NO Mole Fraction Research Articles

NH3 oxidation by NO2 in a jet-stirred reactor: The effect of significant uncertainties in H2NO kinetics

Understanding the kinetics of ammonia (NH3) is becoming increasingly important to a growing variety of applications — ranging from its role as a NOx reduction agent, a key intermediate during combustion of biomass and energetic materials (especially green propellants), and a potential carbon-free energy carrier and storage medium. This wide variety of applications calls for comprehensive NH3 kinetic models that are reliable over wide ranges of temperatures, pressures, and mixtures. Yet, many still consider the present understanding of its kinetics to be incomplete. For example, there are few experimental studies of NH3 oxidation by nitrogen-containing species, which offer the opportunity to probe relatively untested reactions (or combinations thereof) to enable a more comprehensive understanding of NH3 kinetics. To address this gap, we perform jet-stirred reactor experiments of NH3 oxidation by NO2 over an intermediate temperature range (700–1100 K). The mole fractions of NH3, NO2, NO, and O2 are measured through a combination of gas chromatography, chemiluminescence, and infrared absorption. Agreement among different diagnostics (≤4% for NH3 and ≤7% for NO2) and excellent experimental repeatability ensure high confidence in all species measurements. Comparisons of species measurements to model predictions revealed deficiencies in recent kinetic models, particularly for NH3 consumption and NO formation at elevated temperatures (≥900 K). Uncertainty-weighted kinetic analyses point to the importance of reactions that form (NH2 + NO2) and consume (H2NO + NO2, H2NO + OH) H2NO, both of which are uncertain and influential in this system (and many other NH3 oxidation systems). These and other reactions accentuated in the present dataset are also key reactions in NH3/air ignition and N2O formation, both of which remain outstanding challenges for NH3 combustion in engines. Consequently, resolving the modeling deficiencies observed for the present dataset appears especially important to predictive models to enable the use of NH3 as a fuel.

Laminar burning velocities and nitric oxide formation in premixed dimethyl ether/air flames: Experiments and kinetic modeling

Adiabatic laminar burning velocities and post-flame NO mole fractions for flat, non-stretched, premixed dimethyl ether/air flames were experimentally determined with a heat flux burner combined with laser-induced fluorescence diagnostics, over equivalence ratios ranging from 0.7 to 1.6, at atmospheric pressure and initial temperatures from 298 to 338 K. The present burning velocity measurements were then compared with selected data available in the literature obtained with different techniques. The comparison showed reasonably good agreement with recent datasets obtained at different temperatures, as well as possible outliers not suitable for the validation of kinetic models. The detailed kinetic mechanism of the authors was extended by the reactions of dimethyl ether. A comparison of experimental and computational results using two contemporary detailed chemical kinetic mechanisms, along with the one from the authors presented in this work, was also conducted and discussed. Discrepancies between experiments and model predictions and among models themselves were observed, especially under rich conditions. Further numerical analyzes were performed to identify the main causes of the observed differences. Notwithstanding, the present model showed overall good performances in reproducing both laminar burning velocities and nitric oxide mole fractions. Kinetic modeling was also performed to enhance the understanding of the intrinsic NOx emission characteristics of dimethyl ether by comparing the present measurements with those obtained for ethanol/air flames under identical conditions.

Measurement of recreational N2O emissions from an urban environment in Manchester, UK

