Interplay between Densification and Thermochemical Conversion in Sewage Sludge Pyrolysis

Abstract This research investigates sewage sludge pyrolysis as a route for biochar production, comparing in natura and pelletized feedstocks under slow pyrolysis conditions. Experiments were conducted in a batch reactor at 400–500 °C, with a heating rate of 5 °C min–1 and a residence time of 60 min under an inert atmosphere. Product yields, including biochar, bio-oil, and noncondensable gases, were systematically quantified. Product yields and physicochemical properties, including elemental composition, BET surface area, and inorganic composition by X-ray fluorescence (XRF), were determined, supported by SEM/EDS analysis. Pelletization significantly enhanced biochar yield under all operating conditions, increasing the average yield from 52.1% for in natura sludge to 75.1% for pellets, with a maximum of 78.9% at 450 °C. From an operational perspective, 400 °C was identified as the optimal condition, achieving a high biochar yield (77.2%) while requiring a relatively low electrical energy consumption of 1.1 kWh. Under this condition, a mass balance efficiency of approximately 95% was achieved, indicating that most of the initial biomass mass was accounted for among the recovered pyrolysis products. SEM analysis revealed that pelletization promotes a more compact precursor structure, which is preserved after pyrolysis, resulting in denser and more homogeneous biochar. EDS results confirmed the retention of mineral phases typical of sewage sludge, with compositional variations depending on feedstock form. Direct Arrhenius kinetic analysis indicated that, among the evaluated reaction-order models, the fourth-order approximation provided the best fit within the active pyrolysis region. Overall, feedstock densification prior to pyrolysis improves solid yield and enhances the balance between biochar production and energy demand. These findings demonstrate that sludge pelletization is a promising strategy for improving the efficiency and viability of thermochemical conversion processes.