Technical and Energetic Assessment of a Three-Stage Thermochemical Treatment for Sewage Sludge

A three-stage thermochemical process comprising torrefaction,
pyrolysis,
and char activation is proposed for the treatment of dry sewage sludge
or biomass materials. To assess the feasibility of the process, lab-scale
experiments were carried out with dried sewage sludge as feedstock,
and mass and energy balances were calculated. In the process, 19.3%
of the sewage sludge initial weight was transformed into a bio-oil
with three distinct phases and reduced water content (66.1% of water
content in the aqueous phase compared to 73.8% in a single-step fast
pyrolysis). The product gases had a high H2S content but
also enough heating value to be combusted. After being activated by
the torrefaction vapors, the solid fraction (48.2% of the initial
sludge weight) showed certain pore development and might be suitable
for adsorption applications. Regarding the energy balance, it was
found that the combustion of part of the product gas would provide
the necessary heat to drive the process (1019 kJ/kg of dry sewage
sludge).