• Flameless
• FLOX
• Mild
• PCA
• Principal Components Analysis
• Stirling
• Turbulent combustion

The hydrogen economy has received considerable attention during the last years, in the academic, industrial and political fields, for its potential to deal with the urgent issues related to the world energy scenario. Hydrogen can be produced directly from all primary energy sources, allowing fuel diversification and energy independence. In particular, hydrogen containing fuels can be obtained from the thermochemical conversion of coal and renewable sources such as biomasses. The combination of the gasification technologies with carbon capture and storage (CCS) is particularly attracting, to increase H2 purity while reducing greenhouse gas emissions, i.e. CO2.
However, numerous challenges related to production, distribution and end use still need to be faced for H2 to become an energy carrier. For example, hydrogen purity is a major issue in fuel cells, as impurities can adversely affect performances and durability. Fuel composition does not represent a priori a concern for combustion systems; however, H2 properties may negatively affect conventional combustion systems, leading to stability problems and large NOx emissions. Therefore, efforts must be spent to develop technologies able to deal with the complexities of hydrogen containing mixtures. To this purpose, hydrogen/natural gas fuels represent a more realistic alternative to pure hydrogen in a short term perspective, as they provide a ready alternative to pure fossil fuels and retain the H2 potentials from the point of view of greenhouse gas reduction and efficiency increase.
The present Thesis reports numerical and experimental investigations of advanced systems for hydrogen-based fuel combustion. In particular, attention is devoted to a novel combustion technology, named flameless combustion, which allows to control NOx emissions while ensuring very high combustion and thermal efficiencies. Flameless combustion is based on the modification of the traditional flame structure: the system is driven towards homogeneous (temperature and species) conditions, thus allowing to smooth the effect of the oxidation process on the temperature distribution. Such effect is certainly beneficial for controlling NOx formation and shows potentials for limiting the reactivity of hydrogen-based fuels.
Three different case studies have been taken into account. Two of them are semi-industrial systems, both installed at ENEL Ricerca facilities of Livorno, Italy: a self recuperative burner used in the steel industry for heating applications and a micro-CHP unit for the distributed cogeneration of heat and power. The third system is a lab-scale burner, developed at the Politecnico di Milano. An integrated approach, based on both numerical modeling and experiments, has been employed to assess the feasibility of flameless combustion with hydrogen-enriched fuels, being the investigated devices designed for burning natural gas.
Recognizing the complexity of the aforementioned systems, a hierarchical approach is proposed to address the different chemical and physical processes which are involved in the overall operations. In particular, from a modeling prospective, the proper representation of turbulence-chemistry interactions is fundamental to correctly capture the principal features of the combustion process. Therefore, a fundamental study on turbulence-chemistry interactions in turbulent reacting flows has been carried out, to determine the modeling ingredients required for an accurate description of the flameless combustion regime. A novel methodology based on Principal Component Analysis is presented for the identification of the parameters controlling the evolution of a reacting system and for the development of optimal combustion models. To this purpose, high fidelity experimental and numerical data, available in the literature for reference systems, have been used.
The results obtained from such fundamental activity have supported the numerical modeling of the burners for hydrogen-enriched fuel combustion. The major focus of the numerical simulation is represented by the choice of the combustion model and kinetic mechanism and the sensitivity of the final predictions on such choices. It is known that flameless combustion relies on a competition between chemistry and turbulent mixing, ensured by either exhaust recirculation or fuel dilution. Therefore, turbulence-chemistry interactions require special treatment and simple combustion models are unsuited, as they cannot capture the volumetric and diffuse features of the regime. Recent works on the topic have suggested that only the use of detailed kinetic mechanisms can lead to reliable results; however further investigation is needed, especially when hydrogen is added to the fuel.
The numerical simulations of the flameless systems have been carried out with commercial numerical tools; however, state-of-the-art physical models from the literature have been coupled to the main code solvers, to enhance their modeling capabilities. In particular, heat transfer models have been implemented to simulate the system interactions with the surroundings and non-conventional NO formation routes, have been introduced to account for NO formation at low temperatures and with H2 in the fuel.
The quantitative validation of the computational approaches has represented a fundamental moment for the present Thesis, to critically identify potentials and limits in the mathematical models and to plan further improvements and developments. Therefore, the availability of the experimental data has been crucial for judging the actual predictive capabilities of the numerical simulations, over a wide range of operating conditions. In particular, the assessment of the level of agreement between experimental data and numerical simulations has been based on the Verification and Validation (V&V) methodology, which allows to determine the uncertainty in the modeling results by estimating both the experimental and numerical errors.