Oxygen Mole Fraction Research Articles

Reaction zone characteristics, thermal performance and NOx/N2O emissions analyses of ammonia MILD combustion

Pure ammonia MILD combustion is studied numerically in a furnace. The effects of furnace wall temperature and dilution of inlet air with nitrogen on the characteristics of reaction zone (RZ), temperature distribution and exhaust emissions are investigated. The results show that lowering the furnace wall temperature and adding diluent to the inlet air, result in expansion of the RZ, reduction of fuel and oxidizer consumption rates, lower Damkohler, increased turbulent Reynolds and more similar RZ properties to the distributed combustion regime in the combustion regimes diagram. At a lower concentration of oxygen in the oxidizer, a higher threshold wall temperature is required for stable MILD combustion. Reducing the wall temperature and oxygen mole fraction in the oxidizer, increases the uniformity of temperature distribution. Analysis of exhaust emissions shows that reducing wall temperature and oxygen level in the oxidizer stream can significantly reduce NOx pollution, however, N2O emission rises.

Limits of performance of chemical looping air separation in packed bed coupled with electricity production

Oxyfuel combustion, as a carbon capture method, requires oxygen to be separated from nitrogen. Currently, cryogenic air separation is used for this purpose. An alternative is to use chemical looping, where an oxygen carrier is cycled between reducing and oxidising conditions. In this paper, the feasibility of using packed bed reactors in chemical looping air separation is studied. By introducing a compressor and turbine, oxidisers can be operated at an elevated pressure and the proposed scheme can be viewed as a power-station in its own right, in addition to its more recognised application of producing oxygen for a downstream oxy-fuel combustion. For typical oxygen carriers, a single packed bed reactor is not able to meet the 0.30–0.35 oxygen molar fraction needed for an oxyfuel combustor. Therefore, multiple beds must be used in series or heat must be added radially along the bed length to increase the oxygen molar fraction in the bed.

Probing sooting limits in counterflow diffusion flames via multiple optical diagnostic techniques

Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined “nucleation” flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors’ expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot.

The determination method of the ignition modes of single coal particle with a transient coal ignition and combustion model

A transient model for the ignition and combustion of a quiescent coal particle was developed by coupling the mass, energy and species governing equations between the particle phase and the gas phase. The ignition delay times of single coal particle were calculated by using the transient ignition and combustion model with Kobayashi-Sarofim devolatilization model and Chemical Percolation Devolatilization (CPD) model, respectively. The predicted results were compared with the experimental data to further validate better performance of CPD model. The new ignition criteria of single coal particle were proposed to determine the onset of the ignition and subsequently to define the ignition modes of single coal particle by comparing the magnitude of typical ignition time scales, including the start time and the end time of devolatilization, homogeneous ignition delay time and the heterogeneous ignition delay time based on the thermal explosion theory (TET). The ignition modes map of single coal particle under various ambient oxygen molar fractions (O2 = 0–100%) and gas temperatures (Tg = 800–2000 K) was firstly depicted by using the transient ignition and combustion model with the new ignition criteria. The ignition modes of single coal particle transferred from homogeneous gas-phase ignition (GI) to homo-heterogeneous ignition (GI-HI) when the ambient oxygen molar fraction was increased from 5% to 21% under Tg = 1340 K condition, and then they transformed into hetero-homogeneous ignition (HI-GI) and heterogeneous ignition of coal (HI-coal) when the ambient oxygen molar fraction was further increased to 40% and 70%, respectively. Moreover, the simulated results showed that the critical oxygen molar fraction XO2,cr for the transition of ignition mode from homogeneous to heterogeneous ignition was 15% for the particle size of 50 μm, XO2,cr = 20% for the particle size of 100 μm, and XO2,cr = 30% for the particle size of 200 μm, respectively. The critical parameters for the transition of ignition mode of single coal particle were quantitatively determined to give a deep understanding on coal particle ignition process and to provide the guidance for improving the combustion performance of coal particles.

Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.

Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH4 jet flame in an O2/CO2 coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH4 jet flame in O2/CO2 coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH4 jet flame in the O2/CO2 coflow with the O2 mole fraction of 0.35 was conducted to analyze the effects of CO2 dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO2 on the height, width, and temperature distribution of the flame. The results showed that CO2 participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO2 on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH4 jet flame were analyzed in detail to investigate the influence mechanisms of CO2 on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO2 dilution, resulting in different flame heights and temperature distributions.

