Philip C. Malte

A kinetic modeling study of n-pentane oxidation in a well-stirred reactor

Abstract Oxidation of n-pentane in a well-stirred reactor is examined, using a numerical model and a detailed chemical kinetic reaction mechanism, including 53 chemical species and 326 elementary reactions. Temperatures studied range from 1068K to 1253K, and the pressure is atmospheric. The major reaction paths consuming n-pentane are H-atom abstraction by OH radicals, followed in importance by H-atom abstraction by H and O atoms and the unimolecular decomposition to produce ethyl and n-propyl radicals. Logically distinguishable H atom sites in n-pentane are treated separately, and decomposition of the three different types of pentyl radicals via β-scission is found to dominate over isomerization through internal H atom transfer or reaction with molecular oxygen. Computed and measured results for chemical species concentrations show substantial agreement at temperatures below 1200K. However, at higher temperatures, the computed oxidation of the n-pentane is more rapid than measured. In the experiments, n-pentane oxidation does not become so rapid as to cause a clear breakdown is reactor homogeneity until the temperature exceeds 1260K. Modifications in some of the elementary reaction rate constants were found to produce much better agreement between computed and measured results in the temperature range from 1200 to 1260K. Explanations for this behavior are discussed, using a detailed sensitivity analysis of the computational model results.

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

In-stream tidal energy potential of Puget Sound, Washington

Abstract The far-field, barotropic effects of in-stream tidal energy extraction from Puget Sound are quantified using a one-dimensional channel model. In-stream turbines are modelled in regions of energetic flow in northern Admiralty Inlet and Tacoma Narrows. The far-field extraction effects include changes to the tide (amplitude and phase), transport, power dissipation, and kinetic power density. These effects are observed throughout Puget Sound and are dependent on the magnitude and location of extraction. The model indicates that a 5 per cent reduction in transport in the South Sound would correspond to either 260 MW of dissipation by in-stream turbines in Admiralty Inlet, 120 MW in Tacoma Narrows, or an intermediate level of dissipation in both locations. The environmental and economic limits on future developments are discussed. For pilot-scale development, this modelling indicates that the barotropic, far-field extraction effects on Puget Sound will be immeasurably small.

The effect of low-concentration fuels on the conversion of nitric oxide to nitrogen dioxide

The effect of low-concentration fuels on the conversion of NO to NO 2 is studied by an experiment and a chemical kinetic calculation. In the experiment, the NO and NO 2 concentrations are measured for the mixing process of hot combustion gas with cold air in which nine types of fuel (seven hydrocarbons from C1 to C4, H 2 and CO) are added. The hot combustion gas contains from 6 ppm to 79 ppm NO. The experimental results show that the conversion of NO to NO 2 in the mixing process is strongly promoted by the addition of small amount of all nine types of fuel. For example, the addition of only 40 ppm of C 3 H 8 raises the proportion of NO 2 to NO x from 0.24 to 0.90. The effectiveness of promotion strongly depends on the type of fuel. For seven types of fuel showing a similar pattern of promotion, the effectiveness increases as H 2 4 2 H 6 2 H 4 3 H 8 i -C 4 H 10 n -C 4 H 10 . In the chemical kinetic calculation, in order to understand the fundamental chemical kinetic aspect of the effect of fuel on the conversion of NO to NO 2 , a constant-temperature air premixed with NO and a low concentration of hydrocarbons is assumed as a simple model. The reaction mechanism includes 115 reactions for C3 hydrocarbons and NO-NO 2 . The conversion of NO to NO 2 is indicated to proceed mainly through the HO 2 mechanism (NO+HO 2 =NO 2 +OH) even in the presence of fuel. Therefore, high effectiveness is obtained by the fuels which are most easily decomposed to produce HO 2 . Also, the promotion is shown to proceed strongly in a certain temperature range. It is suggested from the results that the presence of fuel in the cool region of the flow in a combustion system should be prevented in order to reduce NO 2 exhaust emission.

