| alleviate the increasingly serious shortage of resources and energy and to protect the atmospheric environment in our country. However, the content of CH4 in biogas is only 50～70%, impurities such as carbon dioxide and hydrogen sulphide take the other 40% in volume. These impurities lead to the low utilization of biogas as well as the corrosion of the equipment. Therefore, it is necessary to separate and purify CH4 from biogas to improve the utilization efficiency and enlarge the application area of biogas.There are many means, such as physical adsorption, chemical scrubbing, pressurized water scrubbing, low temperature separation, membrane separation etc., developed for biogas separation. Among them, physical adsorption technology has been got industrial application in the separation of CO2/CH4, due to the low energy consumption, simple process flow and renewability during the process. For pressure swing adsorption technology, an adsorbent with a large adsorption capacity, high adsorption selectivity, good regeneration performance and low cost is one of the most important factors when applying on biogas separation and purification.In this study, firstly, an agricultural waste employed as raw material to fabricate a steady and appropriate activated carbon (AC) via carbonization/activation using ZnCl2 as an activating agent followed by a heat treatment. The obtained activated carbons were used as precursor to prepare a carbon molecular sieve (CMS) with low high CO2/CH4 separation efficiency via chemical vapor deposition (CVD) technique using methane as a carbon source. It is determined that the prepared soybean straw-based carbon molecular sieves possess a separation ratio as high as 18.25 for CO2/CH4 separation. The value is far higher than the minimum value of 3.0 required in industry. Unfortunately, the CO2 adsorption capacity declines from 6.98 mmol·g-1 to 3.78 mmol·g-1 after the CVD process. Secondly, CO2 was employed as an activating agent to prepare a soybean straw-based activated carbon with ordered pore through carbonization-activation in one-step process. The prepared activated carbon gives a CO2/CH4 separation ratio 3.60 with a high CO2 adsorption capacity of 5.31 mmol·g-1. Finally, in order to improve the CO2 adsorption capacity and CO2/CH4 separation ratio, the soybean straw-based activated carbons obtained by ZnCl2 activation were blended with polyethyleneimine to obtain a soybean straw-based activated carbon/PEI composite material. It is clear that the AC obtained from the soybean straw/ZnCl2 mass ratio of 1.0 has a hierarchical pore structure, which is advantageous to the load of PEI and the maintaining of residual channel. The composite adsorption material with PEI/AC mass ratio of 0.2, gives an adsorption selectivity coefficient of 5.83 for CO2/CH4 separation and a CO2 adsorption capacity of 7.83 mmol·g-1. Moreover, it shows a good regeneration performance. The results suggest that the prepared soybean straw-based activated carbon/PEI composite sorbent is a promising adsorbent applied on PSA process for CO2/CH4 separation. |
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