Pyrolysis-induced lignin transformation and its thermo-kinetic implications for biochar development

• Lignin degradation in TPS exhibited peak shift of 48°C, indicating enhanced thermal stability • Activation energy for lignin degradation in TPS enhanced by 73-81% over RPS • Thermodynamic functions characterized for biochar implications • Lignin degradation followed the diffusion (D) reaction mechanism • SEM images collectively illustrate the progressive decomposition of biomass This study presents anin-depth investigation ofthe transformative alterations in the thermo-kinetic compensation behaviour, functional groups, and morphological evolution induced by pyrolytic stress in biomass.Raw pigeon pea stalk was pyrolyzed at 450°C to produce lignin rich biochar.For systematic characterization raw pigeon pea stalk and obtained biochar both weresubjected tothermogravimetric analysis, BET surface area measurement, SEM analysis and FTIR spectroscopy. TGAspectra were resolved into their bio-components (hemicellulose, cellulose and lignin) using deconvolution techniqueanddeconvoluted spectrarevealed a shift in the lignin degradation peak from 444°C to 492°C in the treated pigeon pea stalk (TPS). Lignin in TPS exhibited significantly higher activation energy (154–155 kJ/mol) compared to RPS (29–37 kJ/mol),corresponding to a73-81% increase after treatment, confirming that thermal treatment at 450°C substantially enhances lignin thermal stability.21%reduction in the Gibbs free energy (ΔG) of lignin, coupled witha substantial increase in enthalpy(ΔH),indicatesimproved thermodynamic suitability for subsequent activation. Negative entropy(ΔS) values for both samples reflect reduced molecular disorder during bond cleavage relative to the initial biomass.Both RPS and TPS followed a three-dimensional diffusion-controlled (D3) reaction mechanism during ligninthermal degradation.Overall, this study correlatesthermally induced biopolymer transformationswith biochar functionality, thereby advancing the scientific understandingand potential applications.