NO Formation Mechanism Research Articles

Evaluation of the Thermal NO formation mechanism under low-temperature diesel combustion conditions

Over the past two decades, the amount of exhaust gas pollutants emissions has been significantly reduced due to the severe emission legislation imposed in most countries worldwide. Initial strategies simply required the employment of simple after-treatment and engine control devices; however, as the restrictions become more stringent, these strategies are evolving in the development of different combustion modes, specially characterized by having low-temperature combustion characteristics. These new working conditions demand the need to check the suitability of the current NO predictive models that coexist nowadays under standard diesel combustion characteristics, paying closer attention to the Thermal mechanism. In order to do so, a common chemical-kinetic software was employed to simulate, for n-heptane and methane fuels, fixed local conditions (standard diesel and low-temperature combustion) described by constant pressure, relative mixture fraction, oxygen mass fraction and initial and final reaction temperature. The study reflects a common trend between all the studied cases, independently of the considered local conditions, making it applicable to more complex situations such as real NO formation processes in diesel sprays. This relationship was characterized by a fourth-degree polynomial equation capable of substantially improving the NO prediction by just using the Thermal NO predictive model.

화학반응기 네트워크을 이용한 희박 예혼합 가스터빈 연소기에서의 오염물질 예측에 관한 연구

A chemical reactor network (CRN) was developed for a lean premixed gas turbine combustor to predict the emission of pollutants such as NOx and CO. In this study, the predictions of NOx and CO emissions from lean premixed methane-air combustion in the gas turbine were carried out using CHEMKIN and a GRI 3.0 methane-air combustion mechanism, which includes the four NO formation mechanisms for various load conditions. The calculated results were compared with experimental data obtained from a modified test combustor to validate the model. The contributions of the four NO pathways were investigated for various load conditions. The effects of nonuniformity of the mass flux and of the equivalence ratio of the injector on the NOx formation were investigated, and a method of reducing the pollutant formation was suggested for the development of a sub-10 ppm gas turbine combustor.

Kinetic study of fuel NO formation from pyrrole type nitrogen

An experimental study using a flow reactor and a numerical simulation with a detailed kinetic model have been performed to investigate the reaction mechanism of NO formation from pyrrole decomposition. The experiments have been conducted in a quartz flow reactor by using a mixture gas of pyrrole, O2 and H2O with N2 as a balance gas at atmospheric pressure and temperature range 800–1400K. The detailed kinetic model consists of 505 elementary reactions and 89 chemical species, and the plug flow calculations have been carried out under same conditions as experiments. Model predictions are in reasonably good agreement with the experimental data for NO formation from pyrrole. Model predictions indicate that the main reaction pathways from pyrrole to HCN are direct reaction by pyrrole pyrolysis and pyrolysis reaction of HNCPROP (HNCHCHCCH2) generated by ring scission reaction of pyrrole. HCN is converted to N2 or NO via NCO and about 90% of nitrogen in pyrrole is transformed to N2 or NO at the temperature of above 1400K. The residence time has little influence on the reaction rate and the reaction rate largely depends on the reaction temperature.

Experimental and numerical investigation of a micro-CHP flameless unit

A numerical and experimental investigation of a system for the micro-cogeneration of heat and power, based on a Stirling cycle and equipped with a flameless burner, is carried out with the purpose of evaluating the system performances with hydrogen-enriched fuels. The numerical model of the combustion chamber gave significant insight concerning the analysis and interpretation of the experimental measurements; however it required considerable efforts, especially regarding the definition of proper boundary conditions. In particular a subroutine was developed in order to couple the oxidation process to the overall operation of the micro-cogeneration unit. This procedure was proved to perform satisfactory, providing values of the heat source for the Stirling cycle that lead to expected thermal efficiencies in agreement with those indicated in the literature for similar systems. The importance of a proper turbulence-chemistry interaction treatment and rather detailed kinetic schemes to capture flameless combustion was also assessed. A simple NO formation mechanism based on the thermal and prompt routes was found to provide NO emissions in relatively good agreement with experimental observations when applied on thermo-fluid dynamic fields obtained from detailed oxidation schemes.

The role of NNH in NO formation and control

One of the remaining issues in our understanding of nitrogen chemistry in combustion is the chemistry of NNH. This species is known as a key intermediate in Thermal DeNO x , where NH 3 is used as a reducing agent for selective non-catalytic reduction of NO. In addition, NNH has been proposed to facilitate formation of NO from thermal fixation of molecular nitrogen through the so-called NNH mechanism. The importance of NNH for formation and reduction of NO depends on its thermal stability and its major consumption channels. In the present work, we study reactions on the NNH + O, NNH + O 2, and NH 2 + O 2 potential energy surfaces using methods previously developed by Miller, Klippenstein, Harding, and their co-workers. Their impact on Thermal DeNO x and the NNH mechanism for NO formation is investigated in detail.

