Prompt NO Formation Research Articles

Insight into premixed diethoxymethane flames: Laminar burning velocities, temperatures, and emissions behaviour

Diethoxymethane ((CH3CH2O)2CH2, DEM) is a promising carbon-neutral fuel. DEM is a diether or acetal with a molecular structure similar to oxymethylene ethers (CH3O–(CH2O)n–CH3, OMEn). Thus, DEM can be expected to have a similar combustion behavior to OMEs, reducing harmful emissions such as NOx and particulate matter (PM) in internal combustion engines. From both experimental and kinetic modeling, fundamental studies on DEM are scarce in the literature. More studies are required to gain a detailed insight into the oxidation kinetics of DEM. Laminar burning velocity (LBV) is a critical property that allows a detailed assessment of the potential application of DEM in combustion devices. Unfortunately, the literature on the LBV of DEM is limited. Therefore, in this study we have investigated the LBV of DEM using two reactors for the first time, namely a heat flux burner and a combustion chamber. The experimental data is reported for equivalence ratio between 0.7 and 1.7, initial temperatures of 368–423 K, and initial pressure of 1–5 bar. In addition, we developed a detailed kinetic model extending our recent work of Shrestha et al. (Combust. Flame. 246 (2022) 112,426) to characterize the combustion behavior of DEM utilizing the new experimental data from this work and the literature data. Our model performs remarkably well in capturing the newly measured LBV experimental data over various experimental conditions. We found that DEM and dimethoxy methane (DMM) have similar values of LBVs (within ±1.5 cm/s) for a given condition, which indicates that intermediate chemistry governs the flame chemistry. Despite DEM being a larger molecule that is expected to have slightly lower LBVs than DMM, its effect on the measured values of LBVs is negligible. Finally, we experimentally measured NOx formation in DEM flame for the first time. The stochiometric flame has the highest NOx formation. The proposed model predicted the equivalence ratio dependence of NOx nicely. However, it overestimates the NOx formation for stoichiometric DEM/air mixtures by ∼30 %. The model suggests that the thermal NO formation route is favored at lean and stochiometric conditions. In contrast, the prompt NO formation route is enhanced for rich mixtures.

Resolving discrepancies between theory and experiment for the NCN + H reaction

Flame modeling studies have highlighted the role of branching in the kinetics for the prompt NO switch reaction, NCN + H. In this reaction, there is strong competition between the CH + N2 (R1) and HCN + N (R2) product channels. Increased branching towards reaction R2 promotes the subsequent formation of Fenimore or Prompt NO. Recent direct shock tube studies on this complex reaction conclude that branching to reaction R2 is much more favorable than the predicted branching from theoretical studies. The experimentally predicted prompt NO switch temperature (TS), i.e., the temperature at which k2/(k1+k2) = 0.5, is at 1670 K, in contrast to theory that predicts Ts > 3200 K. The present theory/modeling work was initiated to highlight potential causes for this significant discrepancy between experiment and theory. Our analysis of the simulations reported in the experimental studies indicate that including a fall-off representation for C2H5 radical dissociation (with C2H5 produced from C2H5I dissociation and acting as an H-atom source) has a noticeable influence on the simulations for NCN decay and HCN formation in the shock tube studies. With the inclusion of a theory-based pressure fall-off representation for C2H5 dissociation, we show that this radical persists for much longer timescales even at the high-temperatures in the shock tube studies. This persistence then facilitates the rapid conversion of reactive H-atoms to CH3 radicals at the shock tube conditions via the H + C2H5 → CH3 + CH3 reaction. An updated theoretical analysis for this classic addition-elimination reaction is also provided in this work. The rapid formation of CH3 radicals necessitates the inclusion of additional reactions to adequately simulate the NCN and HCN data. Our simulations conclude that, with the inclusion of these additional reactions, the NCN and HCN profiles can be reasonably well simulated by the most recent theoretical predictions.

