微藻養殖模組用於工業廢氣和畜產沼氣中二氧化碳的捕捉並利用於微藻生物質的生產與生質氣體的提純

Abstract

微藻養殖可用於廢氣中二氧化碳（CO2）的減量，以及廢氣中部份有毒氣體的減除，而所養殖的微藻細胞中油脂則可被萃取與轉化為生質柴油，據此微藻養殖同時具備有減碳、減廢、生產生物質能等多重效應。本研究中，我們利用化學突變處理與馴養方式，分離得到可分別耐受工業廢氣與畜牧沼氣之微藻Chlorella sp.突變株，並探討通入不同種類的工業廢氣和畜牧沼氣用於微藻養殖對其生長效能，生物質和油脂產量，以及油脂組成的影響。此外，我們也建構了一戶外型的光生物微藻養殖模組，搭配自動氣體切換操作程序，探討戶外實場大規模微藻養殖用以沼氣提純(upgrading)和工業廢氣中CO2減量的效能。 在利用工業廢氣的研究中，Chlorella sp. MTF-15微藻株和中國鋼鐵公司煙道氣被用於各項實驗。煙道氣來源有三種，分別為煉焦爐（主要組成23-27% CO2、70-80 ppm NOX及80-90 ppm SO2）、高爐（24-28% CO2、8-10 ppm NOX及15-20 ppm SO2）及動力場（22-26% CO2、25-30 ppm NOX及15-20 ppm SO2），以這三種工業廢氣進行室內Chlorella sp. MTF-15養殖時，微藻具有相似的生長趨勢，而在通入廢氣與空氣混合稀釋比例約1/4或1/2時，微藻具有較高的生長潛能。Chlorella sp. MTF-15通入煉焦爐、高爐及動力場工業廢氣養殖後，其最大比生長速率與油脂產量分別為0.827、0.762及0.728 d-1與0.668、0.961及0.792 g/L，通入工業廢氣養殖之微藻的C16:0 + C18:1（適合用於生產生質柴油之脂肪酸）含量約為60-65%，其中通入動力場廢氣所養殖之微藻的C18:1含量明顯高於通入煉焦爐與高爐工業廢氣者。Chlorella sp. MTF-15對於煉焦爐、高爐及動力場廢氣中CO2的最佳移除效率分別約為25%、40%及50%；對於煉焦爐廢氣中NOX與SO2的移除效率約為65%與40%，對於高爐與動力場廢氣中NOX與SO2的移除效率為> 80%與> 90%。上述結果顯示，微藻Chlorella sp. MTF-15的生長潛能、油脂產量及脂肪酸組成取決於工業廢氣中的主要氣體組成和微藻養殖模組的操作模式，例如工業廢氣的稀釋比例。 在利用畜牧沼氣中CO2之研究中，由於畜牧廢水經厭氧發酵處理後除了產生含有高濃度甲烷（CH4）之沼氣外，也常伴隨著大量的惰性氣體CO2的產生，因此若要有效的將沼氣應用在引擎燃料上，勢必要先降低沼氣中CO2的濃度以提升沼氣中CH4的濃度。為了使微藻於沼氣中仍保有其生長潛能，我們分離篩選得一株可耐受沼氣之微藻突變株Chlorella sp. MB-9。Chlorella sp. MB-9在通有H2S < 100 ppm和80% CH4的環境之下，相較對照組仍具有70%以上的生長潛能。在實場的戶外操作實驗中，通入分別為0.05、0.1、0.2及0.3 vvm流量之除硫沼氣（~20% CO2、~70% CH4及H2S < 50 ppm），Chlorella sp. MB-9的最高生長率分別為0.320、0.311、0.275及0.251 g/L/d。為了提純沼氣，我們利用所建立的戶外光生物微藻養殖模組，搭配了自動氣體切換程序用於實場微藻養殖試驗，而結果顯示沼氣中的CO2之移除效率能持續維持在50%以上，而CH4的濃度從起始的70%提升至85-90%。 綜上所述，我們的實驗結果證實Chlorella sp. MTF-15與MB-9能直接並有效地利用不同的工業廢氣或畜牧沼氣中的CO2，以作為微藻生長的營養源並生產生物質與脂質。此外，我們所建立之噸級規模的具有自動氣體切換操作功能的戶外光生物反應系統，能有效率地作為一連續式的工業廢氣的生物移除或沼氣提純之CO2捕捉的微藻養殖模組系統。這些結果皆證實了利用微藻進行生物固碳是一個具有潛力的方法，不僅能有效的用來減除工業廢氣中CO2的排放、提升沼氣中CH4的濃度，還能生產富含油脂之微藻生物質作為再生能源的料源。

