collection, transportation, storage, utilisation, and disposal of waste mineral oil.
2	2015	Standardized Management Indicator System for Hazardous Waste	Including the status of hazardous waste identification markings, the formulation of management plans, the implementation of management systems such as reporting and registration, transfer manifests, operating permits, and emergency response plan filings, as well as whether storage, utilisation, and disposal comply with relevant standards and specifications.
3	2016	Measures for the Administration of Permit for Operation of Dangerous Wastes (2016 Revision)	Establishes a tiered approval and categorized management licensing system; requires entities engaged in collection, storage, or disposal to obtain permits; defines application conditions, approval procedures, license content, modification and supervision requirements; imposes strict legal liabilities; prohibits unlicensed operations and illegal transfers.
4	2016	Industry standard conditions for comprehensive utilization of waste mineral oil	Mandates that waste mineral oil recycling enterprises avoid environmentally sensitive areas; sets minimum annual disposal capacity of 30,000 tons for new/renovation projects; requires mandatory environmental protection facilities; limits energy consumption to ≤900 kWh per ton of base oil; mandates possession of a hazardous waste operation license.
5	2020	Standard for pollution control on the hazardous waste landfill	Specifies waste acceptance criteria for hazardous waste landfills and environmental protection requirements for site selection, design, construction, operation, closure, and monitoring.
6	2020	Technical specifications on identification for hazardous waste	Defines technical requirements for sampling, testing, and result determination in hazardous waste characteristic identification.
7	2020	Law of the People’s Republic of China on the Prevention and Control of Environmental Pollution by Solid Wastes (2020 Revision)	Imposes mandatory requirements for the collection, storage, utilization, and disposal of hazardous waste.
8	2021	Pollution Control Standard for Hazardous Waste Incineration	Specifies environmental protection requirements for site selection, operation, monitoring, waste storage, and incineration processes of hazardous waste incineration facilities; includes implementation and supervision provisions.
9	2022	Measures for the Transfer of Hazardous Wastes	Establishes a full-process supervision system for hazardous waste transfers, covering transfer manifests, inter-provincial approvals, stakeholder responsibilities, and risk control requirements.
10	2022	Technical Guideline for Deriving Hazardous Waste Management Plans and Records	Guides hazardous waste generators in formulating management plans, establishing record systems, reporting waste-related data, and enhancing standardized environmental management.
11	2023	Standard for pollution control on hazardous waste storage	Specifies the general requirements, site selection, and pollution control requirements for storage facilities, pollution control requirements for containers and packaging, pollution control requirements for storage process, and environmental management requirements such as pollutant emission, environmental monitoring, environmental emergency planning, implementation, and supervision in terms of the pollution control of hazardous waste storage.
12	2023	Treatment and Disposal Methods for used Mineral Oil Lubricant	Specifies the process routes, method summaries, treatment and disposal methods, finished product control, and environmental protection requirements for the treatment and disposal of used mineral oil lubricant. It applies to the waste category ‘HW08 Waste Mineral Oil and Mineral Oil-Containing Waste’ listed in the National Directory of Hazardous Wastes (2025 Version)
13	2023	Notice by the General Office of the Ministry of Ecology and Environment of Further Strengthening the Standardized Environmental Management of Hazardous Wastes	Enhances full-process informatized supervision of hazardous waste and deepens standardized risk assessment for environmental risk prevention.
14	2024	Guidelines for recycling of used mineral oil	Specifies the general requirements for the recovery and recycling of waste mineral oil, recovery management requirements, recycling process requirements, environmental protection requirements, and recycling product management requirements.
15	2025	National Directory of Hazardous Wastes (2025 Version)	Classifies hazardous wastes by source, waste code, and hazardous characteristics.

lists the specific names and contents in detail.

