Investigating the kinetics and biofuel properties of Alstonia congensis and Ceiba pentandra via torrefaction

Highlights: • Torrefaction rate equations for Araba and Ahun were derived. • The derived n values for the samples indicate their high reactivity. • Coats-Redfern method is inconsistent in the evaluation of the samples' kinetic parameters. • Mass yield during torrefaction of Araba and Ahun are similar to other biomass samples. Alstonia congensis (Ahun) and Ceiba pentandra (Araba) were chosen as representations of tropical wood in this study. The use of untreated wood for energy recovery could lead to a high loss in efficiency. One way of circumventing this in a developing country such as Nigeria is by exposing the fuel materials to a pre-treatment, such as torrefaction, prior to deployment. Attempts were made to improve the combustion properties of these resources and also to investigate their torrefaction kinetics. Derivations of kinetic parameters using Coats-Redfern method were discontinued due to inconsistent results. A non-linear regression method was then employed and the results compared to the average value obtained by the FWO method, which was considered more viable than the Coats-Redfern method. The kinetic parameters ($E_a$,$A$ and n) derived by the regression method are 134.45 kJ/mol, 1.83E+13 min⁻¹ and 2.15, respectively, for Araba and 143.38 kJ/mol, 1.90E+10 min⁻¹ and 2.28, respectively, for Ahun. The thermal behaviour of the samples showed that a lower mass yield resulted in a lower energy yield, while the heating values increased with the temperature of torrefaction. The results obtained in this study affirm the possibility of obtaining an optimum conversion of these resources for energy recovery.