Numerical simulation study on NO generation characteristics of methane/n-heptane dual fuel counterflow flames under different pressures

This paper presents a numerical study of counterflow methane/n-heptane dual fuel flames, investigating the influence of pressure on flame structure and NO generation characteristics. The results demonstrate that, as pressure increases, the peak temperature and heat release rate of the flame increase, while the NO emission index decreases. The prompt NO emission index experiences a monotonic reduction as pressure increases, while the thermal NO emission index initially increases and then decreases. NO emission index via reburn route displays non-monotonicity with increasing pressure. Elevated pressure induces a decline in the mole fraction of radicals (CH, CH2 and HCCO) crucial for prompt NO formation, consequently suppressing the formation of prompt NO. The principal reaction responsible for the consumption of reburn NO at lower pressures is HCCO+NOCO+HCNO, whereas the primary reaction for generating reburn NO is H+HNOH2+NO. However, at higher pressures, the reaction H+HNOH2+NO takes precedence as the dominant reaction consuming NO.

Investigation of the reactions NCN + CH3, NCN + OH, and CH3 + OH behind shock waves

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. As both CH3 and OH radicals are present at high mole fraction level in the reaction zone, their radical-radical cross-reactions with NCN are of special interest, but no experimental rate constants are available. In contrast to the OH reaction, the reaction with CH3 is not considered in common detailed flame mechanisms as yet. It is shown in this study, however, that the NCN + CH3 reaction proceeds close to collision limit and therefore acts as one of the most important reactions to describe the fate of NCN in flames. NCN and OH concentration-time profiles were recorded behind shock waves. The thermal decomposition of cyanogen azide (NCN3) and tert‑Butyl hydroperoxide (TBHP) served as quantitative source of NCN and CH3/OH radicals. Representing a secondary reaction under the applied experimental conditions, the rate constant of the reaction CH3 + OH has been first determined from OH profiles with TBHP as precursor. With k(CH3 + OH) = 4.34 × 10–10 × (T / K)6.38 exp(+68.0 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 925 – 1800 K, p ≈ 393 mbar) it was found to be in good agreement with available literature results. Rate constants of the reaction NCN + CH3 were then determined from NCN profiles by shock-heating gas mixtures with TBHP and NCN3. According to previous as well as our own statistical rate theory calculations, the only relevant product channel is the formation of CH2NH + CN. In the temperature range 857 – 1817 K and at a pressure of p ≈ 314 mbar, a high value for the rate constant k(NCN + CH3) = 5.40 × 1013 exp(+1.77 kJ mol–1 / RT) cm3 mol–1 s–1 (±30%) was found. For future implementation into flame models, we recommend the rate constant expression k(NCN + CH3) = 8.44 × 1019 (T / K) –1.76 exp(–15.2 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 800 – 4000 K), which is based on a theoretically assisted extrapolation of the experimental data. Finally, OH profiles in the presence of CH3 and NCN have been analyzed. In agreement with previous theoretical estimates, the reaction is indeed comparably slow and therefore only a rough upper limit of k(NCN + OH) < 8 × 1012 cm3mol–1s–1 (918 – 1808 K, p ≈ 415 mbar) could be inferred from the experiments.

Bayesian Calibration of Kinetic Parameters in the CH Chemistry Toward Accurate Prompt-NO Modelling

