The Efficiency And Mechanism Of Flue Gas Denox By Tetradesmus Obliquus PF3

In order to solve the serious problem of air pollution in China,the national requirements for flue gas emission from coal-fired power plants are increasingly strict.The ultra-low emission standard(50 mg/m³)of NOx is gradually implemented,but the support of denitration technology is insufficient at present.Microalgae-based denitrification(DeNOx)technology is a new flue gas DeNOx technology,which has the characteristics of mild reaction conditions,great resource potential and high efficiency of advanced treatment.It is suitable for the advanced treatment of low and medium concentration of NOx,which helps to promote the sustainable development of the power industry,and can be used as a technical reserve for the advanced DeNOx of flue gas.However,its basic theoretical research is not perfect,and its mechanism is not clear.Besides,the process design needs to be improved.In order to develop advanced flue gas treatment technology,this paper did the research of microalgae-based flue gas DeNOx technology.Aiming to solve the problems existing in the development of microalgae-based DeNOx technology,the metabolic characteristics of carbon and nitrogen by microalgae were studied,the removal pathways of NO were clarified,and the key limiting factors of NO removal process by microalgae were analyzed.Finally,the microalgae-based DeNOx model with sewage as culture medium and reactors in series was proposed,and the DeNOx efficiency of the model was evaluated.Microalgae-based DeNOx technology depends on the basic carbon and nitrogen metabolism of microalgae cells,which is closely related to carbon metabolism.In this study,the utilization characteristics of carbon and nitrogen by Tetradesmus obliquus PF3(T.obliquus PF3)were studied.The results showed that T.obliquus PF3 could utilize a variety of organic carbon sources and inorganic nitrogen sources.When NO was used as nitrogen source,T.obliquus PF3 showed significant biomass accumulation at the concentration of 100-500 ppm,and NO concentration lower than 200 ppm could promote the growth of T.obliquus PF3.Organic carbon source can promote the absorption and metabolism of nitrate by T.obliquus PF3,but it has no significant effect on the removal of NO.Therefore,T.obliquus PF3 can directly use CO₂ in flue gas as carbon source and realize simultaneous denitration and carbon fixation of flue gas.The fixed rates for NO and CO₂ by T.obliquus PF3 were 2.86±0.23 and 1.29±0.01 g/L/d,respectively.The effects of initial biomass,NO concentration,O₂ concentration,light intensity,gas flow rate,gas residence time and reactor series on the DeNOx efficiency of T.obliquus PF3 were determined.The results showed that in a certain range,increasing biomass,O₂ concentration and light intensity,and reducing gas flow rate had limited effect on NO removal.Increasing the gas residence time and setting reactors in series could significantly promote NO removal.The removal of NO by T.obliquus PF3 followed the first-order kinetics,which was proportional to the gas-liquid contact time and independent of NO concentration.The NO removal efficiecny was 70.8%±3.6%by T.obliquus PF3 in two series reactors,when the initial biomass concentration,NO concentration,gas flow rate,O₂ concentrtaion and light intensity were 0.6 g/L,200 ppm,0.05 vvm and 8000 lx and 1.25 m,respectively.As the cost of culture medium in the process of microalgae DeNOx was high,sewage wastewater was used as the culture medium of T.obliquus PF3.The biomass concentration of T.obliquus PF3 could reach 1.8 g/L after five days growing in unsterilized sewage.Nitrogen and phosphorus were mainly obtained from sewage,while carbon source was obtained from gas by microalgae.When sewage was used as culture medium,ammonia had a certain inhibitory effect on NO removal,and the inhibitory effect disappeared after ammonia was exhausted.Besides,addition of 30mg/L NO₃^--N to wastewater can promote the growth of T.obliquus PF3 and the stable removal of NO.Based on the results above,a DeNOx model with sewage as culture medium and airlift cylindrical photobioreactor in series was constructed,and the DeNOx efficiency was determined using the model under the application conditions of simulated flue gas in laboratory and actual flue gas in outdoor power plant.The removal efficiency of NO was about 70%,and the fixation rates of NO and CO₂ were 2.94±0.16 mg/L/d and 0.46±0.04 g/L/d,respectively.The fixation efficiencies of NO and CO₂ were 70.9±4.8%and 3.2±0.7%,respectively.Under the actual outdoor flue gas conditions,the removal efficiency of NO is between 72.1%±5.4%to 77.9%±3.7%,and the NOx concentration at the outlet is lower than 25 mg/m³,which can reach the ultra-low emission standard of NOx.The average fixation rates of NO and CO₂ were 2.83±0.12 mg/L/d and0.39±0.02 g/L/d,respectively.The fixation efficiencies of NO and CO₂ were77.4±7.6%and 2.24±0.04%,respectively.In addition,the concentration of nitrogen and phosphorus in sewage is greatly reduced,and the concentration of COD is also reduced to a certain extent.The final sewage can reach the secondary discharge standard of urban sewage treatment plant.To provide a theoretical basis for microalgae DeNOx process,this study explored the influence of environmental factors on the growth of T.obliquus PF3,and explored the mechanism of NO removal by T.obliquus PF3 under the optimal growth state of microalgae.The results showed that T.obliquus PF3 had good environmental adaptability,and could accumulate biomass under the conditions of pH 4.5-10.5,4-30?,2000-10000 lx,less than 100 ppm SO₂ and 3 mmol/L NaHSO₃,respectively.The optimum growth conditions of pH,temperature and light intensity were 7.5,25?and 8000 lx,respectively.In the process of NO removal,algae cells are the key factor,which can promote NO removal by secreting extracellular organic matters(EOMs),reducing exogenous iron,adsorbing NO and absorbing NO.By adding ATP hydrolysis inhibitor of HgCl₂ and combining with the nitrogen balance in the system,it was revealed that the main removal pathway of NO in the system was dissolution and oxidation in liquid.T.obliquus PF3 mainly absorbed ionic nitrogen in solution.In summary,T.obliquus PF3 has high flue gas DeNOx efficiency.Microalgae-based DeNOx technology can be used as a technical reserve for power plant flue gas DeNOx and carbon reduction,which is of great significance to reduce the pressure of environmental protection investment in the power industry and realize waste pollutant treatment.