제트 혼합 반응기 내 희박 예혼합 메탄-공기 연소의 NO 생성 예측을 위한 화학 반응기 모델링

제트 혼합 반응기(JSR) 내의 NOx와 같은 배출물질을 예측하기 위해서 화학반응기 모델을 개발했다. 본 연구에서는 JSR에 대한 화학반응기 모델로서 two-PSR 모델이 채택되었다. CHEMKIN 코드와 4가지 NO 생성 메커니즘을 포함한 GRL 3.0 메탄-공기 연소 메커니즘을 이용해서 JSR내의 희박 예혼합 메탄-공기 연소의 NO 생성예측을 실시하였다. 모델의 검증을 위해서 계산된 결과를 Rutar의 실험 데이터와 비교하였다. NO 생성의 중요파라미터와 4 가지 NO 경로의 기여도를 조사하였다. 화염 영역에서는 prompt 메커니즘이 주된 경로이고 화염후 영역에서는 Zeldovich 메커니즘이 주된 경로이다. 희박 예혼합 조건에서는 N2O 메카니즘이가 화염 및 화염후 영역 모두에서 중요한 경로이다.

Smoking Attenuates Increase in Exhaled Nitric Oxide in Atopic but Not in Nonatopic Young Adults with Asthma

Background: Measurement of fractional exhaled nitric oxide (FENO) is useful in assessing eosinophilic airway inflammation. Smoking may modify airway inflammation and reduce FENO levels, compromising the diagnostic value of FENO in smokers. How smoking influences FENO in atopic versus nonatopic asthmatics is unknown. The aim of the present study was to compare FENO in atopic and nonatopic steroid-naive young asthmatic adults and in healthy subjects in terms of smoking. Methods: Forty-six (30 atopic) smoking and 70 (54 atopic) nonsmoking steroid-naive army conscripts (mean age 20 years) with current symptomatic asthma underwent FENO measurement, skin prick tests, spirometry with a bronchodilation test, bronchial histamine challenge, and a standardized exercise test. Ten healthy smokers and 9 healthy nonsmokers underwent FENO measurement, spirometry and bronchial histamine challenge. Results: Smokers with asthma showed significantly higher FENO than did healthy smokers and nonsmokers (p = 0.001, both comparisons). Among atopic asthmatics, FENO was lower in smokers than in nonsmokers (p = 0.002) whereas among nonatopic asthmatics no such difference was detectable (p = 0.89). However, even among nonatopic asthmatic smokers FENO was significantly higher than among healthy controls (p = 0.01). Conclusion: Smoking seems to attenuate the increase in FENO in atopic but not in nonatopic asthmatics. This finding suggests differences in biochemical mechanisms of NO formation in atopic and nonatopic asthma. However, FENO was significantly higher both in atopic and nonatopic asthmatic smokers than in healthy controls. This suggests that FENO can be applied for diagnostic purposes also in young adult smokers.

층류 CH4/Air 예혼합화염의 하류영역에서 NO 농도 예측을 위한 열손실 모델의 검토

One-dimensional modeling of CH4/air premixed flame was conducted to validate the heat loss model and investigate NOx formation characteristics in the postflame region. The predicted temperature and NO concentration were compared to experimental data and previous heat loss model results using a constant gradient of temperature (100 K/cm). The following conclusions were drawn. In the heat loss model using steady-state heat transfer equation, the numerical results using the effective heat loss coefficient (heff) of 1.0 W/m 2 K were in very good agreement with the experiments in terms of temperature and NO concentration. On the other hand, the calculated values using the constant gradient of temperature (100 K/cm) were lower than that in the experiments. Although the effects of heat loss suppress NO production near the flame region, a significant difference in NO concentration was not found compared to that under adiabatic conditions. In the postflame region, however, there were considerable differences in NO emission index as well as the contribution of NO formation mechanisms. In particular, in the range of 0.8

Simulation of turbulent combustion and NO formation in a swirl combustor

A presumed probability density function (PDF) model for temperature fluctuation is proposed and formulated in this paper. It incorporates a two-step reaction mechanism for propane combustion and the thermal and prompt NO formation mechanisms. The present model, together with a new algebraic Reynolds stress model (ASM), is employed to simulate the turbulent combustion and NO formation in a swirl combustor. The calculated propane, carbon monoxide, and carbon dioxide concentrations agree with the measurement. The calculated gas temperature and oxygen and NO concentrations are in general agreement with the measured data. The simulated results show that NO forms mainly in the upstream region of the combustor. The flue gas recirculation effectively abates the nitrogen oxides ( NO x ) emission in the combustor.

