Nitrogen oxides (NOx) are the components of fossil flue gas that result in the most serious environmental concerns, which are restricted by legislation and must be well treated before the emission. The development of efﬁcient low-cost DeNOx technologies is therefore extremely urgent in many countries of the world, especially in China, which is suffering the most due to its fast economic development at the sacrifice of the environment. It is believed that high capital and operating costs would be the key impediment to limit commercial applications of algae-based biodiesel and industrial flue gases treatment, while integrated application of microalgal biotechnology in industrial flue gases sequestration and microalgal biofuel production could be a possible solution (Applied Energy 2015; Applied Microbiology and Biotechnology 2016). In a previous study conducted by the same lab, by using NOx-rich actual flue gas fixed salts as nitrogen source for Chlorella sp. C2 cultivation, 60% of the NOx was removed from the medium with an inoculated cell density of 0.07 g L-1 cell dry weight, together with the production of 33% algae lipids (Environmental science & technology 2014), indicating that microalgae are capable of being applied in an economically viable combination of the biological DeNOx of industrial flue gases and biodiesel production.
However, still considering the unmatched vast amount of NOx-containing flue gases and the relatively low photoautotrophic growth of the algae, a large land area is required for onsite NOx biotransformation, which could be the insurmountable barrier for most industries located within a limited land area, especially in China. To further improve the efficiency of transforming the harmful NOx in flue gas into valuable microalgal biomass, in the present study, mixotrophic cultivation using FGFS as the nitrogen source was optimized in 5 L and further scaled up to 50 L bioreactors, and an impressive nitrogen removal efficiency of over 96%, nitrogen consumption rate of 0.45 g L-1 d-1, along with a biomass productivity of 9.87 g L-1 d-1, were obtained, suggesting that the optimized process mixotrophical culturing of Chlorella sp. C2 was practicable for scaling up, and further proved its applicability in bio-DeNOx. Furthermore, with the optimized process, a conversion ratio of glucose to biomass of 1.77 was achieved, which is much higher than the theoretical value for heterotrophic cultivation, indicating that inorganic (CO2) and organic (glucose) carbon, are simultaneously assimilated in the mixtrophic mode, which in turn will significantly reduce the material cost for carbon.
Moreover, as valuable by-products, a significantly high bio-lipid productivity of 1.83 g L-1 d-1 was finally achieved, and the main components were comparable to conventional diesel fuel with C14~C20. Adding up the environmental and social benefits, this study has demonstrated that the mixotrophic cultivation of the oil-producing green alga Chlorella sp. C2 provides a cost-efficient method for microalgal biodiesel production.
It should be further point out that, at the end of the mixotrophical bio-DeNOx cultivation, only neglectable amount of nutrient elements especially glucose and nitrogen were remained in residual medium, which means no secondary wastes produced and makes it a green process.
Overall, the current research provides for the first time a real possibility and applicability in algal biofuel-based bio-DeNOx for scaling up and practical industrial applications.
The previous studies stated above:
- Zhu X., Rong J., Chen H., He C., Hu W., Wang Q. 2016. An informatics-based analysis of developments to date and prospects for the application of microalgae in the biological sequestration of industrial flue gas. Applied Microbiology and Biotechnology, DOI 10.1007/s00253-015-7277-7.
- Chen, H., Qiu, T., Rong, J., He, C., Wang, Q. 2015. Microalgal biofuel revisited: An informatics-based analysis of developments to date and future prospects. Applied Energy, 155, 585-598.
- Zhang, X., Chen, H., Chen, W., Qiao, Y., He, C., and Wang, Q. (2014) Evaluation of an Oil-Producing Green Alga Chlorella C2 for Biological DeNOx of Industrial Flue Gases. Environmental science & technology 48, 10497-10504.
Chen W1,2, Zhang S1, Rong J3, Li X4, Chen H1, He C1, Wang Q1.Show Affiliations
- Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences , Wuhan 430072, Hubei, China.
- University of Chinese Academy of Sciences , Beijing 100094, China.
- SINOPEC Research Institute of Petroleum Processing , Beijing, China.
- College of Life Sciences, Northeast Forestry University , Harbin 150040, Heilongjiang, China.
Nitrogen oxides (NOx) are the components of fossil flue gas that result in the most serious environmental concerns. We previously showed that the biological removal of NOx by microalgae appears superior to traditional treatments. This study optimizes the strategy for the microalgal-based DeNOx of flue gas by fed-batch mixotrophic cultivation. By using actual flue gas fixed salts (FGFS) as the nitrogen supply, the mixotrophical cultivation of the green alga Chlorella sp. C2 with high NOx absorption efficiency was optimized in a stepwise manner in a 5 L bioreactor and resulted in a maximum biomass productivity of 9.87 g L-1 d-1. The optimized strategy was further scaled up to 50 L, and a biomass productivity of 7.93 g L-1d-1 was achieved, with an overall DeNOx efficiency of 96%, along with an average nitrogen CR of 0.45 g L-1 d-1 and lipid productivity of 1.83 g L-1 d-1. With an optimized mixotrophical cultivation, this study further proved the feasibility of using Chlorella for the combination of efficient biological DeNOx of flue gas and microalgae-based products production. Thus, this study shows a promising industrial strategy for flue gas biotreatment in plants with limited land area.
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