Key Takeaways & Executive Findings
- •• • Molten hydroxide salts (80% NaOH-20% Na2CO3) elevate central heating rate of microalgae pellets to 177 °C/s, mitigating thermal lag in large pellets and enabling faster conversion kinetics. • • Porosity increases by 53.2%–104.3% within 10 s of reaction due to molten salt penetration and erosion, enhancing mass transfer and char reactivity. • • Dominant reaction pathway shifts to char alkalization after only 70 s, indicating accelerated conversion and higher process efficiency. • • Hydrogen yield from char alkalization improves markedly when heating rate is increased above 600 °C, attributed to synergistic catalytic and volatiles reforming effects.
Abstract
This study investigates the thermochemical conversion behavior of microalgae pellets in a molten hydroxide salt (80% NaOH-20% Na2CO3) system and its influence on hydrogen production. By comparing temperature evolution, gas release characteristics, and structural evolution of pellets with and without molten salt, and integrating char alkalization experiments, the regulatory mechanism of molten salt on reaction pathways and hydrogen production was systematically analyzed. Results indicate that molten salt significantly enhances internal heat transfer efficiency, achieving a central heating rate of 177 °C/s, effectively alleviating thermal hysteresis. Concurrently, molten salt promotes pore development through penetration, erosion, and catalytic effects, resulting in a porosity increase of 53.2%–104.3% after 10 s of reaction. Conversion efficiency is markedly improved, with the dominant reaction pathway shifting to char alkalization after only 70 s. Furthermore, when heating rate is increased above 600 °C, hydrogen yield from char alkalization improves more significantly, primarily attributed to the synergistic promotion of molten salt catalysis and rapid heating on volatiles reforming. This study provides a theoretical foundation for understanding efficient hydrogen production from biomass in molten hydroxide salts.
1. Introduction
Global energy transition and carbon neutrality goals drive the need for clean, renewable hydrogen. Biomass thermochemical conversion offers rapid hydrogen production, feedstock diversity, and carbon neutrality. Microalgae, with high hydrogen content and fast growth, are promising. However, conventional pyrolysis or gasification suffers from limited hydrogen yield, low heat transfer efficiency, and high CO2 content, hindering scale-up.
Molten salts, particularly molten hydroxides, present advantages: high thermal conductivity and heat capacity enhance heat transfer; alkali metal ions lower activation energy; strong CO2 absorption shifts water-gas shift toward hydrogen; and they absorb acidic gases like HCN and HCl. Prior work achieved 67 mmol/g hydrogen yield and 80% purity from microalgae in molten hydroxide. Yet, the penetration mechanism, pore evolution, and correlation with hydrogen production in biomass pellets remain unstudied. This work systematically examines these aspects, providing mechanistic insights for process optimization.
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LI Jun, LEI Ling, CAO Wenxuan, ZHU Han, ZHONG Dian, ZENG Kuo, YANG Haiping, CHEN Hanping (2026). Hydrogen Production and Structure Evolution Mechanism during Thermochemical Conversion of Microalgae Pellet in Molten Hydroxide Salts. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0024
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Frequently Asked Questions
What is the maximum central heating rate achieved in microalgae pellets within molten hydroxide salts, and how does it compare to conventional pyrolysis?
The central heating rate reaches 177 °C/s, significantly higher than typical rates in conventional pyrolysis (often <10 °C/s for large particles). This rapid heating minimizes thermal lag and promotes uniform temperature distribution, enhancing reaction kinetics.
How does molten salt affect the pore structure of microalgae pellets during the initial reaction phase?
Within 10 seconds, porosity increases by 53.2%–104.3% due to molten salt penetration and erosion. This rapid pore development enhances mass transfer and exposes more active sites for char gasification, improving overall conversion efficiency.
At what reaction time does the dominant pathway shift to char alkalization, and what is the significance?
The shift occurs after approximately 70 seconds, indicating that molten salt accelerates the transition from devolatilization to char gasification. This early onset of char alkalization contributes to higher hydrogen yields and faster conversion rates.
What is the effect of heating rate above 600 °C on hydrogen yield from char alkalization?
Increasing heating rate above 600 °C significantly improves hydrogen yield from char alkalization. This is attributed to the synergistic effect of molten salt catalysis and rapid heating, which enhances volatiles reforming and char gasification reactions.
What are the practical implications of these findings for scaling up molten hydroxide-based biomass hydrogen production?
The findings demonstrate that molten hydroxide salts can overcome heat and mass transfer limitations in large biomass pellets, enabling faster and more efficient hydrogen production. The quantified improvements in heating rate, porosity, and reaction pathway shift provide design parameters for reactor optimization and process scale-up.
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