Inhibition effect of N2/CO2 blends on the minimum explosion concentration of agriculture and coal dusts

Minimum explosion concentration (MEC) of three agriculture dusts and two coal dusts was studied via a 20-L explosion chamber to reveal the role of gaseous inhibitors. Both active method (CO2 diluting air) and passive method (CO2 replacing O2) were used. The TG and DTG thermal analysis tests were conducted to study the pyrolysis and combustion characteristics of dust samples. An alternative explosion criterion based on combustion duration time was used to determine MEC, and compared with the standardized overpressure method. Under 10-kJ ignition condition, as oxygen mole fraction (XO2) decreased from 21% to 10%, MEC of agriculture dusts and coal dusts respectively increased by around 5 times and 2 times. The active inerting method with a lower N2/CO2 ratio was found to have a better suppression effect on the explosion of the five carbonaceous dusts because the blend has a higher specific heat and a lower oxygen diffusion rate.

Experimental and kinetic modeling studies on oxidation of n-heptane under oxygen enrichment in a jet-stirred reactor

Oxygen-enriched combustion can improve thermal efficiency and reduce CO, unburned hydrocarbons and soot emissions in internal combustion engines. There are significant differences in the combustion process and pollutant emissions between oxygen-enriched and air condition. In order to develop an improved understanding of the effect of oxygen concentration on n-heptane oxidation, the jet-stirred reactor oxidation characteristics of n-heptane were experimentally tested at temperatures of 450-950 K and pressures of 1.03 atm with the residence time (τ) of 2 s, under the condition of oxygen mole fractions from 0.055 to 0.6. Meantime, the simulations were performed on CHEMKIN. Results show that, firstly, with the increase of oxygen concentration, the NTC (Negative Temperature Coefficient) effect is weakened, mainly due to the strengthening of low-temperature reaction path, which is also the reason for the improvement of ignition stability in cold-start condition of engines. Secondly, the activity of the reaction system improves with the increase of oxygen concentration. Within 500-750 K, it is due to enhancement of the low-temperature reaction path. When the temperature is higher than 750 K, the improvement is due to the enhancement of decomposition of H2O2 and addition reaction of H and O2. Thirdly, with the increase of oxygen concentration, both the generation and consumption of CO are strengthened, which leads to the tip of the diesel spray flame changing from soot area under air condition to the CO chemiluminescence.

Proposal and investigation of a novel process configuration for production of neon from cryogenic air separation unit

Cryogenic air separation is an efficient method to produce oxygen and nitrogen that has been investigated extensively. However, the production of rare gas components of air such as neon through air separation has not been explored enough. In this paper, a cryogenic process to produce neon in a recovery column along with nitrogen and liquid oxygen from a two-column air separation unit is proposed. Besides, energy, exergy, and sensitivity analyses of the process are performed. Neon, nitrogen, and oxygen mole fractions obtained were 0.206%, 98.75%, and 74.47% respectively. Results showed that increasing the temperature of the feed streams and the reflux ratio of the columns have a negative effect on the recovery of neon while increasing feed’s pressure up to a point improves the recovery.

Combustion characteristics and synergistic effects during co-combustion of lignite and lignocellulosic components under oxy-fuel condition

Co-combustion of lignocellulosic biomass and coal has been recognized as a promising route to deliver carbon neutrality. In this study, the combustion characteristics of lignocellulosic biomass main components (cellulose, hemicellulose, and lignin), one kind of lignite, and their mixtures were investigated under air and oxy-fuel conditions, respectively. The combustion reactivity of cellulose and hemicellulose was better than that of lignite, while that of lignin was worse. Under the oxy-fuel condition with the same oxygen mole fraction as air, the combustion of cellulose and hemicellulose was slightly delayed, while a distinct combustion delay was observed for lignin similar to lignite. For the co-combustion, compared to lignite, the combustion reactivity could be improved by blending cellulose and hemicellulose, while it got worse by mixing lignin. Cellulose and hemicellulose had a positive synergistic effect on the release of lignite volatiles, while lignin showed a negative synergy on it. All the lignocellulosic components showed positive synergies with lignite on the char oxidation due to the high structural porosity of lignite char and the advanced oxidation of lignocellulosic char.

