Combustion Characteristics for the Stabilization of Nonpremixed Methane-Ammonia/Air Coflow Flames through Their Visualization

In order to confirm the possible use of methane (CH4)-ammonia (NH3) blends as a low carbon fuel, the fundamental combustion characteristics for the stabilization of nonpremixed CH4–NH3/air flames using the coflow jet flame configuration are experimentally studied, observing the flame behaviors by the direct imaging, shadow-graph, and OH radical visualization systems. Two types of flames, attached and lifted flames, are observed, and extinction (blowout) occurs as the injection velocity of fuel ufuel increases. The occurrence of blowout for pure CH4/air flames depends on ufuel regardless of the injection velocity of coflow air ucoflow, while it considerably depends on ucoflow for CH4–NH3/air flames. The OH radical visualization reveals that NH3 addition affects the overall reaction intensity and mechanism but does not reduce the average reaction region near the injector lip. Also, the addition of NH3 reduces the formation of polycyclic aromatic hydrocarbons (PAHs) as a soot precursor. Direct images confirm the effects of NH3 addition, ufuel, and ucoflow on the flame length, and the liftoff height. The measured NOx emissions exhibit that the fuel NOx from NH3 is dominant and the NOx emissions are considerably affected by the flame intensity and fluctuation.