Effect of the combustion model and kinetic mechanism on the MILD combustion in an industrial burner fed with hydrogen enriched fuels

A numerical and experimental investigation of a burner operating in MILD combustion regime and fed with methane and methane-hydrogen mixtures (with hydrogen content up to 20% by wt.) is presented. Numerical simulations are performed with two different combustion models, i.e. the ED/FR and EDC models, and three kinetic mechanisms, i.e. global, DRM-19 and GRI-3.0. Moreover, the influence of molecular diffusion on the predictions is assessed. Results evidence the need of a detailed chemistry approach, especially with H 2, to capture the volumetric features of MILD combustion. The inclusion of molecular diffusion influences the prediction of H 2 distribution; however, the effects on the temperature field and on the major species are negligible for the present MILD combustion system. A simple NO formation mechanism based on the thermal and prompt routes is found to provide NO emissions in relatively good agreement with experimental observations only when applied on temperature fields obtained with the EDC model and detailed chemistry.

The role of methylene in prompt NO formation

We address the plausibility of singlet methylene (1CH2) in the prompt NO formation mechanism via examination of experimental species profiles and kinetic flame modeling of several low-pressure methane–oxygen–nitrogen flames. Existing kinetic models assuming CH as the only prompt NO precursor greatly underpredict NO formation under very fuel-lean conditions. We have constructed a kinetic pathway initiated by the recombination of singlet CH2 with molecular nitrogen to form diazomethane, CH2NN, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N2, a small percentage (approximately 10%) is predicted to react via cleavage of the NN bond leading to NO formation. This leads to accurate prediction of the experimental measurements of NO formation in lean, low-pressure flames. Assuming reasonable kinetic parameters for the reactions of CH2, the large underprediction of NO under lean conditions can be rectified by the inclusion of the 1CH2 prompt NO pathway in the kinetic mechanism.

Nitrous oxide conversion in laminar premixed flames of CH4 + O2 + Ar

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 1atm. 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 (100ppm 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.

A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames

This paper investigates the effects of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames by numerical simulation. The results reveal that flame temperature changes due to the combined effects of adiabatic temperature, fuel Lewis number and radiation heat loss, when hydrogen/reformate gas is added to the fuel of a methane/air diffusion flame. The effect of Lewis number causes the flame temperature to increase much faster than the corresponding adiabatic equilibrium temperature when hydrogen is added, and results in a qualitatively different variation from the adiabatic equilibrium temperature as reformate gas is added. At some conditions, the addition of hydrogen results in a super-adiabatic flame temperature. The addition of hydrogen/reformate gas causes NO formation to change because of the variations in flame temperature, structure and NO formation mechanism, and the effect becomes more significant with increasing strain rate. The addition of a small amount of hydrogen or reformate gas has little effect on NO formation at low strain rates, and results in an increase in NO formation at moderate or high strain rates. However, the addition of a large amount of hydrogen increases NO formation at all strain rates, except near pure hydrogen condition. Conversely, the addition of a large amount of reformate gas results in a reduction in NO formation.

A three-dimensional numerical study on exhaust gas emissions from a medium-duty diesel engine using a phenomenological soot particle formation model combined with detailed chemistry

This paper reports a three-dimensional numerical study, using the KIVA-3V code with modified chemical and physical models, of the exhaust (NO x and soot) emissions from an in-line, four-cylinder, 3.0 l, turbo-charged, direct injection diesel engine operated in spray diffusion combustion mode, by injecting fuel at top dead centre, in typical operating conditions for city driving (70 per cent load, 1500 r/min engine speed) with various exhaust gas recirculation (EGR) rates. To predict soot emission parameters, a gas-phase polycyclic aromatic hydrocarbons (PAH) precursor formation model was coupled with a detailed phenomenological particle formation model, including soot nucleation from the precursors, surface growth/oxidation, and particle coagulation, since changes in the surface area of particles and surface reaction rates had to be taken into account. In order to predict NO x emission parameters, the extended Zeldovich (thermal), intermediate N2O and NO2 from NO formation mechanisms were also included in the modelled elementary reactions. From the numerical simulation results, it was shown that the present code had sufficient performance for predicting the amount of exhaust soot and NO x with practical central processing unit (CPU) time. Comparison of the results of the numerical simulations with data obtained from empirical tests show that the applied code provides adequate predictions of soot and NO x emissions for engine design and optimization within practical CPU times. The results also show that the EGR rate does not greatly affect the soot surface growth rate, although it was previously thought to be a major determinant of soot masses. Instead, soot oxidation rates, especially rates of oxidation by OH radicals, were found to be the most influential factors affecting soot emissions under the tested engine operating conditions.

