| Fossil fuel combustion is also the major source of greenhouse gases responsiblefor global warming. Renewable, carbon neutral, economically viable alternatives tofossil fuels are urgently needed to avert the impending oil crisis and the dramaticconsequences of climate change. At present, the potential value of microbial, andparticularly microalga, photosynthesis to fix CO2in industrial flue-gas and productbiofuel is widely recognized. Photobioreactors (PBR) are the critical equipment in thebiofuels from microalgae process, and are one of technical bottlenecks. Actually,studies have shown that well-designed cultivation systems may lead to significantincrease of CO2fixation efficiency. In this work, Chlorella vulgris was selected fromfour microalgae strains which has the characteristic of resistance for high temperature,high concentrations and low pH value as the CO2fixation and transformation carrier.Based on the advantages of Flat-plate photobioreactors in small occupied area, highmass transfer efficiency, simple structure and easy to scale-up, an innovative closedPBR called internal loop airlift flat plate photobioreactor equipped with interior LEDilluminant (ILA-PBR) was developed. Twice improvements were carried to increasedaily CO2fixation (FD) of Chl. vulgris with low energy consumption from simulatedflue-gas.The first step in developing an algal process is to choose the algal species. Fourstrains of microalgae mentioned in the former part, namely, Chlorella. vulgaris,Dunaliella salina, Chaetoceros gracilis and Cyanobacterium aponinum were studiedin batch mode. Artificial light sources were fluorescence lamp (FL), andflexchromatic light-emitting diode strips of red (LED-R), white (LED-W), and blue(LED-B). Nine light qualities, respectively, LED red light (LR), LED blue light (LB),LED red plus LED blue light (LR+LB), LED white light (LW), fluorescent white light (FW), and the proportions of red or blue light are increased in LW and FW (FW+LR,FW+LB, LW+LR, LW+LB) were provided. Results revealed that LED-W increasedthe maximum photosynthetic action rate of Chl. vulgaris, D. salina, and Cy. aponinum,homoplastically, LED-B played the same role in the growth of Cha. gracilis. In termsof the light qualities, the present data showed species-specific photoacclimationresponses for four species. At the irradiance of60μmol m2s1, the growth rate ofChl. vulgaris was significantly higher under LW and LW+LB than under the otherlight treatments. The highest growth rate of Cha. gracilis appeared under the lightqualities LB or FW+LB. LW+LR was the most favorable for the growth of Cy.aponinum. Be different than other strains, the growth rate of D. salina did not changedistinctly with light qualities. Whatever the light resource and the light quality, theflexible LED strips have more advantages than FL for microalgae growth.To investigate the suitable conditions of CO2supply, the first generationILA-PBR with3.2L of working volume was built. In the microalgal cultures aeratedwith0.04%,1%,2.5%,5%and10%CO2, the maximal FD in fed-batch mode of C.vulgaris was1.18g CO2L-1d-1with1%CO2aeration. The optimum CO2concentration for microalgae growth was, respectively, Chl. vulgaris1~2.5%, D.salina1~2.5%, Cha. gracilis1~5%, and Cy. aponinum0.04%. Uniformly, thelipid-based energy ratio (ERoil) and lipid productivity of Chl. vulgaris was3.5timeshigher than that of D. salina,3.6times higher than Cha. gracilis, and20times higherthan Cy. aponinum. Combined with the previous results, the practicability test of twokinds of light condition (LED-W, LED-W+LED-B) was carried out to explore thefeasibility of application. By comparing with the results obtained under the LED-Wlight, microalgae had similar growth pattern under the LED-W+LED-B light, however,Chl. vulgaris cells attached on the surface of LED-B seriously which might have anegative effect on biomass harvest. On the basis of that, the LED-W was suggested tocultivate C. vulgaris as an interior LED illuminant in ILA-PBR.Secondary, the growth and FD datas from Chl. vulgaris cultivated in theILA-PBR carried by orthogonal experiment were used to evaluate the effects of threemain design parameters including superficial gas velocity (SGV), the ratio of height to diameter (H/D), and the ratio between downcomer and riser cross sectional area(Ad/Ar) on FD. The design parameters were optimized as follows: SGV=0.3vvm,Ad/Ar=3:1and H/D=6:1. It was also demonstrated that SGV played an active role forFD by increasing average gas holdup (g) of microalgae suspensions. Microalgaegrowth was influenced by multiple factors, and only considering reactor performanceascension through volumetric mass transfer coefficient (kLa) was not comprehensive;increasinggcould play a more important role. Aeration was supplemented with1%CO2, and a SGV of0.3vvm was the most beneficial condition for the rapid growth ofC. vulgaris in ILA-PBR. Increasing the initial inoculation optical density (OD680) to0.5, synchronously, improving the incident light intensity to240μmol m2s1couldsignificantly enhance the microalgal resistance to high concentrations of CO2andeffectively improve FD to1.97g CO2L-1d-1in the microalgal cultures aerated with1%CO2at SGV=0.3vvm. Under the same initial OD680and incident light intensity, themaximum FD of1.00g CO2L-1d-1occurred in the first generation ILA-PBR whenSGV was0.3vvm, CO2concentration was15%.In addition, SGV was maintained at0.3vvm. CO2was continuously added to theN2stream to decrease the O2content in the flow agitation. Through this method, FDcould be improved sequentially. In the microalgal cultures aerated with1%and5%CO2with low O2concentration, the maximal FD were increased to2.27g CO2L-1d-1from1.97g CO2L-1d-1,2.12g CO2L-1d-1from1.41g CO2L-1d-1, respectively. Asemi-continuous process was operated under these conditions. The FD was highest incomparison with those in the batch and fed-batch cultivation modes. The results ofFD obtained from semi-continuous mode were1.41g CO2L-1d-1on the first day, andmaintained at the level of1.77-2.42g CO2L-1d-1. Consequently, semi-continuouscultivation mode is conducive to maintaining the high CO2fixtation efficiency of Chl.vulgaris in the ILA-PBR.Finally, the number amplification method was adopted to realize the enlargementof ILA-PBR based on the improved ventilation modes. Mixing aeration mode was putforward to account for it. The third generation ILA-PBR with8.0L of workingvolume was built and simulated flue-gas (15%CO2) was bubbled into microalgae suspension. The FD of this method was1.46g CO2L-1d-1and1.79g CO2L-1d-1when air or N2mixed with15%CO2in ILA-PBR, respectively. Above all, these FDvalues are considerably higher than reported in the published literature for mostclosed photobioreactors. From the perspective of energy conservation, taking LED-Was internal illuminant could saving73.6percent energy than the traditional FLexternal light pattern. It is important to re-iterate that the ERoilof C. vulgaris underformer light pattern is0.011, which is also considerably larger than the latter one(0.002). In the third generation ILA-PBR, the biomass-based energy ratio (ERbiomass)and ERoilof was0.0219-0.0249and0.0091-0.0104, also higher than most PBR.Taking all the evaluation indexes (FD, ERbiomassand ERoil) into consideration, as amore efficient and energy saving CO2fixation system, the ILA-PBR could bepotentially applied in the area of microalgae culturing for industrial flue-gas CO2removal. |
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