1 Introduction
Mineral resources provide the material foundation for the survival and development of human society. Because the mineral resources at the earth’s superficial layer are gradually exhausted, the mining industry worldwide has inevitably sought to exploit resources from the deeper layers in the earth. Theoretically, there are limits to the depths that can be mined. However, since traditional mining theories and technologies are insufficient for handling deeper coal mining, they must be fundamentally reformed.
The mining of mineral resources has a history of at least one century. In the world, the depth of coal mining continues to increase. For example, the mining depth of Suncun Coal Mine in Shandong Province of China has reached 1500 m [
1]. Additionally, the mining depths for geothermal energy, gold, and oil and gas resources have exceeded 3000 m, 4350 m, and 7500 m, respectively. However, the mining of solid minerals (e.g., coal) is essentially different from that of oil and gas. It requires both mining equipment and personnel to enter the coal mines. Under the constraints of technologies (e.g., mining, supporting and hazard-control technologies) and the manual operation environment, traditional mining methods have critical mining depth. Moreover, as the depth increases, the temperature of the rock and soil increases. In recent years, scholars have confirmed that when the vertical buried depth is more than 2000 m, the temperature of rock and soil increases to 70°C, which is far beyond the normal temperature range for human to work [
2]. Human cannot excavate roadway or mine coal because of the high temperature under the ground. Given coal mine safety regulations, the earth’s high temperatures at great depths and the tolerance limits of human body, the critical mining depth is 1618 m when using ventilation cooling [
3]. Moreover, ground-stress measurements in more than 30 countries show that the rock masses are basically in a triaxially equal pressure and a high-stress state at a depth of 6000 m. Thus, they are subject to full plastic flowing deformation. With traditional mining methods, strata control and surrounding rock supporting technologies will fail at the earth’s depth of 6000 m or more [
3–
7].
Aside from the depth, deep coal mining faces a number of seemingly insurmountable obstacles: low mining rates, ecological environment degradation, and frequent disasters. To effectively exploit resources from the deep underground, the concept of mining must be reconsidered and innovative new mining theories, methods and technologies must be applied.
Xie et al. was the first to propose the technological concept of fluidized mining of deep coal resources [
7–
11]. The core principle is from the traditional ‘mining of coal from underground to the ground’ to the ‘
in situ transformation of coal in mines’. This means that deep coal is transformed into gaseous, liquid, or mixed gaseous-solid-liquid substances
in situ for integrated development (e.g., mining, separation, filling, electrification, thermalization and gasification) and transport. This novel integrated, unmanned and intelligent approach to fluidized coal mining includes several new required technologies (e.g., unmanned operation, intelligent mining,
in situ transformation and efficient transport). An integrated approach would thoroughly change the concept of mining, transport and coal resource development. This paper aims to facilitate this to accomplish a green, safe, and efficient method of extracting coal resources, characterized by ‘There are no coal on the ground and no men in the coal mine’.
Traditional coal mining and utilization patterns release substantial CO2, CH4, and CO, which have serious impacts on the environment. In fluidized mining, the coal is not transported to the ground. The raw coal, the CO2, CH4, and CO released during coal mining are transformed into a clean energy source in situ. Moreover, the solid waste produced by the traditional coal mining patterns also has serious impacts on the environment. Various solid wastes (e.g., coal gangue and ashes) generated during traditional coal mining are characterized by their large quantity, low utilization, and difficulty of disposal. In fluidized mining, solid wastes from mining are not hoisted to the ground, but are re-used as aggregate in the in situ backfilling of underground goaf. This practice lessens the pollution of the ground surface environment by harmful solid wastes, accomplishing harmless and in situ processing of solid waste.
This paper systematically deconstructs the deep in situ fluidized coal mining theories. Besides, it presents a theoretical system of mining-induced rock mechanics, three-field visualization, multi-physics coupling of in situ transformation and efficient transport, proclaiming that the future needs a key technologies system of mining, separation, filling, electrification, thermalization, and gasification, including fluidized mining via chemical transformation, biodegradation, physical crushing, etc. In addition, it proposes the strategic roadmap and future research concepts. The aim is to achieve groundbreaking innovation and significantly enhance the capability to acquire the earth’s deep mineral resources.
2 Fluidized mining
Fluidized mining is a system through which deep solid minerals are transformed into gaseous, liquid, or mixed substances
in situ, after which they are subjected to integrated development (e.g., mining, separation, filling, thermalization, electrification, and gasification) in the mines, optimally in an unmanned and intelligent manner [
7–
11].
