Kinetic Modeling of Sewage Sludge Combustion and Gasification for Energy Generation

Abstract

Sewage sludge is a waste that is difficult to valorize due to its high ash content and presence of pollutants. Thermal conversion of sewage sludge in waste-to-energy systems allows to add value to this waste by exploiting its energy content and significantly reducing the amount of final waste to be disposed in landfill, but these processes must be modeled in detail to obtain reliable results. The GasDS simulation suite can be used for such purpose since it is an advanced tool for the detailed simulation of complex kinetic, mass, and energy transfer phenomena happening inside of gas-solid reacting systems. The peculiar characteristics of sewage sludge both in terms of chemical-physical properties and thermal decomposition behavior make it necessary to add new information in the GasDS database. A simplified kinetic model based on lumps (sludge, gas, tar, char, H2O) and two consecutive pyrolysis reactions is proposed and fitted on experimental data, leading to a reaction scheme that can be implemented in the modeling suite. Simulations of air-based sewage sludge thermal conversion systems working at different degrees of oxidation severity are performed, and results regarding flowrate, temperature, and composition of gaseous effluent are shown. Aspen HYSYS simulation software is used to model waste-to-energy systems for the production of electric power. Key performance indicators such as the specific net power output, energy yield, and CO2 emissions are shown. Gasification-based combined cycles perform better with increasing ER and perform worse than combustion-based Rankine cycles at low ER values.