Search

Studying the evolution of the pore structure of coal during spontaneous combustion is of great value in further understanding the mechanism of coal spontaneous combustion (CSC) and its prevention. In this study, we selected three low-rank coals and used nuclear magnetic resonance (NMR) to visualize the macroscopic evolution of the pore structure of coal after heat treatment and to analyze the effect of temperature (25-500 °C) on the pore structure of coal, including porosity, permeability, and fractal dimensions. The obtained results show that the overall NMR signal in coal increases with increasing temperature, indicating that heat treatment can induce the enlargement, opening, and interconnection of pores and fractures in coal. The equivalent average pore radius (r m ) of coal shows a positive correlation with temperature, with a substantial increase in r m , especially after temperatures above 200 °C. During heating, the porosity and permeability of all three coals tended to increase with temperature. At temperatures above 300 °C, the permeability of coal dramatically increases, predicting a higher fluid transport capacity. Furthermore, NMR multifractal theory was proposed for quantitative pore space dimensional characterization. The obtained results show that the fractal dimensions of the adsorption space of coal pores increase and then decrease with temperature during heating, while the fractal dimensions of percolation space are negatively correlated with temperature. In addition, the dimensions of adsorption space vary more strongly than those of percolation space, meaning that the adsorption capacity of low-rank coals is more significantly influenced by temperature., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Accurate identification of the early stages of coal-fire combustion is important for effectively controlling the spread of coal fires. CO 2 and CO, as characteristic gases in the early stage of coal fire combustion, can be effectively monitored by in-situ monitoring near the surface. However, in the previous in-situ monitoring methods, the influence of surface meteorological and soil factors on the release law of characteristic gases is often ignored. Therefore, this paper considers the complexity of the geological conditions in the coal fire area, a system, and equipment for obtaining the near-surface CO 2 and CO variation laws in the early stage of coal fire combustion proposed by the concentration gradient method (CGM). The system and equipment realize the simultaneous online coupling of multi-area and multi-parameter data and conduct field investigations on the Wuda coal fire area. The results show that in the early stage of coal combustion, the change patterns of CO 2 and CO concentrations in different regions are anomalous, and the CO 2 concentration was higher than the CO concentration. The CO 2 and CO concentrations in shallow soil increased with the increase of soil depth, and compared with other areas, the CO 2 and CO concentration was the highest. The shallow soil and CO 2 were identified as the key areas and characteristic gases for identifying the early stage of coal-fire combustion. The CO 2 flux (CF) of different shallow soil depths decreased with increased soil layer depth. Variation of soil-surface CO 2 flux (S-SCF) estimated by flux extrapolation method (FLEM). The change of S-SCF is controlled by meteorological and soil factors, and there is a certain connection between it and the "respiration phenomenon" in the fissure area. Thus, this study provides a theoretical basis for effectively identifying the early stages of coal-fire combustion., Competing Interests: 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., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Understanding the characteristics of coal spontaneous combustion (CSC) in goaf under different porosities is crucial for comprehending the mechanism of CSC and its prevention and control. In this paper, a multi-field coupled model of CSC in the goaf, considering porosity variation, is developed to investigate the effect of porosity on the CSC characteristics in the goaf. The results indicate that, as the goaf depth increases, both porosity and permeability decrease. When the highest goaf porosity is 25%, the average airflow velocity is between 0.00134 and 0.00139 m/s. In contrast, the average airflow velocity in the goaf with a porosity of 40% is approximately six times greater than that of the goaf with a porosity of 25%. As the goaf porosity increases, the overall oxygen concentration, temperature, and oxidized zone area also rise. Moreover, the oxidation zone area can be quantified and visualized, thereby enabling more effective prediction of the CSC risk in the goaf. The findings of the study have a positive significance in guiding the prevention and control of coal fires. [ABSTRACT FROM AUTHOR]

