Experimental study and modeling analysis of sewage sludge smoldering combustion at different airflow rates

Sewage sludge is a major by-product of wastewater treatment, and its unfavorable properties are frequently a key restriction of disposal technologies, resulting in high costs and ineffective waste management. Smoldering combustion is a new technique for disposing of organic solid waste with high moisture content, which efficiently recovers energy with minimal igniting energy requirements. The objective of this study is to investigate the effects of airflow rate on sewage sludge (SS) smoldering combustion by combining experimental and modeling analyses. Results show that air channeling easily forms at the reactor's edge, intensifying the smoldering reaction and forming a concave smoldering front. The minimum airflow rate required for self-sustaining smoldering is 0.3 cm/s. As the airflow rate increases, convective heat transfer becomes dominant over conduction and radiation, resulting in a surge in smoldering temperature and velocity at 0.6 cm/s, followed by a linear increase. The maximum airflow rate at which the smoldering process can propagate stably during SS disposal is 8 cm/s. The expressions of the smoldering characteristics are obtained by using the activation energy asymptotic approach, and the calculated and experimental values show the same trend of variation, with good agreement at low airflow rate conditions. Sensitivity analysis shows that porosity φ is the most crucial parameter affecting smoldering temperature and velocity.