No abstract

No abstract

Since the Industrial Revolution, large amounts of ammonia have been synthesized by the Haber–Bosch process and used as fertilizer and chemical materials. Nitrogen fixation by industrial processes has contributed to the development of human society. However, a serious concern about the nitrogen cycle has been pointed out from the viewpoint of the planetary boundary. The planetary boundary quantifies the allowable limit to maintain a sustainable environment. This nitrogen cycle, which includes the limit of the biogeochemical flow boundary, was suggested to exceed the limits and to pose a serious environmental pollution issue. Therefore, a nitrogen cycle in which nitrogen compounds are collected from the environment and released into the atmosphere has recently been proposed. In this cycle, ammonia–water needs to be processed appropriately. In addition to the standpoint of the planetary boundary, ammonia–water is focused as a fuel that is able to be handled more safely. To design combustion systems that use ammonia–water, its fundamental combustion characteristics need to be elucidated. The objective of the present study is to clarify the effects of water vapor dilution on the laminar flame characteristics of ammonia/water vapor/air premixed flames. A propagating flame in a constant-volume chamber was observed to determine the laminar burning velocity and Markstein length for ammonia/water vapor/air premixed flames for various water vapor dilution ratios. The laminar burning velocity decreased and the Markstein length increased with an increasing water vapor dilution ratio. The effects of water vapor dilution on the laminar burning velocity for ammonia flames were also investigated by numerical calculations. The numerical results show that the decrease in the temperature is the dominant factor of the decrease in the laminar burning velocity with an increase in the water vapor dilution ratio. This study contributes to the understanding of the effects of water vapor dilution on the laminar flame characteristics of ammonia flames up to elevated pressure conditions.

Ammonia (NH3) has been receiving the attention of researchers as an alternative promising green fuel to replace fossil sources for energy production. However, the high NOx emissions are one of the drawbacks and restrictions of using NH3 on a broad scale. The current study investigates NO production/consumption for a 70/30 (vol%) NH3/H2 mixture using kinetic reaction mechanism concepts to shed light on the essential reaction routes that promote/inhibit NO formation. Sixty-seven kinetic reaction mechanisms from the literature have been investigated and compared with recently reported measurements at a wide range of equivalence ratios (ϕ) (0.6–1.4), atmospheric pressure and temperature conditions. Both numerical simulations and experimental measurements used the same combustion reactor configuration (premixed stabilized stagnation flame). To highlight the best kinetic model for the predicting of the NO experimental measurements of NO, a symmetric mean absolute percentage error (SMAPE) has been determined as a preliminary estimation by comparing both numerical and experimental measurements. The results found that the kinetic reaction mechanism of Glarborg showed an accurate prediction with a minor error percentage of 2% at all lean and stoichiometric conditions. Meanwhile, the kinetic model of Wang accurately predicted the experimental data with 0% error at ϕ = 1.2 and underestimated the mole fraction of NO at 1.4 ϕ with an error of 10%. The sensitivity analysis and rate of production/consumption of NO mole fractions analysis have also been implemented to highlight the most important reactions that promote/inhibit NO formation. At lean and stoichiometric conditions, Glarborg kinetic model shows that the kinetic reactions of HNO + H ⇌ NO + H2, HNO + O ⇌ NO + OH, and NH + O ⇌ NO + H are the most important reaction routes with considerable effect on NO formation for 70/30 (vol%) NH3/H2 mixture. In contrast, the reactions of NH2 + NO ⇌ N2 + H2O, NH2 + NO ⇌ NNH + OH, NH + NO ⇌ N2O + H, and N + NO ⇌ N2 + O significantly consume NO to N2, NNH, and N2O. Further, Wang’s mechanism illustrated the dominant effect of each HNO + H ⇌ NO + H2, N + OH ⇌ NO + H, NH + O ⇌ NO + H in NO formation and NH + NO ⇌ N2O + H, NH2 + NO ⇌ NNH + OH, and NH2 + NO ⇌ N2 + H2O in the consumption of NO mole fractions.