Complexity Study on Multi-Field Coupling Systems for Underground Coal Fires

Abstract

Underground coal fires are a major disaster that needs to be urgently addressed in the coal mining industry, as they can cause waste of resources, environmental pollution, threaten the life and health of species, and cause serious damage to society and the economy. Currently, the research on the prevention and control of underground coal fires focuses more on the technical level and lacks scientific guidance at the methodological level. Based on this research gap, this paper uses CiteSpace 6.1.R6 to analyze the research hotspots and subjects in the field of underground coal fires and gives a comprehensive research overview of its microscopic chemical reaction substance and macroscopic multi-field coupling characteristics. Then, from the methodological level, it summarizes the complex system characteristics of underground coal fires and puts forward the elements and principles for the prevention and control of underground coal fire disasters under the paradigm of complex system research. This study proposes a new way of thinking for the prevention and control of underground coal fires and helps to build a whole-process prevention and control system.

1. Introduction

Coal is the world’s basic energy, occupies an important position in the energy structure, and makes a great contribution to maintaining social and economic stability [1]. However, at present, the coal resources of many countries are under the threat of underground coal fire disasters, such as the United States, Australia, China, India, and other countries rich in coal resources [2,3,4]. Underground coal fires will lead to a large waste of coal resources, make mining more difficult, and cause huge direct economic losses [5,6]. In addition, underground coal fires cause indirect damage in other ways, such as the formation of large-scale empty areas underground due to prolonged combustion; the thermal damage effect of a long-term high-temperature environment on rock strata, which causes subsidence and fissures in rock strata and ground surface and brings hidden danger to surface facilities [7]; coal combustion produces large amounts of greenhouse gases (CO₂) and toxic and harmful gases (CO, SO₂, H₂S, and NO_x, etc.), which trigger haze, acid rain, etc., leading to climate deterioration [8,9]; other products from combustion, such as sulfur and coal tar, can deteriorate the soil, thus affecting the life and health of surface plants and animals [10]. In addition, underground coal fires can exacerbate the severity of gas disasters and sudden water disasters in coal seams and trigger more serious compound disasters [11]. The underground coal fire disaster seriously contradicts the concept of green and low-carbon development. Only by realizing the scientific management of underground coal fire disasters can we fundamentally reduce the various hazards generated by underground coal fire disasters.

At present, the research on underground coal fires has been ongoing for a long time [12], and many scholars have not only comprehensively revealed the mechanism of underground coal fire at the micro- and macro-levels [13] but also carried out a lot of research on prevention and control techniques and methods for underground coal fires. Many coal mines predict the risk of coal spontaneous combustion by dividing the “three zones “ of spontaneous combustion in the mining process and then taking measures to prevent and extinguish the fire [14]. Some scholars have proposed injecting new materials such as inert gas or three-phase foam into the fire zone to inhibit coal spontaneous combustion by stopping the oxidization process of coal [15,16]. Szurgacz et al. [17] proposed injecting an ash and water mixture or an ash and water mixture with carbon dioxide into the gob, which not only prevented the entry of oxygen but also interfered with the oxidizing process of coal and achieved a good fire extinguishing effect. In recent years, scholars have also proposed a new concept of resource utilization for thermal energy from coal combustion, which provides a new way to manage underground coal fires [18,19]. Current scholars’ attention on underground coal fires is mainly focused on certain technical issues. These methods and techniques are effective means to prevent and control underground coal fires. However, many issues reduce their generalizability, including the complex cause of disasters, the hidden and complex spatial structure of the disaster site, and the ease of rekindling [20,21,22,23]. There is still a lack of methodological guidance for disaster prevention and control in this complex system. Therefore, it is necessary to analyze the complexity of underground coal fires, dissect their system characteristics, and explore a scientific management approach to mitigate the hazards of underground coal fires.

