Pyrolysis characteristic and kinetic analysis of sewage sludge using model-free and master plots methods

Abstract

The pyrolysis kinetics of two types of sewage sludge (municipal sludge and pharmaceutical sludge) were studied by thermogravimetry. The collected samples were pretreated and tested with nitrogen as carrier gas at different heating rates (10, 20, 30, and 40 °C/min) using a thermogravimetric analyzer. Model-free methods and master plots method were used to calculate the pyrolysis kinetic parameters of the samples. TG/DTG results showed that the thermal decomposition of the two types of sewage sludge may be divided into three stages (Dehydration, Decomposition, and Coking stage). The activation energies of the two kinds of sewage sludge were 366.58 KJ/mol (municipal sludge) and 268.36 KJ/mol (pharmaceutical sludge), respectively. The pyrolysis mechanism of two kinds of sewage sludge was discussed by the master plots method. The results showed that under a low conversion rate, the phase interface reaction model can better explain the pyrolysis mechanism of pharmaceutical sludge; under high conversion rate, the nucleation and growth model was more suitable. For municipal sludge, power law model was most suitable in the whole range. The pyrolysis characteristic of the samples was further studied. The results showed that the organic matter in sewage sludge can be decomposed into bio-oil and bio-gas at elevated temperatures and can be used as an attractive energy source.