Nitrous oxide (N2O) is a potent greenhouse gas that is currently the third largest contributor to anthropogenic radiative forcing. It is also a strong ozone depleting substance. Given this importance, mitigation of N2O emissions remains important and sources must be understood in greater detail. In this study, in situ measurements of N2O alongside a variety of other trace gases and aerosols were made from a ground-based air quality observation site in an urban environment of Fallowfield, Manchester, United Kingdom over a period of 12 months between October 2020 and October 2021. N2O mole fraction was observed to be poorly correlated with other atmospheric pollutant tracers during the measurement period, with little evidence of co-enhancement (and therefore common source relationships) between N2O and other local pollutant trace gases and aerosol. Large N2O enhancements (> 400 ppb above background) over short time scales (< 2 min) were seen with no co-enhancement of other trace gases and aerosol concentrations, suggesting discrete N2O sources in the near vicinity of the measurement site. Measured N2O concentrations showed a consistent temporal pattern over day, week, and year timescales with consistently large weekend enhancements observed between the hours of 18:00 and 02:00 local time, suggesting the source of N2O may be associated with night-time recreational use by nearby residents. These weekend-night-time temporal patterns were not correlated with other trace gases measured at the same location. Analysis of the air transport history of N2O measurements showed high mean nocturnal mole fractions originating from the west and south-west of the observation site, suggesting that emissions may have originated from nearby areas of student accommodation and dense areas of private housing to the west. This study finds evidence for a detectable recreational N2O source that appears to be dominant over other potential N2O sources for the area studied. Further study is needed to quantify the local and national emission rates of this potentially increasing atmospheric pollution source, and to compare the magnitude of this source to other locations within the UK. The study demonstrates an important need to assess and validate National Atmospheric Emission Inventory (NAEI) estimates for recreational N2O emissions.

Experimental and modeling study of NO formation in ethyl formate + air flames

Premixed ethyl formate + air flames have been investigated at atmospheric pressure and initial temperatures from 298 to 348 K. The burning velocities were determined using the heat flux method for equivalence ratios from 0.6 to 1.5 and were compared with limited data available in the literature and with measurements for the C3H6O2 isomer, methyl acetate, obtained under the same conditions. In ethyl formate flames with the initial temperature of 298 K, the concentrations of nitric oxide were measured using laser-induced fluorescence downstream the flame front at heights above the burner of 5 and 10 mm and compared with similar experiments for methyl acetate obtained using the same equipment. It was demonstrated that the burning velocities, as well as NO concentrations in ethyl formate flames, are higher than those in methyl acetate at otherwise identical conditions. The NO mole fraction downstream the flame front of both fuels peaks in stoichiometric mixtures and decreases both in lean and rich flames. The detailed kinetic model developed for the C3H6O2 isomers using rate constants selected from the literature accurately reproduces all the new experimental results reported in the present work.

Monitoring of atmospheric CH4, CO, CO2, N2O and SF6 using three-channel gas chromatography

我国正处于“碳达峰、碳中和”的关键时期,准确认识我国温室气体浓度时空格局以及变化对于评估“碳达峰”和“碳中和”行动成效非常重要。当前我国近地面温室气体高精度监测主要依赖进口的光学监测主机,单台仪器成本高且监测要素有限。为此,该研究基于传统的气相色谱法,自主设计了一套三通道气相色谱分析系统,在单台仪器上实现了5种主要长寿命温室气体(CH4、CO、CO2、N2O和SF6)的高精度监测。对该系统的精密度、线性响应情况和准确度进行的针对性测试实验表明系统检测性能满足世界气象组织/全球大气观测(WMO/GAW)质控标准。针对环境浓度的CH4、CO、CO2、N2O和SF6的连续分析精密度分别达0.08%、1.90%、0.05%、0.08%、0.66%。准确度测试中,5种气体(CH4、CO、CO2、N2O和SF6)使用回归方程计算所得值与标称摩尔分数间的偏差分别达0.15×10-9、0.20×10-9、0.37×10-6、0.35×10-9、0.02×10-12(摩尔分数),CH4、CO、CO2、N2O和SF6仪器响应值与标称摩尔分数的线性拟合相关系数(R2)均为0.9999,线性拟合残差和准确度基本达到WMO/GAW拓展质控目标。该系统对杭州城区大气温室气体在线连续监测结果显示,2021年5~7月期间大气CH4、CO、CO2和N2O呈明显的日变化特征,主要受人为活动影响。综合测试和试运行结果表明,该研发系统具备良好的精密度、准确度、线性和稳定性,与目前国内广泛进口的仪器相比,具有技术自主可控、运行成本更低、自动化水平更高等优势,能满足多种温室气体在线监测研究的需求。

