Abstract Coalbed methane (CBM) reservoirs in China are typically characterized by low permeability, requiring stimulation techniques to improve gas recovery. Supercritical carbon dioxide (SC-CO 2 ) fracturing has emerged as a promising waterless stimulation method, offering advantages such as improved reservoir integrity, the enhancement of complex fracture networks, and potential for CO 2 geological sequestration. As a sedimentary rock, coal exhibits a distinct bedding structure formed during diagenesis, critically affecting permeability enhancement and CBM extractability. To evaluate the effects of SC-CO 2 exposure on coal containing bedding planes with different orientations, a series of experiments is conducted using coal samples with bedding dip angles of 0°, 30°, 45°, 60°, and 90°. A multi-technique approach, including Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), ultrasonic testing, and acoustic emission (AE) monitoring, is employed to characterize changes in functional groups, pore structure, mechanical properties, and acoustic properties before and after SC-CO 2 treatment. Results show that SC-CO 2 significantly reduces the intensity of infrared spectral peaks and alters the abundance of functional groups in coal samples, thereby facilitating the desorption of coalbed methane. Pore structure analysis reveals that SC-CO 2 modifies pore network structures in coal samples with bedding dip angles of 0°, 30°, 45°, and 90°. For the 60° coal samples, SC-CO 2 promotes the development of multiscale pores, thereby providing enhanced pathways for gas migration. Ultrasonic results indicate that P-wave velocity reduction increases with the bedding dip angle, while wavelet packet energy undergoes redistribution and concentration, with the most pronounced fluctuations in sub-bands 1 and 10. Uniaxial compression tests reveal a marked reduction in compressive strength, coupled with decreased energy absorption and increased energy dissipation capacity. AE analysis demonstrates b -value trends across different stress loading stages, reflecting distinct failure modes—progressive and sudden failure. Furthermore, SC-CO 2 treatment increases the number and proportion of AE events concentrated in dominant frequency bands, indicating enhanced macro-crack propagation. This study provides a comprehensive understanding of the multiscale damage characteristics and energy evolution of coal under SC-CO 2 treatment, offering insights into optimizing CBM production and CO 2 sequestration strategies.
Previous studies have shown that rare earth elements (REEs) in the micro-dispersed mineral matter of Fire Clay coal exhibit high leachability. To better understand the fundamental mechanisms underlying this behavior, pH-dependent leaching tests and comprehensive SEM-EDS analyses were conducted. At a pH of 3.3, around 10
The vertical heterogeneity of coal is a crucial factor influencing the development of coalbed methane (CBM) and it is significantly influenced by coal facies. Utilizing the theories of coal facies and variation coefficient to clearly classify and quantitatively characterize the vertical coal facies types and heterogeneity of deep 8# coal in the Yulin area of the Ordos Basin based on the submaceral content, coal quality, and physical property tests. Through correlation analysis, the influence and control mechanism of coal facies on the vertical heterogeneity of coal reservoir characteristics are analyzed. The results indicate that deep 8# coal reservoir exhibits strong intra-layer heterogeneity, and the middle upper parts of the coal seam are locally favorable intervals. The coal-forming swamp of the deep 8# coal seam was moist forest swamp initially and transitioned into low swamp, undergoing water transgression-regression-transgression. Early coal-forming vegetation was woody plants and gradually replaced by herb/herb and woody plant dominance. The Aad is significantly affected by hydrodynamics, increasing with stronger hydrodynamics (higher GWI and lower F/M) and the Vitrinite has a strong positive correlation with GI but is negatively influenced by F/M. Influenced by coal pore development, microfracture formation, and mineral filling, the porosity and permeability of coal are positively correlated with VI, TPI, GI, and F/M but negatively correlated with GWI. Higher TPI correspond to more mesopores and macropores, with a tri-modal pattern pore distribution characteristic and higher GI indicate more developed ultra-micropores (gas pores), typically resulting in a bi-modal pore size distribution. GWI provides a better indication of gas content, and the vertical gas content of the coal seam deviates from its relationship with coal facies indices due to vertical gas migration.
Abstract Hard carbon is prioritized as the anodes candidate in sodium-ion batteries (SIBs) due to its excellent sodium storage performance. The precursors for hard carbon anodes are diverse, including biomass, organic macromolecule polymers, asphalt, and coal. Among these, coal is currently considered an ideal precursor owing to its high content of carbon, rich aromatic structural units, and low cost. This review systematically reviews the types and physicochemical characteristics of coal, preparation methods, advantages and drawbacks of hard carbon from coal (and their pros/cons) for coal-derived hard carbon, and the structural and sodium storage mechanisms of hard carbon. Some key strategies to improve sodium storage performance involving, such as carbon microcrystal orientation control, heteroatoms doping, and closed pores regulation, are highlighted. We have analyzed and summarized the synthesis methods and electrochemical properties of different types of coal, and proposed a coal-specific method for synthesizing hard carbon. By leveraging the characteristics of different coals, we selected the appropriate synthesis path for hard carbon, establishing a connection among raw materials, processes, properties, and industrialization. Furthermore, this review examined the challenges in coal-based hard carbon production, including complex molecular structure evolution of precursors, process controllability of adopted technology method optimization, while proposing future research directions.
Low-carbon sludge dewatering and efficient pyrolysis are crucial for the biomass industry's green development and solid waste resource utilization. This study investigated the combined effects of calcium oxide (CaO) and magnesium chloride (MgCl2) addition during sludge biodrying, focusing on drying efficiency enhancement and greenhouse gas (GHG) emissions reduction, and further evaluated the following pyrolysis behavior of dried material. The combined addition of CaO and MgCl2 created a thermophilic and alkaliphilic environment, consequently increasing the cumulative temperature by up to 69.50