Microalgal cultivation can be used for CO2 and toxic gases mitigation from waste gas and microalgal lipids can be extracted and converted into biodiesel. Accordingly, there are multiple effects such as carbon reduction, waste remediation and biomass production in microalgae cultivation. In this study, using chemical random mutagenesis and domestication, we isolated mutant strains of microalgae Chlorella sp., which were tolerant to flue gas and biogas, respectively. The growth characterization, biomass production, lipid production and lipid composition of Chlorella sp. mutant strains aerated with different kind of gases were investigated. We also established outdoor microalgae-incorporating photobioreactor culture system with a gas cycle-switching operation, which could be continuously used as a CO2 capture model for biogas upgrading or flue gas remediation. In the study of flue gas utilization by microalgal mutant strain, Chlorella sp. MTF-15, aerated with different kind of flue gases from a steel plant, the China Steel Corporation in Taiwan were investigated. There three kind of the flue gases were generated from the coke oven (major components: 23-27% CO2, 80 ppm NOX and 90 ppm SO2), hot stove (24-28% CO2, 10 ppm NOX and 20 ppm SO2) and power plant (22-26% CO2, 30 ppm NOX and 20 ppm SO2). Growth profiles of the microalgal cultures aerated with three different kinds of flue gases were similar and show a higher growth potential at the aeration by 1/4 or 1/2 flue gas dilution ratios than that of with air. The maximum specific growth rate and lipid production of the Chlorella sp. MTF-15 aerated with flue gas from coke oven, hot stove and power plant were 0.827, 0.762 and 0.728 d-1, and 0.668, 0.961 and 0.792 g/L, respectively. The content of C16:0 + C18:1 (the suitable fatty acids for biodiesel production) of the Chlorella sp. MTF-15 cultures aerated with the flue gases was approximately 60-65%. However, C18:1 content of the lipid of Chlorella sp. MTF-15 culture aerated with power plant flue gas was significantly higher than those aerated with coke oven and hot stove flue gases. The optimal CO2 removal efficiency of coke oven, hot stove and power plant flue gas by Chlorella sp. MTF-15 cultures was around 25%, 40% and 50%, respectively. In addition, the NOX and SO2 removal efficiency of coke oven, hot stove and power plant flue gas by Chlorella sp. MTF-15 cultures were approximately 65% and 40% aerated with coke oven flue gas, and > 80% and > 90% aerated with hot stove and power plant flue gases. Those results suggest that growth potential, lipid production and fatty acid composition of the microalgal Chlorella sp. MTF-15 cultures are dependent on the composition of flue gas used and the module operation, at least including flue gas dilution, of the microalgal cultures. In the study of carbon dioxide utilization in livestock biogas, the livestock biogas was produced from anaerobic swine wastewater treatment contains high methane (CH4) but high carbon dioxide (CO2). The presence of high content of CO2, an incombustible gas, in biogas should be mitigated as high calorific value fuel gas. In this study, we isolated a biogas tolerance mutant strain of microalga, Chlorella sp. MB-9. The microalgal mutant strain, Chlorella sp. MB-9, can grow in the presence of gas containing H2S < 100 ppm, and the growth capacity of the microalgal culture aerated with 80% CH4 was approximately 70% of that of the control culture (0% CH4). In the field study of outdoor operation, the maximum growth rates of Chlorella sp. MB-9 aerated with desulfurized biogas (~20% CO2, ~70% CH4 and H2S < 100 ppm) at 0.05, 0.1, 0.2 and 0.3 vvm were 0.320, 0.311, 0.275 and 0.251 g/L/d. To upgrade biogas produced from the anaerobic digestion of swine wastewater, and outdoor photobioreactor was established. The outdoor microalgae-incorporating photobioreactor culture system with a gas cycle-switching operation could be continuously used as a CO2 capture model for biogas upgrading. Furthermore, our field study demonstrated that the efficiency of CO2 capture from biogas could be maintained at 50% on average, and the CH4 concentration in the effluent biogas from the Chlorella cultures increased from its original 70% up to 85-90%. In summary, our experimental results mentioned above confirm that Chlorella sp. MTF-15 and MB-9 can efficiently and directly utilize the CO2 in different kinds of flue gases or livestock biogas for microalgal growth, then produce biomass and lipid. In addition, the established outdoor photobioreactor system using a gas cycle-switching operation could be used as a continuously CO2 capture model for flue gas bioremediation and biogas upgrading. Those results are confirmed that the microalgae-based CO2 biological fixation is regarded as a potential way to not only reduce CO2 emission but also achieve to produce lipid-rich microalgal biomass as a regenerative energy source.