Comparison of air and EGR with different water fractions dilutions on the combustion of hydrogen-air mixtures

Lean combustion and exhaust gas recirculation (EGR) are key strategies for controlling NO and heat release process of hydrogen engines. This paper firstly compared these two strategies on combustion behaviors of hydrogen-air mixtures within dilution ratio ranges from 0 to 50%. Then, the water vapor effects in EGR on combustion of hydrogen-air mixtures are further investigated. All tests are conducted in a constant volume combustion vessel at the initial temperature of 373 K and pressure of 1 bar. CHEMKIN PRO is adopted to predict the hydrogen-air mixtures combustion behaviors under different conditions with one-dimensional premixed adiabatic planner flame model. The unburnt hydrogen concentration and the NO mole fraction in the final products were measured. The results show that laminar burning velocity decreases more obviously with the EGR ratio than air dilution ratio. EGR dilution has much obvious influence on reducing NO mole fraction in the final products. However, both dilution strategies adversely cause the increase of incompletely combusted H2. To balance the NO control and unburned H2 emission, 30% EGR rate with 20% water vapor fraction is recommended based on the testing ranges of this study.

Kinetic and Continuum Modeling of High-Temperature Air Relaxation

Fully kinetic, vibrationally kinetic, and continuum solvers with varying model fidelity are used in this work to model the high-temperature relaxation of air in 7230 and 15,000 K adiabatic heat baths and a hypersonic flow over a cylinder. The results show significant impact of uncertainties in vibrational relaxation times and reaction rate constants on thermal and chemical relaxation, in particular, on gas temperature and species mole fractions. Most notably, these uncertainties need to be reduced for collisions that include nitric oxide. Order-of-magnitude differences in the nitric oxide dissociation and recombination rates have a large impact on the peak NO mole fraction immediately behind the shock and surface-distributed heat flux, respectively. High-fidelity kinetic and continuum approaches are found to have different reaction channels having the largest effect on species mole fractions and gas temperature: exchange and dissociation in the former, and and dissociation in the latter.

On the Vibrational State-Specific Modeling of Radiating Normal Shocks in Air

A comprehensive state-to-state (StS) model for air was formed, including vibration–vibration–translation (VVT) reactions. The generated VVT reaction rates were compared with available first-principles calculations, and reasonable agreement was obtained. The influence of multiquantum transitions revealed that the possible reduction on the number of VVT transitions depends on the application. The influence of VV transitions revealed that the vibrational excitation becomes too fast if VVT transitions are reduced to VT transitions, invalidating this approximation. Comparisons were made with existing NO emission measurements in the ultraviolet and mid-infrared spectrum. At velocities of 3–4 km/s, some of the current dissociation rates may be inaccurate. At a higher velocity of , the NO mole fraction predicted by the StS model is around an order of magnitude greater than that predicted by the two-temperature model. Finally, recommendations were given on the further development of the StS model.

Shock-tube laser absorption measurements of N2O time histories during ammonia oxidation

The oxidation of ammonia was studied experimentally by monitoring the time history of the intermediate N2O species using laser absorption spectroscopy. Experiments were conducted in a shock tube for mixtures of NH3/O2 diluted in ∼96.7% Ar for equivalence ratios of 0.54, 1.03, and 1.84. The equivalence ratios were determined accurately using spectroscopic measurements of NH3 with another laser before each experiment. Experiments were performed at an average pressure of 1.2 atm and covered a temperature range of 1829 to 2198 K. For the same temperature, experiments revealed that increasing the equivalence ratio leads to less N2O formation. The time-history profiles showed that N2O is formed at the beginning of the experiments, mainly from the formed NO, until reaching a peak. The N2O is then fully consumed, mainly via its reaction with H-atom. Characteristic parameters, such as the N2O peak time and mole fraction, were extracted from the N2O profiles and compared with 15 recent NH3 kinetics models. The comparison revealed that none of the existing kinetics models were able to correctly predict both the peak N2O time and mole fraction together. Two of the models were selected to perform a chemical analysis, and an improvement of the predictive capability of one model is proposed. The N2O profiles reported herein are excellent validation targets that offer stringent constraints for the improvement of future NH3 kinetics models.

