Abstract: To investigate the effects of H₂ co-firing on combustion and NO_x emissions in a 600 MW tangentially fired boiler operating at an ultra-low load of 120 MW, numerical simulations were conducted under different H₂ co-firing ratios (heat-input basis) and injection locations. The effects on furnace temperature distribution, flue-gas composition, unburned carbon in fly ash, and NO_x emissions were analyzed. When H₂ was co-fired through both B and C burner layers, increasing the H₂ ratio from 0% to 20% raised the average temperature in the main combustion zone from 1321 to 1380 K and reduced unburned carbon in fly ash from 1.24% to 0.81%, thereby improving combustion stability. At 25% H₂, the temperature decreased and unburned carbon increased slightly compared with the 20% case. As the H₂ ratio increased from 0% to 25%, the furnace-outlet NO_x concentration and CO₂ mole fraction decreased by 25.8% and 25.5%, respectively, while the H₂O mole fraction increased by 31.8%. At 20% H₂, C-layer injection increased the average temperature in the main combustion zone by 71 K compared with pure coal, while reducing unburned carbon and NO_x concentration to 0.71% and 262 mg/m³, respectively. Overall, 20% H₂ co-firing through the C layer showed relatively favorable performance in improving temperature distribution, promoting coal burnout, enhancing combustion stability, and reducing NO_x emissions under low-load conditions.