Kinetic analysis of the combustion process of Nannochloropsis gaditana microalgae based on thermogravimetric studies

- Thermochemical properties of the Nannochloropsis gaditana microalgae were determined.
- Gaseous products of the thermo-oxidative process were identified.
- The comparison of kinetic models was made and the kinetic parameters were determined.
- Consecutive first order reactions (F1F1) is the best fitted reaction model.
- The evaluation of fuel combustibility was determined.

Combustion and co-combustion of biomass are recognized as possible ways to apply renewable energy sources. The application of Nannochloropsis gaditana as a feedstock in thermal processes needs some fundamental research. This study presents the determination of thermochemical properties and kinetic parameters of the thermo-oxidation of Nannochloropsis gaditana using thermogravimetric data with combined evolved gas analysis via FTIR spectroscopy. Three stages of thermo-oxidative degradation were noted: moisture removal, decomposition of intrinsic lipids, carbohydrates and proteins, and oxidation of the formed remaining char. The main gaseous products generated during combustion were water, light hydrocarbons, oxygenated compounds, CO and CO2. However, above 730 K, all intermediates were oxidized to CO and CO2.The parameters for the evaluation of fuel combustibility were determined and compared to those of other biomass sources. Ash-related issues were analyzed and discussed.Isoconversional and “data fitting” methods were used to determinate kinetic parameters. The isoconversional method proves that there is more than a single reaction mechanism of combustion due to the variability of activation energy values. Therefore, this method is not good for determining kinetic parameters of the thermo-oxidative process. The “data fitting” model could be interpreted in two ways: considering consecutive n-order (Fn) or first order (F1) reactions and considering consecutive first order reactions (F1F1) for two fractions of microalgae (first, carbohydrates and proteins; second, lipids). The devolatilization stage of the combustion process occurs with a definitively lower activation energy than the oxidation stage. The F1F1 reaction model could simulate the weight loss a little better than the Fn and F1. Correlation coefficients were all greater than 0.999, which can prove that the kinetic models were correctly found.