AbstractThe present study addresses the ignition characteristics of different ammonia/hydrogen/air mixtures through numerical simulations and determines the minimum ignition energy (MIE) (electrode distance is fixed), which is further compared with experimental measurements [4]. Simulations of the ignition process with a detailed treatment of chemical kinetics [10] and molecular transport were conducted. The MIE for $\phi = 0.9$ (fuel/air equivalence ratio) is analyzed across various parameters including source radii, source time durations, geometric symmetry assumptions, and the presence of shock waves. The simulation results reflect the experimental trend. Ignition scenarios involving shock waves require more energy for successful ignition due to the energy needed for shock formation. Both cylindrical and spherical geometries yield similar MIEs for the same volumes of energy deposition. Heat conduction out of the source volume becomes significant only when the time of energy deposition exceeds $10^{-4}$ seconds. Several possible reasons for the discrepancies between simulation and experiment are discussed.Keywords: ammonia, hydrogen, ignition, numerical simulation, shock wave.IntroductionAmmonia is a promising energy carrier and fuel due to its high energy density, lack of carbon atoms, and the fact that a worldwide infrastructure already exists [1]. ... mehrHowever, combustion processes using ammonia suffer from its low laminar burning velocity and high ignition energy [2]. Hence, hydrogen addition to the fuel either by partial cracking of ammonia or from an external source is considered to improve the ignition and burning characteristics [2, 3]. Studies have shown both experimentally [4-6] and numerically [7, 8] that a hydrogen content in the fuel as small as 10 vol.% already significantly reduces the energy required for ignition. In this study, we examine numerically the ignition of ammonia/hydrogen/air mixtures and compare the results to the experiment in [4]. The influence of various parameters such as hydrogen addition, spark duration time, and spark width on ignition is investigated. The effect of shock waves formed during the energy deposition on the minimum ignition energy is also studied.Numerical MethodsThe numerical simulations were carried out using the in-house program INSFLA [9]. This code solves conservation equations for total mass, species mass, energy, and momentum in one-dimensional configurations by using detailed chemical kinetics and a molecular transport model including thermal diffusion (Soret effect) and differential diffusion [9]. Both the spatial grid and time steps are adaptive, with time stepping coupled to error control. The code provides profiles of temperature, pressure, and species concentrations, varying with both time and space. Planar, cylindrical, and spherical geometries can be studied with or without the assumption of constant pressure. The constant pressure assumption speeds up the computation considerably, but cannot be used when pressure waves develop during the ignition process. The ignition energy deposited by the spark is modelled as $Q(r, t) = \dot{q}_0 \exp \left(- \frac{r^2}{r_s^2} \right)$ during the duration of the spark $t_s$, where $\dot{q}_0$ is the energy density, $r_s$ is the spark width, $r$ is the spatial coordinate, and $t$ is the time [9]. For $t > t_s$, $Q(r, t) = 0$ [9].Simulations were carried out for different mixtures of ammonia, hydrogen, and air using the reaction mechanism by Shrestha et al. [10]. Following the experiments [4], the equivalence ratio was fixed at 0.9. The relative hydrogen content of the fuel, expressed as $X_{\text{H}_2} = \frac{X_{\text{H}_2}}{X_{\text{NH}_3} + X_{\text{H}_2}}$, was varied from 0 to 0.2. Ignition and ignition failure events were determined for all $X_{\text{H}_2}$ with a 1 mJ increment for simulations using the uniform pressure assumption and a 10 mJ or less increment for simulations solving the fully compressible Navier-Stokes equations in the critical energy ranges close to ignition. This allows the numerically calculated MIE for the specific electrode configuration to be narrowed down to a range of 1 mJ and 10 mJ, respectively.ResultsThe trend of a significant decrease in the MIE $W$ with increasing hydrogen content is reproduced in the simulation (although the experiment shows a more profound decrease than the simulation). Further, a shift towards higher energies can be seen in the simulations with shock waves due to the lower temperature in the source volume. The