As a groundbreaking concept, deep in situ fluidized mining is to fluidize the earth’s deep solid minerals in situ, transform them into liquid, gaseous, electrical, thermal, or mixed gaseous-solid-liquid resources, and transport them to the ground in fluidization form efficiently and intelligently. Compared with traditional coal exploitation patterns, deep in situ fluidized mining brings about several changes. For example, fluidized resources (e.g., oil, gas, electricity, and heat) are transported to the surface instead of excavation. Resource utilization (e.g., coal, water, heat and underground space) is integrated. Based on in situ transformation of deep coal resources, existing traditional coal mines can be transformed into regulation and storage bases for power transmission and clean energy, as shown in Fig. 1.
3 Groundbreaking theories of deep in situ fluidized mining
In in situ unmanned operations, intelligent mining and efficient transport, deep solid mineral resources can be gasified, liquefied, electrified, thermalized, and fluidized underground. The methods, the processes, and the patterns greatly differ from those of the traditional coal mining. Traditional coal mining theories, rock mechanics theories, supporting theories, and transport theories are no longer suitable. Therefore, it is necessary to innovatively consider the multi-physics coupling of rock stress-temperature-fracture-seepage field in the context of deep in situ fluidized mining to build a theoretical system.
3.1 Mining-induced rock mechanics
Underground rocks can be crushed by using intelligent unmanned shield tunnelling machines. This differs from conventional mining methods. During gasification, liquefaction, and electrification, a series of disturbances are inevitably produced, thus, affecting the damage, deformation, and failure law of rocks in places. Therefore, in deep
in situ fluidized mining, deep rock masses undergo a succession of mechanical behaviors distinct from those during conventional mining methods. Further, traditional theories on coal mining, rock mechanics, and mining-induced rock mechanics are perhaps no longer applicable and even completely subverted. Therefore, deep
in situ fluidized mining requires that people adopt a new framework of mining-induced rock mechanics and develop novel theories [
5–
7,
9,
11,
12].
Therefore, it is necessary to study the principles, technologies, equipment and methodologies for deep in situ high-fidelity coring, including the preservation of pressure, temperature, humidity, light and quality, as illustrated in Fig. 2. First, a new standard system must be developed for high-fidelity rock mechanics testing. Besides, the in situ nonlinear mechanical behaviors of deep rock mass in their mining-induced stress states must be studied. Moreover, theories and methodologies for rock mechanics under the disturbances of fluidized mining must be proposed and a new constitutive relation must be built for deep rock mass. Furthermore, the energy relationships involved in the stability and rupture extension of rock mass under disturbances of deep in situ fluidized mining must be analyzed, which can help to develop disaster-inducing criteria for the in situ rock. The laws of nonlinear mechanical behaviors of coal and rock mass and those of energy transfer, transformation and distribution must be reexamined so that the instability criteria and behaviors of coal and rock mass in coal mines can be analyzed. The pressure-boost effects of surrounding rocks under chemical and biological liquefaction, gasification and electrification must be considered, and the pressure relief effects of surrounding rocks should be explored while developing theories of structural motion and instability mechanics with supporting methods for large-area strata and caves and the theories and methodologies for the control of strata stability should be proposed.
3.2 Three-field visualization
Deep
in situ fluidized mining integrates a variety of technologies and is a dynamic process involving diverse mining disturbances. Under the influence of mining, the internal structure and
in situ stress field of surrounding deep coal continue to change, generating new mining-induced stress-fracture-seepage field [
11,
13]. However, it is difficult to accurately capture and visually display these fields, thus, restricting theories and technologies of fluidized mining. Therefore, it is necessary to characterize the transparency and visualization of the three-field of rock mass in deep
in situ fluidized mining.
To investigate the evolutionary characteristics of the three-field of rock mass under the disturbance of deep
in situ fluidized mining, it is necessary to develop a fractal restructuring algorithm for the discontinuous structure of deep rock mass and build a high-definition transparent three-dimensional (3D) physical model for the discontinuous structure of deep-strata via 3D printing [
14,
15]. Using 3D stress-freezing technologies and extraction methods, quantitative characterization methodologies and visualization theories can be developed for the three-field to visually and dynamically display the evolution of discontinuous structure, the redistribution of stress field and the interaction of the three-field during deep coal mining and understand the mechanism and spatiotemporal evolution law of various physical phenomena (e.g. rock deformation), as demonstrated in Fig. 3.