Measurement Technique of Oxygen Concentration in Narrow Channels of PEFCs Based on Transmission Laser Absorption Spectroscopy

Oxygen transport at cathodes of polymer electrolyte fuel cells (PEFCs) is one of the key factors for determining their power generation performance. To maintain their stable operations, it’s necessary to establish a technique for real-time monitoring of oxygen concentration inside cathode channels of operational cells. In the previous works, the measurements of oxygen distribution within gas channels of operating PEFCs have been conducted using gas chromatograph (GC) [1], oxygen-quenching of fluorescence and phosphorescence [2], and tunable diode laser absorption spectroscopy (TDLAS) [3]. GC techniques provide better accuracy for gas detection in flow channels, however, it takes a long time (at least a few minutes) to collect a gas sample and identify a species. On the other hand, TDLAS enables the non-invasive in-situ monitoring of chemical species inside cells with a high time resolution of 100 milliseconds. Fujii et al. [3] applied the single-ended TDLAS sensor to measure the oxygen concentration in a PEFC channel. In their experiments, since the laser beam is injected perpendicular to the flow direction, the optical path length is extremely short and the sensitivity is insufficient. The detection sensitivity of oxygen is relatively low due to the weak infrared absorption. Therefore, the optical path needs to be lengthened to measure its concentration with high accuracy. In this study, to solve the above-mentioned problem, we developed the fiber-optic transmission TDLAS system and applied it to the quantitative measurement of oxygen concentration in the narrow channel of a simulated cell.Fig. 1 shows the experimental apparatus for measuring the oxygen concentration in a simulated flow cell using the fiber-optic transmission TDLAS system. TDLAS is one of the high sensitive absorption spectroscopy techniques which measure the absorbance of a chemical species and identify its concentration using infrared diode laser. The triangular modulated beam emitted from a DFB laser diode (wavelength: 764 nm) is directly injected into the straight narrow channel (width and depth: 1.0 or 1.5 mm, length: 50 mm) through the single-mode optical fiber. To collimate the laser beam at the channel inlet, a GRIN lens is attached at the top of the pitch fiber. The injected beam passes one or two channels along the flow direction. In the case of passing two channels, the beam transmitted through the first channel is turned 180 degrees by a triangular prism at the bend section (length: 5 mm). The returned beam is passed through the second channel and detected at a photodiode. The output signal is transmitted to a lock-in amplifier, and then the second harmonic (2f) absorption spectrum can be obtained by phase-sensitive detection. In the experiments, the mixed gas of oxygen and nitrogen was supplied to the channel at 100 ml/min. The oxygen concentration in the channels was adjusted to 0 to 100% by controlling each gas flow rate. This measurement was performed at 25 oC and 1 atm.Fig. 2 presents the 2f absorption signals of oxygen in the narrow channel of the simulated flow cell. The incident laser beam passed two straight channels (channel width and depth: 1.5 mm, optical path: 105 mm). The oxygen mole fraction was varied from 0 to 100%. Although the TDLAS measurements were performed inside the narrow channels, the optical fringe noise was hardly observed and the 2f spectra were clearly obtained. It was also noted that the peak-valley height of 2f spectrum increased with an increase in oxygen mole fraction. The oxygen concentration in fuel cell channels can be quantitatively estimated from only the 2f spectrum data using the fiber-optic transmission TDLAS technique.

Measurement Technique of Oxygen Concentration in Narrow Channels of PEFCs Based on Transmission Laser Absorption Spectroscopy

The oxygen transport on the cathode side of polymer electrolyte fuel cells (PEFCs) is an important factor that determines their performance. This study presents a fiber-optic gas sensing technique for monitoring the oxygen concentration in flow channels of operating PEFCs based on transmission tunable diode absorption spectroscopy (TDLAS). As the first stage, the fiber-optic transmission TDLAS system was developed and applied to the ex-situ measurement of oxygen concentration in the narrow channels of a simulated flow cell. It was found that the second-harmonic (2f) absorption spectra of oxygen can be clearly detected without optical noises even though the channel width is narrowed to 1.0 mm. The calibration results obtained from the TDLAS measurements enable to accurately estimate the oxygen mole fraction within the flow cell. The fiber-optic transmission TDLAS is applicable for the oxygen monitoring in submillimeter-scale channels of actual PEFCs.

Experimental and kinetic study on the pyrolysis and oxidation of isopentane in a jet-stirred reactor

