Thermogravimetric investigation of the co-pyrolysis behavior of oregano and walnut biomass blended with lignite

Abstract

Context—The increasing global energy demand and the environmental impacts associated with fossil fuel consumption have intensified the search for sustainable energy alternatives. Biomass has emerged as a promising renewable resource due to its carbon-neutral nature and wide availability. However, its standalone use has certain limitations, whereas co-utilization with coal can modify feedstock properties and thermal degradation behavior. Despite extensive research on biomass–coal co-pyrolysis, studies involving multiple biomass types, particularly aromatic biomass such as oregano, remain limited. Therefore, characterizing the composition-dependent thermal behavior of multi-component biomass–coal systems is needed.Objective—This study aims to investigate the co-pyrolysis behavior of oregano residue, walnut shell, and lignite blends and to evaluate the effects of biomass ratio and heating rate on thermal degradation characteristics, reactivity, and kinetic parameters.Method—Thermogravimetric analysis (TGA) was performed on pure fuels and ternary blends under a nitrogen atmosphere in the temperature range of 30–950 °C at heating rates of 10, 15, 20, and 25 °C·min-1. Proximate and ultimate analyses were conducted to characterize the fuel properties. TG/DTG-derived reactivity indices (, , , and ), originally developed for combustion systems, were employed in this study as comparative decomposition-related reactivity parameters under inert atmosphere conditions. Additionally, activation energies were determined using the Flynn–Wall–Ozawa (FWO) isoconversional method.Results—Biomass samples underwent intense devolatilization at 200–400 °C, whereas lignite showed broader degradation at 400–800 °C. Walnut shell had the highest average Stage II mass loss across the investigated heating-rate conditions (76.68%), followed by oregano residue. In contrast, lignite was less reactive due to its low volatile matter (36.18%) and high ash content (52.77%). Among the blends, O35–W35–L30 showed the highest thermal reactivity, whereas O15–W15–L70 exhibited greater thermal stability and lignite-like behavior. Average activation energies were 207.06 and 160.75 kJ·mol-1 for oregano residue and walnut shell, 69.67–98.72 kJ·mol-1 for the blends, and 52.06 kJ·mol-1 for lignite. Although activation energy decreased with increasing lignite content, these apparent FWO values were not used alone to assess overall thermal reactivity.Conclusion—The findings demonstrate that the thermal degradation behavior of biomass–lignite systems can be effectively controlled by adjusting blending ratios. Biomass-rich blends exhibited more intense devolatilization in the main decomposition region, whereas lignite-rich blends showed broader degradation over higher temperature intervals. These observations demonstrate composition-dependent thermal behavior under TGA conditions but do not independently establish synergistic interactions or improvements in process-scale conversion efficiency. Future studies should compare experimental curves with mass-weighted theoretical TG/DTG curves and include product distribution analyses to evaluate potential interaction effects.