Complexity science takes a “complex system” as the research object, which is an emerging science that studies and reveals the operation laws of complex systems. The research paradigm of complexity science has been well applied in many complex projects. Sheng et al. [24] used the research paradigm of complex systems to demonstrate the change in the environmental governance system of Lake Tai in China, and Zhang et al. [25] used the research paradigm of complex systems to analyze the principles that need to be grasped in the management of urban compound disasters. The above studies show that complexity science has a good advantage in comprehensively controlling the management elements and principles of complex systems and is a reliable methodology for constructing the whole-process management system of underground coal fires.

This paper analyzes the literature of underground coal fires based on CiteSpace 6.1.R6 and constructs a knowledge map of underground coal fire hotspots and topics. Based on the knowledge map, a comprehensive and in-depth summary of the relevant literature was carried out, and it was found that the research on underground coal fires focuses on the technical level, while the research on the methodological level is relatively small. Therefore, under the research paradigm of complexity science, this paper summarizes the systematic characteristics of underground coal fires and puts forward the elements and principles that need to be controlled in the process of underground coal fire disaster management. This study provides a new way of thinking for the whole-process scientific management of underground coal fires.

2. Analysis of Research Literature on Underground Coal Fires

To understand the current status of research in the field of underground coal fires, and to grasp the frontier issues, hot issues, and challenges in the field, we analyzed the literature in the field of underground coal fires using CiteSpace6.1.R6. The analysis data were obtained from the Web of Science (WOS), and the search terms were “underground coal fire” and “coalfield fire”. The study period was 30 years from 1992 to 2022.

2.1. Publication Volume Analysis

The number of publications in a period represents the level and speed of development in the field. In the WOS database, a total of 773 English-language studies were counted, with an average of 26 published each year, as shown in Figure 1.

Table 1. The cumulative number and accumulation rate of English-language studies published in the field of underground coal fires.

Year	Number	Cumulative Count	Cumulative Rate/%	Year	Number	Cumulative Count	Cumulative Rate/%
1999	6	6	/	2011	18	146	14.1
2000	7	13	116.7	2012	27	173	18.5
2001	4	17	30.8	2013	23	196	13.3
2002	5	22	29.4	2014	27	223	13.8
2003	10	32	45.5	2015	47	270	21.1
2004	18	50	56.3	2016	52	322	19.3
2005	8	58	16.0	2017	58	380	18.0
2006	6	64	10.3	2018	74	454	19.5
2007	17	81	26.6	2019	71	525	15.6
2008	21	102	25.9	2020	83	608	15.8
2009	17	119	16.7	2021	94	702	15.5
2010	9	128	7.6	2022	71	773	10.1

The literature cumulative count is the sum of all literature counts for the current year and the previous year. The literature cumulative rate is the ratio of the current year’s issuance to the previous year’s literature cumulative count.

is obtained by calculating the cumulative number and cumulative rate of English-language studies published in the field of underground coal fires after 2000. To intuitively reveal the development trend of underground coal fires since the 21st century, combined with Figure 1 and Table 1, its development can be divided into two stages: the initial stage and the growth stage, corresponding to 2010 and before and 2011 to 2022.

Initial development period (2010 and before): A total of 128 papers were published in this stage, and the literature accumulation rate varied greatly; the research in the field of underground coal fires gradually attracted attention. The widely recognized and highly cited literature such as ‘Coal fires burning out of control around the world: thermodynamic recipe for environmental catastrophe’ and ‘Research progress of underground coal fire detection, monitoring and fire suppression technology’ have accumulated strength for the further development of this field. Rapid growth period (2011–2022): A total of 645 papers were published in this period, which is five times that of the initial development period. The review paper ‘Coal fires in China over the last decade: A comprehensive review’ describes in detail the progress of underground coal fire detection, modeling, environmental and human health impact assessment, and prevention and control. With the increasing attention on the large-scale exploitation and exploration of underground coal resources and environmental problems, underground coal fires have been paid more and more attention, and the research has increased rapidly.

2.2. Research Subject Analysis

The authors of the literature in a given field are usually the productive output of the field and have a significant impact on its development. In addition, the contribution of research institutions is another angle to analyze the development status of the field. Therefore, to understand the research subjects in the field of underground coal fires, it is necessary to analyze the situation from both the perspectives of the author and the institution.

