Influence of Freeboard Temperature, Fluidization Velocity, and Particle Size on Tar Production and Composition during the Air Gasification of Sewage Sludge

The influence of freeboard temperature, fluidization velocity, and particle size on tar production and composition during the air gasification of dried sewage sludge has been researched using a bench-scale gasifier. Of the operating variables analyzed, the results obtained from a statistical analysis (ANOVA and PCA) indicate that only the freeboard temperature has a significant influence on tar production. The results also show that freeboard temperature is the key factor that influences the tar composition. A high freeboard temperature reduces the amount of tar produced, but the tar compounds obtained are mainly PAH, which could require a more complicated secondary treatment for tar removal. Nevertheless, an increase in the freeboard temperature value implies a substantial increase in both low heating value and cold gas efficiency (6.5% and 10.8%, respectively, when the required freeboard temperature was increased from 873 to 1123 K). Future studies should focus on testing/developing catalytic materials able to degrade (in a secondary treatment) tar with a high content of PAH compounds.