The chemistry of product-gas entrainment in low-nitrous oxides ,multijet, natural-gas burners

The combustion of natural gas in industrial furnaces produces a considerable amount of NO_x because of the high operating temperature. In order to reduce NO_x emissions, a non-premixed, ultra-low-NO_x burner was designed by the Canadian Gas Research Institute (CGRI). The burner reduces NO_x formation by entraining product gases from the furnace cavity by an aerodynamically induced recirculation in the air and fuel jets before they meet in the main combustion zone. The objective of this research is to investigate the role of entrained product gases on the formation of combustion emissions during CH₄ oxidation.; A series of experiments was carved out in a bench-scale, plug-flow reactor under oxygen-rich and fuel-rich conditions with simulated product gas stream pertaining to the CGRI burner operating conditions. Experimental results revealed that the temperature, equivalence ratio and the amount of NO present in the feed have a significant influence on the formation of combustion emissions under oxygen-rich conditions, in which the presence of NO promotes the oxidation of CH₄. However, no significant difference in the concentration of NO between the feed and product gases was observed, except during the partial oxidation of CH₄. At low temperatures and at higher values of equivalence ratio, the concentration of NO_x in the sampling gas was reduced up to ∼50%, when a major portion of the CH₄ was oxidized to CO under oxygen-rich conditions.; Plug-flow simulation was performed using detailed kinetic mechanisms for oxygen-rich (NO-sensitized oxidation mechanism) and fuel-rich (reburning mechanism) conditions to compare the kinetic mechanisms with the experimental results. A sensitivity analysis was performed to identify the main reactions involved in the formation of NO_x under CGRI burner operating conditions. Reaction CH₃ + NO₂ → CH₃O + NO was found to be the predominant route for the NO-sensitized oxidation of CH₄ under oxygen-rich conditions. Nitric oxide acts as a catalyst for the oxidation of CH₄ by producing NO₂ via NO + HO ₂ → NO₂ + OH.; The number of reactions in the detailed kinetic mechanisms was reduced by eliminating the unimportant reactions using principal component analysis (PCA), in which eigenvalue-eigenvector decomposition was performed on the normalized sensitivity coefficient matrix. Also, the loadings obtained in eigenvalue-eigenvector decomposition were used to define the reaction pathways. Considering the spatial (or temporal) variations of the temperature and concentration of species, the important reactions at different spatial (or temporal) positions were identified to construct an adaptive reduced kinetic mechanism using functional PCA.