Abstract

Spontaneous coal combustion is the result of physical and chemical interactions between coal and oxygen over a long period of time. It is a complex physical and chemical process that only occurs under certain conditions (such as low-temperature oxidation and heat accumulation), during which the temperature of the coal rises, eventually reaching the ignition point. It is also a common cause of major disasters in coal mines, as it threatens mine safety, coal production, and coal applications. In order to deeply grasp the research and development status of coal spontaneous combustion in China and point out the direction for future research, firstly, this paper introduces the energy status of China's coal resources, then expounds the development process and theory of coal spontaneous combustion, summarizes the characters and related problems of different spontaneous combustion test methods, and thirdly introduces the main institutions and personnel of coal spontaneous combustion and the corresponding results, finally this paper discusses the existing problems and future development direction of coal spontaneous combustion in China.

1. Introduction

China's total energy consumption reached 4.86 billion tons of standard coal in 2019, with an average annual growth rate of 2.9%, and the use of standard coal increased by 5.3 billion tons compared to 2015. Coal accounted for approximately 57.7% of the total primary energy consumption in 2019 ¹. The energy supply issue is not only related to China's security and economic development, but also to environmental protection and climate change responses.

However, with the continued growth of coal production, various types of coal mine accidents such as mine fires and gas and coal dust explosions have occurred, resulting in high economic losses and a high number of casualties. In recent years, the number of coal mine fire accidents and the resulting deaths have decreased annually through the prevention and control of coal mine fires (Figure 1), however, major fire accidents still occur ². Through the accident data query system of the National Mine Safety Administration ³, we counted the number of coal mine fire accidents from 2000 to 2020. We found that accidents caused by spontaneous coal combustion accounted for the vast majority of coal mine fire accidents. In the past 10 years, the direct cause of coal mine fires in China was spontaneous coal combustion.

2. Development Process of Spontaneous Coal Combustion

Spontaneous coal combustion generally refers to a small amount of heat generated by physical adsorption, chemical adsorption, and oxidation reactions between coal and oxygen in air at room temperature. Under certain conditions, the oxidation heat generation rate is greater than the rate of heat dissipation to the environment, and heat accumulation causes the coal temperature to rise slowly but continuously. When the critical self-heating temperature of coal reaches, the oxidation heating rate accelerates, and finally, the ignition temperature is reached, and the coal is burned. According to existing researches, the oxidation and spontaneous combustion of coal is considered basic chain reactions, and the process of spontaneous coal combustion is generally divided into three stages: low-temperature oxidation, self-heating, and combustion.

On the basis of summarizing the research results of experts at home and abroad, our team summarized and simplified the sectional characters of coal spontaneous combustion by studying the heat production, oxygen consumption, products and so on in the process of coal spontaneous combustion. The details of that research are summarized in Table 1.

3. Development of The Spontaneous Coal Combustion Theory

Plott and Berzehus first proposed the theory of pyrite genesis in the 17th century ⁴. Potter et al. believed that a certain amount of heat released from coal during the fermentation of bacteria plays a decisive role in its self-ignition ⁴. In 1940, Soviet scholar TponoB proposed that the self-heating of coal was due to the strong adsorption of oxygen in the air by unsaturated phenolic compounds in the coal, while releasing a certain amount of heat ⁴. Drehm proposed the theory of electrochemical action in 1990 ⁵, and in 1996, Professor Li Zenghua of China University of Mining and Technology observed that coal broke under the action of an external force, resulting in a large number of cracks, which inevitably caused the fracture of the coal molecules ⁶. In 1998, Lopez et al. proposed the hydrogen atom interaction theory ⁷. In 1870, Rachtan discovered through experiments that the oxygen uptake per 1 g of coal in 24 hours was 0.1–0.5 ml, while that of lignite was 0.12 ml ⁷. In 1945, Jones proposed that the oxygen uptake of bituminous coal can reach 0.4 ml/g in air at room temperature ⁷. In the 1960s, the Fushun Coal Research Institute used the oxygen uptake of coal as an indicator to distinguish the spontaneous combustion tendency of coal. These theories are summarized in Figure 2.

The pyrite action theory holds that the spontaneous combustion of coal is caused by the presence of pyrite in the coal seam, as pyrite interacts with water and oxygen in air and causes thermal reactions. However, further research found that spontaneous coal combustion still occurs without the presence of pyrite, thus setting limits on this theory and ultimately disproved it.

Bacteria may play a role in the self-heating process of coal, but they are likely not the main cause of spontaneous combustion, as the spontaneous combustion of coal is based on a chemical chain reaction process rather than on bacterial action.

Phenol gene theory clarifies that the coal-oxygen composite is the primary cause of spontaneous combustion, which can be considered as a supplement to the coal-oxygen composite theory.

The theory of electrochemical action that coal contains variable valence iron ions, and the redox system Fe2+ and Fe3+ play a catalytic role in the oxidation of coal, causing an electrochemical reaction in the coal and producing a chain reaction with chemical activity, thus greatly accelerating the automatic oxidation process of coal, causing spontaneous combustion.

Free radical theory analyzes the reaction process of various organic compounds in coal and reveals that organic compounds in coal can act in pure oxygen or other oxidants. This theory can be used as an extension of the coal-oxygen composite theory, but this theory has not been widely applied.

