Biological CO₂ fixation using Chlorella vulgaris and its thermal characteristics through thermogravimetric analysis

The present research is focused on cultivation of microalgae strain Chlorella vulgaris for bio-fixation of CO₂ coupled with biomass production. In this regard, a single semi-batch vertical tubular photobioreactor and four similar photobioreactors in series have been employed. The concentration of CO₂ in the feed stream was varied from 2 to 12 % (v/v) by adjusting CO₂ to air ratio. The amount of CO₂ capture and algae growth were monitored by measuring decrease of CO₂ concentration in the gas phase, microalgal cell density, and algal biomass production rate. The results show that 4 % CO₂ gives maximum amount of biomass (0.9 g L⁻¹) and productivity (0.118 g L⁻¹ day⁻¹) of C. vulgaris in a single reactor. In series reactors, average productivity per reactor found to be 0.078 g L⁻¹ day⁻¹. The maximum CO₂ uptake for single reactor also found with 4 % CO_2, and it is around 0.2 g L⁻¹ day⁻¹. In series reactors, average CO₂ uptake is 0.13 g L⁻¹ day⁻¹ per reactor. TOC analysis shows that the carbon content of the produced biomass is around 40.67 % of total weight. The thermochemical characteristics of the cultivated C. vulgaris samples were analyzed in the presence of air. All samples burn above 200 °C and the combustion rate become faster at around 600 °C. Almost 98 wt% of the produced biomass is combustible in this range.