NO Mole Research Articles

Effect of High Pressures on the Formation of Nitric Oxide in Lean, Premixed Flames

Abstract Increasingly stringent regulations are imposed on nitrogen oxides emissions due to their numerous negative impacts on human health and the environment. Accurate, experimentally validated thermochemical models are required for the development of the next generation of combustors. This paper presents a series of experiments performed in lean, premixed, laminar, jet-wall stagnation flames at pressures of 2, 4, 8, and 16 atm. To target postflame temperatures relevant to gas turbine engines, the stoichiometry of the nonpreheated methane–air mixture is adjusted to an equivalence ratio of 0.7. One-dimensional (1D) profiles of temperature and NO mole fraction are measured via laser-induced fluorescence (LIF) thermometry and NO-LIF, respectively, to complement previously published flame speed data (Versailles et al., 2018, “Measurements of the Reactivity of Premixed, Stagnation, Methane-Air Flames at Gas Turbine Relevant Pressures,” ASME. J. Eng. Gas Turbines Power, 141(1), p. 011027). The results reveal that, as the pressure increases, the maximum postflame temperature stays relatively stable, and the concentration of NO produced through the flame front remains constant within uncertainty. Seven thermochemical models, selected for their widespread usage or recent date of publication, are validated against the experimental data. While all mechanisms accurately predict the postflame temperature, thanks to consistent thermodynamic parameters, important disagreements are observed in the NO concentration profiles, which highlights the need to carefully select the models used as design tools. The lack of pressure dependence of NO formation that many models fail to capture is numerically investigated via sensitivity and reaction path analyses applied to the solution of flame simulations. The termolecular reaction H+O2(+M)↔HO2(+M) is shown to hinder the production of atomic oxygen and to consume hydrogen radicals at higher pressures, which inhibits the formation of nitric oxide through the N2O pathway.

Stability limits and NO emissions of premixed swirl ammonia-air flames enriched with hydrogen or methane at elevated pressures

This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, xNH3, was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (xNH3 < 0.9), flames with very lean equivalence ratios (φ < 0.7) can be stabilized and these yield competitively low NO emissions (<200 ppm), regardless of pressure. For this reason, very lean swirl ammonia-hydrogen-air flames are promising candidates for micro gas turbines. However, N2O emissions have the potential to be unacceptably large for these operating conditions if heat loss is too large or residence time is too short. As a consequence, the post flame region must be considered carefully. Due to the lower reactivity of methane compared to that of hydrogen, very lean swirl ammonia-methane-air flames could not be stabilized and good NO performance is limited to rich equivalence ratios for ammonia-methane fuel blends. The equivalence ratio above which good NO performance depends on pressure and bulk velocity.

In Situ Laser-Induced Fluorescence and Ex Situ Cavity Ring-Down Spectroscopy Applied to NO Measurement in Flames: Microprobe Perturbation and Absolute Quantification

Flame-sampling experiments allow for describing the species profiles as a function of the height above the burner. Even with microprobes as those generally used for gas chromatography measurements, the perturbations induced by the microprobe can be non-negligible. Not only will the heat losses as a result of its presence affect the temperature and the spatial position of the species profiles, but the composition of gas sampling may also be altered. To attempt to clarify the induced perturbations by microprobes, NO species profiles are obtained in stoichiometric low-pressure flames using either in situ laser-induced fluorescence (LIF) or ex situ absorption measurements. Gas samplings are performed with two uncooled quartz microprobes characterized by their tip angle (6.6° and 16°) and analyzed in an external absorption cell by cavity ring-down spectroscopy (CRDS). The absolute quantification of the NO mole fractions is undertaken in the burned gas using either the standard addition method by LIF or after gas sampling from the integrated spectral absorptivity by CRDS. In the burned gas, it turns out that the gas sampling is altered by the microprobe having the smallest tip angle, where the absolute mole fraction of NO is found 40% lower than when the quantification is performed either after gas sampling with the largest tip angle microprobe or by in situ LIF. As expected, the comparison of the relative NO profiles along the flame indicates that the profiles measured after gas sampling are shifted downstream. This is all the more so that the tip angle is large and the flame is clearly attached. Kinetic simulations based on the probe geometry and the residence time do not allow us to discern possible homo- or heterogeneous reactions inside the probe. The most disturbing factor is attributed to a cooling effect as a result of the probe