Figure 2 and

Table 2. Statistics of author publications in the field of underground coal fires.

	Author	Number	Area	Year of First Publication
1	Deng, Jun	18	PEOPLES R CHINA	2015
2	Hower, James C	15	USA	2009
3	Wang, Deming	14	PEOPLES R CHINA	2015
4	Liang, Handong	13	PEOPLES R CHINA	2018
5	Zhou, Fubao	11	PEOPLES R CHINA	2015
6	Singh, Gurdeep	10	INDIA	2016
7	Wen, Hu	9	PEOPLES R CHINA	2015
8	Tang, Yibo	9	PEOPLES R CHINA	2018
9	Cheng, Jianwei	9	PEOPLES R CHINA	2012
10	Xiao, Yang	8	PEOPLES R CHINA	2015
11	Shi, Bobo	8	PEOPLES R CHINA	2017
12	Shu, Chi-Min	8	CHINA(TAIWAN)	2019

show that among the English literature counted, the most published authors in the field of underground coal fires are Deng Jun, a Chinese scholar from Xi‘an University of Science and Technology, followed by Hower James C, an American scholar from the University of Kentucky, Wang Deming and Zhou Fubao from China University of Mining and Technology, and Liang Handong from China University of Mining and Technology (Beijing). Among the top 10 authors, Chinese scholars account for 80%, indicating that Chinese scholars have carried out long-term attention and research in the field of underground coal fires, which is reflected in the rich research results.

Figure 3 is the co-occurrence network diagram of cooperation between research institutions in the field of international underground coal fires, in which the nodes represent institutions, and the size of nodes represents the amount of paper output of these institutions. The larger the nodes, the higher the number of published papers; the more connections, which represent the cooperation between institutions, the closer the cooperation between institutions. Among them, the China University of Mining and Technology ranked first with 147 publications, and the China University of Mining and Technology (Beijing) and the Indian Institute of Technology occupied the top three. There are many connections between the Indian Institute of Technology and the Shandong University of Science and Technology, indicating the existence of close research cooperation. There are many connections between the China University of Mining and Technology, the Chinese Academy of Sciences, and the University of Kentucky, indicating that they have carried out more cooperation and exchanges and are active in the field of underground coal fires.

2.3. Research Topic Analysis

Keywords are the core of a paper’s topic content, and the keywords in the research literature constitute the keyword co-occurrence map. In the keyword co-occurrence map, the nodes represent the centrality of the keyword; the greater the centrality, the more related keywords there are; the thickness of the line represents the frequency of keyword co-occurrence, and the thicker the line, the closer the connection between the two keywords. Figure 4 shows the keyword co-occurrence map of research in the field of underground coal fires. The network density is 0.0177, and the value is small, indicating that the research topics in this field are relatively divergent.

Table 3. High-frequency keywords and frequency of research in the field of underground coal fires.

	Keyword	Frequency
1	Spontaneous combustion	144
2	Coal fire	104
3	Fire	89
4	Coalfield	77
5	Jharia coalfield	71
6	Temperature	58
7	Area	44
8	China	43
9	Mine	41
10	Behavior	40
11	Low-temperature oxidation	39
12	Combustion	37
13	Emission	35
14	Model	35
15	Environmental impact	34
16	Trace element	31
17	Inner Mongolia	30
18	Mine fire	28
19	Surface	26
20	Oxidation	25

shows the summary of high-frequency keywords and their occurrence times, and the visualization processing of keyword clustering is carried out.

As shown in Figure 5, 43 keywords in the English literature are clustered, and it can be found that the clustering structure is significant, but the graph structure is scattered. The emergence analysis of keywords was carried out, and the results are shown in

Table 4. Results of keyword emergence in the field of underground coal fires.

