Kinetics, Thermodynamics, and the Reaction Mechanism of Common Nigerian Waste Biomass for Pyrolysis-Based Resource Utilisation

Abstract

Pyrolysis presents a promising approach to harness the bioenergy potential of biomass and promote waste valorisation within a circular economy framework. This study offers a detailed analysis of the thermal characteristics, reaction kinetics, and thermodynamic behaviour of two common waste biomasses in Nigeria, sawdust (SD) and sugarcane bagasse (SB), to assess their bioenergy potential using Thermogravimetric Analysis and the Coats-Redfern integral method. Proximate analyses revealed a high volatile matter content (over 82%) in both feedstocks. The pyrolysis parameters indicated a higher comprehensive pyrolysis (3.486 × 10−6, %3/°C3) and pyrolysis stability (1002.04%/C2) indices for SD, confirming its superior overall thermal reactivity. In contrast, SB demonstrated a superior devolatilisation index (5.0621 × 10−7%/C3) and a lower stability index (869.27%/C2), signifying an intense, concentrated eruption of volatile matter over a narrow temperature window. A multi-stage, segmented approach produced more accurate results than the general single-step model. The diffusion-based D6 model was most suitable for describing the drying and devolatilisation of SD, while the reaction-order R2 model best explained the devolatilisation process of SB. The thermodynamic analysis revealed that formation of the activated complexes for both SD and SB across all stages is strictly non-spontaneous and endothermic, with SD encountering a higher thermal barrier (∆H = 57.57 kJ/mol) than SB (∆H = 37.03 kJ/mol) during the process. These findings indicate that SD possesses properties favourable for fast pyrolysis processes aimed at bio-oil production, whereas SB exhibits characteristics that make it a promising feedstock for biochar-orientated pyrolysis. Further studies involving product yield and quality analyses are recommended to validate these potential applications. These findings provide essential information for designing effective, locally adapted bioenergy systems in Sub-Saharan Africa.