Valorização da silverskin do café por pirólise lenta: modelagem cinética multicomponente e potencial do biocarvão para captura de carbono

Abstract

Slow pyrolysis applied to coffee silverskin, an abundant and underutilized agroindustrial residue, represents a strategic alternative for waste valorization and sustainability promotion. However, the literature still lacks investigations that integrate multicomponent kinetic modeling, the evaluation of thermodynamic activation parameters, and the study of the potential of the biochar generated for carbon capture, especially under experimental conditions that represent scenarios close to large-scale production processes. This study proposes, in an unprecedented way, the application of an integrated and rigorous protocol encompassing the detailed physicochemical characterization of silverskin, including proximate and ultimate analyses, higher heating value, density, Fourier transform infrared spectroscopy, X-ray fluorescence, and thermogravimetric analysis, up to the controlled production of biochars at 450 °C and 550 °C in a fixed-bed reactor, with evaluation of yield, composition, and structure of the obtained products. For an in-depth understanding of the thermal decomposition mechanism, isoconversional methods (Friedman, Flynn-Wall-Ozawa, Kissinger- Akahira-Sunose, and Starink) were employed in conjunction with DTG curve deconvolution using the asymmetric sigmoidal function (Asym2Sig), allowing the determination of the kinetic triplet (activation energy, pre-exponential factor, and reaction model) for each pseudocomponent of the biomass (extractives, hemicellulose, cellulose, and lignin). Prior to kinetic modeling, the physicochemical characterization of the biomass revealed that silverskin presents a high volatile content (73.63%), low ash content (6.64%), and a heating value of 21.03 MJ kg⁻¹. The biochars obtained at 450 °C (B45) and 550 °C (B55) exhibited fixed carbon contents of 50.53% and 52.15%, respectively, accompanied by a marked reduction in the atomic ratios H/C and O/C, indicating a higher degree of aromaticity and thermal stability. Elemental analysis placed the materials in the biochar region with greater thermal and structural stability, indicative of high carbon persistence in the material, a feature favorable for environmental applications such as CO₂ adsorption, contaminant retention, and soil quality improvement. In turn, DTG curve deconvolution performed using the Asym2Sig model allowed the discrimination of pseudocomponents and the application of multicomponent kinetic modeling. The activation energies ranged from 101 to 236 kJ mol⁻¹, following the order P-EX < P-HC < P-CL < P-LG. The isoconversional methods of Friedman, Flynn- Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink showed correlation coefficients higher than 0.98, confirming the robustness of the applied model. The thermodynamic parameters indicated that pyrolysis is an endothermic process (ΔH# > 0) and nonspontaneous (ΔG# > 0), with predominantly negative entropy variations associated with the formation of organized products. The kinetic study of CO₂ adsorption, conducted in TGA, fitted well to the pseudo-second-order model (R² > 0.99), indicating that the process is controlled by the interaction between CO₂ and the active sites of the material. The intraparticle diffusion model suggested three distinct stages (film diffusion, internal diffusion, and equilibrium approach), while the activation energies of 28.02 kJ mol⁻¹ (B45) and 37.04 kJ mol⁻¹ (B55) were consistent with reversible physisorption processes. B55 showed higher adsorptive capacity (44.29 mg g⁻¹ at 30 °C), larger surface area (388.36 m² g⁻¹), and good stability in successive adsorption–desorption cycles, suggesting potential for application in cyclic CO₂ capture processes. Overall, the results demonstrate that slow pyrolysis of silverskin generates biochars with high structural stability and potential for carbon capture, in addition to providing relevant kinetic and thermodynamic insights for a detailed understanding of the process. Thus, the results reinforce the potential of silverskin in biochar production, comprising a low-carbon and promising strategy for mitigating environmental impacts.