NO Formation Mechanism Research Articles

Influence of C 2 and C 3 compounds of natural gas on NO formation: an experimental study based on LIF/CRDS coupling

This study reports CH and NO mole fraction profiles measured in 12 premixed flames representative of natural gas combustion. Four mixtures CH 4/O 2/N 2, CH 4/C 2H 6/O 2/N 2, CH 4/C 3H 8/O 2/N 2, and CH 4/C 2H 6/C 3H 8/O 2/N 2 have been investigated at three equivalence ratios (0.7, 1, and 1.25). The aim of this work was to study the influence of C 2 and C 3 compounds of natural gas on NO formation in flames. Flames were stabilised on a flat flame burner at low pressure (33 torr). The experimental approach was exclusively based on non-intrusive techniques: laser induced fluorescence (LIF) for OH-LIF thermometry and species concentration profiles determination, and cavity ring-down spectroscopy (CRDS) for absolute calibration. Thus, an extended database including a large dynamic of combined CH and NO mole fraction profiles (ranging, respectively, from 0.7 to 10 ppm and from 2.6 to 24 ppm) has been obtained and will serve for further improvement of chemical mechanisms of NO formation in natural gas flames. It is shown that, for a given equivalence ratio, NO quantity remains unchanged when substituting methane by ethane and/or propane, while CH mole fraction exhibits a slight decrease. Predominance of prompt-NO mechanism is shown to be sensitive to the equivalence ratio. These new results have been modelled by using the GRI3.0 mechanism, and a reaction path analysis has been performed in the stoichiometric methane flame to highlight the different routes of NO formation in the different zones of the flame, i.e., the preheat zone, the reaction zone, and the burned gases.

LIF MEASUREMENTS AND CHEMICAL KINETIC ANALYSIS OF NITRIC OXIDE FORMATION IN HIGH-PRESSURE COUNTERFLOW PARTIALLY PREMIXED AND NONPREMIXED FLAMES

We report quantitative, spatially resolved, linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in laminar, methane/air counterflow partially premixed and nonpremixed flames at six pressures up to 15 atm using excitation near 226.03 nm in the γ(0,0) band of NO. For partially premixed flames, fuel-side equivalence ratios (φB) of 1.45, 1.6, and 2.0 are studied at a global strain rate of 20 s−1. For nonpremixed flames, a complete set of NO measurements at global strain rates of 20 s−1, 30 s−1, and 40 s−1 is presented to supplement previously reported data in such flames. The quantitative NO measurements are compared with predictions from an opposed-flow flame code utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on NO predictions is assessed by using a modified version of the code that considers radiation in the optically thin limit. The linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species and temperature profiles generated by the opposed-flow flame code plus quenching cross sections for NO available from the literature. A pathway analysis provides the relative contribution of different NO formation mechanisms to the total amount of NO produced at various pressures. Quantitative reaction path diagrams are used to investigate pictorially species interactions during NO formation. Finally, we identify key reactions controlling NO concentrations in counterflow partially premixed and nonpremixed flames by using a sensitivity analysis. For the nonpremixed flames, NO measurements at pressures of 2–5 atm disagree substantially with predictions from either GRI mechanism. On the other hand, both mechanisms predict observed NO concentrations reasonably well beyond 6 atm. In general, for our nonpremixed flames, NO formation is dominated by the prompt route at lower pressures with an increasing contribution from the N2O pathway at higher pressures. For partially premixed flames, GRI 3.0 mimics the basic quantitative trends found in our LIF measurements at 1–15 atm. The kinetic analysis indicates that the prompt mechanism dominates at lower pressures, whereas the thermal and N2O pathways become more important at higher pressures. At any given pressure, a reduction in partial premixing results in kinetic behavior approaching that of the nonpremixed flames.

