Experimental and Numerical Modeling of NOx Formation in Premixed Combustion of Pure and Renewable Liquid Fuels

Abstract

A jet stirred reactor (JSR) is used to study the effects of fuel type and composition on NOx emissions. The reactor temperature is varied from 1700K to 1900K. Both pure and blended fuels are used in the experiments with the C/H ratio of fuels varying from 0.429 to 0.875. The fuels studied are: methane (as the baseline), n-hexane, n-octane, n-dodecane, isooctane, cyclo-hexane, toluene,1,3,5 tri-methyl benzene (135-TMB), jet-A, jet fuel made by Fischer-Tropsch processing of natural gas, and renewable jet fuel made from camelina, tallow, or bio-alcohol. The combustion of alternative and traditional commercial jet fuels show relatively minor differences in NOx. The trend observed at three different temperatures of the recirculation zone shows: HRJ-tallow < FT-natural gas < alcohol-based jet< HRJ- camelina. The largest difference in NOx is between aliphatic and aromatic fuels. Aromatics produce about 30% more NOx than aliphatic fuels. A Computational Fluid Dynamic (CFD) model of the JSR is created to study the fluid structure and chemistry inside the reactor for gaseous fuels such as hydrogen and methane. Reynolds Average Navier Stokes (RANS) turbulence modeling is initially used for each case to develop a solution that is later utilized to initialize the Large Eddy Simulation (LES) modeling of the flow. Modeling chemistry correctly is essential for this study, therefore, the Complex Chemistry model with the Laminar Flame Concept (LFC) is utilized. This model solves transport equations for species and calculates reaction rates by modified Arrhenius kinetic expressions. LES is used to determine the temperature and composition fluctuations in the recirculation zone, specifically at the location of sampling. LES modeling of H2 combustion with NOx chemistry shows that most of the radicals are mainly formed in the flame brush. O-atom, H-atom, and OH radical have the highest con- centrations in this region. These radicals react with nitrogen molecules and form important NOx species such as N2O, NNH, and NH. Since the concentration of O-atom and H-atom are highest in the flame brush, the Rate of Production (ROP) of NOx species are highest in the flame brush as well. NOx has the highest concentration in the recirculation zone due to the high residence time this zone provides. LES modeling of CH4 combustion shows that the jet occupies a larger volume in the JSR compared to the jet observed in the combustion of hydrogen. This is due to the slower chemistry of methane compared to hydrogen. Based on the CFD results, chemical kinetic modeling is performed to study NOx for- mation from liquid fuel combustion. A 3-zone chemical reactor network (CRN) model is developed and implemented to model the flame zones inside the JSR. The goal of this work is to predict emissions and explain the experimental trends observed for combustion of fu- els such as methane, iso-octane, n-octane, n-dodecane, 135-trimethylbenzene (TMB), and toluene. Two sets of hydrocarbon mechanisms are utilized in the CRN model, the individual fuel hydrocarbon mechanisms and a surrogate jet fuel mechanism. NOx is modeled using 2011 Klippenstein et al. H-N-O mechanism as well as two different prompt mechanisms of Konnov and Glarborg et al. The following findings are observed by analyzing the results of the CRN modeling. NOx is predicted well when the individual fuel hydrocarbon mechanisms or the surrogate jet fuel mechanism are used with Klippenstein et al. H-N-O and Konnov prompt NOx mechanisms. These emissions are slightly under-predicted for n-alkanes and iso-octane. These results are consistent among three different recirculation zone tempera- tures of 1700, 1800, and 1900 K. Sensitivity analysis on NOx pathways shows that N2O is the major pathway to NOx formation in the JSR at the recirculation zone temperature of 1800K. The Zeldovich route is the second most important route followed by the prompt pathway. The NNH route has a small contribution to overall NOx for these fuels in the lean premixed combustion of these fuels. Aromatic fuels emit more CO and NOx compared to aliphatic fuels as shown in both the experiments and the models. CRN results show that this is due to the higher concentrations of O-atom and H-atom present. Having a higher concentration of CO available produces more H-atom from oxidation of CO by the OH radical. The higher concentration of H radical promotes higher O-atom production from the oxidation of H-atom with oxygen. Higher H-atom and O-atom concentrations lead to higher NOx production for aromatics compared to aliphatics.