Abstract
To comprehensively understand the bulk macro conduction characteristics of typical geomaterials existing in deep underground coal mines, i.e. coal, rock, and concrete (shotcrete), meso-scale composition (solid mineral and fluids such as water and air) and structure (particle and pore size, shape, and contact form) analyses were conducted via water absorbability and thermal conductivity measurements, scanning electron microscope, and X-ray diffraction technologies. The results indicate that coal, rock, and concrete's respective meso-scale composition and structures have different degrees of influence on their macro bulk effective thermal conductivity at various conditions. First, the fluid (water and air) composition significantly impacts thermal conductivity at a certain bulk effective porosity: coal < rock < concrete. The ranking of thermal conductivity shows two different forms: concrete < coal < rock in a dry state and coal < concrete < rock in a water-saturated state. Second, the surface morphology and face porosity obtained by scanning electron microscope imaging can explain thermal conductivity fluctuations at various moisture contents and magnification times. The face porosity as well as its quadrant anisotropy degree can also be achieved via scanning electron microscope digital image processing technology, and both of them have a certain relationship with magnification times (scale effect) and effective volumetric porosity, as obtained by water absorbability experiments. In addition, apart from the fluid composition, the mineral composition (species and content of mineral) of the solid skeleton or matrix also significantly influences the macro-scale thermal conductivity of geomaterials. Furthermore, a quantitative comparison analysis can be performed by X-ray diffraction, but further research is needed to improve its accuracy. This study's detailed meso-characterization of the macro effective thermal conductivity for typical geomaterials in an underground coal mine may be of reference value to establish the relationship between different properties of differing scales.
Highlights
At present, many coal mines all over the world (China, Poland, Australia, etc.) are exposed to high temperature and humidity, which has become an important factor that restricts the safe mining of deep coal resources (Amin et al, 2016; Chu et al, 2016; Szlazak et al, 2008; Yang et al, 2011; Kong et al, 2016)
Most studies examine the relationship between thermal conductivity and temperature ranging from room temperature to several hundred degrees, which has considerable relevance to the prevention and control of mine fires caused by spontaneous coal combustion (Deng et al, 2017; Wen et al, 2015)
Based on the above classification, this paper aims to link the macro-scale effective thermal conductivity of three typical geomaterials in underground coal mines to their meso-scale composition and structure via the experiments performed with Scanning electron microscope (SEM) and X-ray diffraction (XRD) technologies
Summary
Many coal mines all over the world (China, Poland, Australia, etc.) are exposed to high temperature and humidity, which has become an important factor that restricts the safe mining of deep coal resources (Amin et al, 2016; Chu et al, 2016; Szlazak et al, 2008; Yang et al, 2011; Kong et al, 2016). In previous studies many researchers have focused on the thermal conductivity of geomaterials, including coal, rock, and concrete. Concrete with low thermal conductivity was used as a type of supporting structure and as an insulating material, to weaken the heat transfer process between the surface of the rock and the mining environment. It showed excellent cooling load reductions and significant operational cost savings for ventilation and cooling systems coal coal concrete shotcrete rock rock concrete shotcrete coal concrete shotcrete
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