Experimental and kinetic modeling study on laminar flame speeds and emission characteristics of oxy-ammonia premixed flames

Abstract

The low flame speed and NOx emissions of ammonia are the key to affecting combustion temperature, stability, flame structure, and pollutant control, limiting its application in power generation devices. This study proposes oxy-ammonia combustion (oxygen content in oxidant = 100%, NH3/O2 combustion) as a novel method for significantly enhancing ammonia combustion. A method for measuring the laminar flame speed using the NH* distribution of the Bunsen burner flame is proposed, and the measured data is close to the constant volume combustion bomb. 21 kinetic models were collected and optimized after comparison with the experimental data of constant volume combustion bombs, and results show that Han et al.'s model has higher prediction accuracy for NH3/O2 flames. The preheating temperature was relevant to gas turbine combustors (298.15–500 K), and the equivalence ratio ranged from 0.7 to 1.6. At this extreme condition (oxygen content in oxidant = 100%), the results show that the maximum laminar flame speed reaches 1.25 m/s at ambient pressure and temperature, which is approximately 18 times that of NH3/air combustion, mainly due to the increased reaction rates of OH, H, O and NH2 radicals in the reaction zone (primary ammonia decomposition and H/O radical pool). The NO emission of NH3/O2 combustion at stoichiometric conditions is about 7216 ppmv@15%O2, 1.5 and 13 times that of NH3/air and CH4/air combustion, respectively. NO production and consumption may follow the N2O and NNH mechanism, and HNO is an important precursor for NO formation. Increasing the preheating temperature has less effect on the reaction pathway directions in the reaction network of nitrogen conversion but affects the path flux significantly.