COMPUTATIONAL MODEL TO INVESTIGATE THE MECHANISMS OF NOx FORMATION DURING WASTE INCINERATION

A numerical model has been developed to study the incineration of solid waste within a fixed-bed reactor. The model simplifies the flow to a one-dimensional succession of perfectly stirred reactors (PSRs) and includes a detailed chemical model. The calculations use the PSR model included in CHEMKIN II. The model predictions or residual nitrogen oxide are in good agreement with the experiments and have permitted validation of the model. The model has been used to investigate the mechanisms of NO formation during the combustion process of solid waste and to determine the principal reactive zones into the setup. NO is formed rapidly as gaseous fuel enters the reactor and occurs in three different steps. The principal intermediates are NCO, NH, and HNO. The rate of formation is dependant on the local oxygen content. A second region of NO production can be identified later in the trajectory of the fuel. Here the NO already formed will react with reducing species such as NH2, NH, NCO, CH3, and HCCO to produce N2.

The mechanism of NO formation from the decomposition of dialkylamino diazeniumdiolates: density functional theory and CBS-QB3 predictions.

The mechanism of decomposition of dialkylamino diazeniumdiolate salts, R(2)N[NONO](-)M(+), to form nitric oxide, NO, was explored using theoretical methods. B3LYP/6-311+G(d) density functional theory calculations gave the optimized geometries, and energetics were further evaluated with the high accuracy CBS-QB3 method, when feasible. Relative pK(a) values were estimated using the PCM model for aqueous solvation. The terminal oxygen is the most basic site, followed closely by the internal oxygen. Protonation of these sites does not lead to decomposition. However, protonation of the weakly basic amino nitrogen leads to very rapid decomposition and NO generation.

New low emission strategies and combustor designs for civil aeroengine applications

In the past, combustion computational fluid dynamics (CFD) in the framework of aeroengines mainly concentrated on high power conditions of combustors using diffusion type flames. In this paper, an overview of novel combustion concepts is given setting the scene for future CFD activities. The demand to lower emissions from jet engines powering passenger aircrafts has prompted a variety of research programmes devoted to this subject. In parallel to the optimisation of conventional combustors with respect to pollutant emissions, particularly nitrogen oxides, novel concepts have been invented, investigated and developed to different degrees of maturity. This presentation starts with a comprehensive overview on the requirements, discusses the NO formation mechanisms and presents concepts to overcome them. An assessment of the various concepts is made taking into account their specific advantages and drawbacks.

Local Flame Structure and NO Formation Mechanism in Methane-Air Turbulent Premixed Flames

Local Flame Structure and NO Formation Mechanism in Methane-Air Turbulent Premixed Flames

Ammonia conversion and NO x formation in laminar coflowing nonpremixed methane-air flames

This paper reports on a combined experimental and modeling investigation of NO x formation in laminar, ammonia-seeded, nitrogen-diluted, methane diffusion flames. The methane-ammonia mixture is a surrogate for biomass fuels, which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas. The computations use adaptive mesh refinement (AMR) to capture fine-scale features of the flame. The model includes a detailed chemical mechanism, differential diffusion, buoyancy, and radiative losses. The model shows good agreement with the measurements over the full range of experimental NH 3 seeding amounts. As more NH 3 is added, a greater percentage is converted to N 2 rather than to NO. The simulation results are analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.

NO x emission characteristics in rich–lean combustion of hydrogen

Characteristics of NO x formation in a gas turbine fuelled with hydrogen were analyzed with both an experimental and a numerical approach. This research experimentally investigated NO x reduction effect of rich–lean combustion in a coaxial burner. Hydrogen emits no Prompt NO even in rich mixture conditions, and can be more effective to reduce NO x in the rich–lean combustion system than hydrocarbons. The results show that the rich–lean combustion of hydrogen successfully reduces NO x emission compared with diffusive combustion. In the rich–lean combustion, hydrogen combustion has lower NO x emission compared to methane combustion, especially with larger equivalence ratio of richer side mixture. Calculations of NO x formation in the rich–lean combustion were also done employing the extended Zel’dovich NO formation mechanism.

