Assessing the Bioenergy Potential of Peanut Shell Waste: High Heating Rate Combustion Behavior and Thermodynamic Analysis

Abstract

This study provides a comprehensive analysis of peanut shell (PnS) combustion behavior using combined physicochemical characterization and non-isothermal thermogravimetric kinetics. To evaluate its potential as a sustainable solid biofuel, PnS was characterized for its proximate and ultimate composition, with its fiber structure analyzed via Van Soest methods and functional groups identified via FTIR spectroscopy. Thermogravimetric analysis (TGA) was performed at high heating rates (20,40,60, and 80 K min−1) to investigate combustion stages under oxidative conditions. The results established PnS as a high-potential energy source, revealing a significant volatile matter content (65.30 wt%) and an exceptionally high heating value (20.87 MJ kg−1), which surpasses many standard agricultural residues. The proximate analysis also indicated a moisture content of 9.61% and an ash content of 6.59%. TGA profiles displayed distinct decomposition stages, with the primary devolatilization occurring between 500 and 700 K. Kinetic analysis was conducted using six model-free methods: Friedman (FR), Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), Starink (STK), Kissinger (K), and Vyazovkin (VY) and the Coats-Redfern model-fitting method. The apparent activation energy Ea was found to vary with conversion (α), reflecting the complex degradation of the lignocellulosic matrix (47.86% cellulose, 28.4% lignin). The activation energy values ranged from approximately 23 kJ mol−1 (VY method at low conversion) to 187 kJ mol−1 (FR method at α=0.5). Model-fitting analysis identified the three-dimensional diffusion (D3) model as the governing reaction mechanism. Thermodynamic analysis indicated positive enthalpy (ΔH:70.7−181.8 kJ mol−1) and Gibbs free energy (ΔG: 116.2−216.7 kJ mol−1), with predominately negative entropy (ΔS), confirming the endothermic and non-spontaneous nature of the reaction activation.