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Formation of Nitrogen Oxides in the Combustion of H2-rich Fuel Mixtures in the presence of Aromatics

par Cafiero, Marianna
Président du jury Coussement, Axel
Promoteur Parente, Alessandro
Co-Promoteur Jeanmart, Hervé
Publication Non publié, 2022-11-28

Thèse de doctorat

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Résumé :

The minimization of nitrogen oxides (NOx) emissions is among the major concerns of combustion-related processes, together with the adverse effects related to the emission of greenhouse gases. NOx emission control can become very challenging for those combustion processes which require high temperatures, and/or involve the exploitation of non-conventional fuel mixtures. This PhD thesis work aims to shed light on NOx formation during combustion of Coke Oven Gas (COG) and particularly to understand the role that aromatic chemical compounds could play during NOx formation. COG is an attractive energy source derived from the coal carbonization, due to its high H2 content and calorific value. It has been widely investigated as a fuel directly burnt in-situ to sustain the energy requirement of the integrated steel production processes, or as an important source of valuable compounds (i.e., H2, CH4, syngas, methanol, etc). The residual presence of aromatic compounds in COG, downstream the raw COG purification line, can potentially affect both its combustion efficiency and pollutant emissions. Therefore, a fundamental understanding of the interactions between NOx formation and aromatic species oxidation is required to both valorize the use of COG as energy carrier, and to highlight promising strategies for the minimization of NOx emissions during the combustion of COG in coke making plants. Benzene (C6H6) has been considered as the representative aromatic compound. The effect of its addition on NO formation chemistry, during combustion of COG surrogate fuel blends has been investigated, under different stoichiometric conditions. A hierarchical approach was used in this thesis work to achieve this objective. Indeed, NOx formation during combustion of a COG surrogate mixture (H2/CH4/CO) has been investigated at i) lab-scale, in low-pressure flat premixed laminar flames through a joint experimental and kinetic modeling study, and at ii) semi-industrial scale, in a 20-kW semi-industrial furnace. The hierarchical approach adopted in this work allowed to highlight the main effects of benzene (C6H6) on NO formation during combustion of COG surrogate mixtures. Particularly, benzene was found to lead to two main competing effects in the COG flames: (a) increase of prompt NO, due to the increased production of the CH radical coming from intermediate species of the aromatic ring oxidation (direct chemical effect); (b) decrease of thermal NO when the local formation of soot particles enhances the emissivity of the flames, thus the efficiency of the heat transfer from the flame to its surroundings (indirect effect due to flame temperature reduction). The relative importance of the two effects determines the final NO x emissions emitted from a combustion system fed with COG.