Numerical Assessment of the Effects of Oxygen-Enriched Secondary Air on Fouling-Related Combustion Conditions in a Grate Furnace

Abstract Heat transfer surface fouling severely threatens the safe and efficient operation of municipal solid waste (MSW) grate incinerators and is closely linked to local oxygen-deficient combustion atmospheres. Oxygen-enriched secondary air has been investigated as an approach for modifying furnace combustion conditions; however, its influence on fouling-related combustion conditions remains insufficiently understood. This study adopts computational fluid dynamics (CFD) to investigate combustion behaviors under four secondary air oxygen mass fraction conditions: 0.21, 0.26, 0.31, and 0.36. The O2 and CO characteristics, temperature distribution, and near-wall gas phase features are compared. Simulation results demonstrate spatially nonuniform responses to secondary air oxygen enrichment. Increasing the inlet oxygen fraction reshapes the overall furnace oxygen field, yet near-wall monitoring points exhibit no monotonic increase in local oxygen content. The CO/CO2 and H2/H2O ratios stay at very low levels for all operating scenarios, and the temperature response is also spatially nonuniform. Since changing the secondary air O2/N2 composition simultaneously changes mixture density, mass flow rate, momentum flux, Reynolds number, and total oxygen supply, the obtained results reflect combined system responses rather than the independent effect of oxygen concentration. The present CFD model focuses on gas-phase combustion and excludes detailed particle transport, ash transformation, mineral phase evolution, and deposition mechanisms; hence, it only assesses relative variations of fouling-related combustion conditions and cannot directly quantify ash deposition rates or fouling suppression performance. No single optimal secondary air oxygen enrichment level is determined among the investigated cases. This work provides a numerical basis for assessing the influence of oxygen-enriched secondary air on fouling-related combustion conditions in the grate furnaces.