玄武岩之二氧化碳礦化封存：常壓與模擬現地高壓環境與灌注水源礦化反應分析

Abstract

自工業革命以來，化石燃料大量燃燒導致大氣中二氧化碳(carbon dioxide, CO2)濃度上升，氣候變遷日益加劇，凸顯淨零排放的迫切性。在臺灣 2050 淨零排放路徑中，碳捕捉與封存(Carbon Capture and Storage, CCS)技術被視為長期減碳關鍵，近幾年CCS於全球快速擴展，各國亦積極推動相關政策與技術開發。其中碳封存可依方法分為海洋封存(ocean storage)、地質封存(geological storage)與礦化封存(mineral storage)，後者因具高穩定與安全性而備受關注。其反應機制為礦物碳酸化(mineral carbonation)，即將CO2與含鎂或含鈣的矽酸鹽礦物（玄武岩）進行反應，生成熱力學穩定的碳酸鹽礦物（碳酸鈣CaCO3、碳酸鎂MgCO3），使碳以固態形式長期固定於礦物中，達到碳封存之目的。目前冰島CarbFix計畫已驗證其可行性，惟該技術需消耗大量淡水資源，若以富含陽離子（鈣Ca2+、鎂Mg2+等）之環境水體替代淡水，進行二氧化碳、水體與玄武岩之礦化反應，除可減少用水競爭與衝突，亦能促進碳酸鹽生成、提升封存效率。 本研究模擬常壓(PCO2=1 atm)反應時間140 天與高壓(PCO2=1000、1500、2000 psi) 反應時間14 天條件下，水體、CO2與玄武岩之礦化反應，探討不同參數條件與環境水體（超純水、海水與鹵水）對礦化封存效率與反應動力學之影響。常壓部分比較封閉與連續採樣系統，結果顯示封閉系統反應平衡穩定，以淡水組70 天為例，在封閉與連續採樣系統玄武岩分別溶出了Mg2+濃度41.84與17.68 mmol/kg，可觀察封閉系統玄武岩溶解效果較佳，能促進碳酸鹽生成，高壓實驗亦採用此設計。高壓條件下，CO2轉為液相提高其在水中溶解度，進而與環境水體中（海水、鹵水）之Mg2+、Ca2+或玄武岩溶出到溶液之Mg2+、Ca2+、鐵Fe2+等陽離子反應生成碳酸鹽沉澱。因玄武岩在反應過程中會溶出金屬陽離子進而影響pH值，所以以pH值變化與陽離子濃度上升作為反應動力指標，常壓下淡水組Mg2+濃度最高達22 mmol/kg，與pH上升趨勢一致，反映玄武岩持續溶解。特別的是，在淡水組別中觀察到Ca²⁺濃度上升緩慢，而海水與鹵水組別中Ca²⁺濃度反而下降，推測在鹽水反應下有碳酸鈣沉澱生成，進一步以X光粉末繞射儀(X-ray diffractometer, XRD)分析確認產物，鹵水常壓反應140 天後之玄武岩粉末可辨識出方解石，特徵峰位於29.6°(2θ)處，與Tilleyite碳酸鹽類特徵峰，證實沉澱生成。 高壓條件下反應進程顯著加快，可透過矽Si4+濃度變化觀察，因其代表玄武岩礦物的溶解情形，在常壓淡水組Si4+上升至0.85 mmol/kg，而高壓淡水組僅14天便達3.04 mmol/kg。進一步以XRD分析確認反應產物，高壓反應後之玄武岩皆可辨識出碳酸鹽類特徵峰，其中Tilleyite、Shortite及Natrite等複碳酸鹽礦物並非玄武岩中的原生相，推測為高壓反應過程中新生成的碳酸鹽礦物。最後，熱重分析結果可估算碳酸鈣平均生成速率，在常壓140 天反應時間內，鹵水為0.058 g/day；而高壓在14 天內1000 psi鹵水亦為0.058 g/day、淡水為0.016 g/day，顯示高壓可於短時間內提升反應效率，並且鹵水能有效促進碳酸鹽生成、提高封存效率。綜合各項分析結果可知，1000 psi鹵水組別在碳酸鹽生成量、反應速率與礦物特徵辨識上皆表現最佳，為本研究中最具潛力之礦化封存條件。本研究結果可提供最適礦化封存參數，並為臺灣碳封存與負碳技術發展提供實證依據。

