Experimental and kinetic modeling study of laminar burning velocity enhancement by ozone additive in NH3 + O-2 + N-2 and NH3 + CH4/C2H6 /C3H8 + air flames

Ammonia (NH 3 ) is regarded as a promising future carbon-free fuel but needs to overcome drawbacks including extremely low burning velocity in practical combustion apparatus. In this study, ozone (O 3 ) additive is used to elucidate one of the mechanisms of potential flame enhancement method based on plasma-assisted combustion. The effects of ozone addition on the laminar burning velocity of premixed NH 3 /(35%O 2 /65%N 2 ) and NH 3 + CH 4 /C 2 H 6 /C 3 H 8 + air flames over a wide range of equivalence ratios were investigated experimentally and numerically. Blending NH 3 with hydrocarbons can decrease the ignition energy and increase the burning velocities of the whole mixture, which may contribute to developing ammonia co-fired mechanisms with varied complex fuels and validating the feasibility of NH 3 using strategies in real applications. Measurements were conducted at atmospheric conditions using the Heat Flux method. For NH 3 /(35%O 2 /65%N 2 ) flames, a significant increase was found on the fuel-lean side. Experimental data showed that maximum enhancement reaches 15.34% at & phi;= 0.6 with 5000 ppm O 3 additive. For NH 3 + CH 4 /C 2 H 6 /C 3 H 8 + air blended flames, the enhancement effect was much more profound under offstoichiometric conditions, being 1.5-4 times higher than that under near-stoichiometric conditions. A 28step O 3 related kinetic sub-mechanism was integrated with fiv e selected NH 3 -oxidation mechanisms to simulate the burning velocities of NH 3 /(35%O 2 /65%N 2 ) flames and CEU-Mech for NH 3 + CH 4 /C 2 H 6 /C 3 H 8 + air flames. Simulation results show improved agreement with the experimental data, especially for fuel-rich conditions as NH 3 blending ratio x NH3 increases from 0 to 0.9. Each of the NH 3 /CH 4 /air, NH 3 /C 2 H 6 /air and NH 3 /C 3 H 8 /air cases fits well between experimental data and numerical results with varied NH 3 -fuel blending ratios. Detailed kinetic analyses adopting the CEU-NH 3 -Mech integrated with O 3 sub-mechanism were carried out and revealed that active radicals such as HNO, which are rapidly produced due to high O concentration from O 3 decomposing in the pre-heating zone, interfered with the ammonia-fuel chemistry and thus evidently promoted the overall combustion process.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study investigates the potential flame enhancement method in plasma-assisted combustion using ozone as an additive. The results show that ozone addition significantly affects the burning velocity of premixed ammonia gases, especially under fuel-rich conditions. The effects of ozone addition on the laminar burning velocity were experimentally and numerically investigated under various equivalence ratios.