Holistic approach for sequential transesterification and pyrolysis of microalgal biomass: Kinetic and thermodynamic analysis of pyrolysis using model-free and model-based approaches

Due to the renewability and sustainability, biofuels derived from algae are considered as competitive substitutes for fossil fuels. Present study investigated a holistic approach for sequential transesterification and pyrolysis of microalgal biomass, i.e. Chlorella minutissima. This study explores the complete utilization of algal biomass through sequential chemical (transesterification) and thermochemical (pyrolysis) processes. First, the lipids were extracted and converted to eco-friendly biodiesel using catalytic route. Then lipid-extracted microalgae were pyrolyzed via thermogravimetric analyzer at four heating rates, namely 5, 10, 20 and 30 °C/min. The model-free isoconversional Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO), and Vyazovkin models were used to analyze the kinetics and thermodynamics of algal pyrolysis. The activation energy obtained for the pyrolysis of Chlorella minutissima utilizing KAS, FWO and Vyazovkin were 146.78, 148.86, 147.11 kJ/mol, respectively. Positive ΔH values from KAS, FWO, and Vyazovkin show an endothermic nature of the process. The mean ΔH was found to be 142.81, 143.97, and 142.82 kJ/mol, while the ΔG was 151.9, 152.20, and 161.40 kJ/mol, respectively for KAS, FWO, and Vyazovkin approaches, at 10 °C/min heating rate. Importantly, this study also revealed that the phase interfacial model was the most appropriate model to represent the degradation process using Coats-Redfern method at 5 oC