	Keyword	Emergence Intensity	Start Time	End Time	1999–2022
1	coal mining	3.55	2000	2009
2	surface	5.47	2004	2012
3	area	3.45	2010	2016
4	origin	4.23	2011	2015
5	powder river basin	4.50	2014	2018
6	impact	4.06	2018	2019
7	underground coal mine	4.04	2018	2019
8	low temperature oxidation	3.90	2018	2020
9	coal	4.02	2019	2020
10	Wuda coalfield	3.59	2019	2022
11	pyrolysis	4.56	2020	2022
12	kinetics	3.34	2020	2022

The size of the emergence intensity indicates the increased use of the keyword in a certain period, and the greater the intensity, the more it can represent being at the forefront of research in the field. The red line indicates the time stage of the keyword as a research hotspot, the light blue line indicates the stage when the keyword does not appear, and the dark blue line indicates the stage when the keyword continues to exist.

, which shows the time stage when each keyword becomes a hot topic. It can be seen from the clustering results and emergence analysis that in recent years, the research on underground coal fires focuses on the dynamic and thermodynamic combustion mechanism of coal, and the impact on the environment is also explored. In addition, underground coal fire prevention and fire extinguishing technology have gradually become hot issues in recent years; however, due to this being a recent development, it is not reflected in the keyword analysis results.

Based on the results of the CiteSpace 6.1.R6 literature analysis, it can be concluded that China, the United States, India, Australia, and other countries with serious underground coal fire disasters have high academic influence in this field. There are a wide range of research topics and hot spots in the field of underground coal fires, such as the study of coal spontaneous combustion mechanisms, fire detection, coal fire prevention and control, fire management, coal fire environmental impact, etc., which have academic research value and significance In the field of engineering. It is worth noting that there is little scientific theoretical guidance on the prevention and control of underground coal fires in the current literature, indicating that there is still room for in-depth research.

3. Literature Review on Underground Coal Fires

Underground coal fire hazards are difficult to monitor and study in a holistic and long-term manner due to their complex causes, long disaster cycles, and hidden locations. Therefore, it is difficult to form a scientific and complete management system. Therefore, based on the conclusion of the analysis of the literature, the key and hot research contents are specifically discussed to provide a basis for analyzing the complex system characteristics of underground coal fires.

3.1. Combustion Characteristics of Coal

The oxidation and combustion characteristics of coal determine the reaction essence of coal and oxygen, which plays a decisive role in the occurrence and development of underground coal fires. Some scholars have experimentally determined the heat flow curves of coal during oxidation, combustion, etc., through thermal analysis techniques to obtain the kinetic parameters at each reaction stage, thus constructing a kinetic model of a coal–oxygen reaction [26,27]; some scholars have also used material analysis techniques to determine the changes in mass and microstructure of coal during oxidation, as well as various reaction products to determine the chemical essence of the coal–oxygen reaction at each stage. The studies related to thermal analysis and material analysis techniques are listed in

Table 5. Summary of the literature on coal oxygen reaction studies.

Analysis Technique	Number	Method	Study Object	Conclusion
Thermal analysis technique	1 [28]	Thermogravimetry (TG); Differential scanning calorimetry (DSC)	Lignite	Three-stage kinetic parameters of coal during low-temperature oxidation at different oxygen concentrations.
	2 [29]	Thermogravimetry (TG)	Jet coal; Gas coal; Fat coal	Two-stage kinetic parameters of coal during low-temperature oxidation and high-temperature combustion at different oxygen concentrations.
	3 [30]	Adiabatic self-heating incubation test	From lignite to low volatile bituminous	Effect of water content and pyrite content of coal on its self-heating behavior
Material analysis technique	1 [31]	X-ray diffraction (XRD); Fourier transform infrared spectroscopy (FTIR); TG-DSC	Bituminous coal; thermally altered coal;	Changes in chemical structure and combustion characteristics of coal during the reaction.
	2 [32]	Scanning electron microscope (SEM); N-2 adsorption; Chromatography oxygen absorption electron; Spin resonance spectrometer (ESR)	Jet coal	Effect of water immersion on the structure and low-temperature oxidation of coal.
	3 [33]	TG-FTIR; TG-GC/TCD	Lignite	Two-stage kinetic parameters and two-step reaction mechanism.

