Kinetic study and pyrolysis: GC/MS products analysis of Spirulina platensis cultivated under a different growing medium

Abstract

Microalgae have a great potential to produce biofuels, but the cost is still too high mainly due to the biomass production. Mixotrophic cultivation has been pointed as microalgae cultivation mode for biomass/bioenergy production with lower cost. The proposals of this work were to cultivate S. platensis in autotrophic and mixotrophic medium using molasses as source of organic carbon and investigate the thermal behavior of obtained biomass by means of thermogravimetric analysis and pyrolysis (Py-GC/MS). The kinetics models proposed by Flynn and Wall and model-free kinetic were used to determine the activation energy. These data are important to projection of design, operation and modeling of thermochemical conversion system for microalgae. The use of molasses as supplement in culture medium for the growth of S. platensis was a good way to increase the biomass productivity and decrease the protein content. The Flynn–Wall and model-free kinetic methods were adequate to calculate activation energy on the conversion range of 0.20 to 0.80. The activation energies were 168–229 kJ mol−1 (Flynn and Wall), 177–238 kJ mol−1 (model-free kinetic) for mixotrophic biomass and 174–220 kJ mol−1 (Flynn and Wall), 181–229 kJ mol−1 (model-free kinetic) for autotrophic biomass. The compositions of volatile compounds produced by S. platensis biomass pyrolysis were different due to cultivation method that influenced the composition of biomass. Hydrocarbons, oxygenated and nitrogenated compounds were produced with different quantities. The volatile compounds content of phenols, oxygenated and nitrogenated increased and non-aromatic and aromatic hydrocarbons content decreased for pyrolysis of mixotrophic biomass. However, the mixotrophic cultivation has a great influence on the microalgae biomass production and should be a factor considered in thermal degradation project for microalgae.