微藻養殖生產油脂並利用微藻油脂產製生質柴油之研究

Abstract

近年來，全球面臨地球暖化危機與石化能源耗竭兩大危機。由於人類大量開採石化能源及發展工業化社會導致溫室氣體CO2大量累積與石化燃料枯竭之嚴重問題。因故低污染的再生能源與CO2減量之策略發展為世界各國所積極研究的議題。然而以海洋微藻培養之利用正是一項高效益的綠色能源開發方法。微藻為能行使光合作用之單細胞植物，能快速且大量生產植物生物質與累積大量油脂於微藻細胞中。有鑑於此，本研究之目的為以高脂海洋微藻利用廢氣CO2培養以進行大量油脂生產並藉由轉酯化反應生成生質柴油之研究。在最佳培養狀況下所篩選海洋微藻生長效率可達一天微藻增生2至3倍。微藻培養時Chlorella sp.與N. oculata之微藻內油脂從生長對數期至生長穩定期，培養狀態進行至氮源缺乏時，可由12%與21%分別提升至21%與50%。然而以半連續式微藻培養於光生物反應器，通入2% CO2之半連續式微藻光生物反應器中，其生長能力與產脂率表現最佳，氮在不同濃度CO2培養下（2至15%），高濃度CO2對於微藻的生長與產脂量能保持穩定並不受高濃度CO2的抑制。然而微藻也能利用不同的有機碳源在混營或異營脂培養狀態下進行生長與油脂堆積。本研究指出，以蔗糖行異營培養之N. oculata其油脂累積雖可達54%，但其生物質產量有降低的情形，而在以蔗糖進行混營之培養下油脂產量可提昇至每公升0.284公克。因此微藻培養時可分為兩個階段：增值階段及肥育階段，先利用最適生長環境來快速培養增加微藻的細胞濃度，再將其轉入低硝酸鹽或以蔗糖混營培養環境中肥育，即可有效率的大量產脂。 本研究中，微藻油脂轉酯化生成生質柴油因其轉化方式可分為化學製程以及生物製程兩種，以最佳油醇莫耳比下，酸化學催化反應與微生物脂解酶催化反應可分別達到88%與82%。酵素催化之轉化率雖佳，但其成本高且反應時間過長，在工業上多以快速簡便之化學催化轉酯化反應達到快速、低成本且高產率的目的。本研究成果證明由藻類油脂製成生質柴油作為再生燃料具可行性與未來性，且在藻類培養過程中能有效的降低環境中的溫室氣體之危害並能提供快速且大量生物油脂之生產以提供生成生質柴油。

In recent years, people over the world face some acute problem with regard to global warming and energy crisis. Humans exploited the fossil energy and developed the industry and civilization well in past hundred years to result in the environmental problems, green-house gas emission rising, and petrochemical fuel exhausting. Photosynthetic organism, microalgae, can use solar energy efficiently to combine water with CO2 to produce biomass. Microalgae can not only produce biomass but accumulate lipid in microalgal cells. Lipids from microalgae can be extracted and converted to biodiesel fuel by transesterification. In the study, the biomass and lipid productivity of Chlorella sp. and Nannochloropsis oculata were evaluated in the different conditions of culture in the closed photobioreactors. Results showed that the lipid accumulation of Chlorella sp. and N. oculata from logarithmic phase to stationary phase were significantly increased from 12% to 24% and 21% to 50%, respectively. In the semiconscious culture of Chlorella sp. and N. oculata, the total lipid productivity was 0.143 and 0.142 (g/L/d), respectively although the cultures were daily replaced half of broth. The results showed that Chlorella sp. and N. oculata were potential candidates for biomass and lipid production by semicontinuous cultures. The comparison of lipid productivities in the semicontinuous systems in which the culture broth were replaced at an interval of 24 h (one-day replacement) or 72 h (three-day replacement) was performed. The result indicated the total lipid yield in the semicontinuous culture operated by one-day replacement was more efficient. Moreover, different carbon sources supplied in the culture could make distinct growing ability and lipid accumulation. The results showed the cultivation of N. oculata using sucrose as carbon source in mixotrophic growth gave the highest maximum biomass (0.80 g/L) and the lipid productivity was up to 0.284 (g/L). Although the lipid contents from the heterotrophic cultivations with sucrose could increase to 54%, the biomass productivity decreased in the heterotrophic cultures. Therefore, the recovery of microalgal cells from semicontinuous cultures can transfer to mixotrophic cultivation for higher lipid production. The fast analysis of lipid contents of live microalgal cells by Nile red staining under fluorescence was established. The linear regression of fluoresce intensity of lipid content was established to measure the lipid contents of microalgae. The typical transesterification method used in this study was chemical and enzymatic processes. The results indicated the efficiency of transesterification by acid catalyst could approach 88% and the efficiency by lipase-catalyzed transesterification could reach 82% by the optimal oil/methanol molar ratio. The high cost and long reaction time of enzymatic processes may be not convenient for usage. In industrial and commercial application, the chemical catalyst is common for transesterification to reach the purposes of low-cost and fast reaction rate. Therefore, the fast and suitable transesterification method in this study is acid-catalytic process.