. These studies show that different coal species have different combustion characteristics during the reaction, and the same coal exhibits different characteristics at different stages. The coals show large differences in microscopic reactions, which in turn lead to large differences in the consumption patterns of coal and oxygen and the production patterns of products.

The rich research results not only reveal the chemical reaction laws from the microscopic point of view but also explain the reasons why the coal–oxygen reaction can accumulate heat and warm up autonomously resulting in spontaneous combustion. The study of coal spontaneous combustion mechanisms has an important role in the study of underground coal fire disasters, and only by clarifying the essence of coal–oxygen reactions at the microscopic scale can we further explore the multi-field coupling laws of underground coal fires at the macroscopic scale.

3.2. Multi-Field Coupling Characteristics of Underground Coal Fires

Considered at the macroscopic level, the study of underground coal fires not only includes the chemical reaction between coal and oxygen but also involves multi-field coupling such as the temperature field, seepage field, concentration field, and stress-strain field. Therefore, underground coal fires are thermodynamic hazards under the coupling effect of multiple fields.

3.2.1. Multi-Field Coupling Law

Unlike ordinary combustion phenomena, underground coal fires occur without a direct ignition source but require a long period for the coal seam to react with oxygen, gather heat, and warm up to reach the ignition point of the coal seam. After that, oxygen must continuously enter the underground to maintain the continuous combustion of the coal seam. In this process, the void structure of the rock layers and the ground surface is the main channel for the contact between oxygen and the coal seam, as well as the heat and mass transfer channel in the combustion process [34]. In addition, the process of the violent combustion of coal changes the temperature distribution of the coal rock seam in a larger area, while changing the pattern of material transport by consuming oxygen and releasing gas products [35,36]. The formation of high-temperature regions, on the one hand, affects the rate of chemical reactions, and, on the other hand, the fire and wind pressure also changes the flow pattern of gases [37]; the different flow states of gases affect the distribution of material concentration, and, in turn, this will have a facilitating or inhibiting effect on coal spontaneous combustion [38,39]. Therefore, as shown in Figure 6 [40], the development of underground coal fires constitutes a complex system with multi-field coupling.

Many scholars have conducted in-depth studies on the multi-field coupling law of underground coal fires. Wang, Wickowski, and Tutak et al. studied the effects of different airflow states on the temperature and material concentration fields during coal spontaneous combustion. The influencing factors included the ventilation volume, airflow speed, and ventilation method in the fire zone [41,42,43]. Wang [44] and Peng [45] focused on the effect of high temperatures on other fields; the former verified that the coal oxidation reaction rate accelerates with increasing temperature and there is a critical temperature from slow to fast oxidation; the latter suggested that the temperature gradient changes the flow rate of the surrounding gas. Plakunov et al. [46], on the other hand, focused on the temperature and gas concentration fields of underground coal fires and analyzed the effect of increased temperature on CO₂ diffusion. Xia, Song, Zheng, and Li constructed a thermal-hydro-mechanical model of underground coal fires using COMSOL Multiphysics. They performed numerical simulations of natural coal fires under different conditions to study their disaster-causing laws [47,48,49,50]. The above study analyzed the multi-field coupling process of underground coal fires and provided ideas for the exploration of the disaster-causing mechanism. However, due to the large number of variables and the interactions between them, it is still difficult to fully grasp their developmental laws. Therefore, the evolution law of coal fires under multi-factor coupling conditions is still the focus and difficulty of underground coal fire research. Scientific theories to guide the prevention and control strategies of underground coal fires still needs to be studied in depth.

3.2.2. The Effect of Void Distribution

In the complex system of the multi-field coupling of underground coal fires expressed in Figure 6, the complex network of voids in the rock layers and surface is not only necessary for the occurrence and expansion of underground coal fires but also for the damage they cause to the external environment. For example, the toxic and hazardous gas products of coal fires must be transported to the surface through these voids and then escape to the atmosphere; in compound disasters, the transport of gas and groundwater, etc., must likewise enter the danger zone through these voids. Therefore, the characteristics of complex voids under the influence of coal seam mining and combustion disturbances are of great significance to the study of the occurrence and development laws of underground coal fires.