Measurements of the laminar burning velocities and NO concentrations in neat and blended ethanol and n-heptane flames

Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 < Φ < 1.2, while under fuel-rich conditions this behavior is due to the lower concentrations of CH radicals, which decrease the production of prompt-NO. At Φ > 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways.

Numerical study of the premixed ammonia-hydrogen combustion under engine-relevant conditions

Recently, ammonia is considered to be an energy medium for hydrogen or energy storage and ammonia/hydrogen can be a potential fuel blend for the spark-ignition engine. Nevertheless, the combustion properties of ammonia/hydrogen mixtures could be distinctive under different initial conditions. In this study, several essential combustion properties, including laminar burning velocity, minimum ignition energy, NOx and ammonia emissions, combustion efficiency, and mixture heating values of ammonia/hydrogen/air premixed combustion were extensively studied under a wide range of equivalence ratios (ϕ), hydrogen fractions (α) and different compression ratio using one-dimensional laminar planar flame and compared with stoichiometric methane, methanol, and ethanol combustion. The behavior of NOx generation changing with ϕ and α was analyzed in detail. Results showed that most properties of ammonia/hydrogen combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions, except for the mixture heating value is slightly lower in all conditions. Besides, the NO mole fraction non-monotonically changes with ϕ and α due to the competition between the effects caused by the reduction of nitrogen-atom species and the enrichment of H/O radicals. The NO mole fraction of stoichiometric ammonia/hydrogen could be even lower than that of hydrocarbons. Considering the properties of hydrocarbon fuels as a reference, promising working conditions for ammonia/hydrogen mixtures are ϕ from 1.0 to 1.05 and α from 40% to 60%. Besides, a high compression ratio is more suitable for ammonia/hydrogen mixtures due to the excellent knock resistance ability of NH3/H2 and better improvement than hydrocarbon fuels under high compression ratio.

Experimental and numerical study of product gas characteristics of ammonia/air premixed laminar flames stabilized in a stagnation flow

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NOx) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.

Stability limits and NO emissions of technically-premixed ammonia-hydrogen-nitrogen-air swirl flames

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames’ propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends.

The Effects of Non-Uniform Magnetic Field on the Concentration of Methane-Air Reaction Species

It is a well-known fact that the effects of magnetic fields on combustion can be used to control and optimize the flame deformation and the flame brightness. The kinetics and equilibrium properties of chemical reactions of combustion are influenced by the magnetic force exerted on paramagnetic species. In this study, the effects of non-uniform magnetic fields on one-stage methane combustion reaction are numerically investigated. It is known that NO, OH, and O₂ are paramagnetic species and the other species and methane have diamagnetic behavior. Considering these facts, the effects of non-uniform magnetic field on 10 main product species of methane combustion are studied, by minimization of the Gibbs free energy. The results indicate that variation of non-uniform magnetic fields from 0 to 0.08 Tesla leads to decrease in NO mole fraction by 99.6 % in temperature range 1500-2500 K. Furthermore, the combination of non-uniform magnetic field and raising the pressure have the beneficial result in decreasing NO and CO mole fractions as well as rise in temperature.

