Fast pyrolysis of torrefied sewage sludge in a fluidized bed reactor

The pyrolysis of dry and of torrefied sewage sludge in a lab-scale fluidized bed reactor has been studied in order to determine whether torrefaction pre-treatment could enhance the properties of the liquid product obtained after pyrolysis. The aim of this work is to evaluate the influence of the torrefaction temperature (220-320 °C) and average solid residence time (3.6-10.2 min) on the product distribution and the properties of the pyrolysis products. Pyrolysis was conducted at 530 °C with an average solid residence time of 5.7 min and a nitrogen volumetric flow per reactor area of 0.074 m3 (STP) m−2 s−1 (measured at 0 °C and 1.01·105 Pa). The experimental results show that torrefaction pre-treatment affects the pyrolysis liquid product, although it does not improve the homogeneity of the liquid. Specifically, it reduces the yields of water and the liquid aqueous phase obtained in the pyrolysis step, especially after torrefaction under the most severe conditions, but it does not have a great effect on the properties of the liquid organic phases obtained. The cumulative yields of gas and organic compounds from the two-step process are not different from the yields obtained from one-step pyrolysis.