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Coal, being one of the major energy sources for power generation, contains several critical trace elements. There is a growing scarcity and expense of these critical elements as a result of the increased demand and limitation of mining sources. To explore the geochemical characteristics of the rare-metal, rare-dispersed (scattered), and rare-earth elements (TREs) in coal, 25 coal seam samples of the Shanxi Formation in the Huainan coalfield were collected. The major element oxides, minerals, and TREs were analyzed by X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and inductively coupled plasma-mass spectrometry (ICP-MS). The results revealed that the coal of the Shanxi Formation had ultra-low moisture and low ash yield and was medium–high-volatility with low sulfur content and high calorific value. Concerning minerals, the coal was mainly composed of kaolinite, illite, quartz, calcite, dolomite, and pyrite. Compared with Chinese coal and world hard coal, rare-metal element Li and rare-dispersed element Se were enriched, whereas Ga and Ta were only slightly enriched. The average content of REYs was 51.34 μg/g, which is lower than the average content of REYs in Chinese coal. It has the enrichment characteristics of light REYs. In the vertical direction, the content of most TREs was higher in the roof and floor of the coal seam and the parting, indicating that the sedimentary microenvironment plays an important role in controlling the migration and enrichment of elements. The experimental results of sequential chemical extraction and correlation analysis showed that the TREs in the Shanxi Formation coal mainly exist in a residual and carbonate bound state, and occur in clay minerals and carbonate minerals. The enrichment of Se may be due to its high organic form ratio. The C-value, B content, w(Sr)/w(Ba), and REY geochemical parameters indicated that the Shanxi Formation Coal seam was developed in a transitional, semi-saline, deltaic sedimentary environment. With their development affected by seawater, REYs in coal are greatly supplied by terrigenous clastics. The complex sedimentary environment is an important reason for the varying occurrence states of TREs in the Shanxi Formation coals.

It is of great significance for the engineering popularization of CO 2 -ECBM technology to evaluate the potential of CCUS source and sink and study the matching of pipeline network of deep unworkable seam. In this study, the deep unworkable seam was taken as the research object. Firstly, the evaluation method of CO 2 storage potential in deep unworkable seam was discussed. Secondly, the CO 2 storage potential was analyzed. Then, the matching research of CO 2 source and sink was carried out, and the pipe network design was optimized. Finally, suggestions for the design of pipe network are put forward from the perspective of time and space scale. The results show that the average annual CO 2 emissions of coal-fired power plants vary greatly, and the total emissions are 58.76 million tons. The CO 2 storage potential in deep unworkable seam is huge with a total amount of 762 million tons, which can store CO 2 for 12.97 years. During the 10-year period, the deep unworkable seam can store 587.6 million tons of CO 2 , and the cumulative length of pipeline is 251.61 km with requiring a cumulative capital of $ 4.26 × 10 10 . In the process of CO 2 source-sink matching, the cumulative saving mileage of carbon sink is 98.75 km, and the cumulative saving cost is $ 25.669 billion with accounting for 39.25% and 60.26% of the total mileage and cost, respectively. Based on the three-step approach, the whole line of CO 2 source and sink in Huainan coalfield can be completed by stages and regions, and all CO 2 transportation and storage can be realized. CO 2 pipelines include gas collection and distribution branch lines, intra-regional trunk lines, and interregional trunk lines. Based on the reasonable layout of CO 2 pipelines, a variety of CCS applications can be simultaneously carried out, intra-regional and inter-regional CO 2 transport network demonstrations can be built, and integrated business models of CO 2 transport and storage can be simultaneously built on land and sea. The research results can provide reference for the evaluation of CO 2 sequestration potential of China's coal bases, and lay a foundation for the deployment of CCUS clusters.

In the coal-grain composite area (CGCA) of eastern China with a high groundwater table (HGT), underground coal mining subsidence has caused extensive submergence of farmland, posing a significant threat to regional food security. Currently, land reclamation techniques in mining subsidence areas primarily focus on post-mining reclamation (PMR) of stable subsidence land with a low reclamation rate. This study investigated the application of concurrent mining and reclamation (CMR) technology for unstable subsidence land in a representative HGT mining area, namely the Guqiao Coal Mine in the Huainan Coalfield. Firstly, mining subsidence prediction and geographic information technology were employed to simulate the spatio-temporal evolution of dynamic mining subsidence, taking into consideration the mining plan. Subsequently, phased reclamation parameters were quantitatively designed by integrating the dynamic mining subsidence and surface reclamation measures. Lastly, scenario simulations were conducted to discuss the effectiveness of CMR in comparison with non-reclamation (NR) and PMR. Additionally, reclamation and ecological restoration strategies for coal mining subsidence areas with comprehensive governance modes were proposed. The findings indicated that mining activities have led to a reduction in both the quantity and quality of original farmland, with 70% of the farmland submerged and rendered uncultivable. In contrast to PMR, which achieved a reclamation rate of 29%, CMR can significantly increase the farmland reclamation rate to 69% while also prolonging the service life of farmland. This study provides theoretical support and technical references for promoting sustainable mining practices, protecting farmland, and facilitating the high-quality development of coal resource-based cities.