Research has found that the management of waste mineral oil from railways is currently limited to collection and storage, with transportation and treatment mostly outsourced to specialized companies with the appropriate qualifications. There is no comprehensive process system in place, and treatment methods are limited. The railway industry also incurs outsourcing costs annually, which increase operational expenses. Establishing an efficient management process for railway waste mineral oil, with specific personnel assigned to each stage of the process, can effectively control environmental risks and prevent environmental violations. This approach enhances the railway industry’s capacity for reducing and recycling hazardous waste, thereby significantly lowering disposal costs.

To ensure the standardization and normalization of governance work, the railway industry needs to establish a more comprehensive workflow for the management of waste mineral oil, enabling full traceability throughout the entire process. Figure 5 shows a more standardized workflow proposed based on the current state of railway management.

All aspects of waste mineral oil are managed by designated personnel from the responsible workshop or department, relying on the coordination of multiple departments (technical department, preparation workshop, materials department, and equipment department). The laboratory conducts the testing and analysis of locomotive diesel engine oil and generates a test report. After the technical department’s dedicated engineer issues an oil change instruction based on the test data, the dedicated administrator in the maintenance workshop carries out the oil change operation in accordance with the instruction, simultaneously recording the amount of oil changed and transferring the waste oil to the hazardous waste temporary storage facility. The materials department verifies and registers the information based on the incoming inventory records, coordinates with the disposal unit, and submits a transfer application (including the disposal unit’s qualifications, transfer weight, and route) to the dedicated engineer of the equipment department. After approval by the equipment department, the application is submitted through the solid waste management system to generate an electronic quadruplicate form, followed by compliant transfer and disposal. The disposal unit is responsible for the final transportation and recycling treatment (such as reuse or harmless disposal), while also establishing records to ensure traceability throughout the entire lifecycle.

The railway system is vast and involves many links, and there is a lack of statistical data on the relevant links. At the same time, after railway waste oil is handed over to disposal units, there is a lack of statistics and tracking information on its specific flow, making it difficult to investigate the disposal methods of railway waste oil after it is discarded and to statistically analyze the specific material flow of railway mineral oil throughout its entire life cycle.

Through research and literature review [30,31,32], the flow process of mineral oil in railways (Figure 6) can be summarized into three main stages: production, circulation, and disposal after waste. The largest source of mineral oil used in railways is petroleum refining enterprises and professional lubricant manufacturers, and it is also used in the maintenance and cleaning of locomotive equipment. The waste mineral oil generated is collected and temporarily stored, then processed and produced by qualified companies. However, due to significant differences in processing technology, the quality varies greatly, and the quantity is difficult to estimate. In some remote areas where regulatory and enforcement efforts are inadequate, waste mineral oil may be sold to unqualified companies, resulting in counterfeit and substandard non-compliant products entering the market. The compliant disposal of waste mineral oil is achieved through disposal and recycling companies, which generate energy value and recycled products that meet the relevant standards before entering the market. The specific disposal method should be selected based on reasonable consideration of indicators such as the oil content, viscosity, pour point (freezing point), flash point, and color of the waste mineral oil. According to the research of Liang Yangyang et al. [16], the mainstream process in China’s waste mineral oil recycling industry is mainly atmospheric and vacuum distillation combined with solvent refining. Hydrogenation refining, molecular distillation, and thin-film evaporation technologies are also used to some extent. In addition, traditional methods such as flocculation, acid–base treatment, and white clay adsorption, as well as end-of-pipe incineration disposal methods, are still used.

Environmental pollution throughout the mineral oil flow process originates from two sources: First, during the maintenance and repair of railway locomotives and their use, mineral oil enters the environment through evaporation, permeation, and leakage; second, during the processing of waste mineral oil by unqualified enterprises, waste oil enters the environment in the form of wastewater, waste gas, and solid waste, and the resulting environmental pollution issues cannot be underestimated [33].

