Experimental and Numerical Investigation of Biosolid Gasification: Equilibrium-Based Modeling with Emphasis on the Effects of Different Pretreatment Methods

In the present study, a modified equilibrium model was developed by Aspen Plus to simulate the gasification of biosolids (treated sewage sludge) in an atmospheric fluidized bed reactor (FBR) using air as a gasification agent. The model, which is based on the Gibbs free energy minimization applying the restricted equilibrium method, was calibrated and validated against a set of experimental data obtained in a pre-pilot-scale FBR. The comparison between the simulation results and the experimental data showed satisfactory agreement, in particular, with respect to the content of combustible gases in the syngas and the heating value. The main objective of the work was to assess the impact of different sludge pretreatment methods (i.e., torrefaction, anaerobic digestion, carbonization, hydrothermal treatment, and copelletization of sewage sludge with solid fuels) and the key operating conditions (gasification temperature and equivalence ratio) on both the syngas quality (composition and heating value) and the performance of the gasifier in terms of the carbon conversion efficiency (CCE), syngas yield (YG), and cold gas efficiency (CGE) under the same operating conditions. Global sensitivity analysis was first performed by varying the gasification temperature and the equivalence ratio (ER). Then, the impact of sludge pretreatments was evaluated by changing the sludge composition in the model in line with experimental data from the pertinent literature. Results point out that the torrefaction pretreatment in combination with gasification exhibits promising results in terms of gas quality (i.e., higher H2/CO ratio and lower CO2/CO ratio) and LHV, even though the carbon conversion efficiency (CCE) slightly decreases presumably because of the lower volatile content in the torrefied material with respect to the parent one.