Pyrolysis of transformer insulating paperboard: determining kinetics, reaction mechanism, thermodynamic characteristics and volatile products using TG-FTIR and Py-GC/MS analyses

Transformer insulating paperboard (TIPB) is widely used in the power transformers. It is crucial to study the pyrolysis of TIPB for energy conservation and risk assessment purposes. As TIPB is made of unbleached sulphate wood pulp, with cellulose as the main component and removing lignin, its pyrolysis behaviours are different from general biomass. In this paper, thermogravimetric (TG) experiments are firstly used to investigate the pyrolysis characteristics of TIPB at multiple heating rates. The results show that the pyrolysis temperature range is in between those of hemicellulose and cellulose. The pyrolysis residue yield is approximately 25%, which is close to the char yield of hemicellulose and much higher than the one of cellulose. The average activation energy calculated using model-free methods is 187.79 kJ mol−1, being close to the activation energy of cellulose. The most suitable average activation energy (E), pre-exponential factor (lnA) and kinetic model of TIPB pyrolysis are determined as 183.84 kJ mol−1, 30.60 s−1 and gα=(1-α)-1/2-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$g\left( \alpha \right) = (1 - \alpha )^{ - 1/2} - 1$$\end{document}, respectively. The FTIR analysis indicates that the pyrolysis volatile products are mainly CO2, H2O and the functional groups containing C=C and C=O bond. The production of CO2 is the highest among all products, while the highest production of organic products belongs to the C=C functional groups of aromatics. The CO2 is produced by the decomposition of cellulose at low temperature and aromatic condensation reactions at high temperature. Combining the results of Py-GC/MS and FTIR shows that the contents of phenols, furans and carbohydrates are lower than the ones of aromatics excluding phenols. The occurrences of ketones, furans, aldehydes and carbohydrates denote the pyrolysis of cellulose, while the palmitic acid proves the existence of hemicellulose in TIPB. The products contain substances with high heat values including the toluene and palmitic acid. Finally, the enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) are calculated. The difference between E and ΔH is ~ 5 kJ mol−1, indicating that the pyrolysis products are easily formed. ΔS drops to negative values when the conversion rate is higher than 0.4, at which point the reaction system reaches thermal equilibrium. ΔG increases with conversion rate, indicating that the total absorbed heat increases during pyrolysis.