Kinetic analysis and thermal characterization of the microalgae combustion process by thermal analysis coupled to mass spectrometry

The thermal characteristics under oxidizing atmosphere of several species of microalgae: Nannochloropsis gaditana (NG), Scenedesmus almeriensis (SC) and Chlorella vulgaris (CV) was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) coupled with mass spectrometry (MS). These microalgae were chosen according to their chemical composition. TGA results showed that microalgae combustion took place in two main stages: devolatilization and char oxidation, which were corroborated by DSC analysis. The former one could be related to the decomposition of their main microalgae components: carbohydrates, lipids and proteins. Samples SC and CV yielded the highest amount of ash, which implied that sample pre-treatment such as washing is required before being used in thermal applications in order to avoid operational problems. Sample NG showed the highest amount of combustion heat. Kinetics were evaluated assuming single separate reactions for each combustion stage. Additionally, the process was successfully modeled obtaining for sample NG a maximum error of ±3.1%. CO, CO2 and H2O were the main components obtained during the combustion process. The evolution of CH4 at high temperatures was mainly attributed to lipid decomposition. Finally, it was noticed that nitrogen compounds (NO, NO2 and HCN) were released in a higher proportion that sulfur ones (SO and SO2).