source radius is identified as a critical parameter, because 29% smaller radii result in significantly smaller MIEs. Note that an overestimation of the ignition source radius would lead to higher MIE. Further, simulations were conducted using a spherical geometry with the same source volume, yielding only minor differences. This observation suggests that diffusion and heat conduction processes do not significantly influence ignition in this configuration.The effect of different $t_s$ in the range $10^{-6}$ s to $10^{-2}$ s on the MIE is analyzed with the constant pressure assumption. It is apparent that for the range between $10^{-6}$ s to $10^{-4}$ s, the effect is negligible. For longer source times ($t_s > 10^{-4}$ s) more energy is required to ignite the mixture, because diffusion and heat conduction processes become important on these time scales.Discussion & ConclusionFor understanding differences in the slope of the MIE dependent of $X_{\text{H}_2}$ between simulation and experiment, several possible reasons can be discussed. From the experimental side, it is likely that the determined ignition energies are indicated too high for smaller values of $X_{\text{H}_2}$. This is because the energy transfer from the capacitive discharge becomes less efficient as the ignition energy increases. Further, the chemical kinetics of the ammonia/hydrogen/air mechanism may introduce uncertainties for mixtures that deviate significantly from pure ammonia or pure hydrogen. The effect of various parameters on the MIE of $\text{NH}_3/\text{H}_2/\text{air}$ mixtures was investigated. When source times are short ($t_s \le 10^{-5}$ s) shock waves are formed and significantly impact the ignition process. The source radius has a strong effect on the MIE. If shock waves are not formed, the source time is not important for $t_s \le 10^{-4}$ s.AcknowledgmentsThe authors gratefully acknowledge the financial contribution from the Deutsche Forschungsgemeinschaft (DFG) under the project MA 1205/32-1, project number 528274426.References1 Valera-Medina, A.; Amer-Hatem, F.; Azad, A.K.; Dedoussi, I.C.; De Joannon, M.; Fernandes, R.X.; Glarborg, P.; Hashemi, H.; He, X.; Mashruk, S. et al. Review on Ammonia as a Potential Fuel: From Synthesis to Economics. Energy & Fuels 2021, 35(9), 6964–7029. DOI: 10.1021/acs.energyfuels.0c03685.2 Li, J.; Lai, S.; Chen, D.; Wu, R.; Kobayashi, N.; Deng, L.; Huang, H. A Review on Combustion Characteristics of Ammonia as a Carbon-Free Fuel. Frontiers in Energy Research 2021, 9. DOI: 10.3389/fenrg.2021.760356.3 Han, X.; Wang, Z.; Costa, M.; Sun, Z.; He, Y.; Cen, K. Experimental and kinetic modeling study of laminar burning velocities of $\text{NH}_3/\text{air}$, $\text{NH}_3/\text{H}_2/\text{air}$, $\text{NH}_3/\text{CO}/\text{air}$ and $\text{NH}_3/\text{CH}_4/\text{air}$ premixed flames. Combustion and Flame 2019, 206, 214–226. DOI: 10.1016/j.combustflame.2019.05.003.4 Essmann, S.; Dymke, J.; Höltkemeier-Horstmann, J.; Möckel, D.; Schierding, C.; Hilbert, M.; Yu, C.; Maas, U.; Markus, D. Ignition characteristics of hydrogen-enriched ammonia/air mixtures. Applications in Energy and Combustion Science 2024, 17. DOI: 10.1016/j.jaecs.2024.100254.5 Lesmana, H.; Zhu, M.; Zhang, Z.; Gao, J.; Wu, J.; Zhang, D. An experimental investigation into the effect of spark gap and duration on minimum ignition energy of partially dissociated $\text{NH}_3$ in air. Combustion and Flame 2022, 241, 112053. DOI: 10.1016/j.combustflame.2022.112053.6 Verkamp, F.J.; Hardin, M.C.; Williams, J.R. Ammonia combustion properties and performance in gas-turbine burners. Symposium (International) on Combustion 1967, 11(1), 985–992. DOI: 10.1016/S0082-0784(67)80225-X.7 Fernández-Tarrazo, E.; Gómez-Miguel, R.; Sánchez-Sanz, M. Minimum ignition energy of hydrogen–ammonia blends in air. Fuel 2023, 337, 127128. DOI: 10.1016/j.fuel.2022.127128.8 Yu, C.; Eckart, S.; Essmann, S.; Markus, D.; Valera-Medina, A.; Schießl, R.; Shu, B.; Krause, H.; Maas, U. Investigation of spark ignition processes of laminar strained premixed stoichiometric $\text{NH}_3-\text{H}_2-\text{air}$ flames. Journal of Loss Prevention in the Process Industries 2023, 83, 105043. DOI: 10.1016/j.jlp.2023.105043.9 Maas, U.; Warnatz, J. Ignition processes in hydrogen-oxygen mixtures. Combustion and Flame 1988, 74(1), 53–69. DOI: 10.1016/0010-2180(88)90086-7.10 Shrestha, K.P.; Seidel, L.; Zeuch, T.; Mauss, F. Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides. Energy & Fuels 2018, 32(10), 10202–10217. DOI: 10.1021/acs.energyfuels.8b01056.
