Energies

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18 pages, 43315 KiB

Open AccessArticle

Comparative Analysis of the Mining Cribs Models Filled with Gangue

by Krzysztof Skrzypkowski

Energies 2020, 13(20), 5290; https://doi.org/10.3390/en13205290 - 12 Oct 2020

Cited by 24 | Viewed by 2712

In the article, comparative laboratory strength tests of three-point and four-point wooden cribs models are presented. In the case of cribs with a triangular cross-section, the notches made at an angle of 60 degrees were used for the first time. The individual beams [...] Read more.

In the article, comparative laboratory strength tests of three-point and four-point wooden cribs models are presented. In the case of cribs with a triangular cross-section, the notches made at an angle of 60 degrees were used for the first time. The individual beams of the three-point and four-point cribs were stacked horizontally and connected to each other by means of a quick-drying adhesive. The main aim of the research was to compare the empty models with cribs filled with a gangue. In order to better understand the mechanism of behavior of models under loads, load-displacement and pressure-compressibility characteristics are presented. It was found that filling the three-point and four-point crib with gangue increases its maximal load several times compared to the empty cribs. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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15 pages, 13511 KiB

Open AccessArticle

Numerical Analysis of Longwall Gate-Entry Stability under Weak Geological Condition: A Case Study of an Indonesian Coal Mine

by Takashi Sasaoka, Pisith Mao, Hideki Shimada, Akihiro Hamanaka and Jiro Oya

Energies 2020, 13(18), 4710; https://doi.org/10.3390/en13184710 - 10 Sep 2020

Cited by 10 | Viewed by 2600

Abstract

The present research primarily focuses on the investigation of gate-entry stability of longwall trial panel under weak geological condition in Indonesia coal mine by means of numerical analysis. This work aims at identifying appropriate roof support at 100 m and 150 m of [...] Read more.

The present research primarily focuses on the investigation of gate-entry stability of longwall trial panel under weak geological condition in Indonesia coal mine by means of numerical analysis. This work aims at identifying appropriate roof support at 100 m and 150 m of depth during gate development. Due to depth depending competency of dominant rock, the stability of gate-entry at 100 m of depth can be optimized by leaving at least 1 m of remaining coal thickness (RCT) above and below the gate-entry. The appropriate support for the trial panel gate-entry is steel arch SS540 with 1 m and 0.5 m spacing for 100 m and 150 m of depth, respectively. The influence of panel excavation on gate-entry is also discussed. Regarding the aforementioned influence, the utilization of additional gate mobile support is recommended at least 10 m from the longwall face. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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14 pages, 4517 KiB

Open AccessArticle

Study on Foamed Concrete Used as Gas Isolation Material in the Coal Mine Goaf

by Changyu Xu, Lijun Han, Maolin Tian, Yajie Wang and Yuhao Jin

Energies 2020, 13(17), 4377; https://doi.org/10.3390/en13174377 - 25 Aug 2020

Cited by 4 | Viewed by 2411

Abstract

In view of the serious threat of gas accumulation in the coal mine goaf and the limitations of the existing gas sealing materials, the orthogonal experiment was developed to study a new type of foamed concrete for mine gas sealing. Dry density, gas [...] Read more.

In view of the serious threat of gas accumulation in the coal mine goaf and the limitations of the existing gas sealing materials, the orthogonal experiment was developed to study a new type of foamed concrete for mine gas sealing. Dry density, gas permeability, and compressive strength were studied as the material indicators according to the demands of the gas isolation material in the coal mine goaf, and the experimental results showed that foam content was the most important factor. Meanwhile, the optimum mix was selected according to the influence of foam content as well as the engineering requirement. Then two application modes of this foamed concrete for goaf gas isolation were put forward, after which the convection-diffusion model of gas was built by COMSOL Multiphysics (COMSOL Inc., Stockholm, Sweden) to reveal the mechanism of different application modes using the parameters of the new foamed concrete. Simulation results showed that this foamed concrete used as isolating material for goaf gas could significantly decrease the gas concentration in workface, which can provide a reference for similar engineering. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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12 pages, 2433 KiB

Open AccessArticle

The Time Duration of the Effects of Total Extraction Mining Methods on Surface Movement

by André Vervoort

Energies 2020, 13(16), 4107; https://doi.org/10.3390/en13164107 - 8 Aug 2020

Cited by 16 | Viewed by 2464

Abstract

Since the 1990s, remote sensing data have been available to monitor the surface movement for long periods of time. The analysis of satellite data shows that there is still residual subsidence (i.e., with average rates of about −10 mm/year) several decades after mining [...] Read more.

