Pyrolysis of the freshwater macroalgae Spirogyra crassa: Evaluating its bioenergy potential using kinetic triplet and thermodynamic parameters

Abstract

The freshwater alga Spirogyra crassa was subjected to pyrolysis to investigate its potential use as a bioenergy feedstock. To do so, the pyrolysis behavior of the Spirogyra crassa under thermogravimetric scale was first determined at the temperature range from 25 to 800 °C, with three slow heating rates (5, 10, and 20 °C min−1) under an oxygen-free atmosphere. It is assumed that the pyrolysis of Spirogyra crassa occurs in four reaction steps with different kinetic triplets. The activation energy was obtained for each reaction step by concurrent use of four isoconversional methods (Friedman, Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink), with average values ranging from 120.6 to 217.0 kJ mol−1. Pre-exponential factors determined from the kinetic compensation effect were found to range between 6.88 × 107 and 7.54 × 1019 min−1. Master plot results indicated that the nth-order-based mechanisms described the pyrolysis behavior of organic matter and, subsequently, the pyrolysis behavior of inorganic matter follows Avrami-Erofeev nucleation mechanisms. According to the thermodynamics parameters, the pyrolysis of Spirogyra crassa verifies to be a non-spontaneous, endothermic, and complex conversion. The summative kinetic expression proposed from the estimated kinetic triplets is a satisfactory option for describing the pyrolysis kinetics of Spirogyra crassa, with a quality of adjustment above 93.3 %. In conclusion, the insights of this study confirm that Spirogyra crassa has considerable potential as a feedstock for bioenergy production, and could be used for engineering purposes in the design or simulation of large-scale pyrolysis reactors.