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).