Efficient Hydrogen Production by Algae Pyrolysis in Molten Salts: Kinetic Analysis Based on the Reducing Charge Demand Method

The kinetic analysis of thermochemical conversion of carbonaceous materials in molten salts is difficult due to the difficulty in obtaining the weight loss characteristics of the feedstock. The novel reducing charge demand (RCD) method is proposed to solve the challenges of molten salt transformation and gas absorption that affect the feasibility and accuracy of traditional analysis methods. The pyrolysis characteristics of algae with different pellet sizes in alkaline molten salts are investigated, and the related kinetic mechanisms are revealed by combining the RCD method and the temperature distribution inside the pellets. Results show that the higher temperature for smaller pellets slightly increases the hydrogen yield/purity. At 750 °C, the hydrogen yield reaches 72.44 mmol/g with carbon capture efficiencies of 69–73%. The pyrolysis starts with algae catalytic pyrolysis (random scission model) and then begins to be dominated by char alkalization (second-order model) at conversion efficiencies of 50–60%. The average activation energy of both stages is reduced by the molten salts to 31.88–35.61 and 56.60–59.69 kJ·mol–1. This study provides a powerful method to explore the kinetic mechanism related to the molten salt system and guide the optimization of the hydrogen production technology.