Nitrogen oxides (NOx) are the components of fossil flue gas that result in the most serious environmental concerns (e.g. smog). The development of efficient low-cost DeNOx technologies is therefore extremely urgent in many countries of the world, especially in China. 

Cooperating with SINOPEC Research Institute of Petroleum Processing, Prof. WANG Qiang, principal investigator of the Research Group of Algal Biochemistry at Institute of Hydrobiology (IHB) of Chinese Academy of Sciences, has conducted researches on "Biological DeNOx of industrial flue gas by the cultivation of oil-producing green algae".    

By the cultivation of energy microalgae, effective biological DeNOx of industrial flue gas as well as production of biofuels and other high-value products could be achieved (Zhang et al. 2014a). The screening of nitrite-tolerant microalgae species becomes an unavoidable step towards an algal bio-DeNOx approach. The previous study showed that most Chlorella strains have an appreciably higher ability to acclimate to high-level of nitrite, and the acclimation is achieved through a three-step process of restrict, acclimate, and thriving.    

By using self-invented high efficient bioreactor (China invention patent, authorization number 201410063589.X), an economically viable algal biofuel-based bio-DeNOx by using Chlorella was evaluated and verified in actual industrial flue gases condition (Zhang et al. 2014b). A technique process Ver1.0 (China invention patent, authorization number 201410063243.X) of biological DeNOx method by microalgae combining of production of biodiesel and high-value products is formed.   

Recently, considering the unmatched vast amount of NOx-containing flue gases and the relatively low photoautotrophic growth of the algae, a study on biological DeNOx of industrial flue gas by mixotrophic cultivation of oil-producing green algae Chlorella is performed. After a stepwise optimization of mixotrophic cultivation of Chlorella using actual flue gas fixed salts (FGFS), an impressive DeNOx efficiency of over 96%, along with a biomass productivity of 9.87 g L^-1 d^-1 and a significantly high bio-lipid productivity of 1.83 g L^-1 d^-1, were obtained. 

Furthermore, inorganic (CO₂) and organic (glucose) carbon are simultaneously assimilated in the mixtrophic mode, which in turn will significantly reduce the material cost for carbon. 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.    

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 within a limited land area (Chen et al. 2016). Based on the study, an improved technique process Ver2.0 of biological DeNOx method by microalgae combining of production of biodiesel and high-value products is further provided.  

The results (Zhang et al.Environmental science & technology2014, 48, 10497-10504; Chen et al.Environmental science & technology2016, 50, 1620–1627), published on Environmental Science & Technology, for the first time proved the real feasibility and practicality of biological sequestration of industrial pollutants by microalgae combining production of high-value products.   

Based on these studies, two invitation reviews from Applied Energy and Applied Microbiology and Biotechnology have been published. 

The studies were supported by the National Program on Key Basic Research Project, the National Natural Science Foundation of China, the Natural Science Foundation of Hubei Province of China, Sinopec, and the Knowledge Innovation Program of the Chinese Academy of Sciences.

Figure: Improved technique process Ver2.0 (Image by IHB)