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.

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Ammonia (NH 3), has been considered as a promising alternative energy carrier for automobile engines and gas turbines due to its production from renewable sources using concepts such as power-togas. Knowledge of the combustion characteristics of NH 3 /air and the formation of pollutants, especially NO x and unburned NH 3 , at intermediate temperatures is crucially important to investigate. Detailed understanding of ammonia reaction mechanism is still lacking. The present study reports ignition delay times of NH 3 /air mixtures over a temperature range of 1100-1600 K, pressures of 20 and 40 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The experimental results are compared to the literature mechanism of Mathieu and Petersen (2015), and reasonable agreement is observed. Detailed modeling for ammonia emissions is performed, and the NH 3 /air combustion is found to be potentially free from NO x and unburned NH 3 at fuel-rich conditions.

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The low reactivity of ammonia (NH 3 ) is the main barrier to applying neat NH 3 as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH 3 -based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H 2 ), methane (CH 4 ), and methanol (CH 3 OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O 3 ) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH 3 consumption can be triggered at lower temperatures than in the neat NH 3 case. CH 3 OH has the most prominent effect on enhancing the reactivity, followed by H 2 and CH 4 . Furthermore, two-stage NH 3 consumption was observed in NH 3 /CH 3 OH blends, whereas no such phenomenon was found by adding H 2 or CH 4 . The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH 3 oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH 2 O + NH 2 ⇄ HCO + NH 3 is responsible for the underestimation of CH 2 O in NH 3 /CH 4 fuel blends. The discrepancies observed in the modeling of NH 3 fuel blends are mainly due to the deviations in the neat NH 3 case. The total rate coefficient and the branching ratio of NH 2 + HO 2 are still controversial. The high branching fraction of the chain-propagating channel NH 2 + HO 2 ⇄ H 2 NO + OH improves the model performance under lowpressure JSR conditions for neat NH 3 but overestimates the reactivity for NH 3 fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH 3 OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH 3 consumption at temperatures below 450 K but unexpectedly inhibit the NH 3 consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH 3 -related species and O 3 is effective for improving the model performance, but their rate coefficients have to be refined.

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