Ammonia, a molecule that is gaining more interest as a fueling vector, has been considered as a candidate to power transport, produce energy, and support heating applications for decades. However, the particular characteristics of the molecule always made it a chemical with low, if any, benefit once compared to conventional fossil fuels. Still, the current need to decarbonize our economy makes the search of new methods crucial to use chemicals, such as ammonia, that can be produced and employed without incurring in the emission of carbon oxides. Therefore, current efforts in this field are leading scientists, industries, and governments to seriously invest efforts in the development of holistic solutions capable of making ammonia a viable fuel for the transition toward a clean future. On that basis, this review has approached the subject gathering inputs from scientists actively working on the topic. The review starts from the importance of ammonia as an energy vector, moving through all of the steps in the production, distribution, utilization, safety, legal considerations, and economic aspects of the use of such a molecule to support the future energy mix. Fundamentals of combustion and practical cases for the recovery of energy of ammonia are also addressed, thus providing a complete view of what potentially could become a vector of crucial importance to the mitigation of carbon emissions. Different from other works, this review seeks to provide a holistic perspective of ammonia as a chemical that presents benefits and constraints for storing energy from sustainable sources. State-of-the-art knowledge provided by academics actively engaged with the topic at various fronts also enables a clear vision of the progress in each of the branches of ammonia as an energy carrier. Further, the fundamental boundaries of the use of the molecule are expanded to real technical issues for all potential technologies capable of using it for energy purposes, legal barriers that will be faced to achieve its deployment, safety and environmental considerations that impose a critical aspect for acceptance and wellbeing, and economic implications for the use of ammonia across all aspects approached for the production and implementation of this chemical as a fueling source. Herein, this work sets the principles, research, practicalities, and future views of a transition toward a future where ammonia will be a major energy player.

Methane oxidation at high pressures and intermediate temperatures was investigated in a laminar flow reactor and in a rapid compression machine (RCM). The flow-reactor experiments were conducted at 700-900 K and 100 bar for fuel-air equivalence ratios (Φ) ranging from 0.06 to 19.7, all highly diluted in nitrogen. It was found that under the investigated conditions, the onset temperature for methane oxidation ranged from 723 K under reducing conditions to 750 K under stoichiometric and oxidizing conditions. The RCM experiments were carried out at pressures of 15-80 bar and temperatures of 800-1250 K under stoichiometric and fuel-lean (Φ=0.5) conditions. Ignition delays, in the range of 1-100 ms, decreased monotonically with increasing pressure and temperature.

a b s t r a c tAmmonia oxidation experiments were conducted at high pressure (30 bar and 100 bar) under oxidizing and stoichiometric conditions, respectively, and temperatures ranging from 450 to 925 K. The oxidation of ammonia was slow under stoichiometric conditions in the temperature range investigated. Under oxidizing conditions the onset temperature for reaction was 850-875 K at 30 bar, while at 100 bar it was about 800 K, with complete consumption of NH 3 at 875 K. The products of reaction were N 2 and N 2 O, while NO and NO 2 concentrations were below the detection limit even under oxidizing conditions. The data were interpreted in terms of a detailed chemical kinetic model.

Ethane oxidation at intermediate temperatures and high pressures has been investigated in both a laminar flow reactor and a rapid compression machine (RCM). The flow-reactor measurements at 600-900 K and 20-100 bar showed an onset temperature for oxidation of ethane between 700 K and 825 K, depending on pressure, stoichiometry, and residence time. Measured ignition delay times in the RCM at pressures of 10-80 bar and temperatures of 900-1025 K decreased with increasing pressure and/or temperature. A detailed chemical kinetic model was developed with particular attention to the peroxide chemistry. Rate constants for reactions on the C 2 H 5 O 2 potential energy surface were adopted from the recent theoretical work of Klippenstein. In the present work, the internal H-abstraction in CH 3 CH 2 OO to form CH 2 CH 2 OOH was treated in detail. Modeling predictions were in good agreement with data from the present work as well as results at elevated pressure from literature. The experimental results and the modeling predictions do not support occurrence of NTC behavior in ethane oxidation. Even at the high-pressure conditions of the present work where the C 2 H 5 + O 2 reaction yields ethylperoxyl rather than C 2 H 4 + HO 2 , the chain branching sequence CH 3 CH 2 OO −→ CH 2 CH 2 OOH +O 2 −→ OOCH 2 CH 2 OOH → branching is not competitive, because the internal H-atom transfer in CH 3 CH 2 OO to CH 2 CH 2 OOH is too slow compared to thermal dissociation to C 2 H 4 and HO 2 .

A detailed chemical kinetic model for oxidation of C 2 H 4 in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C 2 H 3 + O 2 reaction was was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar vinyl peroxide, rather than CH 2 O and HCO, is the dominant product.The experiments, involving C 2 H 4 /O 2 mixtures diluted in N 2 , were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C 2 H 4 are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH 2 CH 2 OH) and 2-hydroperoxyethyl 1 (CH 2 CH 2 OOH) radicals, formed from addition of OH and HO 2 to C 2 H 4 , and vinyl peroxide, formed from C 2 H 3 + O 2 . Hydroxyethyl is oxidized through the peroxide HOCH 2 CH 2 OO (lean conditions) or through ethenol (low O 2 concentration), while 2-hydroperoxyethyl is converted through oxirane. [2][3][4][5][6][7], shock tubes [8][9][10][11][12] and premixed laminar flames [13][14][15][16][17], covering a wide range of stoichiometries and temperatures. Most of the reported work, however, have been carried out at near atmospheric pressure. A few results are available from flow reactor studies at 5-10 bar [6], but despite their relevance for the chemistry in engines, gas turbines, and gas-to-liquid processes, data at high pressures are limited.The objective of the present study is to obtain experimental results for the oxidation of C 2 H 4 at high pressure (60 bar) as functions of temperature (600-900 K) and stoichiometry (lean to fuel-rich) and analyze them in terms of a detailed chemical kinetic model. The oxidation pathways for C 2 H 4 under these conditions are different from those prevailing at higher temperatures and lower pressures and the results of the current work help to extend the validation range for chemical kinetic modeling of C 2 H 4 oxidation. This paper is part of a series investigating the high-pressure, medium temperature oxidation of simple fuels: previously work has been reported for CO/H 2 , CH 4 , and CH 4 /C 2 H 6 mixtures [18,19]. The present kinetic model draws on this work, as well as recent results in tropospheric chemistry. Furthermore, the important reaction of C 2 H 3 with O 2 was characterized from ab initio calculations over a wide range of pressure and temperature. 3 ExperimentalThe experimental setup is a laboratory-scale high pressure laminar flow reactor designed to approximate plug-flow. The setup is described in detail elsewhere [18] and only a brief description is provided here. The system enables well-defined investigations of...