Three-dimensional distribution and oxidation degree analysis of coal gangue dump fire area: A case study

Advanced machine learning schemes for prediction CO2 flux based experimental approach in underground coal fire areas

INTRODUCTION

Underground coal fires pose significant environmental and health risks due to releasing CO2 emissions. Predicting surface CO2 flux accurately in underground coal fire areas is crucial for understanding the distribution of spontaneous combustion zones and developing effective mitigation strategies. In recent years, advanced machine learning techniques have shown promise in various carbon-related studies. This research uses an experimental approach to explore the power of advanced machine learning schemes for predicting CO2 flux in underground coal fire areas.

OBJECTIVES

By leveraging the power of advanced machine learning schemes and experimental approaches, this research aims to provide valuable insights into CO2 flux prediction in coal fire areas and inform environmental monitoring and management strategies.

METHODS

The study involves the collection of an experimental dataset specific to underground coal fire areas, encompassing various parameters related to CO2 flux and underground coal fire characteristics. Innovative feature engineering techniques are applied to capture the unique characteristics of underground coal fire areas and their impact on CO2 flux. Different machine learning algorithms, including Natural gradient boosting regression (NGRB), Extreme gradient boosting (XGboost), Light gradient boosting (LGRB), and random forest (RF), are evaluated and compared for their predictive capabilities. The models are trained, optimized, and assessed using appropriate performance metrics.

RESULTS

The NGRB model yields the best predictive performances with R2 of 0.967 and MAE of 0.234. The novel contributions of this study include the development of accurate prediction models tailored to underground coal fire areas, shedding light on the underlying factors driving CO2 flux. The findings have practical implications for delineating the spontaneous combustion zone and mitigating CO2 emissions from underground coal fires, contributing to global efforts in combating climate change.

Comprehensive study on the spatial distribution of heavy metals and their environmental risks in high-sulfur coal gangue dumps in China

The accumulation of coal gangue (CG) from coal mining is an important source of heavy metals (HMs) in soil. Its spatial distribution and environment risk assessment are extremely important for the management and remediation of HMs. Eighty soil samples were collected from the high-sulfur CG site in northern China and analyzed for six HMs. The results showed that the soil was heavily contaminated by Mn, Cr and Ni based on the Nemerow index, and posed seriously ecological risk depended on the geo-accumulation index, potential ecological risk index and risk assessment code. The semi-variogram model and ordinary kriging interpolation accurately portrayed the spatial distribution of HMs. Fe, Mn, and Cr were distributed by band diffusion, Ni was distributed by core, the distribution of Cu had obvious patchiness and Zn was more uniform. The spatial autocorrelation indicated that all HMs had strong spatial heterogeneity. The BCR sequential extraction was employed to qualify the geochemical fractions of HMs. The data indicated that Fe and Cr were dominated by residual fraction; Cu, Ni and Zn were dominated by reducible and oxidizable fractions; Mn was dominated by reducible and acid-extractable (25.38%-44.67%) fractions. Pearson correlation analysis showed that pH was the main control factor affecting the non-residue fractions of HMs. Therefore, acid production from high sulfur CG reduced soil pH by 2-3, which indirectly promoted the activity of HMs. Finally, the conceptual model of HMs contamination at the CG site was proposed, which can be useful for the development of ecological remediation strategies.

Study on Extraction Valuable Metal Elements by Co-Roasting Coal Gangue with Coal Gasification Coarse Slag

Coal gangue (CG) and coal gasification coarse slag (CGCS) possess both hazardous and resourceful attributes. The present study employed co-roasting followed by H2SO4 leaching to extract Al and Fe from CG and CGCS. The activation behavior and phase transformation mechanism during the co-roasting process were investigated through TG, XRD, FTIR, and XPS characterization analysis as well as Gibbs free energy calculation. The results demonstrate that the leaching rate of total iron (TFe) reached 79.93%, and Al3+ achieved 43.78% under the optimized experimental conditions (co-roasting process: CG/CGCS mass ratio of 8/2, 600 °C, 1 h; H2SO4 leaching process: 30 wt% H2SO4, 90 °C, 5 h, liquid to solid ratio of 5:1 mL/g). Co-roasting induced the conversion of inert kaolinite to active metakaolinite, subsequently leading to the formation of sillimanite (Al2SiO5) and hercynite (FeAl2O4). The iron phases underwent a selective transformation in the following sequence: hematite (Fe2O3) → magnetite (Fe3O4) → wustite (FeO) → ferrosilite (FeSiO3), hercynite (FeAl2O4), and fayalite (Fe2SiO4). Furthermore, we found that acid solution and leached residue both have broad application prospects. This study highlights the significant potential of co-roasting CG and CGCS for high-value utilization.

