Thermal pre-treatment of waste municipal sewage sludge for thermochemical valorizations: An experimental study on moisture reduction and sludge modifications

• Thermal pre-treatment of sewage sludge (SS) for their thermochemical valorization. • Experiments (air flow, 75–250 °C) + SS and products characterization + energy study. • Moisture removal complete at 150 °C or higher temperatures. • SS decomposition with solid loss at higher pre-treatment temperatures. • 120–150 °C: SS moisture OK, decomposition limited, allow plant heat recovery. SS valorization by thermochemical conversions (e.g., gasification) for power, heat or fuels production has an interesting development outlook. The literature suggests that SS moisture must be lowered below 20 wt% for conventional thermochemical conversions such as gasification. This work focuses on the experimental investigation of the effects of a thermal pre-treatment of SS with air, addressed to adjusting SS moisture. Semi-batch tests were performed on SS at a laboratory scale (40 g of SS, 2 NL/min air, 2 h, 75–250 °C). The temperature of thermal pre-treatment significantly influenced the elemental composition of solid residue, its moisture content, and the properties of liquid condensate, characterized by unprecedented analyses. Temperatures of 120 °C or higher ensured adequate moisture removal; however, a significant depletion of SS dry solids (C, H, N, S) was quantified above 200 °C. Condensed liquids were beyond Italian legislative limits for direct discharge into the environment. Finally, estimates of the heat contributions to the pre-treatment were performed. The collected results are a first step between fundamental understanding and process design, suggesting that the investigated pre-treatment may fit seamlessly into the schemes of traditional and innovative thermochemical processing plants.