Study of catalytic post-treatment of the vapours from sewage sludge pyrolysis by means of γ-Al2O3

The present work describes an experimental study on sewage sludge pyrolysis in a Fluidised Bed Reactor (FluBR) and the effect on the pyrolysis liquid product of passing the hot exhaust (pyrolysis) vapours through a Fixed Bed Reactor (FixBR) filled with gamma-alumina (γ-Al2O3). Experiments have been done at temperatures of 450 and 550 °C (FluBR) and 400 and 500 °C (FixBR). In the FixBR experiments, the bed was filled with inert sand and two different bed quantities of γ-Al2O3 (97.3 g, 194.6 g) giving a weight hour space velocity (WHSV) of between 0.6 and 1.8 h−1. By means of this treatment, the pyrolysis liquid phase separation was improved, obtaining a liquid with two phases (organic and aqueous) instead of the three (two organic and an aqueous phase) usually obtained in sewage sludge pyrolysis without applying γ-Al2O3. This organic phase is quite interesting from the point of view of a possible application as bio-oil. Although the product distribution results using γ-Al2O3 show a slight liquid yield decrease for both temperatures (compared to sewage sludge pyrolysis without applying γ-Al2O3), there is a clear increase in the organic phase yield at 450 °C. Therefore, γ-Al2O3 application makes it possible to lower the pyrolysis temperature in order to maximise the organic phase in the sewage sludge pyrolysis liquid. Furthermore, a general improvement in the higher heating value of the organic phases has been achieved.