David G. Nicol

Development of a Five-Step Global Methane Oxidation-NO Formation Mechanism for Lean-Premixed Gas Turbine Combustion

It is known that many of the previously published global methane oxidation mechanisms used in conjunction with computational fluid dynamics (CFD) codes do not accurately predict CH{sub 4} and CO concentrations under typical lean-premixed combustion turbine operating conditions. In an effort to improve the accuracy of the global oxidation mechanism under these conditions, an optimization method for selectively adjusting the reaction rate parameters of the global mechanisms (e.g., pre-exponential factor, activation temperature, and species concentration exponents) using chemical reactor modeling is developed herein. Traditional global mechanisms involve only hydrocarbon oxidation; that is, they do not allow for the prediction of NO directly from the kinetic mechanism. In this work, a two-step global mechanism for NO formation is proposed to be used in combination with a three-step oxidation mechanism. The resulting five-step global mechanism can be used with CFD codes to predict CO, CO{sub 2}, and NO emission directly. Results of the global mechanism optimization method are shown for a pressure of 1 atmosphere and for pressures of interest for gas turbine engines. CFD results showing predicted CO and NO emissions using the five-step global mechanism developed for elevated pressures are presented and compared to measured data.

NOx and N2O in lean-premixed jet-stirred flames

Abstract This study examines NOx/N2O formation mechanisms that are relevant to lean-premixed combustion in practical high-intensity combustors. Both experiments and kinetic modelling are presented. The experimental system for examining high-intensity, lean-premixed combustion is a 1-atm jet-stirred reactor. The reactor burns CH4 and CO/H2 over a fuel-air equivalence range of 0.41 to 0.67, a reactor mean residence time of 1.7–7.4 ms, and measured reactor temperature of 1415 to 1845 K. The CO/H2 fuel is used to eliminate the effects of hydrocarbon attack on the nitrogen system. The NOx mole fraction (wet) measured for the CH4 experiments correlates well with measured temperature (K) and reactor mean residence time τ (s) as follows: XNOx = τ(1.28 × 104)exp(−27230/T). Because of the enhanced O-atom concentration, the NOx mole fraction for the CO/H2 combustion is about threefold higher than for the CH4 combustion. Furthermore, because the free radical behavior in the CO/H2 experiments is complex near blowout, a simple correlation is not available. For the CH4-air experiments, chemical reactor modeling indicates that the approximate percentages of NOx production by the nitrous oxide, Zeldovich, and prompt mechanisms vary from 65:25:10 at 1650 K to 35:50:15 at 1850 K. For the CO/H2-air experiments the nitrous oxide to Zeldovich contributions vary from 95:5 at 1500 K to 65:35 at 1725 K.

The Importance of the Nitrous Oxide Pathway to NOx in Lean-Premixed Combustion

This study addresses the importance of the different chemical pathways responible for NO x formation in lean-premixed combustion, and especially the role of the nitrous oxide pathway relative to the traditional Zeldovich pathway. NO x formation is modeled and computed over a range of operating conditions for the lean-premixed primary zone of gas turbine engine combustors. The primary zone, of uniform fuel-air ratio, is modeled as a micromixed well-stirred reactor, representing the flame zone, followed by a series of plug flow reactors, representing the postflame zone. The fuel is methane. The fuel-air equivalence ratio is varied from 0.5 to 0.7. The chemical reactor model permits study of the three pathways by which NO x forms, which are the Zeldovich, nitrous oxide, and prompt pathways. Modeling is also performed for the well-stirred reactor alone. Three recently published, complete chemical kinetic mechanisms for the C1−C2 hydrocarbon oxidation and the NO x formation are applied and compared. Verification of the model is based on the comparison of its NO x output to experimental results published for atmospheric pressure jet-stirred reactors and for a 10 atm. porous-plate burner. Good agreement between the modeled results and the measurements is obtained for most of the jet-stirred reactor operating range. For the porous-plate burner, the model shows agreement to the NO x measurements within a factor of two, with close agreement occurring at the leanest and coolest cases examined. For lean-premixed combustion at gas turbine engine conditions, the nitrous oxide pathway is found to be important, though the Zeldovich pathway cannot be neglected. The prompt pathway, however, contributes small-to-negligible NO x . Whenever the NO x emission is in the 15 to 30 ppmv (15 percent O 2 , dry) range, the nitrous oxide pathway is predicted to contribute 40 to 45 percent of the NO x for high-pressure engines (30 atm), and 20 to 35 percent of the NO x for intermediate pressure engines (10 atm). For conditions producing NO x of less than 10 ppmv (15 percent O 2 , dry), the nitrous oxide contribution increases steeply and approaches 100 percent. For lean-premixed combustion in the atmospheric pressure jet-stirred reactors, different behavior is found. All three pathways contribute; none can be dismissed. No universal behavior is found for the pressure dependence of the NO x . It does appear, however, that lean-premixed combustors operated in the vicinity of 10 atm have a relatively weak pressure dependence, whereas combustors operated in the vicinity of 30 atm have an approximately square root pressure dependence of the NO x

