Pyrolysis processes in lemon and pomegranate peels: kinetics and mechanisms

Abstract

This article reports a comprehensive investigation of structural aspects and pyrolysis kinetics of lemon and pomegranate peels. The proximate and CHNS analyses reflect distinct biochemical compositions, with FTIR spectra confirming the presence of specific organic components such as hydroxyl groups in cellulose and lignin, acetyl ester units in hemicellulose, alcohols and esters, aromatic compounds, carbonyl groups in hemicellulose and polyphenols. By assessing the morphologies and elemental content, SEM–EDX analysis helps to ascertain their suitability for bioenergy applications and their performance in the pyrolysis process. Thermogravimetric analysis (TGA) along with the differential thermogravimetric analysis (DTGA) show intricate pyrolysis profiles for lemon as well as pomegranate peels. The generalized linear integral isoconversional method reveals four activation energy dependency patterns: for initial moisture removal, lemon and pomegranate peels require 48 and 26 kJ/mol effective activation energies. Moreover, lemon peels require 168, 221, and 491 kJ/mol of energy to thermally degrade pseudo-hemicellulose, pseudo-cellulose, and pseudo-lignin, while pomegranate peels require 134, 215, and 236 kJ/mol, offering mechanistic insights into the complex pyrolysis mechanisms within both the peels. The mechanistic function F(α, T) based on the advanced kinetic approach suggests that the pyrolysis processes in lemon and pomegranate peels predominantly follow the complicated diffusion and nucleation/growth mechanisms. The detailed mechanistic information regarding the pyrolysis of both peels is reported in the present study and their practical valuation in regard to the sustainable conversion of waste into bioenergy and materials is discussed.