Detecting concealed fire sources in coalfield fires: An application study

https://doi.org/10.1016/j.firesaf.2021.103298Get rights and content

Abstract

A majority of coal mines in China are susceptible to fires, especially those affected by concealed fire sources in coalfield fire areas. Hence, to mitigate this problem, the efficient and precise detection of high-temperature anomalous areas is significant and needed, which mainly involves determining the range and depth of concealed fire sources. In this paper, the strengths and weaknesses of each detection method (e.g., remote sensing, electromagnetic radiation, magnetic detection, geological radar detection, resistivity detection, transient electromagnetic, excitation potential method, gas analysis, radon levels detection, infrared images) were investigated, analyzed, evaluated and then compared. Furthermore, after taking their respective practicality and effectiveness into account, an integrated methodology was proposed that uses radon levels, infrared imaging techniques, and drilling methods to detect and verify high-temperature anomalous areas in a coalfield. We carried out the detection in the Ningxia autonomous region. Our testing results revealed that radon levels show positive correspondence with the temperature of potential fire sources. That is to say, higher radon levels usually have higher temperature signal for detection. Accordingly, high-temperature anomalous areas in coalfield fire areas can be distinguished and delineated successfully by their radon levels and verified by infrared imaging techniques and drilling methods.

Introduction

Coalfield fires are common disasters in major coal-producing countries all over the world. The oldest coalfield fire, called Burning Mountain, is located in Australia, where it has been burning for 6000 years [1]. The Ravat coalfield fire in Tajikistan has burned for 2000 years, dating back to the Herodotus Era [2]. In China, several coalfield fires are burning in its northwest provinces (e.g., Xinjiang, Ningxia, Inner Mongolia) [[3], [4], [5]]. Such coalfield fires reportedly will consume 20 million tons of coal resources per year in China, amounting to a direct economic loss of 30 billion RMB [6,7]. Accordingly, coalfield fires now pose a serious threat to China's resource security and the ecological environment of several regions [8], while this worsening problem has increasingly contributed to global warming. Many countries have also held seminars on the prevention and control of coal spontaneous combustion [9].

Coalfield fires mainly result from coal combustion caused by man-made factors or low temperature oxidation. Under low temperature conditions, a coal seam exposed to air and oxygen starts to undergo complex reactions, including physical absorption, chemical absorption, and chemical reactions [[10], [11], [12], [13]], all of which generate a great amount of heat in a concealed coal body. Some of this heat is carried away by wind leakage, but some heat remains stored in coal and the rock material around it via heat conduction processes. Ignition of coal will then occur as heat gradually accumulates to a critical threshold point. Moreover, miners may dig a hole in the coal seam and burn coal for heat to keep warm in winter. Once finished, before leaving the hole they casually pour water on the burning coals. But after that (about several months), the extinguished coal will undergo re-ignition in its natural environment, which causes coalfield fire to start and spread. In both situations described above, there always exist three different temperature districts around the concealed fire sources (Fig. 1): high temperature district, anomalous temperature district, and normal temperature district.

The emissions of coalfield fires pollute the atmosphere, and the released poisonous and injurious gases will degrade the environment that miners live in Ref. [14]. Moreover, coalfield fires will affect the safety of mine tunnels and working faces lying beneath them [[15], [16], [17]]. Therefore, it's imperative to extinguish coalfield fires and to take pre-emptive effective measures to prevent their re-ignition before mining the coal seam. Generally, the main methods of coalfield fire prevention and extinguishment are the stripping method, water injection, drilling and grouting, inert gas method, chemical extinguishing technology, loess cover, and vegetation restoration [18,19]. But these single or integrated methods rarely achieve their intended or expected effects, mainly because of the uncertainty in the location of the concealed fire sources.

Therefore, a significant step towards coalfield fire extinguishment and prevention would be the precise and efficient detection of high temperature districts, which mainly involves determining the range and depth of the concealed fire sources. Currently, detection techniques applied to hidden fire sources and high temperature districts in coalfield fires mainly rely on the gas analysis and temperature-measuring methods [20,21], resistance measurement and geological radar methods, or radon level detection [[22], [23], [24], [25], [26]], magnetic detection and electromagnetic radiation methods [27,28], and radio wave and remote sensing methods [29,30], as summarized in Table 1. Almost every approach has its advantages and disadvantages [31]. Thus, we reasoned it would be more effective to instead adopt existing detection methods in a combined and integrated manner, so as to make full use of their respective advantages while compensating for their disadvantages.

Given the aforementioned aspects, this paper first analyzed the strengths and weaknesses of various detection methods, and estimated the feasibility of the radon level detection technique and infrared imaging technique simultaneously. Then, in the Ningxia autonomous region, our applied work delineated the border of concealed fire sources by using the radon level detection technique, and then verified their ground temperature with the infrared imaging technique, and measured the exact temperature and depth of the hidden fire sources by thermal couple temperature sensors and drilling method.

Section snippets

Feasibility of radon level detection and infrared imaging techniques

Detecting concealed fire sources relies on the evolution rules of characteristic parameters of coal affected by mining during the oxidation and spontaneous combustion process.

Presently, technologies for concealed fire sources detection can be divided into three types: (1) measuring the change in temperature fields in high-temperature anomalous districts; (2) measuring the change to index gases’ concentration in high-temperature anomalous districts; as well as (3) measuring the change in

Testing results of radon levels detection

Radon value represents an exact number, which is used to describe radon concentration.

Radon level represents a degree of radon concentration, which is not a number. High radon level means that radon concentration exceeds a critical value. Low radon level means that radon concentration is below the critical value.

Using SPSS software, we were able to divide the radon concentration results into three categories separated by critical point thresholds. That distinguishing a high temperature from an

Discussion

From the findings of this research, we obtained a general description of the location of hidden fire sources and the distribution of different temperature regions. As a widely used method to resolve the classification process problem, cluster analysis was well able to provide critical threshold values to divide the radon levels into three categories. Fig. 5 delineates the distribution of these different temperature regions found in the field, which revealed that four hidden fire sources in fact

Conclusion

  • (1)

    Within the limitations of this study, a comprehensive understanding of every detection method in Table 1 was provided, including their weaknesses and advantages. We expect this information could be effective for decision makers to use in developing coal mine safety strategies.

  • (2)

    The combined method of radon levels and infrared imaging techniques was carried out to delineate the actual ranges of different temperature regions, and the measurement discrepancies between the two techniques was

Author statement

All authors have contributed to the manuscript.

(1) Bin Du:investigation, resources, data curation, writing-original draft, writing-review & editing, formal and statistical analysis.

(2) Yuntao Liang:conceptualization, methodology, software guidance, supervision, project administration, funding acquisition.

(3) Fuchao Tian:visualization, revision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Youth Project of CCRI Science and Technology Innovation Foundation (Grant No. 2020-QN005); the National Key Research and Development Plan Project (Grant No. 2018YFC0807900); the National Natural Science Foundation of China (Grant No. 51574148).

References (35)

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