Characterization of NOx, N20, and CO for Lean-Premixed Combustion in a High-Pressure Jet-Stirred Reactor

A high-pressure jet-stirred reactor (HP-JSR) has been built and applied to the study of NO{sub x} and N{sub 2}O formation and CO oxidation in lean-premixed (LPM) combustion. The measurements obtained with the HP-JSR provide information on how NO{sub x} forms in lean-premixed, high-intensity combustion, and provide comparison to NO{sub x} data published recently for practical LPM combustors. The HP-JSR results indicate that the NO{sub x} yield is significantly influenced by the rate of relaxation of super-equilibrium concentrations of the O-atom. Also indicated by the HP-JSR results are characteristic NO{sub x} formation rates. Two computational models are used to simulate the HP-JSR and to provide comparison to the measurements. The first is a chemical reactor model (CRM) consisting of two perfectly stirred reactors (PSRs) placed in series. The second is a stirred reactor model with finite rate macromixing (i.e., recirculation) and micromixing. The micromixing is treated by either coalescence-dispersion (CD) or interaction by exchange with the mean (IEM) theory. Additionally, a model based on one-dimensional gas dynamics with chemical reaction is used to assess chemical conversions within the gas sample probe.

Simplified models for NOx production rates in lean-premixed combustion

Simplified models for predicting the rate of production of NOx in lean-premixed combustion are presented. These models are based on chemical reactor modeling, and are influenced strongly by the nitrous oxide mechanism, which is an important source of NOx in lean-premixed combustion. They include 1) the minimum set of reactions required for predicting the NOx production, and 2) empirical correlations of the NOx production rate as a function of the CO concentration. The later have been developed for use in an NOx post-processor for CFD codes. Also presented are recent laboratory data, which support the chemical rates used in this study.Copyright

Development of a five-step global methane oxidation - NO formation mechanism for lean-premixed gas turbine combustion

It is known that many of the previously published global methane oxidation mechanisms used in conjunction with computational fluid dynamics (CFD) codes do not accurately predict CH4 and CO concentrations under typical lean-premixed combustion turbine operating conditions. In an effort to improve the accuracy of the global oxidation mechanism under these conditions, an optimization method for selectively adjusting the reaction rate parameters of the global mechanisms (e.g., pre-exponential factor, activation temperature and species concentration exponents) using chemical reactor modeling is developed herein. Traditional global mechanisms involve only hydrocarbon oxidation; that is, they do not allow for the prediction of NO directly from the kinetic mechanism. In this work, a two-step global mechanism for NO formation is proposed to be used in combination with a three-step oxidation mechanism. The resulting five-step global mechanism can be used with CFD codes to predict CO, CO2, and NO emission directly. Results of the global mechanism optimization method are shown for a pressure of 1 atmosphere and for pressures of interest for gas turbine engines. CFD results showing predicted CO and NO emissions using the five-step global mechanism developed for elevated pressures are presented and compared to measured data.Copyright

NOx Sensitivities for Gas Turbine Engines Operated on Lean-Premixed Combustion and Conventional Diffusion Flames

