| Biogas is mainly composed of methane (CH4,55%-65%) and carbon dioxide (CO2,30%-40%). The energy source of biogas is from CH4, and CO2significantly reduces its energy content, which result in high cost for transportation and storage of the gas. Therefore, it is critical to develop effective methods for the sequestration of CO2from biogas.Adsorption is considered very promising compared with other technologies, because of its intrinsic eco-compatibility and low cost. It is of great importance to develop an efficient CO2adsorbent with a high CO2adsorption capacity, a high selectivity, and good regeneration property. In this paper, the main aim was to find an excellent adsorbent to separate CO2and CH4, which will be benefit for large-scale biogas purification.Three families of adsorbents have been studied. They are molecular sieves, carbon-based adsorbents and amino-functionalized silicas. The separation/regeneration performance of the adsorbents has been compared, and their sensitivity to moisture has been researched. The main research conclusions are as follows:(1) Selective adsorption of CO2from its mixture with CH4on several commercial molecular sieves has been studied at ambient temperature(25℃) and low pressure (0.2MPa). The behaviors of the molecular sieves have been compared by using three basic parameters:selectivity, adsorption capacity and regenerability. The results indicate3A and4A molecular sieves have low CO2adsorption capacity. However5A and13X molecular sieves show high efficiency in dynamic separation of CH4and CO2when the adsorbents were used for the first time. The separation coefficient between CH4and CO2on13X molecular sieve is higher than5A, which prove13X molecular sieve show better performance in separating CO2from its mixture with CH4. But5A and13X molecular sieve could not be regenerated completely and the adsorption/regeneration performance became stable from their second use. The reason why separation performance of5A and13X molecular sieves decrease is explained by FTIR spectroscopy that the partial poisoning of the molecular sieves happened. That is to say, the regenerability of the two molecular sieves need improve. In addition, molecular sieves are sensitive to water vapor, vapor can make adsorbent completely lose the ability of separation of CH4and CO2, which is very disadvantageous to biogas purification technology.(2) The removal of CO2from a CH4/CO2mixture on commercial carbon-based adsorbents has been researched. And the performance in separating CO2from its mixture with CH4on carbon-based adsorbents was compared with13X molecular sieve. The results indicate carbon molecular sieve can separate the mixture well but its selectivity is lower than13X molecular sieve. However carbon molecular sieve can be well regenerated and are not very sensitive to water vapor, which are much better than13X molecular sieve. All the properties of carbon-based adsorbent are mainly governed by its texture. In order to improve the adsorption selectivity of carbon molecular sieve for CH4and CO2, the new preparation process should be urgently researched, which may be benefit for large-scale biogas purification.(3) amino-functionalized silicas were synthesized using MEA and silicas. The adsorbents were characterized by N2adsorption isotherm, thermogravimetry and FTIR, which show that MEA have been successfully modified in the FNG-Ⅱ silica pores. And the performance of adsorbents was researched by adsorption and desorption experiments. The results indicate MEA-functionalized silicas could separation CO2and CH4well in its first or second use, but the performance deducted afterwards. The MEA-functionalized silicas can not regeneration under105℃. When MEA loading is low, the performance improves with MEA loading. However, the performance would not improve when MEA more than20%, because the pores would be occupied so much that the specific surface would decrease in a large number. The most noteworthy is that MEA functionalized silicas can not be affected by vapor.(4) A preliminary study on MEA/MDEA modified silica, the results showed that the addition of MDEA could improve the regeneration performance of adsorbents.In conclusion, molecular sieves can separate CH4and CO2well, but their skeleton chemical structures make them strong interact with CO2and they are too sensitive to vapor. Carbon-based materials have some advantages on regeneration but they also have low recovery rate of CH4. Development of new functional carbon materials may solve this problem, but this also need quite a long time. MEA modified silica can separate CH4and CO2well and has strong tolerance to water, but the disadvantage of the regeneration is very obvious. Too much work should be done to improve the regeneration of amino-functionalized absorbents, and it will be an attractive research field. |
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