Strengthening Generation And Conversion Of CO₂ Microbubbles In Tubular Photobioreactors And Improving Flashing-light Effect In Tubular-pond Photobioreactors For Microalgal CO₂ Fixation From Coal-fired Flue Gas

Microalgae have a fast growth rate,strong carbon fixation capacity and good environmental adaptability,which are of great significance for achieving the national goal of carbon neutrality by the combination of CO₂ emission reduction,conversion and utilization.In the traditional tubular photobioreactor,the bubble residence time is short,the saturated dissolved CO₂ concentration in microaglal solution is low,the gasliquid mixing and mass transfer is poor,and the microalgal culture volume is small due to the limitation of light dilution.However,the raceway pond photobioreactors,which is cheaper and widely applied for large-scale production,require a large area of land and has a low light utilization efficiency.In this study,ZIF-8 nanoparticles were used to enhance gas-liquid mass transfer for increasde the concentration of dissolved inorganic carbon.A ceramic-diaphragm dissolver and a spiral-ascending dissolver were developed for the regulation of the generation,evolution and conversion of CO₂ microbubbles.A tubular-pond hybrid photobioreactor was developed to increase the aeral microalgal biomass yield.Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor,which strengthened the mass transfer and improved microalgal growth rate.The cycle flow field of photobioreactor was investigated with computational fluid dynamics simulation,which significantly increased the horizontal flow rate of microalgal solution from 5.8×10-5 m/s to 0.16 m/s and enhanced the flash-light frequency of microalgae by3.4 times.The accuracy of the simulated results was confirmed by PIV measurements.The optimal ratio（D/w）of the diameter of the paddlewheel to the width of the reactor was obtained as 0.67,which reduced the mixing time by 31.3% and increased the mass transfer coefficient by 41.2%,thereby increasing the growth rate of microalgae by62.3%.Metal-organic framework ZIF-8 nanoparticles were adopted to enhance the CO₂ absorption and transport in the microalgal solution in a horizontal tubular photobioreactor,which improved the CO₂ diffusion and dissolution efficiency and the saturated dissolved inorganic carbon concentration,and promoted the growth and CO₂ fixation of microalgae.The gas-liquid mass transfer coefficient was increased by 45.2%and mixing time was reduced by 9.2% by the addition of ZIF-8 nanoparticles.However,when the concentration of ZIF-8 nanoparticles increased from 0 to 0.2 mmol/L,the average size of microalgae cells decreased from 2.61 to 2.38 μm,and the fractal dimension increased from 1.38 to 1.56,indicating that excessive ZIF-8 nanoparticles might induce potential toxicity.The optimal concentration of ZIF-8 nanoparticles is0.01 mmol/L,which increases the dissolved inorganic carbon concentration in the microalgal solution by 12.9%,thus increasing the microalgal biomass yield by 25.6%.A ceramic-diaphragm dissolver was designed for the regulation of the CO₂ microbubbles generation to improve the growth and CO₂ fixation of microalgae in a horizontal tubular photobioreactor.The bubble formation time and diameter were reduced by 53.7% and 36.4%,thereby reducing the mixing time by 9.8% and enhancing the mass transfer coefficient by 45.2%.As the ratio of the aerator area to the dissolver bas area increased from 0.09 to 0.56,the bubble formation time decreases by 19.6%and the mass transfer coefficient increases by 80.9%,thus the microalgal biomass yield increased by 30%.A spiral-ascending CO₂ dissolver was developed to optimize the microbubble rising trajectory and prolong the gas-liquid contacting time.When the pitch to length（d/L）ratio of helical baffle in the dissolver was 0.33,the bubble formation time and size were reduced by 30.6% and 23.4%,and the residence time of bubbles was significantly increased by 190.2%.Therefore,the mixing time was reduced by 15.8%and the mass transfer coefficient is increased by 69.2%,which improved the microalgal biomass yield by 40.8% in the photobioreactor.A pond-tubular hybrid photobioreactor comprising raceway ponds and horizontal tubes was developed to strengthen flash light effect and improve light utilization efficiency.The single light absorption and utilization mode by conventional raceway pond was converted to sequential absorption and utilization by tubular photobioreactor and raceway pond,which strengthened the flashing-light effect of microalgal cells in the raceway pond.The upper horizontal tubes were optimized.When the ratio of the adjacent tube interval to tube diameter（d/D）was 1 and shaded area percentage was20%,the flashing-light frequency of microalgae in the raceway pond was increased by36.8%,thus the growth rate was increased by 34.7%.The areal microalgal biomass yield was improved by 54.7% in the tubular-pond hybrid photobioreactor.A 1192 m² outdoor tubular-pond hybrid photobioreactor for microalgal CO₂ fixation from coal-fired power plant was established.The cellular photosynthetic pigment synthesis and microalgal growth rules were investigated.The effects of gas flow rate,daily temperature and light intensity on microalgal growth and pigment synthesis were studied.When the average daily light intensity is around 80,000 lux,as the outdoor temperature increased from 16.2 °C to 26.8 °C,the microalgal growth rate in the tubular-pond photobioreactor increased from 13.6 g/（m²·d）to 29.6 g/（m²·d）.