With the increase in mining depth of coal resources, high-temperature thermal damage has become increasingly prominent, and research on geotemperature in mining areas has received increasing attention. At present, simple temperature measurement is the most commonly used well temperature measurement method, but the obtained data cannot be used directly and need to be corrected. According to the thermal recovery law of an approximately steady-state hole, the “three-point” method is often used to correct simple well temperature data. However, the calibration work is complicated, time-consuming and laborious. Moreover, in the “three-point” method, the well depth and the time from when the well fluid stopping circulation to the temperature measurement (TSC) are not considered enough, which easily leads to deviations in the well temperature correction results. Based on the TSC and well depth, a simple well temperature correction equation was obtained by using 50 available data points with TSCs less than 20 h of approximate steady-state temperature measurements collected in the Huainan coalfield (TSC < 20 h in the general simple temperature measurements). The fitting effect of the equation was verified with the original ground temperature and geothermal gradient; in its application to the Huainan coalfield geothermal field, the model has achieved an ideal effect.

Twenty-nine low sulfur coal samples were selected to determine the magnitude and variability of mercury (Hg) content in a well-documented stratigraphy system including ten continuous coal seams in Zhuji Coal Mine, Huainan Coalfield, Anhui Province, North China. Mercury content of samples was measured on a direct mercury analyzer and confident results were obtained as evaluated by standard references, sample replicates and procedural blanks. The calculated overall mine average Hg content is 71.19 ± 9.28 ng/g based on seam averages and weighting by the estimated reserve of each coal seam. The estimated Hg emission potential for Huainan coalfield is obviously lower than that calculated from coal emission factor in industrial use. An increasing trend of Hg content with the evolution of depositional environment was observed from Nos. 3 to 11-2 coal seams. Combining the evidence of sedimentology and paleontology, a better understanding was gained of the mechanism of Hg sequestration in specific coal benches. A large portion of Hg residing in the low sulfur coals presumably integrated to the functional groups of organic constitution, whereas pyrite was generally abundant in the high sulfur coals.

Based on analysis of a large data set and supplementary sampling and analysis for hazardous trace elements in coal samples from the Huainan Coalfield, a generalized contrast-weighted scale index method was used to establish a model to evaluate the grade of coal cleanliness and its regional distribution in the main coal seam (No. 13-1) The results showed that: (1) The contents of Cr, Mn and Ni in the coal seam are relatively high and the average values are greater than 20 μg/g. The contents of Se and Hg are at a high level while most other trace elements are at normal levels. (2) The cleanliness grade of the coal seam is mainly grade III–IV, which corresponds to a relatively good-medium coal cleanliness grade. However, some parts of the seam are at grade V (relatively poor coal cleanliness). (3) Coal of relatively good cleanliness grade (grade III) is distributed mainly in the regions corresponding to the Zhuji-Dingji-Gubei coal mines and in the eastern periphery of the Panji coal mine. Coal of medium cleanliness (grade IV) is distributed mainly in the regions of the Panji-Xiejiaji and Kouzidong coalmines. Relatively poor grade coal (grade V) is distributed in the southwest regions of the coalfield and the contents of Cr, As and Hg in coal collected from the relatively poor coal cleanliness regions often exceed the regulatory standards for the maximum concentration limits.

Minerals in coal provide useful information for not only paleo-environments of peat accumulation, but also for geological evolution during later diagenesis and/or epigenesis. This paper reports new data on coal quality and the mineralogical and geochemical compositions of 17 unaltered (by intrusion) coal samples collected from the Huainan coalfield, providing new insight into the origins and modes of occurrence of the minerals in coal and their geological evolution. The results showed that the studied coal samples were low rank bituminous coal, with low ash yield (11.92–38.31%, average 24.80%) and high volatile content (25.13–43.43%, average 37.29%). Minerals in the coal mainly included kaolinite and quartz; varying proportions of calcite, siderite, ankerite, and pyrite; and traces of chlorite, zircon, strontianite, apatite, and gorceixite. Typical modes of mineral occurrence could be used to determine the formation stage of minerals. The detrital mineral, occurring as sub-angular to rounded discrete fragments or thin layers intimately admixed with organic matter at particular horizons, was of terrigenous origin, deposited during peat accumulation. Cell infillings, as well as nodule siderites and polycrystalline aggregates of pyrite, precipitated during the syngenetic to early diagenetic stages. Cleat infillings, compressed cell infillings, and fracture infillings precipitated in the epigenetic stage. However, the stage of mineral formation of the pore infilling was difficult to determine. Combined with coal quality, mineralogy, and geochemical analysis, the sedimentary environment of Shanxi Formation was affected by seawater, and Fe-rich hydrothermal fluids filled into the No. 3 coal seam in the epigenetic stage. The sedimentary environment of the No. 8 coal seam had widespread reduction and acid conditions due to basin subsidence, and sulfate-rich hydrothermal fluids may have been formed during the peat deposition stage. In contrast, the peat accumulation environment of the Upper Shihezi Formation was oxidized with a low pH condition. Alkaline fluid then flowed into the No. 13-1 coal seam in the epigenetic stage.

