Characterization of thermal NOx formation in hydrogen diffusion burners and analysis of novel design alternatives for boilers

[Publication]

Abstract

In the framework of renewable energies, the potential of green hydrogen as an energy vector is becoming a promising alternative to reduce the greenhouse gas emissions as well as the dependency on fossil fuels. The present doctoral thesis focuses on the use of green hydrogen in domestic and industrial boiler burners as an alternative to natural gas, eliminating the greenhouse gases such as carbon dioxide and toxic gases such as carbon monoxide. The combustion process taking place in the burners depends on the fuel and its characteristics. In that sense, the properties of gaseous hydrogen considerably differ from natural gas, resulting in a different behaviour in the combustion process. Due to its wide flammability limits, premixed and partially premixed hydrogen flames could lead to the so called flame flashback phenomena, compromising the integrity of the system. On the other hand, the high flame temperatures promote the thermal NOx formation, leading to negative environmental effects as well as respiratory diseases. Hence, those characteristics makes it necessary to restate new design alternatives which ensure an optimum combustion process using 100\% hydrogen, eliminating the flashback risk and reducing the NOx formation. In this context, the main objective of the doctoral thesis is to restate new buner designs avoiding or reducing the impact of such phenomena. For that purpose, the diffusion flame technology together with multi-flame burners was selected, eliminating the flashback risk and focusing the core of the thesis on thermal NOx formation. As an starting point, its formation was characterised in diffusion flames considering a simple case study and analyzing the impact of different operating parameters (thermal power, air excess level and flame regime). The obtained results served to raise new design approaches depending on the flame regime (laminar-turbulent). Following the initial characterisation, a turbulent multi-flame diffusion burner was selected, taking as reference the Micromix technology which has already been implemented in gas turbine burners. The concept is based on the formation of miniaturised turbulent diffusion flames, anchored between two vortices generated by the combination of aerodynamic effects induced by bluff bodies and jets in cross-flow configuration. Since domestic and industrial boiler operating conditions differ from gas turbine burners, the feasibility of Micromix technology for the design of domestic and industrial boiler burners was analysed. The results showed that the main characteristics observed in gas turbines were maintained under the desired operating conditions and scaled geometries, showing low NOx levels with respect to the corresponding standards. The general methodology used in the initial characterisation and the feasibility stages was based on CFD simulations carried out through the software ANSYS Fluent. Accordingly, the combustion process was simulated through finite-rate chemistry and FGM models for laminar and turbulent flames respectively. The turbulent flow field was solved following the RANS approach. Numerical results were validated through experimental measurements of temperature and NOx emissions. Burner prototypes were constructed for each research stage along with the corresponding experimental setup. The results from the present thesis have been published in scientific journals with high impact factor.