Design and Characterization of CO2 Sorbents with nano Ca-Al Layered Double Hydroxides as Precursors for CO2 Capture at Elevated Temperature
|關鍵字:||奈米鈣基層狀水滑石;二氧化碳捕獲劑;燒結效應;再生程序;nano Ca-Al Layered hydrotalcite;carbon dioxide sorbent;sintering effects;regeneration process|
|摘要:||全球能源的供應來源目前乃以化石燃料為最大宗，然而人類大量依賴化石燃料與持續排放工業廢氣所產生的溫室氣體而導致溫室效應已嚴重衝擊世界各地之氣候與生態環境。而利用二氧化碳捕捉與封存(Carbon Capture and Storage, CCS)技術，乃為目前國際公認技術可行性最高。目前適用於高溫環境(500－700°C)之CO2捕獲劑，主要包括氧化鈣、氧化鋰與層狀水滑石等材料。整合上述之捕獲劑，其最大問題乃於因燒結效應產生捕獲動力緩慢與捕獲量低及經過多次捕獲循環迴圈後其捕獲效能呈現劣化趨勢。因此，如何改善上述高溫捕碳劑之劣勢，乃為目前主要研究所需突破之關鍵。本論文中，使用各種不同的合成技術，其中包括共沉澱-水熱法、微包乳化法、溶膠-凝膠法與結構重組之水合法製備出具有多功能之性奈米鈣基層狀水滑石材料。藉由粉末X光繞射儀(PXRD)、穿透式電子顯微鏡/電子能量損失光譜 (TEM/EELS)、掃瞄式電子顯微鏡/X光能量分散光譜儀(SEM/EDS)、傅氏轉換紅外線光譜(FTIR)、氮氣吸附等溫線(N2 adsorption isotherm) 深入地鑑定其材料的組成、形貌、化學結構以及多孔結構。
本研究中，將Ca-Al LDH藉由高溫煅燒後所得到之鈣-鋁混合氧化物(CaO-LDO)，利用TGA測試其二氧化碳捕獲效能，結果顯示，與多次捕獲循環迴圈後仍具有非常優異之捕獲維持率。此乃因Al3+ 均勻地插層於CaO中，有效地抑制CaO粉體因燒結效應所產生的聚集現象，故具備優異之捕獲效能。本研究亦將藉由微包乳化法製備奈米尺度之鈣基層狀水滑石。其結構特性乃隨反應溫度及時間影響其形態，而粉體大小約為≈40奈米左右。另外，於研究中的第三部分，係經由溶膠-凝膠法製備出獨特孔洞結構之鈣基層狀水滑石。根據相關文獻報導，二氧化碳捕獲動力約40~60分鐘其捕獲量可達飽和狀態，本研究之二氧化碳捕獲測試結果顯示，不僅具有高捕獲量與優異之捕獲維持率，且擁有快速之捕獲動力，其5分鐘約可達到捕獲飽和狀態。最後考量其捕獲劑之再生再利用；因此本研究於最後的一部分，將上述具有優異捕獲效能之CaO-LDO粉體，經由50次捕獲循環後，其捕獲效能呈現劣化之粉體藉由結構重組之水合作用以進行再生程序。結果顯示，經由再生程序後之Ca-Al LDH 乃具有更細小之層狀平板結晶型態，其大小約為200奈米，不僅結構能回復至Ca-Al LDH之特徵層狀結構型態，其捕獲效能亦可有效地復原至初始之高捕獲量，且捕獲效益為更加化。|
Carbon dioxide (CO2) is a product of complete combustion of fossil fuels and it has been identified as one of the important greenhouse gases (GHGs). Various sorbents have been investigated for CO2 capture at high temperature. Although the CO2 capture capacity of these sorbents can typically be maintained over multiple utilization cycles, there were still some problems, especially in either the carbonation/calcination time period or the slow absorption kinetics due to strong agglomeration or aggregation. The key improvement for such a process is to develop sorbent materials that are capable of capturing CO2 at high temperatures, in which the appropriate sorbent should possess a high capture capacity, a long lifetime, and fast carbonation–calcination kinetics. Therefore, this study is focused on Ca-based LDHs nanostructured materials through various synthesized routes, which include coprecipitation-hydrothermal method, reverse microemulsion, sol-gel method and rehydration method for high temperature CO2 capture. The composition, morphology, chemical structure, and porous structure of these materials were identified in detail using powder X-ray diffraction (PXRD), transmission electron microscopy/electron energy loss spectroscopy (TEM/EELS), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR), and N2-adsorption isotherm. The synthesized mechanisms and CO2 capture of these materials are discussed in respective chapter. In this study, the obtained CaO-LDO samples to maintain high thermal stability over multiple cycles. This mechanism suggests that the Al3+ cations would be incorporated into the matrix by either reacting with CaO or forming a weakly crystalline Al–O phase. However, the weakly crystalline Ca–Al–O phase would perform a similar role as Ca12Al14O33 in increasing sintering resistance and prohibiting degradation during multiple carbonation–calcination cycles. In addition, the study is also focused on the nano-scale Ca–Al LDHs, which was prepared by using reverse microemulsion method. The characterization of the material in this study has been identified to exhibit that no strong agglomeration phenomena occurred in the samples synthesized at higher heating temperatures (80°C, 125°C and 150°C), revealing well-dispersed calcined nanoparticles (≈40 nm) that were likely correlated with the hierarchical LDH structure evolved from the reverse microemulsion reaction at 80–150°C. In the third parts, a scaffold-like hybrid layered double hydroxide is developed by sol-gel method and to integrate the dual functions of sintering-resistant metal oxide supports and a porous CaO/CaCO3-based structure. The results of carbon dioxide capture experiment by thermal gravimetric analysis (TGA) or fixed-bed reactor show that relatively rapid CO2 absorption kinetics and highly stable carbonation/calcination performance at high temperatures for CO2 capture. Although, the Ca-Al LDO system with Ca/Al = 7:1 was used to CO2 capture which display excellent high temperature CO2 capture capacities and remain highly thermal stable carbonation/calcinations performance. In order to, extend the life cycles and reuse efficiency of the Ca-Al LDO sorbents, the regeneration mechanism of the degraded mixed oxides powder will be further investigated as functions of deionized water, sodium carbonate, calcination temperature, react time will be studied and discussed. However, SEM observations show that the sample, both mixed hydroxides and reconstructed LDHs, are composed by aggregates of small plate-like particles (less than 200 nm) with a sub-hexagonal morphology. Therefore, a longer immersed time will be further beneficial for better crystallization of the reconstructed Ca-Al LDH and the sorbent is clearly demonstrates that the capture performance was improved and is largely maintained after regeneration process.