Evaluation of thermochemical properties of raw and extracted microalgae

•Combustion behavior of raw and extracted microalgae was assessed by TGA and FTIR.•Kinetics analysis of microalgae combustion was performed for different heating rates.•Three different decomposition stages could be distinguished.•The raw CV and SC required less energy to initiate the oxidation process than the NOc.•Activation energy values are generally similar for the raw and extracted microalgae.

The thermochemical behavior of three raw and respective solvent extracted (ultrasound assisted) microalgae species – CV (Chlorella vulgaris), SC (Scenedesmus obliquus) and NOc (Nannochloropsis oculata) – was studied in order to evaluate their potential as fuels. Thermograms were obtained for four heating rates (5–25 °C/min). The results reveals that (i) for all microalgae thermal degradation processes of carbohydrates, lipids and proteins were observed for temperatures below 450 °C, while the char oxidation occurred for temperatures between 450 and 600 °C; (ii) the raw CV and SC required less energy to initiate the oxidation process than the raw NOc due to the higher amounts of lipids present in the latter microalgae; (iii) the extracted SC and NOc showed a behavior significantly different than the raw SC and NOc in the stages related to the lipids and proteins decomposition; (iv) the FTIR (Fourier transform infrared spectroscopy) spectra obtained for the raw and extracted microalgae showed significant differences in their oil and polysaccharides contents, revealing that ultrasound extraction using a mixture of n-hexane-isopropanol as solvent is adequate to extract these components from the microalgae studied; and (v) the calculated activation energy values are generally similar for the raw and extracted microalgae.