The characteristics of ruptured deep rock mass structures, in situ stresses and evolution of the three-field caused by fluidized mining should be investigated. The fluidized coal mining should be inferred beforehand by using quantitative characterization methodologies and visualization theories to visually display the possible disasters related to in situ coal mining and pre-judge their impact, forewarn authorities and pre-solve them accordingly. This implies a major change to traditional mining patterns, accomplishing deep in situ fluidized coal mining in an efficient, intelligent, and unmanned manner.
3.3 Multi-physics coupling in in situ transformation
Compared with traditional coal mining patterns, deep
in situ fluidized coal mining can gasify, liquefy, and electrify solid-state coal resources
in situ through a number of mechanical, chemical, and microbiological methods. In addition, deep rock mass are not only subjected to the coupling effects of stress, temperature, and seepage, but are also influenced by other factors, including chemical reactions (e.g., phase transformation of solid-state resources) and microbiological transformations. The process comprises not only coupling effects of stress-seepage-temperature-damage-fracture field, but also the interaction of multiple transformation agents, such as physical crushing, chemical reactions and microbiological reactions [
11–
13]. Therefore, multi-physics coupling principles and methodologies based on traditional coal mining conditions will not be suitable.
Therefore, it is necessary to fully consider the condition of deep in situ fluidized mining, including the coexistence of multi-phase medium, multi-physics coupling and the interaction of multiple transformation agents, as displayed in Fig. 4. Moreover, it is necessary to build a multi-physics coupling model comprising trans-scale fracture structures (e.g., micro, fine and macro structures), a multi-phase coexistence environment (e.g., solid, liquid, gaseous and electrical phases), and multiple action mechanisms (e.g. stress, seepage, temperature, chemical reaction and microbiological reaction) to uncover the constitutive behaviors, seepage mechanisms, deformation characteristics and fracture laws of rock mass under different fluidized mining modes. Then, multi-physics coupling theories can finally be established.
Given the phase transformation and swell effect of in situ liquefaction and gasification of deep coal, it is necessary to develop nonlinear seepage and diffusion theories to uncover the evolutionary processes of boundaries and environmental damage. Using energy theories and nonlinear mathematics, the mechanical behaviors of rock deformation and fractures under the multi-physics coupling effect are studied. In addition, spatiotemporal evolution law and mutation mechanisms of energy accumulation, dissipation, and release during mining are also examined. From the perspective of mechanical behaviors and energy characteristics, the development of disaster-inducing mechanisms and criteria are foreseen, proposing that appropriate methods and technologies for prevention be developed.
3.4 In situ mining, transformation and transport
Based on the technological characteristics of deep in situ fluidized mining and the special environment of deep rock mass (e.g., high ground stress, high gas content, high temperature, high osmotic pressure, and strong time effects), the development of theories of underground mine development and layout, well-suited for deep in situ liquefaction, gasification and electrification are foreseen. Brand-new theories are developed and supported with related technologies, such as mine construction, exploitation, mining, transport, filling and supporting, as depicted in Fig. 5.
In situ fluidization of solid resources is the key phase of this type of coal mining. Sufficiently high rates of fluidization and maximised extraction of useful constituents are the keys to deep in situ fluidized coal mining. Further, they ensure that the fluidized mining technologies can be implemented successfully and applied industrially.
Therefore, it is necessary to further build and develop in situ fluidization theories of deep coal and investigate the chemical and biological mechanisms of fluidization of solid-state resources. In summary, the effect of temperature, pressure, and reaction time on the liquefaction rates of coal resources should be studied. The dissolution and diffusion mechanisms of coal under the application of supercritical extraction solvents must be investigated, their mechanisms of transformation must be studied, and those reactions using bacterial solutions and gases (namely, the reaction principle for chemical and biological liquefaction and gasification of coal under the deep and in situ conditions) must be controlled to uncover the mechanism of deep and in situ fluidization and to build a fluidization theories system for in situ fluidized mining of deep mineral resources. Moreover, it is necessary to develop multi-phase fluid dynamics theories for long-distance pipeline transportation of viscous coal slurry and fine pulverised coal, as exhibited in Fig. 6.
