| The realization of carbon fixation with microalgae and the establishment of a carbon fixation industry with CO2 as a raw material and solar energy as the source to produce large quantities of food, diesel, and chemicals have attracted extensive attention in current research. Improving the yield of microalgal biomass and reducing the cost of its nutrients are crucial for the realization of large-scale applications of carbon fixation technology. Here, an oleaginous green alga (Scenedesmus dimorphus) was studied in two aspects:the method of absorption enhancement was proposed by investigating the mass transfer characteristics of a gas mixture such as flue gas with a low CO2 content, to improve the low utilisation efficiency of carbon in the pilot-scale culture of microalgae; based on the investigation of the influences of oxysulphide and nitrogen oxides in flue gas on the growth of microalgae, and considering the mass transfer characteristics of flue gas and the metabolic kinetics of micro algal cells for nutrients, the relationship governing the adjustment of the components and concentration of flue gas was proposed. On this basis, a moderate pre-treatment process for direct use of flue gas in the culture of microalgae was developed and the biological processes were evaluated.Firstly, the absorption characteristics of gas mixture with low concentration of CO2 were studied in a carbon-supplying device to reveal the influence of flow rate of the gas and pH value of the solution on the absorption rate and absorptivity of CO2. The results indicated that there exist two problems in the cultivation of microalgae using gas mixtures with low CO2 contents:the low utilisation efficiency of CO2, particularly in approximately neutral solutions (with a pH of 6 to 8); and the overall low capacity for carbon supply which fails to meet the demand of microalgae cells for a carbon source at high biomass productivity.Based on the physical absorption of CO2, methanol, propylene carbonate, n-methyl-2-pyrrolidone, polyethylene glycol, dimethyl ether, etc. were added to the culture medium of microalgae as physical CO2 absorption enhancer and their influences on the growth of microalgae and the utilisation of CO2 were explored. The results showed that the addition of these agents improved the absorptivity of CO2 at low CO2 concentrations. Meanwhile, the yield of microalgal biomass and the utilisation efficiency of CO2 were improved.As monoethanolamine (MEA), a chemical absorbent, is commonly used in carbon capture and storage, MEA was applied to the culture of microalgae as a chemical CO2 absorption enhancer in this work. By performing a CO2 absorption experiment, it was revealed that the addition of MEA significantly improved the CO2 absorption in the chosen culture medium, and a high absorption efficiency (>60%) was observed over a pH range of 6.5 to 10.0. In the semi-continuous culture of Scenedesmus dimorphus, the addition of MEA of up to 100 mg/L improved the microalgal biomass productivity and the utilisation efficiency of CO2 without consumption of MEA. Therefore, MEA could be used to fix CO2 repeatedly. However, as the concentration of added MEA exceeded 150 mg/L, it poisoned the microalgal cells and thus influenced microalgal biomass productivity.Tris(hydroxymethyl)aminomethane (Tris) shows a similar absorption behaviour for CO2 as MEA. In addition, Tris also exhibits greater biocompatibility. At concentration of 2 to 8 mmol/L, the added Tris did not poison the microalgal cells. The outdoor experiment conducted in an open raceway pond of 2 m2 verified that the addition of Tris at 6 mmol/L improved microalgal biomass productivity by 32% to 33% and the utilisation efficiency of CO2 by 5.9% to 11.6%. Furthermore, Tris would not be consumed by the microalgal cells; therefore it could be cyclically adopted in the culture medium.Ammonia as CO2 absorbent has many advantages, at the same time, the ammonia can also be used as a cheap nitrogen source to replace the commonly used sodium nitrate in the microalgal cultivation. By culturing microalgae in an open raceway pond of 2 m2, it was proved that, with a similar nitrogen source yield, the biomass productivity was 32.8% higher with ammonia as alternative nitrogen source than that with sodium nitrate:the utilisation efficiency of CO2 was 1.2 times that of the latter.By studying the absorption characteristics of SO2 in a mass transfer device with simulated multi-component flue gas, it was revealed that the existence of SO2 reduced the absorption of CO2, while, a high concentration of O2 improved the absorptivity of NO. Based on this result, and considering the uptake kinetics of microalgal cells for nutrient elements, a method for adjusting the components of the flue gas was proposed as follows:partial desulphurization without denitrification was adopted, and then, to improve the utilisation rate of NO, NO could be oxidised before utilisation.According to the method of adjustment, and by adopting widely used purification technologies for flue gas, a pre-treatment process was developed for microalgal farms allowing them to use flue gas directly:the process was evaluated by culturing microalgae. Experimental results indicated that flue gas containing 10% CO2 and 60 ppm of SO2 exerted no influence on the culture, and SO2 provided the sulphur source needed for the growth of microalgae. When the NO in the flue gas was oxidized, its absorptivity was increased from 47% to 83%; additionally, it contributed to yield a higher biomass concentration and reduced the cost for nitrogen source. When flue gas was used as the sole carbon, nitrogen, and sulphur sources, a lipid productivity of 75.5% of that under normal culture using BG11 medium was obtained. |
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