Given the intensifying global climate crisis, it is extremely important to find ways to reduce greenhouse gas (GHG) emissions from human activities, including carbon dioxide (CO2). The regulations of the European Union aim at decarbonising the transportation and power sectors, involving civil, naval, and aviation systems, along with agriculture and process industries, to achieve a drastic reduction of the carbon footprint resulting in net-zero carbon emissions by the year 2050 [EU Regulation 2021/1119, “European Climate Law”
https://climate.ec.europa.eu/eu-action/european-climate-law_en](si apre in una nuova finestra). Photovoltaic and wind power sources produce electricity with a low carbon footprint, and, in some regions, can even meet total electricity demand during peak production. However, during the off-peak periods, the production of these power sources can be close to zero, and, thus, electric energy has to be stored for days or weeks for later use. Now, and likely in the next decades, storing electrical energy in batteries is expensive, and batteries can store only a relatively small amount of energy for short periods, typically for days. Among the several possible options, the generation of fuels (such as hydrogen and ammonia) from electricity has many advantages, since this way excess energy can be stored in large quantities for a sufficiently long time for on-demand use. Besides electric energy storage, these e-fuels are also well applicable in sectors that are hard to electrify, like heavy transportation, long-haul commercial shipping, and energy-intensive industries.
The combustion of such fuels as ammonia and hydrogen, in the ideal case, produces only N2 and H2O, making them a very environmentally friendly way of energy production since no CO2 is produced. However, the combustion of pure ammonia is also technically challenging due to its high ignition energy, long ignition delay time, and small burning velocity relative to traditional hydrocarbon fuels and hydrogen. These features can be improved by applying H2/NH3 mixtures instead of neat ammonia. Hydrogen is, on the other hand, an extremely reactive gas which requires special safety measures during storage and transportation.
Overall, efficient, safe, and environmentally friendly burners and internal combustion engines can be designed using Computational Fluid Dynamics simulations that incorporate accurate physical and chemical models. Therefore, studies of more and more accurate detailed reaction mechanisms for the combustion of fuels and fuel mixtures remain an important research topic for a long time. In this area, the hydrogen combustion mechanism is a unique object, because it is not only used in systems where combustion of neat hydrogen is considered, but also represents an important constituent part of almost all other combustion mechanisms.
Investigations of combustion kinetics, however, remain challenging, since real combustion processes always occur under elevated pressures and temperatures and involve many different chemical reactions occurring simultaneously and often competing with one another. Therefore, while it's possible to create a detailed combustion mechanism using only theoretically calculated reaction rate constants, it is still necessary to “adapt” the mechanism’s parameters by validating it against experimental measurements performed in controlled environments.
Optimisation of a reaction mechanism means a systematic search of the rate and thermodynamic parameter values within their physically realistic domain of uncertainty in order to achieve the best possible reproduction of selected experimental data. In this project, the procedure received a further refinement by implementing a strict analysis of experimental measurements and their uncertainties, expanding the types of experiments included in the procedure, and developing new mathematical algorithms. The hydrogen combustion mechanism, being relatively small and easy to analyse, was used as a test object in this study, achieving two goals at once: developing a new optimised hydrogen combustion mechanism, and refining the mechanism optimisation procedure itself, to apply it further for more complex fuels.
