A New Assessment Method for the Redevelopment of Closed Coal Mine—A Case Study in Shanxi Province in China

Abstract

It is recognized that a closed mine is a three-dimensional resource that has great potential for redevelopment and/or reutilization. The first and most important step for the redevelopment of a closed coal mine is to perform an evaluation and to select the best strategy/approach for each closed coal mine. Currently, there is no standard assessment method, and different countries use different assessment methods of evaluation. In this study, a five-factor weight analysis method was developed and used for a closed coal mine assessment in Shanxi, China. The case study using this method provides the useful information for the redevelopment of the closed coal mine. The results suggested that the best and or most suitable strategy for the mine is the extraction of coalbed methane (CBM) in the goaf, mine water reuse, and storage for coal solid wastes. The study also suggested that the closed coal mine can be considered for use in culture tourism/education or as space for storage. For some particular closed coal mines, they have potential use for CO₂ sequestration, oil/gas storage, and bauxite resource development.

1. Introduction

At present, more than 100 countries or regions have made carbon neutrality commitments all over the world [1,2]. The United Kingdom, Japan, Mexico, the European Union, South Korea, the Philippines, and other countries and regions have passed special laws to deal with climate change. In 2020, China also made a commitment to the world that its carbon dioxide emissions would peak around 2030, and it would achieve carbon neutrality by 2060. This means that more new energy will be used and traditional energy will be reduced, and coal-based power generation will be especially limited. This will result in more coal mine closures. This trend has already emerged in Shanxi, China, which is a coal-based industrial province. Some scientists study the decarbonization and carbon reduction after coal combustion [3,4,5,6,7], and many countries have started to explore ways to reuse and/or redevelop retired/closed mines [8,9,10].

Closed mines, also called abandoned coal mines, disused coal mines, or shut-down coal mines, refers to the period of time when the operational stage of a mine is ending or has ended and the final decommissioning and mine rehabilitation is being undertaken [11]. A closed mine is a three-dimensional resource in a three-dimensional space, which has a huge utilization value. There are many resources, such as coalbed methane (CBM), mine water, a large amount of unused spatial resources, bauxite, and iron ore under the coal seam [12]. Directly closing or abandoning coal mines is a huge waste of resources, and always induces a series of safety or environmental problems [13,14]. The reutilization of a closed mine can make full use of the remaining resources and avoid environmental and geological problems such as greenhouse gas emissions, mine water pollution, and spontaneous combustion of coal seams caused by the closure of coal mines [8,9,10]; on the other hand, it can also revitalize the surplus value of the mines and resettle the unemployed workers due to coal mine closures [13,14,15].

In recent years, many academics and scientists have focused on the research of this field, and the previous researches on closed mines were mainly focused on estimation methods and control measures for the prevention and control of pollutant leakage from metal mines [16], ground management [17], underground space resource utilization and three-dimensional development [18,19,20], commercial development evaluation and functional transformation [21], and the application of new technologies [22]. In Europe, Germany, the United Kingdom, the Netherlands, and Belgium have rich experience in the reutilization of closed coal mines [23]. The Ruhr region in Germany is a seminal case of transforming abandoned mining areas into industrial museums, and the local government built an industrial heritage road to connect 19 tourism sites, 6 national museums, and 12 typical mining towns [8]. The United States and Canada have rich experience in underground utilization. For example, the United States is the most successful country in the commercialization of methane extraction and utilization from closed coal mines, and it is also the first country in the world to include methane emissions from abandoned coal mines in quantifying total greenhouse gas emissions [24]. In 2011, the United States carried out gas extraction and utilization projects in 38 abandoned coal mines, and the total amount of coal gas used was about 160 million cubic meters. In Asia, Japan has rich experience in utilizing closed coal mines. After the closure of the Hokkaido Coal Mine, a Coal Mine Museum was established using the reserved underground tunnels, and models of workers and equipment were set up. The museum shows the entire history of coal mining, from artificial coal mining to modern mechanized coal mining [25,26,27].

In China, there are three main types of reutilization of closed mine resources—energy utilization, resource utilization, and functional utilization—and the common methods are the construction of mine parks and underground water reservoirs [28,29,30]. For example, a hot spring resort center and a wetland park has been built using closed mines in Xuzhou; an agricultural demonstration area and an ecological tourism area have been established using the coal mining subsidence area of closed mines in Huaibei [31]; 32 underground reservoirs have been built using the underground space formed by coal mining and the purification capacity of the rock in the mining area for mine water by China Energy [32], similar things happened in the nearby Daliuta Coal Mine [33]. Shanxi Province is one province in China with many closed coal mines [34,35]. Shanxi has been experimenting with the secondary use of closed coal mine resources in China, such as the extraction of methane from closed mines, the use of closed mine clean water for agriculture and urban greening and making full use of abandoned coal mine tunnels for cultural tourism. In recent years, new utilization techniques have gradually appeared, such as exploring the use of closed mines for oil and gas storage [36], using abandoned coal mine alleyways to build compressed air energy storage power stations [37], and building large ground-based observation devices for gravitational wave detection [38]. For a large number of closed coal mines in Shanxi, the secondary use of resources is still relatively underutilized and lacking in variety [39,40]. For example, only four closed mines have been exploited and utilized, even though 15 coal mines have been closed since 2016 in Taiyuan, the capital of Shanxi Province [41].