An experimental and kinetic study on the pyrolysis and oxidation of isopentane (2-methylbutane) were conducted in this work. The experiments were performed in a jet-stirred reactor (JSR) at the equivalence ratios of 0.5, 1.0, 2.0 and ∞, across the temperature range from 700 to 1100 K, and at atmospheric pressure. Mole fractions of oxygen, hydrogen, CO, CO2, C1C6 hydrocarbons and methanol were measured using a gas chromatograph (GC), at the initial fuel mole fraction of 0.5% and residence time at 2 s. Three literature kinetic models, named as the Bugler model, the NUIGMech1.1 model, and the LLNL model, were employed to predict the speciation profiles measured in this work, and the ignition delay times in the literature. Based on the model performances and kinetic analysis, some modifications were made to the LLNL model, by supplementing the beta-scission reaction aC5H11 = C4H8–1 +CH3, and updating the rate constants for the reactions iC5H12 + OH = cC5H11 + H2O, iC5H12 + OH = bC5H11 + H2O, C3H4-a = C3H4-p, C3H4-a + H = C3H4-p + H, and C2H6 + CH3 = C2H5 +CH4. After the modifications, the model predictions on mole fractions of 1-butene, ethane, allene and propyne in JSR pyrolysis and oxidation were improved, and the overestimations on the ignition delay times at low temperatures are significantly reduced. Reaction pathway and sensitivity analyses were carried out using the modified model. The results indicated that fuel consumption in pyrolysis is sensitive to the unimolecular decomposition reaction iC5H12 = iC3H7 + C2H5 across the temperature range of 900–1100 K. In addition, fuel low-temperature oxidation reactivity is sensitive to the mutual conversion between HO2 and H2O2, while the competition between OH and HO2 formation has a more pronounced effect at increased temperatures.

Numerical Simulation Study on Coal Spontaneous Combustion: Effect of Porosity Distribution

ABSTRACT The non-uniform distribution of coal porosity has a considerable impact on coal spontaneous combustion (CSC) characteristics. In this study, a one-step global coal oxidation numerical model which took into account the thermal buoyancy and the generation and consumption of multi-component substances was established. The movement of the hot spot and the evolution characteristics of reactants during CSC were researched on. The results suggest that in the early stage of coal self-heating, the hot spots are generated in the high-porosity areas. As coal oxidation intensifies, the hot spots move to the air supply positions. Different coal porosity distributions result in different thermal runaway times, and the coal samples are prone to thermal runaway when the high-porosity areas are near the air supply positions. Resultantly, the movement path of the hot point becomes longer, leading to a larger reduction of average density. Meanwhile, the corresponding thermal runaway and the drastic change in oxygen mole fractions occur later, and the drastic change in thermal runaway is more likely to lag behind that in the oxygen mole fraction. Compared with the gradually changing porosity distribution, the abruptly changing porosity distribution tends to hinder the development of coal oxidation perpendicular to the sudden change cross-section.

The oxygen-deficient combustion and its effect on the NOx emission in a localized stratified vortex-tube combustor

The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13–0.21) on flame configuration, combustion stability, combustion efficiency, and NOx emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NOx generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NOx emission of below 10 ppm (@ 15 vol% O2) due to low oxygen concentration and flame temperature. Furthermore, part of NOx entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H2) to further lowering NOx emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.

Modeling soot formation in laminar coflow ethylene inverse diffusion flames

A numerical study is conducted using the CoFlame code to investigate the sooting characteristics of ethylene-fueled laminar coflow inverse diffusion flames (IDFs), aiming to gain insights into the different flame structure and soot formation features between an IDF and a normal diffusion flame (NDF). The effects of oxygen mole fraction in the oxidizer stream, denoted as Oxygen Index (OI), on flame structure and soot production are studied over the OI range of 21% to 33%. The soot model parameters were chosen to produce overall reasonably good agreement between the predicted and measured in terms of soot volume fraction, mean primary particle size, and the mean number of primary particles per aggregate in both the normal and inverse configurations. A sensitivity study showed that soot nucleation and soot surface growth by PAH adsorption modeling should be considered simultaneously as a strongly coupled process through competing for polycyclic aromatic hydrocarbons (PAHs). The IDF configuration displays distinct characteristics from the NDF one in terms of flame structure as well as soot production. In IDFs soot is formed along the outer edge of reaction front, and does not cross the reaction front to the oxidizer, decoupling completely soot oxidation which leads to soot emission from the open flame tip. Modeling results indicate that in the IDF configuration the hydrogen abstraction acetylene addition (HACA) process is severely suppressed, while the adsorption of PAH molecules is dominant to soot surface growth at relatively low temperatures. Nucleation of new particles is significant all along the flame height, causing emission of young soot. IDF represents an ideal configuration to study soot surface growth by PAH adsorption. Increasing OI enhances all the soot processes, especially surface growth by PAH adsorption. Although soot oxidation rate by OH radicals is also strongly enhanced, it remains about two orders of magnitude lower than soot surface growth rate by PAH adsorption.

