The Staged Thermal Conversion of Sewage Sludge in the Presence of Oxygen

This study compares a staged thermal processing of the sewage sludge, with single step, integrated thermal processing. The aim of this study is to find the optimal conditions for drying and subsequently for carbonization/torrefaction of sewage sludge, regarding the energy consumption. This study presents the results of the drying tests performed at laboratory scale convective dryer for different parameters of drying agent (air). The tests were focused on finding and developing a method of drying that allows to minimize the energy consumption. Subsequently, both dry and vapothermal torrefaction was performed in the presence of oxygen. The kinetics of drying, using low quality heat as well as the properties of products and by-products of torrefaction in both regimes were determined. The process was characterized by mass yield and energy yield in both of the cases. There has been only scarce amount of literature studies published on the torrefaction of sewage sludge so far, without a detailed study of the composition of the torgas and tars of such origin. Performed study enables a comparison of two distinct scenarios of the processing, i.e., drying followed by dry torrefaction with a single stage of vapothermal torrefaction.