4 Key technologies for deep in situ fluidized mining
4.1 Intelligent technologies
Using deep in situ multi-field precise probing technologies, equipment for geological structures and other technologies (e.g., big data, virtual reality (VR) and 3D geographical information systems (GIS)), guaranteed geological technologies, a virtual 3D GIS for coal mines with information systems and tools for building a high-precision 3D dynamic model for mines and equipment can be developed to display transparent mines of deep and in situ rock mass, as presented in Fig. 7. This will help accomplish transparent mining.
To accomplish transparent mines, precise mine navigation and positioning technologies must be developed using wireless long-distance microwave devices, as shown in Fig. 8. Moreover, it is necessary to develop integrated processes for mine construction, tunnelling, mining, transport, filling and supporting while accomplishing intelligent sensing and real-time controls. To automate the coal separation workflow and to make the process parameters more intelligent, intelligent monitoring of physical properties must be implemented, such as raw coal density, intelligent control of the coal separation process, multivariable coupling, intelligent sensing, and remote monitoring. This includes deep and in situ modular intelligent washing and separation equipment with advanced and intelligent control system. The intent is to develop intelligent control technologies that cover the whole process of deep in situ fluidized mining (including physical, chemical, and biological mining, liquefaction, gasification, and electrification), thus, ensuring long-term, continuous, stable, and large-scale mining.
4.2 Unmanned technologies
Developing integrated technologies for deep
in situ fluidized mining is of significant importance (including the technologies for mining, separation, filling, electrification, gasification, and thermalization). Based on the requirements for unmanned and intelligent underground operations, special design technologies for deep and
in situ intelligent shield-mining equipment and integrated technologies for mining, separation, transport, transformation, and filling units are studied to create the development and transformation conditions suitable for deep
in situ fluidized mining [
7,
9,
16]. The deep and
in situ shield-mining unit is responsible for unmanned and intelligent tunnelling and transport. Leveraging its great-depth multi-field-source precise probing device and multi-scale deep spatially-distributed navigation prototype system (Fig. 8), geographically precise information can be dynamically displayed and integrated on a virtual monitoring platform, accomplishing real-time coal mining control. Crushing and separation units are used to crush rocks and separate the clean pulverised coal. A fluidization reaction unit is used for plasma detonation, and a modular generator set is thus built. A small power plant is used to gasify and burn coal underground. Then, the resources can be transferred to the surface efficiently and intelligently. Transformed mineral wastes can be mixed and processed to backfill the goaves to control stratum movements and surface subsidence to ensure safe and green mining.
Unmanned intelligent transportation and lifting technologies can be provided by first developing related systems (e.g., control systems, unmanned intelligent sensors, detection and protection capabilities, and VR-based remote repair and maintenance systems). To meet mining requirements under different geological conditions and depths, deep and in situ intelligent auxiliary transport technologies are studied, including unmanned driving and dual-power traction technologies for auxiliary transport vehicles. The fluidized vertical transport model for ultra-deep coal resources, characterized by the U-shaped balance fluidized cycle and the pump transport of double mine-shafts and powered by compressed air are studied to accomplish continuous and efficient lifting in ultra-deep vertical mines, as shown in Fig. 9. Based on the characteristics of traditional wheel-track transport and pipeline transport, magnetic levitation pipeline transport technologies and linear motor-driven transport technologies can be developed.
4.3 Fluidization technologies
The fluidization system for deep in situ fluidized mining comprises energy-induced physical crushing, chemical transformation, biodegradation and power generated by pulverised coal deflagration.
The energy-induced physical crushing of coal and rock mass requires high stress on deep coal to protrude their fluidized substances. The fluidized substances are then pumped to the surface. To investigate the feasibility of large-scale, large-volume, and high-intensity fracturing and crushing of deep and
in situ coal seams, a waterless ultra-large mass fracture transformation and related technology, integrating various methods (e.g., high-energy pneumatic embrittlement, CO
2-based fracturing, macromolecular proppant and deep earth microbe modification and upgrade) is proposed. Chemical transformation directly transforms coal into liquid or gas
in situ by chemical reaction [
7,
9,
17]. The entire process of chemical transformation must be miniaturised, precise, stable, and controllable. To implement these technologies, it is necessary to study related technologies, including rapid liquefaction of the deep and
in situ fluidization unit, chemical liquefaction (e.g., direct moderate liquefaction, oxidation of coal to produce chemicals, dissolving coal and making coal into oil), deep and
in situ underground gasification, and deep and
in situ cave-based microbiological gasification. Biodegradation implies that deep coal matrices and coal seam gases are degraded
in situ using microbes, thus, transforming coal into gaseous and liquid materials in the mines. The key technologies of biodegradation include cultivation of exogenous efficient bacterial strains and activation of indigenous bacterial strains, multi-branch injection and fracturing diffusion of bacteria solution in coal seams, reaction control for bacteria solution transformation, extracting of transformed gas and liquid products, and control of underground environmental pollution. Additionally, the key technologies include deep and
in situ power generation via pulverised coal deflagration and internal combustion power generation using the pressure boost of pulverised coal. The power generated by pulverised coal deflagration is based on the principle of pulse detonation techniques. After mixing gas and air with an appropriate concentration is input, a sprayed pulverised coal can be ignited using ion ignition, as shown in Fig. 10.
