Pyrolysis of brown macroalgae Dictyota dichotoma and its thermokinetic analysis

Seaweeds are essential autotrophs in the marine environment, resembling submerged forests near shorelines that provide habitats for various organisms. They have now become a significant source of numerous bio-products and are cultivated and harvested in various regions worldwide, contributing to the burgeoning blue economy. Environmentally friendly utilization of marine resources is one solution to meet the increasing demand for bioenergy and bio-products in the advancing blue economy. In this study, we investigated the bioenergy potential of the brown macroalgae Dictyota dichotoma using the slow pyrolysis technique. The pyrolysis process was conducted at three different heating rates (5, 10, and 20 °C min−1) in an inert nitrogen gas atmosphere, starting from room temperature and reaching 800 °C. Four valorization stages were identified, and the data obtained were fitted to various model-free iso-conversional methods, namely Friedman, Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink. The calculated activation energies ranged from 157.32 to 185.71 kJmol−1. The pre-exponential factor, determined from the kinetic compensation effect for the valorization processes, fell within the range of 4.8 × 108 to 6.5 × 1017 min−1. This range demonstrated the thermal conversion of both bioorganic and inorganic molecules, encompassing the relevant reactions throughout the entire process. Analysis using a master plot indicated that the pyrolysis of brown macroalgae primarily follows a diffusion-based process. The reproducibility of the data was further confirmed by fitting the experimental parameters obtained during the simulation process. The thermodynamic analysis revealed values of enthalpy (ΔH) ranging from 146.85 to 179.72 kJmol−1 and values of Gibbs free energy (ΔG) ranging from 143.91 to 288.19 kJmol−1. These findings demonstrate the endothermic and non-spontaneous behavior of seaweed biomass pyrolysis reactions. The entropic values indicated the formation of more ordered molecules during the latter three-pyrolysis stages. Overall, the results highlight the suitability of Dictyota seaweed biomass as a promising feedstock for bioenergy generation, contributing to the development of the blue economy.