Abstract Significant efforts made by the gas turbine industry have helped reduce nitrogen oxides (NOx) emissions considerably. To meet and surpass the increasingly stringent regulations, accurate and robust thermochemical mechanisms are needed to help design future sub-10 ppm combustion systems. Uncertainty in kinetic modeling, however, can result in large prediction uncertainty and significant discrepancy between models that hinder the identification of promising combustors with confidence. Direct reaction rate measurements are seldom available for some reactions, especially when involving short-lived radicals such as methylidyne, CH. As the main precursor to the prompt-NO formation pathway, its large parametric uncertainty directly propagates through the nitrogen chemistry preventing accurate and precise emissions predictions. Recent independent CH concentration measurements obtained at various operating conditions are used as indirect rate measurements to perform statistical, or Bayesian, calibration. A subset of important reactions in the CH chemistry affecting peak-CH concentration is identified through uncertainty-weighted sensitivity analysis to first constrain the parametric space of this prompt-NO precursor. Spectral expansion provides the surrogate model used in the Markov-Chain Monte Carlo method to evaluate the posterior kinetic distribution. The resulting constrained CH-chemistry better captures experimental measurements while providing smaller prediction uncertainty of a similar order as the uncertainty of the measurements, which can increase the confidence in simulation results to identify promising future low-emissions configurations. For the quasi-steady-state species CH, fuel decomposition reactions leading to CH production are constrained while little impact is observed for intermediate reactions within the CH-chemistry. The reduction in prediction uncertainty results mainly from the constrained correlations between parameters which greatly limit the set of feasible reaction rate combinations. Additional independent direct and indirect measurements would be necessary to further constrain rate parameters in the CH chemistry, but this calibration demonstrates that predictions of radical species can be improved by assimilating enough data.

Branching fraction measurement of the prompt-NO switch reaction NCN + H

The branching fraction ϕ of the prompt-NO switch reaction NCN + H (1), essentially yielding either CH + N2 (1a) or HCN + N (1b), is key for modeling prompt-NO formation in flames. Large discrepancies exist between available experimental data and flame modeling studies on the one hand and high-level theoretical predictions on the other hand, resulting in a factor of 5 uncertainty for ϕ1b at T=1500 K. By simultaneous monitoring of NCN and HCN concentration-time profiles during the reaction NCN + H at temperatures 1200K<T<2000K behind shock waves, ϕ1b has been directly measured for the first time. Thermal decomposition of cyanogen azide (NCN3) and ethyl iodide (C2H5I) served as sources for NCN radicals and H atoms. NCN has been detected by UV laser absorption at 329.130 nm and HCN detection was accomplished by mid-infrared frequency modulation spectroscopy at 3228.049cm−1. Kinetic simulations provided both the total rate constant k1 from the NCN and k1b from the HCN profiles. In order to account for a side reaction of residual BrCN from NCN3 synthesis, a rough estimate for the rate constant of the reaction BrCN + H was inferred from the experiments as well. The measured rate constants for k1 confirm earlier results (Faßheber et al., Phys. Chem. Chem. Phys. 16 (2014) 11647) and k1b follows the Arrhenius expression k1b/(cm3mol−1s−1)=4.2×1014exp(−38.2kJmol−1/RT) with moderate uncertainties of ±(37−49%). The branching fraction increases with temperature, yielding ϕ1b=0.22(±46%) at 1200 K, 0.47(±42%) at 1600 K, and 0.63(±53%) at 2000 K. The corresponding prompt-NO switch temperature of TS=1670K (ϕ1b=0.5) shows that the reaction NCN + H advances toward the Fenimore products HCN + N already at typical temperatures of hydrocarbon/air flames, in stark contrast to the most recent theoretical estimate reporting TS=3235K.

Experimental and kinetic modeling study of NO formation in premixed CH4+O2+N2 flames

The nitric oxide (NO) formation in methane (CH 4 ) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2 ), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp ( 185.41 [ c a l / m o l e ] R T ) cm 3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism.

Role of N2O and equivalence ratio on NOx formation of methane/nitrous oxide premixed flames