Experimental and modeling study of nitric oxide formation in premixed methanol + air flames

Validation and further development of models for alcohol combustion requires accurate experimental data obtained under well-controlled conditions. To this end, measurements of nitric oxide, NO, mole fractions in premixed laminar methanol + air flames have been performed using saturated laser-induced fluorescence, LIF. The methanol flames have been stabilized at atmospheric pressure and initial gas temperature of 318 K at equivalence ratios ɸ = 0.7–1.5 using the heat flux method that allows for simultaneous determination of their laminar burning velocity. The LIF signal is converted into NO mole fraction via calibration measurements, which have been performed in flames of methane, methanol and syngas seeded with known amounts of NO. The experimental approach is verified by the measurements of NO mole fractions in the post flame zone of methane flames, investigated in previous studies at similar conditions. Data on the NO formation together with burning velocities for methanol and methane obtained under adiabatic flame conditions provide highly valuable input for model validation. They have been compared with predictions of six different chemical kinetic mechanisms. Summarizing the behavior of all models tested with respect to burning velocities and NO formation in flames of methane and methanol, the mechanism of Glarborg et al. (2018) and the San Diego mechanism (2019) demonstrate uniformly satisfactory performance.

NO formation in high pressure premixed flames: Experimental results and validation of a new revised reaction mechanism

This paper presents an experimental and modelling study of NO formation in high pressure premixed flames. Experiments were performed in a high-pressure counterflow burner in which laminar premixed CH4/air flames were stabilised at equivalence ratios of E.R = 0.7, 1 and 1.2 and for pressures varying from 0.1 to 0.7 MPa. We report quantitative NO mole fraction profiles measured by Laser Induced Fluorescence. The effects of pressure and equivalence ratio on NO formation are discussed. These results are compared to the simulations using two reaction mechanisms: NOmecha2.0 associated to a detailed mechanism for methane oxidation: GDFkin®3.0 and the mechanism from Klippenstein et al., which is the most recent high-pressure NOx formation mechanism available in the literature. In general, both mechanisms are able to predict NO correctly in lean and stoichiometric high pressure flames; however, in rich flames, GDFkin®3.0_NOmecha2.0 gives the best predictions. The performances of these mechanisms are also tested on NO measurements in high-pressure flames from the literature. A kinetic analysis is then presented to identify the main pathways that lead to the formation and consumption of NO and highlight the differences between the two mechanisms, as well as a sensitivity analysis to identify important reactions that influence the formation/consumption of NO in our high pressure flames.

Effect of hydrogen addition on combustion and heat release characteristics of ammonia flame

As a key parameter of fuel, the heat release characteristics of ammonia (NH3) flames under hydrogen (H2)-enriched conditions constitute the emphasis of this research. The effect of H2 addition on heat release characteristics of premixed laminar NH3 flames are numerically investigated by considering the detailed chemical mechanism. The NH3, O2, H2O, NO, and N2O mole fraction distributions based on the Dagaut-Kéromnès mechanism agree well with experimental results, thereby potentially providing repeatable and accurate outcomes with NH3 flames using this mechanism. The OH × N radical positively affects the net heat release rate at various equivalence ratios and H2 addition ratios conditions. The chemical effect is evidently important at low H2 addition ratios, while the transport effect becomes dominant at high H2 addition ratios because of the high mobility of H2. For the three top endothermic elementary reactions of NH3 flames, the contribution of R236: H + O2=OH + O increases, while R171: H + NO(+M) = HNO(+M) and R240: H2O + O=OH + OH decrease with increasing H2 addition ratios. Diverse key exothermic elementary reactions of NH3 flames occur at various equivalence ratios and different H2 addition ratios. The large contribution of R128: N + NO=N2+O at fuel-rich conditions can be applied to reduce NOx emissions of the NH3 flame under pure and H2 addition conditions.

