利用工業煙道廢氣培養微藻對其生物質與油脂產量之最佳化探討

Abstract

近年來，大氣中過量累積的二氧化碳成為全球暖化的主因，其中主要多為工廠所排放含大量二氧化碳之工業廢氣，而微藻養殖則被視為解決此問題的方法之一。微藻可行光合作用，利用光照做為能量，將二氧化碳和水轉化為生物質，達到二氧化碳減量與生產生質能料源的目的。本研究應用本實驗室所篩選出之耐溫、高生長速率，且具高二氧化碳耐受性的微藻株Chlorella sp. TT-1，通入中鋼煙道廢氣做為碳源，進行廢氣養殖微藻之試驗，用以產製微藻生物質，並萃取藻油生產生質柴油。本研究更結合反應曲面法(response surface methodology, RSM) 進行微藻養殖，找出微藻之最適化培養條件，例如微藻養殖之初始濃度、通入廢氣組成比率及廢氣通氣速率等。 本研究利用不同廢氣比例組成的氣體進行微藻養殖，實驗結果顯示以廢氣稀釋為 25 %的混合氣體進行微藻培養時，可達到最佳的生長效率，其生物質產率可達0.421 g/L/day；再則，以廢氣比率為25 %所培養的微藻，其油脂累積量最多，可達約40%的油脂含量，因此在不同的廢氣組成比例的培養下，微藻的生長速率和產脂效應會有所差異，不同的廢氣組成比率並不會顯著的影響微藻脂肪酸甲酯 (fatty acid methyl ester, FAME) 的組成。另外不同的廢氣通氣速率對微藻的生物質產率會有顯著性的影響，在0.3 vvm的通氣速率下微藻有最大的生物質產率，為0.286 g/L/day；在油脂含量的部分，在通氣速率0.3 vvm下的油脂累積可達約40%；通氣速率對FAME的累積也有影響，於0.3 vvm之高通氣速率下有最多之FAME，比低通氣速率下之微藻其FAME在微藻內的含量多約4~5 %。 為探討光照於利用工廠廢氣養殖微藻試驗上之影響，我們將養殖的Chlorella sp. TT-1通入工廠廢氣，並以全日照和半日照兩種不同的光照模式進行微藻養殖。結果顯示微藻之生物質產率在全日照和半日照下，分別為0.299 g/L/day和0.137 g/L/day，明顯可得知微藻在以廢氣培養下，全日照會有較高的產率；另外，微藻之油脂含量在全日照和半日照下分別為37 %和15 %，可看出油脂的合成累積與光照時間是具有相關性；在FAME的部分，以半日照所培養之Chlorella sp. TT-1的飽和脂肪酸(C16:0)比例較多，而以全日照培養則有較多的不飽和脂肪酸(C18:1和C18:2)的累積。 我們根據上述的實驗結果，結合反應曲面法進行微藻養殖最佳化探討，RSM可做實驗因子設計並模擬反應曲面，以較少的實驗成本和時間獲得可信賴且有效的資訊，並可討論因子間的交互作用，進而探討多因子對實驗結果的影響性，並以實驗結果得一模擬公式，找出最適之操作條件。本研究利用微藻培養初始濃度、廢氣組成比率及廢氣通氣速率為實驗因子，探討微藻培養最適化之條件。實驗結果顯示，當微藻株在初始濃度為0.37 g/L、通氣量為0.30 vvm及工業煙道廢氣比率為75 %時，可得到最佳的生物質產率0.486 g/L/day；在初始濃度為0.35 g/L、通氣量為0.24 vvm及工業煙道廢氣比率為75 %時，可得到最佳的油脂產率0.216 g/L/day；在初始濃度為0.37 g/L、通氣量為0.25 vvm及工業煙道廢氣比率為74 %時，可得到最佳的脂肪酸甲酯產率0.157 g/L/day。此外，在上述實驗所得出之最佳微藻養殖參數下，我們另外再增加光照強度至500 μmol/m2/s和700 μmol/m2/s，以探討其對生長速率和產脂效應的影響。在500 μmol/m2/s下微藻有最大的生物質產率0.390 g/L/day，且油脂累積可高達約49 %，另外在FAME的組成部分，500 μmol/m2/s可顯著促進C16:0脂肪酸的累積。

