Enhancing Selective Catalytic Reduction Performance in a Coal-Fired Unit over a Wide Load Range via Static Mixer-Assisted Reactive Mixing: A Full-Process Furnace-to-SCR CFD Analysis

A 660 MW coal-fired unit was investigated to clarify the combustion behavior over a wide load range and the effects of static mixers on selective catalytic reduction (SCR) performance. A full-process CFD model covering the furnace, rear pass duct, and SCR system was established, and the combustion characteristics, NOx formation, and SCR performance were analyzed over a boiler load range of 25–100%. The results showed that, as the boiler load decreased, the furnace heat release weakened, the high-temperature zone contracted, and the flame center shifted downward, with more pronounced flame maldistribution at 25% load. The average NOx concentration at the SCR inlet first decreased and then increased with decreasing boiler load, reaching a minimum at 75% load. Without a static mixer, the NOx concentration at the SCR inlet increased from 238 mg/Nm3 at 100% load to 312 mg/Nm3 at 25% load. After a static mixer was installed, the distance required for NH3 homogenization downstream of the ammonia injection grid was markedly shortened, and the uniformity of the velocity, NH3 concentration, and temperature fields at the SCR catalyst inlet was improved. In particular, the coefficient of variation in NH3 concentration decreased from about 4–5% to about 2–3%, while the denitrification efficiency increased by about 1–5 percentage points compared with the case without a static mixer. The variation in denitrification efficiency among different boiler loads was also significantly reduced, indicating improved adaptability of the SCR system to wide-load operation. Among the tested configurations, the static mixer with small blades and a larger blade angle relative to the vertical plane showed the best overall performance. These results provide useful guidance for SCR system improvement in coal-fired units operating over a wide load range.