Since the 1990s, remote sensing data have been available to monitor the surface movement for long periods of time. The analysis of satellite data shows that there is still residual subsidence (i.e., with average rates of about −10 mm/year) several decades after mining longwall panels in an area. Several years after the underground infrastructure was sealed, the surface started to move upwards. In the past, it often was claimed that movement of the surface was limited in time, i.e., a few years after mining a longwall panel. This is not the case for the conditions of the Campine coal basin, Belgium. This knowledge is important when one wants to design new operations in deep coal seams, but also when planning to stop the underground mining and to seal the access to the mine. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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23 pages, 7967 KiB

Open AccessArticle

Sustainable Rehabilitation of Surface Coal Mining Areas: The Case of Greek Lignite Mines

by Francis Pavloudakis, Christos Roumpos, Evangelos Karlopoulos and Nikolaos Koukouzas

Energies 2020, 13(15), 3995; https://doi.org/10.3390/en13153995 - 3 Aug 2020

Cited by 46 | Viewed by 6156

Abstract

Surface lignite mines of the Balkan Peninsula face growing pressure due to the CO₂ emissions reduction initiatives, rapidly increasing renewable-power capacity, and cheap natural gas. In this frame, the development of a modern mine land rehabilitation strategy is considered as a prerequisite [...] Read more.

Surface lignite mines of the Balkan Peninsula face growing pressure due to the CO₂ emissions reduction initiatives, rapidly increasing renewable-power capacity, and cheap natural gas. In this frame, the development of a modern mine land rehabilitation strategy is considered as a prerequisite for mitigating the social and economic impacts for the local communities. In the case of western Macedonia lignite mines, these prospects are investigated based on a PEST (political, economic, social, technological) analysis of seven alternative land uses. Urban (industrial) development, green houses, and industrial heritage parks are considered as the most appropriate land uses for mitigating the socioeconomic impacts due to the loss of employments. For the land uses occupying large areas (i.e., agriculture, forestry, livestock farming, and photovoltaic parks), an optimisation algorithm is proposed for determining the mix of land uses that maximise revenue, equity, and natural conservation and minimise investment. The algorithm was applied using the opinions of 10 experts, who were involved in mine land reclamation projects carried out in the western Macedonia region in the recent past. According to the results obtained, photovoltaic parks are rated as a more attractive investment than extensive agriculture, as regards the anticipation of revenues, while livestock farming and forests are necessary to safeguard the ecosystem’s functions. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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13 pages, 7255 KiB

Open AccessArticle

Acoustic Emission Characteristics of Coal Samples under Different Stress Paths Corresponding to Different Mining Layouts

by Yiming Yang, Ting Ai, Zetian Zhang, Ru Zhang, Li Ren, Jing Xie and Zhaopeng Zhang

Energies 2020, 13(12), 3295; https://doi.org/10.3390/en13123295 - 26 Jun 2020

Cited by 3 | Viewed by 2396

Abstract

Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well [...] Read more.

Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well to mining layouts and depths under deep mining. The acoustic emission (AE) characteristics of typical coal under deep mining were obtained by conducting laboratory experiments to simulate mining-induced behavior and utilizing AE techniques to capture the variation in AE temporal and spatial parameters in real time, which provide an important basis for studying the rupture mechanisms and mechanical behavior of deep-mined coal. The findings were as follows: (1) AE activity under deep mining was characterized by three stages, corresponding to crack initiation, crack stable propagation, and crack unstable propagation. As the three stages proceeded, the AE counting rate and AE energy rate presented stronger clustering characteristics, and the cumulative AE counting and cumulative AE energy exhibited a sharp increase by an order of magnitude. (2) The crack initiation and the main stages of crack propagation were determined by characteristic points of variation curves in the AE parameters over time. In the main crack propagation stage, the number of cumulative AE events and the cumulative AE counts were similar among the three mining conditions, while coal samples under coal pillar mining released the largest amount of AE energy. The amount of accumulated AE energy released by coal samples increased by one order of magnitude according to the sequence of protective coal-seam mining, top-coal caving mining, and nonpillar mining. (3) Fractal technology was applied to quantitatively analyze the AE spatial evolution process, showing that the fractal dimension of the AE location decreased as the peak stress increased, corresponding to protective seam mining, caving-coal mining, and nonpillar mining. The above results showed that the deformation and fracture characteristics of coal under deep mining followed a general law, but were affected by different mining conditions. The crack initiation and main rupture activity of coal occurred earlier under the conditions of protective seam mining, top-coal caving mining, and nonpillar mining, successively. Moreover, nonpillar mining induced the strongest and highest degree of unstable rupture of the coal body in front of the working face. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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28 pages, 11991 KiB

Open AccessArticle

Water Preservation and Conservation above Coal Mines Using an Innovative Approach: A Case Study

by Yujun Xu, Liqiang Ma and Yihe Yu

Energies 2020, 13(11), 2818; https://doi.org/10.3390/en13112818 - 2 Jun 2020

Cited by 15 | Viewed by 2732

Abstract

To better protect the ecological environment during large scale underground coal mining operations in the northwest of China, the authors have proposed a water-conservation coal mining (WCCM) method. This case study demonstrated the successful application of WCCM in the Yu-Shen mining area. Firstly, [...] Read more.