In the recent years, global warming becomes more serious problem due to the increasing carbon dioxide (CO2) accumulated in the atmosphere, and the plant steel for industry plays the important role in emitting flue gas which is CO2-rich. Microalgae are the candidate to solve the problem by photosynthesis, which use sun light as energy source to convert water and CO2 into biomass, and it can reduce the CO2 emission and produce biomass. We utilized the isolated thermal- and CO2-tolerant mutant microalga Chlorella sp. TT-1 to reduce CO2 in flue gas from the steel plant and produce microalgal biomass which can be extracted oil to produce biodiesel. Furthermore, we cultivated the microalgae combined with response surface methodology to get the optimized cultivation conditions under the specific initial density, aeration rate, and flue gas ratio. To investigate the effect of flue gas ratio, there were different flue gas ratio gases utilized to study the effect of flue gas ratio in microalgal cultivation. The microalga Chlorella sp. TT-1 aerated with 25 % flue gas had more biomass productivity, which was 0.421 g/L/day. Lipid content of Chlorella sp. TT-1 cultures with 25 % flue gas ratio aeration were 40%. In the part of FAME production, there was no difference between the fatty acid methyl ester (FAME) content of Chlorella sp. TT-1 cultivated with different flue gas ratios. In the part about the effect of aeration rate, the biomass productivity of Chlorella sp. TT-1 at 0.3 vvm aeration rate has the maximum value, and it is 0.286 g/L/day. The maximum lipid contents of Chlorella sp. TT-1 was 40 % when it cultivated with 0.3 vvm aeration rates, and the experimental result was obtained that the biomass production and lipid content in microalga cells increased with the increasing aeration rate. The experimental result about FAME content showed it would slightly increase with high aeration rate comparing to low aeration rate, and the variance was 4~5 % in total FAME content. In order to investigate the effect of illumination in microalgae culture aerated with flue gas, Chlorella sp. TT-1 was cultivated under full and half illumination. The biomass productivity of Chlorella sp. TT-1 in full and half illumination were 0.299 g/L/day and 0.137 g/L/day, respectively. The better irradiation time is full illumination for microalgal cultivation, and the light should be devised when the microalgae cultivated with flue gas. The lipid content of Chlorella sp. TT-1 under full and half illumination were approximately 37 % and 15 %, respectively. Obviously, the lipid in microalgal cells under full illumination was more than it cultivated under half illumination. The saturated fatty acid (C16:0) accounted for the most part in Chlorella sp. TT-1 cultivated under half illumination, while the microalgae had more long and unsaturated fatty acid (C18:1 and C18:2) under full illumination. Instead of enormous experiments to test, we cultivated microalga Chlorella sp. TT-1 combining response surface methodology which can reduce the experimental times to diminish the cost, and simulate the experimental formula to get the optimum condition for microalgal cultivation. The optimized biomass productivity of Chlorella sp. TT-1 is 0.486 g/L/day when the initial density of 0.37 g/L aerated with 75 % flue gas ratio at 0.30 vvm aeration rate; the optimized lipid productivity of Chlorella sp. TT-1 is 0.216 g/L when the initial density of 0.35 g/L aerated with 75 % flue gas ratio at 0.24 vvm aeration rate; the optimized FAME productivity of Chlorella sp. TT-1 is 0.157 g/L when the initial density of 0.37 g/L aerated with 74 % flue gas ratio at 0.25 vvm aeration rate. In order to investigate whether the enhancement of the illumination is necessary in microalgal culture aerated with flue gas under the optimum condition of microalgal cultivation and the effect of different illumination intensity for microalgal cultivation, the different illuminations were utilized and irradiated for the microalgal cultivation with flue gas. The microalgal cultivation cultivated with 500 μmol/m2/s had the maximum microalgae biomass productivity 0.390 g/L/day. The illumination of 500 μmol/m2/s could enhance the lipid accumulation, and it could obtain the lipid content of 49%. Furthermore, the culture irradiated under 500 μmol/m2/s had more C16:0 content, and it means higher illumination would induce microalgae to synthesize the saturated carbon compound of shorter chain.