以合成氣製備製程減量二氧化碳之可行性研究

Abstract

由於溫室氣體的議題受到重視，近年來發展了許多技術以求降低大氣中的二氧化碳含量，將捕獲的二氧化碳轉化為有價值的化學品例如合成氣便為其中一種可能的手段。合成氣為多用途的中間產物，依據其組成中氫氣與一氧化碳的比例可以繼續轉化為多種不同的化學品，而二氧化碳可以藉由與甲烷進行乾式重組反應或是與氫氣進行逆向水氣轉化反應轉化成合成氣。然而透過乾式重組反應僅能將二氧化碳轉化為氫碳比為1的合成氣，而且將面臨在高溫下甲烷分解產生大量焦炭的問題，將乾式重組反應與蒸氣重組反應或部分氧化反應整合在同一反應器中或許可解決上述問題。本研究利用文獻中的動力學資料描述反應器的表現以設計製程，並搭配熱整合的方式以回收高溫產物的能量，另為了考慮將二氧化碳轉化時需要額外投入原料與提供能量，本研究利用生命週期評估的資料計算出原料製備階段伴隨的二氧化碳間接排放與估計提供能量時額外排放的二氧化碳，試圖找出在哪些目標氫碳比下合成氣製備製程確實能有減碳的能力，並找出表現最好的聯合反應器。而利用高分子電解質膜電解水或利用蒸氣重組反應器製備氫氣以作為轉化二氧化碳的原料的製程也將在文章中討論。 研究結果顯示三元重組反應器在目標氫碳比小於1.5時能有效將二氧化碳減量，透過反應器的燃燒反應迅速釋放能量可以使反應器內維持高溫以利二氧化碳轉化，而加速甲烷轉化反應的同時能確實減少焦炭生成量。使用氫氣轉化二氧化碳的製程雖然有良好的減碳能力，但由於利用電解水製造乾淨的氫氣成本昂貴，使製程成本遠高於使用天然氣為原料的聯合反應器製程而較不具競爭力，從敏感度分析的結果可以知道若是乾淨的氫氣成本能夠低於每公斤3美元時才能與其他製程有相近的年度總成本。

In recent years, technologies for reducing carbon dioxide in atmosphere have received much attention. Converting captured CO2 into valuable chemicals such as syngas is one possible solution. Syngas is a universal intermediate and could be further converted into various valuable products depending on the H2/CO ratio in syngas. The conversion of CO2 into syngas could be realized by consuming methane (dry reforming) or hydrogen (reverse water gas shift). However, the H2/CO ratio in produced syngas from direct conversion of CO2 with methane is limited to around 1. The concept of combined reforming is to utilize the advantage of steam reforming and partial oxidation simultaneously, which could raise H2/CO ratio and mitigate coke formation on catalyst. This work attempts to use rigorous kinetic model to determine at which H2/CO ratio the process would convert more CO2 than produced, and to select the best combined reforming reactor. All production processes considered in this work are heat-integrated to recover energy from product. The CO2 emission of raw material production evaluated with Life Cycle Assessment data is also considered. Process for conversion of CO2 with hydrogen from polymer electrolyte membrane or from upstream steam reforming reactor is also discussed. The result suggests that process with tri-reforming reactor when target H2/CO ratio lower than 1.5 would have ability to reduce CO2. CO2 conversion could be improved with addition of oxygen because combustion reaction could maintain high temperature in reactor. The sensitivity test for hydrogen cost on total annual cost of process shows that if cleaner hydrogen cost becomes lower than 3 USD/kg, conversion of CO2 with hydrogen is cost competitive with combined reactor process and performs even better in terms of CO2 reduction.