title: Thermal decomposition of sewage sludge under N2, CO2and air: Gas characterization and kinetic analysis.
abstract: Thermochemical valorisation processes that allow energy to be recovered from sewage sludge, such as pyrolysis and gasification, have demonstrated great potential as convenient alternatives to conventional sewage sludge disposal technologies. Moreover, these processes may benefit from CO2recycling. Today, the scaling up of these technologies requires an advanced knowledge of the reactivity of sewage sludge and the characteristics of the products, specific to the thermochemical process. In this study the behaviour of sewage sludge during thermochemical conversion, under different atmospheres (N2, CO2and air), was studied, using TGA-FTIR, in order to understand the effects of different atmospheric gases on the kinetics of degradation and on the gaseous products. The different steps observed during the solid degradation were related with the production of different gaseous compounds. A higher oxidative degree of the atmosphere surrounding the sample resulted in higher reaction rates and a shift of the degradation mechanisms to lower temperatures, especially for the mechanisms taking place at temperatures above 400 °C. Finally, a multiple first-order reaction model was proposed to compare the kinetic parameters obtained under different atmospheres. Overall, the highest activation energies were obtained for combustion. This work proves that CO2, an intermediate oxidative atmosphere between N2and air, results in an intermediate behaviour (intermediate peaks in the derivative thermogravimetric curves and intermediate activation energies) during the thermochemical decomposition of sewage sludge. Overall, it can be concluded that the kinetics of these different processes require a different approach for their scaling up and specific consideration of their characteristic reaction temperatures and rates should be evaluated.
authors.affiliation: University of Johannesburg, Chemical Engineering Department, Doorfontein Campus, PO Box 17011, Doorfontein, 2028, Johannesburg, South Africa; University of Johannesburg, Department of Civil Engineering Science, Kingsway Campus, PO Box 524, Auckland Park, 2006, Johannesburg, South Africa. Electronic address: belen.hernandez313@gmail.com.
authors.forename: Ana Belén
authors.initials: AB
authors.lastname: Hernández
authors.affiliation: University of Johannesburg, Department of Civil Engineering Science, Kingsway Campus, PO Box 524, Auckland Park, 2006, Johannesburg, South Africa.
authors.forename: Felix
authors.initials: F
authors.lastname: Okonta
authors.affiliation: University of Johannesburg, Chemical Engineering Department, Doorfontein Campus, PO Box 17011, Doorfontein, 2028, Johannesburg, South Africa.
authors.forename: Ntuli
authors.initials: N
authors.lastname: Freeman
mesh_terms: D001272:Atmosphere; D005740:Gases; D007700:Kinetics; D012722:Sewage; D013696:Temperature
chemical_list: D005740:Gases; D012722:Sewage
keywords: Bioenergy; Pollutant precursors; Sewage sludge; Thermogravimetric analysis; Waste treatment; Waste-to energy