| In the last few decades, excess carbon dioxide emission has brought serious global warming, which directly threatens the survival and development of human beings. To resolve this critical issue, Carbon Capture and Storage(CCS) has gained a great deal of attention around the world over the past decades. As one of the biological techniques, microalgae carbon capture, which converts CO2 into organics with help of light, bears spectacular advantages of environment-friendly, low cost and safety, and thus is considered as one of the most promising technique for CO2 capture. For a typical microalgae carbon capture process, as the main carbon source for microalgae growth, CO2 is blasted into microalgae suspension by a bubble distributer to form bubble flow rising up through the reactor. Then CO2 molecule in bubble diffuses across the gas-liquid interface and dissolves into surrounding microalgae suspension. Afterwards, the dissolved CO2 is consumed by microalgae cells to produce organic though photosynthesis. As a result, two-phase flow plays an important role in mixing and CO2 transportation, which will significantly affect microalgae growth and CO2 fixation in bubble flow photobioreactors.This work is towards the background of CO2 bio-fixation by microalgae. And the two-phase flow and CO2 transfer in microalgae suspension were studied more deeply. Firstly, the dissolution and consumption of a mixed gas bubble consisting of CO2 and air in microalgae suspension as the most basic problem, was investigated by visualization experiments using the promoted bubble grafting method. Furthermore, a theoretical model based on non-equilibrium theory at the gas-liquid interface was also proposed to predict the CO2 dissolution and fixation characteristics of bubbles in microalgae suspension. Based on this, CO2 bubble dynamic behaviors in microalgae suspension during growth, detachment, coalescence and rising were also visually studied. The movement and distribution of microalgae at the gas liquid interface during bubble growth and coalescence and its effect on CO2 bubble dynamic behaviors were also obtained. Moreover,bubble carrying caused by bubble behaviors and its impacts on microalgal cells distribution in photobioreactors were deeply investigated in this work. The effect of CO2 bubbles behaviors on microalgal cells distribution and growth in various inlet CO2 concentrations, blast orifice sizes and gas flow rates was also discussed to achieve the optimal gas blast condition for microalgae growth and CO2 fixation. Finally, a uniform bubble distributer with multi-orifice plate was designed according to the basic study above. The effect of blast orifice size and spacing of orifice on microalgae growth and CO2 fixation were also analized. And the optimal multi-orifice structure and operating condition for microalgae growth and CO2 fixation was achieved, which can be a guide for gas distributer design and photobioreactors operation during microalgae CO2 capture in a large scale. The main conclusions are as follows.① During dissolution the bubble radius gradually decreased with time and trended towards a constant thereafter. The CO2 concentration near the gas-liquid interface dropped with CO2 diffusion outwards and consumption by microalgae. The bubble with larger initial CO2 volume fraction had faster decreased radius, higher dissolution rate and CO2 fixation efficiency. While when the initial CO2 volume fraction in bubble was 15% and 20%, slightly photosynthesis inhibition emerged at the beginning of dissolution. Smaller bubble experienced a faster shrink and lower dissolution rate but greater CO2 fixation efficiency by photosynthesis. Higher microalgae concentration accelerated bubble shrink, and the acceleration was impeded when OD680 nm went beyond 1.51.② During bubble formation and rising, microalgae cells would like to move to and enrich on the gas-liquid interface where the CO2 concentration was higher. With the increase of the OD680 nm value, the density and surface tension coefficient of microalgae suspension decreased. With the increase of gas flow rate, the bubble growth process gradually transited from the steady state to the unsteady state in microalgae suspension, the departure diameter and rising velocity of bubble in microalgae suspension was smaller than in pure water when the gas flow rate was in the range of steady state. While, an opposite conclusion can be achieved when the gas flow rate was in the range of unsteady state. The amplitude and wavelength of bubble rising trajectory increased with increasing gas flow rate. Larger CO2 concentration and smaller capillary diameter resulted in slower bubble growth rate and smaller bubble detachment diameter, rising velocity and amplitude and wavelength of bubble rising trajectory.③ Before coalescence, the two bubbles grew independently and had similar dynamic behaviors in microalgae suspension. Many microalgae cells and microalgae aggregation adsorbed on the bubble surface and distributed between the two bubbles, resulted in more time was needed for the coalescence of bubbles in microalgae suspension than pure water. In microalgae suspension, the coalesced bubble deformed through three stages, including shrinking, stretching and expansion and then immediately detached. In pure water, the coalesced bubble did not detach immediately after going through these three stages, but continuously grew steadily for a while before detachment. Thus, smaller detachment bubble diameter and lower terminal velocity and acceleration were achived in microalgae suspension than that in pure water. A critical center distance of capillary orifices for bubble coalescence was achieved at 2.5 mm. And the optimal center distances of capillary orifices and gas flow rate for CO2 transfer were also achieved at 1.5 mm and 15 m L/min, respectively.④ During microalgae growth enrichment of microalgal cells on the top of microalgae suspension was observed due to bubble carring leaded to non-uniform distributions of microalgal cells along the photobioreactor. The variation coefficient of the OD680 nm distributions in the vertical direction increased with the microalgae growth due to increased bubble carrying. The effect of blast orifice size on bubble carrying was the most obvious. More serious non-uniform distributions of microalgal cells was achieved, when the inlet CO2 concentration and gas flow rate was higher and blast orifice size was smaller. The optimal operational conditions for Chlorella pyrenoidosa growth were 5%(V/V) in the inlet CO2 concentration, 0.5 mm in the blast orifice size and 40 m L/min in the gas flow rate, considering the CO2 transfer and non-uniform distributions of microalgal cells.⑤ The research of gas distributer design and photobioreactors operation in bubble flow photobioreactors showed that when the blast orifice size was 0.5 mm and spacing of orifice was 1.5 mm, microalgae had the maximum biomass productivity and CO2 fixation rate by photosynthesis of 0.64 g·L-1·d-1 and 961.73 μmol?m-3s-1 respectively. The maximum CO2 fixation efficiency reached 11.31%, when inlet CO2 concentration was 5%. Thus, the multi-orifice structure had great effects on microalgae growth and CO2 fixation. |
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