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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Review on ammonia as a potential fuel: from synthesis to economics

Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames

Abstract A large set of experimental data was accumulated for hydrogen combustion: ignition measurements in shock tubes (770 data points in 53 datasets) and rapid compression machines (229/20), concentration–time profiles in flow reactors (389/17), outlet concentrations in jet-stirred reactors (152/9) and flame velocity measurements (631/73) covering wide ranges of temperature, pressure and equivalence ratio. The performance of 19 recently published hydrogen combustion mechanisms was tested against these experimental data, and the dependence of accuracy on the types of experiment and the experimental conditions was investigated. The best mechanism for the reproduction of ignition delay times and flame velocities is Keromnes-2013, while jet-stirred reactor (JSR) experiments and flow reactor profiles are reproduced best by GRI3.0-1999 and Starik-2009, respectively. According to the reproduction of all experimental data, the Keromnes-2013 mechanism is currently the best, but the mechanisms NUIG-NGM-2010, OConaire-2004, Konnov-2008 and Li-2007 have similarly good overall performances. Several clear trends were found when the performance of the best mechanisms was investigated in various categories of experimental data. Low-temperature ignition delay times measured in shock tubes (below 1000 K) and in RCMs (below 960 K) could not be well-predicted. The accuracy of the reproduction of an ignition delay time did not change significantly with pressure and equivalence ratio. Measured H2 and O2 concentrations in JSRs could be better reproduced than the corresponding H2O profiles. Large differences were found between the mechanisms in their capability to predict flow reactor data. The reproduction of the measured laminar flame velocities improved with increasing pressure and total diluent concentration, and with decreasing equivalence ratio. Reproduction of the flame velocities measured using the flame cone method, the outwardly propagating spherical flame method, the counterflow twin-flame technique, and the heat flux burner method improved in this order. Flame cone method data were especially poorly reproduced. The investigation of the correlation of the simulation results revealed similarities of mechanisms that were published by the same research groups. Also, simulation results calculated by the best-performing mechanisms are more strongly correlated with each other than those of the weakly performing ones, indicating a convergence of mechanism development. An analysis of sensitivity coefficients was carried out to identify reactions and ranges of conditions that require more attention in future development of hydrogen combustion models. The influence of poorly reproduced experiments on the overall performance was also investigated.

Comparison of the performance of several recent hydrogen combustion mechanisms (proceedings)

Experimental and kinetic modeling study of laminar burning velocities of NH3/syngas/air premixed flames

Experimental study and kinetic analysis of the laminar burning velocity of NH3/syngas/air, NH3/CO/air and NH3/H2/air premixed flames at elevated pressures

As a fueling vector, ammonia is gaining more interest for future power generation, transportation and heating systems. Compared to typical hydrocarbons, ammonia's low reactivity characteristics and higher NOx emissions limit its practical implementation. However, the current need for decarbonization of the energy sector puts ammonia as an energy vector and carrier at the far front, making overcoming the challenges of ammonia utilization necessary. In late 2018, Kobayashi et al. presented a review article on the science and technology of ammonia combustion. In continuing their effort, this review is aimed towards giving the most recent updates in ammonia combustion. The article starts with the importance of ammonia as an energy carrier and covers the fundamentals of ammonia combustion. We then present recent activities for addressing the low ammonia reactivity and its impact on NOx emissions. In addition, the chemical kinetics of ammonia oxidation is presented through a discussion of recently proposed chemical kinetics mechanisms. This section emphasizes the role of crucial reaction differences, the updates in the NOx and De-NOx pathways, NHi recombination reactions and H2NO chemistry. Towards utilising ammonia in gas turbines and industrial applications, this review discusses recent efforts and various NO mitigation strategies. This work systematically summarizes the fundamentals of ammonia-based combustion and its application. It serves as a reference for the transition of ammonia into future practical applications. 

Shixing Wang

Papers

Experimental study and kinetic analysis of the laminar burning velocity of NH3/syngas/air, NH3/CO/air and NH3/H2/air premixed flames at elevated pressures

Review on the Recent Advances on Ammonia Combustion from the Fundamentals to the Applications

Parametrization of the temperature dependence of laminar burning velocity for methane and ethane flames

Laminar burning velocities of CH4/O2/N2 and oxygen-enriched CH4/O2/CO2 flames at elevated pressures measured using the heat flux method

Effect of hydrogen addition on laminar burning velocity of CH4/DME mixtures by heat flux method and kinetic modeling