A skeletal mechanism for the prediction of NOx emissions from methane combustion at gas turbine conditions is developed in the present work. The goal is a mechanism that can be used in computational fluid dynamic modeling of lean premixed (LPM) combustors. A database of solutions from 0-D, adiabatic, homogeneous reactors (PSRs) is computed using CHEMKINPRO [1] over a parameter space chosen to include pressures from 1 to 30 atm, equivalence ratios from 0.4 to 1.0, and mean PSR residence times from slightly greater than blowout to 3ms. A resisidence time of 3 ms represents a useful maximum for the super-equilibrium flame zone where most of the NOx forms in LPM combustors. Fuel oxidation and NOx formation are treated separately in the reduction process. The method of Directed Relation Graph (DRG) is applied for methane oxidation and its extension, DRG-aided sensitivity analysis (DRGASA), is used to determine the skeletal NOx mechanism to append to the methane mechanism. Post-processing of the PSR solution database and implementation of the reduction algorithm are accomplished in SAGE [2], a Python based, open-source mathematics software package. The skeletal oxidation and NOx mechanisms are validated against full GRI 3.0 [3] in both PSR and laminar flame speed calculations. When compared with the detailed GRI 3.0 mechanism, NOx emissions are predicted within 7% near blowout and 3% at 3ms, and laminar flame speeds are predicted within 20% over the range of equivalence ratios and pressures. The skeletal mechanism is presented here and it should be noted that all reactions of the H2/CO submechanism are retained. The skeletal mechanism consists of 22 species and 122 reactions for methane oxidation and an additional 8 species and 55 reactions to describe NOx formation (30 species, 177 reactions total). The final skeletal mechanism with NOx chemistry is available for download here [4]. To demonstrate the predictive capability of the validated mechanism in a reactive flow system, it is implemented in an ANSYS Fluent model of a single jet stirred reactor, the results of which are compared to experimental reactor data presented in [5] and [6]. Predicted and measured profiles of temperature and NOx emissions are shown. Temperature and NOx emissions compare well in the recirculation zone of the JSR, although both NOx emissions and temperature are under-predicted in the jet region. Finally, the contribution of each chemical pathway for NOx formation is evaluated.

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NOJ from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet, stirred reactor (JSR). In this study. CH4 is mi.xed with varying levels CO2 and Ni. NO^ measurements are taken at a nominal combustion temperature of 1800K. atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for A'O, emissions as a function of fuel dilution: (I) more NO^ is prodticed per kg ofCH4 consumed with the addition of a diluent. (2) the degree of increase in emission index is dependent on the chosen diluent; N2 dilution increases NO^ production more effectively than equivalent CO2 dilution. Chemical kinetic modeling suggests that NOy production is ¡ess effective for the mixture diluted with CO: due to both a decrease in Nj concentration and the ability of CO2 to deplete the radicals taking part in NO^ formation chemistry. In order to gain insight on flame structure within the JSR. three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH4 combustion. A global CH4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH4 and CO o.xidation quite well. The CFD model illustrates the flow-field, temperature variation, and flame structure within the JSR. A 3-elenient chemical reactor network (CRN), i net tiding detailed chemistry, is constrticted using insight from spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NOy emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.

Gaseous fuels other than pipeline natural gas are of interest in high-intensity premixed combustors (e.g., lean-premixed gas turbine combustors) as a means of broadening the range of potential fuel resources and increasing the utilization of alternative fuel gases. An area of key interest is the change in emissions that accompanies the replacement of a fuel. The work reported here is an experimental and modeling effort aimed at determining the changes in NOx emission that accompany the use of alternative fuels. Controlling oxides of nitrogen (NOx) from combustion sources is essential in nonattainment areas. Lean-premixed combustion eliminates most of the thermal NOx emission but is still subject to small, although significant amounts of NOx formed by the complexities of free radical chemistry in the turbulent flames of most combustion systems. Understanding these small amounts of NOx, and how their formation is altered by fuel composition, is the objective of this paper. We explore how NOx is formed in high-intensity, lean-premixed flames of alternative gaseous fuels. This is based on laboratory experiments and interpretation by chemical reactor modeling. Methane is used as the reference fuel. Combustion temperature is maintained the same for all fuels so that the effect of fuel composition on NOx can be studied without the complicating influence of changing temperature. Also the combustion reactor residence time is maintained nearly constant. When methane containing nitrogen and carbon dioxide (e.g., landfill gas) is burned, NOx increases because the fuel/air ratio is enriched to maintain combustion temperature. When fuels of increasing C/H ratio are burned leading to higher levels of carbon monoxide (CO) in the flame, or when the fuel contains CO, the free radicals made as the CO oxidizes cause the NOx to increase. In these cases, the change from high-methane natural gas to alternative gaseous fuel causes the NOx to increase. However, when hydrogen is added to the methane, the NOx may increase or decrease, depending on the combustor wall heat loss. In our work, in which combustor wall heat loss is present, hydrogen addition deceases the NOx. This observation is compared to the literature. Additionally, minimum NOx emission is examined by comparing the present results to the findings of Leonard and Stegmaier.

The stability of hydrogen combustion under lean premixed conditions in a back-mixed jet-stirred reactor (JSR), is experimentally and numerically investigated. The goal is to understand the mechanism of flame extinction in this recirculation-stabilized flame environment. Extinction is achieved by holding the air flow rate constant and gradually decreasing the flow rate of the hydrogen fuel until a blowout event occurs. In order to gain insight on the mechanism controlling blowout, two dimensional computational fluid dynamic (CFD) simulations are carried out for the lean premixed combustion (LPM) of hydrogen as the fuel flow rate is reduced. The CFD model illustrates the evolution of the flow-field, temperature profiles, and flame structure within the JSR as blowout is approached. A single element chemical reactor network (CRN) consisting of a plug flow reactor (PFR) with recirculation is constructed based on the results of the CFD simulations, and its prediction of blowout is in good agreement with the experimental results. The chemical mechanism of Li et al. is used in both the CFD and CRN models, and GRI is used in the CRN for comparison. The modeling suggests that lean blowout does not occur with the flame in a spatially homogeneous condition, but rather under a zonal structure. Specifically, the flame is stabilized by the entrainment of combustion products from the re-circulation zone into the base of the reactant jet. The mixture of hot products and incoming premixed reactants proceeds through an ignition induction period followed by an ignition event. As the fuel flow decreases, the induction period increases and the ignition event is pushed further around the recirculation zone. Eventually, the induction period becomes so long that the ignition is incomplete at the point where the recirculating gas is entrained into the jet. This threshold leads to overall flame extinction.

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NOX) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet stirred reactor (JSR). In this study, CH4 is mixed with varying levels CO2 and N2. NOX measurements are taken at a nominal combustion temperature of 1800 K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NOX emissions as a function of fuel dilution: (1) more NOX is produced per kg of CH4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N2 dilution increases NOX production more effectively than equivalent CO2 dilution. Chemical kinetic modelling suggests that NOX production is less effective for the mixture diluted with CO2 due to both a decrease in N2 concentration and the ability of CO2 to deplete the radicals taking part in NOX formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH4 combustion. A global CH4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH4 and CO oxidation quite well. The CFD model illustrates the flow-field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constructed using insight from detailed spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NOX emissions for all fuel blends. The experimental and modelling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.