Effects of difference in heating sources on ammonia reactivity: Possibility for photolysis-assisted ammonia combustion

Ammonia (NH₃) reactivity in a micro flow reactor with a controlled temperature profile (MFR) is reexamined through species measurements utilizing two heating sources in the MFR: an H₂/air flat flame and an electric heater. The maximum wall temperatures (T_w,$$_max) formed in the reactor vary in a range of T_w,$$_max = 1100–1400 K. A stoichiometric NH₃/air mixture is tested, and exhaust NH₃ is detected by a quadrupole mass spectrometer (QMS). Unexpectedly, NH₃ is completely consumed at temperatures at least 100 K lower in the H₂/air flat flame case compared to the electric furnace case, despite nearly identical conditions of a MFR characteristic residence time estimated by the wall temperature profiles and the convective flow velocity. Considering the non-thermal characteristics of the two heating sources that the H₂/air flat flame emits ultraviolet light, whereas infrared light as thermal radiation is emitted within the electric furnace, the possibility of NH₃ photolysis in the H₂/air flat flame case is discussed based on literature regarding emissions from the H₂/air flames, the transmittance of the quartz tube, and the photodissociation of NH₃ in the ultraviolet region. When ultraviolet light emitted from the H₂/air flat flame passes through the quartz tube and decomposes NH₃ into NH₂ and H radicals, these produced radicals enhance the growth of OH radicals, resulting in increased NH₃ reactivity. These findings suggest the possibility of photolysis-assisted ammonia combustion, which could be an additional method to overcome the low reactivity of NH₃.