The Mild Combustion is characterized by both an elevated temperature of reactants and low temperature increase in the combustion process. These features are the results of several technological demands coming from different application fields. This review paper aims to collect information which could be useful in understanding the fundamentals and applications of Mild Combustion. The information in this field are still sparse, because of the recent identification of the process, so that many speculative considerations have been presented in order to make the whole framework more consistent and rich with potential new applications. A rigorous definition of Mild Combustion is preliminarily given in order to fix the input variables of the process. Under these constraints the influence of the physical, thermodynamic and chemical variables on the most relevant outlet parameters are analyzed. The physical aspects taken into account are atomization, evaporation, mixing and radiative heat transfer. In particular, the evolution of the mixing layer for high temperature diluted oxidant is analyzed. It is shown that mass fluxes through the stoichiometric isosurfaces are lower than those in not diluted conditions and that the annihilation of these isosurfaces is enhanced in the Mild Combustion conditions. Both effects infer low rates of heat release according to the experimental results reported in the literature. The thermodynamic aspects are dealt through the comparative analysis of the minimum, maximum and equilibrium temperature profiles versus the mixture fraction in the whole allowable range for the diluted and not-diluted cases. The chemical aspects have been analyzed in relation to the chemical kinetics rates for different oxidative routes and the temporal evolution of the self-ignition process. The molecular oxygen addition, the hydroperoxide dissociation and atomic hydrogen oxidation are evaluated in wide pressure and temperature ranges. In such a way self-ignition regimes which rely on different preferential chemical kinetics routes are identified and comparison between diluted/not diluted conditions are performed for a fixed evolution time. In this case it is shown that Mild Combustion conditions extend the pressure–temperature range, in which the oxidation is depressed, at relatively low pressure, whereas the ‘ceiling temperature’ is shifted to lower temperature for Mild Combustion condition at higher pressure. The second part of the review shows the potentialities of the diluted high temperature air combustion in applications related both to efficiency and pollution of thermal generation as well as to abatement of the pollutants along the flue gas stream of a primary combustion system. Some selected examples in these fields as land-base gas-turbines, boiler combustion chamber and domestic heating systems are presented. In these, the emphasis, is put preliminarily on aspects related more to efficiency than to pollution reduction, even though this target is implicitly taken into consideration. Then environme...

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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