Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21-30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0-30 vol.%) at elevated pressure (1-10 bar) and temperature (298-473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N2H2 formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH2+NH=N2H2+H, NH2+NH2=N2H2+H2, N2H2+H=NNH+H2 and N2H2+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.

Due to its high lubricity, nitromethane is a fuel regularly used in model engine or more generally in race engine. The objective of this study is to improve our knowledge and understanding of the combustion of nitromethane for better evaluating its potential as fuel for automotive spark-ignition engines. To achieve this goal, unstretched laminar burning velocities of nitromethane-air mixtures were measured using spherical propagation methodology at 423 K over a pressure range from 0.5 to 3 bar and equivalence ratios from 0.5 to 1.3. The data indicated a typical adverse effect of pressure on laminar burning velocities. Based on the work done by Zhang et al., Proc. Combust. Inst., 33 (2011) 407-414, a modified detailed kinetic model including 88 species and 701 reactions was proposed. Comparisons between experimental and simulated un-stretched laminar flame speed were made and showed good agreement. The new kinetic mechanism was also used to successfully simulate published experiments and rationalize the unusual occurrence of maximum flame speed in the fuellean region.

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Ammonia combustion appears as a meaningful way to retrieve stored amounts of excess variable renewable energy, and the spark-ignition (SI) engine has been proposed as a practical conversion system. The present work aims at elucidating the combustion characteristics of ammonia blends in engine-relevant turbulent conditions. To that end, laminar and turbulent flame experiments were conducted in a constant-volume vessel at engine-relevant conditions of 445 K and 0.54 MPa to assess the combustion behavior of ammonia/hydrogen/air, ammonia/methane/air and methane/hydrogen/air mixtures observed in an all-metal single-cylinder SI engine. Results show that the respective accelerating or decelerating effects of hydrogen or methane enrichment observed in the SI engine could not be sufficiently explained by the laminar burning velocities of the mixtures. Since the latter are very low, the studied combustion regimes are at the boundary between the thin and broken reaction zones regimes, and thus strongly influenced by flame-turbulence interactions. The quantification of the flame response to turbulence shows much higher effects for ammonia blends, than for methane-based fuels.The aforementioned opposite effects of ammonia enrichment with hydrogen or methane are observed on the turbulent burning velocity during the turbulent flame experiments and correlated to the thermochemical properties of the reactants and the flame sensitivity to stretch. The latter may explain an unexpected bending effect on the turbulent-to-laminar velocity ratio when increasing the hydrogen fraction in the ammonia/hydrogen blend. Nevertheless, a very good correlation of the turbulent velocity was found with the Karlovitz and Damköhler numbers, that suggests that ammonia combustion in SI engines may be described following the usual turbulent combustion models. This encourages further investigations on ammonia combustion for the optimization of practical systems, by means of dedicated experiments and numerical simulations.

Ammonia is now recognized as a very serious asset in the context of the hydrogen energy economy, thanks to its non-carbon nature, competitive energy density and very mature production, storage and transport processes. If produced from renewable sources, its use as a direct combustion fuel could participate to the flexibility in the power sector as well as help mitigating fossil fuel use in certain sectors, such as long-haul shipping. However, ammonia presents unfavorable combustion properties, requiring further investigation of its combustion characteristics in practical systems. In the present study, a modern single-cylinder spark-ignition engine is fueled with gaseous ammonia/air mixtures at various equivalence ratios and intake pressures. The results are compared with methane/air and previous ammonia/hydrogen/air measurements, where hydrogen is used as combustion promoter. In-cylinder pressure and exhaust concentrations of selected species are measured and analyzed. Results show that ammonia is a very suitable fuel for SI engine operation, since high power outputs were achieved with satisfying efficiency by taking advantage of the promoting effects of either hydrogen enrichment or increased intake pressure, or a combination of both. The performances under NH3 fueling compare well with those obtained under methane operation. High NOx and unburned NH3 exhaust concentrations were also observed under fuel-lean and fuel-rich conditions, respectively, calling for additional mitigation measures. A detailed combustion analysis show that hydrogen mainly acts as an ignition promoter. In the engine, pure ammonia combustion is assumedly mainly driven by the ignition kinetics of ammonia and the flame response to turbulence rather than by the laminar burning velocity.

Formic acid is a promising fuel candidate that can be generated by reacting renewable hydrogen with carbon dioxide. However, the burning characteristics of formic acid/air mixtures have not been extensively studied. Furthermore, due to its low reactivity, the addition of hydrogen to formic acid/air mixtures may help with improving burning characteristics. This paper presents the first extensive study of formic acid/air premixed laminar burning velocities, as well as mixtures with hydrogen and carbon dioxide. Unstretched laminar burning velocities and Markstein lengths of formic acid in air for two different unburnt gas temperatures and equivalence ratios are presented. Measurements of formic acid mixed with various proportions of hydrogen and carbon dioxide in air are also studied as a potential renewable fuel for the future. Experimental results demonstrate the low burning velocities of formic acid and the ability to significantly enhance flame speeds by hydrogen addition. A modified detailed kinetic model for combustion of formic acid and its mixtures with hydrogen is proposed by merging well-validated literature models. The proposed model reproduces the experimental observations and provides the basis for understanding the combustion kinetics of formic acid laminar premixed flames, as well as mixtures with hydrogen. It is shown that the HOCO radical is the principal intermediate in formic acid combustion, and hydrogen addition accelerates the decomposition of HOCO radical thereby accelerating burning velocities.

International audienceIn a context of decreasing pollutant emissions, the transport sector has to tackle improvements to the engine concept as well as fuel diversification. The use of these different fuels often has an impact on the combustion performance itself. In the case of Spark Ignition (SI) engines, efficiency is a function of the combustion speed, i.e. the speed at which the fresh air–fuel mixture is consumed by the flame front. Every expanding flame is subject to flame curvature and strain rate, which both contribute to flame stretch. As each air–fuel mixture responds differently to flame stretch, this paper focuses on understanding the impact of flame stretch on fuel performances in SI engines. Different air–fuel mixtures (different fuels or equivalence ratios) with similar unstretched laminar burning speeds and thermodynamic properties but different responses to stretch were selected. The mixtures were studied in a turbulent spherical vessel and in an optical engine using Mie-Scattering tomography. The combustion phasing was also investigated in both optical and all-metal single cylinder engines. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion conditions, are relevant parameters that need to be taken into consideration to predict the global performance of fuels, either experimentally or for modeling simulation

Faced with the problem of reducing greenhouse gas emissions and transitioning toward a greater use of renewable energies, ammonia, as an energy carrier, is increasingly seen as a potential "green" fuel for transportation, in particular marine applications. However, its combustion characteristics (high minimum ignition energy and auto-ignition temperature, low combustion speed in comparison to usual hydrocarbon fuels) are drawbacks that have so far limited its use. Due to the evolution of different pollutant standards for road transportation, spark-ignition engines and thus the combustion process itself have been subjected to many changes over the last 20 years (e.g., gasoline direct injection, downsizing). The objective of this article is to discuss the potential of ammonia as a fuel for spark-ignition engines, thanks to the studies carried out so far and to point out directions for future work.