Nowadays, an increase of CO2
levels in the atmosphere is extensively recognized as a major contributor for global warming. Recent reports are highlighted that atmosphere contains CO2
level of 450 ppm [1
]. The atmospheric CO2
can be trapped by green plants via photosynthesis. However, terrestrial plants are estimated to reduce only 3–6% of global CO2
emissions, which is significant given the slow growth rates of plants. On the other hand, microalgae can grow much faster than terrestrial plants, and their CO2
reduction efficacy was 10–50 times higher than plants [2
]. The variety of microalgae cultivated in comfortable environmental condition to produce comparably 15–300 times higher energy sources than plants, which also reduce the land area for cultivation and continuously increase the yield per unit area [2
]. Microalgae can biologically store CO2
through photosynthesis in the form organic compounds and then use microalgal biomass as a feedstock for renewable energy after CO2
]. Moreover, microalgae have been documented as source of valuable biomaterials such as fertilizers, live feed, medicines, and other value-added products.
The large-scale microalgae culture system was divided into two systems, namely the open and closed systems. In the case of the open system, it is difficult to control the amount of light intensity and it may vary depend upon the local time, and also difficult to maintain the temperature. The closed system, which is a device to overcome these limitations, is able to control the light intensity, external influence, and temperature, though the operation cost and the manufacturing cost are high when compared to the open system. Especially, the closed system microalgal growth rate is 1.5–4 times higher than the open system [7
]. The high growth rate of microalgae has a large impact on CO2
capture and may lead to an increase in biomass production. According to the various research condition, closed system may be designed as airlift column, horizontal tube, stirred tank, and flat panel photobioreactor (PBR) [7
Obviously, industrial exhaust gases contain 10–20% of CO2
with trace amounts of SOx
. The selection of microalgae plays a vital role in CO2
reduction efficacy and represents a significantly cost-effective route for biomass production. The desirable qualities of microalgae comprise high growth and CO2
consumption rates, also patience towards trace constituents of exhaust flue gases such as SOx
and production of valuable products. Maeda et al. (1996) used Chlorella
sp. T-1 as a potential microalga for the biological removal of exhausted CO2
from coal-fired thermal power plants. Aslam et al. (2017) have identified that mixed microalgae societies like Desmodesmus
spp. can slowly grow in 100% unfiltered exhausted gas from coal combustion with phosphate buffering condition [9
]. Kassim and Meng (2017) studied biofixation of CO2
by two microalgae species such as Chlorella
sp. and Tetraselmis suecica
with various CO2
]. Even though the above said studies have been carried out in exhausted gas which adversely affects the microalgal growth. To the best of our knowledge, no study has yet reported on the actual injection of exhaust gas, and there is a lack of research on biomass tendency when continuously injecting the gas into large-scale bioreactor.
Hence, the objective of this study is to evaluate the feasibility of microalgae species like Nephroselmis sp. KGE8, Acutodesmus obliquus KGE 17 and Acutodesmus obliquus KGE32, which were cultivated in a laboratory with the supplementation of power plant exhaust gas. Then, evaluate the growth potential of the microalgae in the semi-continuous photobioreactor (PBR) operating with the exhaust gas injection, and evaluate the microalgae productivity at each cultivate-harvest cycle. Finally, we also assessed the feasibility of biodiesel, lipid and C16-18-FAME contents in recovered microalgae.
Batch scale studies reveal that Nephroselmis sp. KGE 8 showed the best growth under exhaust gas conditions. Nephroselmis sp. KGE 8 showed growth potential (0.696 g L−1) in the semi-continuous PBR operation with the exhaust gas injection. The lipid content and C16-C18 FAME content were 39.4% and 77.8% in PBR1, respectively. The microalgae productivity of five reactors showed range from 0.4116 g L−1 to 0.5468 g L−1 at each cultivate-harvest cycles. PBR 1 showed highest microalgae productivity during PBR operation.
When exhaust gas is directly injected, changes in NOx
and temperature condition accelerate the microbial energy conversion. Singh et al. (2014) reported that some algal species obtained maximum biomass in 15% CO2
]. Based on the result, it was concluded that direct injection of exhaust gas is the most suitable condition for utilization of energy source of microalgae. This microalgal cultivation system could be a suitable process for the massive cultivation of microalgae with exhaust gas from power plants.