Research On The Rule Of Microalgal Growth Under The Different Spectral Wavelengths And The Opitmazion Of The Technology For Bio-oil Production Based On Hydrothermal Liquefaction Of Microalgae

With the development of the society, the consumption of energy is excessive, the price of crude-oil keeps rising, the conventional fossil fuels are increasingly scarce and the environmental pollution caused by energy consumption is increasingly severe, and the energy crisis and environmental pollution become a growing problem. The entire world is looking for the renewable and clean energy to replace the traditional fossil energy. Owing to the advantages of the high efficiency in photosynthesis, fast growth rate, good environmental adaptability, high biomass and oil yield, microalgae based biofuel as a renewable clean energy has drawn wide attentions. It is considered as the potential bioenergy to solve the energy crisis in the future, with the environmental friendliness and sustainability. Now, the key issues needed to be solved for commercial use of the microalgae biomass as the energy are how to reduce the costs of industrialization, improve the microalgal biomass and conversion efficiency for bio-oil production, further contribute to the sustainable development of microalgae based energy.Chlorella sp., Isochrysis galbana and Spirulina sp. were used as the objects. In this research, together with the LED as the light source, under different red and bule light combination illumination (100%red light,90%red light+10%blue light,70%red light+30%blue light and50%red light+50%blue light). The effect of the different combinative lights on the microalgal growth and lipid content accumulation was studied. The growth information during microalgae growth including biomass, lipid, protein and chlorophyll was acquired and analyzed. The use of near-infrared spectra and hyperspectral imaging techniques were used to monitor the growth of microalgae that can analyze the dynamic change rules of the biochemical substances in the three microalgaes in vivo. It can provide the theoretical support of adding artificial light during microalgae growth. We also used the Spirulina powder with high protein content, low lipid content and the fast growth rate as the feedstock, designed the hydrothermal liquefaction experiments related to three mainly factors- ratio of material to liquid, temperature and reactor time using quadratic general revolving combination design. The process using Spirulina powder to convert to bio-oil was optimized by response surface methodology method. The major results were obtained as follows:(1) The combination of red light (peak wavelength,660nm) and blue light (peak wavelength,450nm) can improve the growth of the three microalgae in this study. The light effect on microalgae depends on the microalgae species. For the biomass accumulation, Chlorella sp. and Spirulina sp. accumulated the largest biomass at the70%red+30%blue light culture condition, the dry weight of the biomasss was0.400±0.0048and0.455±0.0126g/L, repectively. Isochrysis galbana has the largest biomass at90%red+10%blue light with the biomass of0.779±0.0066g/L. Under the50%red+50%blue light condition, these three microalgae species have the minimum biomass accumulation. For lipid accumulation, the three microalgae has the largest accumulation under the100%red light condition. The lipid content were33.5±0.49、32.3±1.02and8.45±0.225%of Chlorella sp., Isochrysis galbana and Spirulina sp. at the harvest period, respectively. Along with the increase of the ratio of the blue light in the combined light source, the lipid accumulation decrease, and all of them have the minimum lipid accumulation at the condition of50%red+50%blue light. For pigment content, the influence on chlorophyll a accumulation was different among the three microalgae. The content difference of the chlorophyll a among the different combined light source was different at the harvest period. The results of this study indicated that a single red light has a promotion in the microalgal growth and lipid accumulation, the effect of the blue light depends on the microalgal species. When the blue light came to50%proportion and red light reduced to50%proportion, the growth and lipid accumulation of microalgae were decreased;(2) We acquired the visible/near-infrared spectroscopy of the three microalgae at lag phase, logarithmic phases and stationary phase, respectively. We analyzed the spectral absorbance of which wavelength was correlated to the biomass content, and established the multiple linear regression models for chlorophyll a, lipid and protein prediction with a good predictive performance. The results showed that the spectral absorbance at730nm has a good correlation with biomass with R2of0.984,0.972and0.980for Chlorella sp., Isochrysis galbana and Spirulina sp., respectively. Through the predictive model, we can achieve the dynamic detection of the lipid changes in microalgae. Overall, the lipid content in the three microalgae was relatively stable in some time, and when the growth was closed to the stationary phase, the lipid began to increase. For chlorella sp. and Isochrysis galbana, the variation of the lipid was between10%to15%from logarithmic phases and stationary phase. The lipid content in Spirulina sp., was low, the content discrepancy was small from the beginning to the harvest with variation of around1%.(3) Through the visualization analysis of the hyperspectral images of the microalgae at the stationary phase, we found that there were good results in the analysis of the distribution of the biomass and the lipid of Chlorella sp. and Spirulina sp.. In the visualization of the microalgal lipid, the distribution of the lipid showed the content was not uniform. But the visualization method used on Isochrysis galbana was not effective, it is necessary to further optimization of the information acquisition and analysis;(4) The analysis of the oil content and composition of the microalgae powder using Fourier transform infrared spectrometer (FT-IR) and Gas chromatograph-mass spectrometer (GC-MS) methods showed that the different combinations of red light and blue light have different affection on the composition as well as the content of the different components of microalgae cells. FT-IR showed that the component discrepancy of the organic was not big in one strain. The GC-MS showed that the main components and the content of one component of the lipid are different in different microalgae strains. For chlorella sp., the content of unsaturated fatty acids(UFA) was less than saturatedones (SFA); the content of UFA and SFA in the cells were among the different light conditions except the50%red and50%blue LED combined light. There is no eicosapentaenoic acid (EPA) in the70%red+30%blue and the50%red and50%blue combined light source.For Isochrysis galbana, there is no EPA in the50%red and50%blue combined combined light source. The rest of the groups contained small amounts of docosahexaenoic acid (DHA) and EPA. The content of DHA was relatively higher in100%red light and the90%red+10%blue combined light source with0.5%-1%more than other groups. The content of oleic acid (C18:1) is more in every group; for Spirulina sp., compared to the other two species, the vareity of fatty acids was relatively less. C16:0was the main component in SFA while C18:3was the main component in UFA. The content of UFA in Spirulina sp relatively high in100%red light conditon, the ratio of the total fatty acid was more than other groups with changes about3-8%.(5) Based on the single-factor test, through the rotating universal design regression tests with three factors and five levels, we analyzed three main factors affecting the microalgae based hydrothermal liquefaction on bi-oil production. The three factors are the ratio of material to liquid, reaction temperature and reaction time. We adopted the yield of the bio-oil as the response value and the response surface analysis was applied to optimize the technology parameters. The result was that the best experimental conditions are the ratio of material to liquid was10.5/100(algae powder, mg/deionized water, mL), the reaction temperature was357℃, reaction time was37min. Under the optimized conditions, the maximum yield of bio-oil was44.6%. Using the fresh Spirulina culture as the feedstock, the yield of the bio-oil was42.3%. The high heat value of bio-oil was increased to30MJ·Kg-1compared to the HHV18MJ·Kg-1of the feedstock. The yield of the bio-oil was high but the GC-MS analysis of the bio-oil showed that the quality still needed to be further improved.