Biofixation of Carbon Dioxide Using Chlorella vulgaris

Microalgae cultivation is a scalable carbon dioxide removal (CDR) technology for large-scale carbon capture and utilization. In this study, a freshwater strain of C. vulgaris was investigated for its ability to fix CO2 in biomass. Initially, adaptive laboratory evolution (ALE) of the strain was carried out by conducting 7-day cultivation trials. In these trials, the CO2 concentration in the gas stream was gradually increased from 0.03 to 10 vol %. The ALE process was concluded with a fifth-generation strain at 10 vol % CO2. After the adaption of microalgae strain at 10% CO2, further investigations were carried out to understand the effects of inoculum size, culture media, photoperiod, and light intensity on the biomass productivity and biofixation of CO2. The experiments found a 0.015 g/L inoculum size, blue-green-11 (BG-11) culture media, 115 μmol m–2 s–1 light intensity, and a 16:8 phototo:dark period ratio as optimum operating conditions. Under these optimum conditions, the process resulted in the highest biomass productivity of 0.7480 g/(L d) and a biofixation rate of 1.37 gCO2/L/day. Mass balance over the cultivation process under optimum conditions resulted in an ∼0.5% CO2 conversion to biomass. Consequently, a simple flowsheet was developed with the recycling of gases to process a 100 kg/day pure CO2 stream with a 1% purge, and the mass balance over the flowsheet was calculated using the conversion data from the experiments. The experimental data of the current study are critical to understanding the ALE of Chlorella vulgaris and optimum cultivation conditions, whereas the mass balance with recycled gas provides insight into the large-scale deployment of the process.