Kinetic modeling of ammonia/air weak flames in a micro flow reactor with a controlled temperature profile

Ammonia is considered to be one of the promising energy carriers in the future and reliable chemical kinetics to accurately predict ignition characteristics of ammonia/air mixtures is necessary for developing ammonia combustors. However, ignition characteristics of ammonia/air mixtures at low temperatures have not been well studied. The present study employed weak flames in a micro flow reactor with a controlled temperature profile, which have been extensively employed to examine ignition characteristics of hydrocarbons, to investigate ignition characteristics of ammonia/air mixtures at low temperatures. Species measurements for weak flames of ammonia/air mixtures at atmospheric pressure and equivalence ratios of 0.8, 1.0 and 1.2 under a maximum wall temperature of 1400 K were conducted using a mass spectrometer and profiles of the NH3, O2, H2O NO, and N2O mole fractions were obtained. Chemical kinetic modeling was conducted with extensive updates mainly for the N2Hx chemistry because N2Hx species were expected to be produced from the NH2 + NH2 reactions at low temperatures. The mechanism developed in the present study well predicted species profiles of NH3, O2 and H2O for weak flames measured in experiments. The present mechanism also well predicted the final values of the NO and N2O mole fractions behind the reaction zone of weak flames but overestimated these mole fractions in the reaction zone of weak flames. To confirm the existence of N2Hx species in the reaction zone of weak flames, signals from N2H4 were distinguished from measured signals. Sensitivity analysis and reaction flux analysis were conducted and the importance of the N2Hx chemistry in the reaction zone of weak flames at low temperatures was identified. Validation of the present mechanism with literature data on ignition delays and flame speeds were conducted and reasonable agreements with literature data were confirmed. For N2O and NO in the reaction zone of weak flames, however, there was discrepancy between measured and computational mole fractions and further improvements of chemical kinetics related to ammonia ignition are still necessary.

Effects of nitrogen dilution on the NOx formation characteristics of CH4/CO/H2 syngas counterflow non-premixed flames

In the present study, the effects of the composition and nitrogen dilution level on the precise structure and NOx formation characteristics were numerically investigated for ternary syngas composed of H2, CO, and CH4 in laminar counterflow non-premixed flames. Prior to the parametric study, in order to evaluate the prediction capability for the NOx formation characteristics of syngas/air counterflow non-premixed flames, the GRI 3.0 mechanism was validated against measurements. Numerical results indicate that the GRI 3.0 mechanism yields good overall agreement with the measured peak level and distribution of the NO mole fraction over a relatively wide range of H2/CO compositions and strain rates. Computations were carried out for wide ranges of ternary syngas composition and nitrogen dilution level. Based on the numerical results, the detailed discussions are made for the essential features and precise structure of the non-diluted and nitrogen-diluted CH4/CO/H2 syngas counterflow diffusion flames including the temperature, species mole fractions, and NO production rate and its integral. Special emphasis was given to the effects of the fuel-side nitrogen dilution on the NOx formation characteristics of the CH4-rich, CO-rich, and H2-rich syngas diffusion flames for the different ternary syngas compositions. The dominant physical processes and chemical paths to influence the precise flame structure and NO generation encountered under the actual syngas combustion conditions were systematically analyzed and identified.

Effects of CH4 Content on NO Formation in One-Dimensional Adiabatic Flames Investigated by Saturated Laser-Induced Fluorescence and CHEMKIN Modeling

Experiments applying saturated laser-induced fluorescence (LSF) technology were performed focusing on the influence of equivalence ratio, syngas mixture contents, on NO formation in H2/CO/CH4/CO2/N2/O2 premixed flat flames supplied by a heat flux burner. Experimental data were extracted to validate calculation by CHEMKIN software. Both experiments and CHEMKIN calculation draw one conclusion that NO mole fraction in CH4–air flame peaks at stoichiometric and ϕ 1.3, due to thermal NO and prompt NO routes. All mechanisms applied can well-predict the NO mole fraction at the fuel lean side but failed at the fuel rich side. NO mole fraction in syngas flame is propotional to CH4 ratio as shown in the experimental data, but all the mechanisms failed to predict it with some even having a wrong tendency. A rate of NO production and sensitivity analysis suggests that thermal and prompt NO routes play different roles in each mechanism, and modifications to mechanisms are required to improve NO concnetration prediction...

