Pyrolysis of cashew nutshell residues for bioenergy and renewable chemicals: Kinetics, thermodynamics, and volatile products

Abstract

The present study's motivation and novelty are related to the potential of raw (RCNS) and pressed (PCNS) cashew nutshell residues for producing bioenergy and renewable chemicals through their physicochemical characterization and pyrolysis processing (multicomponent kinetic analysis, thermodynamic study, and volatile product analysis). A thermogravimetric analyzer and an analytical pyrolyzer coupled with gas chromatographymass spectrometry (Py–GC/MS) were used to perform the pyrolysis reactions. The pyrolysis behavior of RCNS and PCNS was accurately modeled with the help of the Asym2sig deconvolution function through five and four parallel devolatilization events, respectively. The average activation energies for the pyrolysis of RCNS and PCNS fell in the ranges of 63.8 − 249.3 and 91.1 − 167.4 kJ mol−1, respectively, as determined by four isoconversional methods (Friedman, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink). Pre-exponential factors ranging from 4.2 × 108 to 6.9 × 1016 min−1 (RCNS) and 4.8 × 108 to 3.2 × 1011 min−1 (PCNS) were estimated from the kinetic compensation effect. The master plots method evidenced that the most likely reaction models involved in the pyrolysis pertain to the nucleation-growth and n-order reaction mechanisms. Based on the multiple kinetic triplets acquired, the verification step of the summative rate expressions indicated excellent agreement between the simulated behavior and the experimental data, with a minimum quality of fit of 93.1%. The pyrolysis route can valorize both cashew nutshell residues to obtain renewable chemicals promoting the circular economy, as verified by Py-GC/MS analysis. Aliphatic hydrocarbons were the dominant components of the condensable volatile products at 650 °C, while reaction temperatures of 450 and 550 °C favored the production of oxygenated compounds. Due to the low potential energy barrier, the thermodynamic study attests to the viability of converting the studied residues into valuable products. The present results play an essential role in the utility of both cashew nutshell residues as inexpensive feedstocks for pyrolysis, possibly leading to bioenergy and biobased chemicals, which fall under the principle of valorization of lignocellulosic residues.