建構微藻光生物反應系統並用於二氧化碳減量與微藻生物質的生產

Abstract

以光合生物反應器培養微藻可被用來減量廢氣中的二氧化碳（CO2），且微藻所生產之油脂還可被轉化為生質柴油使用。在本研究中，我們篩選並分離出具有高生長潛能之微藻細胞來減量CO2及生產微藻生物質。此外我們也設計一氣舉式光合生物反應器用以進行微藻的高濃度培養。 首先，我們以CO2減量並生產生物質為目的進行微藻細胞株之篩選，分離得到具有高生長潛能之藻株，再以起始藻細胞濃度為低濃度(i.e., 8 × 10^5 cells/mL)與高濃度(i.e., 8 × 10^6 cells/mL)的培養方式來進行微藻二氧化碳耐受性試驗，試驗結果顯示當微藻之起始細胞濃度較高時，在具有二氧化碳通入培養的環境下，微藻生長速率會較為快速，由上述結果可知微藻對於CO2的耐受能力會因藻細胞濃度的增加而增加，因此本研究將於微藻培養的起始階段，通入適當的CO2 (2%)，使藻細胞適應通有CO2之環境下生長，再配合半連續式的培養方式將微藻轉至10%及15% CO2環境下培養，使微藻細胞能逐漸適應高濃度CO2，進而克服高濃度CO2對於微藻生長之抑制，提昇微藻對於CO2之耐受性。 接著，為了增加生物質生產與CO2移除之效率，我們以半連續式之各種培養策略進行生物質產能評估，每兩天置換四分之一培養液、每三天置換三分之一培養液及每八天置換二分之一培養液，此半連續式培養結果顯示，以每兩天置換四分之一之培養，微藻生物質產能可高達0.61 g/L/d，於本研究中所設計之多孔內管氣舉式光生物反應器不但具有高生物質產能且能維持高密度培養，同時我們也評估多孔內管氣舉式光生物反應器對於CO2之移除效能。由結果顯示，高濃度微藻(5 g/L)培養於通氣為10% CO2的環境下，CO2之移除效能大於60%以上。 最後，我們篩選了一耐溫與耐受CO2微藻突變株Chlorella sp. MTF-7實際應用於實場煙道廢氣養殖。研究結果顯示，我們所篩選之Chlorella sp. MTF-7於室內不同溫度以中鋼煙道廢氣通氣實驗中，Chlorella sp. MTF-7於35oC及40oC之高溫下，微藻生物質產能則為0.32及0.24 g/L/d；直接引中鋼煙道廢氣進行戶外Chlorella sp. MTF-7養殖六天，微藻養殖濃度可達2.87 g/L（起始養殖濃度為0.75 g/L），微藻生物質產能則為0.52 g L-1 d-1。經由兩組間歇煙道廢氣通氣方式進行養殖，其CO2、NO及SO2之移除效能分別約為60%、70%及50%。由結果顯示，所篩選得之Chlorella sp. MTF-7可有效直接利用煙道廢氣養殖，並能穩定生產微藻生物質與有效減除煙道廢氣中之CO2、NO及SO2。

Microalgal cultivated in photobioreactor can be used for CO2 mitigation from waste gas and microalgal lipids can be converted into biodiesel. In this study, we screened and isolated microalgal strains with high potential for CO2 reduction and microalgal biomass production. In addition, we also designed an air-lift photobioreactor for high density microalgal cultivation. First, the high growth potential microalgal cells were screened and isolated as a candidate for CO2 reduction and biomass production. Then, the low (i.e., 8 × 10^5 cells/mL) and high (i.e., 8 × 10^6 cells/mL) density of the microalgal cells inoculums for CO2 tolerance was evaluated. The results indicate that microalgal cells grew rapidly in a high-density culture with CO2 aeration. Thus, the strategy of increasing CO2 tolerance and cell density in the microalgal cultures was performed in this study. At the initiating stage of culture, the microalgal cells were grown and adapted to an enriched-CO2 (2%) environment. Then, the semicontinuous system was performed. The result shows that the microalgal cells can grow well even under the conditions of 10% and 15% CO2 aeration. Then, for increasing biomass production and CO2 removal efficiency, the microalgal cells cultivated in the operation mode that culture broth was replaced by 1/4 (i.e., one-fourth volume of cultured broth was replaced by fresh medium at an interval of 2 days) and 1/3 (one-third broth replaced at 3 days interval) and 1/2 (one-second broth replaced at 8 days interval). The results show that the maximum biomass productivity could achieve 0.61 g/L/d in 1/4 of the culture broth recovered from the culture every 2 days. The CO2 removal efficiency was also evaluated because the high performance of biomass production and high density cultivation. The results show that > 60% of CO2 could be removed from the aerated gas which contains 10% CO2 under high density (approximate 5 g/L) cultivation. Finally, the growth and on-site bioremediation potential of an isolated thermal- and CO2-tolerant mutant strain, Chlorella sp. MTF-7, were investigated. The biomass productivity of Chlorella sp. MTF-7 cultured indoors at 35 and 40oC was 0.32 and 0.24 g/L/d, respectively. The Chlorella sp. MTF-7 cultures were directly aerated with the flue gas generated from coke oven of a steel plant. The biomass concentration, productivity of Chlorella sp. MTF-7 cultured in an outdoor 50-L photobioreactor for 6 days was 2.87 g/L (with an initial culture biomass concentration of 0.75 g/L), 0.52 g/L/d. By the operation with intermittent flue gas aeration in a double-set photobioreactor system, average efficiency of CO2 removal from the flue gas could reach to 60%, and NO and SO2 removal efficiency was maintained at approximately 70% and 50%, respectively. Our results demonstrate that flue gas from coke oven could be directly introduced into Chlorella sp. MTF-7 cultures to potentially produce algal biomass and efficiently capture CO2, NO and SO2 from flue gas simultaneously.