Improvement Of Microalgae Species And Optimal Regulation Of Cultivation Conditions To Fix High Concentrations Of CO₂in Flue Gas

Global warming caused by extensive CO2emissions has become a significant concern in both environmental and energy aspects. For microalgal higher solar energy conversion efficiency and higher growth rate, the biological sequestration of CO2by photosynthetic microalgae is a promising approach of CO2mitigation from different sources, including atmosphere, industrial exhaust gases, especially flue gas of coal-fired power plants. Nuclear irradiation combined with CO2domestication was used to isolated more excellent microalgae species for CO2fixation in this study. The carbon fixation processes were comprehensively optimized to improve microalgal CO2fixation ability.Microalgae species were mutated by nuclear irradiation and domesticated with high concentrations of CO2to improve biomass productivity and CO2fixation. The biomass yield of Chlorella pyrenoidosa mutated by500Gy of60Co-y irradiation was increased by53.1%(to1.12g/L) under air bubbling. A total of2,702genes were up-regulated in mutanted Chlorella PY-ZU1. Nine metabolic pathways involved in cell growth, and carbohydrate and protein syntheses obviously changed. Genes involved in carbon fixation, such as pyruvate kinase,1,7-diphosphate sedoheptulose phosphatase, etc.,were obviously up-regulated. These phenomena promoted Calvin cycle and carbon fixation. When cultivated under15%CO2, carbonic anhydrase of Chlorella PY-ZU1almost did not express. CO2that directly infiltrated into microalgae cells was enough for photosynthesis avoiding the transformation between different forms of inorganic carbon (CO2→HCO3-→>CO2). More ATPs were saved to improve carbon fixation and microalgae growth.The mutants were domesticated with gradually increased high concentrations of CO2, which increased the biomass yield by115%to2.41g/L. The peak CO2fixation rate and the efficiency of domesticated species were1.538g/L/d and32.7%, respectively. It was proposed that high concentration of CO2simultaneously drive lipid accumulation and carbon fixation of microalgae to break through contradictory bottleneck problem between biomass yield and oil content. Continuous aeration of CO2made a natural "nitrogen starvation" condition for microalgae. Microalgae started to enrich fatty acid over cell growth. The maximum fatty acids production rate of microalgae up to192.10mg/L/d with2.81g/L of biomass yield47.04%of oil content.To fix CO2emissions efficiently from flue gas of coal-fired power plants, the culture medium, light intensity and bioreactors were comprehensively optimized in the process of CO2fixation by Chlorella PY-ZUl. To make up for relative insufficiency of nutrients (except for the carbon source) resulting from continuous bubbling of15%CO2, three chemicals were added into the culture to optimize the molar ratios of nitrogen to carbon, phosphorus to carbon, and magnesium to carbon in culture from0.17to0.69, from0.093to0.096, and from0.018to0.030, respectively. Such adjustments led to a1.25-fold increase in biomass (from2.41g/L to5.42g/L). By enhancing light intensity from4,500lux to6,000lux, the peak growth rate of Chlorella PY-ZU1increased by99%and reached to0.95g/L/d. Use of a multi-stage sequential bioreactor notably improved the peak CO2fixation efficiency to85.6%.The toxic nitric oxide (NO) pollutant in flue gas can be converted using a wet UV/H2O2to produce nitrate (NO3), which then be used as a nitrogen source for microalgae. The growth and biomass compositions of the microalgae cultivated with the oxidation product of NO were similar to those of the microalgae cultivated with commercial NaNO3. The NO3concentration produced from NO increased as UV lamp power as well as H2O2and NO concentrations increased, resulting in an improved microalgal growth. The produced NO3-from the oxidation of500ppm NO with6%H2O2and55W UV light was used as supplementary nitrogen source for Chlorella PY-ZUl at15%CO2. The peak growth rate and CO2fixation efficiency of the microalgae were1.18g/L/d and69.6%, respectively. These dependent variables were0.97and1.13times higher than those of the microalgae cultivated with the standard medium, respectively.The toxic nitric oxide (NO) pollutant in flue gas was converted using a wet UV/H2O2to produce nitrate (NO3-), which then be used as a nitrogen source for microalgae. The growth and biomass compositions of the microalgae cultivated with the oxidation product of NO were similar to those of the microalgae cultivated with commercial NaN03. The produced NO3-from the oxidation of500ppm NO was improved the peak growth rate and CO2fixation efficiency of the microalgae to1.18g/L/d and69.6%, respectively. These dependent variables were0.97and1.13times higher than those of the microalgae cultivated with the standard medium, respectively.Applying microalgae decarburization technology into100,000m2demonstration project for microalgae cultivation to capture CO2from coal-fired flue gas in Yantai, China. By optimizing the nutrient ratio of microalgae, the aeration rate of flue gas and the aeration mode in one of the typical open racenway ponds (area of1200m2), the CO2fixation efficiency was40%-50%, increased by92.7%compares to that of microalgae cultivated in the original condition. And the microalgae biomass productivity increased by43.1%.