Since the Industrial Revolution, the extensive combustion of fossil fuels has led to a significant rise in atmospheric carbon dioxide (carbon dioxide, CO2) concentrations, intensifying climate change and highlighting the urgent need for net-zero emissions. In Taiwan’s 2050 net-zero pathway, Carbon Capture and Storage (CCS) technology is considered a key long-term strategy for carbon reduction. In recent years, CCS has rapidly expanded globally, with various countries actively advancing related policies and technological developments. Carbon storage methods can be categorized into ocean storage, geological storage, and mineral storage. Among these, mineral storage has gained considerable attention due to its high stability and safety. Its mechanism is based on mineral carbonation, where CO2 reacts with magnesium- or calcium-bearing silicate minerals (e.g., basalt) to form thermodynamically stable carbonate minerals (CaCO3, MgCO3), enabling long-term solid-state carbon fixation. The feasibility of this approach has been demonstrated in Iceland’s CarbFix project; however, it requires large volumes of freshwater. Replacing freshwater with ion-rich environmental waters (e.g., seawater or brine) may not only reduce water competition and conflict but also enhance carbonate formation and storage efficiency. This study simulates the mineral carbonation of CO2, water, and basalt under atmospheric pressure (PCO2=1 atm) for 140 days and high-pressure conditions (PCO2=1000, 1500, 2000 psi) for 14 days, aiming to investigate the effects of different parameters and water types (ultrapure water, seawater, and brine) on mineral carbonation efficiency and reaction kinetics. Under atmospheric conditions, both closed and continuous sampling systems were compared. The closed system demonstrated better equilibrium maintenance and enhanced basalt dissolution. For instance, in the freshwater group after 70 days, Mg2+ concentrations reached 41.84 mmol/kg in the closed system compared to 17.68 mmol/kg in the continuous sampling system, indicating that closed systems better promote carbonate formation. This design was also adopted for high-pressure experiments. Under high-pressure conditions, CO2 transitions into the liquid phase, increasing its solubility in water and facilitating reactions with Mg2+ and Ca2+ from the water or those leached from basalt (including Fe2+), resulting in carbonate precipitation. Since basalt dissolution alters pH due to the release of metal cations, changes in pH and ion concentrations were used as indicators of reaction kinetics. In the atmospheric-pressure freshwater group, Mg2+ reached up to 22 mmol/kg, accompanied by a pH increase, indicating continued basalt dissolution. Notably, Ca2+ concentrations rose slowly in the freshwater group but decreased in seawater and brine groups, suggesting CaCO3 precipitation. X-ray diffractometer (XRD) analysis confirmed the formation of calcite (peak at 29.6° 2θ) and Tilleyite in the brine group after 140 days under atmospheric conditions. Under high-pressure conditions, the reaction rate was significantly accelerated, as evidenced by the increase in Si4+ concentration, which reflects basalt dissolution. In the atmospheric-pressure freshwater group, Si4+ reached 0.85 mmol/kg, while under high pressure, it reached 3.04 mmol/kg in just 14 days. XRD analysis of post-reaction basalt revealed distinct peaks of carbonate minerals, including Tilleyite, Shortite, and Natrite, none of which are primary phases in basalt, suggesting the formation of new secondary carbonates under high-pressure conditions. Thermogravimetric analysis estimated average CaCO3 formation rates: under atmospheric conditions, brine yielded 0.058 g/day; under high pressure (1000 psi), the brine group maintained 0.058 g/day, and freshwater reached 0.016 g/day. These results indicate that high pressure effectively enhances reaction efficiency within a short period, and ion-rich brine can further promote carbonate precipitation and improve storage performance. In summary, the 1000 psi brine group demonstrated the highest performance in carbonate yield, reaction rate, and mineral identification, representing the most promising mineral carbonation condition in this study. These findings offer valuable insights into optimal mineral carbonation parameters and provide empirical support for advancing carbon storage and negative emissions technologies in Taiwan.