Prediction of N2O and NH3 in fuel-rich gaseous flames

Experimental measurements of the adiabatic burning velocity in neat and NO formation in CH4 + O2 + Ar flames doped with small amounts of N2O are presented. The oxygen content in the oxidizer was varied from 15 to 17%. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + oxygen + argon mixtures were found in satisfactory agreement with the modeling. The NO concentrations in the flames doped with N2O (100 ppm in the argon stream before mixing) were measured in the burnt gases at a fixed distance from the burner using probe sampling. Axial profiles of [NO] were found insensitive to the downstream heat losses. Experimental dependencies of [NO] versus equivalence ratio had a maximum between φ = 1.1 and 1.2. Calculated concentrations of NO were in good agreement with the measurements. In lean flames calculated concentrations of NO strongly depends on the rate constant of reaction N2O + O=NO + NO if too high values proposed in the literature are employed. These new experimental data thus allowed for validation of the key reactions of the nitrous oxide mechanism of NO formation in flames.

NOx Formation Characteristic of Lean-Rich Combustion in Unsteady-State. Numerical Unsteady-State Analysis of Laminor Lean-Rich Combustion Flames.

Recently a lean-rich combustion is drawing attention as a way to decrease the level of NOx. But the flame structure and NOx formation process of the lean-rich flame in the unsteady-state was not made clear sufficiently. So we performed the numerical analysis in the unsteady-state in which we took the NO formation mechanism into the consideration by using the Skeletal Chemistry combined with a part of GRI-mech. And we found that thermal NO is generated not only in the high temperature region formed in the downstream of the rich flame, but also in the swell formed in the downstream of the lean flame. In the swell formed in the downstream of the lean flame, the chemical reaction is occurred actively due to the fluctuation of lateral flow that carries CH4. The faster the lateral flow is, the larger NO mole production rate becomes.

Formation of NO from combustion of volatiles from municipal solid wastes

An experimental and theoretical study has been performed on the formation of NO in the combustion of volatiles from municipal solid wastes. Single components and their mixtures were burned in a small-scale, fixed-bed reactor. Numerical simulations using the opposed flow diffusion flame program OPPDIF were performed to obtain a further understanding of the experimental results. Conversion factors for fuel-N to NO were determined for single components of newspaper, cardboard, glossy paper, low-density polyethylene (LDPE), and poly(vinylchloride) (PVC) and their mixtures, using gases with oxygen concentrations of 12, 21, and 40 vol %. For single components experiments at 100 vol % oxygen were also performed. The conversion factors for paper and cardboard varied from 0.26 to 0.99. The conversion factor for LDPE and PVC varied from 0.71 to 10.09 and 0.04 to 0.37, respectively. Conversion factors higher than 1.0 in the case of LDPE clearly show that NO is formed by thermal and/or prompt mechanisms. For mixtures, calculated conversion factors (based on a weighted sum of the conversion factors for single components) were compared with the experimentally determined conversion factors. Mixtures of paper and cardboard only gave different conversion factors with 40 vol % of oxygen. For mixtures of paper/cardboard and plastics, however, significant differences in the conversion factors were observed at all oxygen concentrations, when comparing experiments on a mixture of paper and plastics with the weighted sum of the single components. The explanation is found in the different combustion properties for paper/cardboard and plastic, which in this case make the formation of thermal NO from LDPE more favorable for the single component than in mixtures with other components. The simulations with OPPDIF confirmed the trends observed experimentally, and allowed an assessment of the contribution of the different mechanisms of NO formation.

The CH+N 2 reaction over the ground electronic doublet potential energy surface: a detailed transition state search

The CH( 2Π)+ N 2 reaction over the ground state potential energy surface has been investigated at the G2M level of theory. This reaction is directly relevant to hydrocarbon combustion chemistry, in particular, to `prompt NO' formation. A detailed mechanism via stepwise and concerted pathways to form HNCN, involving chain and cyclic intermediates, is presented. The proposed mechanism for NO formation is more favorable than the commonly assumed spin-forbidden path producing HCN+ N( 4 S) . The theoretically predicted heats of formation for NCN and HNCN are in excellent agreement with the recently reported experimental values.

