IR and kinetic study of sewage sludge combustion at different oxygen concentrations

Thermal degradation of sewage sludge disposal is attracting more attention due to the increase in municipal wastewater treatment. In this work the performance of the thermochemical processes of sewage sludge at different oxygen concentrations was investigated by thermogravimetric (TG) and Fourier transform infrared analysis (FTIR) study. The oxygen concentrations were varied systematically from 0 to 20%, representing heating process from pyrolysis to full combustion. The evolutions of surface functional groups in these processes were also investigated by in situ diffuse reflectance infrared Fourier transform spectra (DRIFT), which helped to understand the reaction mechanism during the thermal degradation, especially when the reaction conditions were different. The heating process was divided into four stages, dehydration (below 200 °C), devolatilization (200-400 °C), char combustion (above 400 °C), and secondary devolatilization (above 650 °C). Reaction mechanism and kinetic model was proposed based on the stages of heating process. Oxygen concentration was presented explicitly in the reactions and kinetic equations. The model was then developed for the heating processes at different oxygen concentrations, followed by fittings of kinetic parameters. Some of the parameters in the model were fixed as constants to minimize the number of variations. The fitted model agreed well with the TG curves at different oxygen concentrations and could illustrate the evolution of intermediates and products during the heating process. The developed kinetic model could be further applied for the modeling of sewage sludge pellets combustion considering oxygen diffusion process.