Intrinsic kinetics, thermodynamic parameters and reaction mechanism of non-isothermal degradation of torrefied Acacia nilotica using isoconversional methods

Abstract

The objective of this work is to examine the suitability of torrefied biomass for bio-energy generation by investigating its physicochemical characteristics, kinetic and thermodynamic parameters as well as reaction mechanism during pyrolysis. Thus, torrefaction of Acacia nilotica was performed in a fixed bed reactor at 220, 250 and 280 °C, with constant residence time (40 min) and heating rate (15 °C/min). Pyrolysis of torrefied biomass obtained at 220 °C (T-220), 250 °C (T-250) and 280 °C (T-280) was performed using thermogravimetric analyzer at three different heating rate viz. 5, 10 and 15 K/min. Further, isoconversional models namely, Kissinger-Akahira-Sunose (KAS), Ozawa-Wall-Flynn (OWF), Friedman and Starink were employed to calculate the kinetic parameters (activation energy and pre-exponential factor) of raw and torrefied biomass. Using kinetic parameters obtained from KAS method, thermodynamic parameters (enthalpy, Gibbs free energy, and entropy) were calculated at a heating rate of 10 K/min. The activation energy for raw and T-220, T-250, and T-280 using KAS method were found to be 221.49, 241.58, 185.06, and 121.83 kJ/mol, respectively. The increase in activation energy of T-220 might be due to a higher percentage of cellulose content in it than raw biomass. Reaction mechanism during pyrolysis of raw and torrefied biomass was predicted using Criado method (Z-master plot). For raw biomass and T-220, diffusion models were followed; however, for T-250 and T-280, random nucleation models were dominant. Overall, results provide a deep understanding of kinetics and improved characteristics of torrefied biomass as good quality solid fuel.