Screening Of A Microalgae And Its Denitration Efficiency In A Bubble Column Reactor

NOx is one of the main reasons causing the haze and other air pollutions, therefore controlling NOx emission is one of the main methods to solve the air pollution problem in China. Traditional denitration technologies have the problems of energy consumption, high investment in infrastructure, poor security and causing secondary pollution. Microalgae technology is developed as a new flue gas denitrification technology in recent years. Because microalgae conduct NOx absorption and conversion during the multiplication and metabolic processes, no expensive catalyst and corrosion compared to traditional physical and chemical methods. By photosynthesis, denitration and carbon sequestration can be achieved at the same time; in addition, they have products of high economic value. Thus, microalgae denitration technology is more in line with the principles of harmless waste and resource reuse. Efficient denitration algae species and suitable reactor is the key of this technology, this paper focused on screening for algae of high tolerance, its characteristic of denitration, configuration of denitration reactor and its denitration efficiency.An algae strain of high tolerance for flue gas was selected and identified to be a Micractinium sp.. The feasibility of this Micractinium sp. to remove NO from flue gas has been studied and it was found that in the atmosphere of simulated flue gas with 100 ppm NO, Micractinium sp. had a NO removal rate of up to 90% with the growth rate of 0.141 g˙L-1˙d-1. However, if adding 100 ppm SO2 to the simulated flue gas, the growth of Micractinium sp. was suppressed, because p H environment was no longer conducive to the growth of microalgae. Using the 10% CO2 in the flue gas and 2g/L glucose as the carbon sources of Micractinium sp. respectively, the final algae dry weight of glucose group reached 1.448 g/L, growth rate of which was 2.53 times of CO2 group. Both of the glucose group and CO2 group had a high NO removal rate, which were 96.51% and 92.94% respectively. Using 100 ppm NO and 17.65mmol/L NO3-N as the nitrogen sources of Micractinium sp. respectively, NO group grows faster than NO3-N group, probably because NO molecule could promote cell wall relaxation and expand the alga cells, also probably because the dissolved NO in algae solution consumed dissolved oxygen, which could reduce the dissolved oxygen inhibition for algae. However, the ultimately harvested algae dry weight of these two groups were 1.448 g / L and 1.50 g / L, probably because that comparing to NO3-N, NO could just promote microalgae mature earlier and could not increase the algae dry weight.The performance of four photobioreactors(air-lift flat PBR, hollow fiber membrane flat PBR, air-lift cylinder PBR, hollow fiber membrane cylinder PBR) were investigated. The results showed that different photobioreactor algae affected the growth of algae and nutrient metabolic rates with different light conditions and liquid mass transfer effect. When exposing air, the algae dry weight of air-lift and hollow fiber membrane cylindrical bioreactor groups reached 1.0g/L and 0.95g/L respectively, 1.82 times and 1.77 times of air-lift flat and hollow fiber membrane flat groups. Changing air to 10% CO2 significantly improved the growth condition of microalgae, with the harvested dry weight of air-lift and hollow fiber membrane cylindrical bioreactor groups reached 1.50g/L and 1.45g/L, 1.50 times and 1.45 times of air-lift flat and hollow fiber membrane flat groups.A mathematical model for microalgae denitration in an air-lift cylinder PBR was established to analyze the influencing factors of the NO removal efficiency and microalgae growth rate. Then this model was verified by experiments on microalgae cultivation methods, different inlet NO concentration, liquid nitrogen concentration and different height to diameter ratio(H/D) of cylindrical reactor. The results showed that under heterotrophic and polyculture training methods microalgae growth rates reached 0.357 g˙L-1˙d-1 and 0.234 g˙L-1˙d-1, while under heterotrophic condition, the denitration rate was 96.51%, 1.57 times of polyculture condition. Also in the heterotrophic group Micractinium sp. got a higher removal rate of NO3-N than polyculture and autotrophic groups. The removal rate of NO by Micractinium sp. was decreased with NO concentration, with the NO removal rates of 96.51%, 91.99% and 88.57% under 100, 300, 500 ppm inlet NO concentration. When the initial NO3-N concentration in the medium were 17.65 mmol/L, 35.3 mmol/L, 88.25 mmol/L and 176.5mmol/L, NO3-N removal rates by Micractinium sp. were 39.12%, 17.85 %, 27% and 11% respectively. In the atmosphere of simulated flue gas with 100 ppm NO, growth rate of H/D 15 group reached 0.489 g˙L-1˙d-1,1.37 times of H/D 1.1 group, NO removal rates of H/D 15 group and H/D 1.1 group were 96.51% and 97.02% respectively, showing little difference. As for the NO3-N removal rates, H/D 15 group was 39.97%, which was a little higher than that of H/D 1.1 group with 36.84%.