Hydrophobic solid surfaces have been found to promote the formation of gas hydrates effectively and thus help to realize the immense potential applications of hydrates in many sectors such as energy supply, gas storage and transportation, gas separation, and CO2 sequestration. Despite the well-known effectiveness, the molecular mechanism behind the promotion effect has not been thoroughly understood. In this work, we used both simulation and experimental means to gain insights into the microscopic level of the influence of hydrophobic solid surfaces on gas hydrate formation. On one hand, our simulation results show the presence of an interfacial gas enrichment (IGE) at hydrophobic surface and a gas depletion layer at hydrophilic surface. In the meantime, the analysis of water structure near the hydrophobic solid interface based on the molecular trajectories also shows that water molecules tend to get locally structured near a hydrophobic surface while becoming depressed near a hydrophilic surface. On the other hand, the experimental results demonstrate the preferential formation of gas hydrate on a hydrophobic surface. The synergic combination of simulation and experimental results points out that the existence of an IGE at hydrophobic solid surface plays a key role in promoting gas hydrate formation. This work advances the molecular level understanding of the role of hydrophobicity in governing the gas hydrate as well as interfacial phenomena in general.

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The natural fracture system in coal serves as the primary conduit for water and gas flow in coal seam gas fields. For low permeability coal with highly mineralised fracture systems, the dissolution and/or modification of mineral occlusions could potentially enhance permeability and improve stress resilience.This study investigated the effect of mineral alteration by hydrochloric and hydrofluoric acid (HCl -HF) on fracture compressibility and coal permeability. Coal core immersion in 15 % HF-4% KCl solution has enhanced coal permeability to brine from 0.10 to 0.45 mD and reduced fracture compressibility from 0.020 to 0.006 bar -1 . Enhanced permeability and improved stress resilience were attributed to kaolinite (Al2Si2O5(OH)4) dissolution and hieratite (K2SiF6) precipitation, respectively. Geochemical speciation, simulating HF interactions with coal fracture minerals, predicted the occurrence and prevalence of both dissolution and precipitation reactions. Scanning electron microscopy-energy dispersive spectroscopy confirmed the mineral alteration phenomena. Identification of resultant structural changes and the differentiation of chemical from physical effects were elucidated using X-ray computed tomography. The overall findings show that mineral alteration by HF yielded relatively large, crystalline minerals that appeared to provide structural support to fractures, resulting in enhanced fluid flow and improved resistance to compression.

The aim of this work was to determine the influence that an advanced demineralisation procedure has on the combustion characteristics of coal. A high-volatile bituminous coal with 6.2% ash content was treated in a mixture of hydrofluoric and fluorosilicic acids (HF/H 2 SiF 6). Nitric acid was used either as a pre-treatment, or as a washing stage after HF/H 2 SiF 6 demineralisation, with an ash content as low as 0.3% being attained in the latter case. The structural changes produced by the chemical treatment were evaluated by comparison of the FTIR spectra of the raw and treated coal samples. The devolatilisation and combustibility behaviour of the samples was studied by using a thermobalance coupled to a mass spectrometer (TGA-MS) for evolved gas analysis. The combustibility characteristics of the cleaned samples were clearly improved, there being a decrease in SO 2 emissions.

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