Influence of Combustion Initial Conditions on Emissions Production Rates and Released Heat for Methane Fuel Mixtures

An estimation of combustion products (pollutants) which include CO, CO2 and NO mole fraction are reported in this paper for premixed methane/air flames. Different mixtures were used in this study, including lean, stoichiometric and rich subjected to varying degrees of pressures and temperatures ranging from 5 - 40 bars and 350 - 600 K, respectively. In this work, computer software was used to calculate the produced emissions species as well as the final (adiabatic) temperatures for each mixture. Results show that rich mixture of methane fuel produces the highest rate for carbon monoxide (CO) with slight increase as pressure and temperature increase. Where the stoichiometric mixture produces the highest rate of carbon dioxide (CO2). Results showed that this type of emission decreases with the increase of pressure and temperature. On the other hand, nitric acid (NO) was found to be the highest for the lean mixture with sharp increase as pressure and temperature increase. Finally, the combustion heat (Q) for each mixture where plotted against pressure and it was found that the rich mixture of methane produced the highest rates. Results also showed that combustion heat increases sharply with increased pressures and temperatures.

Effects of fuel composition and strain rate on NO emission of premixed counter-flow H2/CO/air flames

NO formation in premixed counter-flow flames of three syngas fuels with different compositions (0.3H2+0.7CO, 0.5H2+0.5CO, 0.7H2+0.3CO) were numerically investigated using the OPPDIF code over a wide strain rate range (a = 10–1000 s−1). The equivalence ratio was fixed at 0.5 and the USC_Mech_II mechanism coupled with NOX reaction mechanism from GRI2.11 was adopted. The strain rates of 10 s−1 and 1000 s−1 were selected to represent small and large strain rates. The results show that the flame temperature and maximum NO mole fraction increase first and then decrease with the increase of strain rate. At a = 10 s−1, the flame temperature decreases with the increase of H2 content in the syngas, and the trend is inversed at a = 1000 s−1. Moreover, it is found that only the H2-lean case (0.3H2+0.7CO) is a single flame at a = 1000 s−1. Furthermore, the less H2 content in the syngas, the more NO is formed at a = 10 s−1; in contrast, more NO is observed for the syngas that contains more H2 at a = 1000 s−1. The reaction pathway analysis reveals that the NNH-intermediate route is the main NO formation route for all the three syngas fuels.

都市ごみ焼却炉におけるFuel-NO&lt;sub&gt;X&lt;/sub&gt;の生成に及ぼす揮発成分の影響に関する詳細素反応解析

Low NOX emission combustion technology is required for Municipal Solid Waste (MSW) incinerators. Comparing investigation between simplified 1D combustion experiment and 1D numerical analysis has been performed to decide detailed reaction mechanism suited for NO emission prediction in MSW combustion. Effects of H2/CO ratio and H2O/CO2 ratio in the modeled pyrolysis gas to NO formation were evaluated in the primary combustion and the subsequent secondary combustion by Premixed Laminar Burner-Stabilized Flame Model. Comparison between measured NO concentration and calculated NO concentration indicated that calculation results obtained by using Kilpinen97 reaction were within a certain error range compared with experimental results in a wider condition range of fuel composition and excess air ratio than the modified GRI-mech3.0. Increased H2/CO and H2O/CO2 led to the decrease in NO mole flux in the primary combustion, respectively by different reaction paths, which increased both H2/CO and H2O/CO2 showed an additive effect on the decrease in NO mole flux. NO conversion ratios after the secondary combustion from NH3 in the modeled pyrolysis gas were still reduced by the increased H2/CO and H2O/CO2 in the modeled pyrolysis gas although the NO conversion ratios after the secondary combustion were larger than those after the primary combustion because of the conversion to NO in the secondary combustion from HCN, NH3 and HCNO remained in the primary combustion. Conversion ratio from HCN, NH3 and HCNO to NO in the secondary combustion can be reduced by the decreased temperature at inlet of the secondary combustion area. Based on the findings above, it is possible to design lower NOX emission MSW incinerators by considering the conditions of the exhaust gas recirculation affecting H2/CO and H2O/CO2 in the primary combustion, and the cooling structure of the primary combustion flue gas.