So many valuable conclusions have been drawn from previous references [42,43,44,45]. However, there is little research on how to choose utilization methods for the closed mines. There are a large number of closed coal mines in China [46,47], and more than 800 coal mines will be closed in Shanxi [48]. Based on this, it is important to study the reutilization of closed mines. The structure of this paper is designed as follows: Section 1 consists of a brief introduction and review on utilization methods all over the world. Section 2 presents the analysis on different utilization methods. Section 3 presents the analysis of resource characteristics and the suitability of different mining areas. Section 4 presents the analysis of suitability for different coal mining areas and cities. Finally, the conclusions are presented.

2. Preferential Analysis of Different Utilization Methods

2.1. General Structural-Logical Algorithm for Comparing Closed Mines

Recently, some novel utility methods have gradually emerged. There was an industrial trial of the underground gasification of residual coal in Chongqing. It operated stably for six months, gasifying a total of nearly 10,000 tons of coal and producing 40,000 m³ of CBM per day [49]. Hongyu Guo et al. attempted to biogasify residual coal from closed mines using microorganisms [50]. In addition, underground space resources in closed mines were used to build large oil and gas storage reservoirs, sealed and modified closed mine shafts and underground car parks were used to store energy power plants, and deep coal seams were used for CO₂ sequestration and methane repulsion [51].

Table 1. Typical cases of re-utilization of closed mines.

Type	Type of Resource	Utilization	Typical Cases
Energy	Residual coal	Underground gasification	Zhongliang Mountain, Chongqing, China
	CBM	Extraction and utilization	Zhongliang Mountain, Chongqing, China
	Geothermal	Geothermal power generation	Netherlands in Heerlen
Natural resources	Mine water	Underground reservoirs	Erdos, Inner Mongolia, China
	Bauxite, iron ore	Mining	Mianchi Yima Mine, Henan, China
Underground space resources	Coal mines tunnels	Underground cultivation	Jingxi Mine, Beijing
	Coal mines tunnels	Storage	United States
	Shaft and tunnel	Pumped storage power generation	Ruhr Mine, Germany
	Shaft and tunnel	Underground experiments	Russia
Above-ground space resources	Subsidence areas	Photovoltaic power generation	United States
	Subsidence areas	Wetland park	Xuzhou, Jiangsu, China
	Open pits	Motor racing tracks	Xinqiu, Liaoning, China
	Buildings	Cultural tourist attraction	Ruhr Essen Coal Mine, Germany

summarizes the typical reutilization of closed mines.

2.2. Methodology for Assessing the Significance of Indicators of Closed Coal Mines

The utilization of closed coal mines has been a research focus in recent years in China’s coal resource-based cities. In Hebei Province, a neighboring province of Shanxi Province, Yifang Gu et al. established an evaluation model for the geological environmental impact of closed coal mines using a fuzzy comprehensive evaluation method and divided the geological environmental impact evaluation index system of closed coal mines into 3 elements and 11 indicators [52]. Shanxi Province and Hebei Province have similar geological conditions and geographical locations.

In order to analyze and evaluate the different utilization methods of closed mines, we interviewed more than 50 professors and experts in coal geology, safety production, ecological environment protection, hydrogeology, CBM geology, coal mine management, and government officials in Shanxi Province. All the interviewed experts have been carefully selected and are proficient in the development and utilization of coal mines, and most of them are professional senior engineers. The selection criteria for the scoring personnel are senior engineers with more than 20 years of experience in coal geology, safety production, ecological environment protection, hydrogeology, CBM geology, coal mine management, etc. All of them have been in the production line of coal and coal mines for a long time. They are familiar with the real situation of closing coal mines in Shanxi and have long-term and rich experience in front-line work and management. It is found that the five-axis evaluation method is more comprehensive and accurate than the three-axis by discussing with the experts. Based on expert opinion, this paper finally selects the five-axis evaluation method to compare and analyze the utilization methods of closed mines. The five indicators are the safety of the method, the environmental benefits of the method, the technical maturity of the method, the geological adaptability of the method, and the economic benefits of the method. Meanwhile, in order to accurately evaluate the importance of these five indicators, the expert scoring method was used to determine the respective weights of the five indicators. The average of all expert scores was used.

2.3. Grading of Each Indicator

The safety indicators means that the utilization method itself is safe, does not cause harm, or will produce safety benefits. The environmental benefits refer to the utilization of closed mines bringing environmental dividends or having a positive effect on the environment.

The utilization of closed mines generally requires certain technical transformation methods, so the maturity of the transformation technology is an important indicator that determines whether the closed mine can be used for secondary use. Any utilization method of closing a mine needs to be based on certain geological conditions, and different utilization methods require different geological conditions. Some utilization methods are highly suitable, such as the extraction of CBM. Others may require more demanding geological conditions. Economic efficiency refers to if the utilization of closed mines can bring a certain economic payback. The input–output ratio of utilization is the basis on which closed mines can be utilized in succession, otherwise they are unsustainable. Most of the re-utilized closed mines should be profitable in the long-term; however, some may require huge investment in some cases in the perspective of short-term benefits. For example, with the purification and utilization of coal mine water, although the safety and environmental benefits are obvious, the investment is huge.