Nitrous oxide (N2O) is often used as an oxidizer in rocket technology with plenty of potential advantages. However, the N2O/hydrocarbon combustion results in the staggering increase of NOx emission. To scrutinize the role of N2O and equivalence ratio on NO formation characteristics of N2O/CH4 premixed flame, mixtures of N2O/CH4 and N2/O2/CH4 were investigated by employing one-dimensional opposed-jet model with detailed reaction mechanism. The flame structures, NO reaction pathways, and NO formation routes were discussed. In the heat release rate analysis, the heat release is dominated by N2O route, resulting in high flame temperature in N2O/CH4 premixed flame. Regarding the mechanism of NO formation, NO is mainly produced from N2 through the thermal routes and from HNO through the HNO-intermediate routes for the N2/O2/CH4 flame but mainly comes from N2O through the N2O and HNO-intermediate routes in the N2O/CH4 flame. The thermal routes alter to the primary chemical mechanism of NO consumption. The NO production in fuel-lean N2O/CH4 flames is enhanced through the N2O route due to the increase of the mole fraction of O atom. The thermal route becomes forward reactions to produce NO for the extreme fuel-rich N2O/CH4 flame. Regarding the reaction pathways of the prompt route, it can be approximately divided into three categories: (1) N2 → CN → NCN, NO (in terminal) → N2; (2) CN → NCN, NO (in terminal) → NO, CN; (3) N2 → CN → NCN, NO (in terminal) → N → N2. In NO production rate analysis, the main NO formation comes from the N2O and HNO-intermediate routes, while NO consumption associates with the thermal and NNH-intermediate routes. In addition, the prompt-NO formation is dominated by species of N2O instead of CH radical in the extreme fuel-rich N2O/CH4 flames.

The Reaction NCN + H2: Quantum Chemical Calculations, Role of 1NCN Chemistry, and 3NCN Absorption Cross Section.

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H2 and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet 1NCN and triplet 3NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the 1NCN + H2 reaction as well as the newly measured high temperature absorption cross section of 3NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(3NCN, λ = 329.1302 nm) = 2.63 × 109 × exp(-1.96 × 10-3 × T/K) cm2/mol (±23%, p = 0 bar, T = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature 3NCN absorption cross section has been resolved. Whereas 3NCN + H2 resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the H2NCN intermediate are HNCN + H as well, with minor contributions of CN + NH2 toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in 1NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of 1NCN and 3NCN chemistry. However, as the main reaction products in case of the H2 reaction are the same and as the population of the 1NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H2 → HNCN + H) = 2.62 × 104 × (T/K)2.78 × exp(-97.6 kJ/mol/RT) cm3 mol-1s-1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for 1NCN chemistry.

Pressure effect on NO emission in methane/air lean-premixed flames

Nitrogen oxides (NOx) from combustion system are one of major pollutants which causes photochemical smog and ozone layer depletion. The interest in lean-premixed combustion has increased to reduce NOx emissions in the industrial gas-turbine as a suitable reduction strategy. The present study investigates methane/air premixed flames with a detailed chemical kinetic model to better understand the pressure effect on NOx formation. A detailed chemical kinetic model is developed by merging AramcoMech 3.0 and recently proposed nitrogen chemistry. The proposed mechanism is first validated against experimental data, including laminar flame speed, ignition delay times, and NO concentration in premixed flames at various pressures. Freely propagating methane/air lean-premixed flames are simulated over a pressure range 1–20 atm and equivalence of 0.5, 0.55 and 0.6. Prompt NO formation is dominant within a narrow heat release region and thermal NO production pathways lead to the total NO formation in the postflame zone. Prompt NO formation rate increases between 1 and 5 atm and then decreases with further increasing pressure due to pressure dependent NCN formation rates. On the other hand, the formation rate of NO in the postflame zone increases monotonically as pressure increases.

Quantitative NH measurements by using laser-based diagnostics in low-pressure flames

NH is a key short-lived radical involved in the prompt-NO formation. Quantification of NH is thus particularly important for testing the NO kinetic mechanisms. However, quantitative measurements of native NH in hydrocarbon/oxygen/nitrogen flames remain very scarce. Therefore, in this work, the mole fractions of native NH were obtained using a combination of laser-based diagnostics; Laser Induced Fluorescence (LIF) and Cavity Ring-Down Spectroscopy (CRDS). The NH species was probed after exciting the transition R1(6) in the A3Π-X3Σ− (0-0) system at 333.9 nm. The mole fraction profiles of NH were successfully obtained in premixed low-pressure flames of CH4/O2/N2 and C2H2/O2/N2 at two equivalence ratios of 1.00 and 1.25. The estimated detection limit for the NH radical was around 4.5 × 108 molecule cm−3 (i.e. 2 ppb in mole fraction at 1600 K), which is nearly 2 orders of magnitude lower than previous values reported in the literature. These new experimental results were compared with predictions by a recently developed NO model (namely NOMecha2.0). In the case of the CH4 flames, a satisfying agreement between the experiment and model was observed. However, in the case of the C2H2 flames, some discrepancies were observed. Model analysis has highlighted the importance of the HCCO radicals in the NH formation through the HCNO→HNCO→NH2 reactions pathway. Modification of the rate constant values of the reactions C2H2 + O and HCCO + O2, which are key reactions for both the acetylene laminar flame speed and the HCCO predictions, has enabled the model to satisfactorily predict the experimental NH and NO profiles also in the C2H2 flames.