HN-1# is the first fully working coring well of the Taiyuan Formation (Ty) in the Huinan Coalfield and exploration studies are currently underway on the associated resources of the coal-bearing strata. The HN-1# well is located in the Fufeng thrust nappe structural belt in the south of the Huainan Coalfield. Three coal samples from the Ty were collected from HN-1# and inductively-coupled plasma mass spectrometry and inductively-coupled plasma atomic emission spectrometry were used to determine the Ge content of each sample. Based on proximate and ultimate analyses, microscopy data, and analyses of the ash products, some important findings were made. The Ty coal samples had a relatively high total sulfur (S t,d ) content (4.24%), thus the coal was considered to be a lower ranked coal (high volatility bituminous coal), which also had a low coal ash composition index ( k , 1.87). Collodetrinite was the main submaceral of the Ty coal. Small amounts of pyrite particles were found in the coal seams of the Ty, while the contents of pyrite and algae in the top and bottom sections of the coal seam were relatively high, which meant that the swampy peat conditions which existed during the formation of the coal seams were affected by seawater; also the degree of mineralization of the coal seam was relatively high, which is consistent with reducing conditions in a coastal environment setting. Atomic force microscopy (AFM) experiments showed that the modes of occurrence of Ge in the Ty coal were mainly those for organic-bound and adsorbed Ge species. The organic carbon isotope values for the Ty coal ranged from − 24.1‰ to − 23.8‰, with an average value of − 24.0‰, which is equivalent to the value for terrestrial plants (average value − 24.0‰). The Ge content of the Ty coal was 13.57 mg/kg. The Ge content was negatively correlated with volatile matter and the ash yield.

The geothermal history and the tectonic subsidence history of the Huaibei-Huainan coalfields were reconstructed by using the vitrinite reflectance data, and their correlative restriction on coalbed gas generation of Huaibei-Huainan coalfields and Qinshui coal basin was discussed. The burial, thermal, and maturity histories of are similar between Huaibei coalfield and Huainan coalfield, obviously different from those of Qinshui coal basin. Based on the tectono-thermal evolution characters of Huaibei-Huainan coalfields and Qinshui basin, the process of coalbed gas generation can be divided into three stages： （1） Dur- ing Early Mesozoic, both in Huaibei-Huainan and Qinshui, the buried depth of Permian coal seams increased rapidly, which resulted in strong metamorphism and high burial temperature of coal seams. At this stage, the coal rank was mainly fat coal, and locally reached coking coal. These created an environment favoring the generation of thermogenic gas. （2） From Late Ju- rassic to Cretaceous, in the areas of Huaibei-Hualnan, the strata suffered from erosion and the crust became thinning, and the Permian coal-bearing strata were uplifted to surface. At this stage, the thermogenic gas mostly escaped. Conversely, in Qinshui basin, the cover strata of coal seams kept intact during this stage, and the thermogenic gas were mostly preserved. Furthermore, with the interaction of magmatism, the burial temperature of coal seams reached higher peak value, and it was suitable for the secondary generation of thermogenic gas. （3） From Paleogene onward, in area of Huainan-Huaibei, the maturity of coal and burial temperature were propitious to the generation of secondary biogenic gases. However, in Qinshui basin, the maturity of coal went against genesis of second biogenic gas or thermogenic gas. By comparison, Huaibei-Huainan coalfields are dominated by thermogenic gas with a significant biogenic gas and hydrodynamic overprint, whereas Qinshui basin is dominated mainly by thermogenic gas.

Coal characteristic and trace elements of 254 coal samples from Zhangji Coal Mine, Anhui Province, China were studied. Compared with average concentrations of trace elements in Chinese, U.S. and world coals, the coals from Zhangji mine contains a higher concentrations of Co, Cu, Sr and Sc and lower concentrations of Ba, Bi, Mn, Cd, Mo and Zn. The concentrations of B and the Sr/Ba ratios showed that the Permian coal-bearing sequences of the coal mine were developed from a brackish-water influenced lower delta plain (No. 6-2 coal) to a freshwater influenced fluvial upper delta plain (No. 9-1 coal) and suffered frequent marine transgression and regression, which may cause many thin and unstable coal seams. The vertical variations of trace elements in different coal seams indicate that the concentrations of most trace elements in coals are significantly related with depositional environments. Some trace elements could be utilized in coal seam correlation.