For convenience of statistics and consistency, the total score for each indicator is set at 100 points. Considering that certain utilization methods may produce zero safety benefits, the safety benefits, the environmental benefits, and the geological adaptability are divided into 5 grades, the scores for each grade are 60, 40, 20, 0, −20. As all the utilization technologies discussed in this article have practical application cases, the technology maturity indicator is not assigned a negative score, and is only divided into mature, basically mature, research stage, and immature technologies. Additionally, 0 points are assigned to the currently immature utilization method. Therefore, the technical maturity level of the current mine closure evaluation is only divided into four grades. The results are shown in

Table 2. Weight of indicators for closed mines.

Evaluation Axis	Representation	Indicators (Weighting)	Quantitative Standard of Index Level	Value
Safety	Safety	Safety benefits (25%)	Huge safety benefit, eliminate security risks	60
			Moderate security benefit	40
			Generate certain safety benefits	20
			No benefit or harm	0
			A certain risk potential for accident	−20
Ecology	Environmentally friendly	Environmental benefits (25%)	Huge benefit not harmful to the environment	60
			Moderate environmental benefit	40
			Have certain environmental benefits	20
			No environmental benefits and hazards	0
			Risk of potential environmental hazard	−20
Technology	Applicability	Technical maturity (25%)	Very mature, lots of application examples	60
			Basically mature, with application examples	30
			Pilot stage, no promotion	10
			Concept stage, in testing	0
Geology	Promotional	Geological suitability (15%)	Applicable to most mines	60
			Only suitable for a part of mines	40
			Only suitable for special mines	20
			Mine needs geological construction renovation	−20
Economic	Inputs and outputs	Economic benefits (10%)	Great, benefit far outweighs investment	60
			Moderate, benefit outweighs investment	40
			Little return on investment	20
			Profit and investment are almost equal	0
			Investment is far less than the return	−20

.

2.4. Scoring of Different Utilization Methods

2.4.1. Energy Utilization

The geothermal gradient of coal mines in most areas of Shanxi is relatively normal, and there are few Class I thermal hazard zones, so energy-based minerals in closed mines in Shanxi generally refer to residual coal and CBM. Considering the reasons for the closure of coal mines and the geological conditions of coal resources in Shanxi, the re-mining of closed coal mines is less likely in Shanxi at present and in the future and is not discussed in this paper. Underground coal gasification is another way to utilize underground residual coal, including industrial gasification and microbial gasification. Industrial gasification has passed the pilot test, and microbial gasification technology is still in the stage of laboratory demonstration and cannot be implemented at present.

2.4.2. Resource Utilization

The main resources in closed mines are mine water and bauxite under coal seams. The high sulfur content of the coal from the Carboniferous Permian Taiyuan Formation has resulted in some of the closed mine water being acid mine water, with a pH value of less than 7, in parts of Shanxi Province. Bauxite and iron ore under coal seam are resources specific to the closed mines in the Carboniferous Permian mining area of North China and refer to the iron ore and bauxite rock section of the Benxi Formation, which is widely distributed in the lower part of the recoverable coal seam of the Early Paleozoic Taiyuan Formation in North China [53]. Mining bauxite and iron ore beneath coal is a relatively small investment and economically beneficial [54]; however, it will destroy the water barrier of the Ordovician tuff water beneath it and lead to the risk of Ordovician water eruptions, so it is suitable for mining in areas where the water-richness of the Ordovician water is weak or the lower coal is not under pressure [55].

2.4.3. Use of Space Underground

The use of underground space in closed mines mainly includes material storage, the construction of underground water reservoirs, the storage of coal gangue fly ash, the construction of underground oil and gas reservoirs, and the storage of CO₂. Most of the coal mines in Shanxi are underground coal mines, and the closures of coal mines leave behind huge underground spaces that can be exploited. Closed mines can be reconstructed as underground reservoirs by taking advantage of the large reserves and low evaporation. Closed mines in plain areas can be used to fill coal-based solid waste, such as coal gangue and fly ash. Closed mine tunnels around cities can be used as underground storage after modification. Closed mines with stable geological conditions can be constructed as underground oil and gas reservoirs. However, the construction of underground oil and gas storage and the storage of CO₂ in closed mines have the highest geological requirements and poor geological adaptability, requiring large investment and extensive renovation. Although China cooperated with Canada in 2002 to conduct a micro pilot test of CO₂ injection to enhance CBM recovery in Well TL-003 in Qinshui Basin, Shanxi Province, coal seam CO₂ storage is still in the stage of exploration and demonstration, and commercialization and large-scale promotion have not yet been carried out [56]. In addition, using the closed mines to seal coal gangue and fly ash has the greatest environmental benefits, and storage has the highest economic benefits.

2.4.4. Use of Space on the Ground

The utilization of the underground space of closed mines mainly consists of using the leftover industrial buildings, industrial equipment, and coal mining subsidence areas to generate solar power or using the water in the areas to build artificial lakes or artificial wetlands, and to develop resorts, racetracks, holiday towns, shopping centers, and training bases. The utilization of the above-ground resources of closed mines does not require favorable geological conditions of the coal mines, has good environmental benefits and good economic benefits, and requires a small investment.

2.4.5. Combined Utilization

The combined utilization of the above- and underground space of a closed coal mine refers to the establishment of mine geoparks, practice bases, pumped storage power stations, and compressed air energy storage power stations. Mine geoparks have good environmental benefits and geological suitability, except that the geographical location needs to be close to cities and in densely populated places. Practice and teaching bases need to be close to universities and colleges. Pumped storage power stations and compressed air storage power stations need to be near thermal power plants or new energy power stations, such as those for solar and wind power; otherwise, the economic benefits are not obvious.

