Life-cycle and economic assessments of an innovative multi-stage bio-thermochemical system for hydrogen production from microalgae

This study introduced an innovative multi-stage bio-thermochemical system for hydrogen production from microalgae, integrating hydrothermal hydrolysis, anaerobic fermentation, and supercritical water gasification, with the aim of establishing a carbon-negative pathway that links flue gas carbon capture, hydrogen production, and resource recycling. Life-cycle and economic assessments were conducted to evaluate the energy, environmental, and economic performance of this proposed system. The results indicated that the coupling hydrogen production system effectively achieved greenhouse gas reduction (-1164.46 g CO2-eq/MJ) through carbon sequestration by microalgae, with 14.98% of the carbon from flue gas being permanently stored in soil conditioner (14.67%) and syngas (0.31%), demonstrating significant environmental advantages. However, the high energy demand of cultivation (29.78%) and supercritical water gasification (31.66%) subsystems led to a negative yield rate (-77.1%), leading to the higher hydrogen cost (1.74 $/kWh) compared with the market price of hydrogen. Sensitivity analysis further identified sedimentation efficiency and total hydrogen yield as key regulatory factors, which could significantly affect the energy conversion and environmental impact. Accordingly, this study highlighted the importance of advancing sedimentation technologies, developing efficient photobioreactors, and designing low-cost catalysts to support the development of economically viable carbon-negative hydrogen production from microalgae.