This chapter delves into the combustion of sewage sludge using a vertical counter-current fixed-bed reactor, with specific emphasis on examining the impact of varying airflow rates (1.5, 3.0, and 5.0 Nm3/h) on the overall process. It commences by conducting physicochemical and thermodynamic analyses of the sludge, aiming to establish a fundamental understanding essential for the experimental phase. The findings indicate that biomass degradation takes place through three primary stages: biomass drying, degradation of volatile and organic matter, and decomposition of inorganic matter. The experimental results reveal two distinct phases of sewage sludge combustion: an ignition propagation phase and a subsequent flame propagation phase occurring within the lower bed layers. The latter phase is characterized by a significant increase in temperature profiles, which is attributed to a second flame propagation toward the top of the bed, burning the remaining biomass not consumed in the first propagation due to the flame’s nonuniformity on the horizontal tube surface. The study also examines the impact of different airflow rates on temperature profiles, central bed temperature averages, ignition front velocity, ignition mass rate, and conversion layer thickness.