Airborne quantification of net methane and carbon dioxide fluxes from European Arctic wetlands in Summer 2019.

Arctic wetlands and surrounding ecosystems are both a significant source of methane (CH4) and a sink of carbon dioxide (CO2) during summer months. However, precise quantification of this regional CH4 source and CO2 sink remains poorly characterized. A research flight using the UK Facility for Airborne Atmospheric Measurement was conducted in July 2019 over an area (approx. 78 000 km2) of mixed peatland and forest in northern Sweden and Finland. Area-averaged fluxes of CH4 and carbon dioxide were calculated using an aircraft mass balance approach. Net CH4 fluxes normalized to wetland area ranged between 5.93 ± 1.87 mg m−2 h−1 and 4.44 ± 0.64 mg m−2 h−1 (largest to smallest) over the region with a meridional gradient across three discrete areas enclosed by the flight survey. From largest to smallest, net CO2 sinks ranged between −513 ± 74 mg m−2 h−1 and −284 ± 89 mg m−2 h−1 and result from net uptake of CO2 by vegetation and soils in the biosphere. A clear gradient of decreasing bulk and area-averaged CH4 flux was identified from north to south across the study region, correlated with decreasing peat bog land area from north to south identified from CORINE land cover classifications. While N2O mole fraction was measured, no discernible gradient was measured over the flight track, but a minimum flux threshold using this mass balance method was calculated. Bulk (total area) CH4 fluxes determined via mass balance were compared with area-weighted upscaled chamber fluxes from the same study area and were found to agree well within measurement uncertainty. The mass balance CH4 fluxes were found to be significantly higher than the CH4 fluxes reported by many land-surface process models compiled as part of the Global Carbon Project. There was high variability in both flux distribution and magnitude between the individual models. This further supports previous studies that suggest that land-surface models are currently ill-equipped to accurately capture carbon fluxes inthe region.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Measurement report: Regional characteristics of seasonal and long-term variations in greenhouse gases at Nainital, India, and Comilla, Bangladesh

Abstract. Emissions of greenhouse gases (GHGs) from the Indian subcontinent have increased during the last 20 years along with rapid economic growth; however, there remains a paucity of GHG measurements for policy-relevant research. In northern India and Bangladesh, agricultural activities are considered to play an important role in GHG concentrations in the atmosphere. We performed weekly air sampling at Nainital (NTL) in northern India and Comilla (CLA) in Bangladesh from 2006 and 2012, respectively. Air samples were analyzed for dry-air gas mole fractions of CO2, CH4, CO, H2, N2O, and SF6 and carbon and oxygen isotopic ratios of CO2 (δ13C-CO2 and δ18O-CO2). Regional characteristics of these components over the Indo-Gangetic Plain are discussed compared to data from other Indian sites and Mauna Loa, Hawaii (MLO), which is representative of marine background air. We found that the CO2 mole fraction at CLA had two seasonal minima in February–March and September, corresponding to crop cultivation activities that depend on regional climatic conditions. Although NTL had only one clear minimum in September, the carbon isotopic signature suggested that photosynthetic CO2 absorption by crops cultivated in each season contributes differently to lower CO2 mole fractions at both sites. The CH4 mole fraction of NTL and CLA in August–October showed high values (i.e., sometimes over 4000 ppb at CLA), mainly due to the influence of CH4 emissions from the paddy fields. High CH4 mole fractions sustained over months at CLA were a characteristic feature on the Indo-Gangetic Plain, which were affected by both the local emission and air mass transport. The CO mole fractions at NTL were also high and showed peaks in May and October, while CLA had much higher peaks in October–March due to the influence of human activities such as emissions from biomass burning and brick production. The N2O mole fractions at NTL and CLA increased in June–August and November–February, which coincided with the application of nitrogen fertilizer and the burning of biomass such as the harvest residues and dung for domestic cooking. Based on H2 seasonal variation at both sites, it appeared that the emissions in this region were related to biomass burning in addition to production from the reaction of OH and CH4. The SF6 mole fraction was similar to that at MLO, suggesting that there were few anthropogenic SF6 emission sources in the district. The variability of the CO2 growth rate at NTL was different from the variability in the CO2 growth rate at MLO, which is more closely linked to the El Niño–Southern Oscillation (ENSO). In addition, the growth rates of the CH4 and SF6 mole fractions at NTL showed an anticorrelation with those at MLO, indicating that the frequency of southerly air masses strongly influenced these mole fractions. These findings showed that rather large regional climatic conditions considerably controlled interannual variations in GHGs, δ13C-CO2, and δ18O-CO2 through changes in precipitation and air mass.

