Life cycle assessment of opencast lignite mining

The life cycle phase of fossil fuel extraction is mainly considered in the life cycle assessment (LCA) when evaluating the energy production processes. It is then only one of many unit processes, which contribute to the blurring of mining-relevant results. There are few items in the literature focusing exclusively on the lignite mining phase and analysing the specific mining conditions and associated environmental impacts. The article focuses on the LCA of lignite mining processes on the basis of data coming from a Polish mine. The technology for opencast lignite mining is noted for its high production efficiency, high level of recovery and lower risk as regards the safety of workers when compared with underground mining systems. However, the need to remove large amounts of overburden to uncover the deposit contributes to a much greater degradation of the landscape. Analysing the results obtained, several key (hot spot) elements of the lignite mining operations were distinguished for modelling the environmental impact, i.e.: calorific value, the amount of electricity consumption, the manner in which waste and overburden are managed. As a result there is a high sensitivity of the final indicator to changes in these impacts.

Characteristics of water contamination in abandoned coal mines: a case study on Yudong River area, Kaili, Guizhou Province, China

The seasonal and spatial changes in the chemical composition of the water in abandoned mine drainages and rivers in Yudong River area in the years of 2017–2018 were analyzed. The effects of mine water drainage on the seasonality and physicochemical properties of the river water after mine closure were evaluated, and the feasibility of irrigation using river water and the degree of pollution to farmland were assessed using the Water Quality Standard for Farmland Irrigation. The results show that the mine water has low pH value (< 3.5–4) and high levels of total hardness, SO42−, Fe, Al, and Zn. In addition, the pH of the mine water is negatively correlated with the presence of other metal ions. The correlation coefficient between the chemical oxygen demand (COD) and Fe reached 0.989. While the pollution levels of Pinglu and Baishui rivers were low, the confluence region of the two rivers was seriously polluted. However, only the levels of Fe and Cd slightly exceeded the Surface Water Environmental Quality Standard after the confluence of Yudong and Chongan rivers. Overall, the heavy pollution type of the confluent river is consistent with mine water pollution. The water quality is slightly better in the dry season compared than in the high-water period. Sulfate and Fe content decreased by 39 and 16 mg/L, respectively, and Cd content decreased two-fold. Despite these findings, this study shows that from 2017 to 2018, the pH and Cd content of the rivers at the confluence exceeded the irrigation limit and the water quality continued to deteriorate, which may pose a soil contamination risk. Long-term use of the river for irrigation water may cause toxic elements such as Cd, Fe, Mn, SO42−, Al, and F- to enter the food chain, thereby endangering the life and health of villagers in Yudong River area.

Numerical simulation of flooding induced uplift for abandoned coal mines: simulation schemes and parameter sensitivity

Numerical simulation approaches have been widely applied to study mining induced subsidence, and they are potential methods to study the flooding induced uplift for abandoned mines. This paper gives an overview about different numerical approaches to simulate uplift induced by flooding abandoned underground mines, including three different hydraulic conditions, considering both unconfined and confined water conditions. Four basic simulation schemes using 1-dimensional rock column models verified by analytical solutions demonstrate these procedures. The results reveal that flooding induced uplift is mainly related to the pore pressure in the mine goaf. The parameter study documents that height and stiffness of the mine goaf have the strongest influence on maximum surface uplift.

An analysis of contributing mining factors in coal workers’ pneumoconiosis prevalence in the United States coal mines, 1986–2018

In the United States, an unexpected and severe increase in coal miners’ lung diseases in the late 1990s prompted researchers to investigate the causes of the disease resurgence. This study aims to scrutinize the effects of various mining parameters, including coal rank, mine size, mine operation type, coal seam height, and geographical location on the prevalence of coal worker's pneumoconiosis (CWP) in surface and underground coal mines. A comprehensive dataset was created using the U.S. Mine Safety and Health Administration (MSHA) Employment and Accident/Injury databases. The information was merged based on the mine ID by utilizing SQL data management software. A total number of 123,589 mine-year observations were included in the statistical analysis. Generalized Estimating Equation (GEE) model was used to conduct a statistical analysis on a total of 29,707, and 32,643 mine-year observations for underground and surface coal mines, respectively. The results of the econometrics approach revealed that coal workers in underground coal mines are at a greater risk of CWP comparing to those of surface coal operations. Furthermore, underground coal mines in the Appalachia and Interior regions are at a higher risk of CWP prevalence than the Western region. Surface coal mines in the Appalachian coal region are more likely to CWP development than miners in the Western region. The analysis also indicated that coal workers working in smaller mines are more vulnerable to CWP than those in large mine sizes. Furthermore, coal workers in thin-seam underground mine operations are more likely to develop CWP.