The coal-oxygen composite theory of coal-oxygen composite has several problems, primarily a lack of understanding regarding the initial cause of the coal-oxygen composite, the process of coal-oxygen composite, how to determine various critical parameters, how to determine the thermal effect in the low-temperature stage, how to determine the shortest spontaneous combustion period of coal, how to transport oxygen in coal.

4. Determination of the Spontaneous Coal Combustion Method

When conditions are appropriate, self-heating occurs, and heat continues to accumulate. When the temperature reaches the ignition point, spontaneous combustion occurs. Such fire-related phenomena not only leads to the loss of available calorific value, but also causes serious safety issues in coal mining, processing, and utilization. In recent years, according to the specific process of spontaneous coal combustion, several major coal-producing countries created methods to detect the tendency of their coal to spontaneously combust and this led to the formation of national standards (Table 2).

The methods described in Table 2 evaluate spontaneous coal combustion and the strength of the combustion tendency by using different approaches. Different test methods have their own functional characteristics, which can only reflect the spontaneous combustion tendency of coal to a certain extent. Overall, the test indices for each method do not form a unified standard. In practical applications, choosing a method becomes difficult, therefore, this study aims to summarize the commonly used methods to detect spontaneous coal combustion both in China and abroad. These methods are divided into four categories according to the types and test methods of evaluation and development indicators; the characteristics and current problems of these methods are also analyzed and compared (Figure 3).

5. Current Status of Spontaneous Coal Combustion Research in China

After consulting the relevant literature, we identified several primary research institutions that have been investigating spontaneous coal combustion measurements in China in recent years. These institutions include six national key laboratories, three provincial and ministerial key laboratories, two ministerial key laboratories, two provincial key laboratories, and one other research institution. The institutions location are mainly distributed in the central, eastern and western parts of China (Figure 4). The specific research contents are shown in Table 3.

Through the analysis of journals, dissertations, and public data released by the laboratory’s official website (Table 3), we concluded that the primary means of studying the characteristics of the coal low-temperature oxidation process in China is the spontaneous coal combustion characteristics test device (thermal reaction furnace) developed independently or jointly by various research institutions. This device was used to carry out experiments, and parameters such as oxygen consumption, products, and temperature change of coal was primarily measured macroscopically. With further development of coal science and technology, as well as coal testing technology, some measurements of microstructure changes, such as functional groups and free radicals, are now widely used by major research institutions in China. The characteristics of the devices are listed in Table 4.

Chinese scholars ²⁸ have found the active sites of coal by modern testing and quantum chemical calculation methods and have mastered and obtained the active site structure and thermodynamic parameters of each element. This has revealed the elemental reaction process of coal oxidation and combustion and established the chain cycle process of the elemental reaction path in spontaneous coal combustion (Figure 5).

Due to numerous cross-wise tasks, projects, invention patents, software copyrights, and related papers of the above research institutions, in order to facilitate statistical collation, this paper only makes a statistical collation of the representative national and provincial longitudinal projects and national, provincial, and association awards of the research institutions in the past ten years, as shown in Table 5.

China's coal reserves are widely distributed, which is the main form of energy supply in China, and this trend will continue for a long time. Correspondingly, China's research institutions for coal spontaneous combustion are widely distributed. In addition, China's economy continues to develop rapidly, and the country continues to increase investment in scientific research. The scientific and technological human resources in the field of coal spontaneous combustion are abundant, and the types and quantities of scientific research equipment are numerous. The research fields are comprehensive, diverse and in-depth. From the point of view of the scientific research projects obtained, the state, industries and enterprises continue to invest in this field, and the research funds are sufficient and the sources are diversified. From the scientific research awards obtained, scientific research projects and achievements have made great progress, great achievements, and continued growth.

6. Conclusion

The spontaneous coal combustion process is a combination of solid pyrolysis and gas combustion chemical reactions. The associated heat and mass transfer processes are complex and difficult to control, resulting in a diversity of spontaneous coal combustion test methods, among which results are often contradictory. Our study on the methods of testing the spontaneous combustion tendency of coal seams provides support and a basis for the targeted formulation of fire prevention and extinguishing measures. The theoretical and practical problems of the current spontaneous coal combustion tendency test methods need to be studied further.

1) The current chromatographic oxygen absorption identification method of spontaneous coal combustion tendency in China has good repeatability, accurate quantification, and easy standardization of tests. However, owing to the unreasonable principle and time-consuming operation, the test results are often inconsistent with the actual situation.

2) The determination method of oxidation kinetics of spontaneous coal combustion tendency is more exact than the principle of the chromatographic oxygen absorption method, but the instrument used in this method is a non-general instrument, which somewhat affects its use.

3) The apparent activation energy can reflect the oxidation dynamic characteristics of spontaneous coal combustion and has the advantage of simple, intuitive, and unified classification. However, this calculation method is insufficient.

Therefore, future researches on spontaneous coal combustion tendency in China should include the detailed kinetic process and characterization method of coal-oxygen chemical reactions, the application of new thermal analysis technology in spontaneous coal combustion tendency tests, and the use of composite indexes to reflect the intrinsic spontaneous combustion tendency of coal.

Acknowledgements

This work is grateful for the financial support from the Major Project of Engineering Science and Technology in Heilongjiang Province in 2020 (Grant number: 2020ZX04A01), and support from the Scientific Research Projects of Undergraduate Universities in Heilongjiang Province (Grant number: 2020-KYYWF-0534).

References