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.