2.5. Selecting the 5-Indicators

According to the method of utilization and current technical level, every expert was invited to score the indicators of the utilization method. The 5-indicator score was selected by averaging the scores arithmetically. Thus, the final weighted value of each utilization method was obtained according to the calculation formula (

Table 3. Index scores and weights for different utilization methods of closed mines.

Utilization Methods	Safety Benefits	Environmental Benefits	Technical Maturity	Geological Suitability	Economic Benefits	Weighted Values
	25%	25%	25%	15%	10%
CBM extraction	60	60	30	60	20	48.5
Mine water utilization	60	60	30	40	−20	41.5
Solid waste storage	40	40	30	60	40	40.5
Cultural tourism	0	20	60	20	20	25
Warehousing	20	20	30	20	40	24.5
Educational practice	0	20	60	20	0	23
Energy storage and power generation	0	60	10	−20	40	18.5
CO2 storage	0	40	0	20	40	17
Oil and gas storage	0	40	0	−20	40	11
Bauxite under coal	−20	−20	30	40	60	9.5
Coal underground gasification	−20	−20	10	−20	40	−6.5

).

As shown in Table 3, it is the highest score for the CBM extraction, followed by mine water utilization and storing coal gangue. The coal seam underground gasification fraction is negative. Considered extreme cases, it is found that the score range for every utilization method would be between −15 and 60. In order to compare and analyze intuitively, according to the final weighted score of every utilization the final scores of every utilization method was divided into four categories according to the interval: the first priority is (40, 60), the second priority (20, 40), the third priority (0, 20), and the fourth priority (−15, 0), which is shown in

Table 4. Priority ranking of utilization methods for closed mines.

Priority Ranking	Score Range	Utilization
First priority	40, 60	CBM extraction, mine water utilization, solid waste storage
Second priority	20, 40	Cultural tourism, warehousing, educational practices
Third priority	0, 20	Energy storage and power generation, storage of CO2, oil and gas storage, bauxite under coal
Fourth priority	−20, 0	Coal underground gasification

.

Safety benefits, environmental benefits, and technological maturity are the three main factors which affect the choice of utilization methods. There is a certain reference value for geological adaptability and economic benefits, but they are not the main factors, with the reason being that they only determine the application scope and promotion of utilization methods. As shown in Table 4, CBM extraction, the mine water utilization, and storing coal gangue have the highest priority. Cultural tourism, educational practice, and storage are the second priority methods. Energy storage and power generation, oil and gas storage, storage of CO₂, mining of bauxite, is the third priority. CBM extraction and utilization can reduce the emission of CH4, which is the main greenhouse gas that escapes into the atmosphere. At the same time, CBM is a green energy, which is important for the establishment of a low-carbon energy system and lifestyle. When a coal mine is closed, a large amount of mine water accumulates [57]. As most of the coal contains sulfur, water in the closed mine is always acidic with a pH below 7, which will cause serious groundwater pollution and need be purified before discharged. Therefore, from the perspective of resource utilization or control of environmental pollution, it is necessary for the utilization of mine water. It has low requirements on mine geological conditions for storing coal gangue in closed mines, which can reduce the occupation of a large amount of cultivated land, with low investment and high environmental benefits.

It is relatively mature for the utilization of closed mines as cultural tourism and education practice technology, with few requirements for mine geology. However, because most of the closed mines are far from the urban area, not all the closed mines have geographical advantages. There is convenient transportation and a large flow of people for the closed mines around the city; thus, they are suitable for use as tourist attractions. Underground warehouses can be built to make full use of the convenient conditions with constant temperature and cheap land prices when the shortage of land around the city is considered.

The use of energy storage in coal mines for power generation is a small investment with large payback and can fully realize the high value use of underground space in mines. Meanwhile, it can alleviate the power tensions during the peak power consumption period and being of great significance to the strategy of carbon peaking and carbon neutral. Nowadays, CO₂ enhanced coalbed methane recovery (CO₂-ECBM) can improve the efficiency of CBM extraction and, on the other hand, achieve geological storage of CO₂ [58]. There are many non-minable coal seams in closed mines, which provides convenient geological storage of CO₂ and CO₂-ECBM. The use of closed mines to construct underground oil and gas storage is of great practical significance to alleviate the pressure of oil and gas peaking and supply protection, but unfortunately this utilization requires very strict geological conditions and high geological stability.

The development and utilization of closed coal mines generally follow the principle of “promoting profit and eliminating harm”, and the purpose of eliminating harm is often greater than promoting profit. Therefore, the first principle of the choice of means of utilization must be to not increase or cause new harm. Therefore, the choice of utilization methods must be conservative and prudent, and the utilization methods must be safe, environmentally friendly, and beneficial to the environment. For the time being, the utilization methods that are immature or have potential environmental hazards should not be selected as much as possible, so as not to cause new damage to the ecological environment of the mining area. Taking the mining of bauxite under coal and the underground gasification of residual coal as examples. Bauxite is widely distributed beneath the Early Paleozoic coal seams in the north of China. Making full use of underground tunnel of the closed coal mine can save mining invest greatly. However, the mining of Bauxite under coal will lead to the karst water of the Ordovician system beneath it, making it is only suitable for mining in places where the water-richness of the Ordovician water is weak. This makes this kind of utilization very limited. The underground gasification of coal can save a lot of underground mining investment and obtain grate economic benefits, but it is more threatening to groundwater and may induce a potential ecological hazard [59,60,61]. Moreover, the large-scale development of underground coal gasification requires high regional tectonic stability [62] and face the challenges of safety, ecology and environmental protection [63]. That is why it gets a negative score and is not recommended.