3.3. Economic and Environmental Benefits of Recycling Waste Mineral Oil from Railways

Used engine oil is a type of waste mineral oil. Due to its high recycling value, the locomotive depot collects waste lubricating oil generated during locomotive inspections into dedicated containers and hands it over to companies with the appropriate hazardous waste disposal qualifications for processing. According to publicly available data from the Tangshan Municipal Ecology and Environment Bureau and the summary of survey data, the generation weight of HW08 category hazardous waste at the Tangshan Locomotive Depot from 2021 to 2024 is as shown in Figure 7 below.

Data analysis indicates that the weight of waste mineral oil generated by the Tangshan Locomotive Depot shows significant fluctuations, with a peak observed in 2024. The generation of waste mineral oil is affected by factors such as the type and number of locomotives in service, operational scale, equipment condition, maintenance cycles, and mileage, resulting in irregular production patterns. Field surveys and online public data reveal that most locomotive depots do not disclose annual waste mineral oil generation, making it difficult to track both the quantities and the flow of this hazardous waste, which creates substantial management risks. The number of locomotive depots managed by each railway bureau varies, and incomplete statistical data prevent accurate calculation of the total waste engine oil output across the national railway system. Nevertheless, one locomotive depot under the Beijing Railway Bureau alone produces nearly 40 tons of waste engine oil and 350 tons of waste mineral oil and related residues annually. Extrapolating to all 18 railway bureaus and 69 locomotive depots nationwide, the cumulative volume of waste engine oil is substantial. If systematically recycled and reused, this waste oil could deliver significant environmental and economic benefits, including cost reduction and efficiency gains for China’s railway sector. In the case of the Tangshan Locomotive Depot, cooperation with local recycling enterprises—under the assumption that all waste engine oil is collected, refined into base oil, and subsequently reused within the railway system—would further enhance these benefits. Large-scale recycling facilities in China typically achieve a base oil yield of 65–80%, depending on feedstock quality and processing technology. Most enterprises employ atmospheric–vacuum distillation in combination with solvent refining, yielding an average base oil recovery rate of approximately 70% [34,35,36].

The economic and environmental benefits of recycling can be estimated by the following equations:

$${\mathrm{Q}}_{\mathrm{r}}={\mathrm{Q}}_{\mathrm{w}}\times \mathsf{\eta }$$

(1)

$$\mathrm{S}={\mathrm{Q}}_{\mathrm{r}}\times ({\mathrm{P}}_{\mathrm{n}}-{\mathrm{P}}_{\mathrm{r}})$$

(2)

$${\mathrm{C}}_{\mathrm{r}\mathrm{e}\mathrm{d}}={\mathrm{Q}}_{\mathrm{r}}\times (\mathrm{E}{\mathrm{F}}_{\mathrm{n}}-\mathrm{E}{\mathrm{F}}_{\mathrm{r}})$$

(3)

• ${\mathrm{Q}}_{\mathrm{r}}$ = Recycled oil production (t)
• ${\mathrm{Q}}_{\mathrm{w}}$ = Waste oil generation (t)
• $\mathsf{\eta }$ = Base oil yield rate (%)
• $\mathrm{S}$ = Cost savings (yuan)
• ${\mathrm{P}}_{\mathrm{n}}$ = Price of new oil (yuan/t)
• ${\mathrm{P}}_{\mathrm{r}}$ = Price of recycled oil (yuan/t)
• C_red = Carbon emission reduction (tCO₂)
• $\mathrm{E}{\mathrm{F}}_{\mathrm{n}}$ = Carbon emission coefficient of new oil (tCO₂/t)
• $\mathrm{E}{\mathrm{F}}_{\mathrm{r}}$ = Carbon emission coefficient of recycled oil (tCO₂/t)

Based on Equations (1)–(3), the estimated economic and environmental benefits for Tangshan Locomotive Depot are summarized in

Table 5. Estimated economic and environmental benefits of waste engine oil recycling for Tangshan Locomotive Depot.

