Addressing NOX formation mechanisms of NH3/H2 dual-fuel flame under DBD plasma-assisted combustion resolved by intermediate radicals analysis

In order to attain carbon-neutrality, the implementation of zero-carbon fuel containing ammonia (NH₃)/hydrogen (H₂) has become more and more practically popular. This work focuses on addressing the combustion performance and nitrogen oxide (NO_X) formation mechanisms of dielectric barrier discharge (DBD) plasma-assisted NH₃/H₂ dual-fuel flames at varied plasma voltages (V_AC) and hydrogen ratios (Z_H2), which were resolved by PLIF and chemiluminescence techniques concurrently. The analytical results obtained show that V_AC had positive effectiveness on contributing to NO_X emissions with a threshold of V_AC = 11 kV found for triggering NO_X formation. With V_AC increased from 11 kV to 12.5 kV constantly, NO_X grew dramatically by 8.3%–12.3% (Z_H2 elevated from 0.2 to 0.3). This phenomenon was mainly because of the growing propagation of OH radicals being faster than that of NH₂ radicals. Besides, three NO_X formation regions, including low-formation region, medium-formation region and high-formation region were determined, which essentially reflected that hydrogen ratio predominated over discharge voltage on forming NOx. Moreover, the inter-relationship between flame surface density (FSD, revealing combustion intensity) and NO_X has been comprehensively explored. And high DBD-V_AC (with low hydrogen blending ratio) was found to significantly promote the FSD resulting in better combustion intensity, but caused inappreciable NO_X formation. In upcoming future application, high voltage of DBD plasma could be utilized for replacing high hydrogen blending ratio in ammonia/hydrogen combustion, to obtain promotional combustion intensity with effective NO_X control.