The re-burning of coal seams still occurs in coal fire areas after firefighting projects. A large amount of spontaneous combustion gas products is released from the surface fissure, which can cause air pollution and even threaten the production of surrounding mining areas. However, the distribution characteristics of fissure channels in the re-burning coal fire areas are local and scattered. Therefore, we developed a system for the in situ monitoring of spontaneous combustion gas emissions from fissure channels to conduct field investigations on the re-burning behavior of coal seams to explore the degree of re-burning in the coal fire area. The results showed that re-burning of the coal fire area induced fissure channels with different shapes. The CO2 concentrations are always higher than the CO concentrations among the spontaneous combustion gas products discharged from the fissure channels. The degree of re-burning of coal seams at different study locations was comprehensively determined using simplified MCE and CO2/CO ratios. The comprehensive factor α was obtained using the generalized principal component analysis (GPCA) by taking into account spontaneous combustion gas, meteorological factors, and the thermal physical characteristics of fissures. A carbon emission model for local fissure channels in the coal fire area was established, and the daily average carbon emissions at the study location were 2.56 t. Therefore, this provides essential theoretical support for taking corresponding fire extinguishing measures according to the degree of re-burning of the coal fire area. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion (CSC) is a complex physical and chemical process. Numerical simulation is an efficient method for visualizing and quantifying CSC. However, in previous models, the property parameters of coal are frequently assumed to be constant and hence cannot be used to predict accurately the thermodynamic behaviors during CSC. In this study, a fully coupled porous media model considering water migration, heat transfer, and thermal deformation was developed to investigate the thermal kinetics of CSC. The porosity and thermal strain during CSC in the model were consistent with the experimental results. Moreover, a method based on nuclear magnetic resonance (NMR) was used to calculate the residual saturation of fluids in porous media during heat treatment. The results showed that porosity increased with temperature during CSC, and the NMR transverse relaxation cutoff values of coal depended nonlinearly on temperature. In the initial stage of CSC, moisture was the dominant factor affecting CSC. As time progresses, moisture and temperature exhibited a synergistic effect. Temperature was the leading factor contributing to CSC in the final stage. The temperature increase during CSC exhibited a 'slow and then fast' trend. In addition, the deformation of coal during CSC caused thermal shrinkage, and its magnitude depended on the temperature increase rate. The results can be used to identify the thermodynamic behaviors during CSC and contribute to a better understanding of the mechanism of CSC. [ABSTRACT FROM AUTHOR]

This paper presents a research on the kinetics and gas release laws of the cooperative spontaneous combustion of coal-polyurethane binary system by means of thermogravimetric analysis and spontaneous combustion simulation experiments. The coal-polyurethane binary system is more prone than individual samples to combustion. The polyurethane mass fraction has a great influence on the cooperative spontaneous combustion process. As the polyurethane mass fraction rises, the activation energies of oxygen absorption stage and pyrolysis stage decrease. The combustion residues of coal-polyurethane binary system scorches into lumps and the surfaces of the particles become rougher. During spontaneous combustion, the initial release temperatures of the four gases CO2, CO, C2H6 and C2H4 drop as the polyurethane mass fraction increases. The gas release amount of them increases with increasing polyurethane mass fraction. And the amount of CH4 decreases as the polyurethane mass fraction increases. The CO/CO2 ratio with temperature for different polyurethane mass fraction have the same trends. The results of this study have implications concerning polyurethane materials application and fire protection in coal mine. [ABSTRACT FROM AUTHOR]

• The evolution law of pore-fracture system during coal spontaneous combustion was studied. • The effect of temperature on the heterogeneity of coal pore-fracture system is analyzed. • The evolution law of coal strength during coal spontaneous combustion was studied. Understanding the variation law of pore–fracture system during coal spontaneous combustion (CSC) plays a crucial role in understanding the mechanism of CSC and its prevention as well as control. In this study, three different ranks of coal samples were heat-treated (25–500 °C). Through nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and uniaxial compression (UC) tests, the changes in the pore-fracture system of different types of coal during CSC were comprehensively summarized. Based on the experimental results, the thermal rupture behavior of coal was simulated using PFC2D software. The results show that During CSC, the pore development of low-rank coal was obvious, the pore development of middle-rank coal was mainly concentrated in the high-temperature stage (T >300 °C), and the pore structure of high-rank coal was more stable. Meanwhile, the heterogeneity of the pore–fracture system of middle- and low-rank coals decreased, and the adsorption and seepage capacities increased. The heterogeneity of the pore–fracture system of high-rank coal was slightly influenced by temperature. The compressive strength of coal decreases with increasing temperature during CSC. The numerical simulation results were similar to the experimental results, i.e., the microcracks in the coal samples increased with temperature during heating; the effect of cracking caused by the high temperature will change the way the sample breaks, and the samples changed from ductile to brittle. The results of this study can serve as a reference for coal-fire control. [ABSTRACT FROM AUTHOR]