Study on polymer-containing oily sludge as a potential fuel by combustion thermochemistry

Abstract

The environmental and resource issues posed by oil sludge have sparked a renewed interest in cleaner and more efficient oil production. The investigation on the combustion process of polymer-containing oily sludge (POS) is expected to open up new markets for POS as an alternative fuel. TGA was used to examine the combustion behaviour, kinetics, and thermodynamics of POS, and X-ray fluorescence (XRF) was used to measure the chemical composition of POS ash to predict ash fouling and slagging. Based on the TG-DTG curves, the mass loss of POS in combustion studies was split into four major areas. The combined calculation method of model-free methods, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), was applied to determine the activation energies from stage 1 to stage 4, which were 47.89 kJ/mol, 66.09 kJ/mol, 190.55 kJ/mol, and 98.37 kJ/mol, respectively. A four-stage kinetic model (D4→D2→D3→R2) was established to describe the combustion process of POS. Compared to other conventional sludges, POS has a relatively low activation energy and is more susceptible to thermochemical transformations. The difference between the enthalpy change (Δ$H$) and activation energy ($E$) indicated that the reactions benefit the formation of the activated complex. Meanwhile, the change in entropy (Δ$S$) implied the thermodynamic disequilibrium in the 3rd stage of POS and completion of the thermal conversion at the end of the reaction. The change in Gibbs free energy (ΔG) increased in a stepwise manner and its lower average value reflected the greater reaction favourability of combustion for POS. The composition of POS ash was mainly metal oxides, with the slagging index, fouling index, and slagging viscosity index of 1.0, 9.1, and 32.0, respectively. POS ash has high fouling and slagging, with special attention in the combustion application.