Hydrogen production and structure evolution mechanism during thermochemical conversion of microalgae pellet in molten hydroxide salts

This study aims to investigate the influence of the thermochemical conversion behavior of microalgae pellet in molten hydroxide salts (80 wt.% NaOH–20 wt.% Na2CO3) on hydrogen production. By comparing the temperature evolution, gas release characteristics, and structure evolution of pellet with and without molten salt, and combining with char alkalization experiments, the regulatory mechanism of molten salt on the reaction pathways and hydrogen production behavior of microalgae pellet was systematically analyzed. The results indicate that the molten salt significantly enhances the internal heat transfer efficiency of the pellet, with a central heating rate reaching 177°C/s, effectively alleviating thermal hysteresis. Meanwhile, the molten salt promoted pellet pore development through penetration, erosion, and catalytic effects, resulting in a porosity increase of 53.2%–104.3% after 10 s of reaction. It also significantly enhanced the conversion efficiency, with the dominant reaction pathway shifting to char gasification after only70 s. Furthermore, when the heating rate was increased above 600°C, the hydrogen yield from char alkalization improved more markedly, primarily attributed to the synergistic promotion effect of the molten salt’s catalytic effect and the rapid heating process on the volatiles reforming. This study provides a theoretical foundation for a deeper understanding of the mechanisms behind efficient hydrogen production from biomass in molten hydroxide salts.