Optimization Strategy via Tiered NH3 Injection for Wide-Proportion and Low-NOx Co-Firing in a 600 MWe Tangential Boiler

NH3/coal co‑firing represents a promising route to reduce CO2 emissions from coal‑fired boilers. Previous studies, however, have mostly adopted uniform NH3 injection across all burners, leaving the optimal co‑firing strategy for achieving safe, complete, and low‑NOx combustion unresolved. In this work, computational fluid dynamics (CFD) simulations were employed to systematically investigate the effects of co‑firing mode, injection position, and NH3 ratio (0–60%) on combustion and NO formation characteristics. For NH3 ratios ≤20%, injecting NH3 exclusively through the lowest burner layer yields the lowest NO emissions. With 20% NH3 introduced at the lowest layer, the outlet NO concentration (339 ppm) is substantially lower than that of uniform injection (408 ppm) and of pure‑coal combustion (419 ppm). When more than 20% NH3 is supplied through a single burner layer, the primary‑air velocity becomes excessively high, impairing the mixing with secondary air and leading to incomplete coal combustion. Hence, the NH3 fraction in any individual burner layer should be capped at 20% to maintain stable and safe operation. For boiler‑wide NH3 co‑firing ratios exceeding 20%, a multi‑layer injection strategy becomes necessary; under such conditions, NH3 should still be injected preferentially from lower burner layers, with each layer limited to 20% NH3. This tiered injection rule enables complete combustion of both coal and NH3 across the entire 0–60% NH3 range, reduces NO emissions, and safeguards the furnace walls and outlet heaters. The findings offer directly applicable guidance for implementing NH3 co‑firing in industrial coal‑fired boilers.