Product distribution and kinetic scheme for the fixed bed thermal decomposition of sewage sludge

In this paper, a new kinetic model for the thermal decomposition of dry sewage sludge was determined. In order to achieve this main objective, various experiments were carried out to collect enough information for the estimation of the different numerical parameters of the model. These experiments include both results from a fixed bed pyrolysis installation and a thermogravimetric analysis device. The experiments allowed for the detailed monitoring of the dynamical evolution of the mass of the sample under investigation, together with the cumulative amounts of tars and permanent gases produced during thermal decomposition of sewage sludge at low heating rates (5–20 °C/min). Solid mass loss during the pyrolysis shows two regions, between 150 °C and 600 °C, where most of the tar is depleted from the solid and non-condensible gases are formed, and a second one between 600 °C and 900 °C where mainly only non-condensible gases are produced. The solid fraction accounted for about 50% of the initial weight, tar around 30% and gases the remaining 20%.Regarding the formation of non-condensible gases from low temperature, a new kinetic scheme was proposed involving an initial decomposition step of the sludge yielding tar and gases as gas phase products and a solid intermediate compound which decomposes at higher temperatures, giving the char fraction and more non-condensible gases. The comparison between the numerical prediction and the experimental results was excellent.