Probing co-pyrolysis of rice straw with sulfur-donor thiourea: Insights into product evolution, reaction kinetics, thermodynamics, and compensation effects

Abstract

Understanding the synergistic interactions between biomass and sulfur-containing additives during co-pyrolysis is crucial for controlling sulfur transformation pathways and tailoring sulfur-doped carbonaceous materials. In the present study, the pyrolysis conversion of rice straw, thiourea, and their blend were probed using thermogravimetric analysis coupled with evolved gas analysis (TG–FTIR–MS and TG–GC/MS), isoconversional kinetics, master-plots method, and thermodynamic evaluation. The thiourea addition could alter the decomposition behavior, shifting the decomposition peaks from 335 to 348°C for rice straw and 255–266°C for thiourea to 206–241°C and 331–347°C for the blend, indicating intermolecular interactions during co-pyrolysis. Product evolution analysis revealed a transition from oxygenated volatiles (e.g., acetic acid and furan derivatives) to sulfur-containing species such as methanethiol and carbonyl sulfide, demonstrating modified reaction pathways induced by thiourea. Thermokinetics analysis showed that the average E_a of the blend (200 kJ mol⁻¹) exceeded those of individual rice straw (157 kJ mol⁻¹) and thiourea (103 kJ mol⁻¹), reflecting kinetic restructuring and the formation of thermally stabilized intermediates. Master-plots analysis identified a three-dimensional phase-boundary (R3) mechanism as dominant at conversions below 0.65, with deviations at higher conversions due to multi-step reactions. Linear correlations between apparent activation energy and pre-exponential factor indicated the occurrence of kinetic compensation effect, while thermodynamic study revealed the enthalpy–entropy compensation, with compensation temperatures matching experimental conditions. The positive values for enthalpy change (140–201 kJ mol⁻¹) and Gibbs free energy change (141–109 kJ mol⁻¹) indicating the non-spontaneous nature of these conversions. These results provided integrated kinetic, thermodynamic, and mechanistic insights into sulfur–biomass interactions during co-pyrolysis and offered guidance for controlling sulfur transformation in biomass-derived carbonaceous materials.