A numerical study on the effect of CO addition on extinction limits and NO x formation in lean counterflow CH4/air premixed flames

The effect of CO addition on extinction and NO x formation in lean premixed counterflow CH4/air flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. A method that gradually switched off the initial reactions of NO formation from different routes was used to analyse the variation of NO formation mechanism. The results indicate that the addition of certain amount of CO increases the strain extinction limits and reduces the radiation extinction limits. As a result, the lean flammability limit of CH4/air premixed flame is extended to leaner side by the addition of CO. The formation of NO in a flame is increased with the addition of CO at a constant equivalence ratio. For an ultra-lean flame, the increase in the formation of NO is mainly because of the increase in the contribution from the NNH intermediate route, while for a near stoichiometric flame, this increase is mainly attributable to the rise in the contribution from the thermal route. With the fraction of added CO being gradually increased, the formation of NO2 in a flame first decreases and then increases at a given equivalence ratio. The addition of CO reduces the formation N2O in an ultra-lean flame, while affects little on the formation of N2O in a near stoichiometric flame.

Diesel combustion: In-cylinder NO concentrations in relation to injection timing

This paper presents local experimental in-cylinder concentrations of nitric oxide, obtained by laser-induced fluorescence measurements in a heavy-duty diesel engine. Quantitative concentration histories during the entire combustion stroke are shown for a number of fuel injection timings. Using images from high-speed combustion visualization experiments, the presence of the diffusion flame is related to the onset of NO formation within the laser probe volume. Further attention is paid to the possible NO formation mechanisms. Off-line characterization of the fuel sprays by means of Schlieren imaging reveals that the initial (premixed) combustion is too fuel-rich for thermal (Zeldovich) NO formation. Furthermore, the experimental NO concentrations are compared to numerical calculations of the thermal NO formation during the mixing-controlled combustion phase. The agreement between model and experiments suggests that the thermal mechanism is the major NO formation pathway. However, it cannot be excluded that transport to the probe volume of early NO, formed under conditions where the thermal mechanism is ineffective, might be of some importance as well.

NO formation mechanisms studied by infrared laser absorption in a single low-pressure plasma pulse

The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 µs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N2(X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N2(A) for the NO formation shows good agreement.

A Reducing Method of a Detailed Reaction Mechanism Based on the QSSA Method

A reducing method for a detailed reaction mechanism is presented. In the reducing procedure, a reduced mechanism is systematically constructed in three steps by using the parameters obtained in the computation of reaction kinetics. The reducing method is applied to the reduction of the detailed mechanism for NO formation from the oxidation of pyridine. The calculated results with the reduced mechanism obtained by the present method are in good agreement with those obtained by the detailed mechanism. The computational time is significantly shorter than that with the detailed mechanism, although the computational time for the reaction kinetics with the reduced mechanism by the present method is larger than that using the conventional method. Furthermore, the computational time required for the construction of the reduced mechanism by the present method is about one tenth of that required by the conventional method. Therefore, the present reducing method is a powerful tool for the construction of the reduced mechanism.

Trace detection of explosives with low vapor emissions by laser surface photofragmentation–fragment detection spectroscopy with an improved ionization probe

Trace explosive residues are measured in real time by surface laser photofragmentation-fragment detection (SPF-FD) spectroscopy at ambient conditions. A 248-nm laser photofragments the target residue on a substrate, and a 226-nm laser ionizes the resulting NO fragment by resonance-enhanced multiphoton ionization by means of its A-X (0, 0) transitions near 226 nm. We tested two probes on selected explosives and modeled their electric field in the presence of a substrate with an ion optics simulation program. The limits of detection range from 1 to 15 ng/cm2 (signal-to-noise ratio of 3) at 1 atm and 298 K and depend on the electrode orientation and mechanism for NO formation.

Modified Isotope Pairing Technique to Study N Transformations in Polluted Aquatic Systems: Theory

Denitrification of nitrate in sediments of polluted aquatic systems has the potential to release considerable nitrogen amounts into the atmosphere in the form of dinitrogen (N2) and/or nitrous oxide (N2O). Nitrate may diffuse into the anoxic sediment layer either from the oxic sediment layer (after being produced there by nitrifiers) or directly from the overlying water. Currently, the most common technique that explicitly distinguishes between the two main nitrate sources is the Isotope Pairing Technique (IPT). 15N-labeled nitrate is added to the water column, and formation of 15N labeled N2 molecules is subsequently monitored. The main shortcoming of the IPT is that the formation of N20 is ignored, thus resulting in an underestimation of sediment denitrification. Another limitation is the inability to account for a possible influx of nitrate through the hyporheic zone (e.g., nitrate-polluted groundwater) into the anoxic sediment layer. We have further developed and analyzed the theoretical basis of the original IPT. The two important factors that we took into account are the isotopic composition of N20 and the input of an external source of nitrate. We also examined the option of adding 15N-labeled ammonium to the water column to specifically adjustthe technique to field studies. The presented modified technique allows us to (i) improve the estimation of sediment denitrification capacity, (ii) gain an insight into the N20 formation mechanism(s) and fluxes, and (iii) assess inputs of nitrate-polluted water through the hyporheic zone.