Detection of Spontaneous Combustion Areas of Coal Gangue Dumps and Comprehensive Governance Technologies: A Case Study

Spontaneous combustion of coal gangue dumps not only releases toxic and harmful gases, polluting the environment, but also leads to explosion accidents and casualties due to improper handling. This paper focuses on delineating the fire area, constructing a comprehensive fire prevention and extinguishing system, and restoring the ecological environment. Infrared thermal imaging was used to detect the shallow fire area, while intensive drilling was conducted to detect the deep fire area. The stability of the coal gangue dump was enhanced by perfusing three-phase foam for cooling and using a curing material to fill the cracks. Land reclamation was then performed to restore the ecological environment. The results indicate that spontaneous combustion of coal gangue dumps can trigger the spread of the fire area from the outside to the inside, gradually expanding due to the 'stack effect'. The sources of spontaneous combustion in gangue fire areas are mainly located 3-5 m below the flat surface, and the shallow and deep fire areas are interconnected, posing a significant danger. These research findings can serve as a reference for detecting fire areas in coal gangue dumps and controlling environmental pollution.

Initial exploration on potential fire hazards detection from coal spontaneous combustion applied by acoustic wave

Coal spontaneous combustion often wastes resources and causes environmental pollution. Rapid and accurate identification of high temperature areas in coal is essential to reducing such combustion and environmental pollution. The acoustic thermometry method has the benefits of large temperature measurement space, non-contact, and high interference resistance. Determining the attenuation characteristics of acoustic waves in loose coal is the basis and premise for realizing acoustic temperature measurement. Four types of bituminous coal were scanned by computer tomography equipment. A self-designed acoustic attenuation test device was used to test coal samples under different temperatures and particle sizes. The study result demonstrates that the distribution characteristics of loose coal voids are mainly related to the particle size. The smaller the particle size range, the more uniform the void distribution. As the size of the coal particles increases, the voids become larger. The acoustic attenuation coefficients of four coal samples showed an increasing trend as frequency increased. The influence of coal particle size distribution on the acoustic attenuation coefficient was greater than that of temperature and metamorphic degree. The peak values of coal sound attenuation for different particle sizes were around 400, 700, 1100, and 1600 Hz. This indicated that the distribution of voids was the main factor affecting the propagation of acoustic waves. By analysing the attenuation mechanism of the acoustic wave in loose coal, the attenuation of acoustic temperature measurement signal was caused by the combined effect of loose coal on acoustic wave absorption and scattering. The study results provide theoretical support for the realization of acoustic wave detection of high temperature point in loose coal spontaneous combustion.

Research progress and visualization of underground coal fire detection methods

Underground coal fires are a widespread disaster prevailing in major coal-producing countries globally, posing serious threats to the ecological environment and restricting the safe exploitation of coal mines. The accuracy of underground coal fire detection directly affects the effectiveness of fire control engineering. In this study, we searched 426 articles from the Web of Science database within 2002-2022 as the data foundation and visualized the research contents of the underground coal fire field using VOSviewer and CiteSpace. The results reveal that the investigation of "underground coal fire detection techniques" is currently the focal area of research in this field. Additionally, the "underground coal fire multi-information fusion inversion detection methods" are considered to be the future research trend. Moreover, we reviewed the strengths and weaknesses of various single-indicator inversion detection methods, including the temperature method, gas and radon method, natural potential method, magnetic method, electric method, remote sensing, and geological radar method. Furthermore, we conducted an analysis of the advantages of the multi-information fusion inversion detection methods, which possesses high precision and wide applicability for detecting coal fires, while highlighting the challenges in handling diverse data sources. It is our hope that the research results presented in this paper will provide valuable insights and ideas for researchers involved in the detection and practical research of underground coal fires.

Ignition control and waste heat assessment of spontaneous combustion gangue hill by gravity heat pipe group: a case study in Shanxi Province, China

Spontaneous combustion gangue hill has attracted great attention due to serious environmental pollution and terrible geological disasters. However, the rich thermal resources inside are often ignored. In order to control the spontaneous combustion of gangue hill and utilize the internal waste heat resources, this project studied the combined treatment effect of 821 gravity heat pipes, laid 47 sets of temperature monitoring devices, evaluated the storage of waste heat resources, and proposed different waste heat utilization methods. The results show that (1) the positions of spontaneous combustion are all located on the windward slope. The highest temperature is in the range of 6 ~ 12 m underground, exceeding 700 ℃. (2) The single-tube experiment of gravity heat pipe shows that the effective temperature control radius is 2 m. The cooling effect is obvious in the range of 3 ~ 5 m underground. However, the temperature rises at the depth of 1 m underground. (3) After 90 days of treatment of the gravity heat pipe group, the temperature at the depths of 3 m, 4 m, 5 m, and 6 m in the high-temperature zone dropped by 56 ℃, 66 ℃, 63 ℃, and 42 ℃, respectively. The maximum temperature drop exceeds 160 ℃. The average temperature drop in the middle- and low-temperature areas is between 9 and 21 °C. (4) The concentration of harmful gases (CO, SO₂, and H₂S) decreases by more than 90%. The hazard level is greatly reduced. (5) The amount of waste heat resources contained within 10 m of the spontaneous combustion gangue hill is 7.83E13J. Waste heat resources can be used for indoor heating and greenhouse cultivation. And, under the temperature difference of 50 °C, 100 °C, and 150 °C, the electric energy generated by the heat through the thermoelectric conversion device in the high-temperature zone of the gangue hill is 4056.8 kWh, 7468.2 kWh, and 10,603 kWh, respectively.

