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. © 2013 Elsevier B.V;