To better protect the ecological environment during large scale underground coal mining operations in the northwest of China, the authors have proposed a water-conservation coal mining (WCCM) method. This case study demonstrated the successful application of WCCM in the Yu-Shen mining area. Firstly, by using the analytic hierarchy process (AHP), the influencing factors of WCCM were identified and the identification model with a multilevel structure was developed, to determine the weight of each influencing factor. Based on this, the five maps: overburden thickness contour, stratigraphic structure map, water-rich zoning map of aquifers, aquiclude thickness contour and coal seam thickness contour, were analyzed and determined. This formed the basis for studying WCCM in the mining area. Using the geological conditions of the Yu-Shen mining area, the features of caved zone, water conductive fractured zone (WCFZ) and protective zone were studied. The equations for calculating the height of the “three zones” were proposed. Considering the hydrogeological condition of Yu-Shen mining area, the criteria were put forward to evaluate the impact of coal mining on groundwater, which were then used to determine the distribution of different impact levels. Using strata control theory, the mechanism and applicability of WCCM methods, including height-restricted mining, (partial) backfill mining and narrow strip mining, together with the applicable zone of these methods, were analyzed and identified. Under the guidance of “two zoning” (zoning based on coal mining’s impact level on groundwater and zoning based on applicability of WCCM methods), the WCCM practice was carried out in Yu-Shen mining area. The research findings will provide theoretical and practical instruction for the WCCM in the northwest mining area of China, which is important to reduce the impact of mining on surface and groundwater. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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14 pages, 4894 KiB

Open AccessArticle

Experimental Study on the Characteristics of Activated Coal Gangue and Coal Gangue-Based Geopolymer

by Weiqing Zhang, Chaowei Dong, Peng Huang, Qiang Sun, Meng Li and Jun Chai

Energies 2020, 13(10), 2504; https://doi.org/10.3390/en13102504 - 15 May 2020

Cited by 55 | Viewed by 4220

Abstract

Coal gangue-based geopolymer (CGGP) is one of the hot spots existing in the recycling of coal gangue resources due to its good comprehensive mechanical properties. However, the coal gangue structure is stable and reactivity is poor, so the coal gangue needs to be [...] Read more.

Coal gangue-based geopolymer (CGGP) is one of the hot spots existing in the recycling of coal gangue resources due to its good comprehensive mechanical properties. However, the coal gangue structure is stable and reactivity is poor, so the coal gangue needs to be activated before utilization. In this paper, the microstructure changes of activated coal gangue by different mechanical and thermal activation methods, as well as the mechanical properties and microstructure changes of the CGGP specimens were studied by experimental investigation. The results indicated that mechanical activation and thermal activation were two effective methods to change the reactivity of coal gangue, which consisted of destroying the stable kaolinite structure and improving the activity of coal gangue. Conversely, part of the amorphous structure in coal gangue was destroyed when the activation temperature reached 900 °C, which was not conducive to the further enhancement of coal gangue activity. For the CGGP prepared by thermally activated coal gangue and modified sodium silicate alkali solution, the uniaxial compressive strength of the CGGP specimens decreased with thermal activation temperatures of the raw coal gangue materials at 700 °C, 800 °C, and 900 °C. The main reason for this was the lower amount of the active metakaolin structure in coal gangue at 900 °C, which was not conducive to the geopolymerization process. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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22 pages, 3290 KiB

Open AccessArticle

A Multi-Criteria Approach for the Evaluation of Low Risk Restoration Projects in Continuous Surface Lignite Mines

by Philip-Mark Spanidis, Christos Roumpos and Francis Pavloudakis

Energies 2020, 13(9), 2179; https://doi.org/10.3390/en13092179 - 1 May 2020

Cited by 24 | Viewed by 3024

Abstract

The restoration of continuous surface lignite mines entering the closure phase is a long-term, complex and multidisciplinary project. During the evaluation of alternative restoration technologies, various environmental, technical, economic and social parameters are investigated. In this framework, for the selection of the lower [...] Read more.

The restoration of continuous surface lignite mines entering the closure phase is a long-term, complex and multidisciplinary project. During the evaluation of alternative restoration technologies, various environmental, technical, economic and social parameters are investigated. In this framework, for the selection of the lower risk restoration alternative, the analysis of the associated risks should be incorporated into the decision-making process. This work provides an overview of practical risk management problems and solutions in mining restoration projects. Moreover, it introduces a multi-criteria methodology for the improvement of the decision-making process in the evaluation of restoration alternatives and the selection of the optimal one, considering a continuous surface mining project. The proposed method is a combination of the analytical hierarchy process (AHP) for the quantification of risk factors and the technique for order of preference by similarity to ideal solution (TOPSIS) for the ranking of restoration alternatives based on a low risk approach. The results of the case study indicate that the proposed approach can be utilized as a low cost and easy-to-apply tool, appropriate for coal mining operators, managers and stakeholders involved in the planning and implementation of post-mining land restoration activities. Furthermore, the suggested methodology could be adopted to support the risk management needs in the restoration stage of complex surface mining projects. Full article

(This article belongs to the Special Issue Green Coal Mining Techniques 2020)

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