3. Resource Characteristics and Suitability

3.1. Classification

Quan Zeng Yin et al. [64] proposed 118 different resource utilization models for closed coal mines in Hebei Province, based on the degree of impact on the geological environment of the mines, the type of problem, and the differences in the locations of the mines. In order to accurately classify the types of geological conditions of the resources of closed mines in the province, the coal characteristics, mine gas content, hydrological characteristics, location characteristics, and natural geographic characteristics of each mining area must be considered comprehensively. The closed mines were divided into electricity-rich, gas-rich, water-rich, bauxite-rich, suburban, and plain mines in Shanxi (

Table 5. Resource types in different mining areas in Shanxi.

Mine Type	Resource Characteristics	Mine Area
Electricity-rich mining areas	Power coal bases	Datong, Pingshuo, Xuangang, Lanxian, Hebaopian
CBM-rich mining areas	High CBM mining areas	Liuliu, Xishan, Yangquan, Wuxia, Lu’an, Jincheng, Huodong
Groundwater-rich mining areas	Rich mine water	Xuangang, Xishan, Dongshan, Fenxi, Huozhou, Xianning, Lu’an, Jincheng
Bauxite-rich mining areas	Rich in bauxite resources	Liuliu, Xuangang, Yangquan, Fenxi, Huodong, Pinglu, Yuanqu
Suburban mining areas	20 km away from the city	Datong, Liliu, Xishan, Dongshan, Huozhou, Yangquan, Lu’an, Jincheng
Plain mining areas	Plains area	Changzhi, Jincheng, Shuonan

).

3.1.1. Electricity-Rich Mining Areas

Power-rich mining areas are located in the five nationally planned mining areas of the Jinbei coal base, including the Datong, Pingshuo, Xuangang, Lanxian, and Hebaopian mining areas (Figure 1a). These five mining areas are rich in power coal resources and are important coal thermal electricity output bases in China. Geographically, they are all located in the northwestern part of the mountain and belong to the arid and semi-arid regions of China, with an average annual rainfall of less than 400 mm. In addition, these areas are important bases for wind power and solar photovoltaic power generation in Shanxi Province due to the high altitude, sufficient wind, and sunlight every year.

Closing mines in power-rich mining areas can make full use of abandoned mine roadways and mine water for water storage, energy storage, and compressed air power generation and can realize the green energy storage of new energy power, such as wind and solar energy. At the same time, taking into account the characteristics of many thermal power plants at the Jinbei base, it is possible to explore and develop the use of closed coal mines to store a large amount of CO₂ produced by coal-fired power plants in non-minable coal seams so that the closed coal mines can be transformed from a “carbon source” into a “carbon sink”.

3.1.2. CBM-Rich Mining Areas

Reducing methane emissions is one of the most economical and effective measures to combat climate change globally [65]. Most of the abandoned coal mines in China are high gas mines and gas outburst mines. There is still a large amount of residual gas in the adjacent coal seams, coal pillars, and underground goafs, which may continue to be slowly released from surface cracks or man-made channels. Methane emissions from abandoned coal mines will not only increase greenhouse gases but may also lead to safety production accidents. During the “14th Five-Year Plan” period, China will continue to eliminate outdated production capacity and close down coal mines that are exhausted and do not meet the requirements of safe production conditions [66]. The methane emissions from abandoned coal mines will increase in the future. There are abundant CBM resources in abandoned coal mines, and the extraction of CBM from abandoned coal mines has become an important CBM resource in coal mining areas. As a province with large coal resources, Shanxi Province has accumulated coal production of about 20 billion tons since the founding of the People’s Republic of China and accumulated many abandoned mine gobs. According to statistics, there are more than 4700 abandoned mines in Shanxi Province, which have value for utilization. The total mined-out area of the mines is about 2052 km², and the residual CBM resources are about 72.6 billion m³ [67]. The seven CBM-rich mining areas were announced by the Department of Natural Resources of Shanxi Province and include the Liliu, Xishan, Yangquan, Wuxia, Lu’an, Jincheng, and Huodong mining areas (Figure 1b).

Table 6. Evaluation of CBM resources in goaf of main mining areas.

Mining Area	Goaf Area (km2)	CBM Resources in Goaf (100 Million m3)	Resource Abundance (100 Million m3/km2)
Xishan	186.02	56.54	0.31
Liliu	76.91	41.26	0.54
Yangquan	195.35	97.33	0.50
Wuxia	75.38	19.01	0.25
Luan	118.98	19.67	0.16
Jincheng	169.08	63.45	0.38
Huodong	52.16	6.70	0.13

shows the CBM resources in the gobs of the seven mining areas with high gas content in Shanxi Province [68].

