From macro perspective, main factors affecting adsorption of CH 4 by coals include coal particle size, pore structure, coal rank, moisture, temperature and pressure, etc.. The adsorption capacity of CH 4 by coals is affected by multiple factors. The adsorption experiment of single component gas showed that the absorption of CO 2 was higher than that of CH 4 by coals, finding that the adsorption capacity of CO 2 by coal surface was higher than that of CH 4 , and the absorption capacity ratio of

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... O 2 /CH 4 changed with pressure and coal rank, but the pressure was not directly proportional to absorption capacity ratio of CO 2 /CH 4 . With the increase of pressure, adsorption ratio of CO 2 /CH 4 decreases gradually. When the pressure is high, the adsorption capacity of CH 4 is higher than that of CO 2 , so it is appropriate to use CO 2 to displace coal seam CH 4 under low pressure. In addition, adsorption-desorption characteristic curve obtained from pure gas and carbon dioxide adsorption isotherm experiment could not demonstrate true adsorption situation of CH 4 and CO 2 mixed gas in coal seam.