Manna, Maria Virginia (2022) Combustion Regimes and Chemical Kinetics of Ammonia. [Tesi di dottorato]

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The compelling requirement to reduce greenhouse gas emissions has led the scientific community to explore alternative energy sources, in particular energy vectors with a minor impact on the environment. The long-term chemical storage represents a key element for the decarbonization of the energy system, to guarantee security and flexibility to the power generation based on renewable sources. In this context, in recent years the interest of the scientific community has been focused on energy vectors whose production and use are able to meet the zero emissions target. Among the available molecules, ammonia is a high density hydrogen carrier and has the advantage of being a carbon-free carrier, with existing delivery infrastructures, consolidated production technologies and well-defined regulations. Ammonia can be efficiently cracked by catalytic processes to recover the hydrogen with high purity. Recently, the interest in ammonia as fuel itself has grown up as well. Nonetheless, while the advantages of ammonia are indisputable, it is known that its combustion properties, as well as the high NOx emissions, represent a serious drawback for its use in conventional combustion processes. Different strategies have been suggested to overcome these limits. Blending ammonia with carbon-based fuels has been proposed as plausible solution to improve ammonia performances in ICEs, boilers and gas turbines. These strategies allow to increase combustion efficiencies and mitigate NOx emissions, but compromise the development of a totally carbon-free energetic system, thus forcing to deal with CO and CO2 emissions. Alternatively, the use of hydrogen as a fuel enhancer can bypass this last drawback. However, this solution comes at a higher NOx specific emission. Due to the above-reported issues, the utilization of pure ammonia as a fuel can only be practicable by implementing new combustion modes. MILD combustion has been proven to be a suitable process to burn ammonia without the addition of fuel enhancers, with low-NOx emissions. The successful application of ammonia as alternative fuel under diluted and pre-heated conditions requires a detailed understanding of its oxidation process. To this aim, several efforts have been recently dedicated to the oxidation characteristics in simple reactor configurations for the evaluation of laminar flame speed, ignition delay times and ammonia oxidation regimes, along with key species and NOx productions in a wide range of operative conditions. The definition of basic characteristics of the ammonia oxidation chemistry has boosted also the development of detailed kinetic mechanisms. Despite all these efforts, the comprehension of the ammonia oxidation chemistry cannot be considered mature. This aspect is even more evident if referring to MILD operative conditions, where high dilution levels impose fuel oxidation to occur under low temperature regimes. The intersection of low combustion temperatures with diluted mixtures alters the evolution of the combustion process with respect to traditional flames, affecting the kinetics involved in fuel oxidation. Due to this background, the present thesis deals with an experimental and numerical characterization of ammonia oxidation in a Jet Stirred Flow Reactor. The experimental analyses have been carried out at nearly atmospheric pressure, covering a wide range of operative conditions. The combustion behavior of ammonia mixtures diluted in Ar and N2 was investigated for temperatures in the range 900-1350K, changing the mixture equivalence ratio from fuel-lean to fuel-rich conditions. Different dilution levels were considered in order to promote the mixture ignition in diverse temperature ranges, bringing to the light peculiar combustion behaviors. Indeed, high dilution levels (<99%) shift the ignition at high temperatures, due to the lower thermal power of the reactant mixtures. On the other hand, a moderate level of dilution, and thus higher fuel concentrations, promote the oxidation process at lower temperatures, favoring completely different kinetic paths. Such operating conditions are also particularly relevant for applications in engines and gas turbines. Temperature and species concentration measurements at steady state condition revealed for the first time the existence of different combustion regimes, namely low, intermediate and high temperature oxidation regimes. At low-intermediate temperatures, H2 and NO concentrations exhibited a non-monotonic trend as a function of the inlet temperatures, independently of the mixture composition, while for higher temperatures the system behavior was more strongly affected by the mixture stoichiometry. The oxidation process was then investigated through transient experimental tests, following the evolution of the reactor temperature in time. These analyses allowed to identify further peculiar behaviors like low-reactivity regime, damped and periodic oscillations, occurring at noticeable temperatures. Such phenomena were deeply explored changing the mixture composition, the residence time, the dilution level and diluent species. The effect of doping ammonia mixtures with NO was also explored in order to highlight the role of NO formation and reduction in ammonia oxidation chemistry. The interaction between ammonia and hydrogen oxidation chemistry was then investigated, highlighting the enhancing role of hydrogen at high temperatures and a peculiar mutual inhibiting interaction of such fuels under low-temperature conditions. The inhibiting effect of ammonia on hydrogen oxidation suggested, for the first time, the plausible role of such molecule as strong collider in third-body reactions, given its physical/chemical properties. This aspect could cover a key-role to improve kinetic models performances for the investigated conditions. Numerical simulations were carried out by means of several kinetic models available in literature. It was shown that kinetic models are able to reproduce the experimental data at high temperature (higher than 1300K), whereas they fail to describe the low temperature reactivity, with severe implications also on the prediction of dynamic behaviors. Kinetic analyses were performed to identify the controlling reaction pathways. It was found that the low-temperature oxidation chemistry depends on different NH2 consumption routes promoted in diverse temperature ranges, whose description changes mechanism by mechanism. In particular, models that envisage the oxidation of ammonia through recombination routes predict higher reactivity at low temperature and the insurgence of instabilities, that derive from a strong competition between NH2 oxidation and recombination routes. Based on the obtained experimental data and numerical analyses, a final effort to model ammonia oxidation was made, improving the performance of an existing detailed kinetic mechanism through the implementation of new reactions and the tuning of the kinetic parameters of the most sensitive ones. The declaration of third-body efficiency for ammonia was essential to correctly reproduce the non-monotonic profiles of the main species in the low-intermediate temperature regime.

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