Item	2021	2022	2023	2024
${\mathrm{Q}}_{\mathrm{w}}$ (t)	19.10	13.24	41.62	45.49
${\mathrm{P}}_{\mathrm{n}}$ (yuan/t)	22,788	22,433	22,433	22,433
${\mathrm{P}}_{\mathrm{r}}$ (yuan/t)	17,091	16,824	16,824	16,824
$\mathrm{S}$ (yuan)	76,200	52,000	163,400	178,600
${\mathrm{C}}_{\mathrm{r}\mathrm{e}\mathrm{d}}$ (tCO2)	32.09	22.24	69.92	76.42

Note: The base oil yield ($\mathsf{\eta }$) is assumed to be 70%. The price of recycled oil is considered 75% of the price of new oil. The carbon emission coefficient of new oil ($\mathrm{E}{\mathrm{F}}_{\mathrm{n}}$) is 3.5 tCO₂/t, and that of recycled oil ($\mathrm{E}{\mathrm{F}}_{\mathrm{r}}$) is 1.1 tCO₂/t. Results are rounded to two decimal places.

.

Calculations show that from 2021 to 2024, using recycled motor oil will save RMB 76,200, RMB 52,000, RMB 163,400, and RMB 178,600, respectively, and reduce carbon emissions by 32.09 tCO₂, 22.24 tCO₂, 69.92 tCO₂, and 76.42 tCO₂, respectively.

4. Recommendations for the Management of Waste Mineral Oil and Future Improvement Measures

4.1. Upgrading Disposal Technology and Standardizing Products

According to life cycle assessment studies, recycling reduces environmental impact compared to producing straight-run base oil from crude oil [26] In view of the current situation regarding the disposal of waste mineral oil in China, enterprises should be required to phase out production processes that are environmentally unfriendly, technologically outdated, or prohibited by the state, and instead adopt safer and more efficient processes and technologies to ensure the quality of recycled oil products. At the same time, it is recommended to expedite the establishment of a comprehensive standardization system for the recycling and reuse of waste mineral oil, establishing uniform and stringent quality standards and environmental emission standards for recycled oil products to ensure they meet various application standards. This will enhance market recognition and expand the scope of application, while reducing the difficulty of environmental regulation and promoting the healthy and orderly development of the industry.

In the treatment of waste mineral oil, it is necessary to comprehensively consider various factors such as changes in lubricant market demand, energy consumption intensity, the layout of treatment facilities, the control of operating costs, quality standards for recycled oil products, and technological development trends. Through systematic assessment, a dynamic balance between economic feasibility and environmental benefits can be achieved [34]. Additionally, mixing waste oils with different characteristics reduces the market value of finished base oil products, which is a critical aspect of waste oil regeneration. The management of railway waste oil should involve the classification, recovery, and disposal of oil products based on their type and characteristics to avoid mixed contamination.

4.2. Closed-Loop Management Throughout the Entire Life Cycle and Quantification of Carbon Emission Reduction Value

The railway sector should establish a closed-loop management system encompassing the entire process of “generation–collection–transportation–regeneration–reuse” of waste mineral oil [37], leveraging Internet of Things (IoT) devices and RFID tagging to enable real-time tracking and monitoring at every stage, thereby preventing unauthorized disposal and loss. The railway group should develop a unified digital management platform to dynamically display the collection, transportation, and regeneration status of waste oil across all regional bureaus, facilitating centralized oversight of the industry. In parallel, a railway-specific carbon accounting model should be developed to quantify the emission reduction benefits achieved by substituting regenerated oil for virgin oil. Verified by third-party certification and integrated into carbon trading mechanisms, this approach can enhance the railway sector’s green transition and expand opportunities for carbon asset development.

