High-Level Energy and Carbon Dioxide Balance Assessment for Microalgae Production as a Negative Emission Technology

Abstract Negative Emission Technologies, i.e., processes that reduce the CO2 concentration of the atmosphere through their operation, are supposed to compensate for hard-to-abate CO2 emissions in the future and thus enable, e.g., countries to achieve climate targets such as net zero. The photoautotrophic production of microalgae biomass and its subsequent partial or complete long-term storage were investigated as a possible Negative Emission Technology. This study analyses from a high-level perspective various combinations of the possible process steps and assesses the effectiveness of this method regarding the energy and carbon dioxide balance. The most efficient pathways sequester approximately 1 kg of CO2 per kg of dried biomass. Key findings highlight the need for natural light in algae growth, with two viable options for achieving a negative CO2 balance: direct sequestration of dried biomass and nutrient recovery followed by carbon sequestration. Critical factors include energy-efficient CO2 delivery, the use of low-carbon electricity, and minimizing the carbon footprint of nutrient inputs.