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 u_fuel increases. The occurrence of blowout for pure CH₄/air flames depends on u_fuel regardless of the injection velocity of coflow air u_coflow, while it considerably depends on u_coflow 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, u_fuel, and u_coflow on the flame length, and the liftoff height. The measured NO_x emissions exhibit that the fuel NO_x from NH₃ is dominant and the NO_x emissions are considerably affected by the flame intensity and fluctuation.