Sewage sludge torrefaction in a fluidized bed reactor

Abstract

Torrefaction of sewage sludge in a lab-scale fluidized bed reactor was studied in order to know if this pre-treatment could enhance the properties of this waste in subsequent thermochemical processing, such as pyrolysis. The influence was studied of two important torrefaction operational parameters, temperature (220–320°C) and solid residence time (3.6–10.2min), on the product distribution and properties. Taking into account the operation conditions evaluated in this work, torrefaction temperature affects solid product properties at long solid residence times (longer than 6.1min) and that the effect of solid residence time is only significant at the highest temperature (320°C). Severe torrefaction conditions result in the release of bonded water which could enhance some properties of the liquid obtained in the ensuing pyrolysis process. However, the torrefaction pre-treatment also implies that part of the extractives is lost from the raw material. Compared to dry raw sewage sludge, the energy density of sewage sludge after torrefaction increases under certain conditions. The removal of H2O and CO2 during the torrefaction step reduces the O/C ratio in the torrefied solid up to 0.12 (66.70% reduction compared to 0.37 in raw sewage sludge) which could be a benefit for subsequent thermochemical treatments. For example, one of the main drawbacks of pyrolysis liquids is their high oxygen content.