Experimental study on the adverse effect of gel fracturing fluid on gas sorption behavior for Illinois coal

Hydraulic fracturing is an effective technology for coal reservoir stimulation. After fracturing operation and flowback, a fraction of fracturing fluid will be essentially remained in the formation which ultimately damages the flowability of the formation. In this study, we quantified the gel-based fracturing fluid induced damages on gas sorption for Illinois coal in US. We conducted the high-pressure methane and CO2 sorption experiments to investigate the sorption damage due to the gel residue. The infrared spectroscopy tests were used to analyze the evolution of the functional group of the coal during fracturing fluid treatment. The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions. As the concentration of fracturing fluid increases, the density of residues on the coal surface increases and the adhesion film becomes progressively denser. The adhesion film on coal can apparently reduce the number of adsorption sites for gas and lead to a decrease of gas sorption capacity. In addition, the gel residue can decrease the interconnectivity of pore structure of coal which can also limit the sorption capacity by isolating the gas from the potential sorption sites. For the low concentration of fracturing fluid, the Langmuir volume was reduced to less than one-half of that of raw coal. After the fracturing fluid invades, the desorption hysteresis of methane and CO2 in coal was found to be amplified. The impact on the methane desorption hysteresis is significantly higher than CO2 does. The reason for the increasing of hysteresis may be that the adsorption swelling caused by the residue adhered on the pore edge, or the pore blockage caused by the residue invasion under high gas pressure. The results of this study quantitatively confirm the fracturing fluid induced gas sorption damage on coal and provide a baseline assessment for coal fracturing fluid formulation and technology.

Characteristic strength and acoustic emission properties of weakly cemented sandstone at different depths under uniaxial compression

As coal mining is extended from shallow to deep areas along the western coalfield, it is of great significance to study weakly cemented sandstone at different depths for underground mining engineering. Sandstones from depths of 101.5, 203.2, 317.3, 406.9, 509.9 and 589.8 m at the Buertai Coal Mine were collected. The characteristic strength, acoustic emission (AE), and energy evolution of sandstone during uniaxial compression tests were analyzed. The results show that the intermediate frequency (125–275 kHz) of shallow rock mainly occurs in the postpeak stage, while deep rock occurs in the prepeak stage. The initiation strength and damage strength of the sandstone at different depths range from 0.23 to 0.50 and 0.63 to 0.84 of peak strength (σc), respectively, decrease exponentially and are a power function with depth. The precursor strength ranges from 0.88σc to 0.99σc, increases with depth before reaching a depth of 300 m, and tends to stabilize after 300 m. The ratio of the initiation strength to the damage strength (k) ranges from 0.25 to 0.62 and decreases exponentially with depth. The failure modes of sandstone at different depths are tension-dominated mixed tensile-shear failure. Shear failure mainly occurs at the unstable crack propagation stage. The count of the shear failure bands before the peak strength increases gradually, and increases first and then decreases after the peak strength with burial depth. The cumulative input energy, released elastic energy and dissipated energy increase with depth. The elastic release rate ranges from 0.46 × 10–3 to 198.57 × 10–3 J/(cm3 s) and increases exponentially with depth.

The view of technological innovation in coal industry under the vision of carbon neutralization

This paper analyzed the current situation and development trends of energy consumption and carbon emissions, and the current situation and development trend of coal consumption in China. In the context of recently established carbon peak and carbon neutralization targets, this paper put forward the main problems associated with the green and low-carbon development and utilization of coal. Five key technological innovation directions in mining were proposed, including green coal development, intelligent and efficient mining, low-carbon utilization and conversion of coal, energy conservation and emission reduction, carbon capture, utilization and storage (CCUS). Focusing on the above technological innovation directions, it is suggested to carry out three basic theories, including the theory of green efficient intelligent mining, clean and low-carbon utilization and transformation of coal, and CCUS. Meanwhile, it is proposed to develop 12 key technologies, including green coal mining and ecological environment protection, efficient coal mining and intelligent mine construction, key technologies and equipment for efficient coal processing, underground coal gasification and mining, ultra-high parameter and ultra-supercritical power generation technology, intelligent and flexible coal-fired power generation technology, new power cycle coal-fired power generation technology, the development of coal-based special fuels, coal-based bulk and specialty chemicals, energy conservation and consumption reduction, large-scale and low-cost carbon capture, CO2 utilization and storage. Finally, necessary measures from the governmental perspective were also proposed.

