To address the challenges of combustion stability and pollutant control during biomass co-combustion in coal-fired boilers under deep peak regulation, a numerical simulation study was conducted on a 660 MW front-and-rear wall opposed-fired pulverized coal boiler using computational fluid dynamics (CFD) technology. First, the reliability of the numerical model was validated under the Boiler Maximum Continuous Rating (BMCR) condition by comparing the simulated results of furnace outlet temperature and NO concentration with on-site operational data, with relative errors of 1.2% and 1.9%, respectively, both within the acceptable range of 5%. Subsequently, the effects of different biomass co-combustion ratios (0%, 5%, 10%, 15%, 20%) and injection positions (primary air nozzles of lower, middle, and upper burners) on the in-furnace velocity field, temperature field, component distribution (O₂, CO, CO₂), and NO emissions were systematically analyzed. The results indicate that increasing the biomass co-combustion ratio does not alter the overall variation trend of flue gas components but significantly affects their concentrations: the O₂ content at the furnace outlet decreases gradually, while the CO₂ content increases, and the NO emission concentration decreases continuously. A 20% co-combustion ratio is identified as the optimal choice, balancing combustion efficiency and NO reduction. Regarding injection positions, biomass injected at the middle burner’s primary air nozzle achieves the best NO control effect, reducing NO emissions by 22% compared to pure coal combustion. This is attributed to the formation of a stable reducing atmosphere in the main combustion zone, which facilitates NO_x reduction. The research findings provide valuable theoretical references and technical support for the parameter optimization and safe, low-emission operation of biomass co-combustion in large-scale coal-fired boilers.

Biomass co-firing is an effective approach for improving the low-carbon and flexible operation of coal-fired boilers under deep peak-shaving conditions. In this study, a 3D CFD model was established for a subcritical four-corner tangentially fired boiler to investigate the effects of biomass blending percentage, injection position, and boiler load on combustion characteristics and NO formation. The results show that increasing the biomass blending percentage improves the uniformity of the flow and temperature fields, promotes more complete pulverized coal combustion, and enhances boiler combustion efficiency. Among the investigated load conditions, the 70% load shows a relatively more favorable combustion state in terms of flow field distribution, temperature level, and species distribution. Compared with B-layer primary air injection, BC-layer secondary air injection shows a stronger tendency to reduce NO formation under the simulated conditions. These results indicate that proper biomass blending and optimization of the injection position can effectively improve combustion uniformity and suppress NO formation.

To clarify the effects of biomass char injection position on combustion characteristics and NO formation, a three-dimensional CFD model of a 300 MW tangentially fired pulverized coal boiler was developed using ANSYS Fluent. The Realizable k-ε model, discrete phase model, species transport model, and P-1 radiation model were employed to describe the flow, combustion, heat transfer, and NO formation processes. The model was validated against field measurements under pure pulverized coal combustion conditions. Based on the validated model, five co-firing cases were designed by injecting biomass char through primary air nozzles located at five different elevations in the burner zone. The results show that biomass char injection position significantly affects furnace flow organization, heat release distribution, carbon conversion, and NO formation, while maintaining the overall tangential swirling structure. Case C achieved the highest outlet velocity of 10.42 m/s, which was 9.1% higher than that of Case E, and exhibited the lowest CO concentration, indicating improved carbon conversion performance. However, the intensified oxidation environment in Case C promoted fuel-N conversion and resulted in the highest NO concentration of 313 ppm. Case B achieved the highest outlet temperature of 1429 K, which was 6.8% higher than that of Case E. In comparison, Case D maintained a similar CO₂ mole fraction of 0.1546 to Case C, which was 0.1545, while reducing the NO concentration from 313 ppm in Case C to 145 ppm, corresponding to a reduction of 53.7%. Therefore, Case D achieved the most favorable balance between carbon conversion and NO emission control under the investigated conditions. This study demonstrates that biomass char injection position is an important operational parameter for optimizing biomass utilization and reducing NO emissions in tangentially fired pulverized coal boilers.

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.