Many scholars have conducted qualitative and quantitative studies on the spatial characteristics of rock layers and ground surfaces during coal combustion. Hao et al. [51] and Su et al. [38] built similar experimental platforms for the fissures in the mined area and overlying strata, respectively. The variation of internal oxygen concentration was analyzed, and the danger zone of coal spontaneous combustion was proposed. Zhuo et al. [34] constructed a fracture model and multi-field coupling model of the mining area using FLUENT software. The distributions of airflow states and gas concentration fields were calculated, and the calculation results were verified in the field. These studies showed that the void rate distribution has a decisive influence on the temperature field, concentration field, and airflow field. Recently, Wang’s team from Central South University conducted an in-depth study on the void rate distribution in the rock layers and the surface under a mining and combustion disturbance. They obtained the void rate distribution models after horizontal and inclined coal seam mining by theoretical derivation, physical experiments, and numerical simulations [40,52,53,54]. Their research results are of great significance for carrying out the study of multi-field coupling characteristics of underground coal fires and the study of fire prevention and suppression.

4. Complexity Study of Underground Coal Fires

Through literature analysis, it is argued that the multi-field coupling and disaster management of underground coal fires is a long-term and complex process. Its study must simultaneously consider chemical reactions, heat mass transfer, and solid deformation. Whether studying the mechanism from a microscopic perspective or the law of multi-field coupling from a macroscopic perspective, the study of underground coal fires has complex characteristics. There is no perfect theoretical system to guide the whole process of disaster prevention and control of underground coal fires. Therefore, to achieve effective prevention and control of underground coal fires, we must first grasp the elements and principles of underground coal fire management from the theoretical level and then guide the practice through scientific theories.

4.1. Complex System Judgment of Underground Coal Fires

Based on the currently accepted scientific methods for judging complex problems [55,56], it is possible to extrapolate the complex problem of underground coal fires from Figure 7.

(1)

Extrapolation from the criteria for judging complex scientific problems

Stochastic problems are complex scientific problems. Underground coal fires are a thermodynamic disaster under the coupling effect of multiple fields; the source of the fire has uncertainty; the evolution process risk is difficult to predict, the degree of disaster is difficult to estimate, and the accident is difficult to prevent, so it has the obvious characteristics of stochastic problems. Therefore, the underground coal fire problem is a complex system problem.

Nonlinear problems are complex problems. In the changes of void rate, temperature field, component field and airflow field and their dynamic relations under the coupling effect of multiple fields, it is impossible to find out the clear mapping relationship between variables, and the coupling effect of multiple fields cannot be completely described by a linear relationship. It has obvious nonlinear characteristics, so the underground coal fire problem is a complex system problem.

People, communities, and social systems are complex problems. In the governance process of underground coal fires, people management, technical specifications, and institutional rules are implemented throughout the process. People and communities are the main objects of study and service in the governance process, so the underground coal fire problem is a complex system problem.

(2)

Inference from the systematic characteristics of underground coal fires

First, there are many factors affecting underground coal fires, and there are complex correlations and interactions. The spatial structure, heat and mass transfer phenomena, and fluid transport are affected by many factors inside and outside the system, such as the chemical properties of coal, oxygen concentration, air flow rate, and air pressure. Secondly, under the influence of multiple field couplings and the external environment, the influencing factors evolve and develop continuously and have dynamic properties. At the same time, there are interactions between the influencing factors, and there is also an exchange of information and materials between the underground coal fire system and the external environment, which has a certain openness, so that the underground coal fire is always in dynamic change. In addition, in the process of underground coal fire prevention and control, the influence of the external environment and the intervention of human measures determine the system development direction with indeterminacy, as well as emergence, mutation, chaos, self-organization, and other phenomena.

