Ignition Behavior of Ammonia-Blended Coal Combustion at Different Coal Combustion Stages

Ammonia-coal co-combustion has been widely investigated as a potential low-carbon combustion strategy for coal-fired power plants, and the precise control of ammonia injection strategies is crucial for optimizing co-combustion performance. This study employs a Hencken flat flame burner system to investigate the ignition and combustion behavior of ammonia-coal co-combustion at different coal combustion stages under controlled variations in oxygen concentrations (10%–30%), ammonia blending ratios (0–100%), volatile matter contents (10%–40%), and char participation (char-added or char-free). Three ammonia co-combustion strategies (NH3–CH4, NH3–CH4–char, and NH3-char) are used to vary the ammonia addition location and isolate ammonia interactions with coal volatiles and char across the different combustion stages. The experimental results reveal distinct ignition behaviors in ammonia-coal co-combustion systems. The char ignition delay distance increases with the increasing ammonia blending ratio and the volatile matter content for the NH3–CH4–char system (representing ammonia addition during the volatile-combustion stage) because the intensified gas-phase reactions create a locally oxygen-depleted zone. Conversely, in the NH3-char system (ammonia added during char combustion), the char ignition delay distance decreases and then increases with the increasing ammonia blending ratio, reflecting competing effects of NH3-induced char preheating and local oxygen depletion. Moreover, the spectral diagnostics identify the CN*/NH* and NH*/CO2* ratios as specific optical indicators of ammonia blending conditions, while the OH*/CO2* ratio depends on both the ammonia blending ratio and oxygen concentration. The maximum particle temperatures fall within the ranges 2080–2273 K for NH3-coal, 1979–2103 K for NH3–CH4–char, and 1708–2013 K for NH3-char, suggesting that delayed ammonia addition helps suppress local temperature peaks and improve the temperature field uniformity. These findings provide mechanistic insights for optimizing staged ammonia injection strategies for coal-fired power plants.