商用沸石對二氧化碳/甲烷吸附分離之效能提升研究

Abstract

沼氣中同時存有甲烷(CH4)與二氧化碳(CO2)溫室效應氣體，為因應全球暖化及能源耗竭的相關議題在全球持續引起重視，本研究乃利用商用沸石來捕捉沼氣中的CO2，以純化CH4並提升沼氣之熱值，做為具高能源價值之CH4燃料。為考慮到成本效益，本研究利用市面上販售的多種商用沸石進行純化沼氣中的CH4，結果顯示13X沸石具良好效果，且較具成本競爭性。故本研究選定13X沸石做為吸附材料，發現在不同的前處理方式下，對於CO2吸附量有顯著差別，在最適化前處理條件下，13X沸石對CH4與CO2之選擇分離性頗佳。以模擬沼氣典型濃度80% (v/v) CH4和20% (v/v) CO2條件進行測試，最適化的前處理下，13X沸石對CO2總飽和吸附容量在27±1℃下可高達145 mg/g，且在CO2吸附效率80%情況下，CH4的純化率將近100%，另經總計210次之循環吸脫附試驗後，CO2吸附量並未出現明顯劣化之良好特性，其工作吸附容量約在75 mg/g維持穩定不衰減，吸附指標則在70%左右。將吸附材料進行物化特性分析結果顯示，13X沸石之所以能夠在經過前處理後達到活化效果，增加吸附反應活性，應為沸石的脫鋁反應(Dealumination)現象，其可減少沸石中的布忍斯特酸(Br□nsted acid)酸基數量，此時陽離子之分佈位置將產生更大的偏移而造成較大的靜電場生成，而進一步增加其吸附選擇性使得活性提高。

Both CH4 and CO2 are greenhouse gases presented in the biogas. If the CO2 in the biogas can be separated from the CH4, then the heating value of biogas can be greatly enhanced and the greenhouse gas emissions can be reduced at the same time. In this study, commercial zeolites were selected as adsorbents. For all tested zeolites, 13X appeared to have the best CO2 adsorption capacity and the lowest purchasing cost as well. Under simulated biogas composition of 80% CH4 and 20% CO2, 13X showed significant enhancement in the CO2 adsorption capacity after proper pretreatment process. The CO2 adsorption capacity could achieve 145 mg/g at 27±1℃. In addition, the selectivity of CO2:CH4 was as high as 36:1. The 13X zeolite demonstrated excellent cyclic adsorption with negligible deterioration in the CO2 adsorption capacity of around 75 mg/g, the adsorption index remained 70% during a total of 210 regenerative cycles. The physical and chemical characterization of 13X zeolite indicated that it has been activated after pretreatment, which may be due to the de-alumination effect. It can reduce the Bronsted acid over zeolite, at the same time the cations will have a greater shift caused by the generation of large electrostatic field, which further increase both its activity and adsorption selectivity.