Coconut fiber pyrolysis decomposition kinetics applying single- and multi-step reaction models

This work aims to evaluate the thermal decomposition kinetics of coconut fiber in a thermogravimetric analyzer using four heating rates (5 - 20 degrees C/min) in nitrogen atmosphere. Three decomposition stages were identified (dehydration, pyrolysis, and carbonization) and the pyrolysis kinetic was performed considering two reaction models (single- and multi-step). In the first model, the apparent activation energy (94.5-210.8 kJ/mol) was determined using five isoconversional methods, and the reaction model representative was the three-dimensional diffusion model, f(alpha) = (3/2) (1-alpha)(2/3) [1 - (1-alpha)(1)(/)(3)](-1), obtained using the master plots method. Through the linearization method of the conversion rate equation, the global activation energy found was 129.8 kJ/mol and the pre-exponential factor (log A) was 18.8 1/s. Due to the complexity of the biomass decomposition, the second model, three independent parallel reactions scheme (IPRS), was applied to describe each pseudo-component (hemicellulose, cellulose, and lignin). It was possible to obtain activation energies from 79.1-196.3 kJ/mol, pre-exponential factors (log A) from 3.1-15.0 1/s, volatilized fractions from 0.3 to 0.5, considering 1st and 2nd reaction orders. Finally, the modeling of conversions and conversion rates of both approaches was in a good agreement with the experimental data, however, the second model proved accuracy in describing the coconut fiber pyrolysis.