Detailed influences of chemical effects of hydrogen as fuel additive on methane flame

A detailed kinetic analysis of chemical effects of hydrogen addition on laminar premixed stoichiometric methane-air flames was conducted at atmospheric pressure. Flame structures and mole fraction profiles affected by chemical effects of hydrogen addition for major species, free radicals and intermediate species are analyzed with particular emphasis on the formations of soot precursor and oxygenated air pollutants. The results illustrate that chemical effects of hydrogen additive lead the methane profile to move towards the upstream side and suppress the formation of acetylene and ketene. The concentrations of free radical H, O and OH increase as methane is replaced by hydrogen mainly due to its chemical effects. In contrast, although the chemical effects of hydrogen addition facilitate the productions of formaldehyde and acetaldehyde, the hydrogen dilution and thermal effects on reducing mole fractions of both species are more significant. As a consequence, the total effects of hydrogen addition lead to a decrease in formaldehyde and acetaldehyde concentrations. Compared to formaldehyde and acetaldehyde, NO mole fraction diminishes in a similar fashion with increased hydrogen additive that the decrease of NO concentration caused by hydrogen dilution and thermal effects is larger than the increase due to its chemical effects.

Study on the combustion characteristics of a premixed combustion system with exhaust gas recirculation

The boiler of a premixed combustion system with EGR (exhaust gas recirculation) is investigated to explore the potential for increasing thermal efficiency and lowering pollutant emissions. To achieve this purpose, a thermodynamic analysis is performed to predict the effect of EGR on the thermodynamic efficiency for various equivalence ratios. Experiments of a preheated air condensing boiler with EGR were conducted to measure the changes in the thermal efficiency and the characteristics of the pollutant emission. Finally, a 1-D premixed code was calculated to understand the effect of the EGR method on the NO reduction mechanism. The results of the thermodynamic analysis show that the thermodynamic efficiency is not changed because the temperature and the amount of the exhaust gas are unchanged, even though the EGR method is implemented in the system. However, when the EGR method is used with an equivalence ratio near 1.00, it is experimentally verified that the thermal efficiency increases and the NOx concentration decreases. Based on the results from numerical calculations, it is shown that the NO production rates of N + O2 ↔ NO + O and N + OH ↔ NO + H are remarkably changed due to the decrease in the flame temperature and the NO mole fraction is decreased.

Effect of cycle time on NH3 generation on low Pt dispersion Pt/BaO/Al2O3 catalysts: Experiments and crystallite-scale modeling

A crystallite-scale model that explicitly accounts for the diffusion of stored NOx in the storage phase is used to explain the effects of cycle time, Pt dispersion and loading on NOx conversion and NH3 selectivity on low Pt dispersion NOx storage/reduction (NSR) catalysts. The low Pt dispersion or aged NSR catalysts are known to give high selectivity to NH3 which is essential for the combined NSR/SCR (selective catalytic reduction) system. It is shown through both experiments and modeling that for a fixed ratio of the moles of NO:H2 fed per cycle, a longer diluted rich feed for fixed lean time and a shorter overall cycle is beneficial in terms of NOx conversion for both 3% and 8% dispersion catalysts. Model predicts that more stored NOx is allowed to diffuse back to the Pt sites during a longer diluted regeneration while excessive NOx slip is prevented in a shorter overall cycle due to frequent regeneration, both of which leads to higher NOx conversion. The NH3 selectivity which depends on local NO:H2 ratio decreased with increase (decrease) in rich time (total cycle time) for 8% catalyst. Similar NH3 selectivity trend as a function of overall cycle time was also observed for 3% catalyst. The findings are helpful in assessing the viability of these catalysts (3% and 8%) to be used in conjunction with the SCR catalyst. Finally, the model is also used to gain additional insights on the impact of Pt loading and dispersion to provide important guidelines for NSR catalyst design for combined NSR/SCR application.