In summary, the underground coal fire system has the typical characteristics of complex systems such as numerous influencing factors, complex associations, openness, dynamics, indeterminacy and emergence, mutation, chaos, and self-organization. These characteristics determine that the underground coal fire system meets the judgment criteria of complex problems, that is, it is stochastic, nonlinear, and composed of people, communities, and social systems.

4.2. Complex System Characteristics of Underground Coal Fires

Combined with the characteristics of complex systems, the typical complexity characteristics of underground coal fire systems are manifested in the compound nature of the cause, the dynamics of the process, the evolution nonlinearity, and the unpredictability of the outcome. The underground coal fire system is shown in Figure 8.

(1)

The compound nature of the cause

Underground coal fire disasters are not the result of a single factor but the result of material reactions, atmospheric and water cycles, and human mining. The most important feature of the cause of underground coal fires is the “nesting” of the causal factors, i.e., the cause –effect relationship is interlocked, which eventually leads to or aggravates the disaster. The disaster-causing factors are compounded and overlapped in time and space. For example, in the process of coal combustion, airflow affects the process of heat transfer, which in turn affects the temperature distribution. The temperature, in turn, affects the rate of airflow, forming a “two-way coupling” chain. The complex multi-field coupling relationship makes it difficult to control and solve underground coal fires using reductionist methods.

(2)

The dynamics of the process

Underground coal fires are, by nature, analyzed as time-dependent kinetic hazards. Various forms of motion are included in the kinetic system of underground coal fires: heat transfer, fluid transport, the deformation of rock layers, and the oxidative decomposition of coal. These forms of motion change slowly or drastically with time and will always develop in a drastic direction if not controlled by humans.

(3)

The evolution nonlinearity

Underground coal fires occur with nonlinear variations in deformation, fluid transport, heat transfer, and other motions in the development space. In the study of multi-field coupling laws, nonlinear problems occur in the control models of each field. For example, the airflow law in a fissure should conform to Darcy’s law, but the effect of temperature increases its nonlinearity when considering the gas seepage problem in underground coal fires [47,50]. Therefore, the nonlinearity of the multi-field coupling system and the governing system determines that the development of underground coal fires is prone to chaos.

(4)

The unpredictability of the outcome

The interaction of the influencing factors of underground coal fires is nonlinear. This leads to hindrance in explaining, controlling, and predicting the evolution of underground coal fires. When conducting disaster control and prediction, the disaster-causing factors cannot be simply decomposed into individual parts by reductionism. Moreover, with the continuous development of underground coal fires, the fracture area expands and the void rate changes. The discontinuity, non-homogeneity, anisotropy, and non-stationary changes of the rock layers become more and more significant. This makes the underground coal fires more difficult to predict. The links involving people and management elements in fire prevention are also difficult to predict. Therefore, the results of underground coal fires are unpredictable.

4.3. Elements and Principles of Underground Coal Fire Disaster Prevention and Control

Based on the complex characteristics of underground coal fires, the complex systems research paradigm [57] is inherently compatible with underground coal fires. The underground coal fire disaster management that absorbs the complex system paradigm contains the following three elements.

(1)

Complex network relationship

The network is composed of “points” and their interconnected “lines”, which is the basic structure of the system. The “fields” involved in underground coal fires and the subsystems in the management chain can be regarded as “points” in the network structure. The interactions of “fields” and the relationships between subsystems are connected as “lines”. Together, they form a complex network for underground coal fire disaster management. In this network structure, the richly connected points indicate that the factor is the key element in the governance link, and the breakthrough of the safety value of the key “point” or the damage to the organization function will have a negative impact on disaster management.

(2)

Spatiotemporal dynamic evolution

The interplay between the causative factors of underground coal fires is interpenetrating and interlocking. After the threshold is exceeded, it still shows an escalating state of “spiraling”. In the evolution of underground coal fires, the causative factors accumulate with time and superimpose with other factors, and the events evolve and circulate in time and space with feedback loops, leading to the generation and aggravation of disasters. The spatiotemporal dynamic evolution and non-linear evolution make the underground coal fire disasters less predictable. Mastering the spatiotemporal dynamic evolution as a governance element can help to achieve governance effectiveness.

