Thermochemical behavior of Chlorella sp. and Chlamydomonas reinhardtii algae: Comparison of laser-driven calorimetry with thermogravimetric analysis

A major focus of algal biofuel research is on maximizing the formation of the energy-containing compounds (i.e., carbohydrates, proteins, fatty acids, and lipids) during algal growth. Measurements were carried out to investigate the thermochemical character of Chlorella sp. and Chlamydomonas reinhardtii. Thermogravimetric analysis was carried out to identify the algal chemical decomposition transitions (stages) from the sample mass loss with temperature, and results were compared to values published in the literature for different algal species. Laser-driven calorimetry was also used to identify the different algal stages, as well as estimate the specific energy release, temperature range, mass loss for each stage, and heating value (i.e., total specific energy release). The estimated total specific energy release was 14.7 kJ g⁻¹ for Chlorella sp. and 9.4 kJ g⁻¹ for C. reinhardtii. The results indicated an endotherm in Stage 1 for both algae due to evaporation of moisture. There was also a small exotherm corresponding to the lighter volatiles. For Stage 2, the largest peak was for the Chlorella sp. carbohydrates with an estimated specific energy release of 2.3 kJ g⁻¹, and the lipids for C. reinhardtii at 1.9 kJ g⁻¹. The Stage 3 carbonaceous materials had the largest specific energy release, containing more than 50% of the energy among all three stages. The estimated value of the apparent activation energy was 168.6 kJ mol⁻¹ for Chlorella sp. and 134.1 kJ mol⁻¹ for C. reinhardtii. Information of this sort is needed for enhancing algal generation of energy-containing compounds that are required for biofuel production.