Theory and modeling of relevance to prompt-NO formation at high pressure

An improved understanding of NOx formation at high pressures would be of considerable utility to efforts to develop advanced combustion devices. A combination of theoretical and modeling studies are implemented in an effort to improve the accuracy of models for the prompt NO process, which is the dominant source of NO under many conditions, and to improve our understanding of the role of this process at high pressures. The theoretical effort implements state-of-the-art treatments of NCN thermochemistry, the interrelated CH + N2 and NCN + H kinetics, and the kinetics of the NCN + OH reaction. For both reaction systems, we implement high level ab initio transition state theory based master equation simulations paying particular attention to the role of stabilization processes. For the NCN + H kinetics we include a treatment of inter-system crossing. The modeling effort focuses on exploring the role of pressure and prompt NO for premixed laminar flames at pressures ranging from 1 to 15 atm, via a comparison with the available experimental data. Additional simulations at higher pressures further explore the mechanistic changes at the pressures of relevance to applied combustion devices (e.g., 100 atm).

Effect of fuel composition on NOxformation in high‐pressure syngas/air combustion

In this study, an experimental investigation of lean premixed syngas/air flames with H2/CO ratio of 1.0 and equivalence ratio of 0.5 has been conducted in a high‐pressure burner facility to investigate the effects of pressure and the presence of hydrocarbons on NOxspeciation. Detailed NOxspeciation measurements in the post‐flame region were conducted for various pressures up to 1.5 MPa (15 bar) using Fourier transform infrared (FTIR) spectroscopy. When the pressure is increased, NO concentration decreases while NO2increases due to pressure dependence of NO to NO2conversion. For a given pressure, the presence of hydrocarbons in syngas leads to an increase in NOxconcentrations possibly due to prompt NO formation. Comparison of NO concentrations in presence of CH4at different pressures shows that the effect of CH4due to prompt NO formation is more dominant than the effect of pressure on NO. © 2018 American Institute of Chemical EngineersAIChE J, 64: 3134–3140, 2018

Thermochemical Mechanism Optimization for Accurate Predictions of CH Concentrations in Premixed Flames of C1–C3 Alkane Fuels

Increasingly stringent regulations on NOx emissions are enforced by governments owing to their contribution in the formation of ozone, smog, fine aerosols, acid rains, and nutrient pollution of surface water, which affect human health and the environment. The design of high-efficiency, low-emission combustors achieving these ever-decreasing emission standards requires thermochemical mechanisms of sufficiently high accuracy. Recently, a comprehensive set of experimental data, collected through laser-based diagnostics in atmospheric, jet-wall, stagnation, premixed flames, was published for all isomers of C1–C4 alkane and alcohol fuels. The rapid formation of NO through the flame front via the prompt (Fenimore) route was shown to be strongly coupled to the maximum concentration of the methylidyne radical, [CH]peak, and the flow residence time within the CH layer. A proper description of CH formation is then a prerequisite for accurate predictions of NO concentrations in hydrocarbon–air flames. However, a comparison against the Laser-induced fluorescence (LIF) experimental data of Versailles, P., et al. (2016, “Quantitative CH Measurements in Atmospheric-Pressure, Premixed Flames of C1–C4 Alkanes,” Combust. Flame, 165, pp. 109--124) revealed that (1) modern thermochemical mechanisms are unable to accurately capture the stoichiometric dependence of [CH]peak, and (2) for a given equivalence ratio, the predictions of different mechanisms span over more than an order of magnitude. This paper presents an optimization of the specific rate of a selection of nine elementary reactions included in the San Diego combustion mechanism. A quasi-Newton algorithm is used to minimize an objective function defined as the sum of squares of the relative difference between the numerical and experimental CH–LIF data of Versailles, P., et al. (2016, “Quantitative CH Measurements in Atmospheric-Pressure, Premixed Flames of C1–C4 Alkanes,” Combust. Flame, 165, pp. 109--124), while constraining the specific rates to physically reasonable values. A mechanism properly describing CH formation for lean to rich, C1–C3 alkane–air flames is obtained. This optimized mechanism will enable accurate predictions of prompt-NO formation over a wide range of equivalence ratios and alkane fuels. Suggestions regarding which reactions require further investigations, either through experimental or theoretical assessments of the individual specific rates, are also provided.