00/01512 Improvement of low load combustion stability for advanced low NO x burner

Local production rates of NO have been derived from experiments in vertical flames on a burner commonly used in central heating equipment. The fuel-air equivalence ratio of around 1.9 caused a double-flame structure to develop above the burner, a phenomenon reflected by the temperature and OH concentration fields. The production rate of NO was a maximum in regions of low temperature and low OH concentration, located in the inner premixed flame and prior to the higher temperatures and OH concentrations of the outer diffusion flame. In addition, the peak rate of disappearance of NO was at the core of the premixed flame. These experimental results supported the assumption that the main mechanism of NO formation was the Fenimore Prompt route; also, an important internal “reburn” mechanism was operating. The opposed flow flame code was used here in the largest part of the simulations with the GRI 2.11 mechanism and with two additional mechanisms for a particular condition of flow and fuel-air equivalence ratio. The code was applied for fuel streams containing methane in air and oxidizer streams containing air only; the aim was to draw analogies with the experimental two-dimensional flame. Results from local rates of production and sensitivity analyses and quantitative reaction path diagrams (QRPD) for the nitrogenous species are presented for conditions representative of domestic appliances (cookers, space and water heaters, and central heating units) fired with natural gas. The co-flow experimental flame and the counter-flow theoretical flame at similar strain rates and fuel richness showed significant similarities.

NOx Formation Characteristic in Lean-Rich Combustion. Numerical Steady-State Analysis of Laminar Lean-Rich Colnbustion Flames.

Nemerical steady-state analysis was performed in order to make clear the flame structure and the formation process of NOχ in the methane-air lean-rich combustion flame. In lean-rich flames, the lean and the rich slit burners were set side by side in the combustor. A two-dimensional numerical analysis was performed based on 235 elementary rections with 52 chemical species, using the CHEMKIN thermodynamic database. In the numerical analysis, both Zeldovich NO and prompt NO formation mechanism were taken into account for NO formation process. We found the region in the base of the lean flame where the chemical reaction occurred actively. It concludes that the region was formed by the diffusion of fuel and various radicals from the rich flame, and by thermal influence from the rich flame. It was the evidence that the rich flame influenced the lean flame. This result is useful not only for decreasing the level of NOχ but also for stabilizing the lean-rich flame.

Simulation and optimization on the regenerative thermal oxidation of volatile organic compounds

Volatile organic compounds (VOCs) are involved in the generation of ozone via reaction with atmospheric NO x in the atmosphere through a photochemical pathway. One of the popular treatment processes for VOCs is thermal oxidation, since it shows high destruction and removal efficiency (DRE). A regenerative thermal oxidizer (RTO) is commonly used because of its high heat exchange efficiency. The process uses ceramic beds to capture heat from gases exiting the combustion zone. Steady and unsteady flow field, distributions of temperature, pressure and compositions of flue gas inside an RTO were simulated by computational fluid dynamics (CFD). The DRE of VOCs and the pollutant concentrations of CO and NO emitted from the RTO are estimated by incorporating two-step incomplete combustion reaction of VOCs and Zeldovich’s NO formation mechanism. The model system was the oxidation of benzene, toluene and xylene by the RTO, which was composed of three beds packed with ceramic beads to exchange heat. The height of the ceramic beds was varied and the effect of change of height was investigated in terms of DRE and pressure drop. The objective of the study was to simulate the performance of the RTO and optimize the height of ceramic beds. In a preliminary calculation, more than 95% of DRE is obtained when the premixed flow velocity of VOCs–air is less than about 3 m/s at the steady state at a constant fuel rate of 0.469 m/s under the given conditions. A weak recirculation zone above the center bed appeared and the intensity of recirculation decreases when no fuel is added in the normal direction. This region represents the putative site for the formation of NO, while most of the CO produced is converted to CO 2 in the recirculation zone. The results show that a level of 1.0% of VOCs is sufficient to provide energy for the oxidation when heat is exchanged through the ceramic bed. A ceramic bed of 0.2 m in height is sufficient to operate properly at these conditions and 5 s is recommended as the stream switching time. The importance of this simulation study is that the performance of RTO can be calculated at any operating conditions without the need for a pilot stage. In addition, the results can provide insight and practical responses involved in the design of an industrial RTO unit.