The role of oxygenated species in the growth of graphene, fullerenes and carbonaceous particles

The growth of carbonaceous materials was studied using a Kinetic Monte Carlo model that captures the growth and oxidation of six-member and partially-embedded five-member rings. A novel algorithm was used to resolve the migration of partially-embedded five-member rings. Circumcoronene molecules were grown at 1500 K and 1 atm in the presence of varying mole fractions of atomic and molecular oxygen and constant mole fractions of hydrogen and acetylene. Four regions of carbon growth associated with different carbonaceous products were identified. Graphene was formed in the presence of high mole fractions of atomic oxygen 10−4<XO≤10−2. Fullerenes were formed in the presence of low mole fractions of atomic oxygen and high mole fractions of molecular oxygen (XO ≤ 10−4 and 10−2 < XO2 ≤ 10−1). Low mole fractions of both atomic and molecular oxygen (XO ≤ 10−4 and XO2 ≤ 10−2) resulted in structures that became curved as time progressed. Small structures were found at the highest mole fractions of atomic oxygen (XO > 10−2). The production and consumption of partially-embedded five-member rings appear to explain the formation of the observed structures. The oxidation of partially-embedded five-member rings produces armchair sites that grow to form flat graphenic structures. Formation and subsequent embedding of partially-embedded five-member rings result in curved structures that resemble fullerenes.

Shipboard observations of atmospheric oxygen in the Southern Ocean during the 2017–2018 austral summer

An in situ measurement system was developed for continuous observations of the mole fractions of atmospheric oxygen (defined as δ(O2/N2)) and carbon dioxide (CO2) and the continuous observation was conducted onboard the research vessel SHIRASE 5003 during its voyage between Australia and Syowa Station, Antarctica, in 2017–2018. The CO2 variation was low with respect to that of δ(O2/N2) in the Southern Ocean, suggesting that the land biospheric and fossil-fuel derived CO2 and O2 emissions negligibly influenced the observations. Therefore, the observed significant variations in the atmospheric O2/N2 can be attributed to the atmosphere-ocean gas exchange. During the southbound voyage in December 2017, we observed large spatial variations in δ(O2/N2) due to marine biological production on the western side of the cruise track. Oceanic O2 fluxes based on a simple model and atmospheric δ(O2/N2) variability were consistent with past oceanic observations. No clear longitudinal gradient in δ(O2/N2) was observed at latitudes toward south of 60° S in December 2017 and February–March 2018. However, local δ(O2/N2) maxima were observed in regions with active marine biological production in December 2017. These observations indicate that local O2 fluxes can also modify the spatial distribution of atmospheric δ(O2/N2) in the Southern Ocean.

The radical index and the effect of oxygen concentration on non-premixed cool flame extinction of large n-alkanes

The effects of fuel and oxygen concentrations on the extinction limits of non-premixed cool flames for large n-alkanes, ranging from n-heptane to n-dodecane, are measured in an atmospheric counterflow burner. The measurements are used (a) to derive a scaling law between the cool flame reactivity and the oxygen concentration and (b) to develop a radical index to rank the cool flame reactivity of n-alkanes, with the assistance of kinetic modeling. First, due to the significant role of multiple oxygen addition reactions in the low-temperature branching sequence, the relationship between cool flame extinction (and hence the chemical time scale) and oxygen concentration are examined. The results suggest that all measured n-alkane cool flame extinction limits exhibit similar nonlinear dependence on oxygen concentration. The results further show that, due to the combined effects of multi-oxygen addition processes and the negative temperature coefficient effect, the cool flame low temperature reactivity of n-alkanes is proportional to nth power of oxygen mole fraction, [O2]n, with n = 1.5–2. Furthermore, by accounting for thermal and mass transport effects using a mass-weighted enthalpy, a cool flame radical index is proposed to evaluate the role of cool flame reactivity on the extinction limits of large n-alkanes. We find that the radical index of large n-alkanes increases with the increase of carbon length of the fuel molecules, which is consistent with stronger low-temperature reactivity.

NOx emission characteristics of CH4 versus O2/CO2 counterflow non-premixed flames at various pressures up to 300 atm

The characteristics of NOx emission of oxy-methane combustor operating over a wide range of pressure (1–300 atm) are investigated by performing one-dimensional numerical simulations of CH4 versus O2/CO2 counterflow non-premixed flames. The amount of NOx emission is estimated by varying the pressure, oxygen mole fraction, strain rate, and N2 amount in the fuel and oxidizer streams. It is found that N2 ingress in the oxidizer stream is more critical to the NOx emission than that in the fuel stream because the reaction zone for NOx formation develops in the oxidizer side. It is also found that the amount of NOx emission non-monotonically varies with the increase of pressure at relatively-low oxygen mole fraction. The NOx reaction pathway analysis demonstrates that such a non-monotonic behavior of NOx emission is mainly attributed to reduction in NOx formation via prompt NOx and NOx reburn pathways at high pressure. The sensitivity of NOx emission to several chemical kinetic mechanisms is examined under various combustion conditions, which reveals important reaction pathways for NOx formation/consumption.