Studying the law of coal adsorption capacity under the action of liquid nitrogen is very important for the extraction of coalbed methane. For this study, the lignite's ability to adsorb methane was studied after it had been frozen by a number of different freezing techniques. In addition, the changes in the organic functional groups on the coal after freezing were identified by using infrared spectroscopy. The results demonstrate that as the duration of liquid nitrogen freezing and the number of freezing cycles increase, the lignite's gas adsorption capacity first decreases, then increases, and finally maintains a stable value. As the liquid nitrogen freezing time increases, the proportion of oxygen-containing functional groups increases, but the percentages of substituted benzene groups and aliphatic hydrocarbon functional groups decrease. The research shows that, compared with a single freezing, cyclic freezing has a greater effect on the lignite's gas adsorption capacity. Freezing influences the proportion of the substituted benzene functional groups on the coal the most; after freezing, the rates of change in the proportions of aliphatic hydrocarbon and oxygencontaining functional groups are only 70.76% and 80.29% as large as the change in the substituted benzene functional groups. As the proportions of the three types of functional groups increase, the lignite's gas adsorption capacity first decreases but then increases. This study will promote the development of liquid nitrogen freezing technology for coalbed methane extraction.
Hot dip aluminizing of mild steel at different temperatures was conducted to reveal the influence of reaction temperature and time on interfacial intermetallic compounds (IMCs). Scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction were employed to investigate the interfacial microstructures. The IMCs of the dipping interface consisted of a thick layer of η-Fe2Al5 between 4.2–132.2 μm next to the steel and a thin layer of θ-Fe4Al13 between 0–5.5 μm close to the aluminum. With increasing dipping temperature and time, the total thickness of IMCs (Fe2Al5 plus Fe4Al13) increased. Specifically, the growth of the Fe2Al5 layer can be described by parabolic rate laws. An activation energy of 93 kJ mol−1 was obtained, combining both the results from the present work and previous studies in the temperature range of 675–900°C. The change in Fe4Al13 thickness is not significant compared with the Fe2Al5. However, the decrease in IMC thickness of the Fe4Al13 with dipping temperature was observed for the first time and had never been reported before. Moreover, it can be clearly observed that the thickness of the Fe4Al13 decreased with dipping time based on the linear fitting results by excluding the result of the initial 1 s. A possible mechanism is that interfacial dynamics and thermodynamics work for the dissolution and decomposition of the Fe4Al13 layer. Higher temperature accelerates the dissolution of the θ layer.
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