Cultivo de microalgas para biofixação de CO2: uma estratégia para captura e conversão de carbono em biocombustíveis e/ou bioprodutos

Abstract

The increasing atmospheric CO₂ concentrations due to human activities have led to the search for alternative CO₂ capture sources, including microorganisms such as microalgae. Studies have shown the effect of carbon dioxide concentration on microalgae growth, even when using residual CO₂ from industries such as cement production, which accounts for more than 6% of global CO₂ emissions. The objective of this study is to evaluate the effect of different CO₂ concentrations on the cultivation of two microalgae strains: Chlorella sp and Spirulina platensis, in order to identify their CO₂ biofixation capacity, potential high-value bioproducts, and system scalability. The microalgae were cultivated in commercial culture media – BG11 (Adapted) for Chlorella and Zarrouk (Adapted) for Spirulina – under different CO₂ concentrations (v/v) (0.04%, 5%, 10%, and 15%). The microalgae were characterized through proximate analysis (ash content, moisture, volatiles, and fixed carbon), elemental analysis (CHNO), thermogravimetric analysis (TGA), X-ray fluorescence (XRF) of the ashes, lipid content, protein content, and both conventional and catalytic analytical pyrolysis. Additionally, during cultivation, daily data on dry biomass and pH were collected. The results indicate that both microalgae achieved the highest biomass yield at 10% CO₂ (v/v), reaching 1.25 g L⁻¹ for Chlorella and 2.15 g L⁻¹ for Spirulina. Despite the high ash content due to the flocculation method, Chlorella exhibited greater potential for biofuel production due to its high lipid content (21.7%) compared to Spirulina (11.2%). Conventional pyrolysis revealed a high amount of oxygenated (>25%) and nitrogenated (>20%) compounds for both microalgae, which were predominantly converted (>90%) into aromatic compounds and hydrocarbons after the use of the HZSM-5 catalyst in the thermochemical conversion process. The system scalability analysis for photobioreactors (80 L) demonstrated economic feasibility when supplemented with an external CO₂ source, making it possible to achieve a self-sufficient system considering only the raw microalgae sale price. This approach can significantly contribute to CO₂ emissions mitigation and the development of a renewable carbon-based economy.