Thermal Degradation and Microwave-Assisted Pyrolysis of Green Algae and Cyanobacteria Isolated from the Great Salt Plains

Abstract

Seven algae strains isolated from the Great Salt Plains of Oklahoma (UTEX SP20, SP22, SP38, SP46, SP47, SP48, and SP50) were examined in this study. Biomass thermal degradation behavior of each strain was evaluated by thermogravimetric analysis. Kinetic parameters were determined using an iso-conversional approach. Algal biomass was used for bio-oil production via microwave-assisted pyrolysis, and the effects of final temperature on product yields and bio-oil composition were evaluated. Thermogravimetric analyses revealed that pyrolysis of algal biomass took place in three stages, with major weight loss occurring at the second stage from around 150°C to 400°C. Biomass of SP38 had the lowest average apparent activation energy (102.8 kJ mol-1), indicating that biomass of SP38 requires the least energy for pyrolysis among the seven strains. SP46 was selected out of the seven strains for microwave-assisted pyrolysis because of its high final biomass concentration and biomass productivity. As the final temperature increased from 450°C to 750°C, the bio-oil yield increased from 4.6% to 22.5% during microwave-assisted pyrolysis of biomass generated by SP46. The major compounds in the bio-oil were acids, aliphatic hydrocarbons, aromatic hydrocarbons, phenols, and organic nitrogen compounds. Microwave-assisted pyrolysis of algal biomass did not produce polycyclic aromatic hydrocarbons at temperatures higher than 450°C.