Effects of strain rate and CO2 on no formation in CH4/N2/O2 counter-flow diffusion flames

The reduction of nitrogen oxides in the high temperature flame is the key factor affecting the oxygen-enriched combustion performance. A numerical study using an OPPDIF code with detailed chemistry mechanism GRI 3.0 was carried out to focus on the effect of strain rate (25-130 s?1) and CO2 addition (0-0.59) on the oxidizer side on NO emission in CH4 / N2 / O2 counter-flow diffusion flame. The mole fraction profiles of flame structures, NO, NO2 and some selected radicals (H, O, OH) and the sensitivity of the dominant reactions contributing to NO formation in the counter-flow diffusion flames of CH4\/ N2 /O2 and CH4 / N2 / O2 / CO2 were obtained. The results indicated that the flame temperature and the amount of NO were reduced while the sensitivity of reactions to the prompt NO formation was gradually increased with the increasing strain rate. Furthermore, it is shown that with the increasing CO2 concentration in oxidizer, CO2 was directly involved in the reaction of NO consumption. The flame temperature and NO production were decreased dramatically and the mechanism of NO production was transformed from the thermal to prompt route.

Entropy generation in turbulent syngas counter-flow diffusion flames

Efficiency is one of the major objectives when designing energy systems. Irreversibilities of combustion processes can be characterized by analyzing entropy generation which is proportional to exergy destruction. In this paper, entropy generation is investigated in turbulent non-premixed counter-flow syngas flames at a high strain rate over a wide range of hydrogen percentage (H2/CO molar fraction from 0.4 to 2.0). The aim is to define the most efficient syngas composition to reduce irreversibilities. Irreversibilities involved in NO formation process are also examined. RANS (Reynolds Averaged Navier Stokes) technique including k-ε turbulence model is used for the flow field estimation. Flame structure is calculated using SLFM (Steady Laminar Flamelet Model) and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Total entropy generation rate accounts for chemical, heat conduction, mixing and viscous effects. Computational results show that the total volumetric entropy generation decreases with H2 enrichment as well as its different contributing effects. Chemical effect is dominant, followed by heat conduction and mixing effects. Viscous effect is negligible. The maximum of both thermal and prompt NO formation routes are influenced by the three main entropy generation modes, with the predominance of the chemical effect. At high strain rates, H2-rich syngas flames are efficient in regards to irreversibilities and NO emissions reduction.

Impact of methyl butanoate oxidation on NO formation in laminar low pressure flames

The impact of the methyl butanoate (MB) oxidation in NO formation is examined in stoichiometric low pressure (5.3kPa) flames. Flame structure analysis in three CH4/MB/O2/N2 flames was performed by using GC measurements. Focus was put on the prompt-NO formation in these flames thanks to laser based diagnostics allowing CH, NO and temperature profiles measurements. Experimental results show a decrease by 20% of the NO formation when MB is substituted to CH4 in the mixture (up to 50% of the fuel). A revised MB oxidation model combined with a detailed NOx chemistry is used to simulate species profiles, and to analyze the routes of formation of NO and of its precursors (CH and CH3 radicals). MB oxidation promotes the formation of CH3 in comparison to methane since it contributes to increase C2 and C3 species. In presence of MB, NO is consumed early in the flame front through reaction NO+HCO=HNO+CO. HCO is formed from formaldehyde that is formed rapidly from MB oxidation.

