| The goal of this research is to characterize experimentally and numerically laminar flames of syngas fuels made of hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). More specifically, the secondary objectives are: 1) to understand the effects of CO2 concentration and H2/CO ratio on NOx emissions, flame temperature, visible flame height, and flame appearance; 2) to analyze the influence of H2/CO ratio on the lame structure, and; 3) to compare and validate different H2/CO kinetic mechanisms used in a CFD (computational fluid dynamics) model over different H2/CO ratios. Thus, the present thesis is divided in three chapters, each one corresponding to a secondary objective. For the first part, experimentations enabled to conclude that adding CO2 diminishes flame temperature and EINOx for all equivalence ratios while increasing the H2/CO ratio has no influence on flame temperature but increases EINOx for equivalence ratios lower than 2. Concerning flame appearance, a low CO2 concentration in the fuel or a high H2/CO ratio gives the flame an orange color, which is explained by a high level of CO in the combustion by-products. The observed constant flame temperature with the addition of CO, which has a higher adiabatic flame temperature, is mainly due to the increased heat loss through radiation by CO2. Because NOx emissions of H2/CO/CO 2 flames are mainly a function of flame temperature, which is a function of the H2/CO ratio, the rest of the thesis concentrates on measuring and predicting species in the flame as a good prediction of species and heat release will enable to predict NOx emissions. Thus, for the second part, different H2/CO fuels are tested and major species are measured by Raman spectroscopy. Concerning major species, the maximal measured H 2O concentration decreases with addition of CO to the fuel, while the central CO2 concentration increases, as expected. However, at 20% of the visible flame height and for all fuels tested herein, the measured CO2 concentration is lower than its stoechiometric value while the measured H2O already reached its stoechiometric concentration. The slow chemical reactions necessary to produce CO2 compared to the ones forming H2O could explain this difference. For the third part, a numerical model is created for a partially premixed flame of 50% H 2 / 50% CO. This model compares different combustion mechanisms and shows that a reduced kinetic mechanism reduces simulation times while conserving the results quality of more complex kinetic schemes. This numerical model, which includes radiation heat losses, is also validated for a large range of fuels going from 100% H2 to 5% H2 / 95% CO. The most important recommendation of this work is to include a NOx mechanism to the numerical model in order to eventually determine an optimal fuel. It would also be necessary to validate the model over a wide range for different parameters such as equivalence ratio, initial temperature and initial pressure. |