NOx Formation in High-Pressure Jet-Stirred Reactors With Significance to Lean-Premixed Combustion Turbines

Measurements of NO x and CO in methane-fired, lean-premixed, high-pressure jet-stirred reactors (HP-JSRs), independently obtained by two researchers, are well predicted assuming simple chemical reactor models and the GRI 3.0 chemical kinetic mechanism. The single-jet HP-JSR is well modeled for NO x and CO assuming a single PSR for Damkohler number below 0.15. Under these conditions, the estimates of flame thickness indicate the flame zone, that is, the region of rapid oxidation and large concentrations of free radicals, fully fills the HP-JSR. For Damkohler number above 0.15, that is, for longer residence times, the NO x and CO are well modeled assuming two perfectly stirred reactors (PSRs) in series, representing a small flame zone followed by a large post-flame zone. The multijet HP-JSR is well modeled assuming a large PSR (over 88% of the reactor volume) followed by a short PFR, which accounts for the exit region of the HP-JSR and the short section of exhaust prior to the sampling point. The Damkohler number is estimated between 0.01 and 0.03. Our modeling shows the NO x formation pathway contributions. Although all pathways, including Zeldovich (under the influence of super-equilibrium O-atom), nitrous oxide, Fenimore prompt, and NNH, contribute to the total NO x predicted, of special note are the following findings: (1) NO x formed by the nitrous oxide pathway is significant throughout the conditions studied; and (2) NO x formed by the Fenimore prompt pathway is significant when the fuel-air equivalence ratio is greater than about 0.7 (as might occur in a piloted lean-premixed combustor) or when the residence time of the flame zone is very short. The latter effect is a consequence of the short lifetime of the CH radical in flames.

Further Observations of the Effect of Sample Probes on Pollutant Gases Drawn from Flame Zones

Abstract The effects of gas sample probes and conditioning techniques on the measurements of flame- generated NO, NO2, and CO have been further examined. (1) A low-pressure, condensation free sampling system was compared with a conventional system, which operated at higher pressure and involved H2O condensation. The conventional system demonstrated apparent losses of CO and NOx, in the range 10 to 50 percent. (2) Sampling from fuel-rich combustion and pyrolysis zones was examined. High NO2-to-NO ratios, approaching unity, were measured repeatedly in the fuel-rich pyrolysis zone of a high-intensity recirculative flame.

Effect of large-scale kinetic power extraction on time-dependent estuaries:

Abstract An open and important question in tidal in-stream energy conversion is the level of kinetic power extraction that is possible without unacceptable environmental degradation. In general, the effects of large-scale kinetic power extraction on estuary-scale fluid mechanics are not well understood. In this paper, these effects are quantified for an idealized estuary using a one-dimensional, time-dependent numerical model. The numerical domain consists of a long, wide inlet and basin connected by a constricted channel. Kinetic power densities within this constriction are suitable for in-stream energy conversion. Modelling shows that the extraction of kinetic power has a number of effects, including: (a) reduction of the volume of water exchanged through the estuary over the tidal cycle; (b) reduction of the tidal range landward of the array; and (c) reduction of the kinetic power density in the tidal channel. These impacts are strongly dependent on the magnitude of kinetic power extraction, estuary geometry, tidal regime, and non-linear turbine dynamics. It is shown that it may be misleading to relate these impacts to the fraction of kinetic energy extracted from the system. Results highlight the importance of time-dependent modelling and the incorporation of non-linear turbine dynamics.

Investigation of NOx and CO formation in lean-premixed, methane/air, high-intensity, confined flames at elevated pressures

The coupling between NOx formation chemistry and the mixing/transport environment is of critical importance to the design of lean-premixed gas turbine combustors but is incompletely understood. In the present research, this problem was addressed via the study of NOx formation in a high-pressure jet-stirred reactor operating on lean-premixed methane/air. These experiments focused on the effects of residence time (0.5–4.0 ms), pressure (3.0, 4.7, and 6.5 atm), and inlet temperature (344–573K). The combustion temperature varied from 1815±5 K at the lowest residence times to 1910±30K at the largest residence times. The NOx was lowest at intermediate residence times, reaching higher values at the extremes. Increasing pressure and inlet temperature tend to reduce NOx concentrations. Concentration profiling in the reactor suggests two general environments: (1) a highly non-equilibrium reaction zone defined by high CO concentrations, and (2) a postflame environment. The NOx formation was concentrated in the region of strongly non-equilibrium combustion chemistry. The Damkohler number was 0.06≤Da≤1, and the ratio of turbulent intensity to laminar burning velocity was 28≤u′/SL≤356, indicating the combustion occurs in the high-intensity, chemical rate-limiting regime. The results were interpreted using a two-environment, detailed chemistry model in which the size and structure of the flame environment were established by matching the measured data, and which were independently verified using turbulent flame velocity/thickness correlations. The modeling suggests NOx formation is controlled by both the specific conditions in the non-equilibrium zone and by the size of the zone. Since both these features are influenced by the experimental parameters, a highly nonlinear scenario emerges with implications for minimizing NOx via combustor design. The modeling also suggests the unique case of well-stirred combustion for NOx at elevated pressure is obtained at low residence time conditions.