A simplified, fundamentally based method for calculating NO x emissions in lean premixed combustors

Recent developments in systematically reduced chemical kinetics are applied to computation of the production of oxides of nitrogen by considering the flame-structure limit of slow CO oxidation along with the thermal mechanism of NO formation. Fuel consumption is presumed to be controlled by turbulent mixing so that the results are applicable to general hydrocarbon fuels. The approach is free of empirical modeling coefficients and relates the rates to elementary chemical-kinetic parameters. Reasonable predictions of motor emissions are obtained when comparisons are made with results of experiments on a dual-fuel diesel engine.

Heterocyclic NO prodrugs

An overview of the different heterocyclic NO-releasing compounds is given. Mesoionic heterocycles like sydnone imines are one example. This class is discussed on the synthesis and the mechanism of NO formation from Molsidomine and its first metabolite SIN-1. Furthermore, 1,2,3,4 oxatriazolium olates and imidates are presented in an example of the synthesis of GEA-3175. Heterocyclic N-oxides are another group of compounds capable of NO release under certain conditions. This class is discussed in the example of furoxane carboxamides like CAS-1609, and some SAR-data show the great impact of intramolecular hydrogen bridges on their in vitro activity. Each class of compounds requires different cofactors for NO release: sydnone imines need oxidants like oxygen, furoxanes are converted to NO via reaction with thioles.

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.

The NO and N2O formation mechanism under circulating fluidized bed combustor conditions: From the single particle to the pilot‐scale

AbstractThe NO and N2O formation mechanism is studied starting from a single fuel particle burning under well‐defined conditions up to a pilot‐scale circulating fluidized bed combustor (CFBC). The fuel, petroleum coke, was the same in all tests and care has been taken to obtain chemical similarity between the different units: a formation rate unit, a laboratory‐scale and a pilot‐scale CFBC. A detailed single particle NO/N2O formation model has been developed and incorporated in a CFBC NO/N2O emission model. To thoroughly test the modeled NO/N2O mechanism, the iodine addition method has been used in all units.

99/01408 Experience with combustion tuning and fuel system modifications to inexpensively reduce NO x emissions from eleven coal-fired tangential boilers : Mazzi, E. A. and Haythornthwaite, S. M. EC (Am. Soc. Mech. Eng.), 1997, 5, (1), 219–229

Research in NOx formation and control has advanced significantly since Zeldovich first postulated the thermal NO formation mechanism in 1943. This paper reviews the history of NOx control implementation over this time period, with an emphasis on the role that research has played on NOx control technology, development and implementation. The discussions are divided by decades, to facilitate comprehension of the progress made. Key representative research during each decade is reviewed, covering topics from fundamental combustion studies to the effect of chemical additives on selective non-catalytic NOx reduction performance. NOx control regulations that drove (and continue to drive) the technology are also discussed. The primary focus is on Southern California and Federal regulations. Southern California was chosen because it has always had the strictest NOx emissions limits in the U.S.A., while other states and the federal government have followed at a somewhat slower pace. Federal regulations are reviewed since they represent the minimum control requirements nationwide. The application of NOx controls discussed ranges from the simple combustion modifications, implemented in the late 1950s and early 1960s, through the development of ultralow NOx burners and the retrofit of advanced selective catalytic reduction systems of today.