In-depth kinetic and thermodynamic evaluation of agricultural biomass gasification in CO2 atmosphere using TGA and pseudo-component deconvolution

The kinetics and thermodynamics of CO2 gasification for four agricultural residues: sunflower hulls (SH), cherry seeds (CS), tomato crop residues (TC) and buckwheat hulls (BH) were investigated using thermogravimetric analysis coupled with multi-peak bi-Gaussian deconvolution. The process was resolved into three pseudo-components for pyrolysis, with two more pseudo-components for subsequent char gasification. Notably, lignin-rich biomass matrices were found to exert an inhibitory effect on the decomposition of hemicellulose and cellulose. Activation energies determined using the Kissinger–Akahira–Sunose method ranged from 159 to 204 kJ/mol for pyrolysis and 236 to 445 kJ/mol for gasification. Master plot analysis revealed that the pyrolysis of pseudo-components is predominantly governed by order reaction models F2/3, F3/4, F1, F2 and F3, with a distinct transition toward diffusion-controlled mechanisms. Consequently, char gasification is governed by different mechanisms: BH is strictly controlled by reaction-order models (F3/4), whereas TC is entirely dominated by diffusion (D3). In contrast, SH and CS exhibit transitional behaviour between reaction-order (F2, F3/2 and F2/3) and diffusion-limited kinetics. Thermodynamic analysis revealed that gasification is strongly endothermic (enthalpy change up to 800 kJ/mol) and non-spontaneous (300–400 kJ/mol). The significant variability in activation entropy, ranging from highly positive (∼300 J/mol·K for CS) to negative (SH and TC), highlights divergent structural evolutions from ordered char to disordered porous matrices. These results demonstrate that feedstock-specific mineral catalysis, particularly the high Ca content in TC, primarily reduces the energy barriers, providing a rigorous framework for optimising CO2-assisted biomass conversion.