5 Strategic roadmap
Based on the groundbreaking theories and key technology systems for deep in situ fluidized mining, this paper presents a strategy roadmap, namely, “Fundamental research in 2025, technological breakthrough in 2035, and integration demonstration in 2050,” as shown in Fig. 11.
In the conceptual stage of developing fundamental theories and technologies (prior to 2025), the main task will be to study the fundamental theories and technologies for fluidized mining. Specifically, the research will focus on informationalized and automated mining with ultralow ecological damage to minimize mining personnel and ensure that the carbon emission level of coal mining is approximate to that of natural-gas exploitation.
In the stage of technological breakthroughs and equipment research and development (2025 to 2035), the main task will be to accomplish intelligent and unmanned mining with near-zero ecological damage using unmanned mining, integration, separation, filling and power generation using pulverised coal deflagration and guided and controlled gasification, to accomplish nearly-unmanned mining and to ensure that the carbon emission level is approximate to that of clean energy.
In the stage of technologies integration and industrial demonstration (2035 to 2050), the main task will be to study the integration of different technologies for deep in situ fluidized mining and mine exploitation and implementation. Specifically, coal-based multi-unit synergy will be conducted using appropriate technologies (e.g., layout of mine exploitation and whole-process controls). Integrated energy systems will be developed and fluidized mining will be performed in situ to accomplish the ultimate goal of ‘There are no coal on the ground and no men in the coal mine’ and to accomplish environmentally friendly development of pure clean energy with zero carbon emissions and zero damage.
During the strategic process, the new theories and technologies for coal mining which need to be implemented focus on: fundamentally new theories of fluidized mining; design theories and new technologies for fluidized mining; precise probing technologies for fluidized mining; power generated by deep and in situ pulverised coal deflagration; deep and in situ reformulation and energy conversion technologies for CO2 and CH4; deep and in situ coal liquefaction technologies; deep and in situ coal gasification technologies; integrated technologies for fluidized mining, in situ energy exchanges, storage and transformation; in situ harmless disposal and technologies utilization for fluidized mining wastes; intelligent transport and lifting technologies for fluidized mining; underground hydraulic power and high stress-induced mining technologies; new technologies for multi-functional shield-machine systems; and health, safety, and environmental management systems for efficient and safe operation of fluidized mining.
6 Conclusions
Deep in situ fluidized mining refers to the fluidized mining technologies through which deep solid coal resources are transformed into gaseous, liquid, or mixed gaseous-solid-liquid substances in situ and are subjected to integrated development (including mining, separation, filling, thermalization, electrification, and gasification) in the mines in an unmanned and intelligent manner. Fluidized mining of coal resources is based on the fluidized mining theoretical system and is underpinned by three technological systems (including intelligence, unmanned, and fluidization). Related technological equipment needs to be developed, thus, providing a foundation for unmanned underground operation and safe and efficient mining of coal resources. Deep in situ fluidized mining poses challenges to the traditional concepts and systems used for coal mining and provides a thorough solution to various extant problems in the industry (e.g., low efficiency, poor safety, severe ecological damage, low resource recovery rates and large energy losses during surface transport and transformation). This will offer new mining theories, taking the lead in a potential mineral-mining revolution.
Deep in situ fluidized coal mining is an important deep earth science. It is a current strategic reserve technology in China for energy development. With the exploration of basic laws of deep earth science and technological innovations, this scientific concept is bound to revolutionized mining technologies, creating a new industry pattern and accomplishing clean, efficient and eco-friendly development of mineral resources from the deep underground.
Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature
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