4.3. Economic Incentive Mechanisms and Digital Regulatory Platform Construction

To promote the standardized collection and utilization of waste mineral oil within the railway system, economic incentive mechanisms should be implemented, including tax benefits, subsidies, or rewards for compliant disposal enterprises and units actively using regenerated oil. Reasonable usage ratios and limits should be established to ensure operational safety. The railway group may establish a dedicated fund to provide volume-based subsidies to enterprises, thereby creating a stable market mechanism. For regulatory purposes, a digital hazardous waste management sub-platform should be integrated into the existing railway operation management system to monitor key indicators in real time, including collection rate, disposal rate, regeneration rate, and carbon reduction. Data should be synchronized with the national solid waste management platform to ensure transparency and compliance and prevent information silos, thereby supporting external supervision and research analyses.

5. Conclusions

This study systematically investigates the generation, management, and resource utilization of waste mineral oil in China’s railway sector, providing insights for both operational practice and policy development. The key conclusions are as follows:

(1) Characterization of waste mineral oil generation and management: Waste mineral oil is primarily produced during locomotive maintenance and repair in depot operations, containing significant metallic and chemical contaminants. Disposal is strictly regulated through hazardous waste transfer manifests and handled by qualified enterprises. The management practices across depots are adapted to local operational conditions, ensuring compliance while incurring additional costs and requiring rigorous storage and oversight.

(2) Spatial alignment and risk assessment: The distribution of HW08-class hazardous waste disposal and recycling enterprises generally aligns with waste mineral oil generation sources in major railway networks, facilitating timely disposal and reducing transportation and environmental risks. However, regions with sparse railway coverage and limited disposal capacity face elevated environmental and operational risks, highlighting the need for strengthened supervision to prevent environmental harm and the circulation of substandard recycled oil products.

(3) Economic and environmental benefits of coordinated recycling: Collaboration between railway units and recycling enterprises via “transportation–disposal” agreements allows the preferential supply of recycled oil at discounted rates. The study demonstrates that increased waste oil generation corresponds to higher economic savings and carbon reduction, with a maximum annual savings of RMB 178,600, with carbon emissions reduced by 76.42 tCO₂ per year.

This research integrates operational data, spatial distribution analysis, and environmental–economic evaluation to provide a comprehensive framework for waste mineral oil management in the railway sector. The study proposes feasible measures for optimizing waste mineral oil disposal and advancing “closed-loop management,” “carbon emission reduction quantification and value transformation,” and “digital regulation.” These findings offer actionable guidance for railway bureaus, locomotive depot, and recycling enterprises, contributing to safer, greener, and more efficient hazardous waste management in the railway industry.

6. Limitations and Future Work

This study provides valuable case-based insights into waste mineral oil management and its environmental benefits within the railway system; however, several limitations should be acknowledged.

First, data representativeness is limited. The analysis primarily relies on a Tangshan case study, supplemented by partial disclosures from selected railway bureaus. Given the heterogeneity across all 18 railway bureaus in locomotive types, operational intensity, maintenance frequency, and waste oil management practices, extrapolations to the national level may introduce uncertainty. Second, the temporal scope is narrow. The dataset covers mainly the past three to five years and may not fully reflect long-term trends or policy impacts, such as equipment upgrades, clean energy substitution, and the implementation of carbon reduction measures. Third, methodological simplifications were applied. Carbon emission factors were adopted from the 2019 Refinement to the IPCC Guidelines for National Greenhouse Gas Inventories and related LCA literature. Specifically, 3.5 tCO₂/t was assigned to virgin base oil and 1.1 tCO₂/t to re-refined base oil. Although not railway-specific, these values represent internationally recognized averages for petroleum refining and re-refining processes. Their use ensures methodological consistency and facilitates global comparability, but deviations from actual railway conditions in China remain possible. In addition, parameters such as base oil yield and the market price of re-refined oil were assumed, introducing further uncertainty.

Future research should address two limitations by (i) strengthening nationwide data collection, particularly through systematic field measurements across all 18 railway bureaus, to improve accuracy and representativeness; and (ii) developing localized carbon emission factors tailored to railway operations, thereby providing a more precise foundation for environmental accounting. The refinement of these parameters based on empirical evidence will enhance both the scientific robustness and policy relevance of future studies.