Influence of external flue gas recirculation on gas combustion in a coke oven heating system

In this study, external flue gas recirculation was considered to reduce nitrogen oxides (NOx) emissions in the heating flues of a coke oven battery. The thermal and prompt NO formation was numerically investigated employing an accurate 3-D representation of the heating flue geometry that the most popular Polish coke oven battery. Originally the developed model was experimentally validated as a transient coupled model for the representative heating flue and the two coke ovens. Then the coupled model was simplified to the heating flue model only with realistic boundary conditions on the heating flue and coke oven interface. As a result of the heating flue model simulations, the range of the reversed flue gas ratio was found for typical thermal loadings of the heating wall to effectively reduce NOx formation. The obtained results showed the significant effect of the considered flue gas recirculation on NOx formation reduction. Namely, the recirculation ratio of 0.2 resulted in 50% of the NOx reduction efficiency.

NO formation in rich premixed flames of C1–C4 alkanes and alcohols

A comprehensive study of prompt nitric oxide (NO) formation is presented for rich (φ=1.3) premixed flames of alkanes and alcohols. Measurements of the flame burning rate, the temperature, and the CH radical and NO species are performed in atmospheric-pressure laminar premixed stagnation flames. New data, obtained for all isomers of the C4 fuels, butane and butanol, are merged with existing data for C1–C3 fuels obtained by the same methodology. The combined dataset consists of all isomeric combinations of the fuel molecules, and provides a complete description of pollutant trends related to fuel type and branching structure for C1–C4 alkanes and alcohols. These data show alcohols produce less NO than alkanes of equivalent carbon chain length. This behavior is linked to the tendency of alcohols to produce lower concentrations of the CH radical, due to inhibition of the formation of methyl radicals. The results show a linear scaling of prompt-NO formation with the CH radical concentrations, when these are scaled by the characteristic residence time in the flame. This residence time depends on the burning velocity of the fuels, which decreases with increasing fuel branching. These data are useful for thermochemical model development using detailed simulations of the experiments and a comparative diagnostics approach. Further adjustments to all of the thermochemical models are needed to capture the observed trends. This dataset is made available and provides validation and optimization targets for future model revisions.

Quantitative CH measurements in atmospheric-pressure, premixed flames of C1–C4 alkanes

The rapid formation of nitric oxide (NO) within the flame front of hydrocarbon flames, occurring via the prompt-NO formation route, is strongly coupled to the concentration of the methylidyne radical, [CH]. This work presents absolute measurements of [CH] taken in atmospheric-pressure, premixed, stagnation flames of methane, ethane, propane, and n-butane. One-dimensional (1D) CH fluorescence profiles are extracted from 2D Planar Laser-Induced Fluorescence (PLIF) measurements made quantitative through normalization by the Rayleigh scattering signal of nitrogen. Axial velocity profiles are measured by Particle Tracking Velocimetry (PTV) and, along with mixture composition and temperature measurements, provide the required boundary conditions for flame simulations based on the 1D hydrodynamic model of Kee et al. (1989). A time-resolved, four-level, LIF model considering rotational energy transfer in both the ground and excited electronic states is used to convert the modeled CH concentration profiles into units compatible with the quantitative CH-LIF measurements. Large variations in the CH concentrations predicted by four thermochemical mechanisms are observed for all fuels and equivalence ratios considered. A detailed study of the mechanisms, through reaction path and sensitivity analysis, shows that the principal reactions impacting CH formation are: (a) involved in the CH formation route (CH3→CH2*→CH2→CH), (b) bypass and remove carbon atoms from the CH formation route, or (c) affect the pool of reaction partners in the aforementioned reactions. The order of magnitude variability in the model predictions is caused by significant disagreements among the mechanisms in terms of rate coefficients and reactions included in these pathways. This data set is made available and provides validation and optimization targets for future combustion model revisions.