Study on the characteristics of radon exhalation from rocks in coal fire area based on the evolution of pore structure

Radon is of great significance as a tracer for the detection of coal fires due to its distinct variations in radon exhalation properties while heating. The research on radon exhalation performance through pore structure is still in its early stages. In this paper, the pore structure and radon exhalation characteristics of heat-treated limestone are studied using indoor tests such as nuclear magnetic and radon measurements. The study's results demonstrate that the radon exhalation rate of limestone initially increases gradually, followed by a steady decline and subsequent increase with the increase in temperature. The radon exhalation rate at 800 °C reaches 2.42 times that at room temperature. The pore structure change within limestone strongly correlates with the radon exhalation rate. The pore volume of micropores (<0.1 μm) plays an essential role in the radon exhalation capacity, which is directly related to the fractal dimension of micropore structure in the heated limestone. The study's findings can be used to identify coal fires.

Dynamic characteristics of near-surface spontaneous combustion gas flux and its response to meteorological and soil factors in coal fire area

Accurate identification of the early stages of coal-fire combustion is important for effectively controlling the spread of coal fires. CO₂ 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 CO₂ 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 CO₂ and CO concentrations in different regions are anomalous, and the CO₂ concentration was higher than the CO concentration. The CO₂ and CO concentrations in shallow soil increased with the increase of soil depth, and compared with other areas, the CO₂ and CO concentration was the highest. The shallow soil and CO₂ were identified as the key areas and characteristic gases for identifying the early stage of coal-fire combustion. The CO₂ flux (CF) of different shallow soil depths decreased with increased soil layer depth. Variation of soil-surface CO₂ 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.

Study on the pore structure and radon release characteristics of coal in northern China

Identifying the release characteristics of radon (Rn-222) in coal mines is critical preventing cancer risks for coal miners and coal fires. The present investigates the pore structure characteristics of coal samples from eleven coal mines in northern China, using low-temperature nitrogen adsorption (LTNA) test, combined with the radon exhalation rate in coal. The findings of the study reveal that the N₂ adsorption isotherms of all the coal samples fall under the inverse S type, with micropores dominating in low-rank coals and mesopores dominating in the medium and high-rank coals, due to the separation of organic matter and quartz by shrinkage of micro-components and the orderly arrangement of aromatic rings as a result of ring condensation and thermal cleavage. The pore diameters of coal samples show similar distribution characteristics for sizes >2 nm, represented by a single peak near the pore diameter of 3 nm. Ash yield controls the mesopore and micropore volumes of medium and high-rank coal samples. The radon emission rate displays positive linear correlation (r² = 0.87) with micropore volumes of analyzed coal samples due to the infillings of free radon in micropores. The radon element is derived by uranium decay, which causes a greater radon exhalation rate of coal mines in areas near the uranium mines. The results of the present study could be helpful to understand the influence mechanism of radon emission processes in coal, which provides an important basis for reducing cancer risks for coal miners and predicting coal fires.

Investigation of thermal behavior and hazards quantification in spontaneous combustion fires of coal and coal gangue

To explore the thermal behavior and hazard during the spontaneous combustion fires (SCFs) of coal and coal gangue (CG), the characteristics of heat release and thermal transfer during the SCFs of coal and CG were tested. The results indicate that coal contains more combustibles and aromatic hydrocarbons, while CG possesses higher contents of ash and inorganic silicate. Coal has a stronger heat release capacity, while CG owns a smaller specific heat capacity, a larger thermal diffusivity and a greater thermal conductivity. Thus, CG performs better with respect to heat transfer. The apparent activation energy of coal is larger in the endothermic stage, whereas that of CG is more notable in the exothermic stage. Based on heat release and heat transfer performance, hazardous zones during the SCFs of coal and CG were identified, and the combustion growth index was established to quantify the hazard of SCF disasters. The results show that the hazard is determined by both heat release and thermal transfer capacities. Coal or CG with a combustible component of 31.3 %, which not only releases massive heat but also transfers heat quickly, corresponds to the most considerable hazard of SCF disasters.