The seven mining areas have a cumulative area of approximately 870 km² of coal mined-out area and a predicted CBM resource of 30.3 billion m³, which is rich in resources and has great prospects for utilization. Among them, the Liliu mine is located at the eastern edge of the Ordos Basin and is a famous planned CBM mining area in China. The Jincheng mine is located in the southern part of the Qinshui coalfield and has a high CBM content in its mines, making it the largest CBM production base in China. At the same time, according to the characteristics of CBM-rich mines with high CBM extraction, CBM-rich mines can also be used to build underground gas storage reservoirs to regulate the production and storage of CBM.

3.1.3. Groundwater-Rich Mining Areas

Groundwater-rich mining areas are delineated by combining the hydrogeological characteristics of the mines, the depth of the burial of the coal seams, and the extent of karst water and mainly include the eight mining areas of Xuangang, Xishan, Dongshan, Fenxi, Huozhou, Xiangning, Lu’an, and Jincheng (Figure 1c). Most of the groundwater-rich mines are located within the karst area or in the transition zone in front of the mountain, with high volumes of mine water. Making full use of the groundwater-rich mine area to close down the mine’s water reserves can reduce regional water stress significantly and improve the efficiency of mine water utilization, reducing the water leakage accidents caused by the water accumulation in the goaf area of the surrounding production coal mines.

3.1.4. Bauxite-Rich Mining Areas

The bauxite-rich mining areas are strategic prospective areas planned by Shanxi Province, mainly including the Liliu, Xuangang, Yangquan, Fenxi, and Huodong mining areas and the Pinglu and Quanqu coal production areas (Figure 1d). In these seven areas, the bauxite under coal is widely distributed, and the ore layer is thicker. Among these areas, the Liliu and Fenxi mining areas are well-known bauxite production bases in China. Bauxite has a high aluminum–silicon ratio and stable deposits, which are suitable for large-scale mining.

3.1.5. Suburban Mining Areas

Suburban mining areas are classified according to whether the mine is close to a city and include the seven mining areas of Datong, Liliu, Xishan, Dongshan, Huozhou, Lu’an, and Jincheng (Figure 1e). Most of the suburban mining areas are located near urban agglomerations, with convenient transportation and a large flow of people. They can make full use of closed mines and auxiliary facilities to build mine parks, resort towns, shopping centers, and expand bases. With the convenience of the proximity to universities and colleges, the closed mines can be transformed into teaching and experimental sites or national geological-mining parks, science and technology practice bases, etc. Considering the characteristics of closed coal mines of constant temperature and humidity and not occupying surface land, it is possible to explore suburban mining areas as storage bases.

3.1.6. Plain Mining Areas

Plain mining areas are mainly delineated according to whether the surface of the mining area is flat, and they are mainly distributed in the Lu’an and Jincheng mining areas in Shanxi (Figure 1f). Most of the coal mines in these two mining areas are located in the geographical plains. The fly ash and coal gangue produced by coal mines and thermal power plants not only affect the ground landscape but also encroach on a large amount of farmland. Therefore, the closed mines in the plain mining areas are suitable for storing and stacking fly ash and gangue, or these fly ash and coal gangue can be used for filling coal mining.

3.2. Scope of Application

The different geological conditions of mine resources are suitable for different utilization methods. Through the analysis of the resource types of closed coal mines in 16 national planning mine areas and 4 coal production areas in the province, it was found that 7 CBM-rich mine areas, including Liliu, Xishan, and Yangquan, are suitable for extracting coal-bed methane from mining areas and establishing underground gas storage. Eight groundwater-rich mine areas, including Xuangang, Xishan, and Dongshan, are suitable for developing and utilizing the water accumulated in the mining areas. Three plain mining areas in Lu’an and Jincheng, Shuonan, are suitable for storing coal-based solid waste and reducing the occupation of surface arable land. Seven suburban mining areas are suitable for industrial and commercial storage, cultural tourism, educational practice, and the establishment of underground oil storage depots. The closed mines in five electricity-rich mining areas, including Datong, Pingshuo, and Xuangang, are suitable for energy storage and power generation and the sequestration of CO₂ from power plants. The closed mines in seven bauxite-rich mining areas, including Liliu, Xuangang, and Yangquan, are suitable for mining bauxite resources under coal seams (

Table 7. Types of utilization and suitable reuse methods of mines closed in the planned mining areas of Shanxi.

Mining Area	Type of Utilization	Development Method
Liuliu, Xishan, Yangquan, Wuxia, Lu’an, Jincheng, Huodong	Energy, underground space	CBM extraction, underground gas storage
Xuangang, Xishan, Dongshan, Fenxi, Huozhou, Xianning, Lu’an, Jincheng	Resource-based use	Mine water utilization
Lu’an, Jincheng	Underground space	Solid waste storage
Datong, Liliu, Xishan, Dongshan, Huozhou, Lu’an, Jincheng	Underground space	Storage, cultural tourism
Datong, Pingshuo, Xuangang, Lanxian, Hebaopian	Underground-above ground	Educational and practical venues underground oil storage
Liliu, Xuangang, Yangquan, Fenxi, Huodong, Pinglu, Quanqu	Underground space	Energy storage, power generation, geological storage of CO2

).