Distribution and modes of occurrence of uranium in coals of Eastern Yunnan, China

Uranium is an environmentally hazardous element, and is commonly present at trace levels (2.4 μg/g for world coals) in coal deposits. However, selected coal deposits could be highly enriched in uranium. In this study, 15 coal samples were collected from Eastern Yunnan coal deposits, China, aiming to characterize the distribution and the occurrence of uranium in those coals. In studied samples, uranium content varied from 0.36 to 8.28 μg/g, with an average value of 3.76 μg/g. Generally, uranium content in coals from northern coal mines (3.02 ± 2.44 μg/g, n = 5) were lower than it in southern coal mines (4.13 ± 2.30 μg/g, n = 10). Uranium in coal samples showed no obvious correlation with total sulfur, whereas was positively correlated with ash yield. The results of sequential chemical extraction procedure confirm that organic-bound is the dominant occurrence of uranium. The slight enrichment of uranium in studied coals was probably attributed to sedimentation processes, hydrological conditions and tectonic structure of the coal deposits.

Insights from principal component analysis applied to Py-GCMS study of Indian coals and their solvent extracted clean coal products

The present work aims at studying five Indian coals and their solvent extracted clean coal products using Py-GCMS analysis and correlating the characterization data using theoretical principal component analysis. The pyrolysis products of the original coals and the super clean coals were classified as mono-, di- and tri-aromatics, while other prominent products that were obtained included cycloalkanes, n-alkanes, and alkenes ranging from C10–C29. The principal component analysis is a dimensionality reduction technique that reduced the number of input variables in the characterization dataset and gave inferences on the relative composition of constituent compounds and functional groups and structural insights based on scores and loading plots which were consistent with the experimental observations. ATR-FTIR studies confirmed the reduced concentration of ash in the super clean coals and the presence of aromatics. The Py-GCMS data and the ATR-FTIR spectra led to the conclusion that the super clean coals behaved similarly for both coking and non-coking coals with high aromatic concentrations as compared to the raw coal. Neyveli lignite super clean coal was found to show some structural similarity with the original coals, whereas the other super clean coals showed structural similarity within themselves but not with their original coal samples confirming the selective action of the e,N solvent in solubilizing the polycondensed aromatic structures in the coal samples.

Stress and deformation analysis of gob-side pre-backfill driving procedure of longwall mining: a case study

At present, non-pillar entry protection in longwall mining is mainly achieved through either the gob-side entry retaining (GER) procedure or the gob-side entry driving (GED) procedure. The GER procedure leads to difficulties in maintaining the roadway in mining both the previous and current panels. A narrow coal pillar about 5–7 m must be left in the GED procedure; therefore, it causes permanent loss of some coal. The gob-side pre-backfill driving (GPD) procedure effectively removes the wasting of coal resources that exists in the GED procedure and finds an alternative way to handle the roadway maintenance problem that exists in the GER procedure. The FLAC3D software was used to numerically investigate the stress and deformation distributions and failure of the rock mass surrounding the previous and current panel roadways during each stage of the GPD procedure which requires "twice excavation and mining". The results show that the stress distribution is slightly asymmetric around the previous panel roadway after the “primary excavation”. The stronger and stiffer backfill compared to the coal turned out to be the main bearing body of the previous panel roadway during the "primary mining". The highest vertical stresses of 32.6 and 23.1 MPa, compared to the in-situ stress of 10.5 MPa, appeared in the backfill wall and coal seam, respectively. After the "primary mining", the peak vertical stress under the coal seam at the floor level was slightly higher (18.1 MPa) than that under the backfill (17.8 MPa). After the "secondary excavation", the peak vertical stress under the coal seam at the floor level was slightly lower (18.7 MPa) than that under the backfill (19.8 MPa); the maximum floor heave and maximum roof sag of the current panel roadway were 252.9 and 322.1 mm, respectively. During the "secondary mining", the stress distribution in the rock mass surrounding the current panel roadway was mainly affected by the superposition of the front abutment pressure from the current panel and the side abutment pressure from the previous panel. The floor heave of the current panel roadway reached a maximum of 321.8 mm at 5 m ahead of the working face; the roof sag increased to 828.4 mm at the working face. The peak abutment pressure appeared alternately in the backfill and the coal seam during the whole procedure of "twice excavation and mining" of the GPD procedure. The backfill provided strong bearing capacity during all stages of the GPD procedure and exhibited reliable support for the roadway. The results provide scientific insight for engineering practice of the GPD procedure.