Numerical and Experimental Analysis of the Effect of a Swirler with a High Swirl Number in a Biogas Combustor

Climate change as a worldwide phenomenon is the cause of multinational agreements such as the Kyoto Protocol and the Paris Agreement with the goal of reducing greenhouse gas emissions. Biogas is one of the most promising biofuels for the integration of clean energy sources; however, biogas has the disadvantage of a low calorific value. To overcome this problem, mechanical devices such as swirlers are implemented in combustion chambers (CCs) to increase their combustion efficiencies. A swirler induces rotation in the airstream that keeps a constant re-ignition of the air–fuel mixture in the combustion. We present the numerical modeling using computational fluid dynamics (CFD) and experimental testing of combustion with biogas in a CC, including an optimized swirler in the airstream with a swirl number (Sn) of 2.48. A turbulence model of the renormalization group (RNG) was used to analyze the turbulence. Chemistry was parameterized using the laminar flamelet model. The numerical model allows visualizing the recirculation zone generated at the primary zone, and partially at the intermediate zone of the CC caused by the strong swirl. Temperature distribution profiles show the highest temperatures located at the intermediate and dilution zones, with the last one being a characteristic feature of biogas combustion. A strong swirl in the airstream generates low-velocity zones at the center of the CC. This effect centers flame, avoiding hot spots near the flame tube and flashback at the structural components. Regarding pollutant emissions, the goal of a biogas that generates less pollutants than nonrenewable gases is accomplished. It is observed that the mole fraction of NO in the CC is close to zero, while the mole fraction of CO2 after combustion is lowered compared to the original mole fraction contained in the biogas (0.25). The mole fraction of CO2 obtained in experimental tests was 0.0127. Results obtained in the numerical model for temperatures and mole fractions of CO2 and NO show a behavior similar to that of the experimental model. Experimental results for mole fraction of CO emissions are also presented and have a mean value of 0.0009. This value lies within allowed pollutant emissions for CO according to national environmental regulations.

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.

Sources of error in open-path FTIR measurements of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emitted from agricultural fields

Abstract. Open-path Fourier transform infrared spectroscopy (OP-FTIR) is susceptible to environmental variables which can become sources of errors for gas quantification. In this study, we assessed the effects of water vapour, temperature, path length, and wind speed on quantitative uncertainties of nitrous oxide (N2O) and carbon dioxide (CO2) derived from OP-FTIR spectra. The presence of water vapour in spectra underestimated N2O mole fractions by 3 % and 12 %, respectively, from both lab and field experiments using a classical least squares (CLS) model when the reference and sample spectra were collected at the same temperature (i.e. 30 ∘C). Differences in temperature between sample and reference spectra also underestimated N2O mole fractions due to temperature broadening and the increased interferences of water vapour in spectra of wet samples. Changes in path length resulted in a non-linear response of spectra and bias (e.g. N2O and CO2 mole fractions were underestimated by 30 % and 7.5 %, respectively, at the optical path of 100 m using CLS models). For N2O quantification, partial least squares (PLS) models were less sensitive to water vapour, temperature, and path length and provided more accurate estimations than CLS. Uncertainties in the path-averaged mole fractions increased in low-wind conditions (&lt;2 m s−1). This study identified the most common interferences that affect OP-FTIR measurements of N2O and CO2, which can serve as a quality assurance/control guide for current or future OP-FTIR users.