4. Methods for Different Areas

4.1. Geographical Distribution

Since 2016, coal mines have closed in 11 areas in the province of Shanxi (Figure 2) [69,70,71,72,73,74,75]. There is a total of 27 closed mines, with the largest number in Linfen, followed by Jincheng and Datong, with 20 and 19 closed coal mines, respectively. The lowest number of closed mines is in Jinzhong, with three. In terms of exiting production capacity, the largest capacity of 18.75 million tons is in Jincheng, followed by Linfen City, with 18 million tons. Jinzhong City has the smallest exit capacity, with 0.84 million tons. In terms of the distribution of closed coal mines, they are relatively concentrated in Datong and Yangquan, while in Linfen, they are the most scattered. The closed coal mines span four planned coal mining areas in Lvliang City and Xinzhou (

Table 8. The numbers and production capacity levels of closed coal mines in different areas of Shanxi since 2016.

City	Number of Closed Coal Mines [69,70,71,72,73,74,75]	Exiting Capacity (Million Tons)	Mining Areas
Datong	19	15.85	Datong, Guangling
Shuozhou	6	9.71	Pingshuo
Xinzhou	15	11.95	Xuangang, Hebaopian, Lanxian, Wutai
Taiyuan	15	9.59	Xishan, Dongshan
Jinzhong	3	0.84	Dongshan, Fenxi
Lvliang	7	6	Liuliu, Fenxi, Xishan, Hebaoquan
Yangquan	11	10.85	Yangquan
Linfen	27	18	Fenxi, Huozhou, Huodong
Yuncheng	4	16.5	Xiangning, Wuanqu, Pinglu
Changzhi	11	5.70	Huodong, Wuxia, Lu’an
Jincheng	20	18.75	Jincheng

).

4.2. Resource Characterization and Suitability Analysis

4.2.1. Datong, Shuozhou, and Xinzhou

Datong, Shuozhou, and Xinzhou are located in the Jinbei coal base in the northern central part of Shanxi Province. Since 2016, 40 coal mines have been closed and withdrawn, with a total output of 37.51 million tons, and distributed across five nationally planned mining areas and two coal production areas. Due to the structure and coal quality in the northern part of Shanxi, the coal seam gas content is low. It is suitable for the development of wind power and photovoltaic solar power generation due to the high altitude, sufficient annual wind, and long sunshine duration. Due to the instability of solar wind energy, it is possible to make full use of the abundant underground coal mines tunnels in the area for mine water energy storage and tunnel compressed air energy storage to realize the “stabilized” output of wind energy and solar energy. The pits formed by open-pit coal mining in Shuozhou are not suitable for CO₂ storage or oil and natural gas storage. However, the pits produced by open-pit coal mining can be used to dispose of coal gangue and fly ash. Closed coal mines in the Datong mining area close to Datong city can be used to develop cultural tourism, industrial storage, or educational practice. The Xuangang mining area in Xinzhou city is rich in resources of sub-coal bauxite, which is suitable for mining sub-coal bauxite.

4.2.2. Taiyuan and Jinzhong

The closed mines in Taiyuan and Jinzhong are basically located near the Taiyuan–Jinzhong basin city cluster, with convenient transportation, especially in the east–west mountain area of Taiyuan, where the closed mines and their ancillary facilities can be fully utilized to build mine parks, resort towns, and shopping centers and expand bases. At the same time, with the convenience of proximity to universities and colleges, teaching and experimental sites can be vigorously developed, and national geological mining parks and internship bases can be built. In addition, the Taiyuan–Jinzhong basin is a world-famous base for the production of white wine and aged vinegar. The convenience of constant temperature and humidity of the closed mines around the city can be fully utilized to build underground wine and vinegar cellars, etc., so as to maximize the value of the closed mines around the area.

4.2.3. Lvliang and Yangquan

The eastern edge of the Ordos Basin, where Lvliang is located, is a famous CBM production area in China. It should make full use of the closed mines to pump out all the CBM from empty areas and build underground gas storage in areas with suitable geological conditions to store the extracted CBM together with shale gas and dense sandstone gas, cutting the peaks and filling the valleys for stable output when the supply and demand of gas sources are out of balance. In addition, Xingxian and Xiaoyi in Lvliang city are famous bauxite production bases in China, with high aluminum-to-silicon ratios and stable seams, making them suitable for large-scale development. Most of the closed coal mines in Yangquan are located near urban areas and belong to the high gas mining areas announced by the Department of Natural Resources of Shanxi Province. In addition to vigorously extracting CBM, it is also possible to use the characteristics of the mines close to urban areas to explore the transformation of closed coal mines into gas and oil storage tanks to alleviate the city’s gas and oil needs.

4.2.4. Linfen and Yuncheng

The CBM content in the Huodong area in Linfen is high, making it suitable for the extraction of CBM from the closed coal mine extraction areas. In addition, the Linfen area is part of the confluence area of Guozhuang Spring, Huo Spring, Longzizi Spring, and Gudui Spring, and has a high volume of water from closed mines, making it suitable for the development of closed mine water. The Huozhou mining area, which is close to the Linfen urban area, can be explored for cultural tourism, storage, and educational practices using the closed mines. In addition, the Huoxi coalfield and the eastern Huodong mining area in the Linfen area are rich in bauxite resources under coal and are suitable for the mining of bauxite under coal. The number of closed mines in the Yuncheng area is relatively small, only located in Pinglu, Yuanqu. The area is rich in bauxite under coal seam and is suitable for successive mine sub-coal bauxite resources after coal resources are depleted.

