Mass transfer characteristics and effect of flue gas used in microalgae culture

Microalgae-mediated biofixation as an innovative technology for flue gases towards carbon neutrality: A comprehensive review

Microalgae-mediated industrial flue gas biofixation has been widely discussed as a clean alternative for greenhouse gas mitigation. Through photosynthetic processes, microalgae can fix carbon dioxide (CO2) and other compounds and can also be exploited to obtain high value-added products in a circular economy. One of the major limitations of this bioprocess is the high concentrations of CO2, sulfur oxides (SOx), and nitrogen oxides (NOx) in flue gases, according to the origin of the fuel, that can inhibit photosynthesis and reduce the process efficiency. To overcome these limitations, researchers have recently developed new technologies and enhanced process configurations, thereby increased productivity and CO2 removal rates. Overall, CO2 biofixation rates from flue gases by microalgae ranged from 72 mg L-1 d -1 to over 435 mg L-1 d-1, which were directly influenced by different factors, mainly the microalgae species and photobioreactor. Additionally, mixotrophic culture have shown potential in improving microalgae productivity. Progress in developing new reactor configurations, with pilot-scale implementations was observed, resulting in an increase in patents related to the subject and in the implementation of companies using combustion gases in microalgae culture. Advancements in microalgae-based green technologies for environmental impact mitigation have led to more efficient biotechnological processes and opened large-scale possibilities.

Scale-up of microalgal systems for decarbonization and bioproducts: Challenges and opportunities

The threat of global climate change presents a significant challenge for humanity. Microalgae-based carbon capture and utilization (CCU) technology has emerged as a promising solution to this global issue. This review aims to comprehensively evaluate the current advancements in scale-up of microalgae cultivation and its applications, specifically focusing on decarbonization from flue gases, organic wastewater remediation, and biogas upgrading. The study identifies critical challenges that need to be addressed during the scale-up process and evaluates the economic viability of microalgal CCU within the carbon market. Additionally, it analyzes the commercial status of microalgae-derived products and highlights those with high market demand. This review serves as a crucial resource for researchers, industry professionals, and policymakers to develop and implement innovative approaches to enhance the efficiency of microalgae-based CO2 utilization while addressing the challenges associated with the scale-up of microalgae technologies.

Microalgae are not an umbrella solution for power industry waste abatement but could play a role in their valorization

Microalgae have long been regarded as a promising solution for biological carbon abatement from the power industry, offering renewable biomass without competing for land or water resources used for food crops. In this study, we extensively examined the application of photosynthetic microorganisms for closing carbon, nitrogen, and micronutrient loops in the power industry. Subsequently, we explored the bottom-up integration of algal biorefineries into power industry waste streams for increased economic benefits and reduced environmental impacts. Analysis of the available data indicated that microalgae integration with the power industry is primarily performed using flue-gas-assisted cultivation. This approach allows for carbon sequestration typically below one gram per liter per day, too low to significantly impact carbon abatement at achievable scales of microalgae cultivation. Alternative approaches are also being explored. For example, soluble bicarbonate platforms allow for higher biomass productivity and temporary carbon storage. Meanwhile, the use of ashes and waste heat and thermophilic strains can result in lower cultivation costs and better control of cultivation conditions. These approaches offer further incremental improvement to microalgae-based carbon abatement systems in the power industry but are unlikely to be an umbrella solution for carbon reduction. Consequently, in the near term, microalgae-based carbon valorization systems are likely to be limited to niche applications involving the synthesis of high-value products. For microalgae to truly transform carbon abatement processes radical improvements in both biology and engineering approaches are urgently needed.