NOx exhaust emissions for gas turbine engines with lean-premixed combustors are examined as a function of combustor pressure (P), mean residence time (τ), fuel-air equivalence ratio (φ), and inlet mixture temperature (Ti). The fuel is methane. The study is accomplished through chemical reactor modeling of the combustor, using CH4 oxidation and NOx kinetic mechanisms currently available. The NOx is formed by the Zeldovich, prompt, and nitrous oxide mechanisms.The combustor is assumed to have a uniform φ, and is modeled using two reactors in series. The first reactor is a well-stirred reactor (WSR) operating at incipient extinction. This simulates the initiation and stabilization of the combustion process. The second reactor is a plug-flow reactor (PFR), which simulates the continuation of the combustion process, and permits it to approach completion. For comparison, two variations of this baseline model are also considered. In the first variation, the combustor is modeled by extending the WSR until it fills the whole combustor, thereby eliminating the PFR. In the second variation, the WSR is eliminated, and the combustor is treated as a PFR with recycle. These two variations do not change the NOx values significantly from the results obtained using the baseline model.The pressure sensitivity of the NOx is examined. This is found to be minimum, and essentially nil, when the conditions are P = 1 to 10atm, Ti = 600K, and φ = 0.6. However, when one or more of these parameters increases above the values listed, the NOx dependence on the pressure approaches P raised to a power of 0.4-to-0.6.The source of the NOx is also examined. For the WSR operating at incipient extinction, the NOx is contributed mainly by the prompt and nitrous oxide mechanisms, with the prompt contribution increasing as φ increases. However, for the combustor as a whole, the nitrous oxide mechanism predominates over the prompt mechanism, and for φ of 0.5-to-0.6, competes strongly with the Zeldovich mechanism. For φ greater than 0.6-to-0.7, the Zeldovich mechanism is the predominant source of the NOx for the combustor as a whole.Verification of the model is based on the comparison of its output to results published recently for a methane-fired, porous-plate burner operated with variable P, φ, and Ti. The model shows agreement to these laboratory results within a factor two, with almost exact agreement occurring for the leanest and coolest cases considered. Additionally, comparison of the model to jet-stirred reactor NOx data is shown. Good agreement between the model results and the data is obtained for most of the jet-stirred reactor operating range. However, the NOx predicted by the model exhibits a stronger sensitivity on the combustion temperature than indicated by the jet-stirred reactor data.Although the emphasis of the paper is on lean-premixed combustors, NOx modeling for conventional diffusion-flame combustors is presented in order to provide a complete discussion of NOx for gas turbine engines.Copyright

Effects of Incomplete Premixing on NOx Formation at Temperature and Pressure Conditions of LP Combustion Turbines

A probability density function/chemical reactor model (PDF/CRM) is applied to study how NOx emissions vary with mean combustion temperature, inlet air temperature, and pressure for different degrees of premixing quality under lean-premixed (LP) gas turbine combustor conditions. Inlet air temperatures of 550, 650 and 750 K, and combustor pressures of 10, 14 and 30 atm are examined in different chemical reactor configurations. Primary results from this study are: incomplete premixing can either increase or decrease NOx emissions, depending on the primary zone stoichiometry; an Arrhenius-type plot of NOx emissions may have promise for assessing the premixer quality of lean-premixed combustors; and decreasing premixing quality enhances the influence of inlet air temperature and pressure on NOx emissions.Copyright

Characterization of NOx, N2O, and CO for Lean-Premixed Combustion in a High-Pressure Jet-Stirred Reactor

A high-pressure jet-stirred reactor (HP-JSR) has been built and applied to the study of NOx and N2O formation and CO oxidation in lean-premixed (LPM) combustion. The measurements obtained with the HP-JSR provide information on how NOx forms in lean-premixed, high-intensity combustion, and provide comparison to NOx data published recently for practical LPM combustors. The HP-JSR results indicate that the NOx yield is significantly influenced by the rate of relaxation of super-equilibrium concentrations of the O-atom. Also indicated by the HP-JSR results are characteristic NOx formation rates. Two computational models are used to simulate the HP-JSR, and to provide comparison to the measurements. The first is a chemical reactor model (CRM) consisting of two perfectly-stirred reactors (PSRs) placed in series. The second is a stirred reactor model with finite rate macromixing (i.e., recirculation) and micromixing. The micromixing is treated by either coalescence-dispersion (CD) or interaction-by-exchange-with-the-mean (IEM) theory. Additionally, a model based on one-dimensional gas dynamics with chemical reaction is used to assess chemical conversions within the gas sample probe.Copyright