Smouldering combustion of sewage sludge: Volumetric scale-up, product characterization, and economic analysis

Abstract

This contribution reports volumetric scale-up, full product characterization, and the economic performance of sewage sludge (SS) treatment via self-sustaining smouldering combustion. Here, we firstly conducted laboratory-scale smouldering experiments, using a local SS supported by sawdust as an auxiliary fuel, to identify optimized operating conditions. Under a Darcy air flux of 3.5 cm/s, the highest wet SS destruction rate of 44.00 kg/(h·m²) is achieved when the mass ratio of sand/SS/sawdust is 18/5/1. These optimized conditions were then employed in a pilot-scale experiment, the result of which suggests a successful scale-up. Systematic characterization of the smouldering products from the laboratory-scale experiment under optimized conditions was then carried out. The results suggest that the content of total organic carbon in the ash residue is very low (3.81 wt%, dry basis), indicating effective destruction of the SS via smouldering. The condensed liquid is dominated by water (96.11 wt%), with organic carbon content of 0.33 wt%. These organic compounds are rich in N-/O-containing monocyclic aromatic and heterocyclic compounds. The non-condensable flue gas mainly consists of O₂ (16.08 vol%), CO₂ (3.72 vol%), CO (8964.69 mg/Nm³), CH₄ (93.38 mg/Nm³), and non-methane hydrocarbon (298.26 mg/Nm³) rich in volatile organic compounds such as benzenes, aldehydes, furans, ketones, and alkanes/alkenes, necessitating a dedicated flue gas treatment facility. The economic analysis demonstrates that for the treatment of SS, the minimum charging price for running a typical smouldering project is 325.5 CNY/ton (wet basis), which is considerably competitive compared with conventional technologies.