Research PaperA lab-scale experiment on low-temperature coal oxidation in context of underground coal fires
Introduction
Underground coal fires (UCFs) pose serious threats to energy resources, environment, human health and mining safety, very common in most coal-producing countries including China, the United States, India, Indonesia and others [1], [2]. At the early stage of UCFs, they are usually very difficult to be detected, while once found, the fires are extremely difficult to be completely extinguished [3]. By tracking the surface thermal anomalies, remote sensing is a very powerful and economical tool for early detection of UCFs because of its low cost and wide availability [4], [5].
The physio-chemical processes between the coal and oxygen during the low-temperature stage are very complicated. Physical and chemical adsorption occur first, followed by oxidizing reactions between the functional groups and oxygen at higher temperatures. Pyrolysis is another process which includes dehydration and release of volatiles. The physical and chemical adsorption as well as the oxidizing reactions are exothermic, while the pyrolysis is endothermic. Low-temperature oxidation of coal and its reaction kinetics under various conditions have been extensively studied [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In their experiments and analyses, forced air flows are directed to pass through the isothermal [6], [7], [8], [9], [10], [11], [12], [13] or adiabatic [14], [15], [16] reactors piled with coal particles. Gas emissions at the exit are collected and analyzed to identify the reaction paths and to derive the reaction mechanisms. Besides coal properties, other factors include the oxygen content, air flow rate, reactor temperature and particle size. However, the dominant driving force in most UCFs is natural convection caused by the density difference between the air and exhaust gases. Hence, the above-mentioned studies are more relevant to coal mine fires in which forced ventilation is present [20].
Because UCFs usually occur in large-scale coal fields, there exists a strong coupling of many factors such as the geological conditions of the overburden, direction and depth of the coal seam, coal properties, ambient temperature, precipitation and so on. Efforts have been made to develop multi-physics models to investigate self-ignition and reaction front propagation in UCFs [21], [22], [23], [24], [25], [26]. More advanced models that incorporate geological mechanics were developed to consider the deformation and subsidence of the roof rocks [27], [28]. The difficulty of modelling UCFs lies in the accurate determination of many physical properties, for example, the thermal conductivity and the permeability of the overburden.
The objective of this study is to develop a lab-scale experimental setup which comprises most essential elements of the large-scale UCFs. Since UCFs are a result of the self-ignition of coal in a semi-open environment, the experimental system must include the overburden to account for the seepage flows of air and exhaust gases. In addition, natural convection of air should be enabled in the experiment. The focus of the present study is placed on the low-temperature oxidation of coal, that is, the early stage of spontaneous combustion, because of its relevance to early detection and prevention of UCFs. The goal of the present study is to acquire fundamental characteristics of low-temperature oxidation of coal in the context of UCFs, including the temporal variation of coal temperature, exhaust gases temperature and CO and CO2 volumetric fractions.
Section snippets
Experimental setup
The occurrence and development of UCFs are mainly controlled by heat and mass transport in the geological and ambient environment. Fig. 1 schematically shows the heat and mass transport in a typical UCF. It is not practical to use soil or rock to simulate the overburden because their properties vary a lot and it is difficult to characterize them. For example, the permeability is a strong function of piling method, which incurs a lot of uncertainty to the experiment and the reproducibility
Coal temperature
Fig. 4 shows the temporal variation of temperatures at different locations in the coal layer. It can be seen that TC3 exhibits a higher level than TC2 and TC1 from the beginning of the experiment till around 20 h. In the case of line-fracture, TC1 starts to rise sharply at around 19 h, and within a short period of time (∼3 h), it overtakes TC3. Comparing the two types of fracture, it is found that the differences between TC3 and TC2 are negligible, indicating that oxygen can only penetrate to a
Conclusions
A lab-scale experimental setup is built to study the early stage, that is, the low-temperature oxidation of coal, in the context of underground coal fires. Alumina porous ceramic blocks with through-cuts are used to represent the overburden above the coal seams. By monitoring the temporal variation of coal temperature, exhaust gases temperature and volumetric fractions of CO and CO2, the main conclusions are obtained as follows:
- (1)
Under the condition of natural convection, air supply is the
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Open Projects of State Key Laboratory of Coal Resources and Safe Mining of CUMT (Grant No. 14KF01).
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