| Energy shortage and environmental disruption attributing to the overuse of traditional fossil fuels have become important issues that hinder the survival and development of human beings.To tune the contradictions between economic development,energy consumption,and environmental pollution,it is urgent to alter the energy consumption structure of our nation by developing renewable green energy and protecting the environment with technology innovation.Microalgae,as a kind of unicellular organism,can use CO2 in flue gas as substrate,nitrogen and phosphorus in wastewater as nutrients and use light as energy source to synthesize bio-oil through photosynthesis.It can realize carbon mitigation,wastewater treatment,and bioenergy production at the same time.Thus,researches on microalgae-based biofuels production have attracted particular interests in recent years.However,the mass and energy transfer efficiency in microalgal bioenergy conversion system are low,leading to a poor economic efficiency of biofuel production from microalgae.To improve microalgae biomass and lipid productivity,investigations and optimizations on the mass transfer in the microalgal bioenergy conversion system are necessary.This work focused on suspended microalgae cultivation technology for biofuels production in photobioreactor(PBR).The mass transfer process of major influencing factors(carbon,light and nutrients)in suspended microalgae cultivation PBR and its effects on microalgae growth and metabolism were systematically investigated.In addition,the mass transfer in PBR were regulated by improving the PBR structure and optimizing the operating conditions to enhance microalgae growth and lipid productivity.Firstly,we analyzed light and carbon transfer characteristics during microalgae growth and metabolism,and investigated the coupled effects of light intensity and dissolved inorganic carbon on microalgae growth and carbon fixation.Microalgae growth and carbon fixation modeling considering the coupled effects of light intensity and dissolved inorganic carbon concentration was constructed,and the time for microalgae to enter stationary phase under different conditions was predicted using the constructed modeling.Secondly,a self-adaptive microalgae anion-exchange-membrane PBR was proposed by adopting the ion permselectivity of anion-exchange-membrane,which optimized the nutrients transfer and distribution in the PBR.Nutrients transfer characteristics in the PBR and its effects on microalgae growth in the proposed PBR was researched and optimized.At the same time,we proposed a nutrients phase-feeding strategy which provided different amount of nutrients in different physiological phases of microalgae according to the requirement.Thirdly,we simultaneously regulated the transfer and distribution characteristics of three major influencing factors(carbon,light anad nutrients)to maximize microalgae lipid productivity.Lastly,to reduce the investment on microalgae lipid production,an annular microalgae ion-exchange-membrane photobioreactor was proposed.The PBR could regulate nutrients transfer and pollutants transfer separately and thus realize the non-touch cultivation of microalgae when using wastewater as nutrients source.In this way,the posioning effects of pollutants in wastewater on microalgae growth was avoided,and efficient microalgae cultivation and wastewater treatment were realized.The main conclusions are as follows.(1)Microalgae growth and carbon fixation kinetics modeling considering the coupled effects of light intensity and dissolved inorganic carbon concentration was constructed.Some additional experiments under random light intensity and dissolved inorganic carbon concentration were conducted to validate the proposed modeling.The results showed that the growth and carbon fixation kinetics modeling fit well with the experimental data.Then the growth kinetics characteristics of microalgae under different light intensity and dissolved inorganic carbon concentration were investigated.We found that microalgal growth rate first increased and then decreased with the cultivation time,while microalgal specific growth rate monotonously decreased with cultivation time.The maximum microalgae biomass concentration(2.303 g/L)and the maximum carbon dioxide fixation amount(4.385 g CO2/L)were obtained under light intensity of 120 ?mol/m~2/s,dissolved inorganic carbon concentration of 17 mM.Meanwhile,the time for microalgae to enter stationary was the shortest as 98 h.When light intensity and dissolved inorganic carbon concentration were higher or lower than the optimum values,the maximum biomass concentration was reduced and the time for microalgae to enter stationary phase was prolonged.(2)A self-adaptive anion-exchange-membrane microalgae photobioreactor(AEM-PBR)was proposed.By controlling the nutrients concentration difference between microalgae cultivation chamber and nutrients feeding chamber,the nutrients concentration in microalgae cultivation chamber could be kept in a suitable level,avoiding the inhibition effect caused by excess nutrients and limitation effects caused by nutrients lack on microalgae growth.Results showed that the maximum microalgae biomass concentration increased from 1.30 g/L in traditional PBR to 2.98 g/L in the AEM-PBR,increased by 129.3%.Then the optimum nutrients feeding rates for the AEM-PBR were investigated,and we found that the maximum biomass concentration of 4.38 g/L were obtained under nitrogen and phosphorus feeding rates of 19 mgN/L/d and 4.2mgP/L/d.(3)A microalgae nutrients phase-feeding strategy was proposed according to different nutrients requirement in different physiological phase of microalgae cultivation.In the adaption phase,relative low nitrogen and phosphorus feeding rates(nitrogen and phosphorus feeding rates were 5.11mgN/L/d and 0.54mgP/L/d,respectively)reduced the lasting time of adaption phase to 34 h.In the growth phase,sufficient nutrients with nitrogen and phosphorus feeding rates of 20.04mgN/L/d and 4.21mgP/L/d enhanced the biomass concentration to around 4.10 g/L.In the stationary phase,nutrients starvation conditions were created by stopping the nutrients feeding and lipid content of microalgae was enhanced,and the microalgae lipid productivity was eventually improved to 132.30 mg/L/d.(4)Under the optimum nutrients feeding strategy,light intensity and CO2 concentration in different physiological phases were synergetically optimized,and the microalgae biomass concentration and lipid productivity were further improved.In the adaption phase,relative low light intensity(60 ?mol/m~2/s)and CO2 concentration(2%)reduced the adaption phase to 24 h.In growth phase,sufficient light(120 ?mol/m~2/s)and carbon(CO2 conentration of 5%)enhanced microalgae maximum biomass concentration to 4.52 g/L.In the stationary phase,excess light(180?mol/m~2/s)triggered the synthesis of lipid in microalgae cells and improved the lipid productivity to 163.4 mg/L/d.(5)An annular microalgae ion-exchange-membrane photobioreactor(IEM-PBR)was proposed that could realize non-touch cultivation of microalgae with wastewater,then the posioning effects of pollutants in wastewater on microalgae growth and metabolism was avoided.The IEM-PBR was well applied to different kinds of un-pretreated wastewater and un-acclimated microalgae strains.When simulated agricultural wastewater,simulated municipal wastewater,simulated industrial wastewater and real municipal wastewater were used as nutrients source for microalgae cultivation,the masimum biomass concentration indreased from 2.34,2.15,0 and 0.52 g/L in traditional PBR to 4.24,3.13,2.01 and 1.71 g/L in the IEM-PBR,respectively.Correspondingly,nitrogen and phosphorus removal efficiency,lipid productvity and lipid quality in the IEM-PBR were also enhanced. |
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