(3)

System Integrity

The complex system problem can be understood in both horizontal and vertical dimensions. The vertical dimension is a reductionist approach that decomposes the system into top-to-bottom levels until a certain element affecting the system is identified. The horizontal dimension is a holistic approach that looks for interaction laws. Combined with the complex systems paradigm, stable and orderly systems can be achieved through the combined action of top-down control and bottom-up self-organization. System integrity is one of the elements of underground coal fire disaster management.

Based on the complex characteristics of underground coal fires and three governance elements, three principles of disaster governance are obtained, and a logical framework for the complex governance of underground coal fire disasters is constructed in Figure 9.

(1)

Focus on complex network relationships rather than individual nodes

The complex system of underground coal fire hazard management is both interconnected and relatively independent in all aspects. The governance system will emerge with different functions and produce different effects under different roles and different modes of association. Combined with the characteristics of complex network relationships, it is more necessary to pay attention to the relational analysis of the complex network, and it is necessary to maintain the synergy and balance of the complex governance network relationships.

(2)

Spiral cycle governance

Underground coal fires evolve with time on the one hand and have multi-field coupling effects in space on the other. Their evolution process changes dynamically, the disaster results are difficult to predict, and the management process has the difficulties of easy reignition and difficult prevention and control. Combined with the spatiotemporal dynamic evolution characteristics, monitoring and control can be carried out in the mode of cyclic management. Through fire prevention technology to control and manage the evolution of underground coal fires, it is possible to continuously improve and implement the management plan and promote a good management effect with the spiral management mode. The spiraling cycle of management can make the system’s regulations and adaptability blend until good disaster management is achieved.

(3)

Aligning micro research and macro governance

In the complex system of underground coal fire disaster management, the holistic changes of the system usually come from subtle changes in the constituent elements. Therefore, to grasp the macroscopic evolutionary law and achieve a better macroscopic governance effect, it is necessary to fully study its microscopic mechanism. Since complex systems are highly nonlinear, they cannot be disassembled into multiple small parts by reductionism. Therefore, combined with the system integrity characteristics, the unification of micro-study and macro-governance should be insisted on. From this perspective, on the one hand, the combination of micro-disaster-causing mechanism research and macro governance research needs to be strengthened. On the other hand, the macro system is embedded in the micro measure actions, so that the macro system supports and also constrains the rationality and coordination of governance measures.

5. Conclusions

(1)

In this paper, the hotspots of underground coal fire research content and research subjects were obtained through CiteSpace 6.1.R6 analysis. The research hotspots in this field are summarized, including the mechanism of coal spontaneous combustion, detection and monitoring of underground coal fires, fire prevention technology, and its environmental impact.

(2)

The current status of research on the combustion characteristics of coal and multi-field coupling laws is demonstrated. It lays the foundation for the study of the complexity of underground coal fires.

(3)

From the perspective of complex science, it is concluded that the underground coal fire system is characterized by complex system features such as the compound nature of the cause, the dynamics of the process, the evolutionary nonlinearity, and the unpredictability of the outcome. Combined with the research paradigm of complex systems, three elements and three principles of underground coal fire disaster management are proposed, which put forward a new thinking path for the prevention and control of underground coal fires.

This paper proposes for the first time to use the research paradigm of complex science to guide the construction of the prevention and control system for underground coal fires. This study fills the gap in scientific prevention and control systems for underground coal fires to a certain extent. The conclusions obtained provide a good focus for the scientific management of the whole process of underground coal fires, which helps managers carry out comprehensive control in engineering practice and build a perfect prevention and control system. At the same time, grasping the above elements and principles in the prevention and control process of underground coal fires will enhance the management of the disaster. This will not only help to save coal resources, but also improve the efficiency of coal mining and promote the green development of the coal industry.