The Effects of Secondary Air Supply on the In-Furnace Combustion Characteristics of Briquettes with Different Moisture Contents

This study investigates the effects of secondary air supply on the combustion characteristics of briquettes with different moisture contents using a coupled FLIC–Fluent framework. FLIC was employed to simulate drying, pyrolysis, volatile release, and char oxidation in the fixed bed, and the resulting bed outlet temperature, velocity, and gas composition were transferred to Fluent as inlet boundary conditions for three-dimensional furnace simulations. At a constant total air supply, all-primary-air operation was compared with staged primary–secondary air supply for briquettes with moisture contents of 10%, 20%, and 30%. Increasing moisture content prolonged drying and preheating, reduced the bed outlet temperature and gas velocity, and weakened fixed-bed combustion. Without secondary air, the furnace’s high-temperature region progressively decreased, while relatively low-temperature regions expanded, and the mean furnace temperature declined from 1125 to 960 K as moisture content increased from 10% to 30%. Although CO release from the fuel bed decreased at higher moisture content, lower furnace temperatures and insufficient gas mixing suppressed subsequent CO oxidation, increasing furnace outlet CO from 820 to 1780 ppm. Redistributing 23.6% of the total combustion air as secondary air improved oxygen–fuel mixing and gas-phase burnout, with a stronger effect at higher moisture content. At 30% moisture, secondary air increased the mean furnace temperature from 960 to 1045 K, reduced outlet CO from 1780 to 980 ppm by 44.9%, and increased the volume fraction above 900 K by approximately 21%. Model validation showed relative errors below 1.0% for mean furnace temperature and below 5.0% for outlet CO and O₂, confirming the reliability of the coupled model. These results demonstrate that the investigated air-staging configuration can effectively mitigate the deterioration of furnace combustion caused by high briquette moisture content.