Improving biomass fuel obtained from Brazil nut residues via torrefaction: A case of kinetic and thermodynamic study

The residues of Brazil nuts (Bertholletia excelsa) were evaluated, in their natural state and in torrefied form, for their energy potential after pyrolysis. Samples of shells (BNS) and husks (BNH) were torrefied at 220, 260 and 300 degrees C for 30 and 60 min, and were characterized for their chemical and energetic composition. It was also determined which samples showed, simultaneously, the lowest hemicellulose content and the highest cellulose content after torrefaction via analysis of the decomposition profile at 10 degrees C/min under pyrolysis. The evolution of gases such as H2O, CO2 and/or C3H8, and CH4 was observed by simulated pyrolysis on a microscale in a thermogravimetric analyzer coupled to a mass spectrometer (TGA-MS). In addition, the kinetics of the pyrolysis of untorrefied and torrefied samples were evaluated using the Flynn-Wall-Ozawa (FWO) and Vyazovkin (VYZ) models in order to obtain the activation energy, pre-exponential factor and thermodynamic parameters. The predominant reaction mechanism was determined by Criado Master-plot method. The mean value of the activation energies ranged from 132.24 to 134.95 kJ/mol for BNS, from 135.53 to 138.47 kJ/mol for ST263, from 149.10 to 150.96 kJ/mol for BNH and from 145.83 to 148.14 kJ/mol for HT263. It was found that D2 and D4 diffusion mechanisms are prevalent in the decomposition of untorrefied samples, while torrefied samples were better adjusted to the F1/A2/A3/A4 overlapping nucleation models. High pre-exponential factor values indicated the presence of multiple intermolecular collisional reactions. The thermodynamic parameters evaluated, as well as the variations in enthalpy, Gibbs free energy and entropy, indicated that the evaluated process is endothermic, though requires little energy. As a whole, in view of the optimized parameters and elevation of the thermal stability of the material, the results demonstrate good potential for the use of Brazil nut shells and husks torrefied at 260 degrees C for 30 min as biochar. In addition, pyrolyzed Brazil nut residues also have great potential in the production of bio-oil and biogas, in view of the high content of volatile matter released during the decomposition of the hemicellulose and lignin.