4.2.5. Changzhi and Jincheng

The southern part of the Qinshui coalfield, where Changzhi and Jincheng are located, has high mine gas content and is suitable for extracting CBM. The closed coal mines in the Jincheng area have good mine water quality and are suitable for the construction of underground reservoirs for industrial and agricultural purposes. When the geological conditions are suitable, large underground oil and gas storage reservoirs can be built. The mine area of Changzhi and Jincheng belongs to the central plain area of the Qinshui Basin, and the piling of the output coal ash and gangue not only affects the ground landscape but also encroaches on a large amount of highly productive agricultural land, so the closed mines in the area can also be used to store and pile coal ash and gangue or use coal ash and gangue to fill in the residual coal mines underground. Most of the coal mines in Changzhi and Jincheng are located in the suburbs of the city, with convenient transportation, a dense population, and high pedestrian flow, and can be explored for the development of cultural tourism and teaching experiments using closed mines.

4.3. Proposals

Combining the natural geographic conditions and the geographic characteristics of the resources in different regions of the province and combining the results of the preferential ranking of the secondary use of closed mines in the province completed in the previous section, the closed mines in Taiyuan, Lvliang, Yangquan, Linfen, Jincheng, and Changzhi are the most suitable for CBM extraction. The closed mines in Shuozhou, Xinzhou, Taiyuan, Jinzhong, Linfen, Changzhi, and Jincheng are suitable for the utilization of mine water. Those in Shuozhou, Changzhi, and Jincheng are especially suitable for the sequestration of coal-based industrial solid waste. All closed mines close to cities are suitable for the utilization of cultural tourism, storage, and educational practices, and oil and gas storage, except those in Xinzhou and Yuncheng. The closed mines in Datong, Shuozhou, and Xinzhou are very suitable for mine energy storage and power generation and the CO₂ storage of power plants. The closed mines in Xinzhou, Lvliang, Yangquan, Linfen, and Yuncheng are suitable for the development of bauxite under coal seams (

Table 9. Suitable utilization methods of closed mines in different areas of Shanxi from 2016 to 2020.

Utilization Rankings	1	2	3	4	5	6	7	8	9	10
	48.5	41.5	40.5	25	24.5	23	18.5	17	11	9.5
Utilization Patterns	CBM Extraction	Mine Water Utilization	Solid Waste Storage	Cultural Tourism	Commercial Storage	Educational Practice	Energy Storage for Power Generation	CO2 Geological Storage	Oil and Gas Storage	Bauxite Mining	Utilization Pattern Sum	Utilization Ranking Score Total
Datong				√	√	√	√	√	√		6	119
Shuozhou		√	√	√	√	√	√	√	√		8	201
Xinzhou		√					√	√		√	4	86.5
Taiyuan	√	√		√	√	√			√		6	173.5
Jinzhong		√		√	√	√			√		5	125
Lvliang	√			√	√	√			√	√	6	141.5
Yangquan	√			√	√	√			√	√	6	141.5
Linfen	√	√		√	√	√			√	√	7	183
Yuncheng										√	1	9.5
Changzhi	√	√	√	√	√	√			√		7	214
Jincheng	√	√	√	√	√	√			√		7	214

). The closed coal mines in Changzhi, Jincheng, and Shuozhou have the highest utilization value and the most utilization methods, followed by Linfen, Taiyuan, Lvliang, Yangquan, and Jinzhong. The closed coal mines in Xinzhou and Yuncheng have fewer utilization methods and less utilization value.

The utilization of closed coal mines in an area needs to consider many factors. The first factor is resource advantage, which is the basis for development and utilization. For example, the extraction of CBM in closed coal mines, the utilization of mine water, and the mining of bauxite. The second factor is the geographic location of the closing coal mines. For example, the closed coal mine is built as cultural tourism center, commercial center, educational practice center. The third factor is social needs, such as closing coal mines to build pumped-storage power plants, sequestering carbon dioxide emissions from power plants. In addition, topographical factors are also important. For example, the storage of coal gangue is only suitable for plain mining areas. Sometimes, several factors may need to be considered. For example, the utilization of store oil and gas, need to consider the resource conditions and location advantages of the mine at the same time. This is because oil and gas storage must be close to both production area and consumer markets, that is, gas-rich and close to urban peripheries.

5. Conclusions

(1)

CBM extraction from mining areas, mine water utilization, and the sequestration of coal-based industrial solid waste are the optimal utilization methods for closed coal mines in Shanxi, followed by cultural tourism, storage, and educational practices. Energy storage and power generation, CO₂ sequestration, oil and gas storage, and bauxite mining under coal are the third preferred utilization methods.

(2)

The closed coal mines in Changzhi, Jincheng, and Shuozhou have the highest utilization value and the most utilization methods, followed by Linfen, Taiyuan, Lvliang, Yangquan, and Jinzhong. The closed coal mines in Xinzhou and Yuncheng have fewer utilization methods and less development and utilization value.

(3)

The utilization of closed coal mines depends macroscopically on the resource and geological conditions of the main mine, social needs, and the maturity of the technology. The resource and geological conditions refer to the geological stability, resource and energy conditions, and economic and geographical conditions. Social needs refer to the external influences, such as safety benefits, environmental benefits, and economic benefits arising from secondary use. The level of economic development, population distribution, scientific education, industrial structure, and national macro policies are also factors that affect the utilization of closed mines.

(4)

The utilization of closed coal mines is the result of the combined effect of resources, technology, environment, and economic development level. As technology advances, some immature technology utilization technology will gradually mature, new utilization technologies will emerge, and the utilization of closed mines will become more diversified and efficient.