Thermal and kinetic pyrolysis behavior of two tropical wood species using thermogravimetric analysis for bioenergy recovery

The objective of this study is to characterize, by thermogravimetric analysis, the thermal behavior of two lignocellulosic biomasses from Gabonese tropical forest species: Aucoumea klaineana (Okoume) and Nauclea diderrichii (Bilinga). These resources generate large volumes of solid residues in industrial wood processing units, thus constituting a promising source for energy recovery. The tests were carried out in an inert nitrogen atmosphere, in a non-isothermal regime, with heating rates of 2.5; 5; 10; 20 and 40 °C/min, over a temperature range of 27 to 1000 °C. The ATG/DTG curves obtained made it possible to identify the different stages of thermal degradation of the main constituents of biomass, namely hemicelluloses, cellulose and lignin. Kinetic analysis was conducted using model-free isoconversional methods, including Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO), to estimate the apparent activation energy as a function of the degree of conversion. The thermodynamic parameters of the process were also determined. The results show that the average activation energy of pyrolysis varies between 171.66 and 176.99 kJ/mol for Aucoumea klaineana, and between 159.71 and 168.01 kJ/mol for Nauclea diderrichii, according to the KAS and FWO methods. The mean values of the enthalpy of activation (ΔH) range from 166 to 167 kJ/mol for Aucoumea klaineana, and from 154 to 164 kJ/mol for Nauclea diderrichii. The high values of the Gibbs free energy (ΔG) reflect the non-spontaneous nature of the thermal decomposition mechanisms, requiring a significant external energy input. In addition, the analysis of the ATG/DTG curves highlights a more marked devolatilization of Aucoumea klaineana at high temperatures, suggesting a high content of volatile compounds, while Nauclea diderrichii has a higher biochar yield, in connection with a more thermally stable residual lignin. The deconvolution of the DTG peaks confirms this better thermal stability for Nauclea diderrichii, while Aucoumea klaineana is distinguished by a higher thermal power density, revealing a more reactive degradation kinetics. All of these results highlight differentiated thermal behaviours between the two species, directing their respective suitability towards different recovery channels, such as the production of biochar or bioenergy.