Effect of wet torrefaction on pyrolysis kinetics and conversion of microalgae carbohydrates, proteins, and lipids

The experimental results of microalgae pyrolysis kinetics are beneficial to the reactor design in the biomass-to-energy process. To understand the complex pyrolysis process of microalgae, pyrolysis kinetics of microalgae components pretreated by wet torrefaction was evaluated using the independent parallel reaction model. Four and five reaction models were implemented to analyze the pyrolysis kinetics of microalgae Chlorella vulgaris ESP-31 (high-carbohydrate) and FSP-E (high-protein), respectively. Five pseudo-components were required to investigate the microalga FSP-E due to the extra carbonaceous material at temperatures higher than 600 °C. In the pyrolysis TGA curves, the first peak of microalga ESP-31 was diminished whereas only a slight decrease in the first peak of microalga FSP-E for the pretreated microalgae in water and H₂SO₄ solutions. From the results, pyrolysis kinetics with a fit quality of at least 97% was predicted for both species of microalgae. The activation energy of carbohydrates for microalga ESP-31 was decreased from 221.33 to 64.59 kJ mol⁻¹ after pretreated in H₂SO₄. In contrast, the activation energies of proteins and lipids were increased for the pretreated microalgae ESP-31. Small changes in the activation energy range of carbohydrates, lipids, and other components were observed for microalga FSP-E compared to microalga ESP-31. On top of that, the thermal degradation temperatures and activation energies of carbohydrates and proteins for the pretreated microalgae ESP-31 and FSP-E displayed the opposite trend. In short, kinetic parameters of microalga ESP-31 could be effectively affected by low-temperature wet torrefaction compared to microalga FSP-E.