以微波鍛燒法回收廢牡蠣殼為氧化鈣並應用於二氧化碳捕集之研究

Abstract

本研究將台灣目前難以解決之水產廢棄物－牡蠣殼，再利用作為捕集二氧化碳之材料，因其成分富含大量的碳酸鈣，經鍛燒熱處理後會轉化為氧化鈣，為二氧化碳吸收劑，具有理論吸收容量高（44/56=0.876 g CO2/g CaO）、孔隙體積高、能迅速吸收及脫除二氧化碳、可長時間循環再生利用等優點，由牡蠣殼製備得氧化鈣，不但成分安定、成本低廉、取得容易，同時亦能解決牡蠣殼成為汙染環境的問題並有效減緩溫室氣體的排放，降低溫室效應對環境的衝擊。 鈣迴路技術補集二氧化碳共分為碳酸化程序及鍛燒程序，氧化鈣在碳酸化程序中與二氧化碳結合生成碳酸鈣，於鍛燒程序中受熱再生為氧化鈣，氧化鈣做為可不斷循環再生補集二氧化碳之載體。本研究將傳統的鍛燒程序改良為微波熱處理程序，微波設備發射出之電磁波經由電磁場方式傳遞，能穿透物質使物質均勻且迅速加熱，特點為反應時間短、反應溫度較低即可達到鍛燒效果。鈣基吸收劑(Ca-based sorbent)在傳統高溫鍛燒時會產生燒結現象(sintering)，因操作溫度過高導致吸收劑顆粒結晶表面變質、孔隙率及比表面積減少造成材料劣化、吸收容量降低，將微波熱處理應用於鍛燒程序，可降低反應溫度、縮短反應時間、提升轉化效率，以期能改善氧化鈣吸收劑的燒結現象、提高捕集二氧化碳的循環次數並增加吸收劑的使用效率。 利用牡蠣殼粉作為二氧化碳捕集的材料為確實可行，在多循環碳捕集中，牡蠣殼粉在第三個循環，碳酸化轉化率可優於商用碳酸鈣，因為其成分含有雜質能使吸收劑在鍛燒過程中不至劣化太快，微波鍛燒程序可以有效降低操作溫度及減緩牡蠣殼粉的燒結現象，使之碳酸化轉化率保持穩定，得到更佳的二氧化碳捕集容量，與傳統鍛燒程序相較之下，微波可有效減少能源消耗節省反應時間，作為取代傳統鍛燒程序相當具有發展的潛力。

Calcium looping cycle, post-combustion is an emerging, energy-efficient, low cost, technology which used CaO as regenerable sorbents of CO2 capturing through the carbonation/calcination reaction cycles. The cost, stability and sustainability of the sorbent are important in the design of these systems. In this study, waste oyster shell was used as raw material for calcium-based sorbent and microwave thermal treatment was applied to the calcination process. The simulated sintering conditions of sorbent occurred in the calcination reaction process and the higher heating temperature resulted in more particle expansion and sintering. Microwave irradiation heating of sorbent was proposed to lower the operating temperature of the calcination and aimed to prevent the absorbent from sintering. The sorbents were characterized using thermogravimetric analysis (TGA), X-Ray diffraction (XRD), scanning electron microscopy (SEM), and tested in the carbonation-calcination cycles. In comparison to the conventional calcination, microwave thermal treated sorbents showed better conversions after a longer series of capturing CO2 cycles and more efficient approach to reduce the electric energy consumption and process time. CO2 capture using oyster shell powder as a sorbent was investigated during the cyclic calcination/carbonation process. Oyster shell powder have higher carbonation conversions than commercial CaCO3 after third calcium looping cycle at the same reaction conditions. Impurity can improve the carbonation conversion of calcium-based sorbents, and this may be a factor contributing to the shells demonstrating a better cyclic CO2 capture performance.