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Coalbed methane (CBM) reserve of low-rank coal (R o,max < 0.65), occupying a proportion of more than 40% of the total CBM reserve of China, is 14.7 × 1012 m3. The exploration degree, particularly in situ gas content estimation of low-rank CBM reservoirs in China, is extremely low. Meanwhile, adsorption characteristics of low-rank coal reservoirs have not been revealed comprehensively. In this study, methane isothermal adsorption experiments of 30 low-rank coal samples collected from typical low-rank coal-bearing basins and coalfields in China were performed under different temperatures and moisture-equilibrated conditions. The results showed that the Langmuir volume, which was found to be relatively low when the inertinite content was above 50% and relatively high when the vitrinite content was above 50%, was positively correlated with the maximum vitrinite reflectance and generally decreased with the increase of the equilibrium moisture content. The adsorbed gas content decreased as the temperature increased, while the relationship between methane adsorption decrement and pressure was described by logarithmic form. The in situ adsorbed gas content of low-rank coal reservoirs increased slightly or almost unchanged with the increase of the burial depth. These characteristics were quite different from those of middle- or high-rank coal reservoirs, and the calculation method for estimation of the in situ absorbed gas content was validated to be correct based on a comparative analysis between the calculated results and actual test results. Findings in this work are helpful to further understand the adsorption characteristics as well as adsorbed gas content of low-rank coal to improve the evaluation technique of the CBM reserve of low-rank coal reservoirs.

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This study presents a wall-climbing robot called Vortexbot. Vortexbot has a suction unit that uses vortex flow to generate a suction force. Unlike the traditional unit based on contact-type suction, the suction unit can maintain a suction force without any contact with the wall surface. Therefore, the suction unit can provide a climbing robot with sufficient stable suction force even on walls with very rough surfaces and raised obstacles/grooves, and there is no wear and tear. Furthermore, the compressed air vents from the gap between the suction unit and the wall surface after rotating in the vortex chamber. Hence, such kind of flow direction can avoid the effect of the dust and dropped items on the wall surface. In this paper, we first introduced the vortex suction unit principle and discuss the feasibility of its application to a wall-climbing robot. Subsequently, the mechanical structure of Vortexbot was designed. After which, we surveyed the suction properties of the suction unit on a smooth wall surface. Then the functional relationship between the percentage change in the suction force and the supply flow rate was obtained. In addition, we studied the effect of the roughness and shape (a raised obstacle and groove) of the wall surface on the suction performance of the suction unit. Finally, we experimentally verified the climbing performance of Vortexbot on several kinds of walls with different surface conditions. It was confirmed that using the suction unit improves the robot’s climbing performance.