Modelling and simulation of fluidized bed steam-oxygen gasification of sewage sludge using thermochemical equilibrium and experimental data

Abstract

This paper presents a gasifier model to describe steam-oxygen gasification of sewage sludge, as well as simulations to assess the process performance. The gasifier model was set up in the Aspen Plus® software and uses a combined equilibrium and empirical approach. A comparison of the model’s predictions with experimental data showed sufficient accuracy to describe the process The oxygen demand (oxygen ratio) of the gasifier and the cold gas efficiency were found to be highly dependent on various operation parameters, thus a comprehensive sensitivity study was presented. To obtain the relevant gasifier temperature of 850 °C using dried sewage sludge, an oxygen ratio of 0.33 and a cold gas efficiency of 0.65 was calculated. Syngas stoichiometry described by the H2/CO-ratio was found to be adjustable by altering the steam to carbon ratio nSC, which enables to tailor the syngas composition to suit the desired downstream application. The simulations predicted that the water-free syngas has a N2 concentration of 0.04 m3m−3 that originated from fuel-bound nitrogen, which needs to be considered for downstream syngas application. A limestone addition of 0.03 kg limestone per kg fuel was calculated to be the optimum in terms of high cold gas efficiency as well as tar and sulfur reduction. Overall, steam-oxygen gasification was characterized as a promising conversion process.