Effects of Erosion Micro-Topographies on Plant Colonization on Weathered Gangue Dumps in Northeast China

Micro-topography has been proved to be beneficial for plant colonization in severe environments. There are numerous micro-topographies caused by erosion of gangue dumps in the Northeast China, which can make plant colonization difficult. To determine how these micro-topographies affect plant colonization, the environment conditions, regeneration characteristics, vegetation characteristics of different erosion micro-topographies, such as bare slope, rill, ephemeral gully and deposit body were studied, and their relationships analyzed. The results showed that the content of particles with a size < 2 mm in the deposit body and bare slope was 33.7% and 7.8% higher than that in the ephemeral gully, respectively (p < 0.05), while the content of particles with a size > 20 mm in the ephemeral gully was 2.24 times higher than that in the deposit body. Except for the substrate water content, the substrate temperature and the surface humidity and temperature of the ephemeral gully were significantly different from those of the deposit body (p < 0.05); the surface temperature was the highest (54.6 °C) while the surface humidity and the substrate water content were the lowest among the erosion micro-topographies. The vegetation coverage, the plant and seedling density of the deposit body were significantly higher than those of the ephemeral gully (p < 0.05), with differences of 5.26, 35.9 and 16.8 times, respectively. The vegetation characteristics (Vdc) were more affected by the regeneration characteristics (Rc) as well as surface humidity and temperature (Sht), while Rc was significantly affected by Sht, which was extremely significantly affected by the soil physical properties and substrate water and temperature (p < 0.01). Different plant species had different responses to the environmental conditions of the erosion micro-topographies. In conclusion, the deposit body and rill are likely to promote plant colonization, which is driven mainly by the seed supply and comfortable growing conditions. The ephemeral gully is not suited to plant colonization because of its unhealthy mechanical composition and strong runoff scouring, and because it is prone to drought, high temperature, and a lack of seeds.

Compressive and Flexural Properties of Ultra-Fine Coal Gangue-Based Geopolymer Gels and Microscopic Mechanism Analysis

Geopolymer gel that possesses advantageous features of fast setting, high strength, and good durability is increasingly used in civil engineering, including rapid retrofit projects, roadway, and other construction projects. Furthermore, geopolymer gel is also a green and economical material as it derives from solid wastes. In this study, activators with different sodium silicate modulus and alkali content were used to activate ultrafine coal gangue and slag powder to prepare coal-gangue-based geopolymers with high strength. To study the influence of slag powder content, sodium silicate modulus, and alkali activator content on strength, a two-stage design was adopted. In the first stage, the orthogonal test with three factors and four levels (10−40% slag, 0.4−1.0 modulus, 16−22%) was used to obtain the influence of each factor on the strength and select the design range of the specimen mix ratio with higher strength. In the second stage, based on the orthogonal experiment, the scope was narrowed to continue to find the optimal excitation scheme and the relationship between the influencing factors and strength. Further, mineral compositional, microstructural, functional group and elemental analyses were performed using X-ray diffraction technique, IR infrared diffraction, electron microscope observation and energy spectrum analysis to elucidate the mechanisms of the strength development. The results show that the factors affecting the geopolymer’s strength were in the order of slag content > alkali content > modulus. The optimum dosage of alkali activator was 18−20%, and the sodium silicate modulus was 0.6−0.8, and the compressive and flexural strength could reach above 40 MPa and 5.9 MPa, respectively. The compressive strength and modulus were in a parabolic relationship. Three types of cementing gels (N-A-S-H, C-A-S-H, and C-N-A-S-H) that were characterized with dense structure and high strength were identified from coal gangue and slag powder after alkali excitation.

The effect of biomass addition on pyrolysis characteristics and gas emission of coal gangue by multi-component reaction model and TG-FTIR-MS

The pyrolysis experiment of biomass added to coal gangue was studied by thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) method. The multi-component reaction model was used to simulate the pyrolysis reaction of coal gangue and biomass. The most suitable model was obtained, and the pyrolysis mechanism was analyzed. According to the two-component reaction model of CG pyrolysis, the decomposition temperature range of components in CG is 340-800 °C and 400-620 °C. The five-component reaction model can well simulate the pyrolysis process of coal gangue and biomass. Meanwhile, the effects of different proportions of biomass in the mixture on the gas products of coal gangue pyrolysis were analyzed. It was found that the addition of biomass to coal gangue could promote the release of gaseous organic matter during pyrolysis. CG75PS25 only has a synergistic effect in the high temperature zone greater than 600 °C. CG25PS75 only has a synergistic effect in a small range of 230-300 °C, and there is an inhibitory effect in other temperature ranges. In general, there is an inhibitory effect between coal gangue and biomass on CO₂ formation, which is of positive significance for greenhouse gas emission reduction.